Accelerated cure of phenol–formaldehyde resins: Studies with model compounds

2‐Hydroxymethylphenol (2‐HMP) and 4‐hydroxymethylphenol (4‐HMP) were used as model compounds to study the reactions that occur during cure of phenol–formaldehyde (PF) resin to which cure accelerators (ethyl formate, propylene carbonate, γ‐butyrolactone, and triacetin) have been added. The addition of cure accelerators significantly increased the rate of condensation reactions. The cure accelerators were consumed during the reaction, indicating that they do not act as true catalysts. Major dimeric and trimeric reaction products were isolated and their structures determined. The results are consistent with a mechanism in which the hydroxymethyl group of 2‐HMP (or 4‐HMP) is first transesterified by the cure accelerator. The ester group is then displaced by reaction with the negatively charged ortho or para position of a second molecule (S_N2 mechanism) or is converted to a reactive quinone methide intermediate, which subsequently reacts with the negatively charged ortho or para position of a second molecule (quinone methide mechanism). When accelerators were added to the reaction mixture, the self‐condensation of 2‐HMP was faster than that of 4‐HMP. As is well documented in the literature, the exact opposite is true without added accelerators. This result would seem to indicate that the phenolic oxygen helps activate the esterified ortho‐hydroxymethyl group. The number and nature of crosslinks in a PF resin cured with added cure accelerator might be different than those in a PF resin cured without an added cure accelerator. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3256–3263, 2002     

The Metabolic Fate of ortho-Quinones Derived from Catecholamine Metabolites

ortho-Quinones are produced in vivo through the oxidation of catecholic substrates by enzymes such as tyrosinase or by transition metal ions. Neuromelanin, a dark pigment present in the substantia nigra and locus coeruleus of the brain, is produced from dopamine (DA) and norepinephrine (NE) via an interaction with cysteine, but it also incorporates their alcoholic and acidic metabolites. In this study we examined the metabolic fate of ortho-quinones derived from the catecholamine metabolites, 3,4-dihydroxyphenylethanol (DOPE), 3,4-dihydroxyphenylethylene glycol (DOPEG), 3,4-dihydroxyphenylacetic acid (DOPAC) and 3,4-dihydroxyphenylmandelic acid (DOMA). The oxidation of catecholic substrates by mushroom tyrosinase was followed by UV-visible spectrophotometry. HPLC analysis after reduction with NaBH₄ or ascorbic acid enabled measurement of the half-lives of ortho-quinones and the identification of their reaction products. Spectrophotometric examination showed that the ortho-quinones initially formed underwent extensive degradation at pH 6.8. HPLC analysis showed that DOPE-quinone and DOPEG-quinone degraded with half-lives of 15 and 30 min at pH 6.8, respectively, and >100 min at pH 5.3. The major product from DOPE-quinone was DOPEG which was produced through the addition of a water molecule to the quinone methide intermediate. DOPEG-quinone yielded a ketone, 2-oxo-DOPE, through the quinone methide intermediate. DOPAC-quinone and DOMA-quinone degraded immediately with decarboxylation of the ortho-quinone intermediates to form 3,4-dihydroxybenzylalcohol (DHBAlc) and 3,4-dihydroxybenzaldehyde (DHBAld), respectively. DHBAlc-quinone was converted to DHBAld with a half-life of 9 min, while DHBAld-quinone degraded rapidly with a half-life of 3 min. This study confirmed the fact that ortho-quinones from DOPE, DOPEG, DOPAC and DOMA are converted to quinone methide tautomers as common intermediates, through proton rearrangement or decarboxylation. The unstable quinone methides afford stable alcoholic or carbonyl products.     

