Chapter 8 — Stereochemistry in Reactions: Why 3D Shape Determines Biological Activity

"We do not make molecules. We make arrangements of atoms." — attributed to several bioorganic chemists

Chapter 7 introduced stereochemistry as a feature of static molecules — chirality, enantiomers, diastereomers, $R/S$. Chapter 8 now addresses: what happens to stereochemistry when reactions run? When you start with a chiral substrate and react it with a reagent, what is the stereochemistry of the product?

Three basic outcomes are possible at any stereocenter:

• Inversion — the configuration flips. $(R) \to (S)$.
• Retention — the configuration stays. $(R) \to (R)$.
• Racemization — the configuration is lost. Mixture of $(R)$ and $(S)$.

And for reactions across double bonds, two more:

• Syn addition — both new bonds form on the same face of the $\pi$ bond.
• Anti addition — new bonds form on opposite faces.

By the end of Chapter 8 you should be able to predict, for any reaction you meet, which of these stereochemical outcomes occurs based on the mechanism.

8.1 The mechanism-stereochemistry connection

The fundamental principle of Chapter 8 is that the geometry of the transition state determines the stereochemistry of the product. A reaction that proceeds through one specific TS geometry gives one stereochemical outcome; a reaction that proceeds through a planar (or otherwise symmetric) intermediate loses stereochemistry.

The corollary: by observing the stereochemistry of the product, we can infer the mechanism. This is one of the most powerful diagnostic tools in physical organic chemistry.

This chapter covers: 1. Stereochemistry from concerted reactions (one-step, defined geometry). 2. Stereochemistry from stepwise reactions (multi-step, possibly planar intermediates). 3. Stereochemistry of additions (syn vs anti). 4. Prochirality and the re/si face convention. 5. Stereoselectivity vs stereospecificity (often-confused terms). 6. Asymmetric synthesis — using chiral catalysts to make pure enantiomers.

8.2 Stereochemistry from one-step ($S_N2$-like) mechanisms

In a concerted, single-step reaction (like $S_N2$, Chapter 10), the nucleophile attacks from the opposite face of the leaving group. The three remaining substituents on the reacting carbon swing to the other side (like an umbrella inverting in the wind). The result: inversion of configuration at the stereocenter.

This is so reliable that it has become a definition — any single-step back-side attack reaction produces inversion, and observed inversion in an experiment is strong evidence for a concerted mechanism.

The Walden inversion

Paul Walden (1896) first observed that converting (R)-malic acid to its derivatives and back gave (S)-malic acid — the configuration had inverted somewhere in the cycle. This was puzzling at the time but is now understood: each substitution step inverted the stereocenter; an even number of steps gave net retention; an odd number gave net inversion.

The Walden inversion is the textbook example of stereochemistry in mechanism.

Geometry of SN2 transition state

The SN2 TS has: - The nucleophile approaching from 180° opposite the leaving group. - The carbon partially sp² (transition between sp³ tetrahedral starting and product). - The three remaining substituents in a plane (passing through the carbon). - The nucleophile and leaving group on the axis perpendicular to that plane.

After the reaction completes: the three substituents have moved to the other side; the carbon is sp³ again; the nucleophile is bonded; the leaving group has left.

Net effect: inversion.

How we know SN2 gives inversion

Multiple lines of evidence: 1. Kinetic isotope effects show single-step mechanism. 2. Cyclic substrates (where back-side attack is geometrically blocked) react slowly or not at all. 3. Stereochemistry observation in chiral substrates: optically pure (R) substrate gives optically pure (S) product (assuming no other complications). 4. Computational studies: ab initio calculations confirm the 180° approach.

Worked example

(R)-2-bromobutane + I⁻ (in DMSO) → (S)-2-iodobutane.

The CH₃CH₂- and CH₃- and H groups stay in place; the Br leaves; the I⁻ takes its place from the opposite face. The configuration inverts.

