Preparation of the first tricoordinate silyl cation

Abstract

The preparation of the first tricoordinate silyl cation was made possible by a sequence of discoveries over a 20 year period. The extremely high electrophilicity of the material required protection from all manner of nucleophiles. Only arenes had sufficiently low nucleophilicity to serve as solvent. Tetrakis(pentafluorophenyl) borate provided the least nucleophilic anion. All manipulations had to be carried out in an inert atmosphere, primarily as protection from moisture. Sterically bulky substituents such as mesityl were needed to protect the cation even from the optimized solvent and anion. The trimesitylsilylium ion was produced from allyltrimesitylsilane by reaction of electrophiles with the allyl double bond and extrusion of the allyl adduct. Copyright © 2001 John Wiley & Sons, Ltd.

A Tamed Reactive Intermediate

“…No atoms can be within bonding distance. This extreme criterion was, for example, fulfilled in the case of the silyl cation (6). In the present case, there is no doubt from the mass spectrum that the imprisoned atom is germanium and from the x-ray data that the triflate counterions are well removed from bonding distances.…”

“…The preparation of the free silylium cation relied on these techniques (6). In 1991, Cram et al, in a paper famously entitled "The Taming of Cyclobutadiene," reported isolation of the highly reactive four-membered ring containing two double bonds by protecting it within a molecular cage or prison (7).…”

Polysiloxanol condensation and disproportionation in the presence of a superacid

“…It has been frequently used in the chemistry of transition metal alkyl and olefin complexes as the counter-ion for highly electrophilic cations [11][12][13]. It was also exploited in synthesis of the silylenium ion R 3 Si þ [14] and used in the first successful synthesis of tertiary trisilyloxonium ion [15]. New very reactive arenium cations (Wheland complexes) have recently been isolated and characterized using B(C 6 F 5 ) À 4 as the counter-ion [16].…”

Studies on the Efficient Generation of PhosphorusCarbon Bonds via a Rearrangement of PIII Esters Catalysed by Trimethylhalosilanes

“…Intermediates such as R 3 Si + are highly improbable in the presence of nucleophiles. [22] It is intriguing why chlorotrimethylsilane (Me 3 SiCl) is not effective in promoting a mechanism similar to that shown in Scheme 8, even at elevated temperatures and under solvolytic conditions. This contrasting behaviour is likely to be due to the strength of the silicon-chlorine bond and the fact that the reaction between Me 3 SiCl and ester 1 (Scheme 9) takes a different course from those described in Schemes 5 and 9.…”