Donor Atom‐Stabilized Aluminum Alkyls as Cocatalysts for the Ziegler–Natta Polymerization of Propene

A number of organoaluminum compounds, stabilized with intramolecular nitrogen‐ or oxygen‐donor functions, have been used as cocatalysts for the MgCl₂/TiCl₄‐catalyzed homopolymerization of propene as well as for the copolymerization of ethene with propene. The polymerization behavior of these aluminum alkyls was examined at different Al/Ti ratios within the range of 2 to 50 and compared with the reference of triethylaluminum (TEA). Especially the alkyls [2‐(N,N‐dimethylaminomethyl)phenyl]dimethylaluminum ( 1 ) and [2‐(N,N‐dimethylaminomethyl)phenyl]diethylaluminum ( 2 ) show the highest activities at very low Al/Ti ratios in the homopolymerization of propene, whereas TEA is almost inactive. The species [8‐(N,N‐dimethylamino)naphthyl]dimethylaluminum ( 4 ) reaches the highest activity of all examined alkyls and is very close to the highest value obtained with TEA. Bulky iso‐butyl groups at the aluminum center are responsible for the very poor performance of the nitrogen stabilized cocatalysts [8‐(N,N‐dimethylamino)naphthyl]diisobutylaluminum ( 5 ) and [2‐(N,N‐dimethylaminomethyl)phenyl]diisobutylaluminum ( 3 ). The properties of the polypropenes synthesized with the stabilized organoaluminum species are similar to those produced with TEA but with a distinctly higher molar mass. In the case of 1 , it was possible to increase the molar mass by a factor of three. For the copolymerizations, the compounds [2‐(N,N‐diethylaminomethyl)phenyl]diethylaluminum ( 7 ) and (2‐methoxybenzyl)diisobutylaluminum ( 8 ) were found to be most suitable, producing polymers with significantly higher activities than TEA. For all copolymers two fractions were obtained, one crystalline fraction with a low and an amorphous part with a high amount of comonomer. In both fractions, 7 and 8 provide a higher comonomer incorporation than TEA.     

Benzene‐di‐N‐substituted carbamates as conformationally constrained analogs of Pseudomonas lipase substrates

Benzene‐1,2‐, ‐1,3‐, and ‐1,4‐di‐N‐substituted carbamates ( 1 – 15 ) are synthesized as the constrained analogs of gauche, eclipsed, and anti conformations of diesters of ethylene glycol, respectively. Carbamates 1 – 15 are characterized as the pseudo‐substrate inhibitors of Pseudomonas species lipase. Long‐chain carbamates are more potent inhibitors than short‐chain ones. Different geometries of benzene‐di‐substituted carbamates, such as benzene‐1,2‐di‐N‐octylcarbamate ( 3 ) (ortho compound), benzene‐1,3‐di‐N‐octylcarbamate ( 8 ) (meta compound), and benzene‐1,4‐di‐N‐octylcarbamate ( 13 ) (para compound), show similar inhibitory potencies for the enzyme. In other words, kinetic data suggest that the enzyme does not discriminate ortho, meta, and para geometries of these constrained analogs.     

Iron‐Catalyzed Oxidative Mono‐ and Bis‐Phosphonation of N,N‐Dialkylanilines

The dehydrogenative α‐phosphonation of substituted N,N‐dialkylanilines by dialkyl H‐phosphonates was achieved under mild conditions by using environmentally benign iron(II) chloride as catalyst and tert‐butyl hydroperoxide as oxidant. The reaction proceeded in the presence of electron‐donating (methoxy, methyl, benzyl) and electron‐withdrawing ring‐substitutents (bromo, carbonyl, carboxyl, m‐nitro) in moderate to good yields. The X‐ray crystal structure of N‐(5,5‐dimethyl‐2‐oxo‐2λ⁵‐[1,3,2]dioxaphosphinan‐2‐yl‐methyl)‐N‐methyl‐p‐toluidine was determined. Bis‐(4‐(dimethylamino)phenyl)methane and bis‐4,4′‐(dimethylamino)benzophenone underwent bisphosphonation selectively by respective monophosphonation at the remote dimethylamino groups. Furthermore, the use of excess dialkyl H‐phosphonate and oxidant allowed us to functionalize both methyl groups of N(CH₃)₂ in N,N‐dimethyl‐p‐toluidine and N,N‐dimethylaminomesidine, respectively, to obtain α,α′‐bisphosphonatoamines in high yield.     

