Investigations on non-classical silylium ions leading to a cyclobutenyl cation

Abstract

Instead of yielding the desired non-classical silylium ions, the reactions of different alkenes/alkynes with several [Me₃Si]⁺ sources mostly led to oligomerization, or – in the presence of Me₃SiH – hydrosilylation of the alkenes/alkynes. Yet, from the reaction of 2-butyne with ion-like Me₃Si–F–Al(OR^F)₃ (R^F = C(CF₃)₃) the salt of the silylated tetramethyl cyclobutenyl cation [Me₄C₄–SiMe₃]⁺[al–f–al]⁻1 ([al–f–al]⁻ = [(R^FO)₃Al–F–Al(OR^F)₃]⁻) was obtained in good yield (NMR, scXRD, Raman, and IR). All the experimental and calculated evidence suggest a mechanism in which 1 was formed via a non-classical silylium ion as an intermediate. The removal of the [Me₃Si]⁺ moiety from the cation in 1 was investigated as a means to provide free tetramethyl cyclobutadiene (CBD). However, the addition of [NMe₄]F, in order to release Me₃SiF and form CBD, led to the unexpected deprotonation of the cation. The addition of 4-dimethylaminopyridine to remove the [Me₃Si]⁺ cation as a Lewis acid/base adduct, led to an adduct with the four-membered ring in the direct neighborhood of the Me₃Si group. By the addition of Et₂O to a solution of 1, the [F–Al(OR^F)₃]⁻ anion (and Et₂O–Al(OR^F)₃) was generated from the [al–f–al]⁻ counterion. Subsequently, the [F–Al(OR^F)₃]⁻ anion abstracted the [Me₃Si]⁺ moiety from [Me₄C₄–SiMe₃]⁺, probably releasing CBD. However, due to the immediate reaction of CBD with [Me₄C₄–SiMe₃]⁺ and subsequent oligomerization, it was not possible to use CBD in follow-up chemistry.