Synthesis and properties of 4,9-methanoundecafulvenes and their transformation to 3-substituted 7,12-methanocycloundeca[4,5]furo[2,3-d]pyrimidine-2,4(1H,3H)-diones: photo-induced autorecycling oxidizing reaction toward amines

The synthesis and properties of 4,9-methanoundecafulvene [5-(4,9-methanocycloundeca-2′,4′,6′,8′,10′-pentaenylidene)pyrimidine-2,4,6(1,3,5H)-trione] derivatives 8a,b were studied. Their structural characteristics were investigated on the basis of the ¹H and ¹³C NMR and UV-vis spectra. The rotational barrier (ΔG^‡) around the exocyclic double bond of 8a was found to be 12.55 kcal mol⁻¹ by the variable temperature ¹H NMR measurement. The electrochemical properties of 8a,b were also studied by CV measurement. Furthermore, the transformation of 8a,b to 3-substituted 7,12-methanocycloundeca[4,5]furo[2,3-d]pyrimidine-2,4(1H,3H)-diones 16a,b was accomplished by oxidative cyclization using DDQ and subsequent ring-opening and ring-closure. The structural details and chemical properties of 16a,b were clarified. Reaction of 16a with deuteride afforded C13-adduct 19 as the single product, and thus, the methano-bridge controls the nucleophilic attack to prefer endo-selectivity. The photo-induced oxidation reaction of 16a and a vinylogous compound, 3-methylcyclohepta[4,5]furo[2,3-d]pyrimidine-2,4(3H)-dione 2a, toward some amines under aerobic conditions were carried out to give the corresponding imines (isolated by converting to the corresponding 2,4-dinitrophenylhydrazones) with the recycling number of 6.1-64.0 (for 16a) and 2.7-17.2 (for 2a), respectively.     

Conformational behavior of dithia[n.3.3](1,3,5)cyclophanes and dithia[n.3.3](1,2,6)cyclophanes

The conformational behavior of a series of crown-fused dithia[n.3.3](1,2,6)cyclophanes (126-CPs) and dithia[n.3.3](1,3,5)cyclophanes (135-CPs) was investigated by variable-temperature ¹H and ¹³C NMR spectroscopy, X-ray crystallography and density functional theory (DFT) calculations. Single crystal X-ray structure analysis showed that two thia-bridges in 126-CPs adopted a pseudochair-pseudochair (cc) conformation and the cyclophane decks underwent a ring-tilting motion in the case of [10.3.3](1,2,6)cyclophane (1a). In contrast, the thia-bridges in 135-CPs took both cc and pseudoboat-pseudochair (bc) conformations, and the ring-tilting process was also found in [10.3.3](1,3,5)cyclophane (2a). Variable temperature ¹H NMR study revealed that there was no wobbling-motion for two thia-bridges in 126-CPs while thia-bridges in 135-CPs experienced a wobbling-process with a conformational barrier of 9.21 and 8.80 kcal mol⁻¹, respectively, for 2a and [13.3.3](1,3,5)cyclophane (2b). DFT calculations for the two cyclophanes series revealed that 126-CPs preferred a cc conformation which was consistent with the experimental observation; similarly, 135-CPs took a preferential cc conformation, agreeing with 2a having a predominant cc conformer (cc:bc ratio=70:30), but not 2b having a predominant bc conformer (cc:bc ratio=15:85) in the solid state.     

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.     

A tandem decarboxylation and inverse electron-demand Diels-Alder reaction of amino-thiophenecarboxylic acids with 1,3,5-triazines

The scope of 1,3,5-triazine inverse electron-demand Diels-Alder (IDA) reactions was expanded to include aminothiophenecarboxylic acids as latent dienophiles. A series of 2-amino-3-thiophenecarboxylic acids (1a-d) and a 3-amino-2-thiophenecarboxylic acid (5) were introduced as productive dienophiles in IDA reactions with various 1,3,5-triazines (2a-e). This method is useful for the one-step synthesis of both thieno[2,3-d]pyrimidines and thieno[3,2-d]pyrimidines, which should complement existing methods.     