Structure of polycondensates from hydroxymethylphenols

The structure of oligomers obtained from mono‐hydroxymethylphenols in melt condensation at 120°C was determined using ¹³C NMR spectra in CD₃OD solution. Alongside of methylene region of spectrum, valuable information was obtained from signals of aromatic carbons. Noncatalytic conditions promote the formation of dihydroxydibenzyl ethers in equilibrium with ortho‐ and para‐benzoquinones of oxymethylene derivatives. The final methylene linked oligomers are formed, mainly, by splitting the ether intermediates with free aromatic positions. In alkaline conditions, highly nucleophilic phenoxide ions of ortho‐hydroxymethyl compounds are responsible for substitution in free aromatic positions. The most favored reaction in the mixture of both hydroxymethylphenols is the formation of p,p′‐methylene. In condensation of para‐hydroxymethylphenol, formation of p,p′‐methylene groups occurs with simultaneous release of formaldehyde. High content of alkali stabilized ortho‐hydroxymethyl groups of fully substituted methylene linked oligomers determines the curing behavior of resol phenol–formaldehyde resins. The role of hemiformals in reactions was insignificant. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Highly Stereoselective Synthesis of Novel Multistereogenic Bis‐Bifunctional Ligands Based on [2.2]Paracyclophane‐ 4,7‐quinone,their Structure Elucidation and Application in Asymmetric Catalysis

Bis‐bifunctional cis‐4,7‐diarylsubstituted‐4,7‐dihydroxy‐4,7‐dihydro[2.2]paracyclophanes 3–6 were synthesized by a highly diastereoselective reaction of ortho‐substituted aryllithium reagents with [2.2]paracyclophane‐4,7‐quinone ( 1 ). Enantiomerically pure diols 3–5 were tested as chiral inductors in the enantioselective addition of diethylzinc to benzaldehyde (up to 93.5% ee). Acid dehydration of cis‐4,7‐di(2‐methoxyphenyl)‐4,7‐dihydroxy‐4,7‐dihydro[2.2]paracyclophane ( 3 ) results in 4,7‐dihydro‐7,8‐di(2‐methoxyphenyl)[2.2]paracyclophane‐4‐one ( 8 ) – a planar chiral cyclohexadienone of the [2.2]paracyclophane series with a para‐semiquinoid substructure. X‐Ray investigations of compounds 3, 4 and 8 were performed.     

Palladium‐Catalyzed N‐Arylation of N,N‐Dialkylhydrazines with Aryl Chlorides

N,N‐Dialkyl‐N′‐arylhydrazines have been prepared usually in high to excellent yields via the reaction of N,N‐dialkylhydrazines with aryl chlorides in the presence of Pd₂(dba)₃, Xphos and NaO‐t‐Bu in dioxane at 120 °C. With ortho‐substituted aryl chlorides best results have been obtained by using 2‐(2′,6′‐dimethoxybiphenyl)dicyclohexylphosphine (ligand d) as the ligand.     

Synthesis,characterization and thermal degradation of dual temperature‐ and pH‐sensitive RAFT‐made copolymers of N,N‐(dimethylamino)ethyl methacrylate and methyl methacrylate

Poly{[(N,N‐(dimethylamino)ethyl methacrylate]‐co‐(methyl methacrylate)} copolymers of various compositions were synthesized by reversible addition‐fragmentation chain transfer (RAFT) polymerization at 70 °C in N,N‐dimethylformamide. The polymer molecular weights and molecular weight distributions were obtained from size exclusion chromatography, and they indicated the controlled nature of the RAFT polymerizations; the polydispersity indices are in the range 1.1–1.3. The reactivity ratios of N,N‐(dimethylamino)ethyl methacrylate (DMAEMA) and methyl methacrylate (MMA) (r_DMAEMA = 0.925 and r_MMA = 0.854) were computed by the extended Kelen–Tüdös method at high conversions, using compositions obtained from ¹H NMR. The pH‐ and temperature‐sensitive behaviour were studied in aqueous solution to confirm dual responsiveness of these copolymers. The thermal properties of the copolymers with various compositions were investigated by differential scanning calorimetry and thermogravimetric analysis. The kinetics of thermal degradation were determined by Friedmann and Chang techniques to evaluate various parameters such as the activation energy, the order and the frequency factor. © 2012 Society of Chemical Industry     

