Pericyclic reactions nonaromatic

According to DFT calculations, the mechanism of this reaction is stepwise [73] and consists of a tandem [2+2] cycloaddition-cycloreversion sequence (Scheme 2.2). Both processes take place via supra-supra mechanisms that are thermally allowed because of the interplay between tt fx and lone pair localized orbitals. The corresponding transition structures TSl and TS2 are nonaromatic as indicated by the low negative values of the nucleus-independent chemical shifts (NICS), thus discarding true pericyclic processes [74,75]. In one special case, a highly hindered 1,3,2-/l -oxaphosphazetidine INT (Scheme 2.2) reaction intermediate was charac-... 

Sigmatropic shifts are not pericyclic because suprafacial overlapping of cyclic p-orbitals is not involved. In these shifts, the orbitals from the breaking bond and the lone pair overlap with the p-orbitals in the TS. These reactions have planar nonaromatic eight-centered cyclic transition states with orthogonal orbital... 

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