Centrally chiral compounds of carbon and silicon

The vast majority of all chiral compounds are centrally chiral and possess one or more tetrahedral stereogenic carbon centres. The compounds (1)-(18) mentioned earlier in this chapter are all of this type. Some additional examples of special interest are mentioned here. One of the simplest possible chiral compounds is (22) which was obtained in enantiomerically pure form only recently and shows a small rotation of [a]o = +1.6° for the enantiomer shown. This brings us to an interesting feature of optical activity the size of the rotation depends to some extent on the presence of a chromophore to interact with the incident light. In particular, chiral compounds which are completely non-polarised, such as saturated hydrocarbons, may have immeasurably small optical rotations. For example, the rotation of enantiomerically pure hydrocarbon (23) which was prepared in 1959 by Wynberg, was predicted to be 0.00002° but (not surprisingly) could not be detected on any available polarimeter. 

Small rotations are also observed for another interesting group of chiral compounds those which are chiral due to the presence of isotopes, llie chiral benzyl alcohol (24) and the diphenylmethanol (25) have both been obtained as pure enantiomers and show rotations of +0.7 and -1.0 respectively. Even the acetic acid (26) chiral due 

Spiro compounds, in which the stereogenic centre is at the fusion of two rings, can also be chiral, as in the example of the olive-fly pheromone (27). 

Silicon compounds which have an asymmetric arrangement of atoms around a tetrahedral silicon centre can, of course, also be chiral. There has been little work on chiral silicon compounds to date but a comprehensive review of this is available, 

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