Amine-Catalyzed Highly Regioselective and Stereoselective C(sp²)-C(sp²) Cross-Coupling of Naphthols with trans-α,β-Unsaturated Aldehydes

A metal-free C(sp²)-C(sp²) cross-coupling approach to highly congested (E)-α-naphtholylenals from simple naphthols and enals is described. The mild reaction conditions with pyridine hydrobromideperbromide (PHBP) as the bromination reagent in the presence of piperidine or diphenylprolinol trimethylsilyl (TMS) ether as promoters enable the process in good yields and with high chemoselectivity, regioselectivity, and stereoselectivity. The process involves an unprecedented pathway of in situ regioselective 4-bromination of 1-naphthols and the subsequent unusual aromatic nucleophilic substitution of the resulting 4-bromo-1-naphthols with the α-C(sp²) of enals through a Michael-type Friedel-Crafts alkylation-dearomatization followed by a cyclopropanation ring-opening cascade process. The noteworthy features of this strategy are highlighted by the highly efficient creation of a C(sp²)-C(sp²) bond from readily available unfunctionalized naphthols and enals catalyzed by non-metal, readily available cyclic secondary amines under mild reaction conditions. The Pied Piperidine: The first organocatalytic para-functionalization of 1-naphthols with enals has been achieved with simple piperidine in an organocatalytic direct C(sp²)-C(sp²) cross-coupling process to generate highly congested (E)-α-naphtholyl enals. The strategy has been extended to the regioselective and stereoselective 1-installation of enals into 2-naphthols.