New oxorhenium complexes with 2-(2′-hydroxyphenyl)-2-benzoxazolinato ligand. X-ray structure and DFT calculations for [ReOX2(hbo)(AsPh3)] and [ReOX2(hbo)(PPh3)] complexes

The reactions of [ReOX₃(AsPh₃)₂] and [ReOX₃(PPh₃)₂] with 2-(2′-hydroxyphenyl)-2-benzoxazoline (Hhbo) have been examined and [ReOX₂(hbo)(AsPh₃)] and [ReOX₂(hbo)(PPh₃)] (X = Cl, Br) complexes have been obtained. The crystal and molecular structures of [ReOCl₂(hbo)(AsPh₃)] (1) and [ReOBr₂(hbo)(PPh₃)] (4) have been determined. The electronic structures of 1 and 4 have been calculated with the density functional theory (DFT) method. The spin-allowed electronic transitions of 1 and 4 have been calculated with the time-dependent DFT method, and the UV–Vis spectra of these complexes have been discussed.     

Interaction of 2-(2′,6′-dialkylphenylazo)-4-methylphenols with iridium. C-H activation and migration of alkyl group

Reaction of 2-(2′,6′-diethylphenylazo)-4-methylphenol (L²) with [Ir(PPh₃)₃Cl] afforded two organoiridium complexes 3 and 4 via C-H bond activation of an ethyl group in the arylazo fragment of the L² ligand. In both the complexes the azo ligand binds to iridium as a dianionic tridentate C,N,O-donor. Two triphenylphosphines and a hydride (in the case of complex 3) or chloride (in the case of complex 4) are also coordinated to the metal center. A similar reaction of [Ir(PPh₃)₃Cl] with 2-(2′,6′-diisopropylphenylazo)-4-methylphenol (L³) yielded another organoiridium complex 5, where migration of one iso-propyl group from its original location (say, the 2′ position) to the corresponding third position (say, the 4′ position) took place through C-C bond activation. In this complex the modified azo ligand binds to iridium as a dianionic tridentate C,N,O-donor. Two triphenylphosphines and a hydride are also coordinated to the metal center. The structures of complexes 3 and 4 have been optimized through DFT calculations. The structure of complex 5 has been determined by X-ray crystallography. All the complexes show characteristic ¹H NMR signals and intense transitions in the visible region. Cyclic voltammetry on all the complexes shows an oxidation within 0.66-1.10 V vs SCE, followed by a second oxidation within 1.15-1.33 V vs SCE and a reduction within −0.96 to −1.07 V vs SCE.     

Application and details of photoinduced oxidative cyclization of 5-(4′,9′-methanocycloundeca-2′,4′,6′,8′,10′-pentaenylidene)pyrimidine-2,4,6(1,3,5H)-triones and related compounds

As novel methodology for synthesizing the furan ring, a photoinduced oxidative cyclization of 5-(4′,9′-methanocycloundeca-2′,4′,6′,8′,10′-pentaenylidene)pyrimidine-2,4,6(1,3,5H)-triones (7a-c) and related compounds 9a-c was accomplished to give 5,10-methanocycloundeca[4,5]furo[2,3-d]pyrimidine-2,4(1,3H)-dionylium tetrafluoroborates (8a-c⁺·BF₄⁻) and related compounds 2a-c⁺·BF₄⁻, respectively. In the photoinduced oxidative cyclization, the molecular oxygen in air is used as oxidant and the reaction proceeds under mild conditions to give desired products without byproducts, and thus, it is interesting from the viewpoint of the green chemistry. On the reactions of the mono-substituted derivatives 7d,e and 9e,f, the selectivity of the photoinduced cyclizations were reversed as compared with those of the DDQ-promoted oxidative cyclizations. By the NMR monitoring of the reactions of 7a and deuterated compound 7a-D₂ under degassed conditions, the details of the reaction pathway were clarified and rationalized on the basis of the MO calculation by the 6-31G^∗ basis set of the MP2 levels as well.     

