Reactions of carbene-stabilized borenium cations

In this paper we probe the reactivity of the borenium cations [C3H2(NCH2C6H4)(NCH2Ph)BH][B(C6F5)(4)] 2 and [C3H2(NCH2C6H4)(2)B][B(C6F5)(4)] 3. The reactions of 2 with cyclohexene or 3,3-dimethyl-1-butene gave the alkyl-aryl borenium salts [PhCH2(CHN)(2)CCH2C6H4BR][B(C6F5)(4)] (R = Cy 4, CH(2)CH(2)tBu 5) while the corresponding reactions with diphenylacetylene, 1-hexyne and 1-phenyl-1-propyne gave the aryl-alkenyl borenium cation salts [PhCH2(CHN)(2)CCH2C6H4BC(R-1)C(H)R-2][B(C6F5)(4)] (R-1 = R-2 = Ph 6, R-1 = H, R-2 = C(4)H(9)7, R-1 = Me, R-2 = Ph 8a, R-1 = Ph, R-2 = Me 8b). In contrast, the reaction of 2 with ethynyldiphenylphosphane or 2-vinylpyridine lead to the formation of the adducts, [PhCH2(CHN)(2)CCH2C6H4B(H)P(Ph-2)CCH][B(C6F5)(4)] 9, [PhCH2(CHN)(2)CCH2C6H4B(H)NC5H4C(H)CH2][B(C6F5)(4)] 10, respectively, while the more bulky donor H2C?C(Ph)PMes(2) gave 1,2-hydroboration of the phosphinoalkene affording [PhCH2(CHN)(2)CCH2C6H4BCH2CH(Ph)PMes(2)][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[tBu(3)PH][C2H2(NCH(B(C6F5)(3))(2)C6H4)(2)CB] 13. The implications of these findings are discussed.