Thermal and photochemical isomerization of exocyclic 1,3-dienes: 1,1-diphenyl-3,4-bis(trimethylsilylmethylene)-1-silacyclopentane s-cis(E,E)

This paper describes the photochemical and the thermal isomerization of s-cis(E,E) 1,1-diphenly-3,4-bis(trimethylsilylmethylene)-1-silacyclopentane (1a). Under thermal conditions a 1,3-sigmatropic of the methylene hydrogen occurs, yielding the s-trans isomer (1b). The photochemical irradiation of (1a) at 300 nm for 1 h in deoxygenated benzene gives the corresponding s-cis(E,Z) isomer (1c) and then the s-cis(Z,Z) isomer (1d) after prolonged irradiation (3 h). There was no evidence for the formation of the corresponding cyclobutene resulting from the ring closure of the exocylic diene.     

Conformational study on some β-phenyl-α,β-unsaturated ketones

The conformation of three (E)-β-phenyl-α,β-unsaturated ketones and their corresponding (Z)-isomers 1, 2, and 3 was established by IR and ASIS. (Z)-1 and (E)-1 have exclusively the s-trans conformation. The two isomers of 2 occur in both conformations but there is a higher s-cis to s -trans ratio with the (Z) than with the (E)-isomer. (Z)-3 appears to exist exclusively in the s-cis conformation, the (E)-isomer has a small content of the s-trans conformation. It was concluded that the ASIS for H_b, is a measure of the s-cis content of the conformational equilibrium.     

Competitive Photoisomerization and Energy Transfer Processes in Fluorescent Multichromophoric Systems

Multichromophoric systems showing both fluorescence and photoisomerization are fascinating, with complex interchromophoric interactions. The experimental and theoretical study of a series of compounds, bearing a variable number of 4-dicyanomethylene-2-tert-butyl-6-(p-(N-(2-azidoethyl)-N-methyl)aminostyryl)-4H-pyran (DCM) units are reported. The photophysical properties of multi-DCM derivatives, namely 2DCM and 3DCM , were compared to the single model azido-functionalized DCM , in the E and Z isomers. The (EE)- 2DCM and (EEE)- 3DCM were synthesized via the click reaction. Steady-state spectroscopy and photokinetics experiments under UV or visible irradiation indicated the presence of intramolecular energy transfer processes among the DCM units. Homo- and hetero-energy transfer processes between adjacent chromophores were confirmed by fluorescence anisotropy and decays. Molecular dynamics simulations for 2DCM were carried out and analyzed using a Markov state model, providing geometrical parameters (orientation and distance between chromophores) and energy transfer efficiency. This work contributes to a better understanding and rationalization of multiple energy transfer processes occuring within multichromophoric systems.     

Photochemische Reaktionen. 118. Mitteilung [1] Zur vinylogen β-Spaltung von Enonen. UV.-Bestrahlung von 4-(3',7',7'-Trimethyl-2'-oxabicyclo [3.2.0]hept-3'-en-1'-yl)but-3-en-2-on

Vinylogous β-Cleavage of Enones: UV.-irradiation of 4-(3′,7′,7′-trimethyl-2′-oxabicyclo[3.2.0]hept-3′-ene-1′-yl)but-3-ene-2-on On ¹π,π*-excitation (λ = 254 nm) in acetonitrile (E/Z)- 2 is converted into the isomers 4–9 and undergoes fragmentation yielding 10 ; in methanol (E/Z)- 2 gives 7–10 and is transformed into 11 by incorporation of the solvent. On ¹π,π*-excitation (λ λ?347 nm; benzene-d₆) (E)- 2 is isomerized into (Z)- 2 , which is converted into the isomers 3 and 4 by further irradiation. ¹π,π*-Excitation (λ = 254 nm; acetonitrile) of 4 gives 6 and (E)- 9 , whereas UV.-irradiation (λ = 254 nm; acetonitrile-d₃) of 5 yields (E)- 7 and 8 . On ¹π,π*-excitation (λ = 254 nm; acetonitrile) of (E/Z)- 12 the compounds (E)- 14 and (E)- 15 are obtained.     

Photocatalytic E→Z Contra-Thermodynamic Isomerization of Vinyl Silanes with Lewis Base

Herein, we disclosed the contra-thermodynamic E→Z isomerization of alkenyl silanes, according to the in situ formation of a chromophoric species, in the presence of rac-BINAP as the catalyst. The reaction carried out in DMSO or CH₃CN under irradiation at 405 nm allowed the interconversion of the E-isomers into the Z-congeners in good to excellent yields and outstanding Z/E selectivities, on 18 examples. Finally, the mechanism of this E→Z isomerization was studied to get insight into the reaction mechanism.     

