Electrophilic boron carboxylate and phosphinate complexes

摘要

The reactions of a series of carboxylic acids with H2B(C6F5)center dot SMe2 are shown to afford species of the form [RC(O)OB(C6F5)](2)O, (R = Tol 1, Ph 2, C6F5 3, Me2BrC 4, Me 5) in 87-95% yields with the concurrent reduction of the carboxylic acid to the corresponding aldehyde. A mechanism for the formation of 1-5 is proposed to proceed via a cyclic eight-membered ring species. Analogues of these species were prepared via reactions of carboxylic and phosphinic acids with HB(C6F5)(2) and H2B(C6F5)center dot SMe2, respectively, to give [TolC(O)OB(C6F5)(2)](2) 6, [(C6F5)C(O)OB(C6F5)(2)](2) 7, and [Ph2P(O)OBH(C6F5)](2) 8. These products react subsequently to give TolC(O)OBH(C6F5)(NC5H4NMe2) 9 and Ph2P(O)OBH(C6F5)(NC5H4NMe2) 10. The acyloxyborate derivatives 1-4 were shown to be inactive in mediating the direct amidation of carboxylic acids, consistent with previous observations that infer the need for a sterically congested environment about the boron centres.