Photocyclization Process Research Articles

Synthesis of an alkylthio-substituted dibenz[a,j]anthracene with improved solubility via the oxidative photocyclization of 1,3-distyrylbenzene derivatives

A series of 1,3-distyrylbenzene compounds bearing different substituents at the 2-position have been synthesized and subjected to an oxidative photocyclization process. The 1,3-distyrylbenzene compounds substituted with chloro, alkylthio, and diphenylphosphino groups at the 2-position afforded dibenz[a,j]anthracene derivatives, whereas those bearing methyl, trimethylsilyl, dimethylamino, butoxy, and fluoro groups gave benzo[c]chrysene derivatives. The solubility of dibenz[a,j]anthracene in organic solvents was improved by the introduction of alkylthio groups.

Studies toward the Synthesis of the Epoxykinamycin FL-120B′: Discovery of a Decarbonylative Photocyclization

Photo-Friedel-Crafts acylation of a naphthoquinone was attempted in an effort to access a diazobenzofluorenone en route to the epoxykinamycin natural product FL-120B'. Photoirradiation of the naphthoquinone substrate which resulted in the unexpected formation of a tetracyclic naphthofuran via a decarbonylative photocyclization process is described.

Photochemically Induced Synthesis of the Topoisomerase I Inhibitors Indeno[1,2-c]isoquinoline-5,11-diones

A convenient synthesis of indeno[1,2- c ]isoquinoline-5,11-diones has been achieved using combinational photochemical and carbocationic cyclization tactics. The synthetic route involved first the construction of adequately functionalized N -styryl benz­amides via Suzuki–Miyaura cross-coupling reaction with enol phosphate combined with a six-π-electron photocyclization process. The assembling of the title compounds was readily ensured through sequential carbocationic annulation reaction and ultimate oxidation of a latent hydroxy functionality.

ChemInform Abstract: Photochemical Approach to the Preparation of Lariat Crown Ethers Containing Peptide Sidearms.

New types of lariat type crown ethers containing peptide sidearms were prepared by using a novel strategy employing single electron transfer (SET)-induced photocyclization reactions of -silylether terminated phthalimides. Reactions of chiral substrates in this series produced diastereomeric mixtures of crown ether products as a result of the formation of new stereogenic center generation in the photocyclization process.

X-Ray Crystal Structure and Photo Chemistry of N-Methyl-N-(1-phenylvinyl)benzamide

N-methyl-N-(1-phenylvinyl)benzamide (I) has empirical formula C16H15NO, crystallizes in the monoclinic space group, P21/n, with unit cell parameters a = 8.9101(2) A ˚ , b = 15.1416(4) A ˚ , c = 9.7737(2) A ˚ , a = 90, b = 109.3320(10), c = 90, Z = 4. Interestingly, the Compound (I) undergoes photocyclization upon irradiation of light to give two cyclic lactams (A and B) below 3100 A ˚ . Compound (I) can be cyclized at either of two equivalent ortho positions. The mechanism for formation of lactam A may involve electrocyclic ring closure reaction upon irradiation of light to give six-membered cyclic zwitterionic intermediate. The ring proton rearranges via (1, 5)-H shift to give isomerized product A. The minor photoproduct B is supposed to be formed via photocyclization process fol- lowed by a loss of proton and the tautomerized product undergoes oxidation for the revival of aromaticity.

Photochemical Approach to the Preparation of Lariat Crown Ethers Containing Peptide Sidearms

New types of lariat type crown ethers containing peptide sidearms were prepared by using a novel strategy employing single electron transfer (SET)-induced photocyclization reactions of <TEX>$\alpha$</TEX>-silylether terminated phthalimides. Reactions of chiral substrates in this series produced diastereomeric mixtures of crown ether products as a result of the formation of new stereogenic center generation in the photocyclization process.

Photocatalytic degradation of the fungicide Fenhexamid in aqueous TiO 2 suspensions: Identification of intermediates products and reaction pathways

Liquid-chromatography interfaced with time-of-flight mass spectrometry (LC–TOF/MS) was used to separate and characterize the transformation products arising from TiO 2-photocatalytic degradation of the fungicide Fenhexamid (FEX) in aqueous solution under simulated solar irradiation. Prior to identification, irradiated solutions of FEX (10 mg L −1) were concentrated by solid-phase extraction. Assignments of the mass spectra ions were aided by elemental composition calculations, comparison of structural analogues and available literature, and acquired knowledge regarding mass spectrometry of related heterocyclic compounds. The primary transformation intermediates identified were hydroxyl and/or keto-derivatives. Several positional isomers are typically produced as a consequence of the non-selectivity of the OH radical attack. Moreover, products resulted from the cleavage of the amide and NH dichlorophenol bonds were formed. Finally, cyclic – benzo[d]oxazole intermediates are also formed through an intramolecular photocyclization process and cleavage of halogen – carbon bond. In the case of the hydroxy and/or keto-derivatives, the generic fragmentation scheme obtained from the interpretation of the ESI–TOF–MS data cannot be diagnostic to precisely localize the position of the entering substituent on the FEX molecule, and thus to characterize all its possible oxygenated derivatives by assigning a plausible structure with confidence. On the basis of identified products different pathways of photocatalytic degradation of FEX were proposed and discussed.

