Arylation of carbonyl compounds catalyzed by rhodium and iridium 1,3-R2-tetrahydropyrimidin-2-ylidenes: Structure-reactivity correlations

Six different well-defined rhodium and iridium N-heterocyclic carbene complexes, i.e., RhCl-(1,3 -dimesityltetrahydropyrimidin-2-ylidene) (COD) (1), RhBr(1,3-dimesityltetrahydropyrimidin-2-ylidene)(COD) (2), RhCl[1,3-di(2-propyl)tetrahydropyrimidin-2-ylidene](COD) (3), IrCl(1,3-dimesityltetrahydropyrimidin-2-ylidene) (COD) (4), Rh(CF(3)COO) (1,3-dimesityltetrahydropyrimidin-2-ylidene) (COD) (5), and IrBr[1,3-di(2-propyl)tetrahydropyrimidin-2-ylidene](COD) (6) (COD = 1,5-cyclooctadiene, mesityl=2,4,6-trimethylphenyl) have been used as catalysts for the arylation of aldehydes and alpha,beta-unsaturated ketones using different arylboronic acids. Compounds 1-4 and 6 were prepared by reaction of [RhCl(COD)](2) and [IrCl(COD)](2), respectively, with a base and the corresponding 1,3-R(2)-tetrahydropyrimidinium salt. Compound 5 was prepared by reaction of 1.0 equivalents of CF(3)COOAg with 1. The use of an excess of CF(3)COOAg resulted in the replacement of Rh(I) by Ag(I) and yielded Ag(1,3-dimesityltetrahydropyrimidin-2-ylidene)(4)Rh(2)(CF(3)COO)(3)(COD)- (8). Compounds 4 and 8 were characterized by X-ray analysis. The activity of the rhodium complexes increased in the order 5 > 3 > 1 > 2, indicating the necessity of strongly electron-withdrawing groups at the metal centers, thus increasing their nucleophilicity. In due consequence, the "softer" iridium complexes 4 and 6 exhibited significantly reduced catalytic activity albeit with enhanced selectivity. The syntheses of the metal complexes as well as a detailed study on their reactivity in the arylation of carbonyl compounds using equimolar amounts of arylboronic acid and carbonyl compound in the presence of 0.08-1 mol % catalyst are presented.