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

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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(ORF)₃ (RF = C(CF₃)₃) the salt of the silylated tetramethyl cyclobutenyl cation [Me₄C₄-SiMe₃]+[al-f-al]- 1 ([al-f-al]- = [(RFO)₃Al-F-Al(ORF)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(ORF)₃]- anion (and Et₂O-Al(ORF)₃) was generated from the [al-f-al]- counterion. Subsequently, the [F-Al(ORF)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.

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