Thermal and sonochemical studies on the Diels-Alder cycloadditions of masked o-benzoquinones with furans:: New insights into the reaction mechanism

被引:21
作者
Avalos, M
Babiano, R
Cabello, N
Cintas, P [1 ]
Hursthouse, MB
Jiménez, JL
Light, ME
Palacios, JC
机构
[1] Univ Extremadura, Fac Ciencias, Dept Quim Organ, E-06071 Badajoz, Spain
[2] Univ Southampton, Dept Chem, Southampton SO17 1BJ, Hants, England
关键词
D O I
10.1021/jo0348322
中图分类号
O62 [有机化学];
学科分类号
070303 ; 081704 ;
摘要
What does a Diels-Alder cycloaddition look like? This question is here addressed in the case of the increasingly significant cycloadditions of masked o-benzoquinones (MOBs), which serve as versatile dienes for the construction of complex and functionalized structures. So first, what the mechanism is not: It is not (by and large) a classical, concerted [4 + 2] cycloaddition. Experimental evidence is now supported by sonochemical studies, which were instrumental in elucidating the pathway. Reactions with furans are accelerated and improved under sonication, even when conducted at - 10 degreesC. Variation of the acoustic energy, temperature, and solvent composition allows us to optimize the yields and provide insights into the mechanism. Ultrasound does not cause sonochemical switching, as an alternative radical pathway should be ruled out. Results are consistent with a polar mechanism as claimed recently in a theoretical study. Moreover, this also does justice to a series of seminal papers, largely ignored, that gave a clue to the crucial issue of furan regioselectivity based on a nucleophilic addition. Most effects caused by ultrasonic agitation are of mechanochemical nature and suggest the existence of a perfectly stirred reactor with enhanced mass transfer.
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收藏
页码:7193 / 7203
页数:11
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