Regioselective monobromination of (E)-1-(2′-hydroxy-4′,6′-dimethoxyphenyl)-3-aryl-2-propen-1-ones using bromodimethylsulfonium bromide and synthesis of 8-bromoflavones and 7-bromoaurones

A wide variety of monobrominated compounds 2a-l have been prepared in good yields from (E)-1-(2′-hydroxy-4′,6′-dimethoxyphenyl)-3-aryl-2-propen-1-ones (1a-l) through regioselective ring bromination using 1.5 equiv of bromodimethylsulfonium bromide (BDMS) at room temperature. Similarly, some of the 2′-hydroxychalcones can be converted directly into tribromides 3 or dibromides 4 by employing 4.0 equiv of BDMS under different reaction conditions which in turn can be transformed into 8-bromoflavones and 7-bromoaurones on treatment with 0.2 M ethanolic KOH solution. Mild reaction conditions, good yields and no chromatographic separation are some of the salient features of the present protocol.     

Synthesis, structural characterization, and reactivity of organolanthanides derived from a new chiral ligand (S)-2-(pyrrol-2-ylmethyleneamino)-2′-hydroxy-1,1′-binaphthyl

Condensation of (S)-2-amino-2′-hydroxy-1,1′-binaphthyl with 1 equiv. of pyrrole-2-carboxaldehyde in toluene in the presence of molecular sieves at 70 °C gives (S)-2-(pyrrol-2-ylmethyleneamino)-2′-hydroxy-1,1′-binaphthyl (1H₂) in 90% yield. Deprotonation of 1H₂ with NaH in THF, followed by reaction with LnCl₃ in THF gives, after recrystallization from a toluene or benzene solution, dinuclear complexes (1)₃Y₂(thf)₂ · 3C₇H₈ (3 · 3C₇H₈) and (1)₃Yb₂(thf)₂ · 3C₆H₆ (4 · 3C₆H₆), respectively, in good yields. Treatment of 1H₂ with Ln[N(SiMe₃)₂]₃ in toluene under reflux, followed by recrystallization from a benzene solution gives the dimeric amido complexes {1-LnN(SiMe₃)₂}₂ · 2C₆H₆ (Ln = Y (5 · 2C₆H₆), Yb (6 · 2C₆H₆)) in good yields. All compounds have been characterized by various spectroscopic techniques, elemental analyses and X-ray diffraction analyses. Complexes 5 and 6 are active catalysts for the polymerization of methyl methacrylate (MMA) in toluene, affording syn-rich poly-(MMA)s.     

An experimental and DFT computational study of a novel zerovalent tetracarbonyl tungsten complex of 2-(2′-pyridyl)quinoxaline

