Investigations towards the reactivity of rhodium bound carbenes
A short series of diazo compounds containing a primary alcohol and one or more other carbene traps were synthesised. The result in each case, from rhodium (11) acetate mediated decomposition where products could be isolated was that O-H insertion is observed at the expense of all other intramolecular processes. A few other catalysts were tried and a similar result was observed. Even catalysts such as copper (11) acetate, commonly used for cyclopropanations did not lead to any cyclopropanes, O-H insertion being the only observed product. The variation of the rate of decomposition of a small selection of diazo compounds with a range of rhodium (11) catalysts is described. The catalysts chosen represent a wide range of ligand pKa, (0.17 -t 12.40, trifluorobutyrate and acetamide respectively). The results show that the rate of decomposition of the diazo substrates is at a maximum when the pKa of the ligand is 10. The relative rates of decomposition of the diazo compounds are unaffected by the catalyst used. The synthesis of a-alkoxyesters by rhodium mediated decomposition of a-diazoesters was considered. Use of ethyl 2-diazo-3-phenylpropanoates leads to the formation of O-H insertion products and ethyl cinnamates. The ratio of insertion product to cinnamate was found to vary with the ligand on rhodium. The use of chiral a-diazophenylacetates was unsuccessful in inducing any chirality to the newly formed chiral centre. The use of chiral rhodium catalysts was also tried but again found to be unsuccessful. The possible reasons for this have been discussed.