Carbocations in superacids

Under the long-lived conditions of carbocations in superacid (Section 5.3, p. 235), 1,2-shifts interconverting ions of like stability also occur and are very rapid. For example, at — 180°C the five methyl groups of 2,3,3-trimethylbutyl cation have only one peak in the nmr. This observation implies that the methyl shift in Equation 6.6 occurs at the rate of 75 x 103 sec -1 with an activation barrier of <5 kcal mole-1.8... 

A book containing authoritative reviews of many aspects of carbocation chemistry has been published.1 These reviews are up-to-date to the end of 1991, as the book is the result of a symposium held in honour of George Olah in 1992 it features many of the world s leading carbocation chemists. At least one book review of it has also appeared.2 Olah has an introductory chapter concerning his decades-long search for stable long-lived carbocations in superacid media,3 and other review chapters include ones on carbocations at surfaces and interfaces,4 on the X-ray structural analyses that have been performed on many carbocation salts and related compounds in recent years,5 and on natural product chemistry in superacids.6 Other review chapters will be referred to below, as appropriate. 

This chapter deals with silyl-substituted carbocations. In Section II results of quantum chemical ab initio calculations of energies and structures of silyl-substituted carbocations are summarized1. Throughout the whole chapter results of ab initio calculations which relate directly to the experimental observation of silyl-substituted carbocations and their reactions are reviewed. Section m reports on gas phase studies and Section IV on solvolytic investigations of reactions which involve silyl-substituted carbocation intermediates and transition states. Section V summarizes the structure elucidation studies on stable silyl-substituted carbocations. It includes ultra-fast optical spectroscopic methods for the detection of transient intermediates in solution, NMR spectroscopic investigations of silyl-substituted carbocations in superacids and non-nucleophilic solvents, concomitant computational studies of model cation and X-ray crystallography of some silyl-substituted carbocations which can be prepared as crystals of salts. 

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