Chapter 37 — Organometallic Chemistry: Transition Metal Catalysis and Modern Synthesis

"Pd-catalyzed cross-coupling has transformed synthetic organic chemistry. From a curiosity in the 1970s to ubiquitous in pharmaceutical synthesis by 2010, Pd chemistry now appears in 20-40% of all drug syntheses. The Nobel Prize in 2010 (Heck, Negishi, Suzuki) recognized just one part of an enormous field." — paraphrase from a synthesis text

"Olefin metathesis (Grubbs, Schrock, Chauvin — 2005 Nobel) lets you break and reform C=C bonds at will. With one catalyst, you can close rings, swap alkene partners, or polymerize cyclic alkenes. The chemistry seemed impossible until it worked."

This chapter introduces transition metal catalysis in organic synthesis. The two most important areas: 1. Pd-catalyzed cross-coupling reactions (Suzuki, Heck, Negishi, Stille, Sonogashira, Buchwald-Hartwig). 2. Olefin metathesis (Grubbs/Schrock catalysts; RCM, cross-metathesis, ROMP).

Both are recognized by Nobel Prizes (2010 for Pd cross-coupling; 2005 for olefin metathesis). Both are used industrially every day.

By the end of this chapter you should be able to: - Identify the elementary steps of transition-metal catalysis: oxidative addition (OA), reductive elimination (RE), migratory insertion (MI), β-hydride elimination, transmetalation. - Predict products of Suzuki, Heck, Negishi, Sonogashira, Buchwald-Hartwig couplings. - Recognize olefin metathesis products (RCM, cross-metathesis, ROMP). - Apply Pd cross-coupling and metathesis to drug and natural product synthesis. - Appreciate the breadth of organometallic chemistry: from polymerization (Ziegler-Natta) to C-H activation (modern).

37.1 The elementary steps of transition-metal catalysis

Transition metals catalyze organic reactions by cycling through oxidation states via a few key elementary steps.

Oxidative addition (OA)

A metal $M^n$ inserts into a bond, increasing its oxidation state by 2:

$$M^0 + R-X \to M^{II}(R)(X)$$

The metal goes from $0$ to $+2$ (or $+1$ to $+3$). The R and X groups are now bonded to the metal.

For Pd cross-coupling: $Pd^0 + Ar-X \to Pd^{II}(Ar)(X)$. The Pd has inserted into the C-X bond.

The reactivity depends on the C-X bond strength: aryl iodides > aryl bromides > aryl chlorides ≫ aryl fluorides. Cross-coupling of aryl iodides is fast; aryl chlorides need special catalysts (e.g., Buchwald's bulky phosphine ligands).

Reductive elimination (RE)

The reverse of OA: two ligands on the metal combine, releasing the product and reducing the metal by 2:

$$M^{II}(R)(R') \to M^0 + R-R'$$

For Suzuki: $Pd^{II}(Ar)(Ar') \to Pd^0 + Ar-Ar'$. The two aryl groups combine to form a biaryl product, regenerating Pd(0).

RE is the C-C bond-forming step in Pd cross-coupling.

Migratory insertion (MI)

A ligand on the metal "inserts" into another bond:

$$M(R)(L) \to M-(R-L)$$

For example, $M-H$ inserting into a $C=C$: $$M-H + C=C \to M-C-C-H$$

The H migrates from M to one carbon of the C=C; the bond between M and the other carbon is now formed.

This is how olefin polymerization works (Ziegler-Natta): an Al-CH₃ group inserts into the C=C of ethylene, generating a longer chain bonded to Al.

β-Hydride elimination

The reverse of migratory insertion: a metal-bound alkyl group has a β-H that can transfer to the metal, leaving an alkene:

$$M-CH_2-CH_2-R \to M-H + CH_2=CH-R$$

This is a major issue in some catalytic cycles (it can short-circuit reactions). Catalysts are designed to suppress β-H elimination when needed.

Transmetalation

Transfer of an organic group from one metal to another:

$$M(X) + M'-R \to M(R) + M'(X)$$

For Suzuki: $Pd^{II}(Ar)(X) + Ar'-B(OH)_2 + \text{base} \to Pd^{II}(Ar)(Ar') + X-B(OH)_2 + ...$. The boron transfers its aryl to the Pd.

Transmetalation is the step where the second nucleophile/coupling partner enters the catalytic cycle.

37.2 The Pd-catalyzed cross-coupling reactions

These are the workhorse C-C bond-forming reactions of modern synthesis. All follow the same general catalytic cycle:

General catalytic cycle (using Suzuki as the example)

1. Oxidative addition: $Pd^0 + Ar-X \to Pd^{II}(Ar)(X)$. Aryl halide adds to Pd.
2. Transmetalation: $Pd^{II}(Ar)(X) + Ar'-B(OH)_2 + \text{base} \to Pd^{II}(Ar)(Ar') + X-B(OH)_2 + base \cdot HX$. Boron's aryl group transfers to Pd.
3. Reductive elimination: $Pd^{II}(Ar)(Ar') \to Pd^0 + Ar-Ar'$. Biaryl forms; Pd(0) regenerated.
4. Pd(0) re-enters the cycle.

Net reaction: $Ar-X + Ar'-B(OH)_2 + \text{base} \to Ar-Ar' + B(OH)_3 + base \cdot HX$.

The catalytic amount of Pd (1-5 mol%) processes many substrate molecules.

Suzuki coupling (boronic acid)

$$Ar-X + Ar'-B(OH)_2 + Pd(0) + \text{base} \to Ar-Ar' + B(OH)_3 + ...$$

• Coupling partner: aryl/alkyl boronic acid or boronate ester.
• Catalyst: Pd(PPh₃)₄ or Pd(OAc)₂ + phosphine ligand.
• Base: K₂CO₃, K₃PO₄, NaOH (varies).
• Solvent: water + organic mixture; ethanol; THF.
• Temperature: room temperature to reflux, depending on substrate.

Suzuki is the most-used Pd cross-coupling. It tolerates many functional groups, gives clean products, and is used at industrial scale.

Heck reaction (alkene coupling)

$$Ar-X + CH_2=CHR + Pd(0) + \text{base} \to Ar-CH=CHR + base \cdot HX$$

• Coupling partner: alkene.
• Catalyst: Pd(0) + phosphine.
• Mechanism: OA → MI (alkene inserts into Pd-Ar bond) → β-H elimination (gives the new alkene + Pd-H) → base regenerates Pd(0).
• Product: substituted styrene-like alkene, with the new aryl group at the position where H was.

Heck reactions give new aryl-vinyl bonds (Ar-CH=CH-R rather than Ar-Ar).

Sonogashira coupling (alkyne coupling)

$$Ar-X + HC \equiv CR + Pd(0) + Cu(I) + \text{amine} \to Ar-C \equiv CR + ...$$

• Coupling partner: terminal alkyne (HC≡CR).
• Catalysts: Pd + Cu(I) co-catalyst.
• Mechanism: Cu deprotonates the terminal alkyne to give Cu-acetylide; the Cu-acetylide transmetalates to Pd; Pd then forms the new C-C bond.

Sonogashira gives aryl-alkyne products (Ar-C≡CR).

Stille coupling (organotin)

$$Ar-X + Ar'-SnR_3 + Pd(0) \to Ar-Ar' + SnR_3X$$

• Coupling partner: organostannane (R₃Sn-Ar').
• Less common today due to tin toxicity.

Negishi coupling (organozinc)

$$Ar-X + Ar'-ZnX + Pd(0) \to Ar-Ar' + ZnX_2$$

• Coupling partner: organozinc.
• Excellent for sp³-sp² couplings (alkyl-aryl bonds).
• 2010 Nobel Prize subject.

