Alkene–Carbene Isomerization induced by Borane: Access to an Asymmetrical Diborene

A 2,3-dihydro-1H-1,2-azaborole derivative 2 was converted to a cyclic
(alkyl) (amino)­carbene (cAAC)
via 1,2-hydrogen migration triggered by boranes to afford cAAC-borane
adducts. This procedure allowed us to develop an asymmetrical diborene
cAAC·(Br)­BB­(Br)·IDip 6, which was isolated
and fully characterized. The 11B NMR spectrum, X-ray diffraction
analysis and computational studies indicate that π-electrons
on the central B2 moiety in 6 are unequivalently
distributed, and thus polarized. A complete scission of the BB
double bond in 6 was achieved by the treatment with an
isonitrile, which led to the formation of a base-stabilized B,N-containing
methylenecyclopropane 7.

将2,3-二氢-1H-1,2-氮杂硼环（2,3-dihydro-1H-1,2-azaborole）衍生物2经硼烷触发的1,2-氢迁移过程，转化为环（烷基）（氨基）卡宾（cyclic(alkyl)(amino)carbene，简称cAAC），从而得到cAAC-硼烷加合物。利用该合成策略，我们成功制备出不对称二硼烯类化合物cAAC·(Br)B=B(Br)·IDip 6，并对其完成了分离与全表征。通过11B核磁共振（NMR）光谱、X射线衍射分析及计算化学研究证实，化合物6的中心B₂结构单元的π电子呈非均匀分布，因此具有极化特性。采用异腈对化合物6进行处理，可实现其B=B双键的完全断裂，最终得到碱稳定的含硼、氮的亚甲基环丙烷类化合物7。