Effect of isocyanate structure on deblocking and cure reaction of N-methylaniline-blocked diisocyanates and polyisocyanates

Two series of N-methylaniline-blocked isocyanates based on monomeric diisocyanates such as 4,4′-methylene bis(phenyl isocyanate), toluene-2,4-diisocyanate, isophorone diisocyanate and 1,6-diisocyanato hexane and their NCO terminated polyurethane prepolymer (polyisocyanates) were prepared and characterized thoroughly by FTIR, ¹H NMR, ¹³C NMR and EI-Mass spectroscopic methods. The blocking reaction of N-methylaniline with aromatic isocyanates and aromatic polyisocyanates occur faster when compared to the aliphatic isocyanates. The deblocking reactions of blocked isocyanates were carried out under dynamic and isothermal conditions using hot-stage FTIR spectrophotometer. The dynamic method was used to determine the deblocking temperature, and the isothermal method was used to calculate kinetics and thermodynamics parameters. Cure reactions of blocked isocyanates with hydroxyl-terminated polybutadiene were also followed to establish the structure-property relationship of the N-methylaniline-blocked isocyanates. The deblocking studies of blocked isocyanates reveal that the aromatic isocyanates undergo deblocking easily compared to aliphatic isocyanates. The rate of deblocking reaction of N-methylaniline-blocked aromatic polyisocyanates was found to be higher compared to N-methylaniline-blocked aromatic monomeric diisocyanate adducts. On the other hand, this trend was just reverse in the cure-reaction studies. The dissolution behavior of N-methylaniline-blocked isocyanates in Terathane-2000, polypropylene glycol-2000, polycaprolactone diol-2000 and hydroxyl-terminated polybutadiene-2500 was also studied and found that all adducts are soluble in these polyols.     

Synthesis and deblocking of cardanol‐ and anacardate‐blocked toluene diisocyanates

Methyl anacardate and secondary butyl anacardate were prepared from anacardic acid and corresponding alcohols and were used, in addition to cardanol, as blocking agents for 2,4‐toluene diisocyanate (TDI). Blocked diisocyanate adducts were characterized via nitrogen estimation, Fourier transform infrared spectroscopy, and proton nuclear magnetic resonance spectroscopy. The deblocking temperatures of the adducts were determined using an FTIR spectrophotometer in conjunction with the carbon dioxide evolution method. The gel times of hydroxyl‐terminated polybutadiene–TDI adducts also were determined. Deblocking temperature and gel time analyses revealed that cardanol‐blocked 2,4‐TDI deblocks at a lower temperature and at a higher rate compared with anacardate‐blocked adducts. In addition, it was found that the electron‐withdrawing ester group reduces the deblocking temperature of the adduct only when it is in solvated form. All adducts were waxy solids that were found to be soluble in polyether polyol, polyester polyol, and polyhydrocarbon polyols. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4047–4055, 2004     

Synthesis and properties of blocked Di- and polyisocyanates

Tricresol-blocked di- and polyisocyanates (BDPs) of variable structure are synthesized, and their main physicochemical and technological properties are defined. They have lower melting points and thermal dissociation (deblocking) temperatures than do the known BDPs based on ε-caprolactam, oximes, and secondary amines.. The new blocked isocyanates are tested as latent curing agents for polyurethane varnish compositions of electroinsulating assignment.     

Catalysis of poly(urethane imidine) copolymer formation from amine‐blocked isocyanate and bisphthalide

The catalysis of imidine formation between an amine‐blocked polyurethane prepolymer and bisphthalide was studied with a series of metal alkoxides, phenoxides, and organotin compounds and tertiary amines. The carbon dioxide released during the reaction was followed for monitoring of the reaction. The metal alkoxides and phenoxides catalyzed the imidine formation reaction but did not catalyze the deblocking reaction, whereas the organotin compounds and tertiary amines showed no catalytic activity in the reaction between isocyanate and phthalide. With tin catalysts, the imidine formation reaction depended on the deblocking of the blocked prepolymer, but it was independent of deblocking with amine catalysts. The resultant poly(urethane imidine) copolymers were characterized with Fourier transform infrared, ¹H NMR, ¹³C NMR, gel permeation chromatography, and thermogravimetric analysis techniques. The thermal stability of polyurethane increased significantly with the incorporation of imidine groups. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 4236–4242, 2001     

Substituent effect of N‐aryl‐N′‐pyridyl ureas as thermal latent initiators on ring‐opening polymerization of epoxide

