Homogeneous Catalysis of Hydrogen Isotope Exchange between D₂ and Ethanol by Chlorotris(triphenylphosphine)rhodium(I). I. in Benzene–Ethanol

Abstract

The kinetics and mechanism of D2–C2H5OH exchange catalyzed by chlorotris(triphenylphosphine)rhodium(I) have been studied in 50 volume percent benzene-ethanol. In this mixed solvent, the rate of exchange was low and was limited by the rate of transfer of deuterium from dideuteridochlorotris(triphenylphosphine)rhodium(III) to the solvent, and not by the rate of activation of D2. The activation energy for the overall process was relatively low, 35 kJ mol−1. In addition, new data have been presented for the catalysis of H2–D2 exchange in benzene by RhCl(PPh3)3. Analysis of the apparent HD:D2 isotope effect that is observed during D2–C2–H5OH exchange suggests that this effect originates with the relative stabilities of the D2, HD, and H2 adducts with RhCl(PPh3)3, or by control of the HD and H2 product distribution by the H2–D2 equilibration reaction.

Rhodium-Phosphoramidite Catalyzed Alkene Hydroacylation: Mechanism and Octaketide Natural Product Synthesis

“…When deuterium-labeled aldehyde d - 1 was subjected to standard reaction conditions in the absence of alkene (Scheme a), a decrease of the aldehyde deuterium content to about 57% was found after 40 h. This result indicates that the phenolic hydrogen atoms can slowly exchange with the aldehyde deuterium via rhodium catalysis . This H/D exchange at the aldehyde does not affect our mechanistic proposal, however, because we derive our conclusions from the relative, not absolute, deuterium content in products 3 and 4 .…”