On the Reactivity of Silylboranes toward Lewis Bases: Heterolytic BSi Cleavage vs. Adduct Formation

Silylboranes are important reagents in a variety of catalytic silylation and silaboration reactions. While transition-metal-catalyzed reactions are well established, organo-/Lewis base-catalyzed reactions of silylboranes have only recently emerged. For both catalytic processes the reactivity of silylboranes toward Lewis bases is of relevance. While for organo-catalyzed reactions Lewis base activation of the silylborane has been proposed, transition-metal- and especially copper-catalyzed reactions also frequently require the presence of Lewis basic alkali metal alkoxides. In the present study we explore the reaction of K(18-crown-6) tert-butoxide and the NHC 1,3-diisopropyl-4,5-dimethyl-imidazol-2-ylidene as exemplary Lewis bases with the two silylboranes pinB-SiMe2Ph and pinB-SiPh3 (pin = OCMe2CMe2O). The reaction with K(18-crown-6) tert-butoxide results in activation of the boronsilicon bond. The isolated product of this activation is either the potassium silyl complex [K(18-crown-6)SiPh3] or [K(18-crown-6)(thf)2][pinB(SiMe2Ph)2], the formal Lewis acid/base adduct of [K(18-crown-6)SiMe2Ph] with pinB-SiMe2Ph. Both complexes react essentially as sources of nucleophilic silyl moieties in reactions with exemplary electrophiles. In contrast, usage of the carbene leads to the formation of isolable Lewis acid/base adducts of the type (NHC)pinB-SiR3, which do not react as sources of nucleophilic silyl moieties. The identification and characterization of these species appears of relevance for the mechanistic understanding and further development of Lewis base/organo- as well as transition-metal-catalyzed silyl transfer reactions.