The photodimerization of aceanthrylene

Sensitized photodimerization of aceanthrylene (AA) with Rose Bengal in methanol produced both syn and anti head-to-head and head-to-tail stereoisomers with slight preference for syn adducts. The stereochemical configurations were assigned by deuterium labeling and by high field homonuclear shift correlation (COSY) experiments and these compare favorably with the assignments made for the photodimers of 1-substituted acenaphthylenes. Currently, AA has photodimerized under direct irradiation only in carbon disulfide.     

Photodimers of cinnamic acid and related compounds. A stereochemical study by NMR

¹H NMR spectra of photodimers of cinnamic acid, chalcone and related compounds were analysed, and the spectral parameters and all the coupling constants were obtained. A clear differentiation among the head-to-head and head-to-tail stereoisomers can be obtained by considering the values of K, L, M and N parameters. The different types of hh and ht isomers can be immediately assigned by considering the relative values of the three-bond couplings. An authentic example of an A₂B₂ spin system is reported.     

STUDIES ON THE PHOTOTOXICITY OF COUMARIN DERIVATIVES — I. PHOTOCYCLODIMERIZATION OF 5,7-DIMETHOXYCOUMARIN

Abstract— 5,7-DimethoxJtcoumarin (DMC) dimerizes through the C₄-photo-cycloaddition of 3,4-double bonds to form a syn head-to-tail dimer on direct irradiation ( Λ≥ 300 nm) in acetonitrile or benzene solution. The quantum yield of the photocyclodimerization in acetonitrile is 0.068 which is greater than that of coumarin.
In the presence of triplet sensitizers such as benzophenone, 5,7-dimethoxycoumarin forms an anti dimer with the quantum yield greater than 0.08. The structure of the photodimers has been elucidated by IR, UV, NMR, and mass spectrometry. The results of luminescence studies, triplet quenching and sensitization revealed that the syn head-to-tail dimer was formed via an excited singlet precursor, while the anti dimer was formed via the excited triplet state.     

Enhancement of diastereoselectivity in photodimerization of alkyl 2-naphthoates with chiral auxiliaries via inclusion within γ-cyclodextrin cavities

Irradiations of alkyl 2-naphthoates are known to result in four isomeric "cubane-like" photodimers: anti(HH)-2, syn(HH)-2, anti(HT)-2, and syn(HT)-2 where the anti(HH)-2, anti(HT)-2, and syn(HT)-2 consist of pairs of diastereomers. Here, chiral auxiliary and chiral microreactor strategies have been combined to achieve high diastereoselectivity in photodimerizations of an enantiomeric pair of 2-naphthoates with (R)- and (S)-1-methoxycarbonylethyl esters as chiral auxiliaries (1R and 1S). Thus, irradiations of their γ-cyclodextrin (γ-CD) complexes have been conducted. Fluorescence, IR, and NMR spectra of both enantiomers of 1 demonstrate that their γ-CD complexes are mainly 2:2 with the molecules of 1 in head-to-head orientations. Irradiation of the complexes in the solid state mainly resulted in anti(HH)-2. The absolute configuration of each diastereomer of anti(HH)-2 has been established for the first time here. The diastereomeric excesses (de's) of anti(HH)-2 from 1R and 1S were 94% and 86%, respectively. These de's are much higher than those found from irradiations in solution (55% for 1R and 1S), where the opposite diastereomeric form is in excess! Calculations of the energies of various conformations of the head-to-head 2:2 inclusion complexes were performed using the PM3 approach. The predicted major diastereomers based on the calculation are consistent with those found experimentally.     

Structures, photoluminescence, and reversible vapoluminescence properties of neutral platinum(II) complexes containing extended pi-conjugated cyclometalated ligands

