Dissociation of Carbanions from Acyl Iridium Compounds: An Experimental and Computational Investigation.

Instead of reductive elimination of aldehyde, or decarbonylation to give a trifluoroalkyl hydride, heating Cp*(PMe3)Ir(H)[C(O)CF3] (1) leads to the quantitative formation of Cp*(PMe3)Ir(CO) (2) and CF3H. Kinetic experiments, isotope labeling studies, solvent effect studies, and solvent-inclusive DFT calculations support a mechanism that involves initial dissociation of trifluoromethyl anion to give the transient ion-pair intermediate [Cp*(PMe3)Ir(H)(CO)]+[CF3]-. Further evidence for the ability of CF-3 to act as a leaving group came from the investigation of the analogous methyl and chloride derivatives Cp*(PMe3)Ir(Me)[C(O)CF3] and Cp*(PMe3)Ir(Cl)[C(O)CF3]. Both of these compounds undergo a similar loss of trifluoromethyl anion, generating an iridium carbonyl cation and CF3D in CD3OD. Three additional acyl hydrides, Cp*(PMe3)Ir(H)[C(O)RF] (where RF = CF2CF3, CF2CF2CF3, or CF2(CF2)6CF3) undergo RF--H elimination to give 2 at a faster rate than CF3H elimination from 1. Stereochemical studies using a chiral acyl hydride with a stereocenter at the β-position reveal that ionization of the carbanion occurs to form a tight ion-pair with high retention of configuration and enantiomeric purity upon proton transfer from iridium. [ABSTRACT FROM AUTHOR]