Catalytic activation of unstrained C(aryl)–C(aryl) bonds in 2,2′-biphenols

Transition metal catalysis has emerged as an important means for C–C activation that allows mild and selective transformations. However, the current scope of C–C bonds that can be activated is primarily restricted to either highly strained systems or more polarized C–C bonds. In contrast, the catalytic activation of non-polar and unstrained C–C moieties remains an unmet challenge. Here we report a general approach for the catalytic activation of the unstrained C(aryl)–C(aryl) bonds in 2,2′-biphenols. The key is to utilize the phenol moiety as a handle to install phosphinites as a recyclable directing group. Using hydrogen gas as the reductant, monophenols are obtained with a low catalyst loading and high functional group tolerance. This approach is also applied to the synthesis of 2,3,4-trisubstituted phenols. A further mechanistic study suggests that the C–C activation step is mediated by a rhodium(i) monohydride species. Finally, a preliminary study on breaking the inert biphenolic moieties in lignin models is illustrated. Catalytic activation of non-polar unstrained C−C bonds remains challenging. Now, the C(aryl)−C(aryl) bonds in 2,2′-biphenols can be cleaved using phosphinites as a recyclable directing group through a rhodium-based spirocyclic intermediate. In particular, the biaryl linkage found in softwood lignin could be cleaved using this method.