Authors: Haqghu, Mohammad; Irani, Mehdi; Gholami, Mohammad Reza

Source: Progress in Reaction Kinetics and Mechanism, Volume 33, Number 2, 2008 , pp. 191-206(16)

Publisher: Science Reviews 2000 Ltd

Abstract:

Ab initio quantum mechanics and ONIOM calculations were used to study solvent and substituent effects on the reactions of 1,4-benzoquinone with cyclopentadiene and cyclohexadiene derivatives in tetrahydrofuran and greater water solvents. These calculations revealed (i) that increasing the number of electron donating methyl group substituents and (ii) the proximity of substituents to the reacting carbons (carbon atoms which contribute to the forming C—C bonds) on the diene, promote charge transfer from the diene to the dienophile in the transition state. These effects increase the negative charge on the oxygen atoms, destabilize the transition state in the organic solvent by increasing steric interactions, and stabilize the transition state in water solvent by increasing the strength of hydrogen bonding. These results, along with consideration of variations in the solvent-accessible surface area (SASA) confirmed that the dramatic acceleration in the rate of the studied reactions in water solvent arise in part from hydrophobic association of the reactants, but predominantly arise from hydrogen bonding between water molecules and the polarized transition state.

Keywords: DIELS - ALDER REACTION; ONIOM; SOLVENT EFFECTS; SUBSTITUENT EFFECTS

Document Type: Research article

DOI: http://dx.doi.org/10.3184/146867808X315742

Publication date: 2008-06-01

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• Progress in Reaction Kinetics & Mechanism is an international journal for the quarterly publication of both in-depth reviews and research articles.

  In-depth reviews are comprehensive accounts bringing together work from many sources with the aim of providing an article of lasting value that will become established as the reference source in the particular subject. Research articles, on the other hand, normally focus on a relatively new or recently developed field or technique giving a state-of-the-art account of the subject and may well refer to a narrower range of existing work. It covers the fields of kinetics and mechanisms of chemical processes in the gas phase and solution of both simple and complex systems.

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