Direct Observation and Reactions of Organic Biradicals Generated from Novel Bicyclic Azoalkanes

Citation

Jain, Rakesh (1987) Direct Observation and Reactions of Organic Biradicals Generated from Novel Bicyclic Azoalkanes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/z16r-7e62. https://resolver.caltech.edu/CaltechTHESIS:02252019-123337280

Abstract

Thermolysis of 2,3-diazabicyclo[2.1.1]hex-2-ene (14) in the gas phase produces highly vibrationally excited (chemically activated) bicyclo[1.1.0]butane (30). The excited 30 rearranges to butadiene, in competition with collisional deactivation by a bath gas. Quantitative modeling using RRKM theory and a stepladder model for collisional deactivation indicates that of the substantial amount of excess energy available to the products of thermolysis of 14, the great majority lies in the hydrocarbon fragment (30). Within the framework of a mechanistic criterion developed previously by Bauer, this result suggests that 14 decomposes by a stepwise, one-bond cleavage involving an intermediate diazenyl biradical. We have also synthesized 16, the dimethyl derivative of 14, and studied its thermal decomposition. As predicted by RRKM calculations, gas phase thermolysis of 16 does not produce chemically activated 1,3-dimethylbicyclobutane (31).

Irradiation of frozen solutions of 1,4-dialkyl-2,3-diazabicyclo[2.1.1]hex-2-enes (16-19) in the cavity of an ESR spectrometer between 4 and 25 K produces the corresponding triplet 1,3-dialkyl-1,3-cyclobutanediyls (20-23). The spectra display zero-field splitting parameters, |D/hc| = 0.112 cm^-1 and |E/hc| = 0.005 cm^-1. Hyperfine coupling can be resolved in some of these spectra and can be simulated using a computer program which has been developed for the purpose. The decay of the ESR signals between 4 and 25 K is nonexponential and shows very little dependence on temperature. A novel approach for analyzing the matrix-site effect is used and evidence is provided for quantum-mechanical tunneling in the decay process.

Matrix isolation photolysis of 2,3-diazabicyclo[2.2.1]hept-2-ene-7 –spiro-cyclopropane (24) and 2,3 diazabicyclo[2.2.1]hept-2-ene-7,5'-spirobicyclo[2.1.0]pentane (25) between 4 and 35 K produces triplet ESR signals which are thermally stable up to 150 K. Independent generation from direct precursors 7-ethylidene-2,3-diazabicyclo[2.2.1 ]hept-2-ene (71) and 7-cyclobutylidene-2,3-diazabicyclo[2.2.1]hept-2-ene (72) confirms the assignment of the ESR signals to triplet biradicals 2-ethylidene-1,3-cyclopentanediyl (28) and 2-cyclobutylidene-1,3-cyclopentanediyl (29), respectively. Interpretable hyperfine coupling is observed and can be simulated, providing direct information about the electronic structures of these biradicals. The mechanism proposed for the formation of 28 and 29 from 24 and 25 involves two biradical rearrangements; a cyclopropylcarbinyl-type ring opening followed by a 1,2-H shift. Neither of these rearrangements can occur via thermal activation at such low temperatures. Chemical activation and quantum-mechanical tunneling are proposed to explain the reactivity of the intermediary biradicals.

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