Widely Electronically Tunable 2,6-Disubstituted Dithieno[1,4]thiazines—Electron-Rich Fluorophores Up to Intense NIR Emission

2,6-Difunctionalized dithieno[1,4]thiazines were efficiently synthesized by (pseudo)five- or (pseudo)three-component one-pot processes based on lithiation-electrophilic trapping sequences. As supported by structure–property relationships, the thiophene anellation mode predominantly controls the photophysical and electrochemical properties and the electronic structures (as obtained by DFT calculations). From molecular geometries and redox potentials to fluorescence quantum yields in solution, the interaction of the dithieno[1,4]thiazine-core with the substituents causes striking differences within the series of regioisomers. Most interestingly, strong acceptors introduced in anti–anti dithieno[1,4]thiazines nearly induce a planarization of the ground-state geometry and a highly intense NIR fluorescence (Φ_F=0.52), whereas an equally substituted syn–syn dithieno[1,4]thiazine exhibits a much stronger folded molecular structure and fluoresces poorly (Φ_F=0.01). In essence, electrochemical and photophysical properties of dithieno[1,4]thiazines can be tuned widely and outscore the compared phenothiazine with cathodically shifted oxidation potentials and redshifted and more intense absorption bands.     

The Structure and Conformation of (CH3)3CSNO

The gas‐phase molecular structure of (CH₃)₃CSNO was investigated by using electron diffraction, allowing the first experimental geometrical parameters for an S‐nitrosothiol species to be elucidated. Depending on the orientation of the ?SNO group, two conformers (anti and syn) are identified in the vapor of (CH₃)₃CSNO at room temperature, the syn conformer being less abundant. The conformational landscape is further scrutinized by using vibrational spectroscopy techniques, including gas‐phase and matrix‐isolation IR spectroscopy, resulting in a contribution of ca. 80:20 for the anti:syn abundance ratio, in good agreement with the computed value at the MP2(full)/cc‐pVTZ level of approximation. The UV/Vis and resonance Raman spectra also show the occurrence of the conformational equilibrium in the liquid phase, with a moderate post‐resonance Raman signature associated with the 350 nm electronic absorption.     

Modulation of Conformational Preferences of Heteroaromatic Ethers and Amides through Protonation and Ionization: Charge Effect

Multiple approaches reveal the strong effects of a positive charge introduced by protonation or ionization on the conformation of o-heteroaromatic ethers and amides. The ethers and amides containing an ortho-N heteroatom are syn-preferring while those containing an ortho-O or ortho-S heteroatom are mostly anti-preferring. However, for all the monocyclic o-heteroaromatic ethers and amides, the protonated ones are all anti-preferring while the ionized ones are all syn-preferring. Interestingly, although both the protonation and ionization introduce a positive charge, they have such different effects on molecular conformation, very informative for understanding the origin of conformational preferences. Detailed analysis shows that the population of the introduced positive charge dictates the conformational preferences via electrostatic and orbital interactions. Compared to ortho-heteroatoms, meta-heteroatoms have weaker effect on conformational preference. Achieved by complete inductive method, the regularity of conformational preferences and switching provides easy ways to modulate conformers (by pH or redox), and makes this kind of ether or amide bond a conformational hinge applicable to design of functional molecules (drugs and materials) and modulation of molecular biological processes.     

Functionalization of one or two methyl groups in the [Cp*<Subscript>2</Subscript>RuBr]<Superscript>+</Superscript>Br<Superscript>−</Superscript> complex in the reaction with bromine

The reaction of [Cp*₂RuBr]⁺Br⁻ with bromine in CH₂Cl₂ (CD₂Cl₂) in an inert atmosphere at room temperature produces the complexes [Cp*Ru(Br)C₅Me₄CH₂Br]⁺Br₃ ⁻ (syn conformer), [Cp*Ru(Br)C₅Me₃(CH₂Br)₂]⁺ (syn and anti conformers), and [Ru(Br)(C₅Me₄CH₂Br)₂]⁺ (syn conformer). All complexes were characterized by ¹H and ¹³C NMR spectroscopy; the former complex, by elemental analysis. These complexes were also prepared by the reaction of [Cp*RuC₅Me₄CH₂]⁺BF₄ ⁻ with bromine in CH₂Cl₂. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2712–2718, December, 2005.     

