Felkin–Anh model template

Newman projections down the C1–C2 bond of (S)-2-phenylpropanal: in the favoured transition state Ph (L) is perpendicular to the carbonyl and anti to the incoming nucleophile, which approaches at the Bürgi–Dunitz angle (about 107°) past H (S) rather than CH₃ (M). MeMgBr gives (2S,3S)-3-phenylbutan-2-ol as the major diastereomer.

What the template shows

• Nucleophilic addition to an α-chiral carbonyl: the large group sits perpendicular to C=O and the nucleophile passes the small group
• Newman projections down C(=O)–Cα (front: C=O carbon; back: α-carbon)
• favoured: Nu passes H (S)
• disfavoured: Nu passes CH₃ (M)
• L perpendicular to C=O, anti to Nu; Nu at about 107° to C=O (Bürgi–Dunitz)
• same face rule, but the other conformer: Nu meets the larger CH₃
• (2S,3S): major (Felkin)
• L, M, S = large, medium, small group on the α-carbon. The nucleophile adds to the face of C=O opposite L.
• Reactive conformation: the largest α-group (L) is perpendicular to the C=O plane, anti to the incoming nucleophile; its σ*(C–L) also lowers the π* of C=O (Anh).
• The nucleophile attacks C=O at the Bürgi–Dunitz angle, about 107°, so it passes close to one of the other two α-groups: it prefers the small one (S) over the medium one (M).
• Example: (S)-2-phenylpropanal with MeMgBr gives (2S,3S)-3-phenylbutan-2-ol as the major diastereomer (Cram’s rule makes the same prediction); the selectivity of simple Grignard additions is modest.
• An electronegative α-group (OR, Cl, NR₂) takes the L position (polar Felkin–Anh); with a chelating metal (Mg²⁺, Zn²⁺, Ti⁴⁺) and an α-OR, the Cram-chelate model can give the other diastereomer.
• Wedge = toward you, hash = away from you · every part of this figure is editable

Drawn with 32 atoms, 108 shapes – every one editable in MolDraw.

Sources

• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 33 Textbook
• Eliel & Wilen, Stereochemistry of Organic Compounds, Wiley 1994, ch. 12 Textbook