Abstract
Three resorcin[4]arene cavitands (1–3) having either one or two resorcinolic C-2 atoms substituted by a CH2PPh2 podand arm were assessed in the Suzuki arylation of aryl bromides with phenylboronic acid. Using P:Pd ratios of 2:1 and operating in dioxane at 100 °C with a catalyst loading of 0.001 mol% resulted in highly efficient catalytic systems. For example, TOFs up to 34570 mol(converted ArBr)⋅mol(Pd)−1⋅h−1 were obtained with the proximally-disubstituted cavitand 3 when using 4-bromotoluene as substrate. The performance was shown to vary in the following order: monophosphine 1<diphosphine 2<diphosphine 3 (where 2 is the distally disubstituted cavitand). A comparison of the catalytic properties of monophosphine-cavitand 1 with those of benzyldiphenylphosphine and o-anisylmethyldiphenyl phosphine suggests that 1 functions as a hemilabile phosphine, the oxygen atoms close to the phosphorus atom behaving as donors able to temporarily increase the electron density on the metal and/or favour the formation of mono-ligand Pd(0) species.
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