Encapsulation-Controlled Photoisomerization of a Styryl Derivative: Stereoselective Formation of the Anti Z-Isomer in the Cucurbit[7]uril Cavity

The photochemical isomerization of a styrylpyridinium dye ( SP ) bearing an unsymmetrically attached benzo-15-crown-5 ether has been studied in aqueous solution in the absence and presence of cucurbit[7]uril (CB[7]). The detailed analysis of the UV/Vis and NMR spectra showes that the isomeric composition of the photostationary mixtures of SP can be modulated by the host-guest complexation with CB[7]. It was found that steric hindrance caused by encapsulation of SP in the host cavity induces the exclusive formation of the anti conformer of Z- SP in contrast with the mixture of both anti and syn conformers obtained during photoisomerization of the dye without CB[7]. Remarkably, the displacement of anti Z- SP from CB[7] does not lead to the transformation of the anti Z-isomer into the syn Z-isomer pointing out the conformational memory of the system. The results provide an interesting example of the supramolecular stereorecognition by the achiral CB[7] host.     

Rotational isomers of small molecules in noble-gas solids: From monomers to hydrogen-bonded complexes

Molecular conformation, quantum tunneling, and hydrogen bonding play important roles in various photochemical processes. We have studied a number of small molecules possessing rotational isomerism (HONO, formic acid, acetic acid, etc.) isolated in noble-gas solid matrices. Selective vibrational excitation efficiently promotes the conformational change in the excited molecule, which allows preparation of higher-energy conformers. Stability of the higher-energy conformers is often limited by quantum tunneling of hydrogen as observed for some carboxylic acids (formic, acetic, etc.). The tunneling mechanism is supported by the strong H/D isotope effect and characteristic temperature dependence with a clear low-temperature limit. The reaction barrier height is an important factor in a tunneling process; however, other factors also play an essential role. The energy mismatch between the initial state of the higher-energy conformer and accepting state of the ground-state conformer is probably important. Hydrogen bonding can change tunneling decay rate of unstable conformers. The trans–cis formic acid dimer was prepared by vibrational excitation of the trans–trans form in neon and argon matrices. Tunneling decay of cis formic acid is substantially slower in the dimeric form compared to monomer, especially in solid neon. This stabilization effect is explained by a complexation-induced increase of reaction barrier, which is confirmed computationally. The complex between cis formic acid and water was prepared in an argon matrix and found to be stable at low-temperatures. These results show that intrinsically unstable conformational structures can be thermodynamically stabilized in asymmetrical hydrogen-bonded network. This effect occurs when the energy difference between conformers is smaller than the hydrogen bond interaction energy, which allows chemistry of unstable conformers to be studied.     

Novel solid-state synthesis of dimeric 4-aryl-1,4-dihydropyridines

On irradiation in the solid state the 4-aryl-1,4-dihydroypridines 1 undergo [2+2] cycloadditon to centrosymmetric head-to-tail dimers 3 and 4a . The almost exclusive formation of the cage dimers 3 via the C₂-symmetric syn-dimers 2 takes place in nearly quantitative yields, in contrast with the cycloaddition reaction of the anti-dimer 4a , which is accompanied by photooxidation to pyridine 5a .     

