1.1- dichloro-2,2-difluoroethylene, addition

Nonfluonnated allenes also readily react with fluoroalkenes to give diverse fluonnated alkylidenecyclobutanes [727, 12S, 129, 130] (equations 55 and 56), except for tetramelhylallene, which rearranges to 2,4-dimethyl 1,3-pentadiene under the reaction conditions prior to cycloaddition (equation 57) Systematic studies of l,l-dichloro-2,2-difluoroethylene additions to alkyl-substituted allenes establish a two-step, diradical process for alkylidenecyclobutane formation [131, 132, 133]... 

The reaction of dihalocarbenes with isoprene yields exclusively the 1,2- (or 3,4-) addition product, eg, dichlorocarbene CI2C and isoprene react to give l,l-dichloro-2-methyl-2-vinylcyclopropane (63). The evidence for the presence of any 1,4 or much 3,4 addition is inconclusive (64). The cycloaddition reaction of l,l-dichloro-2,2-difluoroethylene to isoprene yields 1,2- and 3,4-cycloaddition products in a ratio of 5.4 1 (65). The main product is l,l-dichloro-2,2-difluoro-3-isopropenylcyclobutane, and the side product is l,l-dichloro-2,2-difluoro-3-methyl-3-vinylcyclobutane. When the dichlorocarbene is generated from CHCl plus aqueous base with a tertiary amine as a phase-transfer catalyst, the addition has a high selectivity that increases (for a series of diolefins) with a decrease in activity (66) (see Catalysis, phase-TRANSFEr). For isoprene, both mono-(l,2-) and diadducts (1,2- and 3,4-) could be obtained in various ratios depending on which amine is used. 

Treatment of O-acyl esters (2) with l,l-dichloro-2,2-difluoroethylene provides a,a-difluorocarboxylic acids (37) through photolysis, followed by the hydrolysis of the adducts (36) with AgN03 (eq. 8.17) [53]. Eq. 8.18 shows the preparation of a-keto carboxylic acids (40) from carboxylic acids, by means of the radical addition to ethyl acrylate, oxidation to the sulfoxides by mCPBA, the Pummerer reaction with... 

The product ratios observed for additions between allene and chlorotri-fluoroethylene or l,l-dichloro-2,2-difluoroethylene 126>, 85 15 in favor of the 3-chloro isomer and 95 5 for the 3,3-dichloro adduct, can t be interpreted unambiguously. If both adducts in a set stem from a common type of intermediate, then the product ratios indicate how different that intermediate is from the sorts of diradicals implicated in cydoadditions between, say, l,l-dichloro-2,2-difluoroethylene and the 2,4-hexadienes 16J. But the two adducts may arise throu gh completely independent, dissimilar reaction mechanisms. 

With CF3C=CCF3 or olefins with an internal double bond such as cis- or fraras-F-2-pentene, F-cyclobutene, or l,2-dichloro-l,2-difluoro-ethylene, or with a geminally disubstituted olefin such as 1,1-dichloro-2,2-difluoroethylene, only fully fluorinated products were obtained, likely through a nucleophilic fluorination mechanism. This fact, plus the necessity for a Lewis acid catalyst for addition to proceed, is evidence for an electrophilic addition mechanism. Others have suggested polar addition of BF3 to the olefin with RfBF2 as a reactive intermediate (294). 

Bartlett et al. found that in sealed tubes at 80° with excess l,l-dichloro-2,2-difluoroethylene and a polymerization inhibitor, butadiene, cM-piperylene, and rru/ii-piperylene each yielded a single 1,2-addition product. Wherever the structures have been established they conform to type 1 and not II. 

In the 1,2-cycloaddition of the trans-trans, trans-cis, and cis-cis isomers of 2,4-hexadiene to l,l-dichloro-2,2-difluoroethylene, under conditions of stereochemical stability of reactants and products, it has been ascertained that there is cis-trans isomerisation at the site of addition, and retention of configuration only on the side chain . An equilibrium composition of the product is approached but not attained, proving that in the open-chain (diradical) intermediate there is competition between ring closure and internal rotation around the C-C bond at the reaction site, the latter process being about 10 times faster than the former, at 80°C . 

We noted in this chapter that nucleophilic addition to alkenes occurs when groups capable of electron withdrawal by both induction and resonance are attached to the alkene. The addition of alkoxide anions to 2,2-dichloro-l,l-difluoroethylene occurs on the carbon with the fl uorines. Why does the addition occur to place the negative charge of the carbanion intermediate on the carbon with the chlorines, rather than on the carbon with the more electronegative fluorines ... 

Organolithiums add onto tetrafluoroethylene and eject lithium fluoride far more effectively than Grignard reagents do (Scheme 1-120). With chlorotrifluoro-ethylene and l,l-dichloro-2,2-difluoroethylene as the substrates, the new carbon-carbon bond is established at the chlorine-remote end of the double bond. The element-specific orientation rules out any S v2-like direct substitution (that inevitably would displace the heavier halogen) and argues in favor of an addition/elimination mechanism passing through the ephemeral species 166 (Scheme 1-120). 

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