Buchwald-Hartwig amination (C-N coupling)

$$Ar-X + HNR_2 + Pd(0) + \text{base} \to Ar-NR_2 + base \cdot HX$$

• Coupling partner: amine.
• Mechanism: OA → amide deprotonation by base → coordination to Pd → RE.
• Useful for making aryl-amine bonds (which are otherwise hard, since Ar-X + amine without catalyst doesn't work for non-activated halides).

Why Pd is special

Pd works because: - It's a late transition metal with a stable Pd(0) ↔ Pd(II) cycle. - It tolerates many functional groups (esters, ketones, etc.). - It accepts bulky phosphine ligands that tune reactivity. - The Pd(0) is kinetically inert to most nucleophiles, allowing transmetalation to be the rate-determining step.

Other metals (Ni, Pt, Cu, Rh, Ir) catalyze related reactions, but Pd is the most-developed and most-used.

37.3 Olefin metathesis

A different organometallic chemistry: breaking and reforming C=C bonds. The catalyst (Grubbs or Schrock complex) holds an alkene as a metal-carbene intermediate.

The general mechanism

Olefin metathesis exchanges the substituents on two alkenes:

$$R_1-CH=CH-R_2 + R_3-CH=CH-R_4 \to R_1-CH=CH-R_3 + R_2-CH=CH-R_4$$

Mechanism: a metal carbene (M=CHR₁) reacts with an alkene to form a metallacyclobutane (4-membered ring with M and 3 C). The metallacyclobutane then breaks open in a different direction, giving a new metal carbene and a new alkene.

The metal carbene is a catalyst that catalyzes the alkene scrambling.

Three main applications

Ring-closing metathesis (RCM)

A diene with two terminal alkenes cyclizes to a cyclic alkene + ethylene:

$$\text{H}_2\text{C=CH-(CH}_2\text{)n-CH=CH}_2 \xrightarrow{\text{Grubbs cat.}} \text{cyclic alkene} + \text{H}_2\text{C=CH}_2$$

Used to make medium-large rings (5- to 12-membered) that are otherwise hard to close. Applied in many natural product syntheses.

Cross-metathesis

Two different terminal alkenes give a new alkene:

$$R_1-CH=CH_2 + R_2-CH=CH_2 \to R_1-CH=CH-R_2 + H_2C=CH_2$$

Equilibrium-controlled; ethylene release shifts the equilibrium. Trans alkene is typical product.

Ring-opening metathesis polymerization (ROMP)

A strained cyclic alkene (e.g., norbornene) opens and polymerizes:

$$\text{cyclic alkene} \xrightarrow{\text{Grubbs cat.}} \text{polymer with C=C in backbone}$$

Used to make specialty polymers with controlled molecular weight and tacticity.

The Grubbs and Schrock catalysts

• Grubbs catalyst (Ru-based): more functional-group tolerant; works in air; Grubbs received Nobel in 2005. Generations 1, 2, and 3 have improved activity.
• Schrock catalyst (Mo-based): more reactive; less functional-group tolerant; requires inert atmosphere. Schrock received Nobel in 2005.
• Chauvin (the third 2005 Nobelist): proposed the metallacyclobutane mechanism.

Industrial uses

• Drug synthesis: many natural products (especially macrocycles) made by RCM. Examples: Boceprevir (an HCV protease inhibitor) uses RCM in its synthesis.
• Polymers: ROMP polymers used in specialty applications.
• Fine chemicals: cross-metathesis for making specific alkene products.

37.4 Polymerization with transition metals

Transition metals catalyze alkene polymerization at industrial scale.

Ziegler-Natta catalysis

Ziegler (1953) discovered that TiCl₄ + AlEt₃ catalyzes the polymerization of ethylene to high-density polyethylene (HDPE). Natta extended to propylene → isotactic polypropylene. Both received the 1963 Nobel Prize.

The mechanism: an alkyl group on Al transfers to Ti; ethylene inserts (migratory insertion) into the Ti-alkyl; the chain grows, alternately inserting on Ti and migrating to Al. Polymer molecular weights reach 100,000 to 1,000,000 Da.

Metallocene catalysis

Group IV metallocenes (zirconocene, titanocene) bound to MAO (methylaluminoxane) catalyze alkene polymerization with control of: - Tacticity (isotactic, syndiotactic, atactic). - Molecular weight. - Comonomer incorporation.

Modern PP, PE, PVC, and many specialty polymers are made by metallocene catalysis.

Industrial scale

Polyethylene production: ~120 million tons/year. Polypropylene: ~80 million tons/year. Together, they account for ~50% of all plastic production.

The chemistry is Chapter 37 organometallic chemistry, scaled to industrial production.

37.5 C-H activation: the new frontier

The "holy grail" of organic chemistry has long been selective C-H bond activation: replacing a specific C-H with a new C-X bond. Without C-H activation, you must use halogenated starting materials (or other functionalized substrates).

Modern C-H activation uses transition metal catalysts (Pd, Rh, Ir, Co, Cu, etc.) to functionalize specific C-H bonds:

• Sp² C-H activation (aromatic): proximity-directed; e.g., a directing group on the aromatic ring directs the metal to a specific position.
• Sp³ C-H activation: harder; requires careful catalyst design.

C-H activation has revolutionized late-stage functionalization in drug discovery — you can take a complex molecule and selectively modify one C-H bond without disrupting the rest.

The chemistry: C-H activation typically follows a path: 1. Substrate binds the metal via a directing group. 2. C-H undergoes "concerted metalation-deprotonation" (CMD) or other mechanism. 3. Functionalization step (insertion, transmetalation, etc.). 4. Reductive elimination releases the product.

This is an active research area; many recent papers focus on novel C-H activations.

37.6 Other organometallic methods

Wacker process (industrial)

$Pd^{II}/Cu^{II}$ + ethylene + O₂ → acetaldehyde. The classic homogeneous catalysis, used industrially since the 1950s.

Hydroformylation

Co or Rh + alkene + H₂ + CO → aldehyde. Industrial production of aldehydes from alkenes.

Asymmetric organometallics

• BINAP (Noyori): chiral phosphine for asymmetric hydrogenation (Ch 36 case study 2).
• DiPAMP (Knowles): chiral phosphine for asymmetric hydrogenation; basis of L-DOPA synthesis.
• PHOX (phosphinooxazoline) ligands: for asymmetric Pd allylic alkylations.

The 2001 Nobel Prize (Knowles, Noyori, Sharpless) recognized asymmetric catalysis. Modern asymmetric synthesis relies heavily on chiral organometallic catalysts.

37.7 Industrial and pharmaceutical applications

Buchwald-Hartwig in drug synthesis

The Buchwald-Hartwig amination (Pd + Ar-X + amine → Ar-NR₂) is now a standard method for making aryl-amine bonds. Used in many drug syntheses where direct nucleophilic aromatic substitution doesn't work.

Suzuki in drug synthesis

The Suzuki coupling is used in ~20-40% of drug syntheses. It builds biaryl scaffolds (key in many drugs) and aryl-alkyl bonds.

Ring-closing metathesis in macrocycles

Many macrocyclic drugs (e.g., HCV protease inhibitors like simeprevir) are made by RCM. The RCM closes the large ring efficiently when other methods fail.

Asymmetric metathesis

Modern Mo and Ru catalysts achieve enantioselective metathesis for chiral natural products and pharmaceuticals.

37.8 Why this chapter matters

Transition metal catalysis is essential for: 1. Pharmaceutical synthesis: ~30% of drugs use Pd cross-coupling. 2. Materials chemistry: Ziegler-Natta polyethylene, metallocene PP. 3. Natural product synthesis: many syntheses use RCM for ring closure. 4. Asymmetric chemistry: chiral ligands give enantioselective catalysis. 5. Future directions: C-H activation, photoredox catalysis, biocatalysis.

