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Vinylic Substitution of 1,2-Dibromo-1,2-difluoroethylene and Tribromofluoroethylene. An Intramolecular kBr/kF Element Effect and Apparent Inversion of Configuration in SNV Reactions
Shainyan, Bagrat A.,Rappoport, Zvi
, p. 3421 - 3428 (2007/10/02)
The reactions of (E/Z)-1,2-dibromo-1,2-difluoroethylene(1) and of tribromofluoroethylene (2) with alkoxide ions and of 1 with p-toluenethiolate 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 displacement, together with methyl dimethoxyacetate (3), methyl bromofluoroacetate (4), 1,1,2-trifluoro-2-bromoethyl ether (7), and 1,1-difluoro-1,2,2-trimethoxyethane (8).With 2 equiv of MeO(1-) 3 and 4 are the main products, and at 130 deg C, dimethyl ether 5 is also formed.With EtOCH2CH2O(1-) 1 gave 2-ethoxyethyl bromofluoroacetate (9), bis(2-ethoxyethyl) ether (10), and E/Z mixtures of the substitution products EtOCH2CH2OC(F)=C(F)Br (12) and EtOCH2CH2OC(Br)=C(F)Br (13).Reaction of 2 with excess RO(1-) (R = Me, Et) gives alkyl dibromoacetates, while with 1 equiv of RO(1-) only a bromine from the =C(F)Br carbon is displaced.Reaction of 1 with p-TolSNa in MeOH gives the reduction-substitution product p-TolSC(F)=CHF (18), together with (P-TolS)2 (16) and p-TolSMe (17).The same reaction in DMSO gives E/Z mixtures of the product of displacement of one bromine (19) or two bromines (20).Formation of the products is rationalized by an initial nucleophilic attack on the vinylic carbon followed by leaving group expulsion, giving, e.g., 12, 13, 19, or 20.Hydrolysis of the intermediate or addition of HF to the initial substition product gives saturated products, e.g., 3, 4, 7, or 8, while SN2 reactions on the ether oxygen give ethers 5 and 10.A bromophilic reaction gives the reduction-substitution product 18, while hydrolysis-decarboxylation leads to 17.The regiospecificity of the nucleophilic addition is due to polar and hyperconjugative effects.An intramolecular element effect kBr/kF of > 10 is reported for the first time in the reaction of 1 with EtOCH2CH2O(1-).This value and the absence of such effects in other reactions are consistent with a much higher nucleofugality from a (1-)CC(F)Br system of Br(1-) compared with F(1-).The E/Z compositions of 18-20 indicates an apparent inversion in their formation, but it is not known whether these compositions are thermodynamically or kinetically controlled.