Synthesis and characterization of the new complex W(CO)₄(2,2′-pq), (1), where 2,2′-pq = 2-(2′-pyridyl)quinoxaline, is presented. The non-symmetric ligand 2,2′-pq belongs to the general class of quinoxalines, which are natural products yielding a rich coordination chemistry. Complex (1) crystallizes in space group P2_1/n with α = 9.601(6) Å, b = 16.735(11) Å, c = 10.315(8) Å, Z = 4 and V = 1616.0(19) Å³. Although its structure resembles those of W(CO)₄(phen) and W(CO)₄(bpy), some distortions that stem from 2,2′-pq’s asymmetry are present. DFT calculations reveal a ground state consisting of HOMO, HOMO − 1 and HOMO − 2, mainly of metal and carbonyl character, while LUMO is diimine oriented. The bonding scheme of (1) is illustrated after its consideration as been consisted by two fragments, namely W(CO)₄ and 2,2′-pq, acting as a donor and acceptor of electron density, respectively. In that scheme, back-bonding interaction of the main core to 2,2′-pq is mainly related to the mixing of HOMO − 2 from W(CO)₄ moiety with LUMO from 2,2′-pq moiety. The performed TDDFT calculations, not only in the gas phase but also combined with the conductor like polarizable continuum model (CPCM), reveal that the lowest in energy highly solvatochromic transition of (1) can be ascribed as a HOMO − 2 → LUMO transition and it is better described as MLCT/LLCT, underlying the CO → diimine contribution. The solvatochromic behaviour of (1) is anticipated by DFT/CPCM calculations and is probed in detail by absorption and NMR spectroscopy. The correlation of the lowest-energy-band maximum to the dipole moment of the corresponding solvents provides overall good linear fits, while the correlation to the dielectric constant affords good linear patterns only after the segregation of the solvents into groups. The ¹H NMR data of 2,2′-pq and (1) reveal an increase of the solvent influence to the chemical shifts of the diimine ligand after its coordination to the metal and suggest two different types of solvent-effects for the complex and the ligand, respectively. The observed proton shifts of (1) are related with the results of the Mülliken population analysis in solvents of different polarity; the transition from CCl₄ to MeOH seems to signify a charge transfer from the axial COs and the central metal to the equatorial COs and the internal nuclei of 2,2′-pq.     

A ratiometric fluorescent probe for fluoride ion employing the excited-state intramolecular proton transfer

A ratiometric fluorescent probe 1 for fluoride ion was developed based on modulation of the excited-state intramolecular proton transfer (ESIPT) process of 2-(2′-hydroxyphenyl)benzimidazole (HPBI) through the hydroxyl group protection/deprotection reaction. The probe 1 was readily prepared by the reaction of HPBI with tert-butyldimethylsilyl chloride (TBS-Cl) and shows only fluorescence emission maximum at 360 nm. Upon treatment with fluoride in aqueous DMF solution, the TBS protective group of probe 1 was removed readily and ESIPT of the probe was switched on, which resulted in a decrease of the emission band at 360 nm and an increase of a new fluorescence peak around 454 nm. The fluorescent intensity ratio at 454 and 360 nm (I₄₅₄/I₃₆₀) increases linearly with fluoride ion concentration in the range 0.3-8.0 μmol L⁻¹ and the detection limit is 0.19 μmol L⁻¹. The proposed probe shows excellent selectivity toward fluoride ion over other common anions. The method has been successfully applied to the fluoride determination in toothpaste and tap water samples.     

New and efficient synthesis of 5′-amino-5′-(S)-methyl-2′,5′-dideoxynucleosides

A short, efficient synthesis of 5′-amino-5′-(S)-methyl-2′,5′-dideoxynucleosides 1 has been developed through the diastereoselective addition of methylmagnesium bromide or methyllithium to an intermediate tert-butylsulfinimide.     

Synthesis of (3′R,5′S)-3′-hydroxycotinine using 1,3-dipolar cycloaddition of a nitrone

To synthesize (3′R,5′S)-3′-hydroxycotinine [(+)-1], the main metabolite of nicotine (2), cycloaddition of C-(3-pyridyl)nitrones 3a, 3c, and 15 with (2R)- and (2S)-N-(acryloyl)bornane-10,2-sultam [(2R)- and (2S)-8] was examined. Among them, l-gulose-derived nitrone 15 underwent stereoselective cycloaddition with (2S)-8 to afford cycloadduct 16, which was elaborated to (+)-1.     