Synthesis and properties of 3-arylcyclohepta[4,5]imidazo[1,2-a]-1,3,5-triazine-2,4(3H)-diones and related compounds: photo-induced autorecycling oxidation of some amines

Novel 3-phenyl- and 3-(4-nitrophenyl)cyclohepta[4,5]imidazo[1,2-a]-1,3,5-triazine-2,4(3H)-diones and the corresponding imino derivatives 5a,b and 6a,b were synthesized in modest to moderate yields by the abnormal and normal aza-Wittig reaction of 2-(1,3-diazaazulen-2-ylimino)triphenylphosphorane with aryl isocyanates and subsequent heterocyclization reaction with a second isocyanate. The related cationic compound, 1-methyl-3-phenylcyclohepta[4,5]imidazo[1,2-a]-1,3,5-triazine-2,4(3H)-dionylium tetrafluoroborate 7a, was also prepared. The electrochemical reduction of these compounds exhibited more positive reduction potentials as compared with those of the related compounds of 3,10-disubstituted cyclohepta[4,5]pyrrolo[2,3-d]pyrimidine-2,4(1H,3H)-dione systems. In a search of the oxidizing ability, compounds 5a, 6a, and 7a were demonstrated to oxidize some amines to give the corresponding imines in more than 100% yield under aerobic and photo-irradiation conditions, while even benzylamine was not oxidized under aerobic and thermal conditions at 100 °C. The oxidation reactions by cation 7a are more efficient than that by 5a and 6a. Quenching of the fluorescence of 5a was observed, and thus, the oxidation reaction by 5a probably proceeds via electron-transfer from amine to the excited singlet state of 5a. In the case of cation 7a, the oxidation reaction is proposed to proceed via formation of an amine-adduct of 7a and subsequent photo-induced radical cleavage reaction.     

The (phosphino)tetraphenylborate ligand in ruthenium-arene chemistry

The synthesis of a series of anionic half-sandwich ruthenium-arene complexes [E][RuCl₂(η⁶-p-cymene){PR₂(p-Ph₃BC₆H₄)}] (E = Bu₄N⁺: R = Ph, 1a, ⁱPr, 1b or Cy, 1c; E = bis(triphenylphosphine)iminium or PNP⁺: R = Ph, 1a′, ⁱPr, 1b′ or Cy, 1c′) are reported. X-ray crystallographic studies of 1a′ and 1b′ confirmed the three-legged piano-stool coordination geometry. In solution, complexes 1a-c and 1a′-c′ are proposed to form monomer-dimer equilibria as a result of chloride ligand dissociation. Complexes 1a-c and 1a′-c′ also form the formally neutral zwitterionic complexes [RuCl(L)(η⁶-p-cymene){PR₂(p-Ph₃BC₆H₄)}] (L = pyridine: R = Ph, 2a, ⁱPr, 2b or Cy, 2c; L = MeCN: R = Ph, 3a, ⁱPr, 3b or Cy, 3c) via chloride ligand abstraction using AgNO₃ or MeOTf.     

Regioselective synthesis of novel 4-aryl-2-ethylthio-7-methyl pyrazolo[1,5-a]-[1,3,5]-triazines

Pyrazolo[1,5-a]-[1,3,5]-triazines 6a-d were obtained by an efficient one-step reaction from S,S-diethyl aroyliminodithiocarbonates 4a-d and 5-amino-3-methylpyrazole 5 or by an alternative two-step reaction from 5 and aroyl isothiocyanates 8a-d to give initially the thiourea derivatives 9a-d, which after S-ethylation and cyclization afforded compounds 6a-d. The intermediate 7a isolated from reaction between 4a and 5 permitted us to establish the orientation.     

