Brook Rearrangement

The rearrangement of silicon groups from carbon to oxygen is called the Brook rearrangement. An important driving force for this shift is the increased bond strength of the Si–O bond (110 Kcal/mol) compared with the Si–C bond (76 Kcal/mol). The example given in the following equation is catalyzed by base, and a cyclic transition state is indicated by the high entropy of activation. Retention of configuration at the silicon atom during the rearrangement was demonstrated by the reaction sequence shown by clicking on the diagram. Since the benzylation of the silyl chloride is known to proceed with inversion, and the final hydride reduction with retention, the rearrangement itself must have occurred with retention. Note that the sign of optical rotation for different silyl derivatives does not necessarily correlate with their configuration.

  When alkoxide base is added to silyl ketones, hypervalent silicon intermediates may be formed prior to rearrangement, as shown by one last click on the diagram. Mechanisms for the formation of various products are given by the curved arrows.