66688-23-1Relevant academic research and scientific papers
Studies of the Transition-state Structure by the Method of Volumetric Steric Effects. Part 4. Transition State in Diels-Alder Reactions of (E)-1-Alkyl(alkoxy)buta-1,3-dienes with Alkyl Acrylates
El'yanov, Boris S.,Shakhova, Svetlana K.,Polkovnikov, Boris D.,Rar, Lev F.
, p. 11 - 16 (1985)
A method of studying transition-state structures by analysis of volumetric steric effects is proposed.The method is used to reveal features of the steric structure of the transition state in Diels-Alder reactions.The structure has been found to change from one approximating the structure of the prereactional complex to that resembling the half-chair adduct as the diene substituent changes from methyl to t-butyl.As this takes place, no intermediate half-boat adduct is formed.An increase in steric hindrance during the change in the substituent is regarded as the driving force responsible for the transition-state shift along the reaction co-ordinate.This is consistent with the Hammond postulate.The C=O and C=C groups of the dienophile in the endo-transition state are in the s-cis-conformation.
a(E)-1-methoxy-1,3-butadiene and 1,1-dimethoxy-1,3-butadiene in (4+2) cycloadditions. A mechanistic comparison
Sustmann,Tappanchai,Bandmann
, p. 12555 - 12561 (1996)
The reactions Of (E)-1-methoxy-1,3-butadiene (1) and 1,1-dimethoxy-1,3-butadiene (2) with a series of dienophiles of increasing electrophilicity are described. Stereochemical studies reveal that the cycloadditions of 1 are concerted processes, even for the most electron-deficient olefins dimethyl dicyanofumarate and dimethyl dicyanomaleate. 1,1-Dimethoxy-1,3-butadiene reacts under our conditions (dilute solutions and temperatures ≤60°C) only with those dienophiles which can give zwitterions out of the antiperiplanar conformation of the diene. Zwitterionic intermediates can be trapped by methanol. In the case of tetracyanoethene the kinetics of decay of an intermediate, interpreted as the zwitterion, can be followed by stopped flow techniques: E(a) = 14.8 ± 0.2 kcal mol-1, log A = 11.9 ± 0.1, ΔH(≠) = 10.8 ± 0.1 kcal mol-1, ΔS(≠) = -6.2 ± 0.1 cal mol-1 K-1, and ΔG(≠) = 11.40 ± 0.03 kcal mol-1.
Recyclable organotungsten Lewis acid and microwave assisted Diels-Alder reactions in water and in ionic liquids
Chen, I-Hon,Young, Jun-Nan,Yu, Shuchun Joyce
, p. 11903 - 11909 (2007/10/03)
The water-soluble, organotungsten Lewis acid, [OP(2-py)3W(CO) (NO)2](BF4)2 (1), was synthesized and characterized. A series of 1-catalyzed Diels-Alder reactions were investigated under conventional heating or microwave heating conditions. The cycloaddition reactions were efficiently conducted in either water or in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate. The ionic liquid acts as a powerful medium not only for rate- and selectivity enhancements but also for facilitating catalyst recycling. Dramatic rate acceleration via microwave flash heating as compared to thermal heating was observed. Graphical Abstract