1,N6‐Etheno‐2′‐deoxytubercidin hemihydrate

The title compound [systematic name: 7‐(2‐deoxy‐β‐d ‐erythro‐pentofuranosyl)‐7H‐imidazo[1,2‐c]pyrrolo[2,3‐d]pyrimidine hemihydrate], 2C₁₃H₁₄N₄O₃·H₂O or (I)·0.5H₂O, shows two similar conformations in the asymmetric unit. These two conformers are connected through one water molecule by hydrogen bonds. The N‐glycosylic bonds of both conformers show an almost identical anti conformation with χ = −107.7 (2)° for conformer (I‐1) and −107.0 (2)° for conformer (I‐2). The sugar moiety adopts an unusual N‐type (C3′‐endo) sugar pucker for 2′‐deoxyribonucleosides, with P = 36.8 (2)° and τ_m = 40.6 (1)° for conformer (I‐1), and P = 34.5 (2)° and τ_m = 41.4 (1)° for conformer (I‐2). Both conformers and the solvent molecule participate in the formation of a three‐dimensional pattern with a `chain'‐like arrangement of the conformers. The structure is stabilized by intermolecular O—H...O and O—H...N hydrogen bonds, together with weak C—H...O contacts.     

Synthesis and Photochemistry of syn- and anti-9,10-Epoxy-1, 4-dihydro-1,4-ethanonaphthalene-2,3-diones

The title epoxydiketones were prepared stereoselectively, direct epoxidation of 3a-c with MCPBA produced syn isomers la-c whereas epoxidation of 4a-d followed by saponification of the spirolactone rings gave anti isomers 2a-c. The stereochemistry of 1a and 2a was established by X-ray diffraction, whereas that of the remaining epoxydiketones was determined from the correlation of visible, ¹H, and ¹³C NMR spectral data; the differences between spectra of the corresponding syn and anti isomers are explained in terms of through-space interaction and steric effects. Photolysis of syn isomers 1a-c afforded the corresponding naphthalenes 8a-c in almost quantitative yields; in contrast, irradiation of anti isomers 2a-c gave complicated mixtures. The quantum yield of disappearance of 1a was 52 times that of 2a. Reaction mechanisms are proposed to account for the product formation. The differences in the photochemical behavior of the title syn and anti isomers are rationalized in terms of stereoelectronic effects of the epoxy rings in the syn isomers.     

Electronic structure of π-systems. Studies of electronic structures and tautomeric transformations of a series of 4-methyl-8-(R-phenylazo)dihydrofuro[2,3-<Emphasis Type="Italic">h</Emphasis>]coumarin-9-ones

Tautomeric transformations of a series of 4-methyl-8-(R-phenylazo)dihydrofuro[2,3-h]-coumarin-9-ones were studied by ¹H NMR, IR, electronic absorption spectroscopy, mass spectrometry, and quantum chemistry methods. The obtained results suggest that (p-R-phenyl)-derivatives exist in the quinone hydrazone form, while (o-R-phenyl)derivatives exist as a mixture of quinone hydrazone and azo enol tautomers with domination of the former. The anti-quinone hydrazone tautomers with a strong intramolecular hydrogen bond are more favorable than the syn-quinone hydrazone tautomers.     

Microwave‐Assisted Synthesis of Substituted Dibenzo[b,f][1,4]thiazepines,Dibenzo[b,f][1,4]oxazepines,Benzothiazoles, and Benzimidazoles

A highly efficient synthesis for possessing 7‐membered rings with two heteroatoms is described, using efficient microwave‐assisted one‐pot method to synthesize (substituted) dibenzo[b,f][1,4]thiazepines [1] and dibenzo[b,f][1,4]oxazepines [2] in high yields (up to 99%) by cyclocondensations of o‐aminothiophenol or o‐aminophenol with o‐halobenzaldehydes, o‐fluoroacetophenone, and o‐fluorobenzophenone. In the absence of base, o‐aminothiophenol reacted with o‐halobenzaldehydes to afford benzothiazoles.     

