Concerted multibond reaction

The continuing accumulation of experimental material made it, however, still more apparent that the above criteria, both kinetic and stereochemical [61-64], cannot be regarded as absolute [65], since there are probably both concerted reactions which are forbidden as well as nonconcerted ones which are allowed. An example of the first type of reaction is some 2 + 2 retro-cycloadditions on sterically strained systems [62,66] the second type of processes is then represented by the so-called multibond reactions [67]. This, of course, stimulates to formulate a new and more universal criterion of concertedness as well as a deeper understanding of the very concept of concertedness. 

It should be noted in conclusion that the above-considered (Sect. 9.2.5) double proton migrations may also be regarded as typical multibond reactions. The rule cited above is actually quite applicable to these reactions. At the same time, given a favorable stereochemistry, a concerted synchronous mechanism may be realized for some systems of this type. Analysis of the factors that may provide for the possibility of such exceptions in the case of this and some other multibond reactions would be of particular interest for understanding the nature of cooperative processes. 

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