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 met hylene chloride at room temperature. The major products isolated corre spond to the internal trapping of a carbonyl ylide as well as intramol ecular 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 l ikely due to complexation of the metal with the dipole. A computationa l approach to rationalize the observed product distribution was carrie d out. The thermodynamic stabilities of cycloaddition transition state s were approximated from the computationally derived strain energies o f ground state molecules using traditional force-field techniques. Glo bally minimized ground state energies were obtained for all possible c ycloaddition products, and final strain energies were calculated. In a ll cases studied, the lower energy isomer corresponded to the cycloadd uct 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 carbeno id occurred exclusively across the acetyl rather than the benzoyl grou p. The structure of the internal cycloadduct was assigned on the basis of a proton-detected multiple-bond heteronuclear multiple-quantum coh erence experiment.