Bridged Photochromic Diarylethenes Investigated by Ultrafast Absorption Spectroscopy: Evidence for Two Distinct Photocyclization Pathways

Two photochromic diarylethenes blocked by alkyl bridges in an ideal conformation for photocyclization are studied by stationary and femtosecond transient spectroscopy in order to depict the photocyclization processes: the bistable 1,2-dicyano[2.n]metacyclophan-1-ene with n = 2, abbreviated as [2.2], and its non-bistable analogue with n = 4, abbreviated as [2.4]. The data are interpreted in the light of AM1-CIS calculations and state correlation diagrams based on conclusive TD-DFT calculations. For [2.2], a solvent-sensitive excitation wavelength threshold governing the photocyclization yield is clearly evidenced between the S(1) and S(2) singlet states. Excitation above and beyond this threshold induces two distinct photochemical pathways. The S(1) vertical excitation induces direct efficient (phi approximately = 0.9-1), and ultrafast (approximately 120 fs) photocylization from S(1) open form that leads to a ground-state transition structure, probably through a conical intersection, then to a hot cyclized ground state that relaxes by vibrational cooling. Upon higher excitation energy, the system undergoes internal conversion to the hot S(1) state, then evolves toward the cyclized S(1) state and relaxes by ultrafast S(1)-S(0) internal conversion. Alternatively, the possibility for a second conical intersection near hot S(1) state is discussed. This second photoclosure reaction is less efficient and both the photocylization yield and overall kinetics depend on solvent polarity (phi = 0.49, tau = 2.5 ps in nonpolar solvent; phi = 0.7, tau = 1.5 ps in polar solvent). In the case of [2.4], for which the distance between the two reactive carbons is larger, a unique photoclosure mechanism is found and a structural effect is reported. Indeed, this mechanim is similar to the above second reaction of [2.2] but characterized by much slower kinetics ranging from 12 to 20 ps (depending on the excitation wavelength and solvent polarity). All polarity effects are rationalized in terms of stabilization of the transient states of charge-transfer character involved in the photocyclization process.

Single Electron Transfer-Promoted Photocyclization Reactions of Linked Acceptor−Polydonor Systems: Effects of Chain Length and Type on the Efficiencies of Macrocyclic Ring-Forming Photoreactions of Tethered α-Silyl Ether Phthalimide Substrates

Results of an investigation, aimed at gaining information about the factors governing the efficiencies of single electron transfer (SET)-promoted photocyclization reactions of linked acceptor-polydonor systems, are described. One set of substrates used in this effort includes alpha-trimethylsilyl ether terminated, polymethylene- and polyethylenoxy-tethered phthalimides and 2,3-naphthalimides. Photocyclization reactions of the polyethylenoxy-linked phthalimides and naphthalimides were observed to take place in higher chemical yields and with larger quantum efficiencies than those of analogs containing polymethylene tethers of near equal length. These findings show that the rates of formation of 1,omega-zwitterionic biradicals that serve as key intermediates in the photocyclization processes are enhanced in substances that contain oxygen donor sites in the chain. The findings suggest that these donor sites facilitate both initial SET to acceptor excited states and ensuing intrachain SET, resulting in migration of the cation radical center to the terminal alpha-trimethylsilyl ether position. In addition, an inverse relationship was observed between the quantum yields of photocyclization reactions of the tethered phthalimides and naphthalimides and the length of the polyethylenoxy chain. Finally, the roles played by chain type and length in governing photoreaction efficiencies were investigated by using intramolecular competition in photoreactions of polyethylenoxy and polymethylene bis-tethered phthalimides. Mechanistic interpretations and synthetic consequences of the observations made in this study are discussed.

Photochromic behavior of retinal analogs

New dihetarylethenic retinal analogs were synthesized, and their photochromic behavior was studied. The relationship between the compound structures and their spectral and kinetic parameters was determined due to the comparative spectral-kinetic studies of synthesized dihetarylethenes. It was shown, that all synthesized compounds possess expressed photochromic properties and the thermal irreversibility. The best kinetic characteristics for the photocyclization process as well as a low photodegradation were measured for short-chain nitrile.

