Photoreduction of oxoisoaporphines. Another example of a formal hydride-transfer mechanism

Abstract

Photoreduction of 5,6-dimethoxy-, 5-methoxy- and 2,3-dihydro-7H-dibenzo[de,h]quinolin-7-one (A) by tertiary amines in oxygen-free solutions generates long-lived semi-reduced metastable photoproducts, A-NH(-), via a stepwise electron-proton-electron transfer mechanism with a limit quantum yield of about 0.1 at high TEA concentrations. These metastable photoproducts revert thermally to the initial oxoisoaporphine nearly quantitatively in the presence or absence of oxygen. We present spectrophotometric, NMR and UV-vis data for the metastable photoproducts. The spectrophotometric results and PM3 and ZINDO/S calculations support the proposed mechanism for the photoreduction of the oxoisoaporphines.

Photoreduction of Oxoisoaporphine Dyes by Amines: Transient-Absorption and Semiempirical Quantum-Chemical Studies

“…However, the rate constant for decay of A3 -• is 2.6 times greater than the rate constant of the proton transfer, which should be the rate-limiting step for the formation of the metastable photoproduct A-NH -reported previously. 8 Similar results were obtained with tribenzylamine (data not shown) with the observation of absorption bands maxima slightly shifted to the red at 500 nm for the radical anion, A3 -• , and 415 nm for the N-hydrogen neutral radical, A3-NH • , with respect to TEA. The red shift could be attributed to interaction of the transient species with ground tribenzylamine.…”

“…Isosbestic points indicate the clean formation of a single product that has been inequivocally identified as the semireduced N-hydrogenated anion, A-NH -, of the respective oxoisoaporphine. 8 The strong amine absorption at lower wavelength under our experimental conditions precludes comparisons below 300 nm. Good agreement between experimental and calculated photoproduct spectra can be attributed to the small changes in the dipole moment of the states involved, which precludes large solvent-induced Stokes shifts.…”

“…Imine cation may lead to diethylvinylamine with the final formation of N,N-diethyl-1,3-butadienylamine with quantum yield near 0.03 or react with H 2 O, present in the medium, giving acetaldehyde and diethylamine, as detected by NMR. 8,18 Results Semiempirical Quantum Calculations. Semiempirical molecular orbital calculations at PM3 level 22 (HyperChem 6.0 software) were performed to minimize geometries of the product molecules A-NH -of Scheme 2.…”

Photoreduction of Azaoxoisoaporphines by Amines: Laser Flash and Steady‐State Photolysis and Pulse Radiolysis Studies

“…The 1 H-NMR study of the photoreduction of AOIA and OIA has provided the identity of the metastable photoproducts hydrogenated at the carbonyl O-atom (AOH À ). Their formation rationalizes the observed isotopic H/D exchange and supports the stepwise photoreduction mechanism elaborated earlier for 2,3-dihydro-oxoisoaporphines (2,3-dh-OIA) (21)(22)(23) and OIA (24).…”

Synthesis and total assignment of1H and13C NMR spectra of new oxoisoaporphines by long-range heteronuclear correlations

“…1 Later on, synthetic studies afforded them some unusual derivatives 2 that have been starting material for a photoreduction study by tertiary amines. 3 Thus, we have previously reported the complete 1 H and 13 C NMR spectral assignments of a series of 2,3-dihydrooxoisoaporphines. 4 In this paper, we describe the structure determination, conducted entirely by the use of NMR spectroscopy, and the complete chemical shift assignments of the 1 H and 13 C NMR spectra of new oxoisoaporphine derivatives of the oxidation with Pd/C and the presence of a new isoquinoline skeleton, 6H-dibenzo[de,h]quinolin-6-one.…”