Arylation of Carbonyl Compounds Catalyzed by Rhodium and Iridium 1,3‐R₂‐Tetrahydropyrimidin‐2‐ylidenes: Structure‐Reactivity Correlations

<jats:title>Abstract</jats:title><jats:p>Six different well‐defined rhodium and iridium N‐heterocyclic carbene complexes, i.e., RhCl(1,3‐dimesityltetrahydropyrimidin‐2‐ylidene)(COD) (<jats:bold>1</jats:bold>), RhBr(1,3‐dimesityltetrahydropyrimidin‐2‐ylidene)(COD) (<jats:bold>2</jats:bold>), RhCl[1,3‐di(2‐propyl)tetrahydropyrimidin‐2‐ylidene](COD) (<jats:bold>3</jats:bold>), IrCl(1,3‐dimesityltetrahydropyrimidin‐2‐ylidene)(COD) (<jats:bold>4</jats:bold>), Rh(CF<jats:sub>3</jats:sub>COO)(1,3‐dimesityltetrahydropyrimidin‐2‐ylidene)(COD) (<jats:bold>5</jats:bold>), and IrBr[1,3‐di(2‐propyl)tetrahydropyrimidin‐2‐ylidene](COD) (<jats:bold>6</jats:bold>) (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 <jats:bold>1</jats:bold>–<jats:bold>4</jats:bold> and <jats:bold>6</jats:bold> were prepared by reaction of [RhCl(COD)]<jats:sub>2</jats:sub> and [IrCl(COD)]<jats:sub>2</jats:sub>, respectively, with a base and the corresponding 1,3‐R<jats:sub>2</jats:sub>‐tetrahydropyrimidinium salt. Compound <jats:bold>5</jats:bold> was prepared by reaction of 1.0 equivalents of CF<jats:sub>3</jats:sub>COOAg with <jats:bold>1</jats:bold>. The use of an excess of CF<jats:sub>3</jats:sub>COOAg resulted in the replacement of Rh(I) by Ag(I) and yielded Ag(1,3‐dimesityltetrahydropyrimidin‐2‐ylidene)<jats:sup>+</jats:sup>Rh<jats:sub>2</jats:sub>(CF<jats:sub>3</jats:sub>COO)<jats:sub>3</jats:sub>(COD)<jats:sup>−</jats:sup> (<jats:bold>8</jats:bold>). Compounds <jats:bold>4</jats:bold> and <jats:bold>8</jats:bold> were characterized by X‐ray analysis. The activity of the rhodium complexes increased in the order <jats:bold>5</jats:bold>><jats:bold>3</jats:bold>><jats:bold>1</jats:bold>><jats:bold>2</jats:bold>, indicating the necessity of strongly electron‐withdrawing groups at the metal centers, thus increasing their nucleophilicity. In due consequence, the “softer” iridium complexes <jats:bold>4</jats:bold> and <jats:bold>6</jats:bold> 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.</jats:p>