Arylation of Carbonyl Compounds Catalyzed by Rhodium and Iridium 1,3‐R 2 ‐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 α,β‐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) + 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.