Document Detail


New insight on the origin of the unusual acidity of Meldrum's acid from ab initio and combined QM/MM simulation study.
MedLine Citation:
PMID:  11457147     Owner:  NLM     Status:  MEDLINE    
Abstract/OtherAbstract:
Ab initio molecular orbital and combined QM/MM Monte Carlo simulations have been carried out to investigate the origin of the unusually high acidity of Meldrum's acid. Traditionally, the high acidity of Meldrum's acid relative to that of methyl malonate has been attributed to an additive effect due to the presence of two E esters in the dilactone system. However, the present study reveals that there is significant nonadditive effect that also makes major contributions. This results from preferential stabilization of the enolate anion over that of Meldrum's acid due to anomeric stereoelectronic interactions. To investigate solvent effects on the acidity in aqueous solution, the relative acidities of Z and E conformers of methyl acetate have been determined in combined ab initio QM/MM simulations. There is significant solvent effect on the conformational equilibria for both the neutral ester and its enolate anion in water, leading to stabilization of the E stereoisomer. However, the computed solvent effect of 4.4 kcal/mol in favor of the E isomer of methyl acetate is largely offset by the favorable solvation of 3.4 kcal/mol for the E conformer of the enolate anion. This leads to an enhanced acidity of 3.4 kcal/mol for the (E)-methyl acetate in water over the Z conformer. In Meldrum's acid, it is the preferential stabilization of the enolate anion due to anomeric effects coupled with the intrinsically higher acidity of the E conformation of ester that is responsible for its high acidity.
Authors:
K Byun; Y Mo; J Gao
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Publication Detail:
Type:  Journal Article; Research Support, U.S. Gov't, Non-P.H.S.; Research Support, U.S. Gov't, P.H.S.    
Journal Detail:
Title:  Journal of the American Chemical Society     Volume:  123     ISSN:  0002-7863     ISO Abbreviation:  J. Am. Chem. Soc.     Publication Date:  2001 May 
Date Detail:
Created Date:  2001-07-17     Completed Date:  2001-08-23     Revised Date:  2006-11-15    
Medline Journal Info:
Nlm Unique ID:  7503056     Medline TA:  J Am Chem Soc     Country:  United States    
Other Details:
Languages:  eng     Pagination:  3974-9     Citation Subset:  IM    
Affiliation:
Department of Chemistry and Minnesota Supercomputing Institute, University of Minnesota, 207 Pleasant Street S.E., Minneapolis, Minnesota 55455, USA.
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MeSH Terms
Descriptor/Qualifier:
Acetates / chemistry
Dioxanes / chemistry*
Hydrogen-Ion Concentration
Molecular Structure
Quantum Theory
Solvents
Thermodynamics
Chemical
Reg. No./Substance:
0/Acetates; 0/Dioxanes; 0/Solvents; 2033-24-1/Meldrum's acid; 79-20-9/methyl acetate

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine


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