8.3 Stereochemistry from two-step ($S_N1$-like) mechanisms

In a two-step reaction (like $S_N1$, Chapter 11), the leaving group leaves first, producing a planar, $sp^2$ cation intermediate. The nucleophile then attacks, but the intermediate is planar — so attack can happen from either face.

If attack from either face is equally likely, the product is a racemic mixture (50:50). In practice, modest preferences often exist (the "shielded" face by the departing leaving group is slightly favored — called the "ion pair" effect), but the product is still largely racemized.

Stepwise mechanisms (with a planar intermediate) → racemization (or strong racemization). Concerted mechanisms (with a single step through a defined geometry) → stereospecific outcome (usually inversion).

This is one of the most powerful predictive rules in the book. Once you classify a reaction as concerted or stepwise, you know the stereochemistry.

Why a planar cation racemizes

The carbocation is sp² hybridized: three substituents in a plane, one empty p orbital perpendicular. The two faces of the cation are equivalent (no chirality). A nucleophile can attack from either face with equal probability, giving 50:50 of (R) and (S) products.

If the cation forms a tight ion pair with the leaving group (the leaving group is still close), there's a slight bias toward attack from the face opposite the leaving group (because the leaving group blocks its face slightly). This is the ion pair effect — gives slight preference for inversion (~70:30 inversion:retention).

For loose, "fully ionized" carbocations: full racemization (50:50).

Solvolysis as the typical SN1 example

(R)-3-chloro-3-methylhexane in water gives racemic 3-methyl-3-hexanol. The 3° carbocation forms readily (stable); water attacks from either face; product is racemic.

8.4 Stereochemistry from addition reactions

Alkene reactions (Chapter 15-16) add two groups across a $C=C$. How do they add?

Syn addition — both groups same face

Syn addition: both new groups on the same face of the alkene. Geometrically: they come from the same reagent delivered cyclically.

Examples: - OsO₄ dihydroxylation: gives a cis diol. The osmium adds two oxygens from the same face via a cyclic OsO₄ ester intermediate. - Hydroboration-oxidation: boron and hydrogen add cis; on workup, B is replaced by OH with retention. Net: syn addition of H and OH. - Catalytic hydrogenation: H₂ adds cis to the alkene from the metal surface. - Sharpless asymmetric epoxidation: chiral Ti catalyst delivers oxygen from one face; gives chiral epoxide with high ee.

Mechanistic feature: a cyclic transition state or intermediate that delivers both new groups to the same face simultaneously.

Anti addition — groups on opposite faces

Anti addition: the two new groups go on opposite faces. Geometrically: the two steps happen independently, with one substituent approaching from each side.

Examples: - Bromine (Br₂) addition: gives anti-dibromide. Mechanism: bromonium ion (3-membered cyclic intermediate) attacked by Br⁻ from opposite face → trans/anti. - HBr addition (Markovnikov): gives anti. - Halohydrination (Br₂ + H₂O): bromonium intermediate; OH attacks from opposite face. - Anti dihydroxylation (epoxide + H₂O): epoxide opening gives trans-diol.

Mechanistic feature: a 3-membered cyclic intermediate (bromonium, epoxide, mercurinium) that is attacked from the back face.

How to predict syn or anti

For each reaction: - Cyclic intermediate that opens by back-side attack → anti. - Cyclic TS or surface delivery (catalyst, OsO₄, etc.) → syn. - Carbocation intermediate → typically anti with bias from ion pair, but more variable.

This is a powerful organizing principle for Chapter 15-16.

8.5 Prochirality and re/si faces

Prochiral molecules

A prochiral molecule is achiral but becomes chiral if one of its atoms is replaced. Examples: - An aldehyde (R-CHO) is prochiral at the carbonyl C; reduction with hydride gives a chiral alcohol; the two faces of the C=O give the two enantiomers. - A planar alkene is prochiral; addition gives chiral product (or it can be diastereotopic if there's an existing stereocenter). - The two H's of a -CH₂- group (next to a stereocenter) are diastereotopic.