Hydrogen-Atom Tunneling in Metaphosphorous Acid

Metaphosphorous acid (HOPO), a key intermediate in phosphorus chemistry, has been generated in syn- and anti-conformations in the gas phase by high-vacuum flash pyrolysis (HVFP) of a molecular precursor ethoxyphosphinidene oxide (EtOPO→C₂H₄+HOPO) at ca. 1000 K and subsequently trapped in an N₂-matrix at 2.8 K. Unlike the two conformers of the nitrogen analogue HONO, the anti-conformer of HOPO undergoes spontaneous rotamerization at 2.8 K via hydrogen-atom tunneling (HAT) with noticeable kinetic isotope effects for H/D (>10⁴ for DOPO) and ¹⁶O/¹⁸O (1.19 for H¹⁸OPO and 1.06 for HOP¹⁸O) in N₂-matrices.     

Die Dienol-Benzol-Umlagerung von Allyldienolen: aromatische [1,2]-, [3,3]- und [3,4]-sigmatropische Umlagerungen

The dienol-benzene rearrangement of syn and anti-4-allyl-4-methylcyclohexa-2,5-dien-1-ol (syn and anti 15) occurs by formation of a benzonium ion intermediate in p-toluene-sulphonic acid in ether below 0° and leads to a mixture of 2-, 3- and 4-allyltoluenes in the ratio 54:10:36. By the introduction of ¹⁴C-, D- and methyl labelled dienols it is shown that only the allyl group migrates and that this rearrangement is an intramolecular, one-step process. The formation of 2-allyltoluene occurs with retention, whereas the 3- and 4-allyltoluenes are formed by inversion of the carbon skeleton of the migrating allyl group. These rearrangements can be therefore classified as suprafacial, aromatic sigmatropic reactions of the order [1,2], [3,3] and [3,4]. The transition state can be postulated as representing a positively charged complex consisting of interacting allyl and tolyl radicals. The interaction of the two parts is controlled by the symmetry of the highest occupied π-orbitals (ψ₃ for toluene and ψ₂ for the allyl group) in agreement with the Woodward-Hoffmann rules. The better “distribution” of the charge in the transition state of these reactions in comparison to the ground state is chiefly responsible for the CoPE-like [3,3] sigmatropic reaction occurring at low temperatures. In general, sigmatropic reactions in charged systems are faster. The rearrangement of syn and anti 2-allyl-2-methylcyclohexa-3,5-dien-1-ol (syn and anti 28) gives results similar to those obtained with the para-allyldienols. The thermal rearrangement of 15 and 28 gives 3-allyltoluene by a [3,3] sigmatropic Cope rearrangement followed by elimination of water.     

Photolysis of Carbonyl Diisocyanate: Generation of Isocyanatocarbonyl Nitrene and Diazomethanone

The stepwise decomposition of carbonyl diisocyanate, OC(NCO)₂, has been studied by using IR spectroscopy in solid argon matrices at 16 K. Upon irradiation with an ArF laser (λ=193 nm), carbonyl diisocyanate split off CO and furnished a new carbonyl nitrene, OCNC(O)N, in its triplet ground state. Two conformers of the nitrene, syn and anti, that were derived from the two conformers of OC(NCO)₂ (62 % syn–syn and 38 % syn–anti) were identified and characterized by combining IR spectroscopy and quantum chemical calculations. Subsequent irradiation with visible light (λ>395 nm) caused the Curtius rearrangement of the nitrene into OCNNCO. In addition to the expected decomposition products, N₂ and CO, further photolysis of OCNNCO with the ArF laser yielded NOCN, through a diazomethanone (NNCO) intermediate. To further validate our proposed reaction mechanism, ArF‐laser photolysis of the closely related NNNNCO and cyclo‐N₂CO in solid argon matrices were also studied. The observations of NOCN and in situ CO‐trapped product OCNNCO provided indirect evidence to support the initial generation of NNCO as a common intermediate during the laser photolysis of OCNNCO, NNNNCO, and cyclo‐N₂CO.     

