Structure of a Non‐classically Bridged Methyleneborane in the Crystal

Bridging as a drastic consequence of intense σ-π interaction was predicted as long ago as 1984, and now it has been verified experimentally. Methyleneboranes 1 exist as the bridged structure 1a, a conclusion derived from the unusually small B-C-B angle (calc. for R1 = R2 = H, 78.1/78.8°; obs. for R1 =GeMe3,R2 = 2,3,4,6-Me4C6H, 78.5(4)°). As in the case of non-classical carbocations, the shielding of the formally electron-deficient center in 1 is much greater than that observed with corresponding classical compounds