Novel and Mild Route to Phthalocyanines and 3‐Iminoisoindolin‐1‐ones via N,N‐Diethylhydroxylamine‐Promoted Conversion of Phthalonitriles and a Dramatic Solvent‐Dependence of the Reaction

Refluxing a mixture of phthalonitrile C₆R¹R²R³R⁴(CN)₂ 1 (R¹–R⁴=H), or its substituted derivatives 2 (R¹, R³, R⁴=H, R²=Me), or 3 (R¹, R⁴=H, R², R³=Cl) (1 equiv.) and N,N‐diethylhydroxylamine, Et₂NOH, (4 equivs.) in methanol for 4 h results ( Route A ) in precipitation of the symmetrical ( 6 and 8 ) and an isomeric mixture of unsymmetrical ( 7 ) phthalocyanines, isolated in good (55–65 %) yields. The reaction of phthalonitriles 1 , 2 , or 4 (R¹, R³, R⁴=H, R²=NO₂) (4 equivs.) with Et₂NOH (8 equivs.) in the presence of a metal salt MCl₂ (M=Zn, Cd, Co, Ni) (1 equiv.) in n‐BuOH or without solvent results in the formation of metallated phthalocyanine species ( 9 – 17 ). Upon refluxing in freshly distilled dry chloroform, phthalonitrile 1 or its substituted analogues 2 , 3 or 5 (R¹–R⁴=F) (1 equiv.) react with N,N‐diethylhydroxylamine (2 equivs.) affording 3‐iminoisoindolin‐1‐ones 18 – 21 ( Route B ) isolated in good yields (55–80 %). All the prepared compounds were characterized with C, H, and N elemental analyses, ESI‐MS, IR, and compounds 18 – 21 also by 1D (¹H, ¹³C{¹H}), and 2D (¹H,¹⁵N‐HMBC and ¹H,¹³C‐HMQC, ¹H,¹³C‐HMBC) NMR spectroscopy.     

Synthesis of Reactive Condensation Products of Acetylacetone and Their Transformation into Deeply Coloured Methine Dyes

Starting from acetylacetone 1 , boric acid and a diol, or a 2‐aminothiazole 16 and perchloric acid several dimethyl‐substituted 1,3,2‐dioxaborines 5 and thiazolo[3,2‐a]pyrimidinium salts 17 , resp., have been prepared and transformed by raction with 4‐N,N‐dimethylamino‐benzaldehyde 3a or 3‐methyl‐2‐methylmercapto‐benzthiazolium perchlorate 13 into deeply colored polymethine dyes 11, 12, 14, 15, and 18—21 , and 23—24 their spectroscopic data were recorded.     

Palladium‐Catalyzed Selective Aryl Ring C–H Activation of N‐Acyl‐2‐aminobiaryls: Efficient Access to Multiaryl‐Substituted Naphthalenes

Palladium‐catalyzed cycloaromatization of N‐acyl‐2‐aminobiaryls, through a sequence of ortho C−H bond activation/alkyne insertion/meta C−H bond activation/alkyne insertion, was developed. An efficient synthesis of multiaryl‐substituted naphthalenes, N‐[2‐(5,6,7,8‐tetraarylnaphthalen‐1‐yl)aryl]acetamides, was demonstrated using molecular oxygen as the sole oxidant. Furthermore, through Buchwald's synthetic protocol, two compounds were converted into corresponding fluorescent carbazoles in 30–40% yield by intramolecular C−N bond formation.

