Irradiation-Induced Heck Reaction of Unactivated Alkyl Halides at Room Temperature

The
palladium-catalyzed Mizoroki–Heck reaction is arguably
one of the most significant carbon–carbon bond-construction
reactions to be discovered in the last 50 years, with a tremendous
number of applications in the production of chemicals. This Nobel-Prize-winning
transformation has yet to overcome the obstacle of its general application
in a range of alkyl electrophiles, especially tertiary alkyl halides
that possess eliminable β-hydrogen atoms. Whereas most palladium-catalyzed
cross-coupling reactions utilize the ground-state reactivity of palladium
complexes under thermal conditions and generally apply a single ligand
system, we report that the palladium-catalyzed Heck reaction proceeds
smoothly at room temperature with a variety of tertiary, secondary,
and primary alkyl bromides upon irradiation with blue light-emitting
diodes in the presence of a dual phosphine ligand system. We rationalize
that this unprecedented transformation is achieved by utilizing the
photoexcited-state reactivity of the palladium complex to enhance
oxidative addition and suppress undesired β-hydride elimination.