Amino and Alkyl B-Substituted P-Stabilized Borenium Salts

Abstract

The ability of the phosphino-naphthyl moiety to stabilize amino- and alkylborenium cations has been studied. Surprisingly, the phosphine–aminochloroborane precursor 2 was found to exist in neutral open form (without P→B interaction) in benzene solution and in the solid state but to ionize spontaneously in chloroform to generate the P-stabilized borenium salt 3. Addition of gallium trichloride shifts the process forward and affords the corresponding tetrachlorogallate borenium salt 3′. The phosphine group of 2 remains available for external reactivity, as shown by the ready formation of the corresponding phosphine gold(I) chloride complex 4. The P-stabilized cyclohexylborenium cation 6 has also been prepared by reacting the corresponding bromoborane 5 with gallium tribromide. Compound 6 is a very rare example of an alkyl-substituted borenium salt. The structures of 2, 3′, and 4–6 have been unambiguously ascertained by multinuclear NMR spectroscopy and X-ray crystallography.