Master Chapter 37 to understand modern synthetic chemistry as practiced in industry.

37.9 The 2010 Nobel Prize: Heck, Negishi, Suzuki

The 2010 Nobel Prize in Chemistry was awarded to: - Richard F. Heck (University of Delaware): the Heck reaction (Pd + Ar-X + alkene → Ar-CH=CR-). - Ei-ichi Negishi (Purdue University): Negishi coupling (Pd + Ar-X + R-ZnX → Ar-R). - Akira Suzuki (Hokkaido University): Suzuki coupling (Pd + Ar-X + Ar'-B(OH)₂ → Ar-Ar').

These three reactions revolutionized organic synthesis. Together with Stille coupling (J. K. Stille, 1978; ArX + Ar'-SnR₃ → Ar-Ar'), Sonogashira coupling (Sonogashira, 1975; ArX + HC≡CR → Ar-C≡CR), and Buchwald-Hartwig amination (Buchwald + Hartwig, 1990s; ArX + amine → Ar-NR₂), the family of Pd-catalyzed cross-couplings forms the backbone of modern synthesis.

The big picture

Before Pd cross-coupling (~1970), making a biaryl bond was hard: - Ullmann coupling (1900s): two aryl halides + Cu, very high T → biaryl. Limited scope. - Ferro-coupling: with iron catalysts; specific substrates. - Other methods: lithiation, stoichiometric Cu reagents.

After Pd cross-coupling (1980+): - Make any biaryl bond in mild conditions. - Functional group tolerance. - Predictable regiochemistry. - Gram-scale to ton-scale.

The impact on pharmaceuticals: from ~5% of drug candidates having Pd-coupling steps in 1990 to ~50% in 2020. Modern drug development depends on Pd cross-coupling.

Industrial scale

Pd cross-coupling is used in: - Imatinib (Gleevec) and other kinase inhibitors. - Sitagliptin (Januvia, anti-diabetic). - Many statins (atorvastatin et al.). - Beta-blockers, anti-hypertensives. - Anticancer drugs. - And many more.

Multi-billion dollars of pharmaceutical production each year depends on Pd cross-coupling.

37.10 The 2005 Nobel Prize: Chauvin, Schrock, Grubbs

The 2005 Nobel Prize was awarded to: - Yves Chauvin (Institut Français du Pétrole): proposed the metallacyclobutane mechanism (1971). - Richard R. Schrock (MIT): developed Mo and W metathesis catalysts. - Robert H. Grubbs (Caltech): developed Ru-based catalysts for olefin metathesis.

Olefin metathesis: the chemistry

Olefin metathesis exchanges substituents between two alkenes:

$$R-CH=CH-R + R'-CH=CH-R' \xrightarrow{cat.} R-CH=CH-R' + R-CH=CH-R'$$

The mechanism (Chauvin, 1971): a metal carbene (M=CHR) reacts with an alkene to form a metallacyclobutane (4-membered ring with M); this opens in the opposite direction to give a new metal carbene + new alkene.

Catalysts evolved

• Schrock catalysts (1990): Mo or W carbenes; very active but air-sensitive.
• Grubbs 1st generation (1996): Ru carbene; more functional group tolerant.
• Grubbs 2nd generation (2000): Ru carbene with N-heterocyclic carbene ligand; even more functional group tolerant.
• Hoveyda-Grubbs (2002): modification with isopropoxy group; more recyclable.
• Z-selective Grubbs catalysts: give Z-alkenes selectively (a recent advance).

Applications

• Pharmaceutical synthesis: many ring-closing metathesis (RCM) steps in drug syntheses.
• Natural product synthesis: macrolides, alkaloids, terpenes.
• Polymer chemistry: ROMP for cyclic alkene polymerization (e.g., norbornene → polynorbornene).
• Industrial chemistry: butene-to-propene metathesis (Lyondell process).

37.11 Detailed mechanisms

Suzuki coupling mechanism

1. Oxidative addition: Pd(0) + ArBr → Pd(II)(Ar)(Br). Pd inserts into C-Br bond.
2. Transmetalation: Pd(II)(Ar)(Br) + Ar'-B(OH)₂ + base → Pd(II)(Ar)(Ar') + B(OH)₂(Br)(OH⁻). - The base (NaOH or K₂CO₃) coordinates to B; activates B-Ar bond for transmetalation. - Aryl group transfers from B to Pd.
3. Reductive elimination: Pd(II)(Ar)(Ar') → Pd(0) + Ar-Ar'. New C-C bond formed.
4. Catalyst regeneration: Pd(0) is back; cycle continues.

The base is critical for the transmetalation step. Without base, the boron species is too unreactive.

Heck reaction mechanism

1. Oxidative addition: Pd(0) + Ar-X → Pd(II)(Ar)(X).
2. Alkene coordination: alkene binds to Pd.
3. Migratory insertion (MI): Ar migrates from Pd to one carbon of the alkene; Pd moves to the other.
4. β-Hydride elimination: Pd-H bond forms; new C=C bond forms (anti-Markovnikov regiochemistry).
5. Reductive elimination: Pd(0) regenerated; HX released.

The Heck reaction gives an aryl group on a vinyl carbon; the original alkene's H ends up bonded to Pd then released as HX.

Buchwald-Hartwig amination mechanism

1. Oxidative addition: Pd(0) + Ar-X → Pd(II)(Ar)(X).
2. Amine coordination: HNR₂ binds to Pd.
3. Deprotonation: external base removes H from N; gives Pd(II)(Ar)(NR₂).
4. Reductive elimination: Ar-NR₂ + Pd(0).

This forms a C-N bond from an aryl halide and an amine — a powerful method for aryl amine synthesis.

Olefin metathesis mechanism (Chauvin)

1. Coordination: alkene binds to M=CHR carbene.
2. [2+2] cycloaddition: gives metallacyclobutane.
3. Retro-[2+2]: ring opens in the opposite direction.
4. Result: original alkene's substituents have been swapped; new metal carbene formed.
5. Cycle continues.

The key intermediate: the metallacyclobutane (4-membered ring with M, 3 C). It can open in two directions, giving either the starting materials or the products.

37.12 Industrial applications in detail

Pharmaceutical manufacturing

Sitagliptin (Januvia)

A blockbuster anti-diabetic drug. The key step uses Buchwald-Hartwig amination + Pd-catalyzed asymmetric reduction.

Sorafenib (cancer drug)

Heck reaction used to install a chlorinated aryl-vinyl bond.

Dasatinib (cancer drug)

Suzuki coupling installs the biaryl scaffold of the kinase inhibitor.

Crizotinib (lung cancer drug)

Buchwald amination installs a key amine bond.

Multiple kinase inhibitors

Almost every modern kinase inhibitor uses Suzuki, Buchwald, or Heck coupling somewhere in the synthesis.

Materials science

OLED materials

Organic light-emitting diodes (OLEDs) use polymers and small molecules synthesized by Suzuki coupling. The biaryl scaffold gives the right π-conjugation for visible emission.

Conductive polymers

Conductive polymers like PEDOT:PSS use coupling chemistry. The Sonogashira coupling installs alkyne linkers.

Liquid crystal materials

Many LCD materials use Suzuki coupling to install biaryl backbones with specific properties.

Polymer industry

Ziegler-Natta polymerization

The classical method for polyolefins (HDPE, polypropylene). Uses TiCl₃ + Al(C₂H₅)₃ catalysts at industrial scale (~150 million tons/year of HDPE and PP combined).

Metallocene catalysts

Modern metallocene catalysts (Cp₂ZrCl₂ + MAO) give more controlled stereochemistry: isotactic, syndiotactic, or atactic polypropylene depending on catalyst design.

