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Studies on the Mechanism of Metal‐Catalyzed Hydrogen Transfer from Alcohols to Ketones
Author(s) -
Pàmies Oscar,
Bäckvall JanE.
Publication year - 2001
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20011203)7:23<5052::aid-chem5052>3.0.co;2-z
Subject(s) - chemistry , transfer hydrogenation , racemization , catalysis , ruthenium , deuterium , rhodium , hydrogen , iridium , carbon fibers , inorganic chemistry , organic chemistry , materials science , physics , quantum mechanics , composite number , composite material
The mechanism of metal‐catalyzed hydrogen transfer from alcohols to ketones has been studied. Hydrogen transfer (H‐transfer) from ( S )‐ α ‐deutero‐ α ‐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 α ‐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 α ‐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 α ‐carbon and another that gives about half of the deuterium content at α ‐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 α ‐position of the alcohol, and involvement of an electron transfer pathway was suggested to account for the loss of deuterium.