Thiocyanate Ion Promoted E ⇌ Z Isomerization of a‐Bromo‐β,β‐Diactivated Electrophilic Olefins: An Apparent High Nucleofugality of NCS−

Abstract

Abstract(E)‐β‐Chloro‐α‐phenylcinnamaldehyde and (E)‐ and (Z)‐methyl α‐tert‐butoxycarbonyl‐p‐nitro‐β‐bromocinnamates in the presence of NCS− ion in acetonitrile undergo an (E) ⇌ (Z) isomerization which is faster than the vinylic substitution. (E)‐ and (Z)‐methyl α‐cyano‐β‐mesyloxy‐ (and β‐chloro‐) p‐nitrocinnamates undergo substitution under similar conditions without preceding isomerization. MO calculations gave the energy differences between the eclipsed and the perpendicular conformers of carbanions −CH2CH2X and (NC)2ČH2X (X = Cl, NCS). These rotational barriers denote the hyperconjugation stabilization energy resulting from interaction of the C‐X bond and the 2p(C−) orbital. Both HCA values are high, those for X = Cl being 15–20% higher than for X = NCS. The limited literature data on the nucleofugality of NCS− suggests that it can have a nucleofugality which is not much lower than that of Cl− or Br−. Consequently, NCS−‐catalyzed isomerizations of electrophilic vinyl chlorides and bromides, which involve a 180° internal rotation followed by a faster expulsion of NCS− than of Cl− from the initially formed intermediate carbanion Y'Y−C‐C(R)(Cl)SCN, are not unreasonable, especially if solvation, which was not included in the calculations, plays an important role in the nucleofuge expulsion process.