Stereochemical investigations of the mechanism of C-H bond activation. Diastereomeric and isotopic scrambling in (hydrido)alkyliridium complexes

The diastereomeric complexes (RS),(SR)-((2,2-dimethylcyclopropyl)(Cp*) (PMe3)IrH) (2a) and (RR),(SS)-((2,2-dimethylcyclopropyl)(Cp*) (PMe3)IrH) (2b) and their alpha-deuterated analogues (2a-alpha(d1), 2b-alpha(d1)) were synthesized in racemic form and separated by low-temperature (-80 degrees C) column chromatography. Thermolysis (140 degrees C) of diastereomerically pure 2a or 2b in C6D6 results in its interconversion to the other diastereomer. Thermolysis of the deuterium-labeled analogues 2a-alpha(d1) and 2b-alpha(d1) 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 C6D6. 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 far comparison of the rearrangement processes in the current iridium system and the previously reported analogous rhodium system.