Re and si face nomenclature

For the two faces of a sp² carbon (carbonyl, alkene), use the CIP priority of the three substituents on the sp² C: - Look at the face from one side; if the priorities go clockwise: re face. - Look from the other side; if priorities go counterclockwise: si face.

Attack on the re face vs si face gives the (R) vs (S) product (with appropriate analysis). This is the language of asymmetric synthesis.

Enantiotopic vs diastereotopic

Two protons (or two faces) are: - Enantiotopic: they give enantiomers when distinguished. A reaction with an achiral reagent treats them equivalently. With a chiral reagent (catalyst), they can be distinguished. - Diastereotopic: they give diastereomers when distinguished. Even an achiral reagent can react with them at different rates (because the diastereomeric TSs are not equivalent in energy).

This distinction matters in NMR (Ch 9): enantiotopic protons appear identical in normal NMR; diastereotopic protons appear as separate signals.

8.6 Stereoselectivity vs stereospecificity

These terms are easy to confuse:

• Stereospecific: different stereoisomers of the substrate give different stereoisomers of the product. (E.g., (E)-alkene → one diastereomer; (Z)-alkene → different diastereomer.)
• Stereoselective: a single substrate gives one stereoisomer of product preferentially over another. (E.g., asymmetric hydrogenation gives mostly one enantiomer.)

Stereospecific is a stronger claim than stereoselective. Stereospecific implies a fully concerted, predictable mechanism. Stereoselective is a kinetic preference (often expressed as % ee or % de).

Examples

• SN2 is stereospecific: (R) substrate → (S) product, always.
• Asymmetric hydrogenation with Rh-BINAP is highly stereoselective: prochiral alkene → 95% one enantiomer + 5% the other. Not 100%; not stereospecific.
• Diels-Alder is stereospecific: (E,E)-diene + (Z)-dienophile → specific diastereomer (cis substituents).

Pay attention to the language in journal articles: "stereoselective" with high % ee is the goal of asymmetric synthesis; "stereospecific" usually implies a concerted mechanism.

8.7 Asymmetric synthesis — extending Ch 7's case study

Asymmetric synthesis is the deliberate use of stereoselective reactions (often with chiral catalysts) to make a desired enantiomer. The general strategy:

1. Choose a prochiral starting material.
2. Use a chiral catalyst that distinguishes the two faces of the prochiral center.
3. The catalyst makes one TS lower in energy than the other → one enantiomer is the major product.

Chiral auxiliaries

Older approach: temporarily attach a chiral group to the substrate. The chiral group biases the reaction toward one diastereomer; remove the chiral group at the end. This is a stoichiometric use of chirality.

Examples: Evans oxazolidinone auxiliaries; Oppolzer's sultam.

Chiral catalysts

Modern approach: a small amount of chiral catalyst (often Pd-, Ru-, Ti-, or Rh-based with chiral phosphine or amine ligands) controls many turnovers of substrate. Catalytic in chirality.

Examples (covered in later chapters): - Sharpless asymmetric epoxidation: chiral Ti-DET catalyst. - Knowles/Noyori asymmetric hydrogenation: chiral phosphine + Rh or Ru. - Jacobsen-Katsuki asymmetric epoxidation: chiral salen-Mn catalyst. - Asymmetric organocatalysis: chiral amines (proline; MacMillan's imidazolidinone).

Kinetic resolution

If a chiral catalyst reacts faster with one enantiomer of a racemate, the reaction can stop midway, leaving the unreactive enantiomer pure. This is kinetic resolution. Used industrially with lipases.

Dynamic kinetic resolution (DKR)

If the racemate also racemizes faster than it reacts, the equilibrium continually replenishes the consumed enantiomer; both enantiomers can be converted to one enantiomer of product. The combination of kinetic resolution + racemization is DKR, and it can give 100% theoretical yield of a single enantiomer.

8.8 Diastereoselectivity in carbonyl additions

When a nucleophile adds to a chiral aldehyde or ketone (one with an existing stereocenter), the new stereocenter can be (R,R) or (R,S) — two diastereomers. Predicting which one is the major product is diastereoselectivity.

