On treatment with [Ir(PMe 3 ) 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 complex containing cis-phosphine ligands. Studies with 2 H-and doubly 13 C-labeled vinylcyclopropenes, the syntheses and characterization of which are also reported, show that this process involves a rearrangement of the carbon skeleton and not a substituent shift. The corresponding 1,2-diphenyl-3-vinylcyclopropene undergoes iridacyclohexadiene formation without any rearrangement. On heating at 90 °C, each iridacycle converts to its corresponding isomer containing trans-phosphine ligands without any skeletal or substituent rearrangement of the metallacycle, as evidenced by absence of change in the labeling pattern. At higher temperatures, further rearrangement occurs in the case of each metallacycle, which does not alter the metallacyclohexadiene backbone, but rather exchanges the substituents of the R and R′ carbon atoms. This rearrangement is shown to occur even when there is no driving force due to relief of steric effects. Mechanisms for each rearrangement are proposed and discussed.
Under conditions of kinetic control, 1,2-diphenyl-3-vinyl-1-cyclopropene undergoes ring
opening with the [Rh(Cl)(PMe3)2] fragment to give two isomeric η3:η1-1,3-pentadienediyl
compounds: the expected 1,2-diphenyl isomer, and the 2,3-diphenyl isomer resulting from
an apparent skeletal rearrangement reaction. The latter complex has been characterized
by X-ray crystallography. Both complexes underwent ring closure to give the same
1,2-diphenylcyclopentadienyl complex on treatment with silver ion. Addition of a third
equivalent of trimethylphosphine to the 2,3-diphenyl isomer produced two meridional
rhodacyclohexadienes, which exhibit facile solvent-dependent chloride dissociation. In
contrast, phosphine added reversibly to the 1,2-diphenyl isomer to give only the chloride-dissociated compound, and the tris(phosphine) product could only be isolated after anion
exchange with hexafluorophosphate. No deprotonation to give rhodabenzene complexes could
be achieved. The mechanism of rearrangement is proposed to involve a carbocation
rearrangement during the ring-opening reaction and is compared to other metal-promoted
reactions of vinylcyclopropenes.
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