Radical-Friedel-Crafts benzylation of arenes with benzyl ethers over 2H-MoS2: ether cleavage into carbon- and oxygen-centered radicals.

Abstract

The selective activation of C-O ether bonds is an essential tool in organic synthesis and natural polymer depolymerization. However, the direct cleavage of the ether bond is still challenging work, especially breaking this inert and redox-neutral bond to provide one active carbon radical and another oxygen-centered fragment with oxidation capacity that can participate in the controllable radical reaction. We herein report that commercial 2H-MoS2 with negligible acidity can efficiently catalyze the benzylation of arenes with benzyl ethers, and a new Radical-Friedel-Crafts mechanism is proposed, which is quite different from the strong acid-catalyzed Friedel-Crafts mechanism. With dibenzyl ether as the model benzylation reagent, 2H-MoS2 can achieve the homolytic cleavage of the Bn-OR bond to generate the benzyl carbon radical and RO˙ species, identified by EPR measurement and radical trap experiments. The following radical-involved benzylation is confirmed by the Hammett results and a plausible pathway is proposed to clarify the Radical-Friedel-Crafts process. Heterogeneous 2H-MoS2 can be consecutively used four times without regeneration and it offers 94-95% yields of 2-benzyl-1,4-dimethylbenzene from dibenzyl ether and p-xylene in 30 min at 140 °C. Furthermore, this mechanism can provide some inspiration to activate the ether bond and to utilize ether as an oxidant in C-H bond activation.