Study of the kinetics of the [1,5]-sigmatropic phenyl rearrangement in 3,4-bis(para-substituted phenyl)-1,2,5-triphenyl-2,4-cyclopentadien-1-ols

Abstract

A kinetic study of the suprafacial [1,5]-sigmatropic phenyl rearrangements of 3,4-bis- (para-substituted)-1,2,5-triphenyl-2,4- cyclopentadien-1-ols to 3,4-bis-(para-substituted)-2, 2,5-triphenyl-3-cyclopenten-1-ones, where the substituent were, t-butyl, methyl, hydrogen, chloro and bromo, has been performed at 180, 190, 200 and 210 ± 0.2° in tetraethylene glycol. The rearrangement was observed to be first order throughout the temperature range investigated, and the rate constants (k) at the temperatures used were found to be 8.057, 2.143, 2.930 and 6.456 x 10⁻⁴ sec⁻¹ for the t-butyl substituent; 4 7.700, 2.110, 3.434, and 6.581 x 10⁻⁴ sec⁻¹ for the methyl substituent; 0.909, 2.166, 3.678 and 6.448 x 10⁻⁴ sec⁻¹ for the unsubstituted alcohol; 1.469, 2.992, 5.989 and 11.12 x 10⁻⁴ sec⁻¹ for the chloro substituent; and 1.955, 3.215, 6.215 and 13.01 x 10⁻⁴ sec⁻¹ for the bromo substituent, respectively. . Calculations of the activation energy of this phenyl [1,5]-sigmatropic shift from the Arrhenius equation give values of 28.71, 30.31, 28.01, 29.38 and 27.49 kcal·mol⁻¹ for the above substituents respectively, while ΔH‡ these phenyl migrations were 27.78, 29.39, 27.93, 28.45 and 26.56 kcal·mol⁻¹ respectively. ΔS‡ values were also calculated to be -16.6, -13.1, -16.1, -14.0 and -17.8 eu, respectively for a mean temperature range of 453-483 ± 0.2°K. The isokinetic temperature was calculated to be 320°. Calculated rho (ρ) values for the sigmatropic migrations were -0.813, -0.400, -0.690 and -0.658 at 180, 190, 200 and 210 ± 0.2°C. These results are used to discuss both the mechanism of this rearrangement and the transition state for rearrangements in the pentaphenylcyclopentadienol system. Ina much broader sense these results and the results of previously documented sigmatropic rearrangements exhibiting electronic influences were used to formulate a general scheme to predict the outcome of those sigmatropic rearrangements that entail an electronic effect as seen in the rates of the above reactions. This scheme used as its basis the transition state approach invoking inductive and mesomeric arguments to rationalize the observed rates of the sigmatropic rearrangements of this dissertation, those of previously documented sigmatropic rearrangements and finally to predict a relative rate of rearrangements in general. Finally a mass spectral investigation of the fragmentation patterns of 1-para-phenylsubstituted 2,3,4,5-tetraphenyl-2,4-cyclopentadien-l-ols and 3,4-bis(para-substituted phenyl)-1,2,5-triphenyl-2,4- cyclopentadien-1-ols was undertaken. A continuum of two super imposable pathways with the choice of the major decomposition mode being determined by the electron donating or withdrawing ability of the substit- vent was established. Linear free energy relationships for mass spectral decomposition were presented as well as a justification for this behavior.