Meerwein-Ponndorf-Verley reductions, aryl ketones

Kinetic studies of the Midland reduction confirmed that the reduction of aldehydes is a bimolecular process and the changes in ketone structure have a marked influence on the rate of the reaction (e.g., the presence of an EWG in the para position of aryl ketones increases the rate compared to an EDG in the same position). However, when the carbonyl compound is sterically hindered, the rate becomes independent of the ketone concentration and the structure of the substrate. The mechanism with sterically unhindered substrates involves a cyclic boatlike transition structure (similar to what occurs in the Meerwein-Ponndorf-Verley reduction). The favored transition structure has the larger substituent (Rl) in the equatorial position, and this model correctly predicts the absolute stereochemistry of the product. 

---