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.
Highlights
The formulation of acyloxyboranes generated a lot of confusion in the early literature, over the formulation of the product derived from the reaction of orthoboric acid and acetic anhydride
We previously reported the preparation of the salt [Cp*2Fe][PhCO2B(C6F5)3] via the one electron reduction of a peroxide in the presence of a borane
Repeated reactions showed that all the reagents were consumed when combined in an acid to borane ratio of 3 : 2. The 11B NMR spectrum revealed the complete conversion of H2B(C6F5)·SMe2 into a new fourcoordinate boron species that exhibited a broad resonance at 5.1 ppm
Summary
The formulation of acyloxyboranes generated a lot of confusion in the early literature, over the formulation of the product derived from the reaction of orthoboric acid and acetic anhydride. 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]2 6, [(C6F5)C(O)OB(C6F5)2]2 7, and [Ph2P(O)OBH(C6F5)]2 8. 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.
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