Ir-mediated nucleophilic ortho-functionalization of phenols: Syntheses, structures, scope, and limitation

Treatment of [Cp*Ir(eta(5)-PhO)][BF4] (1) with hydride, deuteride, and C-, N-, and S-centered nucleophiles affords the stable eta(4)-phenol tautomers of the type [Cp*Ir(eta(4)-exo-2-(Nu)-C6H5O)] (2-6) {Nu = nucleophile}. In all cases regiospecific nucleophilic addition occurs at the ortho-position relative to C=O with exo-stereochemistry. The X-ray molecular structure of the first neutral phenol tautomer [Cp*Ir(eta(4)-exo-2-(CH(COMe)(2))-C6H5O] (4) was determined and provides valuable crystallographic information for an organic phenol tautomer. Oxidation of the novel dienone iridium complexes [Cp*Ir(eta(4)-exo-2-(Nu)-C6H5O)] by iodine provided a different type of products depending dramatically on the nature and electron properties of the 2-exo-nucleophile. For instance R3C-, RO-, and R3P-centered nucleophiles gave the related ortho-functionalized phenols along with the starting material recycled in the form of [Cp*Ir(mu-I)I](2). In dramatic contrast N- and S-centered nucleophiles showed a retronucleophilic addition or C-Nu bond cleavage as demonstrated by complexes 5 and 6 to give the starting material identified spectroscopically and by X-ray structure as [Cp*Ir(eta(5)-PhO)][I] (8). In the latter, a rationale involving a one-electron oxidation process is proposed to explain the experimental results.