Reacting K2PtCl4 with the tridentate R-C(wedge)N(wedge)C-H2 ligands 2,6-di-(2'-naphthyl)-4-R-pyridine (R = H, 1a; Ph, 1b; 4-BrC6H4, 1c; 3,5-F2C6H3, 1d) in glacial acetic acid, followed by heating in dimethyl sulfoxide (DMSO), gave complexes [(R-C(wedge)N(wedge)C)Pt(DMSO)] (2a-d). In the crystal structures of 2a-c, the molecules are paired in a head-to-tail orientation with Pt...Pt separations >6.3 A, and there are extensive close C-H...pi (d = 2.656-2.891 A), pi...pi (d = 3.322-3.399 A), and C-H...O=S (d = 2.265-2.643 A) contacts. [(Ph-C(wedge)N(wedge)C)Pt(PPh3)] (3) was prepared by reacting 2b with PPh3. Reactions of 2a-d with bis(diphenylphosphino)methane (dppm) gave [(R-C(wedge)N(wedge)C)2Pt2(mu-dppm)] (4a-d). Both head-to-head (syn) and head-to-tail (anti) conformations were found for 4a.6CHCl3.C5H12, whereas only one conformation was observed for 4b.2CHCl3 (syn), 4c.3CH2Cl2 (syn), and 4d.2CHCl3 (anti). In the crystal structures of 4a-d, there are close intramolecular Pt...Pt contacts of 3.272-3.441 A in the syn conformers, and long intramolecular Pt...Pt separations of 5.681-5.714 A in the anti conformers. There are weak C-H...X (d = 2.497-3.134 A) and X...X (X = Cl or Br; d = 2.973-3.655 A) interactions between molecules 4a-d and occluded CHCl3/CH2Cl2 molecules, and their solvent channels are of varying diameters (approximately 9-28 A). Complexes 2a-d, 3, and 4a-d are photoluminescent in the solid state, with emission maxima at 602-643 nm. Upon exposure to volatile organic compounds, 4a shows a fast and reversible vapoluminescent response, which is most intense with volatile halogenated solvents (except CCl4). Powder X-ray diffraction analysis of desolvated 4a revealed a more condensed molecular packing of syn and anti complexes than crystal 4a.6CHCl3.C5H12.     

Mass spectrometric investigation of polymers: Thermal degradation of truxillic and truxinic polyamides

The thermal degradation mechanism of four isomeric truxillic and truxinic polyamides were investigated by direct pyrolysis in the ion source of a mass spectrometer. Thermal degradation reactions were followed directly by this method by detecting the thermal and electron impact-induced fragments. The results obtained have shown that the thermal degradation products are sensibly different for the head-to-head (hh) and head-to-tail (ht) polymers and that the predominant pyrolytic process is the cyclobutane ring cleavage. In the hh isomers, both symmetrical and asymmetrical cyclobutane ring cleavage was detected, while in the ht isomers only symmetrical cleavage occurs; this explains the noticeable difference found in the thermal stability of the two polymer types.     

Azolium-linked cyclophanes: effects of structure, solvent, and counteranions on solution conformation behavior

This paper describes the synthesis, structural characterization, and solution behavior of some xylyl-linked imidazolium and benzimidazolium cyclophanes decorated with alkyl or alkoxy groups. The addition of alkyl/alkoxy chains to the cyclophanes allows for studies in chlorinated solvents, whereas previous solution studies of azolium cyclophanes have generally required highly polar solvents. The azolium cyclophanes may exist in a syn/syn conformation (azolium rings mutually syn, arene rings mutually syn) or a syn/anti conformation (azolium rings mutually syn, arene rings mutually anti). The preferred conformation is significantly affected by (i) binding of bromide (ion pairing) to the protons on the imidazolium or benzimidazolium rings, which occurs in solutions of bromide salts of the cyclophanes in chlorinated solvents, and (ii) the addition of alkoxy groups to the benzimidazolium cyclophanes. These structural modifications have also led to cyclophanes that adopt conformations not previously identified for similar azolium cyclophane analogues. Detailed (1)H NMR studies for one cyclophane identified binding of bromide at two independent sites within the cyclophane.     

Highly stereoselective photocyclodimerization of alpha-cyclodextrin-appended anthracene mediated by gamma-cyclodextrin and cucurbit[8]uril: a dramatic steric effect operating outside the binding site

Photocyclodimerization of alpha-cyclodextrin (CD)-appended anthracene was studied in the presence of gamma-CD and cucurbit[8]uril (CB[8]) hosts to manipulate the stereodifferentiating photoreaction occurring inside the cavity by the bulky attachment located outside. The gamma-CD-mediated photodimerization afforded the head-to-tail photodimers in 98% combined yield, in particular, the syn-head-to-tail photodimer of 91% ee in 68% yield, which are much greater than 32% ee and 44% yield obtained with unmodified anthracene carboxylate. The use of CB[8] also led to a striking inversion of the head-to-tail/head-to-head selectivity, affording exclusively the head-to-head photodimers in 99% combined yield.     

Enhancement of intramolecular excimer formation and photodimerization of anthrylmethyl α,ω-alkanedioates via hydrophobic interactions

The fluorescence spectra and photodimerization of anthrylmethyl a,w-alkanedioates (A-Mn-A) both in organic and in aqueous organic mixed solvents have been studied.In aqueous organic mixed solvents strong intramolecular excimer emission is detected and the quantum yield for the intramolecular photodimerization is significantly greater than those in organic solvents.These observations suggest that hydrophobic interactions force A-Mn-A molecule to self-coil.The ratio of the head-to-head to head-to-tail products in the intramolecular photodimers of A-Mn-A depends on the length of the linking chain.This work presents a successful example of application of hydrophobic interactions to enhancement of large-ring formation.     

