PHOTOREDUCTION OF AZOALKANES BY DIRECT HYDROGEN ABSTRACTION FROM 1,4-CYCLOHEXADIENE, ALCOHOLS, STANNANES, AND SILANES

A mechanistic investigation of the photoreduction of the n,pi triplet -excited azo chromophore has been carried out on azoalkanes 1, which e xhibit efficient intersystem-crossing quantum yields (ca. 0.5). The az oalkanes 1a and 1b undergo facile photoreduction to the corresponding hydrazines in the presence of a variety of hydrogen donors, which incl ude 2-propanol, benzhydrol, 1,4-cyclohexadiene, tributylstannane, and tris(trimethylsilyl)silane. In contrast, the hydrazine yields derived for the azoalkanes Ic and Id are significantly lower even at high hydr ogen donor concentrations due to their lower triplet yields and shorte r 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 constant s 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 rat e constants for quenching of the triplet-excited azoalkane 1a and of t he singlet-excited state of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) rev eals a similar reactivity of excited azoalkanes toward hydrogen donors ; differences can be accounted for in terms of variations in the energ ies of the excited states. The interactions of the excited azoalkanes with tributylstannane and benzhydrol produce the radicals characterist ic for hydrogen abstraction from these substrates, namely tributylstan nyl and hydroxydiphenylmethyl radicals, which were detected through th eir transient absorptions at 390 and 550 nm, respectively. Interesting ly, compared to the photoreduction of benzophenone with benzhydrol, fo r which the quantum yield for conversion to radicals is unity, between the azoalkane 1a and benzhydrol this efficiency is only ca. 12%. An a ssociative 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 emp loyed to monitor the formation of benzyl radicals (at 317 nm). The pho tolysis of DBO in tetrahydrofuran as solvent and quencher produced an absorption at ca. 290 nm, which was tentatively assigned to the corres ponding hydrazinyl radical.