Cycloreversion, asynchronous

Cycloreversion of 4-methoxystyrene from [Re(0)(OCH2CHPhO)-(Tp )] has been investigated and it has been proposed that the reaction proceeds via a significantly asynchronous but concerted transition state. Computation modeling studies as also a Hammett study on cycloreversions of substituted styrene from a series of different [Re(0)(Tp )(diolato)] complexes have also been reported.214... 

A computational and experimental study of the mechanism of the aza-Wittig reaction between phosphazenes and aldehydes showed it to consists of two consecutive asynchronous thermally allowed (because they do not correspond to the n-systems) [2 + 2] cycloaddition (i)-cycloreversion (ii) processes (the second of which controls the stereochemical outcome of the whole reaction) via the relatively stable intermediate Ii (Scheme 4). The results indicate that P-trimethyl-X -phosphazenes are more reactive than their P-triphenyl analogues, and that the formation of the corresponding ( )-imines is preferential or exclusive. 

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