Synthesis and properties of 5-[bis(1-heteroazulen-3-yl)methylidene]pyrimidine-2,4,6(1,3,5H)-triones

The synthesis and properties of a novel type of 5-[bis(1-heteroazulen-3-yl)methylidene]pyrimidine-2,4,6(1,3,5H)-triones (13a-c) and (14a-c) are studied. The synthetic procedure is based on addition of bis(1-heteroazulen-3-yl)methyl cations with barbituric acid and subsequent oxidation by o-chloranil. Structural characteristics of 13a-c and 14a-c were clarified on inspection of the ¹³C NMR spectral data and X-ray crystal analysis. Based on the investigation of the UV-vis spectra of 13a-c and 14a-c and their protonated cations, conformational change of the heteroazulene-moiety and the barbituric acid-moiety is suggested. In the CV measurements of 13a-c and 14a-c, two reversible reduction waves are observed, indicating the stabilizing ability of heteroazulenes toward the corresponding radical and anion species. Furthermore, 13a-c and 14a-c exhibit one irreversible oxidation wave and the corresponding reduction wave appearing in a far negative region, which suggested a conformational change in the radical cation during the redox process. The conformational change is rationalized on the basis of the MO calculations.     

Asymmetric Friedel-Crafts alkylation of activated benzenes with methyl (E)-2-oxo-4-aryl-3-butenoates catalyzed by [Pybox/Sc(OTf)3]

The asymmetric Friedel-Crafts reaction between methyl (E)-2-oxo-4-aryl-3-butenoates (1a-c) and activated benzenes (2a-d) has been efficiently catalyzed by the Sc^III triflate complex of (4′S,5′S)-2,6-bis[4′-(triisopropylsilyl) oxymethyl-5′-phenyl-1′,3′-oxazolin-2′-yl]pyridine (pybox 3). The 4,4-diaryl-2-oxo-butyric acid methyl esters (4) are usually formed in good yields and the enantioselectivity is up to 99% ee. The sense of the stereoinduction can be rationalized with the same octahedral complex (10) between 1, pybox 3 and Sc triflate already proposed for other reactions involving pyruvates, and catalyzed by the same complex.     

Efficient synthesis of novel dipyridoimidazoles and pyrido[1′,2′;1,2]imidazo[4,5-d]pyridazine derivatives

Novel dipyrido[1,2-a;3′,4′-d]imidazoles 7a-d, dipyrido[1,2-a;4′,3′-d]imidazoles 8a,c and pyrido[1′,2′;1,2]imidazo[4,5-d]pyridazine derivatives 9a-d were synthesized by two pathways: thermal electrocyclic reaction of 3-alkenylimidazopyridine-2-oximes 10 and direct condensation of ethyl glycinate (or hydrazine) with 2,3-dicarbonylimidazo[1,2-a]pyridines 11.     

The stereodynamics of 3,5-bis(trifluoromethylsulfonyl)-1,3,5-oxadiazinane and 1,3,5-tris(trifluoromethylsulfonyl)-1,3,5-triazinane—an experimental and theoretical study

Multinuclear dynamic NMR spectroscopy of 3,5-bis(trifluoromethylsulfonyl)-1,3,5-oxadiazinane (3) revealed the existence of two conformers with differently oriented CF₃ groups with respect to the ring, and two dynamic processes: ring inversion and restricted rotation about the N-S bond. Two transition states connecting the two conformers and corresponding to clockwise and counterclockwise rotations about the N-S bond were found; the calculated activation barriers of about 12 kcal/mol are in excellent agreement with those measured experimentally for the related molecule 1,3,5-tris(trifluoromethylsulfonyl)-1,3,5-triazinane (1). X-ray analysis proved the existence of the symmetric isomer of 3, which is the minor isomer in solutions but the only one in the crystal due to packing effects. The normal Perlin effect (JCH_ax<JCH_eq) was observed for 2(6)-CH₂ in 3, whereas the reversed Perlin effect was found for the 4-CH₂ group in 3 as well as for all CH₂ groups in 1 both experimentally and theoretically. The latter effect in compounds 1, 3, and 1-(methylsulfonyl)-3,5-bis(trifluoromethylsulfonyl)-1,3,5-triazinane (2) can be considered as a genuine reverse Perlin effect since larger values of ¹J_CH are observed for longer C-H bonds.     