     

Synthesis,photochemical characterization,and thermal stability of a series of substituted formamidines as ultraviolet light absorbers

The three‐step reactions of ethyl 4‐aminobenzoate, formic acid, and halohydrocarbons afforded 10 N‐substituted‐N,N′‐diaryl‐formamidine derivatives ( F1 – 10 ) as ultraviolet absorbers. These N‐substituted formamidines were characterized by ¹H NMR, ¹³C NMR, FT‐IR, and ESI‐MS spectroscopies. The UV–vis absorbance and fluorescence properties of the compounds F1 – 10 were investigated in different solvents and in the presences of different metal ions. The effects of the amount of Al³⁺, Pb²⁺, Zr⁴⁺ ions on the UV–vis absorbance and fluorescence properties of compound F1 were also investigated. Moreover, the thermal stability of the compounds F1 – 10 was evaluated as well as the intermediate N,N′‐bis(4‐ethoxycarbonylphenyl)‐formamidine. J. VINYL ADDIT. TECHNOL., 25:E108–E113, 2019. © 2019 Society of Plastics Engineers     

Organocatalytic Asymmetric Vinylogous Michael Addition of Dicyanoolefins to Imine Intermediates Generated in situ from Arenesulfonylalkylindoles

An organocatalytic asymmetric vinylogous Michael addition of dicyanoolefins to vinylogous imine intermediates generated in situ from arenesulfonylalkylindoles has been developed. This protocol provides an easy and convenient approach to C‐3 alkyl‐substituted indole derivatives with high yields (up to 93%), diastereomeric ratios (up to 99:1 dr) and enantioselectivities (up to 99% ee). The resulting adducts can be also readily converted to pyrazolo derivatives or α‐alkylation products of ketones without any decrease of the diastereoselectivities and enantioselectivities.     

On the Trapping of Vinylgold Intermediates

An internal aryl‐substituted ortho‐alkynylphenol and a similar aniline with stoichiometric amounts of N,N′‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene‐gold tosylate [(IPr)AuOTs] and triethylamine gave the aurated heterocycles as stable intermediates of the corresponding gold(I)‐catalysed hydrooxylation and hydroamination reactions. X‐ray crystal structure analyses of both products could be obtained. A similar internal alkyl‐substituted ortho‐alkynylphenol gave only the cycloisomerised product, no aurated intermediate could be detected.     

Synthesis of C8‐N‐Arylamine‐Modified 2′‐Deoxyguanosine‐5′‐Triphosphates and Their Effects on Primer Extension by DNA Polymerases

C8‐N‐arylamine adducts of 2′‐deoxyguanosine (2′‐dG) play an important role in the induction of the chemical carcinogenesis caused by aromatic amines. C8‐N‐acetyl‐N‐arylamine dG adducts that differ in their substitution pattern in the aniline moiety were converted by cycloSal technology into the corresponding C8‐N‐acetyl‐N‐arylamine‐2′‐deoxyguanosine‐5′‐triphosphates and C8‐NH‐arylamine‐2′‐deoxyguanosine‐5′‐triphosphates. Their conformation preference has been investigated by NOE spectroscopy and DFT calculations. The substrate properties of the C8‐dG adducts were studied in primer‐extension assays by using Klenow fragment exo^? of Escherichia coli DNA polymerase I and human DNA polymerase β. It was shown that the incorporation was independent of the substitution pattern in the aryl moiety and the N‐acetyl group. Although the triphosphates were poor substrates for the human polymerases, they were incorporated twice before the termination of the elongation process occurred; this might demonstrate the importance of C8‐N‐arylamine‐2′‐deoxyguanosine‐5′‐triphosphates in chemical carcinogenesis.     

Synthetic Studies toward Communesins

Communesins A–H are a growing family of natural products isolated from a marine fungal strain of Penicillium species. Preliminary biological evaluation has revealed that these compounds possess insecticidal activity and cytotoxicity against several tumor cell lines. Their interesting biological activities and unique structures have attracted considerable attention of synthetic chemists worldwide. To date, several elegant protocols for assembling the core structure of these indole alkaloids have been described, including intermolecular Diels–Alder reaction of methylated aurantioclavine with quinine methide imine and intramolecular hetero Diels–Alder reaction of the azaortho-xylylene intermediates. Recently, three completed total syntheses for communesin F have been disclosed, in which the crucial vicinal quaternary stereogenic centers were constructed by employing an intramolecular cyclopropanation; an intramolecular Heck reaction of a tetrasubstituted alkene, or an oxidative coupling of a 3-substituted indole, as the key step. Accompanying the total synthesis, the absolute configuration of natural communesin F was established as 6R,7R,8R,9S,11R. However, total syntheses of other members of communesin family that contain an epoxide moiety have not been achieved, which will stimulate more synthetic studies.     

