Reactivity of C,N-chelated organoboron compounds with lithium anilides – formation of unexpected 1,2,3-trisubstituted 1H-2,1-benzazaboroles

Abstract

A set of C,N-intramolecularly coordinated boranes containing various C,N-chelating ligands L(1-3) (where L(1) = [o-(CH=NtBu)C6H4], L(2) = [o-(CH=N-2,6-iPr2C6H3)C6H4], L(3) = [o-(CH2NMe2)C6H4]); L(1-3)BCl2 (for 1 L = L(1), for 2 L = L(2), for 5 L = L(3)), L(1)BPhCl (3) and L(1)BCy2 (4) (where Cy = cyclohexyl) were synthesized and fully characterized by multinuclear NMR spectroscopy and in cases of 1 and 3-5 by the single crystal X-ray diffraction analysis. The reaction of with the anilides ArNHLi (Ar = 2,6-Me2C6H3 or 2,6-iPr2C6H3) proceeded via unexpected addition of anilide across the C=N bond yielding 1,2,3-trisubstituted 1H-2,1-benzazaboroles 6-11, whose structures were unambiguously established by single crystal X-ray diffraction analysis (except for 11) and multinuclear NMR spectroscopy. In contrast, compounds 4 and 5 were inert towards ArNHLi. The investigation dealing with the reaction mechanism between the parent boranes 1-3 and ArNHLi revealed that amidolithiation of the C=N double bond involved in the ligand backbones is the crucial step of the whole reaction. The C=N double bond in 1-3 is activated by its coordination to the ortho bonded Lewis acidic boron center, which was also proven by the fact that the non-substituted ligand L(1)H did not react with ArNHLi under the same reaction conditions in an analogous reaction.