Photoredox-Assisted Reductive Cross-Coupling: Mechanistic Insight into Catalytic Aryl–Alkyl Cross-Couplings

Here, we describe
a photoredox-assisted catalytic system for the
direct reductive coupling of two carbon electrophiles. Recent advances
have shown that nickel catalysts are active toward the coupling of
sp3-carbon electrophiles and that well-controlled, light-driven
coupling systems are possible. Our system, composed of a nickel catalyst,
an iridium photosensitizer, and an amine electron donor, is capable
of coupling halocarbons with high yields. Spectroscopic studies support
a mechanism where under visible light irradiation the Ir photosensitizer
in conjunction with triethanolamine are capable of reducing a nickel
catalyst and activating the catalyst toward cross-coupling of carbon
electrophiles. The synthetic methodology developed here operates at
low 1 mol % catalyst and photosensitizer loadings. The catalytic system
also operates without reaction additives such as inorganic salts or
bases. A general and effective sp2–sp3 cross-coupling scheme has been achieved that exhibits tolerance
to a wide array of functional groups.