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Thermodynamics of Rhodium Hydride Reactions with CO, Aldehydes, and Olefins in Water: Organo-Rhodium Porphyrin Bond Dissociation Free Energies

Xuefeng Fu and Bradford B. Wayland*

Contribution from the Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323

J. Am. Chem. Soc., 2005, 127 (47), pp 16460–16467

DOI: 10.1021/ja054548n

Publication Date (Web): November 4, 2005

In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed.

, wayland@sas.upenn.edu

Abstract

Tetra(p-sulfonato-phenyl) porphyrin rhodium hydride ([(TSPP)Rh−D(D₂O)]^-⁴) (1) reacts in water (D₂O) with carbon monoxide, aldehydes, and olefins to produce metallo formyl, α-hydroxyalkyl, and alkyl complexes, respectively. The hydride complex (1) functions as a weak acid in D₂O and partially dissociates into a rhodium(I) complex ([(TSPP)Rh^I(D₂O)]^-⁵) and a proton (D⁺). Fast substrate reactions of 1 in D₂O compared to reactions of rhodium porphyrin hydride ((por)Rh−H) in benzene are ascribed to aqueous media promoting formation of ions and supporting ionic reaction pathways. The regioselectivity for addition of 1 to olefins is predominantly anti-Markovnikov in acidic D₂O and exclusively anti-Markovnikov in basic D₂O. The range of accessible equilibrium thermodynamic measurements for rhodium hydride substrate reactions is substantially increased in water compared to that in organic media through exploiting the hydrogen ion dependence for the equilibrium distribution of species in aqueous media. Thermodynamic measurements are reported for reactions of a rhodium porphyrin hydride in water with each of the substrates, including CO, H₂CO, CH₃CHO, CH₂CH₂, and sets of aldehydes and olefins. Reactions of rhodium porphyrin hydrides with CO and aldehydes have nearly equal free-energy changes in water and benzene, but alkene reactions that form hydrophobic alkyl groups are substantially less favorable in water than in benzene. Bond dissociation free energies in water are derived from thermodynamic results for (TSPP)Rh-organo complexes in aqueous solution for Rh−CDO, Rh−CH(R)OD, and Rh−CH₂CH(D)R units and are compared with related values determined in benzene.