Active catalysts for the Suzuki coupling: Palladium complexes of tetrahydropyrimid-2-ylidenes

An alternative way to synthesize a mixed N-heterocyclic carbenes (NHC) phosphine palladium(II) complex is described in this work. A free carbene, generated by deprotonation of a tetrahydro-pyrimidinium salt, was reacted with Pd(PPh3)2Cl2 to yield the desired mixed NHC phosphine palladium(II) complex.In this work, we have combined the superior activity of mixed NHC phosphine palladium complexes in Suzuki-coupling reactions with the additional advantage of a stronger σ-donating ligand. The combination of these two effects results in an increased catalytic activity in Suzuki reactions than found for corresponding mixed imidazole–ylidene phosphine complexes.Using this new class of catalysts, TONs of approximately 1 Mio. after a reaction time of 14 h can be achieved. Desactivated bromoarenes could also be coupled efficiently, using quite a low amount of catalyst (0.005 mol%). Even aryl chlorides can be coupled: TONs of approximately 6000 can be reached after only 14 h without any detectable catalyst deterioration using only 0.01 mol% of catalyst.

Tetrahydropyrimid-2-ylidenes form excellent palladium catalysts for the Suzuki–Miyaura reaction, especially when these N-heterocyclic carbene derivatives are combined with phosphines at the metal centre. A comparison with a corresponding mixed imidazol-2-ylidene palladium(II) complex clearly shows that the stronger σ-donating tetrahydropyrimid-2-ylidene ligand is superior with the regard to the catalytic performance. A specific advantage is the fact that chloro arenes work well with the new catalyst, even at low concentrations of ca. 0.01 mol%. Figure optionsDownload as PowerPoint slide