Vinylcyclopropane derivatives radical ring-opening

Figure 4. Radical ring-opening polymerization of vinylcyclopropane derivatives.

MO calculations have been carried out on the isomerization of cyclopropane to propene, and the MNDO method has been used to study the reaction pathway and to optimize the structure of reactant, transition structure, and product of the ring opening reaction of bicyclo[1.1.0]butane. Various methods have been employed to estimate the rate constants for ring opening of the 2-cyclopropyl-2-propyl radical. 1-Acceptor-1-sulfenyl-substituted 2-vinylcyclopropanes of the type (430) have been found to afford 6-sulfenyl-a,jS y, -unsaturated carboxylic esters and nitriles (431) upon treatment with acid, by a process which involves C(l)—C(2) bond fission and a novel 1,5-sulfenyl rearrangement (see Scheme 110). It has been shown that the benzophenone-sensitized photolysis of vinyl norcaradiene derivatives, such as 5-(2-methylprop-l-enyl)-3-oxatricyclo[4.4.0.0 ]deca-7,9-dien-4-ones (432), results in the regioselective cleavage of only one of the cyclopropyl c-bonds to afford isochroman-3-one derivatives (433). It has been reported that the major product obtained from the reaction of structurally diverse a-diazo ketones with an electron-rich alkene in the... 

With vinylcyclopropanes thioselenation proceeds via ring-opening of cyclopropyl-carbinyl radicals. An example of the thioselenation of isocyanides by means of a (PhS)2-(PhSe)2 mixed system is shown in Scheme 15.69 the product is a useful building block for TS-lactarn derivatives, e.g. carbacephems. 

Feldman s group was one of the most active in this field, and published several applications of the synthesis of cyclopentanoid compounds via [3 + 2] annulation [60]. Thus, the PhS radical-catalyzed reaction of substituted vinylcyclopropanes with functionalized alkenes affords vinylcyclopentane derivatives (equation (24)). The reaction mechanism is shown in Scheme 9 [60, 61]. Initiation occurs by PhS radical addition to the vinylcyclopropane (step a), followed by three additional steps prior to termination via ejection of the PhS radical to afford the vinylcyclopentane product (step e). The intermediate steps include ring opening to afford the homoallylic radical (step b), bimolecular addition of the alkene to produce the 5-hexenyl radical (step c), and cyclization to cyclopentanyl carbinyl radical (step d). 

The addition of arenesulfonyl derivatives to the double bond of vinylcyclo-propanes leads to 1,5-adducts via a ring opening of cyclopropylcarbinyl radicals. Equation (58) shows an example of copper (I)-catalyzed addition of benzenesul-fonyl chloride to poly substituted vinylcyclopropanes [117]. Similar chemistry has been observed for the addition of selenosulfonates to cyclopropylidene derivatives and cyclopropylacetylene [118]. 

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