Thermal cyclopropyl-allyl anion

It was only after Woodward and Hoffmann in 1965 had predicted a conrotatory mode for the thermal cyclopropyl-allyl anion transformation that a new interest developed in this reaction. By means of the iso-7c-electronic aziridine 310 Huisgen and coworkers succeeded in demonstrating that the thermal recation gave a conrotatory formation of azomethine ylid (311) and that the light-induced reaction resulted in a disrotation to give 312. 

These results have been attributed to an increased ionicity of the carbon-lithium bond in the case of 331 and 340 as compared to 342 and 343 . This conclusion is supported by electrochemical measurements and MNDO calculations A similar conclusion had been reached earlier by Boche and Martens for thermal cyclopropyl-allyl anion transformations. 

Thus with the cyclopropyl anions 331- and 340 - it has been established that the disrotatory mode, as predicted by Woodward and Hoffmann , is the preferred one. It is however not clear whether a photochemical cyclopropyl-allyl anion or a thermal cyclopropyl radical ring-opening (the latter caused by photochemical electron ejection) takes place. It has also been realized that systems with X = vinyl or Br (342- and 343-, respectively) do not open photochemically . 

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. 

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 . 

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 electrocyclic reactions of 3-membered rings, cyclopropyl cation and cyclopropyl anion, may be treated as special cases of the general reaction. Thus the cyclopropyl cation opens to the allyl cation in a disrotatory manner (i.e., allyl cation, n = 0), and the cyclopropyl anion opens thermally to the allyl anion in a conrotatory manner (i.e., allyl anion, m = 1). Heterocyclic systems isoelectronic to cyclopropyl anion, namely oxiranes, thiiranes, and aziridines, have also been shown experimentally and theoretically to open in a conrotatory manner [300]. 

The cyclopropyl anion 320 should be the intermediate both in the cis-trans isomerization to give 322 (NaOMe in MeOH) and in the ring-opening reaction (NaH in DMF) to give 321" whose stereochemistry is unknown. From these studies it seemed reasonable that cyclopropyl anions did indeed thermally isomerize to give allyl anions. 

Conrotation was predicted for the thermal ring-opening of the cyclopropyl anion to the allyl anion. ... 

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

No thermal ring openings are observed for conventional cyclopropyl anions, even if they carry two phenyl groups to provide stabilization of the potential terminus of the allyl anion. CH acidic optically active precursors of cyclopropyl anions exhibit a very high degree of retention of configuration when treated with bases in polar protic solvents and in most cases in aprotic solvents, too. - ... 

By these simple rules Woodward and Hoffmann predicted a disrota-tory course for the opening of the cyclopropyl cation in its ground state to the corresponding allyl cation, while the thermal opening of cyclopropyl radical and anion to allyl radical and anion is conrotatory. A glance at Fig. 2 clearly shows the reason. Reverse predictions can be made for photochemically induced reactions. 

Trans- and cw-2-phenylcyclopropyl bromides undergo nucleophilic ring opening of the cylopropane ring to give trans- or cis-cinnamyl acetate, respectively when treated with KOAc and 18-crown-6 in DMSO (eqs 11 and 12). A thermal, disrotatory solvolysis of the cyclopropyl halide, wherein the acetate anion stabilizes the incipient allylic cation, is proposed for this stereospecific reaction. Notably, solvolysis of either trans- and cis-2-phenylcyclopropyl bromides in AcOH (i.e., without KOAc) leads to the formation of fro/w-cinnamyl acetate exclusively. 

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