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. T hree catalysts, rhodium(II) acetate, rhodium(II) trifluoroacetate, and rhodium(II) acetamide, were investigated. The effect of solvent was a lso studied on the reactions with rhodium(II) acetate as catalyst. The relative rates were analyzed by the Hammett equation. The relative ra tes are found to correlate better with sigma than with sigma(+) for al l three catalysts when CH2Cl2 is the solvent. For the insertion reacti ons catalyzed by Rh-2(OAc)(4), Rh-2(O2CCF3)(4), and Rh-2(acam)(4) in C H2Cl2, the Hammett correlations of the relative rates with a give reac tion 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 correla tions is discussed.