Understanding and Expanding Zinc Cation/Amine Frustrated Lewis Pair Catalyzed C–H Borylation
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Abstract
[(NacNac)Zn(DMT)][B(C6F5)4], 1, (NacNac = {(2,6- i Pr2H3C6)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.
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