Electrophilic boron carboxylate and phosphinate complexes

Abstract

The reactions of a series of carboxylic acids with H₂B(C₆F₅)·SMe₂ are shown to afford species of the form [RC(O)OB(C₆F₅)]₂O, (R = Tol 1, Ph 2, C₆F₅3, 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)·SMe₂, respectively, to give [TolC(O)OB(C₆F₅)₂]₂6, [(C₆F₅)C(O)OB(C₆F₅)₂]₂7, and [Ph₂P(O)OBH(C₆F₅)]₂8. 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.