Lateral Stereocontrol Using pi-Allylmolybdenum Systems: The Use of Methyl Substitution To Effect Conformational Control Leading to High Levels of Stereoselectivity.

Stereocontrol exerted by a pi-allyl-Mo(CO)(2)Tp system (Tp = hydrotris(1-pyrazolyl)borato), during addition of Grignard reagents to neighboring aldehyde functionality and during dihydroxylation of alkenes, is found to be dependent on conformational preferences of the substituent relative to the organometallic moiety. Incorporation of a methyl group at C(2) of the pi-allyl ligand leads to excellent conformational control when the lateral substituent is a vinyl group, and this results in a diastereomer ratio of 25:1 for the diol that is obtained from osmylation. Poorer conformational control is observed for an aldehyde, and nucleophile additions show correspondingly lower stereoselectivity.