CHAPTER 9: Question 7

2D Explanations

Osmium tetroxide and meta-chloroperbenzoic acid both react with alkenes by a syn-addition as shown below. In the case of osmium tetroxide, this directly produces the (racemic) l-diastereomer of the 1,2-dihydroxy-1-phenylpropane. In the case of meta-chloroperbenzoic acid however, the initial product is the (racemic) l-diastereomer of an epoxide. This reacts with hydroxide by an S_N2 reaction to give the racemic u-diastereomer of 1,2-dihydroxy-1-phenylpropane. Thus, the two processes do not give the same product.

If cis-1-phenylpropene was used as the starting material, then the reactions would still proceed by syn-additions (followed by an S_N2 reaction in the case of meta-chloroperbenzoic acid / hydroxide) and the osmium tetroxide reaction would now give the u-diastereomer of the product whilst the meta-chloroperbenzoic acid / hydroxide reaction would give the l-diastereomer as shown below.

Both reactions are both stereoselective since each stereoisomer (cis or trans) of the starting material gives a single diastereomer (l or u) of the product. The reactions are also stereospecific since each stereoisomer (cis or trans) of the starting material gives a different stereoisomer (l or u) of the product.

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