ROMP polymers

Ring-opening metathesis polymerization (ROMP) of cyclic alkenes: - Norbornene → polynorbornene (high-impact polymer; Vestenamer). - Cyclooctene → polycyclooctene. - Other cyclic alkenes with various substituents.

37.13 Comparison of Pd cross-couplings

Most use the same OA → transmetalation → RE cycle but with different transmetalating reagents and conditions.

When to use which?

• Suzuki: most popular; arylboronic acids are stable, non-toxic, easy to make.
• Heck: when you need an aryl-alkene bond (vs aryl-aryl).
• Sonogashira: when you need an aryl-alkyne bond.
• Negishi: for sp³-sp² couplings (e.g., alkyl Zn + aryl halide).
• Stille: for special cases where boronic acid doesn't work; tin reagents are more nucleophilic.
• Buchwald-Hartwig: for C-N bond formation (replaces SNAr in many cases).

37.14 C-H activation: the new frontier

Traditional cross-coupling needs a pre-functionalized substrate (Ar-X). C-H activation lets you couple at unactivated C-H bonds:

$$Ar-H + R-X \xrightarrow{Pd, \text{ligand}} Ar-R$$

The challenges: - Aryl C-H bonds are unreactive (~110 kcal/mol). - Selectivity: which C-H gets activated? - Catalyst design.

Directing groups

Most C-H activation uses a "directing group" (DG) — a coordinating group near the C-H that brings the metal close. Examples: - 2-pyridyl: directs to C-H ortho. - Amide: directs to ortho. - Carboxylate, carbonyl: similar.

Examples

• Ortho-functionalization of arenes: with DG; pioneer work by Murai (1993) on Ru-catalyzed.
• Late-stage diversification of drugs: a drug late in synthesis is functionalized at specific C-H bonds without re-doing the whole synthesis.
• Asymmetric C-H activation: chiral catalysts give enantioselective C-H functionalization.

Modern catalysts

• Pd-catalyzed: with directing groups.
• Rh-catalyzed: especially for ortho-functionalization.
• Ir-catalyzed: with N-heterocyclic carbene ligands.
• Ni-catalyzed: cheaper alternative to Pd.

C-H activation is a rapidly developing field. By 2030, expect to see widespread industrial use.

37.15 Photoredox catalysis (preview)

A sister field of organometallic catalysis: photoredox, using visible light + photocatalysts (often Ru or Ir bipyridyl complexes) to generate radical intermediates.

The general scheme: 1. Light excites photocatalyst. 2. Excited catalyst oxidizes (or reduces) substrate by single-electron transfer. 3. Substrate radical reacts with another reagent. 4. Radical chain or photo-redox cycle continues.

This enables: - Mild radical reactions. - Photo-induced atom transfer. - New mechanism options not available to closed-shell chemistry.

Photoredox is covered in Ch 40. It's complementary to organometallic catalysis: similar effects (new bonds, new selectivity) via different mechanisms.

37.16 Asymmetric organometallic catalysis

Many of the modern asymmetric methods use chiral organometallic catalysts:

Chiral phosphine ligands

• BINAP: chiral biaryl phosphine; used by Noyori.
• DiPAMP: chiral phosphine; used by Knowles.
• PHOX: chiral oxazoline phosphine.
• Walphos, Josiphos: ferrocene-based chiral phosphines.

These ligands are coordinated to Pd, Rh, Ru, or other metals to give chiral catalysts.

Asymmetric reactions using these

• Asymmetric hydrogenation (Knowles, Noyori): Rh-BINAP for alkenes.
• Asymmetric ketone reduction (Noyori): Ru-BINAP.
• Asymmetric C-C bond formation: Pd-BINAP for cross-coupling.
• Asymmetric allylic alkylation: Pd + chiral ligand.
• Asymmetric Heck, Suzuki, etc.: chiral catalysts give enantioselective C-C bonds.

These methods are workhorses of modern pharmaceutical synthesis.

37.17 Catalyst design principles

For a transition-metal catalyst to work well, several factors:

Bond strengths

• M-X bonds shouldn't be too strong (won't release product).
• M-X bonds shouldn't be too weak (won't form intermediate).

Sterics

• Bulky ligands push the substrate to the right position.
• Too bulky and the substrate won't fit.
• Buchwald's bulky phosphines: sweet spot.

Electronics

• Electron-rich metals: undergo OA more readily.
• Electron-poor metals: undergo RE more readily.
• Tunable via ligand choice (electron-donating vs -withdrawing).

Stereochemistry

• Chiral ligands give chiral catalysts.
• Bidentate vs monodentate.
• Effective ligand bite angle.

Modern catalyst design uses combinatorial methods, computational screening, and iterative optimization. Companies like Strem and Sigma-Aldrich sell hundreds of well-characterized catalysts.

37.18 Practical considerations

Catalyst loading

Typical Pd cross-coupling: 1-5 mol% Pd loading. Higher loading speeds the reaction; lower is more economical.

For sensitive substrates: 10-20 mol% might be needed.

Solvents

Common solvents: - THF, dioxane, toluene (for Pd cross-coupling). - DMF, DMSO (for some specialized cases). - Aqueous mixtures for Suzuki (since boronic acid is water-soluble).

Bases

Common bases for cross-coupling: - K₂CO₃, Cs₂CO₃ (for Suzuki). - KOtBu, NaOtBu (for Buchwald-Hartwig amination). - Et₃N (for Sonogashira).

Temperature

Mild T (50-100 °C) is typical. Higher T speeds the reaction but can cause decomposition.

Atmosphere

Most Pd cross-couplings need anaerobic conditions (no O₂). Otherwise, Pd(0) gets oxidized to Pd(II) without substrate.

37.19 Common mistakes in organometallic chemistry

Common Mistake 37.1 — Forgetting to add a base in Suzuki coupling. The base activates the boronic acid for transmetalation. Without it, the reaction doesn't proceed.

Common Mistake 37.2 — Using O₂-saturated solvents for Pd cross-coupling. Dissolved O₂ oxidizes Pd(0) to Pd(II), inactivating the catalyst.

Common Mistake 37.3 — Confusing Suzuki and Negishi: Suzuki uses ArB(OH)₂; Negishi uses ArZnX. Both give similar products but use different transmetalating reagents.

Common Mistake 37.4 — Forgetting that Heck gives an aryl-vinyl product (Ar-CH=CR), not an aryl-alkyl. The alkene's H is not retained at the vinyl C; it's released as HX.

Common Mistake 37.5 — Using Grubbs catalyst with strong Lewis acids or oxidizing conditions. Grubbs 1st generation is sensitive; Grubbs 2nd generation is more robust but still has limits.

37.20 Future directions

Earth-abundant metal catalysis

Replacing Pd, Pt, Rh, Ru, Au (precious metals) with Fe, Co, Ni, Cu, Mn (earth-abundant). Major active research; some industrial use.

Mechanochemistry

Ball-mill chemistry: solid + solid + catalyst + grinding. Reduces solvent use.

Continuous flow

Continuous flow Pd cross-coupling: better heat control, scalability, safety.

Electrochemistry

Use electrons (from electrolysis) instead of stoichiometric oxidants/reductants.

Photoredox-organometallic synergy

Combining photoredox with organometallic catalysis: new reaction options.

AI/ML for catalyst discovery

Machine learning models predict catalysts for new reactions; combinatorial libraries; high-throughput screening.

37.21 The mechanism-first thesis applied

The Chapter 37 chemistry can seem like a "list of named reactions." But the mechanism-first thesis applies:

The same elementary steps (OA, MI, β-H elim, transmetalation, RE) cycle in essentially every Pd-catalyzed reaction. Differences come from: - The transmetalating partner (B(OH)₂, ZnR, SnR₃, alkene, alkyne, amine). - The exact intermediates and energetics. - The product type.

