Koko Maeda

Photochemical reaction of 2,4,4,6-tetrasubstituted 1,4-dihydropyridines in deaerated media: photocolouration and photorearrangement accompanying dehydrogenation

The photochemical colour change of 2,2,4,6-tetraphenyl-1,4-dihydropyridine (1) in deaerated solutions from colourless to violet, which has been reported as a photochromic phenomenon, was found to be an irreversible change and to give 2,3,4,6-tetraphenylpyridine (7) as a main product. Spiro-[2,6-diphenyl-1,4-dihydropyridine-4,9′-fluorene](2), 4-p-bromophenyl-2,4,6-triphenyl-1,4-dihydropyridine (3), and 2,4,6-triphenyl-4-p-tolyl-1,4-dihydropyridine (4) also showed a similar photocolour change in deaerated solutions and gave 1,3-diphenyl-2-azatriphenylene (8), 3-p-bromophenyl-2,4,6-triphenyl-pyridine (9), and 2,3,6-triphenyl-4-p-tolylpyridine (11), respectively. These reactions proceed through di-π-methane rearrangement accompanying dehydrogenation. 4-Benzyl-2,4,6-triphenyl-1,4-dihydro-pyridine (5) and 4-benzyl-1-methyl-2,4,6-triphenyl-1,4-dihydropyridine (6) showed very faint colouration on irradiation. Compound (5) gave 2,4,6-triphenylpyridine (12) produced by elimination of the benzyl group and a hydrogen. For a compound (6) products have not been isolated.

Structures and Photochemical Reactions of 1-(9-Anthryl)-2-aroylethylenes: Competing cis-trans Isomerization and Skeletal Rearrangement

These compounds rapidly undergo photochemical cis-trans isomerization in solution, while irradiation of cis-(I) in the solid state almost exclusively afforded an intramolecular [4+2] cyclization product, suggesting that the isomerization would be inhibited by crystal lattice constraints. cis-(IV) also underwent [4+2] cyclization in the solid state, while in the case of cis-(III) only cis-trans isomerization was observed both in the solid state and in solution

Structural aspects of the mechanical and thermal dissociation of the central bond in 2,2′-bis(2,3,4-triarylchromenyl)s

2,2′-Bis(2,3,4-triphenylchromenyl)(1a) has been shown to undergo reversible homolytic cleavage of the C(2)–C(2′) bond to give green-coloured chromenyl radicals when subjected to mechanical force or heating in the solid state. 2,4-Bis-(p-chlorophenyl)(1b), 2,4-bis-(p-tolyl)(1c), 2,4-bis-(p-methoxyphenyl)(1d), and 2,4-bis-(p-bromophenyl)(1e) derivatives have been synthesized, and substituent effects on the radical dissociation investigated. Upon pressing 1b exhibits the highest degree of dissociation, while thermal dissociation is facilitated by electron-releasing methyl (1c) and methoxy (1d) groups. Dissociation enthalpies (ΔH) for 1a–e in dichloromethane solutions are evaluated to be 12–15 kcal mol–1. The small values of ΔH are considered to be mainly due to overcrowding around the C(2)–C(2′) bond. MMP2 calculation predicts that meso-1a is more stable than the (±)-isomer and the lowest-energy conformation of meso-1a is a centrosymmetric anti form. In the optimized geometry of meso-1a C(2)–C(2′) the bond length is calculated to be 1.584 A, which is rather longer than a normal Csp3–Csp2 bond. AM1 calculations give similar results and suggest that through-bond interactions between the lone-pairs on the oxygen atoms and/or 2- and 2′-aryl groups are not significant compared with the steric repulsion.

Photochemical reactions of anthracene–naphthalene bichromophoric systems linked by a three-carbon chain

Photochemical reactions of bichromophoric compounds having 9-anthryl and 1-naphthyl groups linked by a three-carbon chain have been investigated in solution and in the solid state. Direct photolysis of 1a–c,e–g in degassed benzene gave both cyclomers 2a–c,e–g and head-to-tail anthracene dimers 3a–c,e–g through intra- and inter-molecular [4 + 4] cycloaddition, respectively, while 1-acetoxy derivative 1d gave only cyclomer 2d. The reactivity for the cyclomerisation of 1a–g was lower than that of the bisanthracene system. The conjugated carbonyl group in 1e and 1f significantly retarded the reactions. On biacetyl sensitisation neither cyclomerisation nor dimerisation occurred, but alkene 1e underwent cis–trans isomerisation. In the solid state, (E)-1e gave dimer 3e in 22% yield, while the other compounds were photostable or gave the corresponding dimer in a lower yield. X-Ray structure analysis showed that in the crystal of (E)-1e adjacent anthracene rings related by an inversion centre were stacked with 3.45 A separation, which was favourable for head-to-tail dimerisation.

Photochemical reaction of lucigenin : electron-transfer reduction of 10,10′-dimethyl-9,9′-biacridinediium by counter anions, followed by photocyclization

Lucigenin (1) is a charge-transfer complex between 10,10′-dimethyl-9,9′-biacridinediium (DMBA2+) and two nitrate anions. When the complex (1) was irradiated in the charge-transfer absorption band (>510 nm) in deaerated solutions, DMBA2+ was converted into the reduced form, 10,10′-dimethyl-9,9′-biacridylidene (2). On the other hand, irradiation of (1) with visible light (>420 nm) gave a red product assumed to be 7,16-dimethylbenzo[1,2,3-kI:6,5,4-k′I′] diacridine (4). A mechanism is suggested which involves electron-transfer photoreduction of DMBA2+ by counter anions, followed by photocyclization of (2) to give a dihydrophenanthrene-type intermediate (3), and subsequent dehydrogenation.

