Mechanistic study of electrochemical oxidation of catechols in the presence of 4-hydroxy-1-methyl-2(1H)-quinolone: Application to the electrochemical synthesis

Electrochemical oxidation of catechols (1a-1c) has been studied in the presence of 4-hydroxy-1-methyl-2(1H)-quinolone (3) as a nucleophile in aqueous solution using cyclic voltammetry and controlled-potential coulometry. The results indicate that the quinones derived from catechols (1a-1c) participate in Michael addition reactions with 3 to form the corresponding benzofuran (or isochromeno[4,3-c]quinoline) derivatives (6a-6c). The electrochemical synthesis of (6a-6c) has been successfully performed in an undivided cell in good yield and purity. The oxidation mechanism was deduced from voltammetric data and by coulometry at controlled-potential. The products have been characterized after purification by IR, ¹H NMR, ¹³C NMR and MS.     

Electrochemical synthesis of new catechol derivatives

Electrochemical oxidation of catechols (1a-e) has been studied in the presence of 2-acetylcyclopentanone (3) as a nucleophile in aqueous solutions, using cyclic voltammetry and controlled-potential coulometry. The results indicate that the quinones derived from catechols (1a-e), participate in Michael addition reactions with anion of 2-acetylcyclopentanone (3) and via EC mechanism pathway, converted to corresponding catechol derivatives (4a-e). In this work, we derived novel catechol derivatives with good yields based on electrochemical oxidation in aqueous solutions, at carbon electrode in an undivided cell, using environmentally friendly method.     

A new facile electrochemical method for the synthesis of 4-(pyridine-2-ylthio)benzene-l,2-diols

Electrochemical oxidation of catechols (1a-1c) has been studied in the presence of 2-mercaptopyridine (3) as a nucleophile in water solution using cyclic voltammetry and controlled-potential coulometry. The results revealed that the quinones derived from catechols (1a-1c) participate in Michael addition reaction with 2-mercaptopyridine (3) and converted to the corresponding (pyridine-2-ylthio)benzene-l,2-diol derivatives (4a-4c), via an EC mechanistic pathway. The electrochemical synthesis of compounds 4a-4c has been successfully performed at a carbon rod electrode and in an undivided cell with good yields and high purity.     

Electrochemical study of stable N-alkoxyarylaminyl radicals

The electrochemical study of N-tert-butoxy-2,4-diphenyl-6-tert-butylphenylaminyl (1a), N-tert-butoxy-2,4-bis(4-chlorophenyl)-6-tert-butylphenylaminyl (1b), N-[2-(methoxycarbonyl)-2-propyl]-2,4-diphenyl-6-tert-butylphenylaminyl (2), and N-tert-butoxy-2,4,6-tris(4-chlorophenyl)phenylaminyl radicals (3) was performed by cyclic voltammetry using acetonitrile as the solvent and Bu₄NPF₆ as the supporting electrolyte. On cathodic scan (100 mV/s), all the radicals gave chemically reversible cyclic voltammograms, and the were determined to be −1.405 V (1a), −1.310 V (2a), −1.282 V (2b), and −1.195 V (3) (versus Fc⁺/Fc), respectively. On anodic scan (100 mV/s), on the other hand, 1a, 1b and 2 showed chemically reversible cyclic voltammograms, but 3 exhibited a partially reversible couple even on a scan rate of 500 mV/s, indicating that the cation species of 3 was less stable. The determined for 1a, 1b, 2 and 3 were 0.220, 0.280, 0.318 and 0.294 V (versus Fc⁺/Fc), respectively. The electrochemical data were compared with those of thioaminyl radicals, the corresponding sulfur analogues of 1-3.     

