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Felkin-Anh model 2026-10-20
The Felkin-Anh model predicts facial preference in nucleophilic addition beside a stereogenic centre under suitable open, non-chelating conditions. Orient the large alpha substituent approximately perpendicular to C=O, then favour approach opposite it and past the small substituent. It is a predictive model rather than a universal guarantee.
Past exam of the chemistry B course of the University of Cambridge 2022 Paper 1 B4 a Solution 2026-10-20
A is the predicted major diastereomer. At the carbon adjacent to the ketone, take phenyl as the large group, vinyl as medium, and hydrogen as small. There is no adjacent heteroatom donor that would force a five-membered chelate. The appropriate first model is therefore the non-chelating Felkin-Anh model.
View along the carbonyl-carbon–stereocentre bond. Rotate to place the large alpha phenyl group approximately perpendicular to the carbonyl plane. Of the two resulting approaches opposite that large group, the favourable Bürgi-Dunitz trajectory passes nearer hydrogen rather than vinyl. Propyl transfer to the carbonyl and protonation of its oxygen give the configuration labelled A: in the printed projection the new OH projects forward and the propyl bond backward, while the original vinyl bond remains backward. Rotation used to construct a Newman projection must not be mistaken for inversion of the original stereocentre.
This is a relative-selectivity prediction, not a numerical product ratio. Simple attack on the face that happens to look less crowded in the initial flat drawing is unreliable because the relevant C–C single bond can rotate. B is the competing product from the less favourable approach beside vinyl. The model and its preference for the trajectory beside the small group are described in the institutional treatment.[ref]
Felkin-Anh and Cram Chelate by University of Windsor. pp. 1–2, section 1, nonpolar alpha substituents and the small-group trajectory
Original Newman construction of the preferred Felkin–Anh approach and the local stereochemistry corresponding to product A
. Details. Past exam of the chemistry B course of the University of Cambridge 2025 Paper 1 B4 a Solution 2026-10-20
Apply the Felkin-Anh model to the stereocentre alpha to the ketone. Its large group is the chain continuing through the phenyl-bearing carbon, its medium group is methyl, and its small group is hydrogen. In the preferred transition-state arrangement, put the large group approximately perpendicular to C=O and approach from the opposite face, passing the small hydrogen along a Bürgi-Dunitz trajectory. The remote ether cannot form the usual small alpha-alkoxy chelate, and the coordinating solvent further supports using an open-addition prediction.[ref]
Felkin-Anh and Cram Chelate by University of Windsor. pp. 1–2: perpendicular large group and nucleophilic approach past the small group
The predicted new stereocentre is . In the question's zigzag projection, this corresponds to the new OH bond directed behind the page and the newly introduced methyl toward the reader, with the original stereocentres unchanged. The new-centre priority order is OH phenyl the substituted alpha-carbon chain methyl. This is a model-based major-product prediction, rather than a quantitative diastereomeric ratio.
