Ligand-induced selectivity in the rhodium(II)-catalyzed reactions of alpha-diazo carbonyl compounds

3-Allyl-2,5-diazopentanedione and 3-butenyl-2,5-diazopentanedione were allowed to react with a trace amount of a rhodium(II) catalyst in methylene chloride at room temperature. The major products isolated correspond to the internal trapping of a carbonyl ylide as well as intramolecular cyclopropanation. Changing the catalyst from Rh-2(OAc)(4) to Rh-2(cap)(4) to Rh-2(tfa)(4) caused a significant alteration in product distribution. A rather unusual and unexpected regiochemical crossover in the cycloaddition occurred when Rh-2(tfa)(4) was used and is most likely due to complexation of the metal with the dipole. A computational approach to rationalize the observed product distribution was carried out. The thermodynamic stabilities of cycloaddition transition states were approximated from the computationally derived strain energies of ground state molecules using traditional force-field techniques. Globally minimized ground state energies were obtained for all possible cycloaddition products, and final strain energies were calculated. In all cases studied, the lower energy isomer corresponded to the cycloadduct actually isolated. A study of the regiochemical aspects of the Rh(II)-catalyzed reaction of 1-diazo-3-(2-oxo-2-phenylethyl)hexane-2, was also carried out. Cyclization of the initially formed rhodium carbenoid occurred exclusively across the acetyl rather than the benzoyl group. The structure of the internal cycloadduct was assigned on the basis of a proton-detected multiple-bond heteronuclear multiple-quantum coherence experiment.