Reactivity of vinyl sulphonic esters. Part XVI. Solvolytic reactivity of β-halogenovinyl derivatives

Abstract

The solvolytic reactivity of trans-(I) and cis-(II)β-halogenovinyl 2,4,6-trinitrobenzenesulphonates, R²(Hal)C:C(O₃SAr)R¹[(I; Hal = Cl, Br, or I, R¹,R²= alkyl or aryl), (II; Hal = Cl or Br, R¹,R²= aryl)], has been investigated in acetic acid and in nitromethane–methanol. The kinetics and products indicate that the solvolysis in both solvents proceeds by S_N1 and S_N1–E1 mechanisms in the case of some β-iodovinyl derivatives. The reactivity of esters (I) strongly depends on the nature of the halogen and follows the order IBr > Cl. The trans-esters (I; R¹,R²= aryl) are more reactive than the corresponding cis-isomers when Hal = Br [k_(I)/k_(II)= 10–15] and almost equally reactive when Hal = Cl. The substitution products from (I; Hal = I) retain the trans-geometry, whereas the substitutions of esters (I) and (II)(R¹,R²= aryl), when Hal = Br or Cl, are not stereospecific. Anchimeric assistance due to iodine and, to a much lesser extent, to bromine are suggested to be involved. The results are also discussed in terms of bridged versus open geometry for the intermediate vinyl cations and compared with those for electrophilic additions of halogens to the CC triple bond.