Palladium‐Catalysed C?H Bond Electrophilic Fluorination of Highly Substituted Arylpyrazoles: Experimental and DFT Mechanistic Insights

A general protocol for palladium‐catalysed C H mono‐ and di‐fluorination of highly substituted arylpyrazoles is reported. Coupling pathways and substrate limitations are discussed in the light of complementary mechanistic experimental and density functional theory (DFT) studies. The mono‐ and di‐ortho‐fluorination of arylpyrazoles having substituted pyrazole groups and ortho‐, meta‐, or para‐substituted arene moieties is achieved. Various pyrazole groups can efficiently promote the direct C H activation/fluorination of substrates bearing valuable reactive ester, cyano, halide and nitro functions. The presence of methoxy, methyl and trifluoromethyl is tolerated on the pyrazole directing groups. However, steric substituent effects have a marked influence which is evidenced by calculations. DFT modelling suggested also a previously unseen outer‐sphere oxidative addition of N‐fluorobenzenesulfonimide (NFSI) to Pd(II) as an alternative mechanism to the commonly assumed Pd(II)/Pd(IV) process. This unprecedented proposal, which is supported by the mass spectrometry identification of a key Pd(II) monomer under the stoichiometric conditions deserves more attention. The influence of elaborate highly substituted directing groups on the course of Pd‐catalysed fluorination has generally received limited attention although this question has a crucial synthetic utility; herein, appropriate conditions for isolating pure products are reported.

     

Synthesis and characterization of novel polyaspartimides derived from 2,2′‐dimethyl‐4,4′‐bis(4‐maleimidophenoxy)biphenyl and various diamines

A novel bismaleimide, 2,2′‐dimethyl‐4,4′‐bis(4‐maleimidophenoxy)biphenyl, containing noncoplanar 2,2′‐dimethylbiphenylene and flexible ether units in the polymer backbone was synthesized from 2,2′‐dimethyl‐4,4′‐bis(4‐aminophenoxy)biphenyl with maleic anhydride. The bismaleimide was reacted with 11 diamines using m‐cresol as a solvent and glacial acetic acid as a catalyst to produce novel polyaspartimides. Polymers were identified by elemental analysis and infrared spectroscopy, and characterized by solubility test, X‐ray diffraction, and thermal analysis (differential scanning calorimetry and thermogravimetric analysis). The inherent viscosities of the polymers varied from 0.22 to 0.48 dL g⁻¹ in concentration of 1.0 g dL⁻¹ of N,N‐dimethylformamide. All polymers are soluble in N‐methyl‐2‐pyrrolidone, N,N‐dimethylacetamide, N,N‐dimethylformamide, dimethylsulfoxide, pyridine, m‐cresol, and tetrahydrofuran. The polymers, except PASI‐4, had moderate glass transition temperature in the range of 188°–226°C and good thermo‐oxidative stability, losing 10% mass in the range of 375°–426°C in air and 357°–415°C in nitrogen. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 279–286, 1999     

Synthesis of α‐Carbolines Starting from 2,3‐Dichloropyridines and Substituted Anilines

9H‐α‐Carbolines have been prepared via consecutive intermolecular Buchwald–Hartwig reaction and Pd‐catalyzed intramolecular direct arylation from commercially available 2,3‐dichloropyridines and substituted anilines. The combination of a high reaction temperature (180 °C) and the use of DBU were found to be crucial for the intramolecular direct arylation reactions of the 3‐chloro‐N‐phenylpyridin‐2‐amines as no reaction was observed at 120 °C and 180 °C using different inorganic and other organic bases. On the other hand, nitrogen‐methylated pyridine analogues of these substrates {N‐[3‐chloro‐1‐methylpyridin‐2(1H)‐ylidene]anilines} do undergo ring closure at 120 °C, with K₃PO₄ as base, affording the respective 1‐methyl‐1H‐α‐carbolines in good yields.