Reaction of 1,1-Dichlorodifluoroethylene with 1,3-Butadiene

The second problem in the addition of 1,1-dichlorodifluoroethylene to 1,3-butadiene is regiospecificity. Carbon 1 of 1,3-butadiene may become attached either to the carbon holding two chlorines or to the carbon with two fluorines. The cycloaddition of fluorinated alkenes is usually not a concerted four-center reaction in which the bonds are formed simultaneously or nearly so. Instead, the reaction is a stepwise biradical process in which the first step is formation of a free-radical intermediate with a single electron at that end of the double bond that can better accomodate it. That happens at the carbon linked to two chlorine atoms. Thus, a biradical is formed that cyclizes to form 2,2-dichloro-l,l-difluoro-3-vinylcyclobutane M [118, 119, 720]. 

At the temperature of 180-200°C, perfluoropropylene oxide ejects diflu-orocarbene and forms N, trifluoroacetyl fluoride (general method for the preparation of difluorocarbene). Difluorocarbene adds to 1,2-dichlorodi-fluoroethylene and yields 0,1,2-dichloroperfluorocyclopropane [121]. 

As in Explanation 89, these two compounds do not react in the sense of the Diels-Alder reaction. Instead, a cyclobutane derivative is formed by cycloaddition. The reaction is a free-radical process starting with single electrons on carbons 1 and 2 of perfluorovinylsulfur pentafluoride. Carbon 1 of the 1,3-butadiene joins the difluoromethylene group of the perfluorovinylsulfur pentafluoride because in this way, the newly formed biradical can better accommodate the single electron on the carbon next to the SF5 group. The biradical closes the ring in such a way that two stereoisomers, P and Q, cis- and trans-2,3,3-trifluoro-l-vinyl-2-cyclobutyl-sulfur pentafluoride, are formed in equal amounts [122]. 

Free radicals with single electrons on the carbons of the trifluoroethylene combine with the double bond of butadiene in both possible ways carbon of the difluoromethylene joins either the terminal carbon of the butadiene or the internal end of the double bond of the butadiene. The biradicals thus formed cyclize to R, 2,3,3-trifhioro-l-vinylcyclobutane, and S, 2,2,3-trifluoro-l-vinylcyclobutane, both as 1 1 mixtures of cis- and trans-isomers. 

In addition, a small amount of a six-membered ring compound is formed by the Diels-Alder reaction as a third product to give T, 4,4,5-tri-fluoro-1-cyclohexene. It is likely that both concerted and biradical mechanisms are involved [123]. 

---