RESUMEN
Hydrogen bond and potassium cation mediated preorientation were detected for phthalimido acetic acid and the corresponding acetate. Evidence for these phenomena came from X-ray structure analysis as well as cyclic voltammetric and IR spectroscopic measurements. These interactions rationalize the photoinduced electron transfer (PET) reactivity of the substrates in photodecarboxylation reactions.
RESUMEN
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.
RESUMEN
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.
RESUMEN
The photochemistry of five linear methylthiomethyl (MTM)-esters of omega-phthalimido carboxylic acids Pht=N-(CH(2))(n)()COOCH(2)SCH(3) 1a-e (n = 1, 2, 3, 5, and 10), of the two methylthioethyl (MTE)-esters Pht=N-(CH(2))(n)()COOCH(2)CH(2)SCH(3) 2a,b (n = 1, 2), and of the two methyl-substituted MTM/MTE esters 3a and 3b was investigated. Two reaction channels were observed: (i) photocyclization to give medium-sized and macrocyclic rings, (ii) photochemical deprotection to give the free carboxylic acids. Photocyclization of 1b and 1c (n = 2, 3) resulted in 4b,c in excellent yields whereas the substrates 1a and 1d,e with shorter as well as longer spacer groups (n = 1, 5, 10) gave preferentially the deprotected products 5a,d,e. Subsequent photolysis afforded N-methylphthalimide (6) from 5a. The MTE-esters 2a and 2b gave the macrocyclic lactones 7 and E-8. Thus, the competition between cyclization and deprotection strongly depended on the chain length of the hydrocarbon linker between phthalimido chromophore and ester group. To examine the influence of the position of the ester group in the linker chain the model substrates 3a and 3b with identical number of atoms separating electron donor and acceptor group were investigated. The more flexible MTE-derivative 3b cyclized to give a 4:1 diastereoisomeric mixture of cis/trans-9b, whereas photolysis of the more reluctant MTM-ester 3a resulted in cis-9a only after prolonged irradiation. These results show that MTM can function as a photolabile protecting group whereas MTE cannot be removed photochemically. The distance dependence of the secondary reaction steps indicates that the primary electron transfer is not necessarily induced starting from close contact geometries.