Formylation of activated arenes by phenyl formate: implications for the mechanism of the Fries rearrangement of aryl formates

We present an NMR and DFT investigation of the reaction of phenyl formate with 3‐methoxyphenol and 3,5‐dimethoxyphenol with excess BCl 3 . The products obtained (3‐methoxy‐ and 3,5‐dimethoxy‐salicylaldehyde, respectively) are the same as those resulting from the Fries rearrangement of 3‐methoxy‐ and 3,5‐dimethoxy‐phenyl formate. These results represent a novel regioselective synthetic route to aromatic aldehydes, using phenyl formate as a source of formylating agent. They also unambiguously prove that the Fries rearrangement of aryl formates (that we recently investigated in J. Org. Chem . 71 , 9331–9340, 2006) is intermolecular: the intermediate formyl chloride is released in situ and, in turn, it formylates the intermediate dichloroborate ester of 3‐methoxy‐ and 3,5‐dimethoxy‐phenol in a second independent step. The BCl 2 moiety bound to the aryl oxygen of the substituted phenol interacts with the formyl chloride strongly favouring the ortho substitution. Copyright © 2008 John Wiley & Sons, Ltd.