Structural Effects on the Ground and Excited-state Properties of Photoswitchable Hydrogen-Bonding Receptors

The ground- and excited-state properties of a series of photochromic barbiturate receptors (N,N'-bis{6-[omega-(anthracen-9-yloxy)alkanoylamino]pyridin-2-yl}-5-t-butyl-isophthalamide, Tn), in which anthracene chromophores are tethered via (CH2)n (n = 1, 3-6) polymethylene linkers to the H-bond receptor moiety, are described. In these systems, the thermally reversible [4pi + 4pi] photodimerization of the anthracenes yields macrocyclic receptors (TnC) that possess significantly reduced affinity toward barbital as compared to their acyclic counterparts. The length of the tether not only determines the overall binding ability of the cyclized receptor but also has a profound influence on the photochemical and photophysical properties of the anthracene chromophores. The reduced mobility experienced by the covalently bound anthracenes controls the reactivity of a fluorescent excimer that is proposed to be an intermediate in the photocyclization process.

Electron transfer-initiated asymmetric photocyclization of chiral auxiliary-substituted N-acyl-α-dehydro(1-naphthyl)alaninamides to the corresponding 3,4-dihydrobenzo[ f]quinolinone derivatives

Photoinduced electron transfer reactions of the title N-acyl-α-dehydronaphthylalaninamides [ ( Z)- 1 ] with ( S)-1-phenylethylamino and ( S)-alaninamide auxiliary groups in methanol containing a tertiary amine were shown to form ( R, S)- and ( S, S)-3,4-dihydrobenzo[ f]quinolinone derivatives ( 2 ) in excess at rt, respectively. The magnitude of diastereomeric excess (de) was varied in the range of −5–26% for ( R, S)- 2 and 16–92% for ( S, S)- 2 , depending on the chiral auxiliary and reaction temperature. The mechanism of asymmetric induction in the photocyclization process eventually affording diastereomeric 2 was discussed based on solvent, tertiary amine, chiral auxiliary and temperature effects on the de value as well as on MM2 and PM5 calculations for the diastereomeric enol intermediates.

Photocyclization of 2-vinyldiphenylacetylenes and behavior of the isonaphthalene intermediates

The conformation, electronic structure, spectroscopy, and unimolecular photoisomerization of 2-vinyldiphenylacetylene and two derivatives have been investigated. 2-Vinyldiphenylacetylene exists predominantly in a planar anti conformation. Introduction of an alpha-methyl substituent results in increased phenyl-vinyl dihedral angles for both syn and anti conformers, whereas a cyclic analog is constrained to a syn conformation with a large phenyl-vinyl dihedral angle. All three molecules undergo photocyclization to yield unstable cyclic allene (isonaphthalene) intermediates which undergo further reactions leading to stable products. Both the photocyclization process and behavior of the allene intermediate are dependent upon ground state conformation. The photophysical behavior of the 2-vinyl derivative, namely its short singlet lifetime and low fluorescence quantum yield, is similar to that of diphenylacetylene. It also has a low quantum yield for photocyclization. The 2-isopropenyl derivative and conformationally locked cyclic analog have relatively long singlet lifetimes and large quantum yields for fluorescence and cyclization. The difference in excited state behavior of the planar 2-vinylacetylene and its non-planar analogs is attributed to the effect of the phenyl-vinyl dihedral angle on the barriers for activated decay of the linear singlet state. However, the behavior of the 2-isopropenyl derivative does not appear to be dependent upon ground state conformation (synvs.anti). The cyclic allene intermediates undergo sequential protonation-deprotonation in methanol solution to yield stable products. The 2-vinyl derivative yields only the fully aromatized 2-phenylnaphthalene. However, the 2-isopropenyl and cyclic derivatives yield mixtures of fully and partially aromatized products. Preferential formation of the partially aromatized products is attributed to a stereoelectronic effect on the deprotonation step. In diethyl ether solution only the fully aromatized product is formed via a free radical mechanism.

Heck Vinylation of Aryl Iodides by a Silica Sol—Gel Entrapped Pd(II) Catalyst and Its Combination with a Photocyclization Process.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Relative Contributions of Diabatic and Adiabatic Channels to Photoisomerization and Photocyclization of Diarylethylenes

Relative contributions of diabatic and adiabatic channels to photoisomerization and photocyclization processes were measured for stilbene, 1-phenyl-2-(2-naphthyl)ethylene (Ph-2N), 1,2-di-(1-naphthyl)ethylene (1N-1N), 1-(1-naphthyl)-2-(2-naphthyl)ethylene (1N-2N), and 1,2-di-(2-naphthyl)ethylene (2N-2N) by the steady-state spectrophotometric method in solutions at room temperature. The effects of solvent nature and atmospheric oxygen on the quantum yields of these processes in 1N-1N and 2N-2N were studied.

