Rationally designed pincer-type Heck catalysts bearing aminophosphine substituents: Pd-IV intermediates and palladium nanoparticles

The aminophosphine-based pincer complexes [C6H3-2,6-{XP(piperidinyl)(2)}(2)Pd(Cl)] (X=NH 1; X=O 2) are readily prepared from cheap starting materials by sequential addition of 1,1',1 ''-phosphinetriyltripiperidine and 1,3-diaminobenzene or resorcinol to solutions of [Pd(cod)(Cl)(2)] (cod=cyclooctadiene) in toluene under N-2 in one pot. Compounds 1 and 2 proved to be excellent Heck catalysts and allow the quantitative coupling of several electronically deactivated and sterically hindered aryl bromides with various olefins as coupling partners at 140 degrees C within very short reaction times and low catalyst loadings. Increased reaction temperatures also enable the efficient coupling of olefins with electronically deactivated and sterically hindered aryl chlorides in the presence of only 0.01 mol% of catalyst. The mechanistic studies performed rule out that homogeneous Pd-0 complexes are the catalytically active forms of 1 and 2. On the other hand, the involvement of palladium nanoparticles in the catalytic cycle received strong experimental support. Even though pincer-type Pd-IV intermediates derived from 1 (and 2) are not involved in the catalytic cycle of the Heck reaction, their general existence as reactive intermediates (for example, in other reactions) cannot be excluded. On the contrary, they were shown to be thermally accessible. Compounds 1 and 2 show a smooth halide exchange with bromobenzene to yield their bromo derivatives in DMF at 100 degrees C. Experimental observations revealed that the halide exchange most probably proceeded via pincer-type Pd-IV intermediates. DFT calculations support this hypothesis and indicated that aminophosphine-based pincer-type Pd-IV intermediates are generally to be considered as reactive intermediates in reactions with aryl halides performed at elevated temperatures.