Vinylcyclopropanes ring opening

An efficient carboannulation proceeds by the reaction of vinylcyclopropane (135) or vinylcyclobutane with aryl halides. The multi-step reaction is explained by insertion of alkene, ring opening, diene formation, formation of the TT-allylpalladium 136 by the readdition of H—Pd—I, and its intramolecular reaction with the nucleophile to give the cyclized product 137[I08]. 

Reaction of the cyclopropyl-substituted pivalate (25) with dimethyl benzylidenema-lonate in the presence of a palladium catalyst gave a mixture of alkylidenecyclo-propane (26) and vinylcyclopropane (27). The ratio of these two adducts is found to be quite sensitive to the choice of ligand and solvent. While triisopropyl phosphite favors the formation of the methylenecyclopropane (26), this selectivity is completely reversed with the use of the bidentate phosphite ligand dptp (12). Interestingly there was no evidence for any products that would have derived from the ring opening of the cyclopropyl-TMM intermediate (Scheme 2.8) [18]. 

The direction of ring opening by homolytic cleavage of a cyclopropane bond is controlled by the stability of the diradical species formed. Upon heating of the mono-deuterated vinylcyclopropane 3, a mixture of the two isomeric mono-deuterated cyclopentenes 4 and 5 is formed ... 

Stansbury and Bailey. A review by Colombam on addition-fragmentation processes is also relevant. Monomers used in ring-opening are typically vinyl (e.g. vinylcyclopropane - Scheme 4.20 Section 4.4.2.1) or methylene substituted cyclic compounds (e.g. ketene acetals - Section 4.4.2.2) where addition to the double bond is followed by p-scission. 

Vinylcyclopropanes bearing a cis alkyl substituent undergo a competitive prototropic shift accompanying ring opening (Eq. 14)14). In such cases, temperature... 

The reactions of vinyl ethers with vinyldiazoacetates unsubstituted at the vinyl terminus result in a very interesting outcome because either regioisomer of the [3 + 2] cycloadduct can be obtained (Scheme 14.16) [104]. An example is the reaction with 2,3-dihydrofuran where regioisomer 122 is formed via the established ring-opening reaction of the donor/acceptor-substituted vinylcyclopropane 121 under Lewis acidic conditions (Scheme 14.14) [104, 105]. The cyclopropanation step has been optimized to... 

A more detailed evaluation of the diverse structures proposed for the secondary species goes beyond the scope of this review. We mwely emphasize that the ESR results provide detailed evidence for the nature of the radical center, but fail to elucidate the cationic site. The identity of this center is left to secondary considerations or speculation. We also note that any alternative structure has the virtue of not contradicting the ab irutio calculations the potential c ture of chloride ion has precedent in the nucleophilic substitution at a cyclopropane carbon (see Section 7). Another type of ring-opened structure has been postulated as an intermediate in the aminium radical cation catalyzed rearrangement of l-aryl-2-vinylcyclopropanes (see Section 5). 

Suzuki has shown that vinylcyclopropane 143 behaves both as an electrophile and a nucleophile and thus undergoes palladium-catalyzed ring-opening polymerization as shown in Equation (66). Vinyl cyclopropane 143 first reacts with palladium(O) to induce ring opening of the cyclopropane ring and forms zwitterionic TT-allylpalladium/molonate anion species. Repeated intermolecular attack of the malonate anionic moiety to the 7r-allylpalladium part through bond formation of an r/i -carbon atom affords finally the polymer 142. ... 

Numerous examples of vinylcyclopropane to diene rearrangement have been studied (e.g. equation 134)178 and thoroughly reviewed in the aforementioned surveys and in this series , and hence will not be repeated here. The reader will also find in this series180 ample examples of typical ring-opening reactions of cyclic vinylcyclopropanes (equation 135)181... 

The vinylcyclopropane radical cation, generated at 77 K by X-irradiation of (139) in a Freon-113 matrix, was shown to rearrange at 105-110 K to afford two ring-opened distonic radical cationic species.300 The rearrangement reactions of the radical cations of 1,3- and 1,4-pentadiene and cyclopentene and the formation of spin adducts with 2.4.6-tri-/-butylnitrosobenzene (BNB) are discussed. The pulse radiolysis of 1,1 -binaphthyl-2,2,-diyl hydrogenphosphate (BiNPCUH) (140) in deaerated f-butanol at... 

Ring-opened cyclopropane radical cations have also been postulated to account for the stereochemistry of the aminium radical cation catalyzed rearrangement of l-aryl-2-vinylcyclopropanes [225]. These systems, of course, contain substituents that may veil the true nature of the cyclopropane radical cations by delocalizing spin and charge. In addition, we note that the experimental findings allow some latitude in their interpretations. 

A severe limitation of this method, however, is the failure of the ylide 103 to yield oxaspiropentanes vide supra) from a,p-unsaturated ketones and the poor yields of vinylcyclopropanes obtained from its reactions with hindered ketones or with con-formationally rigid six-membered rings. Moreover, attempts to extend the oxaspiro-pentane ring opening to compounds containing an adjacent tertiary center have failed thus, oxaspiropentane 110 did not lead to 111, Eq. (33) 57). 

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