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Hexadehydro Diels-Alder reaction
In organic chemistry, the hexadehydro-Diels-Alder (HDDA) reaction is an organic chemical reaction between a diyne (2 alkyne functional groups arranged in a conjugated system) and an alkyne to form a reactive benzyne species, via a () cycloaddition reaction.〔Hoye, T. R.; Baire, B.; Niu, D.; Willoughby, P. H.; Woods, B. P. ''Nature'', 2012, ''490'', 208 ()〕〔Holden, C.; Greaney, M. F. ''Angew. Chem. Int. Ed. Engl.'', 2014, ''53'', 5746 ()〕〔 Yeoman, J. T. S.; Reisman, S. E. ''Nature'', 2012, ''490'', 179 ()〕 This benzyne intermediate then reacts with a suitable trapping agent to form a substituted aromatic product. This reaction is a derivative of the established Diels-Alder reaction and proceeds via a similar () cycloaddition mechanism. The HDDA reaction is particularly effective for forming heavily functionalized aromatic systems and multiple ring systems in one synthetic step.
== Reaction Mechanism ==
Depending on the substrate chosen, the HDDA reaction can be initiated thermally or by the addition of a suitable catalyst, often a transition metal.〔〔〔Yun, S. Y.; Wang, K.-P.; Lee, N.-K.; Mamidipalli, P.; Lee, D. ''J. Am. Chem. Soc.'', 2013, ''135'', 4668 ()〕〔Vandavasi, J. K.; Hu, W.-P.; Hsiao, C.-T.; Senadi, G. C.; Wang, J.-J. ''RSC Adv.'', 2014, ''4'', 57547 ()〕 The prevailing mechanism for the thermally-initiated HDDA reaction is a () cycloaddition between a conjugated diyne (1,3-dialkyne) and an alkyne (often referred to as a “diynophile” in analogy to the Diels-Alder “dienophile”) to form an ortho-benzyne species.〔〔 The metal-catalyzed HDDA is thought to proceed through a similar pathway, forming a metal-stabilized benzyne, which is then trapped.
The simplest model of an HDDA reaction is the cycloaddition of butadiyne and acetylene to form ortho-benzyne (o-benzyne, shown below).〔Ajaz, A.; Bradley, A. Z.; Burrell, R. C.; Li, W. H. H.; Daoust, K. J.; Bovee, L. B.; DiRico, K. J.; Johnson, R. P. ''J. Org. Chem.'', 2011, ''76'', 9320 ()〕 This reactive intermediate (denoted by brackets) subsequently reacts with a generalized trapping reagent that consists of a nucleophilic (Nu-) and electrophilic (El-) site, giving the benzenoid product shown.
The o-benzyne intermediate can be visualized in the two resonance (chemistry) forms illustrated above. The most commonly depicted form is the alkyne (1), but the cumulene (1’) form can be helpful in visualizing ring formation by () cycloaddition.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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