Photoactive building blocks for coordination complexes: Gilding 2,2′:6′,2″-terpyridine

The alkyne unit of 4′-ethynyl-2,2′:6′,2″-terpyridine has been functionalized with Ph₃PAu, (2-tolyl)₃PAu or Au(dppe)Au units to produce compounds 1-3, respectively. These derivatives have been characterized by electrospray mass spectrometry, solution ¹H and ¹³C NMR, UV-Vis and emission spectroscopies, and single crystal X-ray diffraction. In the solid state, molecules of 1 or 2 pack with separated domains of tpy and R₃PAu units; the tpy units in 2 (but not 1) exhibit face-to-face π-stacking. Compound 3 crystallizes as 2(3)^.CHCl₃, and the folded conformation of the dppe backbone results in a short (2.9470(8) Å) aurophilic interaction. Folded molecule 3 captures CHCl₃, preventing intramolecular face-to-face π-interactions between the tpy units. In CH₂Cl₂ solution, 1-3 are emissive when excited between 230 and 300 nm, but over minutes when λ_ex = 230 nm, the emission bands decay as the compounds photodegrade.     

Solvent-promoted chemiluminescent decomposition of a bicyclic dioxetane bearing a 4-(benzothiazol-2-yl)-3-hydroxyphenyl moiety

An aprotic polar solvent such as N-methylpyrrolidone (NMP) promoted the thermal decomposition of bicyclic dioxetane bearing a 4-(benzothiazol-2-yl)-3-hydroxyphenyl moiety 1 without the addition of any base. This solvent-promoted decomposition (SPD) gave light as effectively as the base-induced decomposition (BID) in an aprotic polar solvent. SPD caused intramolecular CT-induced chemiluminescence similar to BID, but, in contrast to BID, SPD proceeded through a pathway with a large negative entropy of activation. Dioxetane 1 was also shown to give light due to excited state intramolecular proton transfer (ESIPT) upon heating in p-xylene.     

Crystallization-based optical resolution of 1,1′-binaphthalene-2,2′-dicarboxylic acid via 1-phenylethylamides: control by the molecular structure and dielectric property of solvent

In the recrystallization of a diastereomeric mixture of amides (RS_a,S)-1 formed from racemic 1,1′-binaphthalene-2,2′-dicarboxylic acid and (S)-1-phenylethylamine, either of the diastereomers crystallizes out as a diastereomerically pure form, depending on the solvent employed; sterically undemanding solvents, acetone, dichloromethane, and acetonitrile, afford crystals formulated as (S_a,S)-1·solvent with an exception of ethanol, which affords (R_a,S)-1·EtOH, while sterically bulkier solvents afford (R_a,S)-1 including no solvent. The stereoselectivity can be rationalized by the crystal structures. A dielectrically controlled resolution (DCR) can also be carried out by using mixed solvents, which contain, for example, 25 vol % of acetone and varying ratios of hexane and 1-propanol in total 75 vol %; (S_a,S)-1·acetone is deposited as crystals from the solvents with a dielectric constant (ε) range 8.9 ? ε ? 10.2, while (R_a,S)-1 is deposited from the solvents with 14.8 ? ε ? 20.3.     

Spectral-luminescent and solvatochromic properties of 2-(3-coumarinyl)-5-(2′-(R-amino)-phenyl)-1,3,4-oxadiazoles

Spectral-luminescent properties of the newly synthesized 2-(3^′-coumarinyl)-5-(2′-(R-amino)-phenyl)-1,3,4-oxadiazoles has been investigated in solvents of various polarity and hydrogen-bonding ability. It has been found that for all the studied compounds no excited state intramolecular proton transfer occurs despite the presence of coumarinyl fragment - electron acceptor effect of the coumarinyl fragment is not sufficient to increase the excited state acidity of the amino group. It has been found that the absorption spectra of the studied compounds shift to higher energy with increase in solvent polarity, whereas corresponding fluorescence spectra shift to lower energy with solvent polarity increase. It has been suggested that long-wavelength shifts of the fluorescence spectra of the studied compounds with increase in solvent polarity is caused by the solvent relaxation. The observed solvent relaxation effect allow us to propose some of the studied compounds as potential probes to monitor changes in solvent relaxation in low-polar media and as potential probes for rigidochromic effect.     