The synthesis and photodimerization of 2h-2-benzazepine-1,3-diones

The photochemical dimerization of 2$\text{H}$-2-benzazepine-1,3-diones has been described. The structures of the major products are assigned as the head-to-tail, 5, and the head-to-head. 7, photodimers on the basis of the analysis of the coupling constants in the NMR spectra. In addition, the structures of the minor products and attempts to convert 5 to 7 are discussed.     

Self-organization and photoreactivity of anthryl dendron having perfluoroalkyl chains at the terminals

A newly designed anthryl dendron having perfluoroalkyl chains at terminals showed thermotropic liquid crystallinity, which was characterized using polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction. The anthryl dendron forms S_B phase at room temperature below 53 °C, at which temperature a phase transition to Col_rd takes place. In a fluorous solvent, regioselective photodimerization of the anthryl dendron occurred to give head-to-head photodimers, although photodimerization in the S_B phase and chloroform gave both head-to-head and head-to-tail photodimers.     

PHOTOCYCLOADDITION REACTION OF 5,7-DIMETHOXYCOUMARIN TO THYMIDINE

Abstract— The photocycloaddition reaction of 5,7-dimethoxycoumarin to thymidine on direct irradiation (λ > 300 nm) is studied as a model for photosensitization reaction of furocoumarins. The major photoadducts were isolated by silica gel column and gel permeation chromatography. Each component of the photoadducts was further separated by reverse phase, paired-ion high performance liquid chromatography. The structure of these photoproducts isolated is consistent with 1:1 C₄-cycloadducts in accordance with characteristics of their UV, IR, NMR and mass spectra and elemental analysis data. The stereochemistry of each isomer was studied by Fourier transform NMR, UV and IR spectra. The fraction C has the anti head-to-tail configuration and the fraction D has the configuration of anti head-to-head. The fractions A and B probably have the syn configuration.     

Photodimers of cinnamic acid and related compounds. A stereochemical study by electron-impact and field desorption mass spectrometry

The low energy (13 eV) electron-impact and field desorption mass spectra of some photodimers of cinnamic acid and related compounds containing the cyclobutane ring are reported and the fragmentation patterns analysed in order to obtain stereochemical information on the substituent position on the cyclobutane ring. Both symmetrical and asymmetrical splittings of the cyclobutane ring were detected, allowing characterisation of the head-to-head and head-to-tail structures of the title compounds. A ring opening rearrangement of the McLafferty type was also found.     

One-pot synthesis of pentacyclic diamines from quinolines by a new Zn/AcOH-promoted cascade reaction

A simple and efficient method for the one-pot synthesis of pentacyclic diamines from quinolines is described. It involves a new Zn/AcOH-promoted cascade reaction, in which two C[bond]C bonds and four to five stereogenic centers are established under mild conditions. The regiochemistry of the dimerization and cyclization step is governed by substituent effects, allowing access to a head-to-head (2, 3) or head-to-tail skeleton (4, 5).     

Preparation and photolysis of polyamides having coumarin dimer component

The ring-opening polyaddition reaction of anti and syn head-to-head Coumarin dimers with diamines and the photocleavage behavior of the resulting polyamides were investigated. Anti head-to-head Coumarin dimer successfully reacted with aliphatic and aromatic diamines in an aprotic polar solvent to give corresponding high molecular weight polyamides. The polyamides showed good film forming ability and exhibited solubility behavior typical of polyelectrolytes. Furthermore, these polyamides were found to undergo exclusively asymmetric photocleavage on the cyclobutane rings in the polymer main chain to give a fumaramide unit with elimination of 2,2′-dihydroxystilbene. On the other hand, syn head-to-head Coumarin dimer gave only low molecular weight polyamides. These polymerization and photocleavage behaviors were elucidated in comparison with those of model reactions.     

Centrosymmetric and Non-centrosymmetric Packing of Aligned Molecular Fibers in the Solid State Self Assemblies of Cyclodextrin-based Rotaxanes

Two [2]-rotaxanes each comprising α-cyclodextrin as the rotor, and with either 3,3′-difluoro- or 3,3′-dichloro-stilbene as the axle and trinitrophenylamino substituents as the blocking groups at the 4- and 4′-positions, were prepared and their structures analyzed in solution and the solid state using ¹H NMR spectroscopy and X-ray crystallography, respectively. With each rotaxane, in solution the stilbene rotates freely within the cyclodextrin annulus. In the solid state the 3,3′-dichlorostilbene-based rotaxane adopts two very similar conformations, each having the chlorines in the anti,anti-orientation. By comparison, the 3,3′-difluorostilbene-based rotaxane adopts anti,anti-, anti,syn- and syn,syn-orientations of the substituents. The crystal packing of each rotaxane displays aligned molecular fibers, which are centrosymmetrically orientated in the case of the difluoride due to the head-to-head/tail-to-tail alignment of the cyclodextrins. By contrast, all of the cyclodextrins in the dichloride are aligned head-to-tail along a single axis to give a polar, non-centrosymmetric crystal.     