Heck vinylation of aryl iodides by a silica sol-gel entrapped Pd(II) catalyst and its combination with a photocyclization process.

[reaction: see text] Silica sol-gel encaged PdCl(2)(PPh(3))(2) is a recyclable catalyst for the Heck vinylation of aryl iodides. It is possible to couple the Heck reaction with photocyclization in a one-pot process.

Asymmetric transformation of chiral auxiliary-substituted N-acyl-α-dehydro(1-naphthyl)alanines into 3,4-dihydrobenzo[ f]quinolinone derivatives via photoinduced electron transfer

Electron transfer-initiated photocyclizations of the title compounds [ ( Z)- 1 ] with an ( S)-alanine methyl ester auxiliary in methanol containing a tertiary amine were found to give ( S, S)- and ( R, S)-3,4-dihydrobenzo[ f]quinolinones ( 2 ) as major products. The magnitude of diastereomeric excess (de) for ( S, S)- 2 was varied from 0 to 55%, depending on the properties of the amine and solvent employed. The mechanism of asymmetric induction in the photocyclization process eventually affording 2 was discussed based on solvent, tertiary amine and temperature effects on the de value.

A synthetic strategy for the preparation of cyclic peptide mimetics based on SET-promoted photocyclization processes.

A novel method for the synthesis of cyclic peptide analogues has been developed. The general approach relies on the use of SET-promoted photocyclization reactions of peptides that contain N-terminal phthalimides as light absorbing electron acceptor moieties and C-terminal alpha-amidosilane or alpha-amidocarboxylate centers. Prototypical substrates are prepared by coupling preformed peptides with the acid chloride of N-phthalimidoglycine. Irradiation of these substrates results in the generation of cyclic peptide analogues in modest to good yields. The chemical efficiencies of these processes are not significantly affected by (1) the lengths of the peptide chains separating the phthalimide and alpha-amidosilane or alpha-amidocarboxylate centers and (2) the nature of the penultimate cation radical alpha-heterolytic fragmentation process (i.e., desilylation vs decarboxylation). An evaluation of the effects of N-alkyl substitution on the amide residues in the peptide chain showed that N-alkyl substitution does not have a major impact on the efficiencies of the photocyclization reactions but that it profoundly increases the stability of the cyclic peptide.

Entropy-controlled diastereoselectivity in the photocyclization of rigid derivatives of o-allylaniline.

Two rigid derivatives of o-allylaniline, namely 8-allyl-2-phenyl-1,2,3,4-tetrahydroquinoline (1b) and 7-(trans-2-cinnamyl)-2-methylindoline (1c), have been chosen as suitable systems to study the potential stereoselectivity of the photocyclization process. Photolysis of 1b leads to a mixture of diastereomeric lilolidines 4 (trans/cis), while 1cproduces a mixture of 4 (trans/cis) and the tetrahydropyrrolo[3,2,1-hi]indole derivatives 5 (trans/cis). To disclose whether the diastereoselectivity could be entropy dependent, photolysis of 1b and 1c has been performed at several temperatures. In both cases, linear relationships have been observed when ln(k(t)/k(c)) (the relative reaction rate constants calculated from the diastereomeric excess) is plotted against the reciprocal temperatures. However, significant entropy-controlled diastereoselectivity has only been found for the photocyclization of 1c to 4. The fluorescence spectra of 1b,c show formation of intramolecular charge-transfer exciplexes, which is in agreement with the proposed excited-state electron-transfer mechanism for photocyclization.

The synthetic potential of phthalimide SET photochemistry.

The authors' studies in the area of phthalimide photochemistry are discussed in the context of the development of new methods for N-heterocycle synthesis. Emphasis is given to reactions which are initiated by both intermolecular and intramolecular SET from silicon-containing electron donors to excited states of phthalimides and related maleimides and conjugated imides. The photoaddition and photocyclization processes which ensue follow mechanistic pathways, in which efficient desilylation of initially formed radical cation occurs to generate radical pair and biradical intermediates that serve as precursors of the products. Several examples that demonstrate the preparative potential of these reactions are presented. These are taken from the authors' investigations of (1) phthalimido-alkylsilane photocyclization reactions, (2) azomethine ylide-forming excited-state processes of N-(trimethylsilylmethyl)phthalimide, and (3) photoaddition and photocyclization reactions of phthalimide alpha-silyl ether, thioether, amine, and amide systems.