By understanding the elementary steps, you can predict reactions you've never seen before.

Similarly, olefin metathesis is mechanistically a single thing (M=CHR carbene + alkene → metallacyclobutane → new M=CHR + new alkene). RCM, cross-metathesis, and ROMP are just different applications of this single mechanism.

This unification is why organometallic chemistry, despite its complexity, is teachable: a few key principles cover a vast range of reactions.

37.22 Olefin metathesis in deeper detail

Types of metathesis

Cross-metathesis (CM): $$R-CH=CH-R + R'-CH=CH-R' \to R-CH=CH-R' + (R-CH=CH-R \text{ or } R'-CH=CH-R')$$

A direct exchange of substituents between two alkenes.

Ring-closing metathesis (RCM): $$\text{diene with two terminal alkenes} \to \text{cyclic alkene + ethylene (gas)}$$

The diene's two ends are linked, with ethylene gas released. Used for synthesizing macrocycles. Released ethylene drives the equilibrium forward (Le Chatelier).

Ring-opening metathesis (ROM): $$\text{cyclic alkene} + \text{another alkene} \to \text{linear alkene}$$

A strained cyclic alkene opens; the strain drives the reaction.

Ring-opening metathesis polymerization (ROMP): $$\text{strained cyclic alkene} \xrightarrow{cat.} \text{linear polymer with C=C in chain}$$

The cyclic alkene's strain drives polymerization. Used for polynorbornene, polyoctene.

Catalysts in detail

Schrock catalyst (Mo(=CHR)(=N-Ar)(OR)₂): - Very active, fast. - Air-sensitive, sensitive to functional groups. - Used in research labs.

Grubbs 1st generation (RuCl₂(=CHPh)(PCy₃)₂): - Robust, air-stable. - Compatible with many functional groups. - Industrial standard for many years.

Grubbs 2nd generation (RuCl₂(=CHPh)(IMes)(PCy₃)): - Even more functional group tolerant. - Higher activity. - N-heterocyclic carbene (NHC) ligand replaces one phosphine.

Hoveyda-Grubbs (RuCl₂(=CHC₆H₄(OiPr))(IMes)): - Recyclable (the chelating ligand keeps Ru on the substrate). - Modified for many applications.

Z-selective Grubbs (modified Ru catalysts): - Give Z-alkenes selectively. - Rare and useful for natural product synthesis (some natural products have Z-alkenes specifically).

Industrial use of metathesis

• Cross-metathesis of olefins (Lyondell process): converts butenes to propene at industrial scale.
• Pharmaceutical RCM: macrocyclic drug synthesis.
• Polymer ROMP: polynorbornene (Vestenamer).

Examples in synthesis

• Macrolide antibiotics: erythromycin, oleandomycin (RCM of diene precursor → 14-membered ring).
• Insect pheromones: many specific Z-alkene pheromones made by Z-selective metathesis.
• Boniva (osteoporosis drug): contains alkene installed by metathesis.

37.23 Ziegler-Natta polymerization in detail

The 1963 Nobel Prize was awarded to Karl Ziegler (Germany) and Giulio Natta (Italy) for the discovery of stereoregular alkene polymerization.

The catalyst

TiCl₄ (or TiCl₃) + Al(C₂H₅)₃ in alkane solvent. The actual active species is a Ti-CH₃ or Ti-Et carbene/alkyl that undergoes migratory insertion with ethylene or propylene.

Mechanism

1. Coordination: alkene binds to Ti.
2. Migratory insertion (MI): the alkyl on Ti migrates to one C of the alkene; the alkene's other C now bonded to Ti.
3. Repeat: with another alkene, MI again. Chain grows.
4. Termination: chain transfer to monomer or β-H elimination eventually.

Stereoregular polymers

Ziegler-Natta gives stereoregular polymers: - Isotactic polypropylene (all CH₃ groups on same side of polymer chain): high crystallinity, hard. - Syndiotactic polypropylene (CH₃ alternating): different properties. - Atactic (random): less common with proper catalyst.

The stereoregularity comes from the well-defined Ti coordination sphere directing each new alkene's geometry.

Modern metallocene catalysts

Cp₂ZrCl₂ + MAO (methylaluminoxane) gives even better-controlled polymerization: - Single-site catalysts (vs heterogeneous Ziegler-Natta). - Highly defined stereochemistry. - Tunable via Cp ligand modification.

Industrial impact: ~150 million tons/year of Ziegler-Natta + metallocene-derived polymers.

37.24 Heck reaction in detail

The Heck reaction (1968) couples an aryl halide with an alkene:

$$Ar-X + CH_2=CHR \xrightarrow{Pd, base} Ar-CH=CHR + HX$$

Mechanism

1. Oxidative addition: Pd(0) + ArX → Pd(II)(Ar)(X).
2. Alkene coordination: alkene binds to Pd.
3. Migratory insertion (MI): Ar migrates from Pd to one C of the alkene; Pd now bonded to the other C.
4. β-Hydride elimination: the Pd takes an H from the β-position (the C that originally was the alkene's other end).
5. Reductive elimination: Pd(0) regenerated; HX released.

Regiochemistry

The Heck reaction gives the aryl group at the terminal C of the alkene (anti-Markovnikov-like). The β-H elimination determines the geometry of the new C=C: typically E-isomer (trans) is favored.

For example: - Ar-X + CH₂=CHR → Ar-CH=CH-R (E predominates). - Ar-X + CH₂=CH-CO₂Me (methyl acrylate, Michael acceptor) → Ar-CH=CH-CO₂Me (E-cinnamate analog).

Application: cinnamate esters

The classic Heck product is a cinnamate ester (PhCH=CH-CO₂R), made from PhBr + CH₂=CH-CO₂R + Pd. Used in fragrances (cinnamic acid esters smell sweet) and many natural products.

Asymmetric Heck

Chiral Heck catalysts give enantioselective addition. Not as widespread as asymmetric hydrogenation but emerging.

37.25 Application to natural product synthesis

Taxol synthesis

The cancer drug Taxol (paclitaxel) has been synthesized many ways. Several key syntheses use Pd-catalyzed steps: - Suzuki couplings to install biaryl segments. - Heck reactions for stereochemically defined cyclizations. - Buchwald amination steps.

Erythromycin synthesis

The classic 14-membered macrolide. Modern syntheses use RCM (Grubbs) for the macrocyclic ring closure.

Halichondrin B / Eribulin

Halichondrin B is a complex marine natural product with potent anticancer activity. Eisai's synthesis of eribulin (the simplified analog) uses many Pd-catalyzed cross-couplings.

Azadirachtin

A complex insecticide; synthesized by Steve Ley using elaborate Pd-catalyzed steps.

Strychnine, vinca alkaloids

Many alkaloid syntheses use Pd catalysis for key C-C bond formations.

37.26 Comparison: Pd vs Cu vs other metals

The choice depends on substrate, conditions, and economics.

37.27 Spectroscopy of organometallic intermediates

NMR

• 31P NMR: identifies phosphine ligands; characteristic signals.
• 1H NMR: detects M-H species at unusual shifts (often negative ppm due to anisotropic shielding).
• 13C NMR: distinguishes metal carbene C (very downfield, 250-400 ppm).

IR

• M-CO stretching: characteristic at ~1900-2100 cm⁻¹.
• M-H stretching: ~1800-2200 cm⁻¹.
• M-C(=N)-related: various.

Mass spectrometry

• ESI-MS for soluble organometallic intermediates.
• MALDI for organometallic polymers.

These tools verify proposed mechanisms by detecting intermediates.

