Radical Cyclization and Fragmentation of Azoxy Compounds

Abstract

Photolysis of azoazoxyalkane 9 and thermolysis of β-azoxyperester 13 afford β-azoxy radicals 1 and 14, respectively. One reaction pathway of these radicals is cyclization to azoxy oxygen to form cyclic hydrazyl radicals 2 and 16 that fragment to a ketone or aldehyde plus hydrazonyl radical 3. The analogous hydrazyl radical 6 need not be invoked in the case of γ-azoxy radical 5, which instead undergoes a rare solution phase β-scission to lose ethylene. Surprisingly, the same β-scission was found in the 3,3-dimethyl-4-pentenyl radical (34), a hydrocarbon analogue of 5.

An Aliphatic Bifunctional Free Radical Initiator. Synthesis of a Block Copolymer from an Azoperester by Sequential Thermal and Photochemical Initiation

“…Thus, the longer chain in 7 compared to 6 successfully prevented cyclization. The thermal and photochemical reactions of 7 are similar to those of azo and azoxy peresters that we studied earlier 16,23 where disproportionation of the tert-butoxy-alkyl radical pair and formation of tert-butyl ethers were dominant processes. In the thermolysis of 7, the sizable yield of methane indicates substantial fragmentation of the tert-butoxy radical and indeed, when 7 was thermolyzed with styrene, the polymer did not contain any detectable tertbutoxy end groups.…”

“…Turning to the products that were formed, we found that all samples contained acetone, tert -butyl alcohol, ether 16 , carboxylic acid 17 , and an alkene assigned as 18 on the basis of its GC retention time and GC/MS spectrum. These types of products, including carboxylic acids, are typical for peresters studied in this laboratory . Since carboxy radicals lose CO 2 very rapidly, it is unlikely that 17 arises by O−O homolysis of 7 .…”

“…These types of products, including carboxylic acids, are typical for peresters studied in this laboratory. 23 Since carboxy radicals lose CO 2 very rapidly, 24 it is unlikely that 17 arises by O-O homolysis of 7. Instead, we attribute 17 to hydrolysis of 7 by adventitious water, based on our observation that thermolysis of another perester in scrupulously dried benzene greatly reduced the yield of carboxylic acid.…”

Photorearrangement of α-Azoxy Ketones and Triplet Sensitization of Azoxy Compounds

“…Several years ago, we reported that γ-azoxy radical 1 generated from a perester precursor undergoes fragmentation to ethylene with a rate constant below 2 × 10 5 s -1 at 120 °C . Mysteriously, we were unable to find any product attributable to the expected α-azoxy radical 2 , also named hydrazonyl oxide 3 .…”

The free radical chemistry of the azoxy group

“…Thermolysis of 59 gave typical perester products along with a 61% yield of ethylene. 29 This fragmentation, whose rate constant was 2 x 10 5 s -1 at 120 °C , might proceed via β-scission of 60 or it could involve prior cyclization to 61, analogous to 33 → 42. Theoretical calculations yielded no low energy transition structure for cyclization of 60, suggesting that it fragments directly to 62, 29 a radical whose chemistry will be discussed below.…”