Studies on the mechanism of metal-catalyzed hydrogen transfer from alcohols to ketones.
The mechanism of metal-catalyzed hydrogen transfer from alcohols to ketones has been studied. Hydrogen transfer (H-transfer) from (S)-alpha-deutero-alpha-phenylethanol ((S)-1) to acetophenone was used as a probe to distinguish between selective carbon-to-carbon H-transfer and nonselective transfer involving both oxygen-to-carbon and carbon-to-carbon H-transfer. The progress of the reaction was monitored by the decreasing enantiomeric excess of (S)-1. After complete racemization, the alcohol was analyzed for its deuterium content in the alpha-position, which is a measure of the degree of selectivity in the H-transfer. A number of different rhodium, iridium, and ruthenium complexes (in total 21 complexes) were investigated by using this probe. For all rhodium complexes a high degree of retention of deuterium at alpha-carbon (95-98%) was observed. Also most iridium complexes showed a high degree of retention of deuterium. However, the results for the ruthenium complexes show that there are two types of catalysts: one that gives a high degree of deuterium retention at alpha-carbon and another that gives about half of the deuterium content at alpha-carbon (37-40%). Two different mechanisms are proposed for transition-metal-catalyzed hydrogen transfer, one via a monohydride (giving a high D content) and another via a dihydride (giving about half of D content). As comparison non-transition-metal-catalyzed hydrogen transfer was studied with the same probe. Aluminum- and samarium-catalyzed racemization of (S)-1 gave 75-80% retention of deuterium in the alpha-position of the alcohol, and involvement of an electron transfer pathway was suggested to account for the loss of deuterium.
Pàmies
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Department of Organic Chemistry, Arrhenius Laboratory Stockholm University, Sweden.
Bäckvall
J E
JE
D000438:Alcohols / Q000737:chemistry*; D002384:Catalysis; D002626:Chemistry, Pharmaceutical; D003903:Deuterium; D006859:Hydrogen / Q000737:chemistry; D007659:Ketones / Q000737:chemistry*; D010084:Oxidation-Reduction; D028561:Transition Elements / Q000737:chemistry*
D000438:Alcohols; D007659:Ketones; D028561:Transition Elements; D006859:Hydrogen; D003903:Deuterium