37.28 Computational organometallic chemistry

DFT (density functional theory) calculations have become the standard for organometallic mechanism studies:

• Calculate full catalytic cycle energies for OA, MI, β-H elim, RE, etc.
• Identify the rate-determining step (highest TS).
• Predict ligand effects on rate and selectivity.
• Design new catalysts computationally before synthesizing.

For Pd cross-coupling: typical activation barriers are 15-25 kcal/mol; explains why moderate temperatures (50-100 °C) are needed.

For metathesis: barriers are 15-20 kcal/mol; metallacyclobutane intermediate is energy-similar to TS; the geometry is well-defined.

These calculations have become routine; most published Pd cross-coupling papers include DFT analysis.

37.29 Common ligands

Each Pd cross-coupling needs the right ligand. Some commonly used:

Phosphines

• Triphenylphosphine (PPh₃): classical, mildly electron-rich.
• Tri-tert-butylphosphine (PtBu₃): very electron-rich; activates Cl-Ar bonds.
• Trifuryl phosphine, tri-2-furylphosphine: less basic, more selective.
• JackiePhos, RuPhos, SPhos (Buchwald ligands): bulky biaryl phosphines for difficult couplings; often work with aryl chlorides.
• DiPAMP: chiral phosphine for asymmetric hydrogenation.
• BINAP: chiral biaryl phosphine for asymmetric reactions.

N-heterocyclic carbenes (NHCs)

• IMes, SIMes: imidazolylidene-based.
• IPr, SIPr: similar.
• Used in second-generation Grubbs catalysts and modern Pd catalysts.

Bidentate ligands

• dppe, dppp, dppb: bisphosphines for various couplings.
• bpy, phen, terpy: bipyridines for special applications.

The choice depends on the substrate and reaction type. Often, a screen of several ligands is needed to find the best combination.

37.30 Beyond Pd: other transition metals

Modern organometallic chemistry uses many metals:

Rhodium (Rh)

• Asymmetric hydrogenation with chiral phosphines (Knowles, Noyori).
• Hydroformylation (alkene + CO + H₂ → aldehyde): industrial scale (~10 million tons/year).
• C-H activation with directing groups.

Ruthenium (Ru)

• Olefin metathesis (Grubbs catalysts).
• Asymmetric ketone hydrogenation (Noyori, BINAP-Ru).
• Photoredox catalysis (Ru(bpy)₃²⁺).

Iridium (Ir)

• C-H borylation: install boron at specific C-H sites.
• Asymmetric hydrogenation (often more active than Rh).
• Photoredox (Ir(ppy)₃).

Nickel (Ni)

• Cross-coupling (cheaper alternative to Pd).
• Earth-abundant electrocatalysis.
• Negishi-type couplings.

Iron (Fe)

• Earth-abundant catalysis: Sonogashira-type, Suzuki-type.
• Alkene hydroboration alternatives.

Cobalt (Co)

• Pauson-Khand reaction (alkyne + alkene + CO → cyclopentenone).
• Earth-abundant alternative for hydrogenation.

Manganese (Mn)

• Asymmetric epoxidation (Jacobsen-Katsuki).
• CO₂ activation.

Each metal has its specialty. Modern synthesis uses combinations.

37.31 Pharmaceutical case study: sitagliptin (Januvia) green synthesis

The diabetes drug sitagliptin (Januvia, Merck) was first synthesized in 2003 using a stoichiometric chiral auxiliary approach. By 2010, Merck had partnered with Codexis to develop a much greener route using an engineered transaminase enzyme + asymmetric catalysis.

Original route (2003)

• Chiral auxiliary (β-hydroxy acid) directs asymmetric synthesis.
• Multiple protecting group steps.
• Several solvent-intensive steps.
• E-factor (waste/product) ~250 — very high.

Codexis biocatalytic route (2010)

• Engineered transaminase enzyme directly converts the prochiral ketone to the chiral amine.
• Single step, high ee (>99%).
• Reduces waste by ~50%.
• Green chemistry award (EPA, 2010).

Asymmetric Pd-catalyzed route

For the carbon-skeleton assembly: - Pd-catalyzed asymmetric reduction of an enamide. - BINAP-Rh catalyst for enantioselectivity. - One-step from prochiral substrate.

This is modern process chemistry at its best: combining biocatalysis with asymmetric organometallic catalysis to achieve high efficiency and low waste.

The sitagliptin story is covered in detail in Ch 40 (green chemistry) but it's worth previewing: organometallic asymmetric synthesis is a key tool for green pharmaceutical manufacturing.

37.32 Why organometallic catalysis matters

Modern drug development depends on Pd cross-coupling (Suzuki, Heck, Buchwald, etc.), olefin metathesis, and asymmetric organometallic catalysis. These methods enable:

1. Drug discovery acceleration: build complex molecules in fewer steps.
2. Asymmetric synthesis: install chirality in one step instead of resolution.
3. Sustainable manufacturing: lower waste, lower energy.
4. Late-stage diversification: modify a near-complete drug to test analogs.
5. Functional group tolerance: operate on complex molecules without protecting groups.

The 20th-century synthesis paradigm was built on single-bond chemistry. The 21st-century synthesis paradigm depends on transition-metal catalysis. Organometallic chemistry is at the heart of modern synthesis.

37.33 Connections to other chapters

• Chapter 5: thermodynamics and kinetics (governs catalytic cycles).
• Chapter 7: chirality (asymmetric catalysis).
• Chapter 8: stereochemistry of reactions.
• Chapter 10: SN2 (Pd cross-coupling is conceptually like SN2 at sp² C).
• Chapter 15: alkenes (substrates for Heck, metathesis, hydrogenation).
• Chapter 17: alkynes (Sonogashira, alkyne metathesis).
• Chapter 21: aromatic chemistry (substrates for cross-coupling).
• Chapter 23: alternative aromatic substitution (SNAr; Pd is the modern alternative).
• Chapter 26: acyl substitution (Pd-catalyzed acyl halide chemistry).
• Chapter 31: retrosynthesis (Pd disconnections).
• Chapter 35: drug design (Pd cross-coupling in drug synthesis).
• Chapter 36: oxidation/reduction (Pd-catalyzed).
• Chapter 38: total synthesis (heavy use of Pd).
• Chapter 40: green chemistry (Pd as a green catalysis platform).

37.34 Other organometallic reactions

Beyond Pd cross-coupling and metathesis, transition metals enable many other reactions:

Hydroformylation

$$\text{alkene} + CO + H_2 \xrightarrow{Co \text{ or } Rh} \text{aldehyde}$$

The Oxo process: industrial scale (~10 million tons/year). Used to make detergents, plasticizers, and many others.

Catalysts: - Co(CO)₈ (classical, harsh conditions). - Rh-phosphine complexes (mild, modern).

Wacker oxidation

$$\text{terminal alkene} + O_2 + H_2O \xrightarrow{Pd, Cu} \text{methyl ketone}$$

The industrial route to acetaldehyde (CH₃CHO). Pd cycles through Pd(II)/Pd(0); Cu cycles to reoxidize Pd. Annual scale: ~3 million tons of acetaldehyde.

CO/CO₂ activation

Transition metals activate C-O bonds: - Rh-catalyzed carbonylation: methanol + CO → acetic acid (Monsanto/Cativa process; major industrial route). - Cu-catalyzed CO₂ reduction: CO₂ + H₂ → methanol or formic acid. - Photocatalytic CO₂ reduction: emerging green chemistry.

Hydrogenation in detail

Beyond simple H₂/Pd, modern hydrogenation includes: - Asymmetric hydrogenation: Knowles/Noyori; chiral phosphine + Rh or Ru. - Transfer hydrogenation: 2-propanol or HCOOH as H₂ source; Ru-catalyzed. - Heterogeneous Pd, Pt, Ni: industrial hydrogenation. - Iron-catalyzed: emerging earth-abundant alternative.

