Electronic Effects of Rh(II)-Mediated Carbenoid Intramolecular C-H Insertion: A Linear Free Energy Correlation Study

The relative rates of the Rh(II)-mediated carbenoid insertion into the benzylic C-H bonds of a series of para-substituted phenyl substrates have been measured by an indirect intramolecular competition method. Three catalysts, rhodium(II) acetate, rhodium(II) trifluoroacetate, and rhodium(II) acetamide, were investigated. The effect of solvent was also studied on the reactions with rhodium(II) acetate as catalyst. The relative rates were analyzed by the Hammett equation. The relative rates are found to correlate better with σ than with σ+ for all three catalysts when CH2Cl2 is the solvent. For the insertion reactions catalyzed by Rh2(OAc)4, Rh2(O2CCF3)4, and Rh2(acam)4 in CH2Cl2, the Hammett correlations of the relative rates with σ give reaction constants of -1.26 (r ) 0.98), -0.66 (r ) 0.97), and -1.39 (r ) 0.98), respectively. The Hammett analysis also indicates no significant solvent effect. The mechanistic significance of these Hammett correlations is discussed.