Photoinduced Electron-Transfer Reactions with Quinolinic and Trimellitic Acid Imides:  Experiments and Spin Density Calculations¹

Abstract

The regioselectivity of photoinduced electron-transfer (PET) reactions of unsymmetrical phthalimides is controlled by the spin density distribution of the intermediate radical anions. ROHF ab initio calculations were found to be most suitable for atomic spin density analysis. Intramolecular PET reactions of quinolinic acid imides were studied with the potassium butyrate and hexanoate 1a,b and a cysteine derivative 3. The photocyclizations products 2a,b and 4 were formed with moderate regioselectivities (68:32, 57:43, and 81:19) showing preferential ortho cyclization. The intermolecular reaction of potassium propionate and potassium isobutyrate with N-methylquinolinic acid imide (5) yielded as addition products the dihydropyrrolo[3,4-b]pyridines 6a,b with slight ortho regioselectivity (55:45). In contrast to these low regioselectivities, the PET reaction of potassium propionate with the methyl ester of N-methyltrimellitic acid imide (9) yielded solely the para addition product 10. Likewise, the intramolecular photoreaction of the cysteine derivative 7 gave a 75:25 (para/meta) mixture of regioisomeric cyclization products 8. The regioselectivity originates from donor-acceptor interactions prior to electron transfer and differences in spin densities in the corresponding imide radical anions. The results of DFT and ab initio calculations for the radical anions of the quinolinic acid imide (11(*)(-)) and the methyl ester of trimellitic acid imide (12(*)(-))( )()were in agreement with the latter assumption: spin densities in 11(*)(-) were higher for the imido ortho carbon atoms (indicating preferential ortho coupling); for 12(*)(-) the spin densities were higher for the imido para carbon atoms (indicating preferential para coupling). These correlations became more significant when the additional spin densities at the carbonyl oxygen and the adjacent carbon atoms were taken into account. The cyclization selectivities for 2, 4, and 8 deviate from the intermolecular examples probably because of ground-state and solvent effects.

Photodecarboxylation Study of Carboxy-Substituted N-Alkylphthalimides in Aqueous Solution: Time Resolved UV−Vis Spectroscopy and Conductometry

“…10 Significantly lower triplet yields were determined for 3a-c and even lower values for 1, 2a-c, and 4a-e (Table 2). Photoinduced intramolecular charge separation between the thioether and the phthalimide moieties has been proposed to account for both decarboxylation and cyclization [1][2][3][4][5][6][7][8] which may be coupled processes. The cyclization products of 1, 2, and 4 are sulfur-containing heterocycles with five-, six-, and seven-membered ring systems.…”

“…The phthalimides were used as described previously; [1][2][3][4][5][6][7][8][9] N-methylphthalimide (5, EGA) and acetonitrile (Merck, Uvasol) were used as received. Water was deionized by a Millipore (Milli Q) system and 2-naphthylglyoxylic acid (2-NG) was the same as used previously.…”

“…12 This reaction has been applied for ω-phthalimido alkylcarboxylic acids, where numerous applications for medium and large ring synthesis were developed. 5,6,8 The photochemistry of N-phthaloylmethionine (1), 2 N-phthaloyl-S-methyl-L-cysteine (2a), 4 and of phthalimidocarboxylic compounds containing a trans-cyclohexane-1,4-diyl spacer group between the nitrogen and the carboxyl group (3a-c) 11 has recently been described in detail. For the N-phthaloylvaline methyl ester we have proposed that the formation of a photoisomer as the main product upon direct excitation occurs via an upper excited n,π * triplet state.…”

Oxophlorin and Metallooxophlorin Radicals—DFT Studies

“…Importantly, the distribution of the spin density has been investigated for chlorophyll a and the pheophytin model, revealing the satisfactory prediction of the hyperfine coupling constants, as determined by electron paramagnetic resonance (EPR). [46±48] DFT studies of the spin density distribution have been also carried out for a variety of radicals including: a-nitronyl radicals, [49] the p-benzosemiquinone anion radical, [50,51] the plastosemiquinone anion radical, [52] the radical anion of the quinolic acid imide, [53] and 4-hydroxyimino-TEMPO (TEMPO 2,2,6,6-tetramethylpiperidin-1-yl oxide). [54] The DFT calculations reproduced the main features of the spin density distribution as reflected by the hyperfine coupling constants, [49±52] paramagnetic shifts, [54] or reactivity pattern.…”

“…[54] The DFT calculations reproduced the main features of the spin density distribution as reflected by the hyperfine coupling constants, [49±52] paramagnetic shifts, [54] or reactivity pattern. [53] The spin density map, as obtained by a polarized neutron study, has been qualitatively accounted for by DFT calculations, for instance for m-azido dicopper(II ) complexes. [55] The concise overview of the oxophlorin chemistry presented in this introduction led us to conclude that the DFT methodology is suitable for analyzing some aspects of the molecular and electronic structure of oxophlorin and its derivatives.…”