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Synthetic Scope and Mechanistic Studies of Ru(OH) x /Al 2 O 3 ‐Catalyzed Heterogeneous Hydrogen‐Transfer Reactions
Author(s) -
Yamaguchi Kazuya,
Koike Takeshi,
Kotani Miyuki,
Matsushita Mitsunori,
Shinachi Satoshi,
Mizuno Noritaka
Publication year - 2005
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/chem.200500539
Subject(s) - racemization , chemistry , catalysis , isomerization , transfer hydrogenation , hydrogen , acetophenone , ruthenium , organic chemistry
Three kinds of hydrogen‐transfer reactions, namely racemization of chiral secondary alcohols, reduction of carbonyl compounds to alcohols using 2‐propanol as a hydrogen donor, and isomerization of allylic alcohols to saturated ketones, are efficiently promoted by the easily prepared and inexpensive supported ruthenium catalyst Ru(OH) x /Al 2 O 3 . A wide variety of substrates, such as aromatic, aliphatic, and heterocyclic alcohols or carbonyl compounds, can be converted into the desired products, under anaerobic conditions, in moderate to excellent yields and without the need for additives such as bases. A larger scale, solvent‐free reaction is also demonstrated: the isomerization of 1‐octen‐3‐ol with a substrate/catalyst ratio of 20 000/1 shows a very high turnover frequency (TOF) of 18 400 h −1 , with a turnover number (TON) that reaches 17 200. The catalysis for these reactions is intrinsically heterogeneous in nature, and the Ru(OH) x /Al 2 O 3 recovered after the reactions can be reused without appreciable loss of catalytic performance. The reaction mechanism of the present Ru(OH) x /Al 2 O 3 ‐catalyzed hydrogen‐transfer reactions were examined with monodeuterated substrates. After the racemization of ( S )‐1‐deuterio‐1‐phenylethanol in the presence of acetophenone was complete, the deuterium content at the α‐position of the corresponding racemic alcohol was 91 %, whereas no deuterium was incorporated into the α‐position during the racemization of ( S )‐1‐phenylethanol‐OD. These results show that direct carbon‐to‐carbon hydrogen transfer occurs via a metal monohydride for the racemization of chiral secondary alcohols and reduction of carbonyl compounds to alcohols. For the isomerization, the α‐deuterium of 3‐deuterio‐1‐octen‐3‐ol was selectively relocated at the β‐position of the corresponding ketones (99 % D at the β‐position), suggesting the involvement of a 1,4‐addition of ruthenium monohydride species to the α,β‐unsaturated ketone intermediate. The ruthenium monohydride species and the α,β‐unsaturated ketone would be formed through alcoholate formation/β‐elimination. Kinetic studies and kinetic isotope effects show that the RuH bond cleavage (hydride transfer) is included in the rate‐determining step.