deposit_your_work

Thermolysis and photolysis of bifunctional initiators: Free radical centers beta to the azo, azoxy and perester groups

Files in this item

Files Size Format View
9514248.PDF 4.479Mb application/pdf Thumbnail

Show full item record

Item Metadata

Title: Thermolysis and photolysis of bifunctional initiators: Free radical centers beta to the azo, azoxy and perester groups
Author: Wu, Aiyin
Advisor: Engel, Paul S.
Degree: Doctor of Philosophy thesis
Abstract: Peresters $\beta$ to azo or azoxy groups are potentially interesting bifunctional initiators. They decompose thermally at essentially the same rate as the analogous simple perester. It follows that decomposition occurs stepwise; that is, the observed rate constants are determined by the rate of oxygen-oxygen bond cleavage. Using 1,4-cyclohexadiene as a radical scavenger, we found that the lifetimes of $\beta$-azo radicals 59B and 59P and $\beta$-azoxy radicals 62B and 62P were 0.4 ns, 1.3 ns (110$\sp\circ$C), 2.4 ns and 6.6 ns (120$\sp\circ$C), respectively. The nature of the products generated on thermolysis of azoxyperesters depended on the position of oxygen and on the substituents on the azoxy group. tert-Butyl 3-tert-butylazoxy-NNO-3-methylperbutanoate (9B) afforded nitrous oxide in nearly 80% yield, showing that the bonds break in the same pattern as those of azo peresters. For tert-butyl 3-phenylazoxy-NNO-3-methylperbutanoate (9P), the NNO fragment appeared as nitrogen and water instead of nitrous oxide because the (phenylazo)oxyl $\pi$-radical (63P) is relatively stable to fragmentation. For tert-butyl 3-tert-butylazoxy-ONN-2-methylperbutanoate (8B) and tert-butyl 3-tert-butylazoxy-ONN-2,2-dimethylperpropanoate (28B), the $\beta$-azoxy radical is proposed to attack the azoxy group to form an aziridine substituted nitroxide (86). This unstable species undergoes ring opening to afford primary and tertiary $\beta$-azoxy radicals (82, 83) which were trapped with 1,4-cyclohexadiene. The rate constant of 82 going to 83 is 1.5 $\times$ 10$\sp7$ s$\sp{-1}$ and from 83 to 82, it is 1.5 $\times$ 10$\sp3$ s$\sp{-1}$ at 120$\sp\circ$C. Only trans to cis isomerization of the azo group could be observed upon 405 nm irradiation of tert-butyl 3-phenylazo-3-methyperbutanoate (2P) at room temperature. However, 366 nm irradiation of tert-butyl 3-tert-butylazo-3-methylperbutanoate (2B) at room temperature generated $\beta$ perester radicals, which might be trapped by a radical scavenger. In order to obtain the lifetime of the $\beta$-perester radical, tert-butyl 3-tert-butylazo-3-cyclopropylperbutanoate 43 was synthesized and by using the cyclopropylcarbinyl (CPC) rearrangement as a radical clock, the lifetime of 44 was determined as 200 ns at 25$\sp\circ$C.
Citation: Wu, Aiyin. (1994) "Thermolysis and photolysis of bifunctional initiators: Free radical centers beta to the azo, azoxy and perester groups." Doctoral Thesis, Rice University. http://hdl.handle.net/1911/16793.
URI: http://hdl.handle.net/1911/16793
Date: 1994

This item appears in the following Collection(s)