Reactivity and electrochemical behavior of ruthenium dithiolene complexes with coordinatively unsaturated metal centers: cycloaddition and dimerization reactions

The novel ruthenium dithiolene complexes [(arene)Ru{S₂C₂(COOMe)₂}] (arene = C₆H₆ (1a), C₆H₄(Me)(iPr) (1b), C₆Me₆ (1c)) were synthesized. The equilibrium between complex 1a and the corresponding dimer [(C₆H₆)Ru{S₂C₂(COOMe)₂}]₂ (1a′) was confirmed in solution. The reaction of complex 1a with dimethyl- or diethylacetylene dicaboxylate gave the alkene-bridged adducts [(C₆H₆)Ru{S₂C₂(COOMe)₂}{C₂(COOR)₂}] (R = Me (2a), Et (3a)) as [2 + 2] cycloaddition products formally. The reactions of complex 1a with diazo compounds also gave the alkylidene-bridged adducts [(C₆H₆)Ru{S₂C₂(COOMe)₂}(CHR)] (R = H (4a), SiMe₃ (5a), COOEt (6a)) as [2 + 1] cycloaddition products. The electrochemical behavior of complex 1a was investigated. The reductant of complex 1a was a stable species for several minutes. The oxidant of complex 1a was very unstable; the cation 1a⁺ formed was immediately converted to the corresponding cationic dimer 1a′⁺. The cationic dimer 1a′⁺ was stable for several minutes, and it was rapidly and quantitatively converted to the neutral complex 1a when it was reduced.     

(N-7-Azaindolyl)oligothiophenes: synthesis, characterization, and photophysical properties

The synthesis of a new series of mono- and oligothiophenes capped by 7-azaindoles such as 2-(N-azaindolyl)thiophene (1), 2-(N-azaindolyl)-5′-(bromo)oligothiophenes (2a-4a), and 2,5′-bis(N-azaindolyl)oligothiophenes (2b-4b) has been investigated. The reaction of 7-azaindole with 2-bromothiophene under the modified Ullmann condensation conditions led to the formation of 1. Simple extension of the same method to the reaction of 2,5′-dibromooligothiophenes in the presence of 4-5 M excess of 7-azaindole led to the formation of 2a-4a and 2b-4b in moderate overall yields (40-55%). All compounds were fully characterized by analytical and various spectroscopic techniques. The structures of 2b, 3b, and 4b were determined by X-ray diffraction analyses. All three compounds show several intermolecular C(π)?H interactions leading to the formation of herringbone packing in the solid-state structure. The UV absorption spectra of 1-4 consist of three characteristic electronic transitions corresponding to n→π^∗ and π→π^∗ transitions arising out of the π-conjugation of the entire molecule as well as local aromatic units. The emission spectra of the same compounds show intense fluorescence bands at the wavelengths between 422 and 495 nm. The length of the thiophene chain and the presence of bromine atom influence the band position of both absorption and emission spectra. While the extension in π-conjugation causes the reduction in the band gap, the bromine atom shifts the electronic transition energy to the blue region. The cyclic voltammetric measurements were performed with 1-4, which show that the compounds exhibit a typical pseudo-reversible redox wave with E_ox in the range 0.6-1.2 V.     

1,1-(1-Propene-1,3-diyl)-ferrocene: modified synthesis, crystal structure, and polymerisation behaviour

The dehydro[3](1,1^′)ferrocenophanes, 1,1^′-(1-propene-1,3-diyl)-ferrocene (3a), and 1,1^′-(3-phenyl-1-propene-1,3-diyl)-ferrocene (3b) were synthesised under Shapiro conditions from the tosylhydrazones of the corresponding α-oxo-[3](1,1^′)ferrocenophanes. Electrochemistry shows 3a is oxidised at smilar potential to ferrocene; according 3a can be chemically oxidised using silver trifluoromethanesulfonate. The structure of 3a shows a ring tilt of 11.3°. Attempts to polymerise 3a using the ROMP initiator Mo(CHCMe₂Ph)[N(2,6-ⁱPr₂C₆H₃)][OCMe(CF₃)₂]₂ led to a mixture of insoluble material and a soluble mixture of apparently cyclic oligomers ([3a]_n).     