2‐(4′‐Pyridyl‐N‐oxide)‐Substituted Hemithioindigos as Photoresponsive Guests for a Super Aryl‐Extended Calix[4]pyrrole Receptor

We report the synthesis of two 2‐(4′‐pyridyl‐N‐oxide)‐substituted hemithioindigos (HTIs). We probed their photoisomerization by using UV/Vis and ¹H NMR spectroscopy techniques. Light irradiation at λ=450 nm provoked the isomerization of the HTI Z isomer to the E counterpart to a large extent (≈80 % at the photostationary state). ¹H NMR titration experiments revealed the formation of thermodynamically and kinetically stable 1:1 inclusion complexes of the (Z)‐HTI isomers with a super aryl‐extended host (association constant>10⁴ m ^?1). Photoirradiation at λ=450 nm of the inclusion complexes induced the isomerization of the bound HTI N‐oxide to afford the (E)‐HTI?calix[4]pyrrole complex. We determined accurate association constant values for the 1:1 inclusion complexes of the (Z)‐ and (E)‐HTI isomers by using isothermal titration calorimetry experiments. The results showed that the stability constants of the (E)‐HTI complexes were 2.2–2.8‐fold lower than those of the (Z)‐HTI counterparts, which explains the lack of light‐induced release of the former to the bulk solution.     

Kombinierte d-NMR- und LIS-Untersuchungen zum konformativen Verhalten von Formylmethylenthiopyranen

The rotational barriers ΔG ^≠ toE,Z- ands-cis, s-trans-isomerisation in formylmethylenthiopyranes are determined and discussed with regard to their dependence on substituent effects. Preferential conformers are distinguished by lanthanide induced shifts. Best fits between calculated and experimental shifts are obtained forE,s-trans andZ,s-cis isomers. The results indicate a nonbonding interaction between aldehyde oxygen and ring sulphur, favourings-cis conformation in case ofZ configuration.     

Conformational analysis of some (E)-α-phenyl-β-(2-thienyl)- and -(2-furyl_acrylic acids

NMR spectral data of some (E)-α-phenyl-β-(2-thienyl) acrylic acids indicate that these compounds exist in the preferred s-trans conformation. In the case of (E)-α-phenyl-β-(2-furyl)acrylic acids and their methyl esters the presence of only s-cis rotamer has been established.     

Azobenzene: a Visible-Light Chemical Actinometer for the Characterization of Fluidic Photosystems

(E)-Azobenzene is introduced as a suitable chemical actinometer in the visible spectral range (440–540 nm) for photon flux determination of fluidic microphotoreactors or for assessing efficiency of visible light photo-induced reactions, its evaluation is straightforward without tedious analytics. Photoisomerization quantum yields (Φ_E→_Z) of (E)-azobenzene were accurately determined upon irradiation at several wavelengths and in different solvents based on well-known diarylethene.     

Patterned retarders prepared by photoisomerization and photopolymerization of liquid crystalline films

Isomerizable diacrylates derived from cinnamic acid are designed, synthesized and mixed with liquid crystalline diacrylates with the aim of making films with alternating birefringent and isotropic domains by applying the E–Z isomerization process at room temperature. The effects of the structure of the isomerizable‐mesogenic group on the isotropization efficacy, the efficiency of the E–Z isomerization reaction, and film formation are discussed. Compounds derived from cyclohexyl cinnamate are proved to be good candidates that meet a whole set of parameters related to processing and application. These compounds exhibit a low nematic‐to‐isotropic transition temperature. In addition, they show no yellowing upon irradiation, unlike similar compounds derived from phenyl cinnamate. To elucidate the origin of isotropization of the film by irradiation, the pure Z‐isomer is prepared by photolysis of the E‐isomer and subsequent chromatographic separation of both isomers. Analysis of reference samples containing the pure isomers reveals that the decrease in transition temperature can be attributed exclusively to the E–Z photoisomerization process. Finally, thin films with alternating birefringent and isotropic parts of 100×100 µm² are obtained by using a combination of photoisomerization in air and photopolymerization in a nitrogen atmosphere, which is referred to as photo‐patterning.     

Control of the Helical Chirality of Enantiopure Sulfinyl (Z)‐Azobenzene‐Based Photoswitches

A new class of enantiopure ortho,ortho‐disubstituted azobenzene photoswitches has been synthesized from (S)‐2‐(p‐tolylsulfinyl)benzoquinone and arylhydrazines. The sulfoxide acts as a unidirectional controller of the helical chirality that arises in the Z isomer after photoisomerization. Highly congested E‐azobenzenes 5 c showed two atropisomeric diastereoconformers in the solid state that converged upon irradiation into a unique Z isomer with defined helicity (M), as evident in the X‐ray structure. The chiroptical properties of this three‐state enantiopure switch can be externally tuned both photochemically and/or thermally. Theoretical CD spectra calculated by using time‐dependent DFT methods support the existence of two atropoisomeric E isomers and only one Z isomer with (M) helicity. Complementary to the classical azobenzene‐based switches, the photoswiching event is promoted under green/blue light and do not occur under UV irradiation.     

