We present comprehensive computational and experimental studies on the mechanism of an asymmetric photoredox/Pd dual-catalytic reductive C(sp3)–C(sp3) homocoupling of allylic electrophiles. In stark contrast to the canonical assumption that photoredox promotes bond formation via facile reductive elimination from high-valent metal–organic species, our computational analysis revealed an intriguing low-valent allylpalladium pathway that features tandem operation of Pd(0/II/I)–Pd(0/II/I/II) cycles. Specifically, we propose that (i) the photoredox/Pd system enables the in situ generation of allyl radicals from low-valent Pd(I)-allyl species, and (ii) effective interception of the fleeting allyl radical by the chiral Pd(I)-allyl species results in the formation of an enantioenriched product. Notably, the cooperation of the two pathways highlights the bifunctional role of Pd(I)-allyl species in the generation and interception of transient allyl radicals. Moreover, the mechanism implies divergent substrate-activation modes in this homocoupling reaction, suggesting a theoretical possibility for cross-coupling. Combined, the current study offers a novel mechanistic hypothesis for photoredox/Pd dual catalysis and highlights the use of low-valent allylpalladium as a means to efficiently intercept radicals for selective asymmetric bond constructions.
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Li et al. (2024) studied this question.
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