Computational study of sulfoxides of thiacyclohexane, 4‐silathiacyclohexane, 4‐fluoro‐4‐silathiacyclohexane,and 4,4‐difluoro‐4‐silathiacyclohexane: Relative energies of conformations and sulfinyl oxygen stabilized pentacoordinate silicon in boat and twist structures

Second‐order Møller‐Plesset theory (MP2) has been used to calculate the equilibrium geometries and relative energies of the chair, 1,4‐twist, 2,5‐twist, 1,4‐boat, and 2,5‐boat conformations of thiacyclohexane 1‐oxide (tetrahydro‐2H‐thiopyran 1‐oxide), 4‐silathiacyclohexane 1‐oxide, cis‐ and trans‐4‐fluoro‐4‐silathiacyclohexane 1‐oxide, and 4,4‐difluoro‐4‐silathiacyclohexane 1‐oxide. At the MP2/6‐311+G(d,p) level of theory, the chair conformer of axial thiacyclohexane 1‐oxide is 0.99, 5.61, 5.91, 8.57, and 7.43 kcal/mol more stable (ΔE) than its respective equatorial chair, 1,4‐twist, and 2,5‐twist conformers and 1,4‐boat and 2,5‐boat transition states. The chair conformer of equatorial thiacyclohexane 1‐oxide is 4.62, 6.31, 7.56, and 7.26 kcal/mol more stable (ΔE) than its respective 1,4‐twist and 2,5‐twist conformers and 1,4‐boat and 2,5‐boat transition states. The chair conformer of axial 4‐silathiacyclohexane 1‐oxide is 1.79, 4.26, 3.85, and 5.71 kcal/mol more stable (ΔE) than its respective equatorial chair, 1,4‐twist, and 2,5‐twist conformers and 2,5‐boat transition state. The 2,5‐twist conformer of axial 4‐silathiacyclohexane 1‐oxide is stabilized by a transannular interaction between the sulfinyl oxygen and silicon, to give trigonal bipyramidal geometry at silicon. The chair conformer of equatorial 4‐silathiacyclohexane 1‐oxide is 2.47, 7.90, and 8.09 kcal/mol more stable (ΔE) than its respective 1,4‐twist, and 2,5‐twist conformers and 2,5‐boat transition state. The chair conformer of axial cis‐4‐fluoro‐4‐silathiacyclohexane 1‐oxide is 4.18 and 5.70 kcal/mol more stable than its 1,4‐twist conformer and 2,5‐boat transition state and 1.51 kcal/mol more stable than the chair conformer of equatorial cis‐4‐fluoro‐4‐silathiacyclohexane 1‐oxide. The chair conformer of axial trans‐4‐fluoro‐4‐silathiacyclohexane 1‐oxide is 5.02 and 6.11 kcal/mol more stable than its respective 1,4‐twist conformer and 2,5‐boat transition state, but is less stable than its 2,5‐twist conformer (ΔE = ?1.77 kcal/mol) and 1,4‐boat transition state (ΔE = ?1.65 kcal/mol). The 2,5‐twist conformer and 1,4‐boat conformer of axial trans‐4‐fluoro‐4‐silathiacyclohexane 1‐oxide are stabilized by intramolecular coordination of the sulfinyl oxygen with silicon that results in trigonal bipyramidal geometry at silicon. The chair conformer of axial 4,4‐difluoro‐4‐silathiacyclohexane 1‐oxide is 3.02, 5.16, 0.90, and 6.21 kcal/mol more stable (ΔE) than its respective equatorial chair, 1,4‐twist, and 1,4‐boat conformers and 2,5‐boat transition state. The 1,4‐boat conformer of axial 4,4‐difluoro‐4‐silathiacyclohexane 1‐oxide is stabilized by a transannular coordination of the sulfinyl oxygen with silicon that results in a trigonal bipyramidal geometry at silicon. The relative energies of the conformers and transition states are discussed in terms of hyperconjugation, orbital interactions, nonbonded interactions, and intramolecular sulfinyl oxygen–silicon coordination. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Remote substituent effect favoring the formation of syn-adducts in the chelation controlled radical reactions of γ-benzyloxy-α-methylenecarboxylic acid esters