Geometric Isomerisation of Bifunctional Alkenyl Fluoride Linchpins: Stereodivergence in Amide and Polyene Bioisostere Synthesis

Amide groups are pervasive across the chemical space continuum, where their structural and pharmacological importance, juxtaposed with the hydrolytic vulnerabilities, continues to fuel bioisostere development. Alkenyl fluorides have a venerable history as effective mimics (Ψ[CF=CH]) owing to the planarity of the motif and intrinsic polarity of the C(sp²)−F bond. However, emulating the s-cis to the s-trans isomerisation of a peptide bond with fluoro-alkene surrogates remains challenging, and current synthetic solutions only enable access to a single configuration. Through the design of an ambiphilic linchpin based on a fluorinated β-borylacrylate, it has been possible to leverage energy transfer catalysis to affect this unprecedented isomerisation process: this provides geometrically-programmable building blocks that can be functionalised at either terminus. Irradiation at λ_max=402 nm with inexpensive thioxanthone as a photocatalyst enables rapid, effective isomerisation of tri- and tetra-substituted species (up to E/Z 98 : 2 in 1 h), providing a stereodivergent platform for small molecule amide and polyene isostere discovery. Application of the methodology in target synthesis and initial laser spectroscopic studies are disclosed together with crystallographic analyses of representative products.     

Synthesis and photochromic properties of 4-[2-(anthracen-9-yl)-5-methyloxazolyl] fulgide

New photochromic fulgide, viz., 4-{1-[2-(anthracen-9-yl)-5-methyloxazol-4-yl]ethylidene}-3-isopropylidenetetrahydrofuran-2,5-dione, with fluorescent properties was synthesized. Studies by electronic, IR, and ¹H NMR spectroscopy and X-ray diffraction demonstrated that this fulgide exists in the Z form. Light irradiation of its solutions at a wavelength of 365 nm causes Z/E isomerization giving rise to the thermally stable cyclic form. The latter is transformed into the starting E isomer under light irradiation at λ = 436 nm. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 97–101, January, 2006.     

Photochemische Reaktionen. 88. Mitteilung [1]. Zur Photochemie des Iso-methyl-β,(E)-jonon-epoxids

Photolysis of iso-methyl-β,(E)-ionone-epoxide On n, π*-excitation (λ ≥ 347 nm) the title compound 7 isomerizes to the (Z)-enone-epoxide 8 , which yields the bicyclic alkohol 9 in a second photochemical step. The photoisomerization 8 → 9 is a further example for the influence of a methyl substituent at C(α) of an enone-chromophore on the nature of the photochemical processes. On UV. irradiation in the presence of traces of hydrochloric acid 7 gives quantitatively the furane 10 .     

Reversible Photoswitching of Isolated Ionic Hemiindigos with Visible Light

Indigoid chromophores have emerged as versatile molecular photoswitches, offering efficient reversible photoisomerization upon exposure to visible light. Here we report synthesis of a new class of permanently charged hemiindigos (HIs) and characterization of photochemical properties in gas phase and solution. Gas-phase studies, which involve exposing mobility-selected ions in a tandem ion mobility mass spectrometer to tunable wavelength laser radiation, demonstrate that the isolated HI ions are photochromic and can be reversibly photoswitched between Z and E isomers. The Z and E isomers have distinct photoisomerization response spectra with maxima separated by 40–80 nm, consistent with theoretical predictions for their absorption spectra. Solvation of the HI molecules in acetonitrile displaces the absorption bands to lower energy. Together, gas-phase action spectroscopy and solution NMR and UV/Vis absorption spectroscopy represent a powerful approach for studying the intrinsic photochemical properties of HI molecular switches.     

NMR stereospecific long-range coupling and preferred conformations in some (E)- and (Z)-α-phenyl-β-[2-(N-methyl)nitropyrrolyl] acrylic acids

NMR data have been used to assign the stereochemistry to some new (E)- and (Z)-α-phenyl-β-[2-(N-methyl)nitropyrrolyl]acrylic acids. The (E)-molecules are biased in the s-cis conformation showing NMR spectroscopic features strictly depending on the conformation.The analysis of the NMR spectra reveals that the (Z)-isomers exist also in the s-cis conformation.     

