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

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) (R-F = C(CF3)(3)) the salt of the silylated tetramethyl cyclobutenyl cation [Me4C4-SiMe3](+)[al-f-al](-) 1 ([al-f-al](-) = [((RO)-O-F)(3)Al-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.