Interception of Transient
Allyl Radicals with Low-Valent
Allylpalladium Chemistry: Tandem Pd(0/II/I)–Pd(0/II/I/II) Cycles
in Photoredox/Pd Dual-Catalytic Enantioselective C(sp3)–C(sp3) Homocoupling
posted on 2024-02-22, 15:03authored byBo Li, Hong-Hao Zhang, Yongrui Luo, Shouyun Yu, William A. Goddard III, Yanfeng Dang
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.