Solution Conformations,Photophysics, and Photochemistry of Bile Pigments; Bilirubin and Biliverdin,Dimethyl Esters and Related Linear Tetrapyrroles

Bilirubin and biliverdin dimethyl esters (BRE and BVE, respectively) and related linear tetrapyrroles have been studied using a combination of photochemical and spectroscopic techniques, the latter including absorption, fluorescence fluorescence excitation, medium-induced circular dichroism, and proton magnetic resonance. Both types of tetrapyrroles form mixtures of different topological isomers in very dilute solutions. In the case of the bilirubins the heterogeneity of the solutions is caused by two coexisting conformers with different orientations of the A/B and C/D pyrromethenone moieties with repect to each other. The spectral properties of one conformer resemble the isolated parent pyrromethenone, whereas those of the other result from electronic coupling of the two subchromophores presumably held in a “ridge tile” -like orientation. C? C rotations at the C-5 and C-15 bridges substantially compete in both components with the photochemical channels (E→Z isomerization and lumirubin formation) for the radiationless deactivation of the excited singlet state. The more rigid “ridge tile” component additionally undergoes hydrogen bond-mediated deactivation, and it photoisomerizes more efficiently. The situation is markedly more complex with the biliverdins. In order to obtain a more detailed insight into the mechanisms of the radiationless excited-state processes, time-resolved optoacoustic spectroscopy and ultrafast absorption (pump-probe) and fluorescence detection (single-photon-timing) techniques were used to supplement the stationary methods. The solution mixtures are composed of a (family of) helically coiled all-Z, all-syn species, and of species differing from the former by stretched arrangements of the rings B and C around the central C-10 bridge (E-anti, E-syn, and Z-anti). Two excited singlet states with picosecond lifetimes are attributed to either one or two coiled ground-state forms, and two remarkably long-lived nanosecond excited states arise each from a stretched ground state. The radiationless deactivation of the shorter-lived of the picosecond states is brought about by ultrafast intramolecular proton transfer between the B/C nitrogen atoms, in addition to the C? C rotational modes operative in both. Z→E photoisomerization is also an appreciable deactivation channel of excited biliverdin dimethyl ester. It is confined to the central C-10 double bond and selectively affords a stretched isomer (10E-anti), which thermally reforms the coiled starting meterial at room temperature via a sequence of tautomerization and C? C rotation. Heating or ultrasonic treatment can reverse this sequence and drive it farther to populate another stretched isomer (10E-syn) which is thermally stable at room temperature. This stretched form aggregates (presumably to dimers) already at concentrations at which the coiled species still appears to be fully monomeric.     

Contra‐Thermodynamic,Photocatalytic E→Z Isomerization of Styrenyl Boron Species: Vectors to Facilitate Exploration of Two‐Dimensional Chemical Space

Designing strategies to access stereodefined olefinic organoboron species is an important synthetic challenge. Despite significant advances, there is a striking paucity of routes to Z‐α‐substituted styrenyl organoborons. Herein, this strategic imbalance is redressed by exploiting the polarity of the C(sp²)−B bond to activate the neighboring π system, thus enabling a mild, traceless photocatalytic isomerization of readily accessible E‐α‐substituted styrenyl BPins to generate the corresponding Z‐isomers with high fidelity. Preliminary validation of this contra‐thermodynamic E→Z isomerization is demonstrated in a series of stereoretentive transformations to generate Z‐configured trisubstituted alkenes, as well as in a concise synthesis of the anti‐tumor agent Combretastatin A4.     

Contra‐Thermodynamic,Photocatalytic E→Z Isomerization of Styrenyl Boron Species: Vectors to Facilitate Exploration of Two‐Dimensional Chemical Space

Designing strategies to access stereodefined olefinic organoboron species is an important synthetic challenge. Despite significant advances, there is a striking paucity of routes to Z‐α‐substituted styrenyl organoborons. Herein, this strategic imbalance is redressed by exploiting the polarity of the C(sp²)?B bond to activate the neighboring π system, thus enabling a mild, traceless photocatalytic isomerization of readily accessible E‐α‐substituted styrenyl BPins to generate the corresponding Z‐isomers with high fidelity. Preliminary validation of this contra‐thermodynamic E→Z isomerization is demonstrated in a series of stereoretentive transformations to generate Z‐configured trisubstituted alkenes, as well as in a concise synthesis of the anti‐tumor agent Combretastatin A4.     

Photochemische Reaktionen 114. Mitteilung [1]. Zur Photochemie cyclischer Acetale vom 1,3-Dioxa-4,6-cycloheptadien-Typus

Photochemistry of Cyclic Acetals of the 1,3-Dioxa-4,6-cycloheptadiene Type UV.-irradiation (λ=254 nm) of 3 gives the isomers (E)- 5 (4%), (Z)- 5 (60%) and 6 (3%). On triplet sensitization (acetone; λ ≥ 280 nm) 3 is converted to (E)- 5 (3%), (Z)- 5 (7%) and 7 (9%). ? The ¹π,π*-excitation (λ=254 nm) of 4 yields the isomers 2 (9%), 8 (10%), 9 (34%), 10 (20%) and 11 (3%). On thermolysis (200°) 4 gives 10 (87%) by a Claisen-rearrangement.