Synthesis, Spectroscopy and Reactivity of Phenyl-Substituted 1,3-Biradicals

Citation

Coms, Frank D. (1989) Synthesis, Spectroscopy and Reactivity of Phenyl-Substituted 1,3-Biradicals. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/1xkp-ft85. https://resolver.caltech.edu/CaltechETD:etd-05302007-152726

Abstract

Irradiation of frozen MTHF solutions of 1,4-diphenyl- and l-methy-4-phenyl-2,3-diazabicyclo[2.l.l]hex-2-enes(14 and 15) at cryogenic temperatures produces the corresponding triplet 1,3-diphenyl- and l-methyl-3-phenylcyclobutanediyls (24-Ph and 24-MP) as monitored by EPR spectroscopy. The spectra of 24-Ph and 24-MP exhibit zero-field splitting (zfs) parameters of |D/hc| = 0.060 cm⁻¹, |E/hc| = 0.001 cm⁻¹ and |D/hc| = 0.0082 cm⁻¹, |E/hc| = 0.003 cm-1, respectively. The triplet EPR spectra provide valuable information concerning biradical geometry and spin distribution. The trends in the observed D values can be quantitatively modeled using a relatively simple scheme for zfs calculations. Analysis of the hyperfine splitting pattern observed in the Δmₛ = 2 transition of ³24-Ph reveals that the four-membered ring is planar.

Upon warming biradicals ³24-Ph and ³24-MP to 20-50 K, nonexponential decay to the corresponding bicyclobutanes commence. The decay kinetics of ³24-Ph were quantitatively studied from 27 to 54 K with the explicit inclusion of matrix-site effects. The kinetics analysis revealed that the variation of the decay rates with temperature follows the Arrhenius law, producing activation parameters of log A = 7.8, Eₐ° = 2.29 kcal/mol. These activation parameters are compared with those of other cyclobutanediyls.

Photolysis of matrix-isolated 1,4-diphenyl-2,3-diazabicyclo[2.2.1]-hept-2-ene (21) in the cavity of an EPR spectrometer at 3.8 and 77 K produces the triplet spectrum of 1,3-diphenylcyclopentanediyl 20. The zfs parameters of the spectrum are |D/hc| = 0.045 cm⁻¹, |E/hc| = 0.001 cm⁻¹. The observed zfs parameters and hyperfine splitting pattern are completely in line with expectation and are consistent with a planar cyclopentanediyl geometry. Biradical ³20 is considerably more persistent than its four-membered ring analog ³24-Ph.

1,4-Diphenylbicyclo[2.l.0]pentane (22), the closed-shell isomer of 20, possesses a very weak C1-C4 bond. This weak bond is manifested in the low activation barrier for a degenerate bridge-flip process. Through a combination of ¹H NMR complete lineshape analysis and magnetization transfer studies, the activation parameters for this process are determined to be ΔH^‡ = 12.2 kcal/mol, ΔS^‡ = -16.4 eu. At room temperature, bicyclopentane 22 reacts rapidly to form 2,3-dioxa-1,4-diphenylbicyclo[2.2.l]heptane(23). Using the activation parameters for the bridge-flip reaction along with other data results in the development of a detailed model of the kinetic and thermodynamic relationships among ¹20, ³20, and 22.

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