The oxidative coupling of methyl benzoate

The reaction of methyl benzoate (MBA) with pressurized (50 atm) artificial air (50 mol% O-2-N-2 mixture) at 150-200 degrees C over a soluble palladium catalyst [PdLL2'] (L = phen, bipy, dppe; L' = OAc, TFA) afforded isomeric dimethyl bibenzoic acid eaters (DMBBA). Different factors affect the reactivity and regioselectivity of the process. The reactivity is enhanced by trifluoromethanesulfonic acid, arguably by activating the C-H bond of the aromatic ring by its protonation. The increase of temperature not only increases the reaction rate (E-A = 5.5 kcal/mol) but also favors a formation of an ortho-coupled product. Ligands with strong trans influence (L = phen, bipy, or dppe) direct the oxidative coupling away from 2,X'-isomers (X = 2, 3, or 4). Overall, electronic properties of a palladium catalyst are more important than steric restriction of the same catalyst for altering the activity and regioselectivity for MBA coupling. (C) 2000 Academic Press.