Selective hydroboration of electron-rich isocyanates by an NHC-copper(i) alkoxide

The (IPr)CuOtBu catalysed reduction of 11 aryl and alkyl isocyanates with pinacolborane gave only the boraformamides, pinBN(R)C(O)H, in most cases. Overreduction, which hampers almost all isocyanate hydroborations, was restricted to electron poor aryl isocyanates (4-NC-C6H4NCO, 4-F3C-C6H4NCO, 3-O2N-C6H4NCO). Computational analysis showed stability of [(IPr)CuH](2), which was proposed to be the catalyst resting state, drives selectivity, suggesting an approach to prevent overreduction in future work. In the case of iPrNCO, formation of this species renders overreduction kinetically inaccessible. For 4-NC-C6H4NCO, however, the barrier height for the first step of over-reduction is much lower, even relative to [(IPr)CuH](2), resulting in unselective reduction.

Automated summary

The (IPr)CuOtBu catalysed reduction of 11 aryl and alkyl isocyanates with pinacolborane resulted in the formation of boraformamides, pinBN(R)C(O)H, in most cases. Overreduction was observed only for electron poor aryl isocyanates (4-NC-C6H4NCO, 4-F3C-C6H4NCO, 3-O2N-C6H4NCO). Computational analysis revealed that the stability of [(IPr)CuH](2), proposed as the catalyst resting state, influences selectivity and offers a potential solution to prevent overreduction in future work. In the case of iPrNCO, the formation of [(IPr)CuH](2) prevents overreduction kinetically. However, for 4-NC-C6H4NCO, the barrier height for the first step of overreduction is much lower, resulting in unselective reduction.