Scope and Mechanistic Studies on the Ruthenium-Catalyzed Multicomponent Deaminative C–H Coupling Reaction of Phenols with Aldehydes and Enamines for the Formation of Xanthene and Dioxacyclic Derivatives

Abstract

The in situ generated catalytic system from the tetranuclear Ru–H complex [(PCy3)(CO)RuH]4(O)(OH)2 (1) with 3,4,5,6-tetrachloro-1,2-benzoquinone (L1) has been found to mediate a multicomponent deaminative coupling reaction of phenols with aldehydes and enamines to form xanthene products. The multicomponent C–H coupling reaction of phenols with 2-hydroxybenzaldehydes and cyclic enamines efficiently installed the tricyclic 1,3-dioxacin derivatives, while the analogous coupling reaction of phenols with 2-hydroxybenzaldehydes and triethylamine selectively formed bicyclic 1,5-dioxacyclic derivatives. The density functional theory (DFT) calculations established two energetically viable mechanistic pathways for the formation of xanthene products, in which both pathways identified the C–O bond cleavage step as the turnover limiting step. A Hammett plot from the coupling reaction of 3,5-dimethoxyphenol with an enamine and para-substituted benzaldehydes p-X-C6H4CHO (X = OMe, Me, H, Cl, CF3) showed a negative slope (ρ = −0.98). The calculated energy analysis showed a similar trend (ρ = −0.59) for the mechanism via the C–O cleavage rate-limiting step. The combined experimental and DFT computational results support a mechanistic path that involves the dehydrative C–H coupling of phenol with aldehyde, followed by the deaminative coupling reaction with an enamine in forming the xanthene product.