quid-catalyzed oxidative esteri fi cation of carboxylic acids with benzylic hydrocarbons via benzylic Csp 3 – H bond activation under metal-free conditions †

It is well known that benzyl esters are prevalent functional groups in medicinal agents and natural products. The traditional methods of preparing benzyl esters by utilizing benzyl alcohols/bromides/chlorides with carboxylic acids as coupling partners have been reported over the decades (Scheme 1a). Furthermore, Khan's group proposed benzyl esters can also be obtained by the reaction of aromatic carboxylic acids with substituted toluenes in the presence of NaBrO3/NaHSO3 in early 2003. However, some approaches have disadvantages of long reaction time or sensitivity to the environment. With increasing interest in transition metal-catalyzed C–H bonds activation to construct C–O bonds, several types of transition metals, including Cu, Pd, Pt and Rh have been recently revealed admirable catalytic activity in esterication. Among them, the approach of Pd-catalyzed benzylation of toluene with carboxylic acids under 1 atm of oxygen has been disclosed by Zhang's group (Scheme 1b). It did not need to be pre-functionalized and provided a more facile, atom-economic method to prepare benzyl esters. Despite their obvious potential application, these transition metal-catalyzed C–H bond activation methods suffered from a few drawbacks such as expense and toxicity of transition metals, which limit their practicability. Hence, the development of a facile and practical approach for constructing benzyl esters under metal-free conditions is extremely desirable.