1.1.3.3- Tetrakis allyl cation

Protonation of tetrakis(trimethylsilyl)allene 33 with HSO3F/ SbF5 (1 1) gives the 1,1,3,3-tetrakis(trimethylsilyl)-l-propen-2-yl cation 34. The isomeric allyl cation 35 is not formed (12, 44). 

The synthesis of a trisilaallyl cation with an allylic cation character is described. The allylic cations are considered important intermediates in organic transformations. The preparation of the trisilaallyl cationic species, cyclotetrasilenylium ion is performed by the reaction of tetrakis(di-tbutylmethylsilyl) disilene (6) with [EtsSifbenzene)]" - TPFPB TPFPB = tetrakisfpenta-fluorophenyl)borate in toluene (Scheme 6.3.1.1). The cyclotetrasilenylium ion is demonstrated to exist as a cation free of anion interaction in the solid state (5). 

The cyclotetrasilenylium ion with the tetrakis(2,3,5,6-tetrafluorophenyhborate counter anion could also be synthesized by reaction with [EtaSiibenzene)]" TTFPB (TTFPB = tetrakis(2, 3,5,6-tetrafluorophenyl)borate) in toluene. Based on the chemistry of the lighter allyl cation congeners, the allylic character of the cyclotetrasilenylium ion may lead to interesting reactivity. 

This complex is not the actual catalyst for the hydrovinylation, but needs to be activated in the presence of a suitable co-catalyst. The role of this additive is to abstract the chloride ion from the nickel centre to generate a cationic allyl complex that further converts to the catalytically active nickel hydride species. In conventional solvents this is typically achieved using strong Lewis acids such as Et2AlCl. Alternatively, sodium or lithium salts of non-coordinating anions such as tetrakis-[3,5-bis(trifluoromethyl)phenyl]borate (BARF) can be used to activate hydrovinylation... 

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