Part I. Thermal Rearrangements of Small Ring Hydrocarbons. Part II. Photosensitized Rearrangements of Small Ring Hydrocarbons - Nonvertical Energy Transfer

Citation

DeBoer, Charles David (1966) Part I. Thermal Rearrangements of Small Ring Hydrocarbons. Part II. Photosensitized Rearrangements of Small Ring Hydrocarbons - Nonvertical Energy Transfer. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ffvv-9988. https://resolver.caltech.edu/CaltechETD:etd-02092009-135708

Abstract

PART I

The thermal rearrangements of cis- and trans-1, 2-divinylcyclobutane, cis- and trans-8-vinylbicyclo(4, 2, 0)-2-octene, trans1, 2-dimethy1-1, 2-divinylcyclobutane and trans-1-isopropeny1-2- methyl-2-vinylcyclobutane have been studied in detail. The kinetic parameters indicate that the rearrangements of the cis isomers occur by way of a concerted transition state while the trans isomers rearrange through a diradical intermediate. Optically active trans-1, 2-divinylcyclobutane undergoes racemization as well as rearrangement, and produces 4-vinylcyclohexene having a trace of residual optical activity. Consequently we infer that the rates of ring closure in the intermediate are of the same order of magnitude as the rates of internal rotation about single bonds.

We have also studied the rates for the reversible isomerization of cis- and trans-1, 2-diphenylcyclopropane. Here the concerted mechanism is not possible for the cis isomer; both isomers appear to go through a diradical intermediate.

PART II

We have observed the reversible photosensitized isomerization of cis- and trans-1, 2-diphenylcyclopropane. In contrast to the direct irradiation of diphenylcyclopropane, which gives several products, the sensitized reaction is very clean, giving only cis-trans isomerization. Comparison of the product ratios from the photosensitized decomposition of 3, 5-dipheny1-1-pyrazoline with the intercepts of Stern-Volmer plots for the sensitized isomerization of diphenylcyclopropane indicates that the same rotationally equilibrated species, 1, 3-diphenyltrimethylene, is produced in both cases. Apparently the triplet state of the diradical undergoes cis-trans equilibration before ring closure, while the singlet state, produced by either thermal decomposition or direct irradiation of the pyrazoline, does not. We infer from this that the rate of spin inversion is slower than the rate of internal rotation.

The quantum yields for the reaction with many different sensitizers were measured, and the rate constants for the energy transfer step calculated from these. These rate constants show an approximate correlation with the triplet energy of the sensitizer.

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