The chelation controlled radical reactions of ethyl γ-benzyloxy-α-methylenecarboxylates bearing a bulky γ-substituent, such as CHMe₂, CHPh₂, c-C₆H₁₁ and CH(Ph)OTBDMS, with alkyl iodides gave the syn-adducts with high diastereoselectivities. However, the diastereoselectivity for the substrates bearing a γ-substituent CH(i-Pr)OTBDMS depended critically on the configuration of the substituent; the substrate bearing the OTBDMS group anti to the γ-benzyloxy group showed poor diastereoselectivity, but its diastereomer gave syn-adduct exclusively. The high syn-selectivitiy is referred to the H-atom transfer to the outside face of radical center in the sharply folded seven-membered chelate intermediate bearing the ethoxy group with Z-geometry. The corner flapping of the radical center atom of the global minimum energy conformer generates a local minimum conformer and the H-atom transfer to the outside face of the radical center of the newly formed structure gives the anti-adduct. The poor diastereoselectivity is due to the very small energy difference between the two conformers and consequently both the syn- and anti-adducts are yielded in nearly equal amounts.     

The vibrational and NMR spectra,conformations and ab initio calculations of aminomethylene,propanedinitrile and itsN-methyl derivatives

The IR and Raman spectra of aminomethylene propanedinitrile (AM) [H₂N-CH=C(CN)₂], (methylamino)methylene propanedinitrile (MAM) [CH₃NH-CH=C(CN)₂] and (dimethylamino)methylene propanedinitrile (DMAM) [(CH₃)₂N-CH=C(CN)₂] as solids and solutes in various solvents have been recorded in the region 4000-50 cm^–1. AM and DMAM can exist only as one conformer. From the vibrational and NMR spectra of MAM in solutions, the existence of two conformers with the methyl group orientedanti andsyn toward the double C=C bond were confirmed. The enthalpy difference H ⁰ between the conformers was measured to be 3.7±1.4 kJ mol^–1 from the IR spectra in acetonitrile solution and 3.4±1.1 kJ mol^–1 from the NMR spectra in DMSO solution. Semiempirical (AM1, PM3, MNDO, MINDO3) and ab initio SCF calculations using a DZP basis set were carried out for all three compounds. The calculations support the existence of two conformersanti andsyn for MAM, withanti being 7.8 kJ mol^–1 more stable thansyn from ab initio and 8.6, 13.4, 11.6, and 10.8 kJ mor^–1 from AM1, PM3, MNDO, and MINDO3 calculations, respectively. Finally, complete assignments of the vibrational spectra for all three compounds were made with the aid of normal coordinate calculations employing scaled ab initio force constants. The same scale factors were optimized on the experimental frequencies of all three compounds, and a very good agreement between calculated and experimental frequencies was achieved.     

Conformation analysis of formic acid. Extended basis set SCF and CEPA calculations

SCF computations using extended DZP and TZP basis sets have been performed to determine the structure of syn and anti formic acid and the transition state for rotation of the OH group. Effects of electron correlation were accounted for by means of CEPA calculations which predict the anti conformer to be 5 kcal/mol and the transition state 14.7 kcal/mol higher in energy than the syn conformer, with probable error estimates of 0.7 kcal/mol and 2 kcal/mol respectively.     

A suggested revised conformation of methylthionitrite as seen by its spectra and preliminary ab initio calculations

A proof has recently been given that gaseous methylthionitrite, CH₃SNO, occurs exclusively (or mainly) in its anti conformation [1]. The present paper claims that existing spectral evidence and ab initio calculations now performed suggest that gaseous methylthionitrite is mainly the syn conformer, the extra stability being of the order 1–2 kcal mol^?1. Methyl group rotation in the syn conformer is hindered by a three-fold barrier of height 689 cm^?1 while the methyl group rotation in the anti conformer is hardly hindered. The syn/anti energy difference and the barriers hindering methyl group rotation closely parallel the corresponding measured quantities in methylnitrite, CH₃ONO.