A pyrazolyl-terminated 2,2′:6′,2″-terpyridine ligand: Iron(II), ruthenium(II) and palladium(II) complexes of 4′-(3,5-dimethylpyrazol-1-yl)-2,2′:6′,2″-terpyridine

The preparation of 4′-(3,5-dimethylpyrazol-1-yl)-2,2′:6′,2″-terpyridine (2) under acidic conditions results in the formation of the salts [H₂2][MeOSO₃]₂ and [H₂2][EtOSO₃]₂, treatment of which with base leads to neutral 2. The structure of [H₂2][EtOSO₃]₂ · H₂O has been established by single crystal X-ray diffraction. The complexes [Fe(2)₂][PF₆]₂ and [Ru(2)₂][PF₆]₂ have been prepared and characterized, and the single crystal structure determination of [Ru(2)₂][PF₆]₂ is reported; [Fe(2)₂][PF₆]₂ is isostructural with [Ru(2)₂][PF₆]₂. Treatment of [Fe(2)₂]²⁺ with PdCl₂ produces [Pd(2)Cl]⁺, isolated and structurally characterized as the hexafluoridophosphate salt, illustrating that metal exchange within the tpy-binding domain occurs in preference to palladium(II) coordination by the N-donor atom of the pendant 3,5-dimethylpyrazol-1-yl unit in 2. [Pd(2)Cl]²⁺ can also be prepared from PdCl₂ and [H₂2][MeOSO₃]₂ in refluxing methanol.     

1-(2′-Pyridylazo)-2-naphtholate (PAN) complexes of rhodium(III): Synthesis,structure and spectral studies

The ligating properties 1-(2′-pyridylazo)-2-naphthol (HPAN) toward Rh(III) have been examined. The reaction of RhCl₃·3H₂O with HPAN in presence of excess PPh₃ afforded trans-[Rh(PAN)Cl(PPh₃)₂]PF₆ (3PF₆). Intermediate cis-[Rh(PAN)Cl₂(PPh₃)] (4) has also been isolated. Solid state structures were authenticated by X-ray analyses revealing that monoanionic PAN is coordinated to rhodium in meridional fashion. Both the compounds were spectroscopically characterized in both solution and solid states, which include IR, NMR (¹H and ³¹P), and optical spectra. The diamagnetic complexes show multiple CT transitions in the visible region. Low-energy transitions (λ ≈ 550–650 nm) occurred in the absorption spectra are predominantly ligand centered in nature. The rhodium(III)–PAN compounds are red emissive (λ_em ≈ 650 nm) at room temperature and the nature of the emission level is probably an ILCT level. Complexes are electro-active in acetonitrile and display irreversible oxidative and reductive waves and these responses are ascribed to be PAN ligand centered in character.     

Reactivity studies of cyclopentadienyl ruthenium(II), osmium(II) and pentamethylcyclopentadienyl iridium(III) complexes towards 2-(2′-pyridyl)imidazole derivatives

The reaction of [CpRu(PPh₃)₂Cl] and [CpOs(PPh₃)₂Br] with chelating 2-(2′-pyridyl)imidazole (N ∩ N) ligands and NH₄PF₆ yields cationic complexes of the type [CpM(N ∩ N)(PPh₃)]⁺ (1: M = Ru, N ∩ N = 2-(2′-pyridyl)imidazole; 2: M = Ru, N ∩ N = 2-(2′-pyridyl)benzimidazole; 3: M = Ru, N ∩ N = 2-(2′-pyridyl)-4,5-dimethylimidazole; 4: M = Ru, N ∩ N = 2-(2′-pyridyl)-4,5-diphenylimidazole; 5: M = Os, N ∩ N = 2-(2′-pyridyl)imidazole; 6: M = Os, N ∩ N = 2-(2′-pyridyl)benzimidazole). They have been isolated and characterized as their hexafluorophosphate salts. Similarly, in the presence of NH₄PF₆, [Cp∗Ir(μ-Cl)Cl]₂ reacts in dry methanol with N ∩ N chelating ligands to afford in excellent yield [Cp∗Ir(N ∩ N)Cl]PF₆ (7: N ∩ N = 2-(2′-pyridyl)imidazole; 8: N ∩ N = 2-(2′-pyridyl)benzimidazole). All the compounds have been characterized by infrared and NMR spectroscopy and the molecular structure of [1]PF₆, [2]PF₆ and [7]PF₆ by single-crystal X-ray structure analysis.     

