Recent Advances in Triplet Free‐Carbene Transfer Chemistry

Abstract

Photoinduced free‐carbene transfer reactions have emerged as a powerful and practical method for enabling a wide range of C─C and C─X bond‐forming transformations and synthesizing structurally diverse and complex molecules. However, the spin state of free‐carbene intermediates is a crucial parameter for understanding their chemical properties and reactivity. Selective generation of triplet free‐carbene intermediates, which feature diradical character, is challenging but highly desirable. Such protocols are intriguing for developing novel synthetic methods and achieving unprecedented selectivity. Over the past 5 years, the development of photocatalysis has unlocked the capability of triplet carbene intermediates in synthetic applications. In this review article, we categorize these recent advancements into four classes, including (a) [3 + 2]‐cyclization with alkynes, (b) stepwise addition with alkenes, (c) C─C bond insertion with BCBs, and (d) C─H/X─H/C─X bond insertion and coupling reactions. The discussion focuses on the catalytic generation of triplet carbene species, reaction design, mechanistic pathways, and substrate scope. We aim to present a unified perspective on the current state‐of‐the‐art in triplet free‐carbene chemistry under light‐driven conditions, while also highlighting future research opportunities in this emerging field.