Cover Picture: (Adv. Synth. Catal. 10/2012)

The cover picture, provided by Qi-Lin Zhou, shows the example of an exceptionally highly efficient chiral catalyst. The catalyst is a very stable dihydride iridium complex of a tridentate chiral spiro P-N-N ligand based on the 1,1′-spirobiindane skeleton, which exhibits excellent enantioselectivity and extremely high turnover number (in excess of one million) for the hydrogenation of both aryl ketones and β-aryl-β-keto esters. Although little is known about the mechanism of this iridiumcatalyzed carbonyl group hydrogenation reaction, a non-classical metalligand bifunctional mechanism deserves to be recommended to interpret the high efficiency of this catalyst. With the help of KO-t-Bu and under an H₂ atmosphere, the dihydride iridium complex is converted to a trihydride iridium complex. After combination of the substrate with the trihydride iridium complex, a cyclic six-membered transition state is formed. The hydridic IrH and protic NH of the catalyst are simultaneously transferred to the carbonyloxygen double bond of the substrate, resulting in a chiral alcohol and a hydrido amido iridium complex. Finally, the hydrido amido iridium complex is hydrogenated with H₂ to regenerate the trihydride iridium complex.