Unusual Selectivity-Determining Factors in the Phosphine-Free Heck Arylation of Allyl Ethers

Abstract

The Heck reaction of aryl iodides and bromides with allyl ethers has been investigated. Using phosphine-free Pd(OAc)2 in DMF at 90 °C in the presence of Bu4NOAc, the reaction gave cinnamyl derivatives, usually in good to high yields, with a wide range of aryl halides. The reaction tolerates a variety of functional groups, including ether, amide, alcohol, aldehyde, ketone, ester, cyano, carboxylic acid, and nitro groups. Ortho-substituted arylating agents afforded moderate yields in some cases, though good to high yields were obtained with o-iodotoluene, iodovanillin, and 1-iodonaphthalene. Several pieces of experimental evidence suggest that the observed selectivity in formation of the vinylic substitution products is kinetic in origin under these conditions. A DFT investigation has been performed to clarify the source of product selectivity and, in particular, the preference for cinnamyl ether over enol ether products. Interestingly, it was found that the product selectivity does not arise from competing β-hydride eliminations but rather from a competition between β-elimination and hindered single-bond rotation in the initial carbopalladation product.