1.3- disubstituted 2-vinylcyclopropanes

Some care must be taken in drawing conclusions from the E/Z or syn/anti selectivity of a given catalyst/alkene combination. The intrinsic stereoselectivity may be altered in some cases by subsequent isomerizations initiated by the catalyst. For example, epimerization of disubstituted vinylcyclopropanes is effectively catalyzed by palladium compounds the cis - trans rearrangement of ethyl chrysanthemate or of chrysanthemic acid occurs already at room temperature in the presence of PdCl2 L2 (L = MeCN, EtCN, PhCN)96 Oxycyclopropane carboxylic esters undergo metal-... 

Another approach to such vinylcyclopropanes involved the addition of Grignard reagents, e.g. n-amylmagnesium bromide to 171b leading to the cyclopropylcarbinol 175. Oxidation at —60 °C in methylene chloride with DMSO-(COCl)2 109) led to the ketone 176, which was treated with ethylidenetriphenylphosphorane for instance to produce in 69% overall yield from 171b a. (Z, E) mixture of the disubstituted vinylcyclopropanes 177, precursors of dihydrojasmone (vide infra, Sect. 5.5). 

More examples have been quoted of special growth reactions for molecules having two carbon-carbon double bonds. Cyclopolymerization occurs for diallylaminomethyl phenols, allyl esters of unsaturated acids, 2-(2-vinyl-phenoxy)ethyl methacrylate and acrylate, 2-divinylbenzene and some silicon-containing monomers. - Polymerizations of p-xylylene and its derivatives have been considered further. - The radical polymmzation of some disubstituted vinylcyclopropanes has been represented as shown in equation (2), where X and Y=CN or COjEt. The involvement of the ring must be connected with stabilization by the substituents of the product radical. 

Ferrate 38 also turned out to catalyze [5 + 2]-cycloadditions. In this context, various vinylcyclopropane derivatives 52 were converted into the corresponding cycloheptadiene derivatives 53. The products were obtained in good to excellent diastereoselectivites favoring the 1,2-trans-disubstituted isomer 53a. 

Thus a substantial body of experimental evidence shows that 1,2-disubstituted cyclopropanes, including vinylcyclopropanes, react thermally to give isomeric cyclopropanes through both one-center and two-center epimerizations, with (kt + k2) kl2 ratios from 1.4 to 4. Rate constants for both (, + k2) and kl2 events respond to the capacity of substituents to stabilize adjacent radicals in a regular fashion consistent with trimethylene diradical transition structures. Rate constants for vinylcyclopropane structural isomerizations do as well, thus reinforcing the notion that these reactions are nonconcerted diradical mediated reactions. 

All of the experimental and theoretical work on the stereomutations of cyclopropanes and vinylcyclopropanes covered above seems consistent with and understandable in terms of kinetically significant involvements of Cj(ts), Cs(ts) and EF(ts) structures and partitionings of EE trimethylene intermediates resulting in the formation of klt k2 and kl2 products at comparable rates. For trans-1,2-disubstituted cyclopropanes, neither the Smith mechanism (one-center stereomutations only) nor any two-center-only formulation can be correct, as demonstrated by Crawford and Lynch in 1968143 and reinforced by numerous subsequent studies (Figures 2 and 3). 

The intermediacy of >/2-cyclopropene complexes of nickel has been proposed in catalyzed 2+1 reactions of free cyclopropene with electron-poor olefins, to give vinylcyclo-propanes. For example, the reaction of fumarate esters with 3,3-disubstituted cyclopropenes in the presence of Ni(COD)2 catalyst gave vinyl-substituted trans-2,3-cyclopropane dicarboxylate esters (equation 235)72 302. However, when maleic esters were used instead, a mixture of both cis and trans vinylcyclopropane diesters is obtained. 

Most of the radical stabilization energy parameters utilized in Figure 2 (R = H or D, -2.65 Me or Et, - 0.5 CN, 6.6 vinyl or propen-2-yl, 13.2 (E )-propenyl, 15.4) are SE values calculated by Leroy and coworkers " those for phenyl (9.4) and cinnamyl (19.1) were set empirically. These particular parameters are serviceable and reasonable, but cannot be considered preferred . Other plausible radical stabilization energy values lead to very similar correlations which demonstrate, as Figure 2 demonstrates, that the rate constants for two-center epimerizations of cyclopropanes and vinylcyclopropanes are sensitive to the radical stabilizing capacities of substituent groups in a simple additive fashion. Whatever 1,3-disubstituted trimethylene diradical intermediates or transition structutes may be involved respond kinetically to substituents as though each locus of radical character were thermochemically independent. 

Cho and Ahn studied the radical polymerization of 1,1-disubstituted 2-vinylcyclopropanes (X = COOEt, or CN)... 

The thermal C3 -> 5 ring expansion of 1-siloxy-l-vinylcyclopropanes occurs either from the Z or from the E isomers. It leads to cyclopentanone silyl enol ethers that are able to undergo either further regiospecific alkylation into 2,3-disubstituted cyclopenta-nones 145.209,213 dehydrosilylation into cyclopentenones The overall process constitutes an efficient three-carbon annelation process (equation 129). 

The reaction which uses the phosphorus ylide also produces31 35,77 highly stereo-selectively 1 -heterosubstituted-1 -vinylcyclopropanes, but now the (Z)stereoisomers are the major ones (Schemes 61, 62 a, 63 a). It is well-known that in the Wittig reaction the betaines which lead to the (Z)-disubstituted olefins are formed under kinetically controlled conditions and that any feature which favours the elimination of triphenyl-phosphine oxide from these betaines should increase the amount of the (Z)stereoiso-... 

For the reaction of alkyl-substituted alkenes, chloro(triphenylphosphite)copper can be used as a catalyst.Vinylcyclopropanes are formed at low temperature in yields of 55-77% for disubstituted alkenes (see Houben-Weyl, Vol. E19b, p763). 

The stabilized carbanion that is formed as an intermediate in the addition of the cuprate reagent to diethyl 2-vinylcyclopropane-l,l-dicarboxylate (6) can be alkylated by various alkyl halides prior to workup. A number of different disubstituted malonates 12 were obtained by this procedure. 

The diazotization route is frequently accompanied by products derived from solvolysis of the initially formed cyclopropylidene or the rearranged cyclopentenylidene (Skattebol rearrangement) in the case of vinylcyclopropylidene with alcohol solvent, although allenes still account for the major products in the case of vrc-disubstituted cyclopropylidenes. It is noteworthy that the stereochemistry of the ring substituents (Table 2, entries 2 and 3) is an important factor in affecting yields of allenes. gcw-Disubstitution of vinylcyclopropanes diminishes formation of allenes in favor of products from the Skattebol rearrangement (entry 6). 

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