Iridium-promoted reactions of carbon-carbon bonds. Skeletal rearrangement of a vinylcyclopropene during iridacyclohexadiene formation and subsequent isomerization of iridacyclohexadienes via alpha,alpha '-substituent migrations

On treatment with [Ir(PMe3)(2)(acac)] at room temperature, 1,2,3-triphenyl- 3-vinylcyclopropene undergoes ring opening accompanied by rearrangement to give, instead of the expected 1,2,3-triphenyliridacyclohexadiene complex, a crystallographically characterized 1,2,4-triphenyliridacyclohexadiene comp lex containing cis-phosphine ligands. Studies with H-2- and doubly C-13-lab eled vinylcyclopropenes, the syntheses and characterization of which are al so reported, show that this process involves a rearrangement of the carbon skeleton and not a substituent shift. The corresponding 1,2-diphenyl-3-viny lcyclopropene undergoes iridacyclohexadiene formation without any rearrange ment. On heating at 90 degrees C, each iridacycle converts to its correspon ding isomer containing trans-phosphine ligands without any skeletal or subs tituent rearrangement of the metallacycle, as evidenced by absence of chang e in the labeling pattern. At higher temperatures, further rearrangement oc curs in the case of each metallacycle, which does not alter the metallacycl ohexadiene backbone, but rather exchanges the substituents of the alpha and alpha' carbon atoms. This rearrangement is shown to occur even when there is no driving force due to relief of steric effects. Mechanisms for each re arrangement are proposed and discussed.