SIGMATROPIC ISOMERIZATION OF TETRAPHENYLCYCLOPENTADIENES - REACTION-MECHANISM AND QUANTUM-CHEMICAL TREATMENT

Thermal isomerizations of 2.3.5.5-tetraphenylcyclopenta-1,3-diene (Ia) or 1,2.4.5-tetraphenylcyclopenta-1,3-diene (IIa) in melts lead to ide ntical 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 besi des minority isomers IIb, IIc. On the contrary, the 1,2,4,5-tetrasubst ituted phenanthrene hydrocarbon IXa having forced coplanarity of both benzene rings only partially isomerizes to IXb. Equilibrium constants of the isomerization IIa half arrow right over half arrow left IIIa in diphenyl ether at 323 to 433 K were measured (K = 5.8 - 2.3). Relativ e stability of hydrocarbons Ia, IIa, IIIa, and VIIa is interpreted on the basis of quantum chemical AM1 calculations. The time course of iso merization of the hydrocarbon IIc allowed us to propose two reaction m echanisms based on the [1,3]- and [1.5]-sigmatropic steps. The prepara tion of 2,3,5.5-tetrasubstituted cyclopentadienes Ia - Ic by dehydrati on of cis-1.2-diols IVa - IVc is described.