A series of N‐aryl‐N′‐pyridyl ureas were synthesized by the reactions of 4‐aminopyridine (4AP) with the corresponding isocyanates such as phenyl isocyanate, 4‐methylphenyl isocyanate, 4‐methoxyphenyl isocyanate, 4‐chlorophenyl isocyanate, 4‐(trifluoromethyl)phenyl isocyanate, and 4‐nitrophenyl isocyanate. Bulk polymerization of diglycidyl ether of bisphenol A (DGEBA) in the presence of the ureas as initiators was evaluated by differential scanning calorimetry (DSC) at a heating rate of 10 °C/min. The resulting DSC profiles indicated exothermic peaks above 140 °C, while the DSC profile measured for a formulation composed of DGEBA and pristine 4AP indicated an exothermic peak at around 120 °C, implying that the derivation of 4AP into the corresponding ureas is a useful strategy to achieve thermal latency. The peak top temperatures were correlated with the electron density of the aromatic ring of the ureas, that is, as the electron‐withdrawing nature of the substituent on the aromatic ring became larger, the peak increases. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2569–2574     

Study on synthesis,characterizations, and properties of pyridinol-blocked isocyanates and waterborne polyurethane

A series of pyridinol-blocked isophorone isocyanates were synthesized through esterification reaction, Fries rearrangement, and blocking reaction and characterized by ¹H-NMR, ¹³C-NMR, and Fourier transform infrared spectra. Based on the synthesized blocked isocyanates, the blocked waterborne polyurethane (BWPU) was prepared by the self-emulsification method. The deblocking studies revealed that the deblocking temperature reduces with electron-withdrawing and steric hindrance substituents on the ortho position of pyridinol. The stability, molecular weight (M_w), particle size, viscosity, and hydrophilicity of BWPU were studied and compared. The results showed that with an increased amount of blocking agents, molecular weight, particle size, and viscosity decrease and the hydrophilicity increases.     

Synthesis and characterization of N‐substituted hyperbranched polyureas

N,N′‐disubstituted hyperbranched polyureas with methyl, benzyl, and allyl substitutents were synthesized starting from AB₂ monomers based on 3,5‐diamino benzoic acid. Carbonyl azide approach, which generates isocyanate group in situ on thermal decomposition, was used for the protection of isocyanate functional groups. The N‐substituted hyperbranched polymers can be considered as the new class of internally functionalized hyperbranched polyureas wherein the substituent can function either as receptor or as a chemical entity for selective transformations as a tool to tailor the properties. The chain‐ends were also modified by attaching long chain aliphatic groups to fully realize the interior functionalization. This approach opens up a possible synthetic route wherein different functional substituents can be used to generate a library of internally functionalized hyperbranched polymers. All the hyperbranched polyureas were characterized by FTIR, ¹H‐NMR, DSC, TGA, and size exclusion chromatography. Degree of branching in these N,N′‐disubstituted hyperbranched polyureas, as calculated by ¹H‐NMR spectroscopy using model compounds, was found to be lower than the unsubstituted hyperbranched polyurea and is attributed to the lower reactivity of N‐substituted amines compared to that of unsubstituted amines. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5134–5145, 2004     

Competitive proton and hydride transfer reactions via ion‐neutral complexes: fragmentation of deprotonated benzyl N‐phenylcarbamates in mass spectrometry

The gas‐phase chemistry of deprotonated benzyl N‐phenylcarbamates was investigated by electrospray ionization tandem mass spectrometry. Characteristic losses of a substituted phenylcarbinol and a benzaldehyde from the precursor ion were proposed to be derived from an ion‐neutral complex (INC)‐mediated competitive proton and hydride transfer reactions. The intermediacy of the INC consisting of a substituted benzyloxy anion and a phenyl isocyanate was supported by both ortho‐site‐blocking experiments and density functional theory calculations. Within the INC, the benzyloxy anion played the role of either a proton abstractor or a hydride donor toward its neutral counterpart. Relative abundances of the product ions were influenced by the nature of the substituents. Electron‐withdrawing groups at the N‐phenyl ring favored the hydrogen transfer process (including proton and hydride transfer), whereas electron‐donating groups favored direct decomposition to generate the benzyloxy anion (or substituted benzyloxy anion). By contrast, electron‐withdrawing and electron‐donating substitutions at the O‐benzyl ring exhibited opposite effects. Copyright © 2015 John Wiley & Sons, Ltd.     

