UV-laser photochemistry of the azoalkane 1-phenyl-2,3-diazabicyclo[2.2.2]oct-2-ene: detection and lifetime determination of the triplet diradical 1-phenyl-1,4-cyclohexadiyl by time-resolved UV-spectroscopy (flash photolysis) and by trapping with molecular oxygen

Abstract

ChemInform Abstract Direct laser photolysis of azoalkane (I) at 364 nm gives the diene (III), presumably via the singlet diradical S-(II). Prolonged photolysis gives the unknown (IV) as secondary product while under Ph2CO sensitization small amounts of (V) are formed. In the presence of O2 (10 atm), however, the sensitized laser photolysis of (I) gives via the triplet diradical T-(II) (lifetime 275 and 248 ±15 ns in benzene and MeCN, resp.), besides hexadiene (III), the peroxides (VI)-(VIII) formed at an essentially diffusion-controlled rate.

Time-resolved studies of biradical reactions in solution

“…Photoinduced nitrogen loss from azo compounds has been frequently employed as a source of many biradicals.32"34 Scheme IV illustrates the reaction for the case of a typical azoalkane. 34 The thermolysis and photolysis of cyclic azo compounds lead to different degrees of stereospecificity,33 suggesting the involvement of triplets in the photochemical process. High yields of triplet reaction can of course be achieved by triplet sensitization.34 Photochemical (frequently triplet sensitized) ring cleavage has occasionally been employed.…”

“…57,58 That is, the orbitals in XI are parallel whereas those in XII (in its boat conformation) are more favorably oriented for ISC, according to Salem's orbital orientation rule.53 Similar conformational arguments have been used to explain the lengthening of the lifetime by the 1-phenyl substituent in XIII. 34 Although magnetic field effects on biradical lifetimes have not been detected for small biradicals (e.g., Norrish Type II), they have been observed for VII (n = [10][11][12].50a For these biradicals application of a magnetic field leads to an initial increase in lifetime followed by a decrease at higher fields. The occurrence of magnetic field effects for larger biradicals is readily understood on the basis of a change in the relative contributions of SOC and HFC to the ISC mechanism.…”

Synthesis of Cyclic Peroxides

“…For example, even under 10 atm oxygen pressure, Ph 2 CO-sensitized photolyses of the azoalkanes 42 afford the dienes 43 and bicyclo[2.1.1]hexanes 44 as the major products while the bicyclic peroxides 45 are formed in very poor yields ( 1.5%) (Scheme 8) 64,71,72 . Failure in oxygen trapping was attributed to an extremely rapid spin intercrossing in the initial triplet 1,4-cyclohexadiyl diradicals 46 to generate the corresponding singlet diradicals 47, which are transformed into oxygen-free products of types 43 and 44 72 .…”

“…For example, even under 10 atm oxygen pressure, Ph 2 CO-sensitized photolyses of the azoalkanes 42 afford the dienes 43 and bicyclo[2.1.1]hexanes 44 as the major products while the bicyclic peroxides 45 are formed in very poor yields ( 1.5%) (Scheme 8) 64,71,72 . Failure in oxygen trapping was attributed to an extremely rapid spin intercrossing in the initial triplet 1,4-cyclohexadiyl diradicals 46 to generate the corresponding singlet diradicals 47, which are transformed into oxygen-free products of types 43 and 44 72 . In contrast, when azocompound 48 is irradiated with a medium-pressure mercury lamp using a cutoff filter (λ > 380 nm), or when it is subjected to thermolysis at 80 • C under 4.6 atm of oxygen pressure, it generates 1,4-perinaphthadiyl diradical 49, which readily reacts with triplet oxygen to give polycyclic peroxide 50 in 57% and 41% yield, respectively (Scheme 9) 60 .…”

2-Chloro-1,1-dimethyl-1H-cycloprop[e]azulene, the first cyclopropa-fused azulene

“…The formation of 18 can be rationalised in terms of interception of the diradical intermediate 13 by oxygen to give the cyclic peroxide 17, a process well-documented in the photolysis of other cyclic azo compounds. 7 Acid-catalysed or homolytic cleavage of the peroxide bond of 17 followed by cyclisation onto the reactive position of the five-membered ring of the azulene system would then deliver 18. Significantly, the alkene 15, resulting from intramolecular hydrogen atom transfer within intermediate 13 or 14, was not detected.…”