Synthesis and properties of novel 5-(cyclohepta-2′,4′,6′-trienylidene)pyrimidine-2(1H),4(3H),6(5H)-triones: methodology for synthesizing cyclohepta[b]pyrimido[5,4-d]furan-8(7H),10(9H)-dionylium tetrafluoroborates

Novel condensation reaction of tropone with N-substituted and N,N′-disubstitued barbituric acids in Ac₂O afforded 5-(cyclohepta-2′,4′,6′-trienylidene)pyrimidine-2(1H),4(3H),6(5H)-trione derivatives (8a-f) in moderate to good yields. The ¹³C NMR spectral study of 8a-f revealed that the contribution of zwitterionic resonance structures is less important as compared with that of 8,8-dicyanoheptafulvene. The rotational barriers (ΔG^‡) around the exocyclic double bond of mono-substituted derivatives 8a-c were obtained to be 14.51-15.03 kcal mol⁻¹ by the variable temperature ¹H NMR measurements. The electrochemical properties of 8a-f were also studied by CV measurement. Upon treatment with DDQ, 8a-c underwent oxidative cyclization to give two products, 7 and 9-substituted cyclohepta[b]pyrimido[5,4-d]furan-8(7H),10(9H)-dionylium tetrafluoroborates (11a-c·BF₄⁻ and 12a-c·BF₄⁻) in various ratios, while that of disubstituted derivatives 8d-f afforded 7,9-disubstituted cyclohepta[b]pyrimido[5,4-d]furan-8(7H),10(9H)-dionylium tetrafluoroborate (11d-f·BF₄⁻) in good yields. Similarly, preparation of known 5-(1′-oxocycloheptatrien-2′-yl)-pyrimidine-2(1H),4(3H),6(5H)-trione derivatives (14a-d) and novel derivatives 14e,f was carried out. Treatment of 14a-c with aq. HBF₄/Ac₂O afforded two kinds of novel products 11a-c·BF₄⁻ and 12a,c·BF₄⁻ in various ratios, respectively, while that of 14d-f afforded 11d-f. The product ratios of 11a-c·BF₄⁻ and 12a-c·BF₄⁻ observed in two kinds of cyclization reactions were rationalized on the basis of MO calculations of model compounds 20a and 21a. The spectroscopic and electrochemical properties of 11a-f·BF₄⁻ and 12a-c·BF₄⁻ were studied, and structural characterization of 11c·BF₄⁻ based on the X-ray crystal analysis and MO calculation was also performed.     

Thermal and microwave assisted reactions of 2,5-disubstituted thienosultines with [60]fullerene: non-Kekulé biradicals and self-sensitized oxygenation of the cycloadduct

Refluxing an o-dichlorobenzene solution of 2,5-disubstituted thienosultines 10a-f with [60]fullerene for 2-24 h gave both 1:1 and 2:1 cycloadducts in 37-79% isolated yields. The reaction was highly accelerated by microwave irradiation giving comparable yields of cycloadducts. Sultines 10a-f underwent cheletropic extrusion of SO₂ to form the corresponding non-Kekulé biradical intermediates 11a-f, which were subsequently trapped by [60]fullerene to form corresponding cycloadducts. The activation energy barriers (ΔG_c^≠) determined for the boat-to-boat inversion of these 4′,5′,6′,7′-tetrahydrobenzo[c]thieno-[5′,6′:1,2][60]fullerene adducts 12a-f were found to be in the range of 13.5-14.8 kcal/mol. Unexpectedly, one of the monoadduct 12a was found to be labile when kept in air under ambient light. Two new products 15 (a sulfine-enone) and 16 (an endione) were isolated from the decomposed 12a and were found to derive from self-sensitized singlet oxygen reaction on the 2,5-dimethylthieno moiety of 12a.     

