Pericyclic pseudo reactions

This reaction has been suggested to be a [l,9]-sigmatropic rearrangement, but has been proved to be a pseudo-pericyclic reaction from computational studies,as illustrated here. 

Pseudo-pericyclic reactions are reactions that appear at first glance to be pericyclic, in that arrow pushing shows a cyclic array of electrons at the transition state. Several of the other facets of pericyclic reactions are also commonly evident, such as low barriers if the geometries are favorable. However, these reactions are never forbidden based upon a symmetry argument. Examples of such reactions are given below. Write the proper arrow pushing for these reactions, and identify... 

As a consequence of the pericyclic reaction path, the addition of a-stereogenic allylmctals to carbonyl compounds is accompanied by an effective 1,3-chirality transfer in the allylic moiety combined with 1,4-chira induction at the prostereogenic carbonyl group3032. The scheme also demonstrates the importance of the orientation of the substituent X in the six-membered transition state. By changing from a pseudo-axial to a pseudo-equatorial position, the cation-induced sy/i-attack addresses opposite faces of both prostereogenic moieties, leading to a Z-and an -isomer, these being enantiomeric in respect to the chiral moiety. 

The formation of cyclic nitrones (150) from pericyclic mechanism. Kinetic and computational studies have provided evidence for the involvement of a novel pseudo-pericyclic electrocyclization in the conversion of o-vinylphenyl isocyanates into quinolin-2-ones. " Such reactions have also provided evidence of torquoselectivity in a 6jt system. Hash vacuum thermolysis of triazoles (151) has been found to afford dihydroquinolines (155), presumably by generation of a-oxoketenimines (152) which can undergo a [1,5]-hydrogen shift to the o-quinoid imines (153)7(154) and subsequent electrocyclization (see Scheme 57). 

Ab initio and density functional calculations have been carried out on the mechanism of the 1,5-electrocyclization reactions of conjugated nitrile ylides (190). The results indicate that vinyl-conjugated systems (306) (X = CH2) cyclize via the classical electrocyclization pathway—a pericyclic, monorotatary process with a relatively early transition state in which there is substantial torsion of the vinyl group as well as pyramidalization at C(5). In contrast, systems with a heteroatom at the cyclization site 306 (X=NH, O) react via a pseudo-pericyclic process that is characterized by the in-plane attack of the lone pair of the heteroatom on the nitrile ylide. Such reactions have a lower activation energy. 

Ab initio MO calculations (MP2/6-31G ) of the reactions of nitrosoketene with formaldehyde, acetone and prop-2-enal indicate a 3 + 2-pathway via a concerted, planar, and pseudo-pericyclic transition state.59... 

Finally, the use of internal conversion as a cosmetic term for vague mechanisms should be avoided. In the cases where radiationless internal conversion is definitely known to occur, the cascade of states can often be discovered, or the processes may be pericyclic pseudo reactions. 

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