Reactions of carbene-stabilized borenium cations

Abstract

In this paper we probe the reactivity of the borenium cations [C₃H₂(NCH₂C₆H₄)(NCH₂Ph)BH][B(C₆F₅)₄] 2 and [C₃H₂(NCH₂C₆H₄)₂B][B(C₆F₅)₄] 3. The reactions of 2 with cyclohexene or 3,3-dimethyl-1-butene gave the alkyl-aryl borenium salts [PhCH₂(CHN)₂CCH₂C₆H₄BR][B(C₆F₅)₄] (R = Cy 4, CH₂CH₂tBu 5) while the corresponding reactions with diphenylacetylene, 1-hexyne and 1-phenyl-1-propyne gave the aryl–alkenyl borenium cation salts [PhCH₂(CHN)₂CCH₂C₆H₄BC(R¹)C(H)R²][B(C₆F₅)₄] (R¹ = R² = Ph 6, R¹ = H, R² = C₄H₉7, R¹ = Me, R² = Ph 8a, R¹ = Ph, R² = Me 8b). In contrast, the reaction of 2 with ethynyldiphenylphosphane or 2-vinylpyridine lead to the formation of the adducts, [PhCH₂(CHN)₂CCH₂C₆H₄B(H)P(Ph₂)CCH][B(C₆F₅)₄] 9, [PhCH₂(CHN)₂CCH₂C₆H₄B(H)NC₅H₄C(H)CH₂][B(C₆F₅)₄] 10, respectively, while the more bulky donor H₂CC(Ph)PMes₂ gave 1,2-hydroboration of the phosphinoalkene affording [PhCH₂(CHN)₂CCH₂C₆H₄BCH₂CH(Ph)PMes₂][B(C₆F₅)₄] 11. In another vein of reactivity, one or two equivalents of the FLP, PtBu₃/B(C₆F₅)₃ is shown to react with 3 to give the zwitterionic borenium-borate species [C₂H₂(NCH(BC(CHNCH₂C₆H₄)₂)C₆H₄)(NCH(B(C₆F₅)₃)C₆H₄)CB] 12 and the anionic bis-borate species[tBu₃PH][C₂H₂(NCH(B(C₆F₅)₃)₂C₆H₄)₂CB] 13. The implications of these findings are discussed.