Superelectrophilic intermediates in nitrogen-directed aromatic borylation.

The first examples of borylation under conditions of borenium ion generation from hydrogen-bridged boron cations are described. The observable H-bridged cations are generated by hydride abstraction from N,N-dimethylamine boranes Ar(CH(2))(n)NMe(2)BH(3) using Ph(3)C(+) (C(6)F(5))(4)B(-) (TrTPFPB) as the hydride acceptor. In the presence of excess TrTPFPB, the hydrogen-bridged cations undergo internal borylation to afford cyclic amine borane derivatives with n = 1-3. The products are formed as the corresponding cyclic borenium ions according to reductive quenching experiments and (11)B and (1)H NMR spectroscopy in the case with Ar = C(6)H(5) and n = 1. The same cyclic borenium cation is also formed from the substrate with Ar = o-C(6)H(4)SiMe(3) via desilylation, but the analogous system with Ar = o-C(6)H(4)CMe(3) affords a unique cyclization product that retains the tert-butyl substituent. An ortho-deuterated substrate undergoes cyclization with a product-determining isotope effect of k(H)/k(D) 2.8. Potential cationic intermediates have been evaluated using B3LYP/6-31G* methods. The computations indicate that internal borylation from 14a occurs via a C-H insertion transition state that is accessible from either the borenium pi complex or from a Wheland intermediate having nearly identical energy. The Ar = o-C(6)H(4)SiMe(3) example strongly favors formation of the Wheland intermediate, and desilylation occurs via internal SiMe(3) migration from carbon to one of the hydrides attached to boron.