Substitution Reactions at Tetracoordinate Boron: Synthesis of N-Heterocyclic Carbene Boranes with Boron−Heteroatom Bonds

Boryl halide, carboxylate and sulfonate complexes of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (dipp-Imd-BH2X, X = halide or sulfonate) have been prepared from the parent borane dipp-Imd-BH3 by (1) substitution reactions with R−X (X = halide or sulfonate), (2) reactions with electrophiles (like I2 or NIS), or (3) acid/base reactions with HX (provided that HX has a pKa of about 2 or less). Dipp-Imd-BH2I is most conveniently prepared by reaction with diiodine while dipp-Imd-BH2OTf is best prepared by reaction with triflic acid. These and other less reactive complexes behave as electrophiles and can be substituted by a wide range of heteroatom nucleophiles including halides, thiolates and other sulfur-based nucleophiles, isocyanate, azide, nitrite, and cyanide. The resulting products are remarkably stable, and many have been characterized by X-ray crystallography. Several are members of very rare classes of functionalized boron compounds (boron azide, nitro compound, nitrous ester, etc.).

以母体硼烷dipp-Imd-BH₃为原料，可制备得到1,3-双(2,6-二异丙基苯基)咪唑-2-卡宾（1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene）配位的卤代硼基、羧酸硼基及磺酸硼基配合物（dipp-Imd-BH₂X，其中X为卤离子或磺酸根），具体合成路径包括：(1) 与R−X（X为卤离子或磺酸根）发生取代反应；(2) 与亲电试剂（如碘单质(I₂)或N-碘代琥珀酰亚胺(NIS)）反应；(3) 与HX发生酸碱反应（需满足HX的pKa约≤2）。其中dipp-Imd-BH₂I最便捷的制备方法是与二碘单质反应，而dipp-Imd-BH₂OTf的最佳合成路径则为与三氟甲磺酸反应。上述配合物及其他反应活性较低的同类物种均表现出亲电性，可与多种杂原子亲核试剂发生取代反应，涵盖卤离子、硫醇盐及其他含硫亲核试剂、异氰酸根、叠氮根、亚硝酸根与氰根离子。所得产物稳定性优异，其中多数已通过X射线晶体衍射（X-ray crystallography）完成结构表征，部分产物属于极为罕见的官能化硼化合物类别（如硼叠氮化物、硝基硼化合物、亚硝酸酯基硼化合物等）。