Novel Z-selective head-to-head dimerization of terminal alkynes catalyzed by lanthanide half-metallocene complexes

Various terminal alkynes have been cleanly dimerized into the corresponding head-to-head (Z)-enynes by use of the half-metallocene lutetium alkyl complexes Me2Si(C5Me4)(NAr)Lu(CH2SiMe3)(THF) (Ar = Ph, C6H3Me2-2,6, C6H2Me3-2,4,6) as catalysts. Aromatic C-Cl, C-Br, and C-I bonds, which are known to be extremely susceptible to reductive cleavage by transition metals, survived in the present reactions. The corresponding dimeric alkynide species [Me2Si(C5Me4)(NAr)Lu(mu-CCR)]2 are thought to be the true catalysts, some of which have been isolated and structurally characterized. These alkynide species were thermally stable and soluble at the reaction temperatures (80-110 degrees C), but they precipitated upon cooling to room temperature after completion of the reaction. Therefore, this catalyst system works homogeneously but can be separated and reused, thus constituting the first example of a recyclable catalyst system for the dimerization of terminal alkynes and also the first example of (Z)-selective head-to-head dimerization of aromatic terminal alkynes.     

Thermal and radiation-induced dehydrochlorination of poly(vinyl chloride). II. Head-to-head structures

The mechanism of dehydrochlorination of 2,3-dichlorobutane and chlorinated polybutadiene which are model compounds of head-to-head poly(vinyl chloride) has been investigated by pyrolysis, thermal, and ultraviolet-induced decomposition. The activation energy of dehydrochlorination for head-to-head poly(vinyl chloride) in nitrogen was 23 kcal/mole at temperatures of 150–190°C, which is slightly smaller than that (29 kcal/mole) for head-to-tail poly(vinyl chloride). The conjugated double bonds were formed by thermal and radiation decomposition of head-to-head poly(vinyl chloride), similar to head-to-tail poly(vinyl chloride). The probability of polyene formation by radiation-induced dehydrochlorination is larger than that by thermal decomposition and is affected by the conformation and the molecular motion of the main chain. This may be due to the alternative mechanism of dehydrochlorination in the thermal and radiation decomposition. The amount of head-to-head linkage of poly(vinyl chloride) samples prepared with various catalysts is dependent on polymerization temperature rather than the kinds of catalyst. Commercial poly(vinyl chloride) has 6–7 head-to-head linkages per 1000 monomeric units.     

Investigating the impact of regiochemistry in ester functionalized polyfurans

In this study, the impact of ester side group regiochemistry on electronic and conformational properties of regioregular polyfurans was explored. A derivative with alternating head-to-head and tail-to-tail orientation of the side groups was synthesized and compared with its head-to-tail analogue, and these were benchmarked against the related regioregular poly(3-hexylfurans). The study revealed that alkyl ester side groups in a head-to-head orientation will create significant steric strain as compared with linear alkyl chains with identical regiochemistry. Though the head-to-head ester side groups lead to twisting along the polymer backbone, they also produce a protective effect against photodegradation in comparison with a poly(3-hexylfuran). Altogether, the work highlights how side groups can impact conformational properties and stability of polyfurans.     

Structure and photophysics of 2-(2'-pyridyl)benzindoles: the role of intermolecular hydrogen bonds

The photophysical properties of two isomeric 2-(2'-pyridyl)benzindoles depend on the environment. Strong fluorescence is detected in nonpolar and polar aprotic solvents. In the presence of alcohols, the emission reveals an unusual behavior. Upon titration of n-hexane solutions with ethanol, the fluorescence intensity goes through a minimum and then increases with rising alcohol concentration. Transient absorption and time-resolved emission studies combined with ground- and excited-state geometry optimizations lead to the conclusion that two rotameric forms, syn and anti, coexist in alcohols, whereas in nonpolar and aprotic polar media, only the syn conformation is present. The latter can form cyclic complexes with alcohols, which are rapidly depopulated in the excited state. In the presence of excess alcohol, syn --> anti rotamerization occurs in the ground state, promoted by the cooperative action of nonspecific and specific effects such as solvent polarity increase and the formation of hydrogen bonds to both donor and acceptor sites of the bifunctional compounds.