Cross-Dehydrogenative Coupling Reactions With Redox-Active Guanidine Reagents.

Cross-dehydrogenative coupling (CDC) reactions are attractive and environmentally friendly ways to form new C-C and other element-element bonds. Here, we report the first CDC reactions triggered by a redox-active guanidine reagent. N-phenyltetrahydroisoquinoline and derivatives with electron-donating and withdrawing substituents at the N-phenyl group are reacted with nitromethane, dimethylmalonate and methylphenyl ketone in the presence of oxidized, dicationic 1,4-bis(tetramethylguanidino)benzene as redox-active guanidine reagent. The CDC products are obtained in good yield. The diprotonated 1,4-bis(tetramethylguanidino)benzene by-product can be easily separated from the product, and subsequently recycled through oxidation to the CDC reagent. The reaction mechanisms are discussed, and compared to the corresponding reactions with the high-potential quinone 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The analysis discloses important advantages of redox-active guanidine reagents compared to quinone reagents such as DDQ, arising from shorter lifetimes of radical intermediates.