Vinylcyclopropanes special

The vinylcyclopropane rearrangement is an important method for the construction of cyclopentenes. The direct 1,4-addition of a carbene to a 1,3-diene to give a cyclopentene works only in a few special cases and with poor yield. The desired product may instead be obtained by a sequence involving the 1,2-addition of a carbene to one carbon-carbon double bond of a 1,3-diene to give a vinylcyclopropane, and a subsequent rearrangement to yield a cyclopentene ... 

The thermal expansion of a vinylcyclopropane to a cyclopentene ring is a special case of a [l,3]-sigmatropic migration of carbon, although it can also be considered an internal [ 2 + 2]-cycloaddition reaction (see 15-63). It is known as a vinylcyclopropane rearrangementThe reaction has been carried out on many vinylcyclopropanes bearing various substituents in the ring or... 

Compounds with an additional 2-vinyl group, easily available in two steps from a,J -unsaturated ketones, are of special interest. If the reactive vinyl ketone moiety is liberated, it can be trapped in situ by suitable nucleophiles, e.g. CH-acids, generating polyfunctional compounds or by a diene unit which undergoes an intramolecular Diels-Alder reaction (equation 93). Besides, radical additions to the vinylcyclopropane are also possible giving silyl enol ethers as ring-opened products . Future synthetic applications of theses processes are obvious. 

Special attention should be given to the MIRC reaction of 2-bromobut-2-enoates with cyclohexenones, e.g. 1 and Table 10. When first cyclohexenone was added to a lithium diiso-propylamide solution, tricyclo[3.2.1.0 ]octan-6-ones were formed in 20-55% yield via double Michael addition, followed by intramolecular displacement. However, when ethyl 2-bro-mobut-2-enoate was first added to a lithium diisopropylamide solution, vinylcyclopropanes were formed in good yield (Table 10, entries 1-2). Other chalcones have also been used to produce vinylcyclopropanes (entries 3-8) with reasonable success. 

The 1,4-addition of carbenes to 1,3-dienes to give cyclopentenes is extremely rare, since 1,2-addition to give a vinylcyclopropane is much more favorable. Unfortunately, vinylcyclopropanes can be converted into cyclopentenes on heating at higher temperature. In special conditions direct 1,4-addition reactions are observed, but only in poor yield (Scheme 5.40). 

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. 

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