Hydroamination

$$\text{alkene or alkyne} + R_2NH \xrightarrow{cat.} \text{amine}$$

Catalyzed by lanthanides, late transition metals. Direct route to amines.

Carbonylative coupling

Combining Pd cross-coupling with CO insertion: $$ArX + R-Nu + CO \xrightarrow{Pd} Ar-C(=O)-Nu$$

Gives carbonyl products in one step (e.g., aryl ketones, esters).

Hydroboration / Hydrosilylation

$$\text{alkene} + RBH_2 \xrightarrow{cat.} \text{alkylborane}$$

$$\text{alkene} + R_3SiH \xrightarrow{cat.} \text{alkylsilane}$$

Catalyzed by Pt, Rh, Ir; selective addition.

Cycloadditions catalyzed by metals

• Pauson-Khand reaction: alkyne + alkene + CO + Co → cyclopentenone.
• Trost trimethylenemethane cycloaddition: Pd-catalyzed.
• Nicholas reaction: cobalt-stabilized propargyl cation + nucleophile.
• [2+2+2] cycloaddition: 3 alkynes → arene with Co or Rh catalyst.

Comprehensive scope

Organometallic chemistry is not just Pd cross-coupling and metathesis. It includes: - All catalytic hydrogenation. - All catalytic carbonylation (Oxo, Wacker, etc.). - All asymmetric catalysis (hydrogenation, oxidation, cycloaddition). - All cross-coupling (many variants). - All olefin polymerization (Ziegler-Natta, metallocene, ROMP). - All C-H activation. - Many natural product synthesis steps.

A modern synthetic chemist must be fluent in organometallic chemistry.

37.35 Hydroformylation in industrial scale

The Oxo process (or hydroformylation) is one of the largest-scale industrial uses of homogeneous organometallic catalysis. Industrial production: ~10 million tons/year.

Reaction

$$\text{alkene} + CO + H_2 \xrightarrow{Co \text{ or } Rh \text{ catalyst}} \text{aldehyde (linear or branched)}$$

For propene: CH₂=CH-CH₃ + CO + H₂ → CH₃CH₂CHO (n-butanal) + (CH₃)₂CHCHO (iso-butanal). The ratio of n/iso depends on catalyst: - Co-catalyzed (BASF/Ruhrchemie): 80:20 to 75:25 n:iso. - Rh-catalyzed (Monsanto/Celanese): 95:5 n:iso (more linear-selective).

Products

The aldehydes are converted to: - Alcohols (by hydrogenation): butanol, propanol, etc. Used as solvents, plasticizers. - Plasticizers (esterification with phthalic acid): industrial scale. - Acrylates (further oxidation): polymer precursors.

Catalysts

Modern Rh-based catalysts use chiral ligands for asymmetric hydroformylation: - PHOX: chiral phosphine-oxazoline. - DIPHOS: bisphosphines. - CC-BIPHEP: phosphite-phosphine.

These give chiral aldehydes for fine chemicals.

Mechanism (Co catalyst)

1. Oxidative addition: HCo(CO)₃(R)(H) generated from H₂ + Co(CO)₈.
2. Alkene coordination: alkene binds.
3. Migratory insertion: H migrates from Co to alkene; Co now bonded to other alkene C.
4. CO insertion: CO migrates between alkyl and Co; gives acyl-Co.
5. Hydrogenolysis: H₂ + acyl-Co → aldehyde + Co.

This catalytic cycle has been operating in industry since 1938 (BASF).

37.36 The Monsanto / Cativa acetic acid process

Acetic acid (CH₃COOH) is produced industrially at ~10 million tons/year. Monsanto's Rh-catalyzed methanol carbonylation (1960s) was the first commercial process; the modern Cativa process (2000s) uses an Ir catalyst.

Reaction

$$CH_3OH + CO \xrightarrow{Rh \text{ or } Ir, MeI \text{ promoter}} CH_3COOH$$

A 100% atom-economical process — every C atom in the methanol + CO ends up in the acetic acid.

Catalysts

• Rh complex with MeI promoter (Monsanto, 1968): 99% selectivity for AcOH.
• Ir complex with MeI promoter (Cativa, 1996): even higher rate, more thermally stable.

Mechanism

1. Methanol + HI → MeI (in situ).
2. MeI + Rh(I) → Me-Rh(III)(I) (oxidative addition).
3. CO insertion: Me-Rh(III)(I)(CO) → AcCO-Rh(III)(I).
4. Reductive elimination: AcOH + Rh(I).
5. Regenerated Rh(I) + MeI repeats.

The Rh cycles through Rh(I) and Rh(III) oxidation states. The MeI promoter is critical — it activates the methanol for the oxidative addition.

Industrial impact

This is the largest-scale homogeneous catalysis in the world. Acetic acid is used for: - Food preservation (vinegar; 4% acetic acid). - Chemical feedstock (esters, anhydride, vinyl acetate for polymers). - Industrial solvent.

The Rh/Ir-catalyzed carbonylation is a textbook example of organometallic catalysis at industrial scale.

37.37 The take-home for organometallic chemistry

Organometallic chemistry is at the heart of modern synthesis. Key principles:

1. Cycle through oxidation states: OA, MI, β-H elim, transmetalation, RE.
2. Tune the catalyst: ligands, metal, conditions all matter.
3. Pd cross-coupling is the workhorse of modern synthesis.
4. Olefin metathesis lets you break and reform C=C.
5. Asymmetric organometallic catalysis gives chiral products.
6. Industrial scale: acetic acid, hydroformylation, polyolefins, pharma, all rely on organometallics.
7. Future directions: C-H activation, photoredox, earth-abundant metals.

Combined with the rest of organic chemistry, organometallic catalysis enables the construction of essentially any molecule. From simple aspirin to complex Taxol, transition metals are part of the synthetic toolbox.

The Nobel Prizes (2001, 2005, 2010, 2021, 2022) reflect this importance. Organometallic chemistry has been recognized repeatedly as a transformative field.

37.38 Mechanistic worked problems

Problem A: Predict the Suzuki coupling product

PhBr + 4-methylphenyl-B(OH)₂ + Pd(PPh₃)₄ + K₂CO₃ in EtOH/H₂O at 80 °C.

Mechanism: 1. Pd(0) + PhBr → Pd(II)(Ph)(Br) (OA). 2. K₂CO₃ activates the aryl boronic acid. 3. Transmetalation: Pd(II)(Ph)(Br) + 4-methylphenyl-B(OH)₂ → Pd(II)(Ph)(4-methylphenyl) + B(OH)₃ (with K, etc.). 4. RE: Pd(0) + Ph-(4-methylphenyl) (= 4-methylbiphenyl).

Product: 4-methylbiphenyl.

Problem B: Predict the Heck product

PhBr + CH₂=CH-CO₂Me (methyl acrylate) + Pd(OAc)₂ + Et₃N at 80 °C.

Mechanism: 1. Pd(0) + PhBr → Pd(II)(Ph)(Br). 2. Alkene coordinates. 3. MI: Ph migrates from Pd to one C of alkene; Pd to other C. 4. β-H elim: Pd-H + new C=C bond (anti-Markovnikov regiochemistry, E-isomer preferred). 5. RE: Pd(0) + HBr (Et₃N takes up HBr).

Product: methyl cinnamate (E-isomer; Ph-CH=CH-CO₂Me).

Problem C: Predict the Buchwald amination

Bromobenzene + morpholine + Pd₂(dba)₃ + BINAP + NaOtBu in toluene.

Mechanism: 1. Pd(0) + PhBr → Pd(II)(Ph)(Br). 2. Morpholine binds. 3. NaOtBu deprotonates the morpholine-Pd intermediate; gives Pd(II)(Ph)(NR₂). 4. RE: Pd(0) + Ph-NR₂ (aniline).

