Time-Resolved Spectroscopy of Sulfur- and Carboxy-SubstitutedN-Alkylphthalimides

Abstract

The photophysical and photochemical properties of N-phthaloyl-methionine (1), S-methyl-N-phthaloyl-cysteine methyl ester (2) and N-phthaloyltranexamic acid (3) were studied by time-resolved UV/Vis spectroscopy, using laser pulses at 248 or 308 nm. The quantum yield of fluorescence is low (phi(f)< 10(-2)) for 1-3 in fluid and glassy media, whereas that of phosphorescence is large (0.3-0.5) in ethanol at - 196 degrees C. The triplet properties were examined in several solvents, at room temperature and below. The spectra and decay kinetics are similar, but the population of the pi(pi*) triplet state, as measured by T-T absorption, is much lower for 1 and 2 than for 3 or N-methyltrimellitimide (5') at ambient temperatures. The quantum yield (phi(delta)) of singlet molecular oxygen O2(1deltag) formation is substantial for 3 and 5' in several air- or oxygen-saturated solvents at room temperature, but small for 2 and 1. The quantum yield of decomposition is substantial (0.2-0.5) for 3 and small (<0.05) for 2 and 1. It is postulated that photoinduced charge separation in the spectroscopically undetectable 3n,pi* state may account for the cyclization products of 1 and 2. In aqueous solution, this also applies for 3, whereas in organic solvents cyclization involves mainly the lower lying 3pi,(pi*) state. Triplet acetone, acetophenone and xanthone are quenched by 1-3 in acetonitrile; the rate constant is close to the diffusion-controlled limit, but smaller for benzophenone. While the energy transfer from the triplet ketone occurs for 3, a major contribution of electron transfer to the N-phthalimide derivative is suggested for 1 and 2, where the radical anion of benzophenone or 4-carboxybenzophenone is observed in alkaline aqueous solution.

Synthesis of Sulfur-Containing Tricyclic Ring Systems by Means of Photoinduced Decarboxylative Cyclizations

“…[15] Under the sensitization conditions used in the experiments described here, an intermolecular photoinduced electron transfer (PET) involving triplet excited acetone was confirmed. [15] The thioether is thus oxidized by PET to a sulfur-centered radical cation. Unlike the case of the simple dialkyl sulfide substrates, [5] no deprotonation from the respective α-CH position occurred, but regioselective extrusion of CO 2 took place, with formation of an α-thio C radical.…”

“…The recently postulated intramolecular photoinduced electron transfer (PET) mechanism involving the thioether as the electron-donating species and the triplet excited phthalimide as the electronaccepting one [5] has been established for direct excitation of the chromophore. [15] Under the sensitization conditions used in the experiments described here, an intermolecular photoinduced electron transfer (PET) involving triplet excited acetone was confirmed. [15] The thioether is thus oxidized by PET to a sulfur-centered radical cation.…”

“…For the corresponding phthalimido-ω-carboxylic acids, this electron transfer is assumed to proceed in an intramolecular fashion involving the 3 π,π* state. [15] The remarkable drop in efficiency can be interpreted in terms of the weaker donor properties of the carboxylate anion in comparison to the thioether group. Thus, the low yields for cyclization, and also the low degrees of conversion (52Ϫ71% reisolated starting material), indicate that the primary electron transfer still occurs from the thioether function (path A), but is mainly followed by rapid nonproductive BET.…”

“…The quantum yields of selected substrates were measured upon excitation at 308 nm in argon-saturated aqueous solutions at pH ϭ 9 or in acetonitrile (Table 7). [29] In pure acetonitrile, in which the acids are present in their nondissociated forms, the quantum yields are low, and in the same range as the thioalkyl derivatives. [4,5] Obviously, the carboxylate group is unreactive under these conditions and electron transfer is followed by rapid nonproductive BET.…”

Diastereo- and Enantioselective Synthesis of Pyrrolo[1,4]benzodiazepines through Decarboxylative Photocyclization

“…[7] Intramolecular electron transfer and extrusion of carbon dioxide lead to the 1,(w 1)-triplet diradical which, after spin inversion, combines to form the closed-shell products. This process allows, for example, the synthesis of diastereo-and enantiomerically pure benzopyrrolizidines from glutamic acid derivatives.…”