Concertedness in Pericyclic Reactions

Although such an understanding of the reaction mechanism is in principle applied also in the theory of pericyclic reactions, the above general picture is in this case slightly complicated by the specific, in the course of the historical development created classification of reaction mechanisms in terms of concertedness and/or nonconcertedness. As concerted are intuitively understood the reactions, for which the scission of old bonds and formation of the new ones is synchronized, whereas for the nonconcerted reactions the above bond forming processes are completely assynchronized. Moreover, since the above assynchronicity is also intuitively expected to induce the stepwise nature of the process, the nonconcertedness is frequently believed to require the presence of intermediates, whereas the concerted reactions are believed to proceed in one elementary step. 

Although such a definition is seemingly quite clear and unique, the practical exploitation of the above criterion is complicated by the fact that the scission and the formation of the bonds is a microscopic process, inaccesible to direct experimental observation.This, of course, suggests the nesessity of a search for other, more easily exploitable criteria of concert. As one such criterion is usually regarded the 

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Vivanco et queried the origin of the loss of concertedness in pericyclic reactions. They studied several [3+2] thermal cycloadditions between azomethine ylides and nitroalkenes both computationally and experimentally. An analysis of the energy density H at relevant BCPs reveals that the lithium-heteroatom interactions are electrostatic. 

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