UV-laser photochemistry of azoalkanes: surprising effects of phenyl substitution on the lifetimes of 1,3-cyclopentanediyl and 1,4-cyclohexanediyl triplet diradicals

Synthesis of Cyclic Peroxides

“…In contrast, benzophenone-sensitized laser photolysis (λ 350 nm) of azoalkane 16 generates the more persistent triplet diradical 19, which reacts with molecular oxygen (150 psi) to give the bridged bicyclic endoperoxide 20 63 . Due to the photolability of many endoperoxides 51,52 , the yield of bicyclic peroxide 20 dramatically decreases as the reaction proceeds, resulting in 73% yield of 20 at 21% conversion and 54% yield at 43% conversion 63 Increasing the stability of diradicals through aryl substitution at the radical centers augments the efficacy of intermolecular oxygen trapping 58,61,64 . For example, benzophenone-sensitized laser photolyses of azoalkanes 21a and 21b at −10 • C under 10 atmospheres pressure of oxygen afford 1-phenyl-22a and 1,4-diphenyl-2,3-dioxabicyclo[2.2.1]heptanes 22b as the only reaction products (Scheme 3) 64 .…”

“…Due to the photolability of many endoperoxides 51,52 , the yield of bicyclic peroxide 20 dramatically decreases as the reaction proceeds, resulting in 73% yield of 20 at 21% conversion and 54% yield at 43% conversion 63 Increasing the stability of diradicals through aryl substitution at the radical centers augments the efficacy of intermolecular oxygen trapping 58,61,64 . For example, benzophenone-sensitized laser photolyses of azoalkanes 21a and 21b at −10 • C under 10 atmospheres pressure of oxygen afford 1-phenyl-22a and 1,4-diphenyl-2,3-dioxabicyclo[2.2.1]heptanes 22b as the only reaction products (Scheme 3) 64 . Peroxides 22a and 22b derive from the intermediacy of the corresponding triplet 1-phenylcyclopentane-1,3-diyl radical and 1,3-diphenylcyclopentane-1,3-diyl radical which efficiently trap molecular oxygen.…”

“…Under similar conditions also alkyl-substituted azo-compounds 21c and 21d afford the corresponding bridged bicyclic endoperoxides 22c and 22d 58,65,66 . The benzophenone-sensitized photolysis of phenyl-substituted azoalkanes 21a and 21b under argon results in the ring contraction products 23a and 23b respectively 64 . Noteworthy is that 1,4-diphenylbicyclo[2.1.0]pentane 23b, prepared from 21b under oxygen-free conditions, is readily oxidized by triplet oxygen at room temperature to give endoperoxide 22b (Scheme 3) 64 .…”

“…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 .…”

“…The effectivity of such a cycloaddition depends on the relative stability of the substrate and on reaction conditions. Thus, potassium tert-butoxide induced prototropic rearrangement of cis-4-octene-1,7-diyne (64) in aerobic conditions generates the cis-bis(allene) derivative 65 that cyclizes to a highly unstable ortho-quinodimethane 66, which could not be trapped by triplet oxygen (Scheme 12) 78 . However, under similar conditions the more stable orthonaphthaquinodimethane (70), generated from 68, affords the endoperoxide 71 in a moderate yield.…”