Photoreduction of Azoalkanes by Direct Hydrogen Abstraction from 1,4-Cyclohexadiene, Alcohols, Stannanes, and Silanes

Abstract

A mechanistic investigation of the photoreduction of the n,pi triplet-excited azo chromophore has been carried out on azoalkanes 1, which exhibit efficient intersystem-crossing quantum yields (ca. 0.5). The azoalkanes 1a and 1b undergo facile photoreduction to the corresponding hydrazines in the presence of a variety of hydrogen donors, which include 2-propanol, benzhydrol, 1,4-cyclohexadiene, tributylstannane, and tris(trimethylsilyl)silane. In contrast, the hydrazine yields derived for the azoalkanes 1c and 1d are significantly lower even at high hydrogen donor concentrations due to their lower triplet yields and shorter triplet lifetimes. A clear dependence of the hydrazine yields on the bond dissociation energies of the hydrogen donors has been observed, which is reflected in the quenching rate constants obtained from time-resolved transient absorption spectroscopy. The absolute rate constants for interaction of the triplet azoalkane 1a with hydrogen donors are generally lower (ca. 10-100-fold) than for benzophenone, in line with the less favorable reaction thermodynamics. The comparison of the rate constants for quenching of the triplet-excited azoalkane 1a and of the singlet-excited state of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) reveals a similar reactivity of excited azoalkanes toward hydrogen donors; differences can be accounted for in terms of variations in the energies of the excited states. The interactions of the excited azoalkanes with tributylstannane and benzhydrol produce the radicals characteristic for hydrogen abstraction from these substrates, namely tributylstannyl and hydroxydiphenylmethyl radicals, which were detected through their transient absorptions at 390 and 550 nm, respectively. Interestingly, compared to the photoreduction of benzophenone with benzhydrol, for which the quantum yield for conversion to radicals is unity, between the azoalkane 1a and benzhydrol this efficiency is only ca. 12%. An associative effect through N.H-O bonding is held responsible, which promotes hydrogen transfer versus diffusion out of the caged radical pair. The quenching of the singlet-excited DBO by toluene was also employed to monitor the formation of benzyl radicals (at 317 nm). The photolysis of DBO in tetrahydrofuran as solvent and quencher produced an absorption at ca. 290 nm, which was tentatively assigned to the corresponding hydrazinyl radical.

Radical mechanism of azo cleavage for monoazo reactive dyes, a QSPR study: the similarity between photoreduction on polyamide substrates and thermal reduction in aqueous dithionite solutions

“…The HTs of azo dyes bearing a keto group at the O13 position have a strong tendency to adopt the enol structure (due to a p K a shift ), resulting in ATs in the excited states. H‐abstraction has been confirmed to occur from the S 1 state of the azo group . (The photochemistry of azo groups has been extensively studied for azoalkanes, including thermal reactions between photoinduced radicals.…”

“…The HTs of azo dyes bearing a keto group at the O13 position have a strong tendency to adopt the enol structure (due to a pK a shift [57,58]), resulting in ATs in the excited states. Photochemical and thermal decompositions of photoinduced radicals derived from cyclic azoalkanes in the liquid phase were shown to occur in the excited and ground states with large rate constants [59][60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75][76].) (The photochemistry of azo groups has been extensively studied for azoalkanes, including thermal reactions between photoinduced radicals.…”

“…(The photochemistry of azo groups has been extensively studied for azoalkanes, including thermal reactions between photoinduced radicals. Photochemical and thermal decompositions of photoinduced radicals derived from cyclic azoalkanes in the liquid phase were shown to occur in the excited and ground states with large rate constants [59][60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75][76].) Some ATs generated may undergo no intramolecular H-abstraction when in the excited state.…”

Cover Picture: Studying the Origin of the Antiferromagnetic to Spin‐Canting Transition in the β‐p‐NCC6F4CNSSN. Molecular Magnet (Chem. Eur. J. 9/2010)

“…Thus, a hydrogen donor with lower interaction efficiency, that is, a chiral tetrahydrofuran derivative, seemed to be a promising choice to circumvent this problem. The intermolecular rate constant for the triplet quenching of benzophenone by this ether is known to be 30 times lower than for 1,4‐cyclohexadiene (9.6×10 6 M −1 s −1 versus 2.9×10 8 M −1 s −1 ) in benzene 28. Furthermore, a higher selectivity in the quenching reaction can be anticipated as a result of the lower reactivity of tetrahydrofuran which might result in a significant discrimination between both diastereomers (similar to the reactivity‐selectivity principle).…”

Diastereomeric Differentiation in the Quenching of Excited States by Hydrogen Donors

“…The intermolecular rate constant for the triplet quenching of benzophenone by this ether is known to be 30 times lower than for 1,4-cyclohexadiene (9.6 10 6 m À1 s À1 versus 2.9 10 8 m À1 s À1 ) in benzene. [28] Furthermore, a higher selectivity in the quenching reaction can be anticipated as a result of the lower reactivity of tetrahydrofuran which might result in a significant discrimination between both diastereomers (similar to the reactivity-selectivity principle).…”