Chapter 37 — Key Takeaways

What you should leave Chapter 37 with

  1. Transition metal catalysis uses the cycle: oxidative addition (OA) → ligand combination/migration → reductive elimination (RE), with optional steps including transmetalation, β-hydride elimination, and migratory insertion.

  2. Oxidative addition (OA): $M^0 + R-X \to M^{II}(R)(X)$. Metal inserts into a bond; oxidation state increases by 2.

  3. Reductive elimination (RE): $M^{II}(R)(R') \to M^0 + R-R'$. Two ligands combine; metal reduces by 2; new C-C or C-X bond forms.

  4. Migratory insertion (MI): an alkene or alkyne inserts into a metal-ligand bond. Used in polymerization and cross-coupling.

  5. β-hydride elimination: reverse of MI. Used in Heck reaction (gives alkene product).

  6. Transmetalation: organic group transfers between metals. Critical in Suzuki, Negishi, Stille couplings.

  7. The Pd-catalyzed cross-coupling family: - Suzuki: Ar-X + Ar'-B(OH)₂ → Ar-Ar'. Most common. - Heck: Ar-X + alkene → aryl-substituted alkene. - Sonogashira: Ar-X + terminal alkyne (HC≡CR) → Ar-C≡CR. Cu co-catalyst. - Negishi: Ar-X + Ar'-ZnX → Ar-Ar'. 2010 Nobel. - Stille: Ar-X + Ar'-SnR₃ → Ar-Ar'. Less used due to tin toxicity. - Buchwald-Hartwig: Ar-X + amine → Ar-NR₂. C-N bond formation.

  8. The 2010 Nobel Prize (Heck, Negishi, Suzuki) recognized Pd cross-coupling. ~30% of approved drugs use Pd cross-coupling somewhere in their synthesis.

  9. Mechanism of Suzuki coupling (the model cycle): - Step 1: Pd(0) + Ar-X → Pd(II)(Ar)(X) (OA). - Step 2: Pd(II)(Ar)(X) + Ar'-B(OH)₂ + base → Pd(II)(Ar)(Ar') (transmetalation). - Step 3: Pd(II)(Ar)(Ar') → Pd(0) + Ar-Ar' (RE).

  10. Why Pd is the metal of choice: stable Pd(0) ↔ Pd(II) cycle; tolerant of many functional groups; bulky phosphine ligands tune reactivity for difficult substrates (Ar-Cl).

  11. Olefin metathesis (Chauvin, Schrock, Grubbs — 2005 Nobel) exchanges substituents on two alkenes via a metallacyclobutane intermediate.

  12. Three main applications of olefin metathesis:

    • Ring-closing metathesis (RCM): diene + Grubbs cat → cyclic alkene + ethylene.
    • Cross-metathesis (CM): two alkenes exchange substituents.
    • Ring-opening metathesis polymerization (ROMP): cyclic alkene → polymer.

  13. The Grubbs catalyst (Ru-based) is functional-group tolerant, works in air, and is the workhorse of modern synthesis.

  14. The Schrock catalyst (Mo-based) is more reactive but requires inert atmosphere.

  15. RCM in natural product synthesis: epothilones (anticancer), boceprevir/simeprevir (HCV), eribulin (anticancer), and many other macrocyclic drugs use RCM in their synthesis.

  16. Ziegler-Natta polymerization (TiCl₄ + AlEt₃) makes high-density polyethylene (HDPE) with linear chains. 1963 Nobel (Ziegler, Natta).

  17. Metallocene catalysis (group IV metallocenes + MAO) gives stereoregular polypropylene (isotactic, syndiotactic) with control of:

    • Tacticity.
    • Molecular weight.
    • Comonomer incorporation.

  18. C-H activation is the new frontier: selective functionalization of specific C-H bonds using Pd, Rh, Ir, Co, Cu catalysts. Enables late-stage functionalization in drug discovery.

  19. Asymmetric organometallic catalysis:

    • BINAP (Noyori, Nobel 2001): asymmetric hydrogenation of ketones with Ru.
    • DiPAMP (Knowles, Nobel 2001): asymmetric hydrogenation; basis of L-DOPA synthesis.
    • DuPHOS (Burk): used in sitagliptin synthesis.
    • PHOX: chiral Pd ligands.

  20. Mastery of Chapter 37 connects to:

    • Pharmaceutical synthesis (Pd cross-coupling, asymmetric hydrogenation).
    • Natural product synthesis (RCM for macrocycles).
    • Materials chemistry (polymers).
    • Modern catalysis research (C-H activation, photoredox).
    • Industrial chemistry (sitagliptin, polyethylene, etc.).

Cross-references

  • Chapter 10 — SN2 chemistry (cross-coupling competes with SN2 for alkyl halides).
  • Chapter 15-17 — Alkenes and alkynes (substrates for metathesis and Heck).
  • Chapter 25 — Grignard reagents (Mg-based, simpler organometallics).
  • Chapter 36 — Oxidation/reduction; asymmetric hydrogenation.
  • Chapter 38 — Total synthesis using Pd cross-coupling and metathesis.
  • Chapter 40 — Green chemistry; catalytic methods.
  • Appendix C — Reaction summary.
  • Appendix F — Named reactions (Suzuki, Heck, RCM, etc.).

Study tip

For each Pd cross-coupling problem: 1. Identify the aryl halide (Ar-X). 2. Identify the coupling partner (boronic acid, alkene, amine, alkyne). 3. Predict the product: replace X with the coupling partner's organic group. 4. Check the conditions: appropriate base, solvent, ligand for the substrate.

For metathesis problems: 1. Identify the alkene partners (or the diene for RCM). 2. Predict the new alkenes: substituent exchange. 3. Note any byproducts: ethylene (RCM) or two new alkenes (CM).

Memorize the 6 main Pd cross-couplings and their bond-forming roles. With this, you can read modern synthesis papers fluently.