Photochemical reaction of 2,4,4,6-tetraaryl-4H-pyrans and -4H-thiopyrans with colour change by a 1,5-electrocyclic reaction. X-Ray molecular structure of 4-methyl-2,3,6-triphenyl-2H-thiopyran

2,4,4,6-Tetraphenyl-4H-pyran and -4H-thiopyran exhibited a photochemical colour change in the solid state. On irradiation in solution, 2,4,4,6-tetraphenyl-4H-pyran gave 1,3,5,6-tetraphenyl-2-oxabicyclo[3.1.0]hex-3-ene, while the 4H-thiopyran gave 1,3,5,6-tetraphenyl-2-thiabicyclo-[3.1.0]hex-3-ene as an initial photoproduct, followed by further transformation into 2,3,4,6-tetraphenyl-2H-thiopyran. 4-(4-Bromophenyl)-2,4,6-triphenyl-4H-pyran, 4-(4-methylphenyl)-2,4,6-triphenyl-4H-pyran, and 4-(4-bromophenyl)-2,4,6-triphenyl-4H-thiopyran also showed photochemical colour changes and finally gave both the phenyl-migrated and the (substituted phenyl)-migrated products in comparable yields in each case. 4-Methyl-2,4,6-triphenyl-4H-thiopyran showed no photochemical colour change in the solid state, but in solution it afforded 4-methyl-2,3,6-triphenyl-2H-thiopyran, whose structure was confirmed by X-ray analysis. Based on comparison with the photochemical behaviour of 2,4,4,6-tetraphenyl-1,4-dihydropyridine and 2,2,4,6-tetraphenyl-1,2-dihydro-1,3,5-triazine, a six-membered ylide was proposed as the coloured photochemical intermediate.

The conformations of cis- and trans-isomers of benzylideneanilines: 1H nuclear magnetic resonance and optical spectroscopic studies

The conformations of the cis- and trans-isomers of benzylideneanilines were studied by two spectroscopic approaches. Effects of trans–cis isomerization on diamagnetic shielding in the 1H n.m.r. spectra suggest only a limited deviation of the aniline ring from the 1-α-α′-1′ plane in the trans-isomers in the absence of steric hindrance due to methyl substitution. Similar conclusions are suggested by the intensity-lowering effects of trans–cis isomerization and of methyl substitution on the first electronic absorption band. Such effects also indicate that the first electronic transition extends over the whole molecule as in stilbene and rule out the possibility that the aniline ring is perpendicular to the 1-α-α′-1′ plane in either isomer of benzylideneaniline.

Solid-State Photodimerization of Anthracenes Having a Chiral Substituent

Abstract Anthracenes containing a chiral substituent at the 9-position, AnCH(OAc)R (An = 9-anthryl; R = Me, Et, i-Pr, t-Bu, CF3), dimerized on UV irradiation in benzene to give two head-to-tail dimers, meso- and dl- forms with 1:1 ratio. In the solid state, some (R˭Et, i-Pr) gave one dimer exclusively, while in another case (R˭Me) dimerization proceeded slowly to give both dimers. The solid-state photochemical behavior can be explained based on the crystal structure.

Mechanism of the chemiluminescence of biisoquinolinium salts

1,1′-Biisoquinolinium salts (BIQ2+2X–) are known to exhibit chemiluminescence on addition of hydrogen peroxide in an alkaline solution. Reduction of BIQ2+ with Na2S2O4–Na2CO3 or Na–Hg gave the radical cation BIQ+˙ and then the neutral two-electron reduced species, 1,1′-biisoquinolylidene (BIQ), which was characterised by mass spectrometry, UV–VIS and 1H NMR spectroscopy. This electron-rich olefin instantaneously reacted with molecular oxygen showing a blue luminescence in organic solvents. Treatment of BIQ2+ or BIQ+˙ with KO2 in dimethyl sulfoxide (DMSO) or acetonitrile also caused light emission. The chemiluminescence spectra of both BIQ–3O2 and BIQ2+–KO2 reactions were the same as that of BIQ2+–H2O2–OH– and the main product was an isoquinolinone derivative in either case. In the case of 2,2′-ethylene-bridged BIQ2+, the chemiluminescence spectrum coincided with the fluorescence spectrum of the corresponding biisoquinolinone. A plausible reaction pathway for the chemiluminescence of the BIQ2+–H2O2–OH– system is as follows: BIQ2+ is reduced by electron donors such as OH– and OOH– to BIQ, which reacted with molecular oxygen to give a 1,2-dioxetane. Thermal decomposition of this intermediate generates the excited state of isoquinolinone. The ‘photoproducts’ from isoquinolinones were obtained in the chemiluminescence reaction as minor products, confirming the chemical formation of the excited state of the isoquinolinones.

Novel photochemical rearrangement of 2,4,4,6-tetraphenylpyridin-3(4H)-one to an oxazole derivative

Irradiation of 2,4,4,6-tetraphenylpyridin-3(4H)-one 1 in solution and in the solid state gave 1,3,6,6-tetraphenyl-2-azabicyclo[3.1.0]hex-2-en-4-one 4 as a primary photoproduct, which underwent a novel photorearrangement to yield 5-(2,2-diphenylethenyl)-2,4-diphenyloxazole 3 along with 3-hydroxy-2,4,5,6-tetraphenylpyridine 2: the structures of 3 and 4 were determined by X-ray structure analysis.