Silver(I) complexes of bis[2-(diphenylphosphino)phenyl] ether

The reaction of AgOTf in dichloromethane with bis(2-(diphenylphosphino)phenyl) ether (DPEphos) in an equimolar ratio afforded a dinuclear complex [Ag₂(κ²-P,P′-DPEphos)₂(μ-OTf)₂] (1), whereas the similar reaction in a 1:2 molar ratio resulted in the formation of a bis-chelating complex [Ag(κ²-P,P′-DPEphos)₂][OTf] (2). The silver(I) complex 1 was obtained as a dimer, in which two silver atoms are bridged by two triflate groups to form three adjacent eight-membered spirocyclic rings. The mixed-ligand complex [Ag(κ²-P,P′-DPEphos)(2,2′-bpy)][OTf] (3) was obtained in the reaction of 1 in dichloromethane with 2,2′-bipyridine. The crystal structures of complexes 1–3 were determined by single crystal X-ray analyses.     

Total synthesis of (±)-luminacin D

A 15-step synthesis of (±)-luminacin D from ethyl pent-2-ynoate is reported. The pivotal step involves the formation of the central C-2′/C-3′ bond of the natural product by condensation of the titanium enolate derived from aromatic ketone 1 with aldehyde 2a. A remote asymmetric centre in aldehyde 2a exerts control over the stereochemical course of this reaction, with the major adduct (3a, 54% yield) possessing the required (2′S^∗,3′R^∗,5′R^∗)-stereochemistry. This assignment was unambiguously established by X-ray crystallography of late stage synthetic intermediate, 17. Further manipulation of 3a (six steps) yielded synthetic (±)-luminacin D spectroscopically identical to material isolated from Streptomyces sp. Mer-VD1207 by Naruse et al.     

Synthesis and thermal reaction of hydrido(selenolato) platinum(II) complex having a 9,10,11,12,14,15-hexahydro-9,10[3′,4′]-furanoanthracenyl group

The oxidative addition of selenol, HhfSeH (2, Hhf = 9,10,11,12,14,15-hexahydro-9,10[3′,4′]-furanoanthracenyl) with [Pt(η²-nb)(Ph₃P)₂] (nb = norbornene) in toluene afforded the corresponding hydrido(selenolato) Pt(II) complex [cis-PtH(SeHhf)(Ph₃P)₂] (3) as a stable compound. Refluxing a xylene solution of 3 produced two isomers of five-membered selenaplatinacycles 4 in moderate yield as an inseparable mixture. In addition, the molecular structures of HhfSeH 2 and the minor selenaplatinacycle 4a were determined by X-ray crystallography.     

1D coordination polymers of silver(I) and copper(II) based on bis(N-heterocyclic carbene) ligands: Synthesis and structural studies

The alkyl chain-linked diimidazolium (or dibenzimidazolium) salts, 1,1′-diethyl-4,4′-tetramethylene-diimidazolium-diiodide (L¹H₂·I₂) and 1,1′-diethyl-3,3′-trimethylene-dibenzimidazolium-diiodide (L²H₂·I₂), and their silver(I) and copper(II) coordination polymers, [L¹AgI]_n (1) and [L²Cu₂I₄]_n (2), have been prepared and characterized. Complex 1 is a 1D helical polymer generated by bidentated carbene ligands (L¹) and Ag(I) atoms. The 1D polymer of 2 is formed by bidentated carbene ligands (L²) and coplanar quadrilateral Cu₂I₂ units. 3D supramolecular frameworks in the crystal packings of 1 and 2 are formed via intermolecular weak interactions, including C–H···π contacts, π–π interactions and C–H···I hydrogen bonds.