Product: N-phenylmorpholine.

Problem D: Predict the RCM product

Diethyl di(allyl)malonate + Grubbs 2 catalyst.

Mechanism: 1. Grubbs catalyst binds one alkene of the diene. 2. [2+2] gives metallacyclobutane. 3. Retro-[2+2] in reverse direction. 4. New M=CHR carbene + new alkene; ethylene released. 5. The remaining diene is now a single C=C; intramolecular [2+2] with the next alkene. 6. Closes the ring; ethylene released; cyclic alkene formed.

Product: 5-membered ring with 1 C=C (a substituted cyclopentene).

Problem E: Predict the metathesis product

(Z)-2-butene + (E)-2-pentene + Grubbs 2.

Mechanism: cross-metathesis. The two alkenes exchange substituents. Multiple products possible: - 2-butene (E or Z). - 2-pentene (E or Z). - Mixed alkenes (E or Z propenes, etc.).

Product: a mixture; equilibrium of all possibilities. Often, this is the bottleneck of cross-metathesis — getting selectivity for one product is hard.

Problem F: Sonogashira coupling

PhBr + HC≡C-CH₃ + Pd(PPh₃)₂Cl₂ + CuI + Et₃N.

Mechanism: 1. Pd(0) + PhBr → Pd(II)(Ph)(Br) (OA). 2. CuI-catalyzed deprotonation: HC≡C-CH₃ + Et₃N + CuI → Cu-C≡C-CH₃ + Et₃N·H+I-. 3. Transmetalation: Pd(II)(Ph)(Br) + Cu-C≡C-CH₃ → Pd(II)(Ph)(C≡C-CH₃) + CuBr. 4. RE: Pd(0) + Ph-C≡C-CH₃ (an aryl-alkyne).

Product: 1-phenyl-1-propyne (Ph-C≡C-CH₃).

These mechanisms can be applied to many cross-coupling problems.

37.39 Common mistakes in organometallic chemistry

Common Mistake 37.6 — Forgetting to include base in Suzuki coupling. Without base, the boronic acid doesn't transmetalate; reaction stalls at the OA intermediate.

Common Mistake 37.7 — Using too much Pd. Higher loading speeds the reaction but also increases the amount of Pd that must be removed from the product (key for pharmaceuticals; <10 ppm Pd allowed).

Common Mistake 37.8 — Forgetting to deoxygenate. Pd(0) is sensitive to O₂. Industrial reactors use N₂ or Ar atmosphere.

Common Mistake 37.9 — Using the wrong ligand for the substrate. Different aryl halides need different ligands; aryl iodides work with simpler ligands than aryl chlorides.

Common Mistake 37.10 — Confusing the products of Heck and Suzuki. Heck gives Ar-vinyl bond (Ar-CH=CR); Suzuki gives Ar-Ar (biaryl). Different mechanisms (β-H elim vs none).

37.40 Asymmetric organometallic catalysis in detail

The 2001 Nobel Prize was awarded to Knowles, Noyori, and Sharpless for the development of asymmetric catalysis. Knowles and Noyori specifically worked on organometallic asymmetric catalysis.

Knowles' DIPAMP-Rh catalyst

DIPAMP is a chiral phosphine ligand with two phosphorus stereocenters (P-chirality, which is unusual). Coordinated to Rh, it gives a chiral Rh catalyst that asymmetrically hydrogenates prochiral alkenes.

The first commercial use: L-DOPA synthesis for Parkinson's disease (1974). Before DIPAMP-Rh, L-DOPA was made by chemical resolution (50% wasted). After DIPAMP-Rh, asymmetric hydrogenation gives directly the (S)-product.

Noyori's BINAP-Ru and BINAP-Rh catalysts

BINAP (2,2′-bis(diphenylphosphino)-1,1′-binaphthyl) is a chiral biaryl phosphine. Atropisomerically stable; (R)-BINAP and (S)-BINAP are enantiomerically pure.

Coordinated to Ru: catalyst for asymmetric ketone hydrogenation. Used industrially for many drug intermediates.

Coordinated to Rh: catalyst for alkene hydrogenation. Used for naproxen and many others.

Sharpless asymmetric oxidation

Although primarily organic (Ti-tartrate complex isn't strictly transition metal), Sharpless asymmetric epoxidation is in the same family. Industrial scale use for many natural products.

Modern asymmetric catalysis

Beyond hydrogenation: - Asymmetric Heck: chiral catalysts give enantioselective C-C bond formation. - Asymmetric Suzuki: chiral phosphines give enantioenriched biaryls. - Asymmetric C-H activation: chiral catalysts target one C-H over another. - Asymmetric organocatalysis (Nobel 2021): proline/imidazolidinone catalysts; not strictly organometallic but combines well.

These methods are workhorses of modern asymmetric synthesis.

37.41 Final overview

The chemistry of Chapter 37 transformed synthetic organic chemistry. The Nobel Prizes (2001 asymmetric catalysis, 2005 metathesis, 2010 Pd cross-coupling) reflect this. Key takeaways:

• Transition metal catalysis enables otherwise difficult or impossible bond formations.
• The catalytic cycle (OA, MI, β-H elim, transmetalation, RE) is the unifying framework.
• Pd cross-coupling (Suzuki, Heck, Negishi, Stille, Sonogashira, Buchwald) is the workhorse of modern synthesis.
• Olefin metathesis (Grubbs, Schrock, Chauvin) lets chemists break and reform C=C bonds.
• Asymmetric organometallic catalysis enables enantioselective drug synthesis at industrial scale.
• Industrial processes (acetic acid, hydroformylation, polyolefins) depend on transition metal catalysis.
• Modern frontiers include C-H activation, photoredox catalysis, earth-abundant metals, and combinations of these.

Chapter 38 continues with the art of synthesis — applying all the tools we've learned (organic + organometallic + asymmetric catalysis + biocatalysis) to total synthesis of complex natural products.

37.42 Summary

1. Transition metal catalysis uses elementary steps: oxidative addition (OA, +2 oxidation state), migratory insertion (MI, alkene/alkyne coupling), β-hydride elimination, transmetalation, reductive elimination (RE, -2 oxidation state).
2. Pd cross-coupling general cycle: Pd(0) → OA with ArX → Pd(II)(Ar)(X) → transmetalation → Pd(II)(Ar)(Ar') → RE → Ar-Ar' + Pd(0).
3. Suzuki coupling (Ar-X + Ar'-B(OH)₂): biaryl formation. Most common cross-coupling.
4. Heck reaction (Ar-X + alkene): aryl-vinyl bond. Goes through MI + β-H elimination.
5. Sonogashira coupling (Ar-X + terminal alkyne): aryl-alkyne bond. Cu co-catalyst.
6. Negishi coupling (Ar-X + Ar-ZnX): biaryl or aryl-alkyl bond. 2010 Nobel.
7. Buchwald-Hartwig amination (Ar-X + amine): aryl-amine bond.
8. Olefin metathesis (Grubbs, Schrock catalysts): exchange of alkene substituents. - RCM: ring-closing metathesis (diene → cyclic alkene + ethylene). - Cross-metathesis: two alkenes exchange substituents. - ROMP: ring-opening metathesis polymerization.
9. Ziegler-Natta (TiCl₄ + AlEt₃): alkene polymerization to HDPE, PP. 1963 Nobel.
10. Metallocene catalysis: stereoregular polymerization with controlled tacticity.
11. C-H activation: emerging frontier; selective functionalization of specific C-H bonds.
12. Asymmetric organometallic catalysis (BINAP, DiPAMP, PHOX): enantioselective C-C bond formation.

Chapter 38 brings everything together in the art of synthesis — the capstone chapter on total synthesis of complex natural products.