Allyl cations electrocyclic ring closure

Anodic oxidation of enamines in NaCN-MeOH with iodide ion as mediator provides ring-fused cyclopropane aminonitriles with the nitrile group exclusively in exo position [Eq. (29)] [256]. This reaction is possibly an electrocyclic ring closure of an intermediate allyl cation LXXIII. 

This looks as though each of the C—C bonds is independently the result of both HOMO/LUMO interactions, with an endo selectivity as well. In the presence of dienes, these species behave as allyl cations (see p. 259) and undergo clean [4 + 2] cycloadditions, as in the reaction of the oxyallyl 6.372 giving the tricyclic ketone 6.373, which is similar to the diene 6.369. Normally, oxyallyls are in equilibrium by disrotatory electrocyclic ring closures with cyclopropanones and with allene oxides, but the presence of the five-membered ring in these particular examples makes these pathways counter-thermodynamic. 

Ring strain is important in preventing a reaction that would otherwise change your view of a lot of the chemistry you know. Allyl cations are conjugated systems containing 2k electrons, so if you knew no other chemistry than what is in this chapter you might expect them to cyclize via disrotatory electrocyclic ring closure. The product would be a cyclopropyl cation. 

The three-fragment MOVB theory described and applied above to the case of the allyl cation electrocyclization can be easily applied to any electrocyclic ring closure of a closed shell system with the conclusion always being that the stereoselectivity of the reaction is dictated by the symmetry labels of the pi HOMO and pi LUMO of the core pi system in the following way ... 

Other chemists prefer a pericyclic description of the ring-closure step. The same enolate simply loses chloride to give an oxyallyl cation —a dipolar species with an oxyanion and a delocalized allylic cation. This species can cyclize in a two-electron disrotatory electrocyclic reaction (Chapter 36) to give the same cyclopropanone. We shall return to this discussion in the next chapter but, whatever the mechanism, there is no doubt that a cyclopropan one is an intermediate. 

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