VINYLIC SUBSTITUTION OF 1,2-DIBROMO-1,2-DIFLUOROETHYLENE AND TRIBROMOFLUOROETHYLENE - AN INTRAMOLECULAR KAPPA(BR) KAPPA(F) ELEMENT EFFECT AND APPARENT INVERSION OF CONFIGURATION IN SNV REACTIONS/

The reactions of (E/Z)-1,2-dibromo-1,2-difluoroethylene (1) and of tri bromofluoroethylene (2) with alkoxide ions and of 1 with p-toluenethio late ion give multiplicity of products. The reaction of 1 with 1 equiv of NaOMe gives mainly a 2:1 mixture of the product of one bromine dis placement, together with methyl dimethoxyacetate (3), methyl bromofluo roacetate (4), 1,1,2-trifluoro-2-bromoethyl ether (7), and 1,1-difluor o-1,2,2-trimethoxyethane (8). With 2 equiv of MeO- 3 and 4 are the mai n products, and at 130-degrees-C, dimethyl ether 5 is also formed. Wit h EtOCH2CH2O- 1 gave 2-ethoxyethyl bromofluoroacetate (9), bis(2-ethox yethyl) ether (10), and E/Z mixtures of the substitution products EtOC H2CH2OC(F)=C(F)Br (12) and EtOCH2CH2OC(Br)=C(F)Br (13). Reaction of 2 with excess RO- (R=Me, Et) gives alkyl dibromoacetates, while with 1 e quiv of RO- only a bromine from the =C(F)Br carbon is displaced. React ion of l with p-TolSNa in MeOH gives the reduction-substitution produc t p-TolSC(F)=CHF(18), together with (p-TolS)2(16) and p-TolSMe (17). T he same reaction in DMSO gives E/Z mixtures of the product of displace ment of one bromine (19) or two bromines (20). Formation of the produc ts is rationalized by an initial nucleophilic attack on the vinylic ca rbon followed by leaving group expulsion, giving, e.g., 12, 13, 19, or 20. Hydrolysis of the intermediate or addition of HF to the initial s ubstitution product gives saturated products, e.g., 3,4,7, or 8, while S(N)2 reactions on the ether oxygen give ethers 5 and 10. A bromophil ic reaction gives the reduction-substitution product 18, while hydroly sis-decarboxylation leads to 17. The regiospecificity of the nucleophi lic addition is due to polar and hyperconjugative effects. An intramol ecular element effect k(Br)/k(F) Of >10 is reported for the first time in the reaction of 1 with EtOCH2CH2O-. This value and the absence of such effects in other reactions are consistent with a much higher nucl eofugality from a -CC(F)Br system of Br- compared with F-. The E/Z com positions of 18-20 indicates an apparent inversion in their formation, but it is not known whether these compositions are thermodynamically or kinetically controlled.