Photoreduction of Polycyclic DBO-Type Azoalkanes with Amines to Hydrazine Products

Abstract

The n,π*-singlet-excited polycyclic, α-substituted DBO-type azoalkanes 1 are reduced by amines to form the corresponding hydrazines 2 as photoreduction products in competition to denitrogenation to the bicyclo[2.2.0]hexane photoproducts 3. The photophysical, photochemical, and electrochemical properties of the azoalkane determine the product ratio of the hydrazine 2 versus the bicyclo[2.2.0]hexane 3. The quantum yields of the photoreduction are quite low (<0.1), for which efficient deactivation of the singlet excited state through charge transfer is postulated. The charge transfer in the exciplex between the azoalkane and the amine promotes the hydrogen abstraction of the α-CH bond from the amine by the azoalkane and, thus, facilitates its photoreduction, a general photochemical process applicable to singlet- as well as triplet-excited azoalkanes.

Quenching of n,?*-excited azoalkanes by amines: structural and electronic effects on charge transfer

“…11,13,43 This 'photoreduction' is known to be induced by an initial charge-transfer interaction between the reactants, with the amine as electron donor. While free ions may be formed in thermodynamically favorable cases (for strong acceptors), the formation of exciplexes becomes the presumed quenching pathway for weak acceptors, [44][45][46] e.g.…”

“…for the azoalkane DBO, where free ions have not been detected in the quenching by amines. 47 The observed photoreduction products of the azoalkanes (hydrazines) 13 and the low photoreaction quantum yields (Φ r`5 %, Table 5) are diagnostic for a photoreaction, which involves a competition between exciplex-induced hydrogen abstraction from the a C-H or N-H bonds of the amines and exciplex-induced deactivation (Scheme 1). 11,13 The low rate constants for the quenching of DBO by amines (Tables 1 and 2) allow one to examine detailed structural and electronic effects of the donor compound on these reaction pathways.…”

“…11,13 Interestingly, the quantum yield decreases with increasing solvent polarity, i.e. cyclohexane % benzene b acetonitrile b water (Table 5).…”

“…1: although the primary amines cyclohexylamine (1) and tert-butylamine (2) have the same ionization potential, amine 1 quenches DBO three times as fast. Even among the tertiary amines some inconsistencies are found: tri-npropylamine (13) and N-ethyldiisopropylamine (14) have similar donor strengths but amine 14 has a two times higher quenching rate constant. The quenching rate constants for triplet benzophenone (BP) are included in Tables 1 and 2 to illustrate the diffusion-controlled nature of the quenching of this excited ketone by most amines.…”

“…intermediates with partial charge-transfer character, has been suggested in the quenching of n,p*-excited ketones and azoalkanes by amines. 3,7,[9][10][11][12] Since direct spectroscopic detection through exciplex emission has not been achieved, reduced quantum yields 11,13 , small or 'inverted' solvent effects, 9,12,14 steric hindrance effects 15 , a low sensitivity of the quenching rate constants on the donor propensity of the quencher, 9 and related observations for olefins and aromatic quenchers [16][17][18][19][20][21] have been taken as indirect experimental indications for exciplexes.…”