Stereochemical Investigations of the Mechanism of C−H Bond Activation. Diastereomeric and Isotopic Scrambling in (Hydrido)alkyliridium Complexes

Abstract

The diastereomeric complexes (RS),(SR)-((2,2-dimethylcyclopropyl)(Cp{sup *})(PMe{sub 3})IrH) (2a) and (RR),(SS)-((2,2-dimethylcyclopropyl)(Cp{sup *})(PMe{sub 3})IrH) (2b) and their {alpha}-deuterated analogues (2a-{alpha}{sub d{sub 1}}, 2b-{alpha}{sub d{sub 1}}) were synthesized in racemic form and separated by low-temperature ({minus}80 C) column chromatography. Thermolysis (140 C) of diastereomerically pure 2a or 2b in C{sub 6}D{sub 6} results in its interconversion to the other diastereomer. Thermolysis of the deuterium-labeled analogues 2a-{alpha}{sub d{sub 1}} and 2b-{alpha}{sub d{sub 1}} results additionally in scrambling of deuterium from the {alpha}-position of the dimethylcylopropyl ring to the metal hydride position. Diastereomer interconversion and isotopic scrambling occur at similar rates, which are faster than the rate observed for the reductive elimination of dimethylcyclopropane and subsequent oxidative addition of C{sub 6}D{sub 6}. Quantitative analysis of these rate data is reported. The similarity of these rates is discussed in terms of a common intermediate mechanism involving a metal alkane (or {sigma}-alkane) complex. This mechanism is used as a basis for comparison of the rearrangement processes in the current iridium system and the previously reported analogous rhodium system.