Substrate-Paired Electron Donor-Acceptor Complexes Enable Photochemical Alkyl-Alkyl Cross-Coupling.

Selective C(sp3)-C(sp3) cross-coupling of two distinct alkyl radicals is difficult because homocoupling and radical decomposition compete with heterocoupling. We report a catalyst-free, redox-neutral photochemical coupling of alkylpyridinium salts with alkylboron reagents under mild conditions. Amino-acid-derived pyridinium salts and benzylic or nonbenzylic alkylboron partners participate with useful cross-selectivity. Mechanistic studies support photoexcitation of a substrate-paired EDA complex, followed by substrate-to-substrate electron transfer and radical-pair recombination.