Silylium ion Lewis acid catalyst

So far, the first approach, i.e. the enantioselective Sakurai reaction using chiral Lewis acid catalysts has been accomplished by Yamamoto s chiral (acyloxy)borjme (CAB) catalysis [40] and others that are not included in this chapter. Progress in developing chiral silicon Lewis acids for the reaction is now awaited but a recent publication by Jprgensen and Helmchen on a chiral silylium ion appears in a later section on ring construction [41]. Thus, other two approaches are shown herein. 

Recently, this concept could be transferred to the homologous silylium ions 3-Si (Si in cx-position to the ferrocene core) [18, 19], which were found to be potent catalysts for Diels-Alder reactions at low temperature [19]. The electron-rich ferrocene core buffers the Lewis acidity, thus avoiding the irreversible formation of Lewis pairs. 

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