Thermal and Sonochemical Studies on the Diels−Alder Cycloadditions of Maskedo-Benzoquinones with Furans:  New Insights into the Reaction Mechanism

Abstract

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 degrees C. 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.

(3aR,6S,7aR)-7a-Chloro-2-[(4-nitrophenyl)sulfonyl]-1,2,3,6,7,7a-hexahydro-3a,6-epoxyisoindole

“…For Diels-Alder reactions, see: Winkler (1996); Paulvannan (2004); Norton (1942); Fraile et al (2001); Padwa et al (2003); Medimagh et al (2008); Avalos et al (2003). For the thermal IMDA reaction of furan-cored compounds, see: ; Koşar et al (2006Koşar et al ( , 2007Koşar et al ( , 2011; Arslan et al (2008); Temel et al (2012).…”

“…The feasibility of employing IMDA has been considered the main application often becomes the preparation of the substrates in synthetic pathways (Fraile et al, 2001;Padwa et al, 2003). Thermal intramolecular Diels Alder reaction has also been popular and facile methodology since early 1980's Furan is also one of the most used diene part in thermal IMDA cycloaddition (Medimagh et al, 2008;Avalos et al, 2003).…”

Five‐Membered Heterocycles: Furan

“…The a-hydroxyl group controlled the facial selectivity of the Diels-Alder type reaction. Recent studies, however, suggest a stepwise, double Michael addition as the mechanism for this type of reaction [188]. Scheme 6.97 Furans are less reactive dipolarophiles.…”

Atom Economy – Principles and Some Examples

“…Under microwave heating [35], some reactions can be performed in the absence of a solvent that would be otherwise required for heat transfer [36]. Sonochemistry [37] is also a way to enhance, for instance, Diels-Alder reactions, which are very atom economical reactions [38,39].…”