Silyl-substituted carbocations

Chapter 1 by H.-U. Siehl discusses parallel stable ion NMR spectroscopic and computational studies on various classes of silyl-substituted carbocations. Chapter 2 by K. Komatsu focuses on unusually stable n-conjugated carbocations that are formed as a result of annelation to bicyclic frameworks. 

Early attempts to generate ot-aryl-(3-silyl substituted carbocations by ionization of 1,1-diphenyl-2-(trimethylsilyl)ethanol 10 usipg FS03H in S02C1F even at very low temperature of -140 °C were unsuccessful (77). Only 1,1-diphenylethyl cation 11 and trimethylsilyl fluorosulfate, fhe products of P-silyl cleavage were observed. 

The ferrocenyl group is a very good electron donor. The a-ferrocenyl P-silyl substituted carbocation 20 is accessible by protonation of ( )-1 -ferrocenyl-2-(triisopropylsilyl)alkene 21 with trifluoroacetic acid in SO2CIF at - 95 °C (13, 22). 

In addition to its interesting structure, the triethylsilylium-aromatic complex has proved useful in preparing other cations. Reaction with 1,1-diphenylethylene, for example, provided the cation 95, the first example of a persistent p-silyl substituted carbocation (i.e., where decomposition by loss of the silyl group did not occur). 

The first X-ray structure of an a-silyl-substituted carbocation (33) is reported its pXR+ value is predicted to be 4.73 The trimesitylsilylium cation is proposed to be a nearly free, tricoordinate species.74 The dimethylsilylium cation undergoes isomer interconversion via (34), according to high-level calculations the most stable structure is... 

Provided that the silicon-carbon bond can be coplanar with the vacant p orbital, the /J-silyl substituted carbocation should be stabilized by hyperconjugation, and this has been demonstrated by Kresge and coworkers47,49. 

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. 

In this brief part only the most important results of ab initio calculations regarding the energies and structures of silyl-substituted carbocations are summarized1. 

Most of the gas phase studies concerning the /J-silyl-substituted carbocations were directed towards a quantification of the /J-silyl group effect and to assign a structure to the observed ions. We will first report the results of the experiments in which only the energy of these type of cations have been determined and we will then discuss the results of the structure elucidations. 

TABLE 1. Experimental stabilization energies (kcalmol ) of j6-silyl-substituted carbocations Me3SiCHR R2-CR3R4 and Me3Si-CR =CR2 according to equations 13 and 14, respectively... 

In this section, solvolysis reactions are described which are thought to proceed via silyl-substituted carbocations. The reader should be aware of the fact that nearly all effects which are described here are of purely kinetic origin and therefore refer to energy differences between ground states and transition states. Hence they are not strictly applicable to the intermediate silyl-substituted carbocations, although the Hammond postulate suggests a close structural resemblance between the transition state for the ionization and the formed carbocation. 

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