Polybinaphthyls incorporating chiral 2,2′-binaphthyl and isoquinoline moieties by Sonogashira reaction

Polymers P-1, P-2, P-3, P-4 and P-5 were synthesized by the polymerization of 5,8-bis(ethynyl)isoquinoline (M-1) with (R)-3,3′-diiodo-2,2′-bisbutoxy-1,1′-binaphthyl ((R)-M-2), (S)-3,3′-diiodo-2,2′-bisbutoxy-1,1′-binaphthyl ((S)-M-2), (R)-6,6′-dibromo-2,2′-bisbutoxy-1,1′-binaphthyl ((R)-M-3), (S)-6,6′-dibromo-2,2′-bisbutoxy-1,1′-binaphthyl ((S)-M-3), and rac-6,6′-dibromo-2,2′-bisbutoxy-1,1′-binaphthyl (M-4) under Sonogashira reaction, respectively. Both monomers and polymers were analyzed by NMR, MS, FT-IR, UV-vis spectroscopy, DSC-TGA, fluorescence spectroscopy, GPC and circular dichroism (CD) spectroscopy. CD spectra of polymers P-1 and P-2, P-3 and P-4 are almost identical except that they gave opposite signals at each wavelength. The long wavelength CD effect of P-1 and P-2 can be regarded as the more extended conjugated structure in the repeating unit and the helical backbone in the polymer chain. All five polymers have strong blue-green fluorescence due to the efficient energy migration from the extended π-electronic structure of the repeating unit of the polymers to the chiral binaphthyl core and are expected to provide understanding of structure-property relationships of the chiral conjugated polymers.     

Mechanistic study of electrochemical oxidation of 4-tert-butylcatechol: A facile electrochemical method for the synthesis of new trimer of 4-tert-butylcatechol

The electrochemical trimerization of 4-tert-butylcatechol via anodic oxidation of 4-tert-butylcatechol (1) is described. The mechanism of trimerization has been studied in aqueous solution using cyclic voltammetry and controlled-potential coulometry. The electrochemical synthesis of trimer 3a has been successfully performed in an undivided cell in good yield and purity.     

Mechanistic study of electrochemical oxidation of o-dihydroxybenzenes in the presence of 4-hydroxy-1-methyl-2(1H)-quinolone: Application to the electrochemical synthesis

Electrochemical oxidation of o-dihydroxybenzenes (1a and 1b) has been studied in the presence of 4-hydroxy-1-methyl-2(1H)-quinolone (3) as a nucleophile in aqueous solution using cyclic voltammetry and controlled-potential coulometry. The results indicate that the o-quinones derived from o-dihydroxybenzenes (1a and 1b) participate in 1,4-(michael) addition reactions with 3 to form the corresponding new o-dihydroxybenzene derivatives (6a and 6b). We propose a mechanism for the electrode process. The efficient electrochemical synthesis of 6a and 6b has been successfully performed at carbon rod electrodes in an undivided cell in good yield and purity. The products have been characterized after purification by IR, ¹H NMR, ¹³C NMR and MS.     

Electrooxidation of 4-methylcatechol in the presence of barbituric acid derivatives

Electrooxidation of 4-methylcatechol (1) in the presence of 1,3-dimethylbarbituric acid (2a) and 1,3-diethylthiobarbituric acid (2b) as nucleophiles has been studied in detail by cyclic voltammetry and controlled-potential coulometry. The results indicate that 1 can be oxidized to its related o-benzoquinone (1a) and without conversion to its quinone methide tautomeric form, via an ECEC mechanism pathway, is converted to barbiturate derivatives (5a-b). The electrochemical synthesis of 5a-b have been successfully performed in one-pot in an undivided cell.     

Polarity reversal induced by electrochemically generated thiazol-2-ylidenes: The Stetter reaction

The inversion of the normal reactivity (umpolung) of aldehydes has been induced via N-heterocyclic carbenes (NHCs) thiazol-2-ylidenes 2a or 3a, generated by simple electrolyses of solutions containing thiazolium salt 2 or 3. Accordingly, 1,4-dicarbonyl compounds have been obtained, in mild conditions and in moderate to very high yields, via 1,4-addition of the Breslow intermediates to the suitable Michael acceptor. The procedure has been performed in classical organic solvents (VOCs) as well as in room temperature ionic liquids (RTILs). The different reactivity of aliphatic aldehydes vs the one of aromatic and heteroaromatic aldehydes has been emphasized.     

