Investigations on <i>non-classical</i> silylium ions leading to a cyclobutenyl cation

Abstract

Instead of yielding the desired <i>non-classical</i> 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 <i>ion-like</i> Me₃Si-F-Al(OR^F)₃ (R^F = C(CF₃)₃) the salt of the silylated tetramethyl cyclobutenyl cation [Me₄C₄-SiMe₃]⁺[<i>al</i>-<i>f</i>-<i>al</i>]^- <b>1</b> ([<i>al</i>-<i>f</i>-<i>al</i>]^- = [(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 <b>1</b> was formed <i>via</i> a <i>non-classical</i> silylium ion as an intermediate. The removal of the [Me₃Si]⁺ moiety from the cation in <b>1</b> 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 <b>1</b>, the [F-Al(OR^F)₃]^- anion (and Et₂O-Al(OR^F)₃) was generated from the [<i>al</i>-<i>f</i>-<i>al</i>]^- 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.