Cyclopropyl anion ring opening

The competing side reaction is a cyclopropyl anionic ring opening (for a review see ref 13). In the case of isomeric l-benzoyl-2,3-diphenylcyclopropanes, no base-induced isomerization (MeOH/ NaOMe, 65 °C, 4 h) was detected. In many cases these rearrangements require rather drastic conditions but the interconversion can be facilitated by the participation of an adjacent substituent. For example, with an acyl-substituted cyclopropane, enolization permits cisitmns isomerization to proceed without ring opening and therefore without competing anionic structural isomerization. 

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 . 

It is not clear, however, whether a photochemical anion or a thermal cyclopropyl radical ring-opening — the latter caused by photochemical electron ejection — took place. 

A number of intermediates related to the cyclopropylmethyl radical undergo rapid ring opening. 1-Cyclopropylvinyl radicals 35 rearrange to 2-(allenyl)ethyl radicals 36, while radical anions 37, formed by one-electron reduction of cyclopropyl ketones, ring open to give enolate radicals... 

For a review of ring opening of cyclopropyl anions and related reactions, see Boche, G. Top. Curr. Chem., 1988, 146, 1. 

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 equilibria between the allyl and the cyclopropyl cations and the corresponding equilibria of the anions, are not seen as such, but they are found in disguise. Thus the cationic reaction is seen when cyclopropyl halides are heated or treated with silver ions. The cyclopropyl cation itself is not an intermediate, because the ring opening (4.23, arrows) takes place at... 

There has been considerable interest in ring opening of cyclopropyl-containing ketyl anions from both a mechanistic and synthetic viewpoint. Cyclopropyl substituents are often... 

In 1990, Tanko and Drumright51 presented evidence that the ring opening of the radical anion generated from phenyl cyclopropyl ketone (Scheme 15) is reversible, with an... 

Cyclopropyl groups have been used to probe for ketyl anion intermediacy in reactions of Sml2 with ketones. In 1991, Molander reported that treatment of cyclopropyl ketone 15 with Sml2 yields ring-opened product 16 in 81% overall yield (equation 10)56 Timberlake and Chen reported that several cyclopropyl-ring-opened products result from treatment of 17 with Sml2 (Scheme 17)57. 

Fig. 9. Conversion of oyclopropyl species to allyl species. Conrotatory changes are on the left disrotatory changes are on the right. Upper section alternative modes of ring opening and orbital correlation of cyclopropyl species. Middle section symmetry state correlation of radical. Bottom section symmetry state correlation of anion.

Tolbert [1-3] has summarized the various photochemical pathways open to allyl anions in general, including electron photoejection. Other pathways include ring closure to a cyclopropyl anion, E-Z isomerization, protonation and a-bond cleavage. With the availablity of a greater number of photochemical pathways for allyl anions in general, it is not surprising that electron photoejection is not an important pathway for aryl-stabilized allyl anions. One of the most studied... 

Cyclopropyl-substituted ketones are suitable substrates for generating distonic radical anions from ketyl radical anions. A series of cycloalkanone substrates with unsaturated side-chains, to trap the primary radical formed after cyclopropylcar-binyl ring opening, has been investigated (Scheme 31) [118, 119]. For the first electron-transfer step triethylamine is used as electron donor. The reaction sequence is terminated by proton or hydrogen transfer from the solvent or the a-amino radical formed after deprotonation of the amine radical cation. 

The conrotatory ring opening predicted by correlation diagrams based on the Woodward-Hoffmann rules for cyclopropyl anions have their conterpart in the isoelectronic aziridines. The interconversions outlined below were investigated by Huisgen et and photochemical ring opening was shown to follow a... 

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