Electrochemical Allene C─H Functionalization via Carbanion Sampling

Functionalization of C─H bonds represents a direct and practical avenue to upgrade simple organic molecules to value-added products, whereas the reaction selectivity is largely governed by the acidity and bond dissociation energy (BDE) of C─H bonds. Developing a novel strategy to break these constraints is highly desirable, as it would open a new horizon for conventionally challenging molecules. Herein, we disclosed a carbanion sampling strategy that enables unconventional selectivity in the allene C(sp²)─H functionalization harnessing chromium-catalyzed hydrogen evolution. The electrochemical protocol provides in situ access to diverse allene carbanions and facilitates a dynamic carbanion sampling process to give thermodynamically unfavored products. 1,3-Disubstituted allenes were precisely functionalized at the less acidic and stronger C(sp²)─H site to yield abnormal site-selective allene products, while the 1,1-substituted allenes proceeded isomerization to deliver functionalized alkynes. All of the products presented in this work are unprecedented and exhibit remarkable versatility in late-stage derivatization. The superiority of the carbanion sampling strategy was demonstrated in the convergent synthesis of functionalized allenes from alkyne or mixed substrates. More importantly, we conducted a series of density functional theory (DFT) calculations and control experiments to rationalize the unconventional selectivity.