Understanding and Expanding Zinc Cation/Amine Frustrated Lewis Pair Catalyzed C–H Borylation

[(NacNac)Zn(DMT)][B(C6F5)4], 1, (NacNac = {(2,6-iPr2H3C6)N(CH3)C}2CH), DMT
= N,N-dimethyl-4-toluidine), was synthesized via
two routes starting from either (NacNac)ZnEt or (NacNac)ZnH. Complex 1 is an effective (pre)catalyst for the C–H borylation
of (hetero)arenes using catecholborane (CatBH) with H2 the
only byproduct. The scope included weakly activated substrates such
as 2-bromothiophene and benzothiophene. Computational studies elucidated
a plausible reaction mechanism that has an overall free energy span
of 22.4 kcal/mol (for N-methylindole borylation),
consistent with experimental observations. The calculated mechanism
starting from 1 proceeds via the displacement of DMT
by CatBH to form [(NacNac)Zn(CatBH)]+, D,
in which CatBH binds via an oxygen to zinc which makes the boron center
much more electrophilic based on the energy of the CatB-based LUMO.
Combinations of D and DMT act as a frustrated Lewis pair
(FLP) to effect C–H borylation in a stepwise process via an
arenium cation that is deprotonated by DMT. Subsequent B–H/[H-DMT]+ dehydrocoupling and displacement from the coordination sphere
of zinc of CatBAr by CatBH closes the cycle. The calculations also
revealed a possible catalyst decomposition pathway involving hydride
transfer from boron to zinc to form (NacNac)ZnH which reacts with
CatBH to ultimately form Zn(0). In addition, the key rate-limiting
transition states all involve the base, thus fine-tuning of the steric
and electronic parameters of the base enabled a further minor enhancement
in the C–H borylation activity of the system. Outlining the
mechanism for all steps of this FLP-mediated process will facilitate
the development of other main group FLP catalysts for C–H borylation
and other transformations.

[(NacNac)Zn(DMT)][B(C6F5)4]（记为配合物1，其中NacNac配体的结构为{(2,6-二异丙基苯基)N(甲基)C}2CH，DMT为N,N-二甲基对甲苯胺）通过两条路线合成，起始原料分别为(NacNac)ZnEt或(NacNac)ZnH。配合物1是使用邻苯二酚硼烷（catecholborane, CatBH）进行（杂）芳烃C–H硼化反应的高效（预）催化剂，唯一副产物为氢气（H2）。该催化体系的底物适用范围涵盖弱活化底物，例如2-溴噻吩与苯并噻吩。计算研究阐明了一条合理的反应机理：以N-甲基吲哚的硼化反应为例，该机理的总自由能垒为22.4 kcal/mol，与实验观测结果一致。以1为起始的计算机理遵循如下路径：CatBH取代DMT，生成[(NacNac)Zn(CatBH)]+（记为物种D）；在此物种中，CatBH通过氧原子与锌配位，基于CatB基最低未占据分子轨道（lowest unoccupied molecular orbital, LUMO）的能量可知，该配位作用使硼中心的亲电性显著增强。D与DMT的组合可作为受阻路易斯对（frustrated Lewis pair, FLP），通过分步路径实现C–H硼化反应：反应经由芳鎓阳离子（arenium cation）中间体，最终由DMT完成去质子化过程。随后发生B–H/[H-DMT]+脱氢偶联反应，同时CatBH取代锌配位球内的CatBAr，由此完成催化循环。计算研究还揭示了一条可能的催化剂分解路径：硼原子向锌转移氢负离子，生成(NacNac)ZnH；该中间体可与CatBH进一步反应，最终生成零价锌（Zn(0)）。此外，所有关键限速过渡态均涉及该碱；因此，对该碱的空间位阻与电子参数进行精细调控，可进一步小幅提升该体系的C–H硼化催化活性。阐明该受阻路易斯对介导反应的全步骤机理，将有助于开发其他主族元素受阻路易斯对催化剂，用于C–H硼化反应及其他转化过程。