Cram and Felkin-Anh models (Ch 25)

For a chiral α-substituted carbonyl (R-C(=O)R'), where R contains a stereocenter, the Felkin-Anh model predicts: - The largest substituent on the α-C is perpendicular to the C=O plane. - The nucleophile attacks from the side away from the largest group. - This gives one diastereomer preferentially.

The Cram chelation model applies when the α-C has a coordinating group (OH, OR) that can chelate to a Lewis acid: the chelate locks the conformation, and the nucleophile attacks from the predicted face.

These models are explored in Ch 25 (carbonyl additions).

Practical importance

Diastereoselective reactions are crucial in natural product synthesis. Building a complex natural product with 5-10 stereocenters requires controlling each new stereocenter relative to the existing ones. Felkin-Anh, Cram, and related models guide synthetic design.

8.9 Why it matters — a tour through the rest of the book

Every reaction you will learn in the rest of the book has characteristic stereochemistry:

Every one of these rules can be derived from mechanism. Mastering Chapter 8's mechanism-stereochemistry connection gives you predictive power across all of Chs 10-37.

8.10 The thalidomide story revisited

Chapter 7 introduced thalidomide. Chapter 8 explains why prescribing only the (R) enantiomer would not have prevented the disaster.

Thalidomide has an α-carbonyl chiral center: the chirality is at a carbon adjacent to a C=O. This α-carbon has an acidic H (pKa ~ 11-13, due to the α-position; Ch 27). In aqueous solution at physiological pH, the proton can be removed and re-added — this enolization racemizes the stereocenter.

A pure (R) thalidomide tablet, taken with water, partially racemizes within hours in the body. Some (R) is converted to (S), and (S) is the teratogen.

Therefore: - Synthesizing pure (R) thalidomide is possible but does not make the drug safe. - The fundamental issue is the α-carbon stereolability — the ease of racemization at the α-position. - Chapter 27 covers α-carbon chemistry and enolization in detail; Chapter 35 returns to drug design.

This is one of organic chemistry's great cautionary tales: even with pure enantiomers, biological systems can undo the chirality if the molecule has labile stereocenters. Modern drug design avoids α-carbonyl chiral centers when possible.

8.11 Summary

1. Mechanism determines stereochemistry. Concerted reactions give specific stereochemical outcomes; stepwise reactions with planar intermediates lose stereochemistry.

2. One-step (concerted) mechanisms: stereospecific. SN2 gives inversion at carbon; Diels-Alder gives syn cis addition.

3. Two-step (stepwise) mechanisms with planar intermediates: racemization. SN1 with a carbocation gives ~50:50 (R/S) product.

4. Addition reactions: - Syn (both groups same face): cyclic TS, single-reagent delivery (OsO₄, hydroboration, hydrogenation). - Anti (groups opposite faces): cyclic intermediate opened by back-side attack (bromonium, epoxide).

5. Prochirality: a planar sp² C (carbonyl, alkene) has two faces — re and si. Attack on different faces gives different enantiomers/diastereomers.

6. Stereoselective vs stereospecific: - Stereoselective: kinetic preference for one stereoisomer. - Stereospecific: different starting stereoisomers give different products. (Stronger.)

7. Asymmetric synthesis uses chiral catalysts to selectively make one enantiomer. Modern industrial drug synthesis relies on this.

8. Diastereoselectivity in carbonyl additions follows Felkin-Anh or Cram chelation models — predictable from the chiral environment of the substrate.

9. The thalidomide story is mechanistically understandable: α-carbon stereolability via enolization racemizes the drug in vivo. Pure (R)-thalidomide is not safe.

10. Mastery of Chapter 8 lets you predict the stereochemistry of any reaction once you know its mechanism. This predictive power is the goal of mechanism-first chemistry.

Chapter 9 next — NMR spectroscopy. Stereochemistry will show up directly in NMR coupling constants (3J couplings depend on dihedral angle; cis/trans alkene protons couple differently; chiral environments produce distinct NMR signals).