Synthesis and properties of ornithine- and lysine-based poly(N-propargylamides). Responsiveness of the helical structure to acids

Ornithine- and lysine-based novel N-propargylamides, N-α-tert-butoxycarbonyl-N-δ-fluorenylmethoxycarbonyl-l-ornithine-N′-propargylamide (1), N-α-tert-butoxycarbonyl-N-?-fluorenylmethoxycarbonyl-l-lysine-N′-propargylamide (2), N-α-fluorenylmethoxycarbonyl-N-δ-tert-butoxycarbonyl-l-ornithine-N′-propargylamide (3), and N-α-fluorenylmethoxycarbonyl-N-?-tert-butoxycarbonyl-l-lysine-N′-propargylamide (4) were synthesized and polymerized with a rhodium catalyst. Polymers with moderate molecular weights were obtained in good yields. Poly(1)-poly(4) showed strong Cotton effects in THF, whose sign and wavelength depended on the substituents. They were satisfactorily converted into the corresponding polymers [poly(1a)-poly(4a)] with free amino groups. Poly(1a) and poly(2a) also formed a helix, while poly(3a) and poly(4a) did not. Poly(1a) and poly(2a) decreased the CD intensity by the addition of m- and o-phthalic acids.     

Preparation of polysilsesquioxane having dimethylamino group and grafted thermoresponsive polymer

The polysilsesquioxane having mercapto, dimethylamino, and phenyl groups (PAMPSQ) was prepared from the corresponding trimethoxysilanes by co-condensation under basic conditions. The mercapto group on PAMPSQ was utilized as an initiator for graft copolymerization of N-isopropylacrylamide and N,N-dimethylacrylamide. The grafting of the copolymer proceeded effectively to give the thermoresponsive polysilsesquioxane derivative (GrPSQ) without the formation of gel product. The obtained GrPSQ was amphiphilic and provided the property of thermoresponsive reversible aggregation. The aggregation temperatures of GrPSQ varied in buffer solutions showing different pH values. Furthermore, the formation of ammonium salts utilizing dimethylaminio groups with carboxylic acids or halogen compounds led to a rise in aggregation temperature.     

On the conformation of the cellulose solvent N-methylmorpholine-N-oxide (NMMO) in solution

The N-oxide group of the cellulose solvent N-methylmorpholine-N-oxide showed a strong preference for the axial position compared with N-methyl as determined by NMR experiments and computational studies. In solvents with negligible solvent-solute interaction, about 95% of the NMMO molecules obtained a typical chair conformation with an axial N-O (1) while 5% had an equatorial N-O (2) at room temperature (25 °C). Other conformations (boat and twist) are energetically largely disfavored. N-Benzylmorpholine-N-oxide was prepared as reference compound possessing 100% axial N-O. Aprotic solvents of increasing polarity slightly shifted the conformation equilibrium towards the more polar conformer 2. The effect of protic solvents, capable of forming H-bonds, was more pronounced, with water increasing the percentage of 2 to 25% of the total population. Addition of sugar model compounds reversed this effect, so that formation of 2 was suppressed and exclusively 1 was found, indicating a strong interaction between the latter conformer and the carbohydrate.     

Polybinaphthyls incorporating chiral (R) or (S)-2,2′-binaphthyland oxadiazole moieties by Stille reaction

Chiral polymers P-1 and P-2 were prepared by the polymerization of (R)-3,3′-diiodo-2,2′-bisbutoxy-1,1′-binaphthyl ((R)-M-1) and (S)-3,3′-diiodo-2,2′-bisbutoxy-1,1′-binaphthyl ((S)-M-1) with 2,5-bis[(4-tributylstannyl)phenyl]-1,3,4-oxadiazole (M-2) via Pd(PPh₃)₄ catalyzed Stille coupling reaction. 1,3,4-Oxadiazole unit not only has high electron affinity, high thermal and oxidative stability, but also serves as a good chromophore. Polymers have strong blue fluorescence due to the efficient energy migration from the extended π-electronic structure of the polymers to the chiral binaphthyl core and can be expected to have potential application in the materials of fluorescent sensors. Circular dichroism (CD) spectra of polymers P-1 and P-2 are almost identical except that they gave opposite signals at each wavelength. The long wavelengths CD effect of P-1 and P-2 can be regarded as the more extended conjugated structure in the repeating unit and a high rigidity of the polymer backbone.     

