Resonance structure B is not significantly involved in the π-etectron distribution of the deprotonated borylborataalkynes 1, as shown by structure analysis and NMR spectroscopic data. This finding, together with the fact that the BspCsp distance found for 1b · 2Et2O is in exact agreement with that calculated for HB⊖ CH, supports the presence of a true BC triple bond in 1, as shown in A. a, R1 = 2,4,6-Me3C6H2; b, R1 = 2,3,5,6-Me4C6H; R2 = Me3Si.
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Allwohn et al. (1990) studied this question.