Skeleton-retentive C-H functionalization and deracemization of allenes

Allenes have garnered considerable scientific interest as versatile synthetic building blocks and important structural motifs in numerous biologically active molecules. Consequently, the development of efficient strategies for selective functionalization and asymmetric induction has become a major focus in modern synthetic chemistry. Owing to their unique reactivity and propensity to undergo competing reaction pathways, achieving site-selective allene functionalization or deracemization while preserving the allene skeleton remains a significant challenge. Deracemization represents a powerful strategy for accessing enantiomerically enriched allene derivatives, which are highly valuable in allene chemistry. Therefore, the development of diverse strategies in this field has become a recent focus of research. This review uncovers concepts by providing a comprehensive overview of selective C-H functionalization and deracemization of allenes, while retaining the valuable allene core. The discussion includes organolithium chemistry, peptide- and phase-transfer catalysis, transition-metal (TM) catalysis, hydrogen atom transfer (HAT) and hydrogen evolution reaction (HER) strategies, Electrochemical methods, N-heterocyclic carbene (NHC) catalysis, and the photochemical deracemization of allene. Moreover, this summarisation covers the state of the art from the past couple of decades and future opportunities in selective allene functionalization and deracemization, highlighting key mechanistic insights, synthetic applications, and emerging trends.