Structure-reactivity relationships in the photoreduction of n,π*-excited ketones and azoalkanes: the effect of reaction thermodynamics, excited-state electrophilicity, and antibonding character in the transition state

Abstract

The reaction enthalpies for the photoreduction of n,pi*-excited states (acetone, benzophenone, 2.3-diazabicyclo[2.2.2]oct-2-ene, and a 2,3-diazabicyclo[2.2.1]hept-2-ene derivative) by model hydrogen donors (methanol and dimethylamine) were calculated on the basis of a critically evaluated data set of bond dissociation energies for donors and reduced acceptors. These were compared with the observed photoreactivity, which can be assessed through quenching rate constants of the excited states by hydrogen donors. The intriguing observation of a preferential attack at electrophilic hydrogen atoms, i.e., N-H or O-H, by n,pi*-excited azoalkanes is rationalised on the basis of the calculated thermochemical data, differences in electrophilicity, and varying contributions of antibonding character in the transition state. Singlet-excited azoalkanes act as nucleophilic species, while excited ketones display an electrophilic reactivity. This is in line with the pictorial description of the electron distribution in these excited states.

Investigation of Polar and Stereoelectronic Effects on Pure Excited-state Hydrogen Atom Abstractions from Phenols and Alkylbenzenes†

“…The more negative ρ values for triplet ketones are in line with the participation of charge transfer (exciplex formation) in the quenching. In contrast, the variance in ρ values between singlet‐excited DBO and the tert ‐butoxyl radical must be related to polar effects in the direct hydrogen abstraction process (2, 13, 18, 33), because exciplexes or ground‐state charge transfer complexes are not postulated for these two reactive species. Such philicity effects are commonly understood in terms of contributions of polar resonance structures in the transition state (Scheme 1).…”

“…Such philicity effects are commonly understood in terms of contributions of polar resonance structures in the transition state (Scheme 1). The polar structure contributes to a different extent for the excited azoalkane versus the tert ‐butoxyl radical, owing to the nature of the abstracting atoms (N versus O) and it has in fact been previously suggested that the excited azo chromophore is a less electrophilic species, implying a lower contribution of the polar structure (in which the abstracting atom bears a partial negative charge) (33). Scrutiny of the results of high‐level ab initio calculations on the hydrogen abstraction of pyrazoline (as a model for DBO) from methylene chloride supports the view of a low degree of charge separation in the transition state (N=N–H–CHCl 2 ) (42), because the partial charges on the azoalkane, the hydrogen to be transferred, and the dichloromethyl moiety are very small and amount to –0.09, +0.24 and –0.15, respectively.…”

“…Previous photochemical reactivity data have suggested that DBO not only is less electrophilic than ketones, but is in fact nucleophilic, based on an unusual preference of DBO for abstraction of electrophilic hydrogen atoms like NH or OH (17, 33). DBO abstracts even hydrogen atoms from otherwise photochemically inert solvents like water, chloroform and acetonitrile (29, 33).…”

“…Previous photochemical reactivity data have suggested that DBO not only is less electrophilic than ketones, but is in fact nucleophilic, based on an unusual preference of DBO for abstraction of electrophilic hydrogen atoms like NH or OH (17, 33). DBO abstracts even hydrogen atoms from otherwise photochemically inert solvents like water, chloroform and acetonitrile (29, 33). Benzophenone behaves towards these quenchers as a typical n,π*‐excited ketone and prefers to abstract nucleophilic hydrogen atoms like α‐C–H in alcohols (33), because of the electron‐deficient character of the carbonyl oxygen in the excited state (10, 33).…”

“…DBO abstracts even hydrogen atoms from otherwise photochemically inert solvents like water, chloroform and acetonitrile (29, 33). Benzophenone behaves towards these quenchers as a typical n,π*‐excited ketone and prefers to abstract nucleophilic hydrogen atoms like α‐C–H in alcohols (33), because of the electron‐deficient character of the carbonyl oxygen in the excited state (10, 33). The switch‐over in philicity may even lead to an inverted selectivity, e.g.…”