No abstract

The Rh(III)-catalyzed C-H functionalizations of benzamide derivatives with olefin were studied by DFT calculations to elucidate the divergent pathways controlled by the N-OR internal oxidants. For substrates of N-OMe and N-OPiv internal oxidants, the energy profiles for consecutive N-H deprotonation/C-H activation/olefin insertion sequences were similar, and different properties and reactivities of the generated 7-membered rhodacycles were predicted. When N-OMe is involved, this intermediate is generally unstable, and the olefination occurs easily via a β-H elimination/reductive elimination (RE) sequence to generate the Rh(I) intermediate, which is then oxidized to the active Rh(III) via MeOH elimination from the N-OMe reduction in the presence of a HOAc. However, for a 7-membered rhodacycle containing a N-OPiv moiety, the coordination of the acyloxy carbonyl oxygen stabilizes this intermediate and increases the barrier of the olefination pathway. Instead, the migration of the acyloxy from N to Rh(III) via a 5-membered ring TS to form a cyclic Rh(V) nitrene intermediate is more kinetically favorable, then the facile RE of this Rh(V) species forms the heterocycle product and regenerates Rh(III). Notably, for both reactions, the direct C-N formation from intermediates containing a C(sp(3))-Rh(III)-N(sp(3)) unit would be very difficult with barriers over 40 kcal/mol.

In the context of modular and rapid construction of molecular diversity and complexity for applications in organic synthesis, biomedical and materials sciences, a generally useful strategy has emerged based on boron-selective chemical transformations. In the last decade, these types of reactions have evolved from proof-of-concept to some advanced applications in the efficient preparation of complex natural products and even automated precise manufacturing on the molecular level. These advances have shown the great potential of boron-selective reactions in simplifying synthetic design and experimental operations, and should inspire new developments in related chemical and technological areas. This tutorial review will highlight the original contributions and representative advances in this emerging field.