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Origins of enantioselectivity in asymmetric ketone hydrogenation catalyzed by a RuH(2)(binap)(cydn) complex: insights from a computational study.
MedLine Citation:
PMID:  23187862     Owner:  NLM     Status:  Publisher    
Abstract/OtherAbstract:
In this paper, the origins of enantioselectivity in asymmetric ketone hydrogenation catalyzed by RuH(2)(binap)(cydn) (cydn = trans-1,2-diaminocyclohexane) were discussed. Fifteen substrates involving aromatic, heteroaromatic, olefinic and dialkyl prochiral ketones were used to probe the catalytic mechanism and find an effective way to predict the chirality of the products. The calculated results demonstrate that the hydrogen transfer (HT) step from the Ru complex to the ketone substrate is the chirality-determining step in the H(2)-hydrogenation of ketones. The hydrogenation of aromatic-alkyl ketones can give higher enantiomeric excess (ee) values than that of dialkyl ketones. An interesting intermediate (denoted as ) could be formed if there is an α-hydrogen for R/R' groups of the ketone due to the H(2)-H(α) interaction. Two substituent groups of the ketone could rotate around the C[double bond, length as m-dash]O axis in two directions, clockwise or counter-clockwise. This rotation, with the big or conjugative substituent group away from/toward the closer binap ligand of the Ru catalyst, will form favorable/unfavorable chiral products with an Re-/Si- intermediate structure. On the contrary, if there is no such α-hydrogen in any substituent group of the ketone, and another intermediate (denoted as ) would not exist. This study indicates that the conjugative effect of the substituent groups of the ketone play an important role in differentiating the R/R' groups of the ketone, while steric and electrostatic effects contribute to a minor extent. Furthermore, the disparity of the R and R' groups of the ketone is of importance in the enantioselectivity and the favorable chiral alcohol is formed when the structure of the conjugative/big substituent group is away from the closer binap ligand of the RuH(2)(binap)(cydn) catalyst. According to the three factors of the substituent group and the fourth quadrant theory, the enantioselectivity of 91 prochiral ketones catalyzed by a series of Ru catalysts were predicted. All of the predictions are consistent with the experimental results.
Authors:
Ran Feng; Ang Xiao; Xin Zhang; Yanhui Tang; Ming Lei
Publication Detail:
Type:  JOURNAL ARTICLE     Date:  2012-11-28
Journal Detail:
Title:  Dalton transactions (Cambridge, England : 2003)     Volume:  -     ISSN:  1477-9234     ISO Abbreviation:  Dalton Trans     Publication Date:  2012 Nov 
Date Detail:
Created Date:  2012-11-28     Completed Date:  -     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  101176026     Medline TA:  Dalton Trans     Country:  -    
Other Details:
Languages:  ENG     Pagination:  -     Citation Subset:  -    
Affiliation:
State Key Laboratory of Chemical Resource Engineering, Institute of Materia Medica, College of Science, Beijing University of Chemical Technology, Beijing 100029b, PR China. leim@mail.buct.edu.cn zhangxin@mail.buct.edu.cn.
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