Synthesis and gas permeation properties of various Si-containing poly(diarylacetylene)s and their desilylated membranes

Diarylacetylene monomers having trimethylsilyl groups and other substituents (substituted biphenyl, 1a and 1b; trimethylsilylmethylphenyl, 1c-e) were synthesized and polymerized with TaCl₅-n-Bu₄Sn catalyst to produce the corresponding poly(diarylacetylene)s (2a-d). Polymers 2a-c had high molecular weights and were soluble in common organic solvents. Free-standing membranes of 2a-c as well as previously reported 2f-h were prepared by the solution-casting method. Desilylation of these Si-containing polymer membranes was carried out with trifluoroacetic acid to afford 3a, 3b, and 3f-h. Upon desilylation, biphenyl-containing membranes became less permeable (3a, b), whereas fluorene-containing membranes became more permeable (3f-h). In particular, 3h exhibited extremely high gas permeability (PO₂ = 9800 barrers), which is about the same as that of poly(1-trimethylsilyl-1-propyne). Desilylated membranes 3a and 3f-h showed different gas permeability from that of polymers 2i-k which have the identical chemical structures and obtained directly by the polymerization of the corresponding monomers.     

Novel poly(diphenylacetylene)s with both alkyl and silyl groups as gas permeable membranes: Synthesis, desilylation, and gas permeability

Diphenylacetylenes having a dimethyloctylsilyl group and an alkyl group at para positions [Me₂n-C₈H₁₇SiC₆H₄CCC₆H₄R; R = H (1a), i-Pr (1b), t-Bu (1c), n-Bu (1d)] and having only an alkyl group [PhCCC₆H₄R; R = i-Pr (1B), t-Bu (1C)] were synthesized and then polymerized with TaCl₅/n-Bu₄Sn catalyst to provide the corresponding poly(diphenylacetylene)s (2a, 2b, 2c, 2d, 2B, and 2C). The formed polymers afforded tough free-standing membranes by casting from toluene solutions. Desilylation reaction of Si-containing membranes (2a-d) was carried out with trifluoroacetic acid to give the desilylated membranes (3a-d). The permeability of these membranes to O₂, N₂, and CO₂ were determined. All the Si-containing membranes exhibited almost the same gas permeability. The desilylation of Si-containing membranes of 2a-c resulted in large increase of gas permeability. No apparent increasing of gas permeability was observed in the desilylation of 2d. To clarify the effects of desilylation, CO₂ diffusivity (D(CO₂)), CO₂ solubility (S(CO₂)), and fractional free volume (FFV) of the polymer membranes were investigated. The S(CO₂) values of desilylated membranes were much larger than that of Si-containing counterparts. The D(CO₂) and FFV of membranes of 2a-c increased through desilylation. The desilylated membrane of 3d had small D(CO₂) value and almost the same FFV compared with 2d. Further, the comparison of the permeability between three types of membranes with the same chemical structure revealed that the microvoids were not generated by the desilylation of membranes of poly(diphenylacetylene)s containing alkyl groups.     

Alkyloxy substituted organic dyes for high voltage dye-sensitized solar cell: Effect of alkyloxy chain length on open-circuit voltage

Three novel organic dyes (SB1, SB2, and SB3) containing 4-(hexyloxy)-N-(4-(hexyloxy)phenyl)-N-phenylaniline as electron donor and cyanoacrylic acid as electron acceptor bridged by alkyloxy (methyl = SB1, propyl = SB2 and hexyl = SB3) substituted p-phenylenevinylene linkers have been synthesized. Density functional theory (DFT) has employed to study electron distribution and intramolecular charge transfer. Increase in alkyl chain length in alkyloxy substituent leads to increase in open-circuit voltage (V_OC), which is found to be related to the increased electron lifetime at open-circuit condition. Under AM 1.5 G 1 sun light illumination (100 mW/cm²), an optimized SB3-sensitized cell show a short-circuit photocurrent density (J_SC) of 12.83 mA/cm², an open-circuit voltage (V_OC) of 0.745 V and a fill factor (FF) of 0.64, corresponding to an overall conversion efficiency (η) of 6.12%. Little degradation in η observed over 40 days is indicative of long-term stability of the SB-series dyes.     

