Collect. Czech. Chem. Commun. 1993, 58, 882-892

Sigmatropic Isomerization of Tetraphenylcyclopentadienes: Reaction Mechanism and Quantum Chemical Treatment

Pavel Šebeka, Petr Sedmerab, Stanislav Böhma and Josef Kuthana

a Department of Organic Chemistry, Prague Institute of Chemical Technology, 168 28 Prague 6, Czech Republic
b Institute of Microbiology, Academy of Sciences of the Czech Republic, 142 20 Prague 4, Czech Republic


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 IIaIIIa 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.