Electron Transfer Induced Isomerization of cis-4,4'-Diphenylstilbene into Its Trans Form

Article abstract of DOI:10.1021/ja00529a036

The 4,4'-diphenylstilbenes, denoted by C and T for the cis and trand isomers, as well as T^-. and T^2-, were characterized spectroscopically and electrochemically.The disproportionation 2T^-.,Na⁺ <*> T + T^2-,2Na⁺ has equilibrium constant in THF 0.04, ΔH = 18.2 kcal/mol, and ΔS = 54 cal/mol<*>deg, and its rate constant is 9.0 <*> 10⁸ M^-1 s^-1.The cis-trans isomerization was studied in THF at ambient temperature in three systems: T + T^-.,Na⁺ + C, T^2-,2Na⁺ + C, and B^-.,Na⁺ + C, B denoting biphenyl.In the first two systems the reaction is governed by the three interrelated equilibria, namely, C + T^-.,Na⁺ <*> T + C^-.,Na⁺ (K₁), C + T^2-,2Na⁺ <*> C^-.,Na⁺ + T^-.,Na⁺, (K₂ = K₁/K_dispr), and 2T^-.,Na⁺ <*> T + T^2-,2Na⁺ (K_dispr), the rate-determining step being C^-.,Na⁺ -> T^-.,Na⁺ (k_i).The results give k_iK₁ or k_iK₂.In the last system the rapid electron transfer B^-.,Na⁺ + C -> B + C^-.,Na⁺ produces momentarily high concentration of C^-.,Na⁺ and, since its disproportionation is favored, a relatively large amount of C^2-,2Na⁺.The latter rapidly isomerizes into T^2-,2Na⁺.Thus, in early stages of the reaction, the concentration of T^2-,2Na⁺ exceeds that expected for the disproportionation equilibrium had the reaction with B^-.,Na⁺ formed only C^-.,Na⁺ and then T^-.,Na⁺.This observation provides the evidence for the existence of C^2-,2Na⁺.