Stereochemistry to Deduce Mechanism

Several common examples show the power of diis reasoning. The reaction of osmium tetroxide widi olefins followed by reduction gives diols resulting exclusively from syn addition to the double bond. The reaction is hence stereospecific The addition stereochemistry is clearly seen in cyclic olefins, but it is also seen in acyclic olefins where single diastereomers are produced [reaction (6.4)]. 

The results from the cyclic series show that both oxygens come from the same side of tile double bond, probably from a single 0s04 molecule. The results in the acyclic series demonstrate that both oxygens add to the ends of the double bond at the same time. If one oxygen added first, an intermediate with a single carbon-carbon bond would be formed which could isomerize by rotation around that bond. The observation of complete diastereoselectivity requires that both C-0 bonds be formed simultaneously. Thus a concerted addition across the double bond is the most reasonable pathway consistent with these results. The stereochemical analysis is shown below for the cis starting material. 

An analogous analysis for the trans olefin would predict only the d,l diastereomer for concerted addition of 0s04 to the it bond, but die same meso-d,l mixture would be obtained if die addition were stepwise. 

Walden inversion was the term given to the change in stereochemistry observed in bimolecular nucleophilic substitutions. For example, reaction of (2S)-2-triflyloxyesters with sodium azide gives (2R )-2-a/,idocstcrs. 

Inversion of configuration requires that the nucleophile adds electrons to die a orbital of die carbon-tridate bond from die side opposite diat bond. As required by die stereochemistry, formation of die bond from azide to carbon is concurrent widi cleavage of the carbon leaving group bond. 

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