Piano-Stool Rhodium(V) and Rhodium(IV) Intermediates in C─H Functionalization and B─H Functionalization: Mechanistic Insights and Emerging Reactivity

Rhodium(III) catalysis is widely adopted for C─H functionalization due to its operational simplicity and broad substrate scope, with the Rh(III)/(I) catalytic manifold being the most commonly encountered. However, an increasing number of studies associate new or enhanced reactivity with high-valent Rh(V) and Rh(IV) intermediates. This review compiles the evidence for both, with a focus on methodologies that proceed via the Rh(III)/Rh(IV)/Rh(II) catalytic cycle. While Rh(V)-mediated catalysis is supported by computational studies, experimental validation remains limited. In contrast, catalysis involving Rh(IV) intermediates is supported by both computational and experimental evidence.