Thermolysis of polyazoalkanes and the radical stabilizing effect of azo and azoxy groups

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Title: Thermolysis of polyazoalkanes and the radical stabilizing effect of azo and azoxy groups
Author: Pan, Li
Advisor: Engel, Paul S.
Degree: Doctor of Philosophy thesis
Abstract: An unique linear aliphatic trisazoalkane was synthesized. Its thermolysis proceeded stepwise to yield seven fragments, with an overall reaction enthalpy of -93.6 kcal/mol, 66.6 kcal/mol lower than that for azo-t-butane. Due to the stepwise feature, the difference in overall reaction enthalpy resulted in only an 8.6 fold rate increase for the trisazoalkane relative to azo-t-butane at 153.46°C. Photolysis of a novel cyclic azimine obtained during the synthesis of the trisazoalkane yielded a highly strained triaziridine and an unusual triazane. The triaziridine rapidly reverted to the azimine with a rate constant of 2.05 x 10-4 s-1 at 24.4°C. Arylmethylenecyclopropanes with para azo, azoxy and other related N-containing substituents were synthesized to study radical stabilizing effects. The order of increasing radical stabilizing ability was determined to be -N(O)=NBu-t, -N3, CH=NBu-t, -N=NBu-t, -NH2, -CH=NOMe, -CH=NNMe 2, -N=N(O)Bu-t, -N=NPh and -CH=N(O)Bu-t. The last three groups, azoxy, nitrone and phenylazo, were the best non-ionic radical stabilizers ever studied. The large rate enhancements in the thermal rearrangement resulted from radical stabilization through spin delocalization by the para-substituents in the transition state. A cumyl type 1,3,5-trisazoalkane and its monoazo, para-bisazo and meta-bisazo analogs were synthesized. Thermolysis of C3 and C4 followed a stepwise mechanism; that is, the two azo groups decomposed sequentially. Mathematical models were developed for analyzing the kinetics data. The rate constants for these azoalkanes increased statistically with the number of -N=N-groups. In addition to the stepwise mechanism, the para-bisazo compound also decomposed concertedly to a quinodimethane. To demonstrate that the 1,3,5-triazoalkane could serve as a radical initiator, different trapping products were made by thermolysis of the 1,3,5-trisazoalkane in the presence of PhSH and nitroxyl radicals. In the binary system of C4 and TEMPO, low polydispersity (1.13) was obtained for polystyrene.
Citation: Pan, Li. (2000) "Thermolysis of polyazoalkanes and the radical stabilizing effect of azo and azoxy groups." Doctoral Thesis, Rice University.
Date: 2000

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