Tyrosine-based poly(m-phenyleneethynylene-p-phenyleneethynylene)s. Helix folding and responsiveness to a base

The Sonogashira-Hagihara polymerization of 3′,5′-diiodo-N-α-tert-butoxycarbonyl-l-tyrosine methyl ester (1) and 3′,5′-diiodo-N-α-tert-butoxycarbonyl-O-methyl-l-tyrosine methyl ester (2) with para-diethynylbenzene (3) was carried out to obtain optically active poly(m-phenyleneethynylene-p-phenyleneethynylene)s [poly(1) and poly(2)] with M_n’s ranging from 9900 to 15,000 in 80-87% yields. Poly(1) exhibited intense CD signals in DMSO and THF, but did not in CH₂Cl₂, indicating that it took a predominantly one-handed helical conformation in the former two solvents. On the other hand, there was no evidence for poly(2) to take a helical structure in these solvents. Poly(1) turned the CD sign at 390 nm from plus to minus in DMSO/H₂O = 9/1 (v/v) by the addition of NaOH. Alkaline hydrolysis of ester moieties of poly(1) and poly(2) gave the corresponding polymers having carboxy groups [poly(1a) and poly(2a)]. Poly(1a) and poly(2a) increased the CD intensity by the addition of NaOH.     

Synthesis and characterization of biodegradable polylactides and polylactide-block-poly(Z-lysine) copolymers

The reaction of (R,R)-trans-1,2-bis(2,4,6-triisopropylbenzenesulfonamidato)cyclohexane (^RRTBSC-H₂, 1) with MN[Si(CH₃)₃] in tetrahydrofuran (THF) produces [(^RRTBSC)₂M₄(THF)₄] (2: M = Li, 3: M = Na, 4: M = K). Experimental results show that all three complexes 2-4 are active toward the ring-opening polymerization of l-lactide and compound 2 efficiently catalyzes the polymerization of l-lactide in the presence of a variety of alcohols in a controlled fashion with very narrow polydispersity index. In addition, a variety of biodegradable poly(l-lactide)-block-poly(N_ξ-carbobenzyloxy-l-lysine) block copolymers with different ratios have also been synthesized using poly(l-lactide) containing amino chain end (PLLA-NH₂) as a macroinitiator.     

Synthesis and photovoltaic properties of low-bandgap polymers based on N-arylcarbazole

Low-bandgap poly(2,7-carbazole) derivatives with variable N-substituent of ethyl (PEtCzBT), phenyl (PPhCzBT) and 4-diphenylaminophenyl (PTPACzBT) on the carbazoles, were synthesized through Suzuki coupling reaction. The polymers show excellent solubility in organic solvents (readily soluble in chloroform, THF and toluene etc.), good thermal stability (5% weight loss temperature of more than 417 °C), and electrochemical properties (reversible redox process with narrow bandgap), and deep HOMO energy levels (∼5.1 eV), allowing them promising candidates in the solar cell fabrication. Bulk-heterojunction solar cells with these polymers as electron donor and (6,6)-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as electron acceptor exhibit high V_oc (0.91-0.95 V) and good power conversion efficiency (PCE) of 1.69% for PEtCzTB, 2.01% for PPhCzTB, and 2.42% for PTPACzTB.     

Synthesis and gas permeability of ester substituted poly(p-phenylene)s

Polymerizations of various ester substituted 2,5-dichlorobenzoates [substituent: linear alkyl groups (1a-f), branched alkyl groups (1g-l), cyclohexyl groups (1m-o), phenyl groups (1p-r), and oxyethylene units (1s-v)] were investigated with Ni-catalyzed/Zn-mediated system in 1-methyl-2-pyrrolidone (NMP) at 80 °C. Most of monomers bearing linear and branched alkyl groups successfully polymerized to give relatively high-molecular-weight polymers (M_n = 10,000-20,800). However, the molecular weight of the polymer having eicocyl groups was low because of steric hindrance of long alkyl chain. The polymerizations of cyclohexyl 2,5-dichlorobenzoate and phenyl 2,5-dichlorobenzoate produced low-molecular-weight polymers, while the polymerizations of monomers with alkyl cyclohexyl and alkyl phenyl groups proceeded to afford polymers with relatively high-molecular-weights. The polymers possessing oxyethylene units were obtained, but the molecular weights were low when the oxyethylene chains were long. The gas permeability of membranes of poly(p-phenylene)s with alkyl chains increased as increasing the length of alkyl chain. The membranes of poly(p-phenylene)s with phenyl groups and oxyethylene units exhibited high densities and relatively low gas permeability. However, the CO₂/N₂ separation factor of membrane of poly(p-phenylene) having oxyethylene units was as large as 73.6.