Competitive C-H Activation and C=C Coordination in the Reactions of Acetylenes with a Binuclear Rhodium Complex.
Interim technical rept.,
ROCHESTER UNIV NY DEPT OF CHEMISTRY
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The reaction between phenylacetylene and the binuclear complex Rh2CO3dppm2, 1, has been studied in detail. At 28.5 C in benzene, the reaction leads to formation of a phenylvinylidene bridged A-frame complex Rh2CO2dppm2C double bond CHPh, 2a, while in refluxing acetone it yields the alkyne bridged complex Rh2mu2-Eta2-PhCCHCO2dppm2, 3. The kinetics of the reaction show that vinylidene formation is cleanly first order in PhCCH and in 1 with k sub HK sub D of 2.7, while formation of 3 proceeds with no significant isotope effect and a kinetic dependence on PhCCH suggestive of a preequilibrium involving 1 only. The effect of CO on the reaction leading to 2a and 3 supports the notion that this preequilibrium involves CO dissociation from 1. The results clearly establish that the mu2-Eta2-alkyne bridged complex does not lie on the reaction path of the metal-promoted acetylene-to-vinylidene transformation, and suggests that vinylidene formation proceeds with initial C-H activation.
- Organic Chemistry