Electrophilic osmylation

E)- and (Z)-Allylsilanes with bulky alkyl groups are osmylated with the same sense of induced diastereoselectivity as observed for allylic alcohols and derivatives. In contrast to allylic alcohols, the preferred ground state conformation of these substrates with the allylic hydrogen atom in the plane of the double bond is also expected to be the most reactive one. Assuming electrophile approach from the opposite side to the silicon atom, the outside position is usually preferred by the alkyl groups3 32,38,39. 

Due to the electrophilic nature of osmium tetroxide, electron-withdrawing groups connected to the alkene double bond retard the dihydroxylation. This is in contrast to the oxidation of alkenes by Potassium Permanganate, which preferentially attacks electron-deficient double bonds. However, in the presence of a tertiary amine such as pyridine, even the most electron-deficient alkenes can be osmylated by osmium tetroxide (eq 4). The more highly substituted double bonds are preferentially oxidized (eq 5). 

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