Synthesis and crystal structure of 2′-deoxy-2′-fluoro-4′-thioribonucleosides: substrates for the synthesis of novel modified RNAs

We report herein the synthesis of appropriately protected 2′-deoxy-2′-fluoro-4′-thiouridine (5), -thiocytidine (7), and -thioadenosine (35) derivatives, substrates for the synthesis of novel modified RNAs. The synthesis of 5 and 7 was achieved via the reaction of 2,2′-O-anhydro-4′-thiouridine (3) with HF/pyridine in a manner similar to that of its 4′-O-congener whereas the synthesis of 35 from 4′-thioadenosine derivatives was unsuccessful. Accordingly, 35 was synthesized via the glycosylation of the fluorinated 4-thiosugar 25 with 6-chloropurine. The X-ray crystal structural analysis revealed that 2′-deoxy-2′-fluoro-4′-thiocytidine (8) adopted predominately the same C3′-endo conformation as 2′-deoxy-2′-fluorocytidine.     

Isolation, X-ray structure and properties of an unusual pentacarbonyl(2,2′-pyridyl-quinoxaline) tungsten complex

The first example of a monodentate complexation of 2-(2′-pyridyl)quinoxaline (pq) to a metal centre through N₄ is reported. Photochemical exchange of the THF ligand in W(CO)₅THF by pq yields W(CO)₅(N₄-pq) (1), where the potentially bidentate pq ligand coordinates in an unusual monodentate fashion. Complex 1 is isolated as orange crystals and fully characterized on the basis of NMR, IR, UV-Vis and emission spectroscopy. The structure of 1 was determined by X-ray analysis. W(CO)₅(N₄-pq) (1) crystallizes in space group P2_1/n, monoclinic crystal system with α = 7.0237(5) Å, b = 10.4618(8) Å, c = 23.7768(18) Å, Z = 4 and V = 1731.9(2) Å³. Complex 1 exhibits intramolecular CH?N and intermolecular CH?O hydrogen bonds between the CH groups and nitrogen atoms of quinoxaline and CH groups and oxygen atoms of carbonyls, respectively, resulting in a supramolecular architecture in solid state. The preference to N₄ as coordination site is discussed in terms of electronic interactions. Solutions of 1 emits dually at 77 K while they are moderately instable at room temperature, as 1 undergoes chelation via a first-order kinetic process to form W(CO)₄pq (2). The determined reaction rate of 1 in toluene is 2.3 × 10⁻⁵ s⁻¹ (at 298 K) and is compared with literature values for other W(CO)₅L (L:diimine) complexes.     

Facile acid-catalyzed condensation of 2-hydroxy-2,2′-biindan-1,1′,3,3′-tetrone with phenols, methoxyaromatic systems and enols

Various phenols, methoxy aromatic compounds, 3- and 4-hydroxycoumarins and enols smoothly condense with 2-hydroxy-2,2′-biindan-1,1′,3,3′-tetrone 1 in an acid medium producing 2-aryl/alkyl-2,2′-biindan-1,1′,3,3′-tetrones in high yields. The adducts of resorcinol, 1,3,5-trihydroxybenzene and α- and β-naphthols of 1 preferably remain in the intramolecular hemi-ketal form, confirmed by X-ray diffraction studies. On the other hand para and meta substituted phenols condense with 1 in an acid medium to produce 6 or 7 substituted 2′,4-spiro(1′,3′-indanedion)-indeno[3,2-b]chromenes in good yields.