Cyclopropyl-Allyl-Anion Rearrangements

2 Rearrangements Involving Three-Membered Rings 2.1 Cyclopropyl-Allyl- Anion Rearrangements 

It was only after Woodward and Hoffmann had predicted a conrotatory mode for the thermal cyclopropyl-allyl anion transformation in 1965 9) that new interest developed in this reaction 10). But it was first shown by Huisgen and coworkers U) by means of the iso-rc-electronic uncharged aziridine 1 which gives the azomethine ylids 2 and 3, respectively, that the stereochemistry of the thermal and the photochemical reactions agrees with the prediction. 

Kauffmann and coworkers 12) studied another hetero-analogous but in this case an anionic system, N-lithio-cis-2,3-diphenylaziridine (4). 4 transforms thermally into endo,exo-l,3-diphenyl-2-azallyllithium (5). 

The rearrangement of 5 to 6 competes successfully with the trapping of 5 with frans-stilbene which established the stereochemistry of 5. The stereochemistry of 2 and 3 has similarly been determined by 1,3-dipolar cycloaddition reactions. 

Why is it that the predicted modes of rearrangement were first confirmed by means of these two systems and not with a real cyclopropyl-allyl anion system The long history of the cyclopropyl-allyl anion rearrangement shows that this is because of several problems 10b). The first of these, which is also indicated from the results with 1 and 4, and which is confirmed by MO calculations, is the slow thermal conrota-tion of a cyclopropyl anion to give the corresponding allyl anion , as compared to the fast isomerization of the allyl anion to give the most stable isomer. 

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It was Smith and Showell who inferred that 195 is deprotonated to give the cyclopropyl anion 196 which isomerizes to the allyl anion 197 (the cyclopropyl allyl anion rearrangement is treated in detail in Section IV.C.). 

Table 1. Relative energies (AAHf, kcal/mol) with respect to the cyclopropyl anion of the transition state of the conrotatory cyclopropyl-allyl anion rearrangement A, the allyl anion B, and the transition state of the allyl anion isomerization C.

In spite of these difficulties a kinetical criterium has been elaborated for the thermal conrotation of a cyclopropyl anion . The result of this study has recently been confirmed by a special cyclopropyl-allyl anion rearrangement allowing trapping reactions of the allyl anion . 

One can therefore conclude that the base-catalyzed cyclopropane - propene rearrangement 93a(b) - 96a(b) is induced by an acid-base and not by an electron transfer reaction. The mechanism of the facile cyclopropyl - allyl anion rearrangement 94a(b) - 95a(b), however, is not clear74). 

In previous chapters the influence of the gegenion and the solvent on rearrangements of carbanions mostly concerned the rate of a certain reaction (and thus kinetic aspects), as, e.g., in the case of the cyclopropyl-allyl anion rearrangement (Sect. 2.1), the ally anion isomerization (Sect. 2.2), the cyclization of 5-hexenyl alkali metal compounds (Sect. 4) or the configurational isomerization of a-substituted vinyl-lithium compounds (Sect. 5). 

What is the preferred mode of rearrangement after electron transfer to (derivatives of) 102 or to 104 and 1051 Does the ring-opening reaction occur on the radical anion or the dianion stage These questions have been dealt with in a series of papers first by Bauld and coworkers 77). They are of general interest because the theoretical treatment of electrocyclic reactions of open-shell systems, as e.g. the cyclopropyl allyl radical rearrangement, led to ambiguous results 9,78 81). 

The thermal cyclopropyl anion to allyl anion rearrangement is predicted to be conrotatory and the photochemical rearrangement to be disrotatory. ... 

The stereochemistry of the reaction has been examined both theoretically92 and experimentally.93 It has been found to be stereospecific, with disrotatory motion of the methylene groups. Cyclopropyl anions also undergo thermal rearrangements to allyl anions.94... 

The formation of 15 on treatment of 14 with lithium 2,2,6,6-tetramethylpiperidide is clearly a rearrangement of an allyl anion into a cyclopropyl anion, that is trapped by the electrophile iodomethane. However, since it is also an azacycloheptatriene to azanorcaradiene rearrangement, it will be covered in Section 2.3.2.1. 

The anion of 3-alkoxycarbonyl-3 f-azepines exhibits a reaction that is an allyl anion to cyclopropyl anion rearrangement (see Section 2.3.1.2.) and is also formally an azacyclohep-tatriene to azanorcaradiene rearrangement with an additional alkoxycarbonyl shift. Treatment of 13 (X-ray analysis) with lithium 2,2,6,6-tetramethylpiperidide (LTMP), followed by iod-omethane, provides 14 (X-ray analysis of picrate) in 50-60% yield. The mechanism as suggested by the authors is shown on the following page. 

Gas-phase unimolecular rearrangements of a series of substituted cyclopropyl anions 14, generated by electron ionization of the respective neutral reagents, to the corresponding allyl anions 15 were found to have ring-opening barriers in the range 19 to >36 kcal mol ... 

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