ELECTRONIC EFFECTS IN DIRHODIUM(II) CARBOXYLATES - LINEAR FREE-ENERGYRELATIONSHIPS IN CATALYZED DECOMPOSITIONS OF DIAZA COMPOUNDS AND CO AND ISONITRILE COMPLEXATION

A linear free energy analysis of substituent effects in rhodium carbox ylate complexes has been conducted. Two reactions of diazo compounds i nvolving intramolecular competition between (1) O-ylide formation and secondary C-H insertion, and (2) tertiary C-H and primary C-H insertio n were studied as well as complexation effects on the IR frequencies o f CO and tert-butyl isocyanide. Aliphatic and aromatic carboxylate com plexes were included. Regression equations that describe these process es include contributions from sigma(alpha) (polarizability) and sigma( F) (field effects) and, for the benzoates, sigma(R) (resonance). Compl exes that deviate from the LFER include rhodium trifluoroacetate and r hodium triptycenecarboxylate, and their behavior was explained through (partial) release of the free carbene. The effect of ligand polarizab ility on selectivity in these reactions was interpreted as evidence fo r the importance of backbonding from rhodium to the carbene carbon in the product-determining step. The ability of these complexes to backbo nd was shown through the complexation study. Higher selectivity with i ncreasing ability to backbond is analogous to other carbenes such as d ifluorocarbene. 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 selecti vity engendered by field effects and polarizability could be explained by increased selectivity for electron-rich sites.