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 L¹ = [o-(CHNtBu)C₆H₄], L² = [o-(CHN-2,6-iPr₂C₆H₃)C₆H₄], L³ = [o-(CH₂NMe₂)C₆H₄]); L^1–3BCl₂ (for 1 L = L¹, for 2 L = L², for 5 L = L³), L¹BPhCl (3) and L¹BCy₂ (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 1–3 with the anilides ArNHLi (Ar = 2,6-Me₂C₆H₃ or 2,6-iPr₂C₆H₃) proceeded via unexpected addition of anilide across the CN 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 CN double bond involved in the ligand backbones is the crucial step of the whole reaction. The CN 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 L¹H did not react with ArNHLi under the same reaction conditions in an analogous reaction.