Thermal isomerization of 2,3,5,5-tetraphenylcyclopenta-1,3-diene (Ia) or 1,2,4,5-tetraphenylcyclopenta-1,3-diene (IIa) in melts lead to identical equilibrium mixtures of hydrocarbon IIa with prevailing 1,2,3,4-tetraphenylcyclopenta-1,3-diene (IIIa). The isomerization of bis-tert-butyl cyclopentadienes Ib or Ic produces an equilibrium mixture with preponderant 1,2,3,4-tetrasubstituted cyclopentadienes IIIb, IIIc besides minority isomers IIb, IIc. On the contrary, the 1,2,4,5-tetrasubstituted phenanthrene hydrocarbon IXa having forced coplanarity of both benzene rings only partially isomerizes to IXb. Equilibrium constants of the isomerization IIa → IIIa in diphenyl ether at 323 to 433 K were measured (K = 5.8 -2.3). Relative stability of hydrocarbons Ia, IIa, IIIa and VIIa is interpreted on the basis of quantum chemical AM1 calculations. The time course of isomerization of the hydrocarbon IIc allowed us to propose two reaction mechanisms based on the [1,3]- and [1,5]-sigmatropic steps. The preparation of 2,3,5,5-tetrasubstituted cyclopentadienes Ia - Ic by dehydration of cis-1,2-diols IVa - IVc is described.
2,3,5,5-四苯基环戊-1,3-二烯(Ia)或1,2,4,5-四苯基环戊-1,3-二烯(IIa)在熔融状态下的热异构化导致与主要的1,2,3,4-四苯基环戊-1,3-二烯(IIIa)相同的平衡混合物。叔丁基环戊二烯(Ib)或(Ic)的异构化产生与主要的1,2,3,4-四取代环戊二烯(IIIb,IIIc)的平衡混合物,除了少量异构体(IIb,IIc)。相反,具有两个苯环强制共面性的1,2,4,5-四取代菲烃(IXa)只部分异构为IXb。在323至433K的二苯醚中测量了异构化IIa→IIIa的平衡常数(K=5.8-2.3)。通过量子化学AM1计算解释了烃Ia、IIa、IIIa和VIIa的相对稳定性。烃IIc的异构化时间过程使我们能够提出基于[1,3]-和[1,5]-迁移步骤的两种反应机制。描述了通过脱水法制备2,3,5,5-四取代环戊二烯Ia-Ic的方法,通过对顺式-1,2-二醇IVa-IVc的脱水。