Synthesis, spectroscopy and reactivity of phenyl-substituted 1,3-biradicals
Abstract (Summary)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 [delta]ms = 2 transition of (3)24-Ph reveals that the four-membered ring is planar. Upon warming biradicals (3)24-Ph and (3)24-MP to 20-50 K, nonexponential decay to the corresponding bicyclobutanes commence. The decay kinetics of (3)24-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, EaÂ° = 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 (3)20 is considerably more persistent than its four-membered ring analog (3)24-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 [...] = 12.2 kcal/mol, [...] = -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 (1)20, (3)20, and 22.
Advisor:Dennis A. Dougherty
School Location:USA - California
Source Type:Master's Thesis
Date of Publication:05/05/1989