Electronic effects in dirhodium(II) carboxylates. Linear free energy relationships in catalyzed decompositions of diazo compounds and CO and isonitrile complexation

Article abstract of DOI:10.1021/ja00099a017

A linear free energy analysis of substituent effects in rhodium carboxylate complexes has been conducted. Two reactions of diazo compounds involving intramolecular competition between (1) O-ylide formation and secondary C-H insertion, and (2) tertiary C-H and primary C-H insertion were studied as well as complexation effects on the IR frequencies of CO and tert-butyl isocyanide. Aliphatic and aromatic carboxylate complexes were included. Regression equations that describe these processes include contributions from σ_a(polarizability) and σ_F(field effects) and, for the benzoates, σ_R (resonance). Complexes that deviate from the LFER include rhodium trifluoroacetate and rhodium triptycenecarboxylate, and their behavior was explained through (partial) release of the free carbene. The effect of ligand polarizability on selectivity in these reactions was interpreted as evidence for the importance of backbonding from rhodium to the carbene carbon in the product-determining step. The ability of these complexes to backbond was shown through the complexation study. Higher selectivity with increasing ability to backbond is analogous to other carbenes such as difluorocarbene. Increased selectivity engendered by backbonding could occur by facilitating the reversal of an intermediate complex between the carbenoid and the C-H bond undergoing insertion. Increased selectivity engendered by field effects and polarizability could be explained by increased selectivity for electron-rich sites.