The Substituent Effect of 1‐Arylazonaphthen‐2‐ols on Azo‐hydrazone Tautomerization According to NMR Analysis

The substituent effect on azo‐hydrazone tautomerization of 1‐arylazonaphthen‐ols is studied by means of NMR analysis. Among the ¹³C chemical shifts, the C(2) of this series compound is the most sensitive to the variation in the nature of substituent on the phenyl ring. Therefore, the variation in the chemical shifts of C(2) is used to probe the substituent effect by using the substituent chemical shifts and free energy vs. Hammett’s constant (χρ⁺). Both methods give a negative correlation slope, indicating the electron‐with‐ drawing groups favor the hydrazone tautomer form. The effect on the chemical shifts of C(2) of compound 8 in ten solvents can be classified as the solvent with a proton‐donor, proton‐acceptor and arenes system. The substituent with electron‐donating character is more sensitive to the nature of solvent and it favors the hydrazone form. Free energy obtained from the dynamic NMR technique indicates the tautomerization favors the hydrazone‐form for the substituent with electron‐withdrawing character.     

SN1‐SN2 and SN2‐SN3 mechanistic changes revealed by transition states of the hydrolyses of benzyl chlorides and benzenesulfonyl chlorides

Hydrolysis reactions of benzyl chlorides and benzenesulfonyl chlorides were theoretically investigated with the density functional theory method, where the water molecules are explicitly considered. For the hydrolysis of benzyl chlorides (para‐Z? C₆H₄? CH₂? Cl), the number of water molecules (n) slightly influences the transition‐state (TS) structure. However, the para‐substituent (Z) of the phenyl group significantly changes the reaction process from the stepwise (S_N1) to the concerted (S_N2) pathway when it changes from the typical electron‐donating group (EDG) to the typical electron‐withdrawing one (EWG). The EDG stabilizes the carbocation (MeO? C₆H₄? CH₂⁺), which in turn makes the S_N1 mechanism more favorable and vice versa. For the hydrolysis of benzenesulfonyl chlorides (para‐Z? C₆H₄? SO₂? Cl), both the Z group and n influence the TS structure. For the combination of the large n value (n > 9) and EDG, the S_N2 mechanism was preferred. Conversely, for the combination of the small n value and EWG, the S_N3 one was more favorable. © 2014 Wiley Periodicals, Inc.     

Polymerization of o‐quinodimethanes. III. Polymerization of o‐quinodimethanes bearing electron‐withdrawing groups formed in situ by thermal ring‐opening isomerization of corresponding benzocyclobutenes

The substituent effect on the radical polymerization of o‐quinodimethanes, generated by thermal isomerization of benzocyclobutenes, was investigated. Polymerizations of three benzocyclobutenes bearing electron‐withdrawing groups were studied, namely 1‐cyanobenzocyclobutene (1), 1‐chlorobenzocyclobutene (2), and 1‐bromobenzocyclobutene (3). While radical polymerizations of 2 and 3 did not afford any polymer, radical polymerization of 1 afforded n‐hexane‐insoluble polymer(M_n = 5000) in moderate yields at temperatures above 120°C. The structure of the obtained polymer was confirmed to be a ring‐opened polymer(4) by IR, ¹H‐, and ¹³C‐NMR. The yield of the polymer increased with an increase in the initiator concentration. The polymer yield reported in this paper is higher than those of benzocyclobutenes bearing electron‐donating groups, reported previously by the authors. The semi‐empirical molecular orbital calculation supported the contribution of ring‐opening polymerization of spiro‐compounds, rejecting the possibility of 1,4‐polymerization. Lastly, radical copolymerizations of 1 with various comonomers were also performed to obtain the corresponding copolymers. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1555–1563, 1999     

A Facile and Efficient Synthesis of Arylsulfonamido‐Substituted 1,5‐Benzodiazepines and N‐[2‐(3‐Benzoylthioureido)aryl]‐3‐oxobutanamide Derivatives

A facile and efficient synthesis of 1,5‐benzodiazepines with an arylsulfonamido substituent at C(3) is described. 1,5‐Benzodiazepine, derived from the condensation of benzene‐1,2‐diamine and diketene, reacts with an arylsulfonyl isocyanate via an enamine intermediate to produce the title compounds of potential synthetic and pharmacological interest in good yields (Scheme 1). In addition, reaction of benzene‐1,2‐diamine and diketene in the presence of benzoyl isothiocyanate leads to N‐[2‐(3‐benzoylthioureido)aryl]‐3‐oxobutanamide derivatives (Scheme 2). This reaction proceeds via an imine intermediate and ring opening of diazepine. The structures were corroborated spectroscopically (IR, ¹H‐ and ¹³C‐NMR, and EI‐MS) and by elemental analyses. A plausible mechanism for this type of cyclization is proposed (Scheme 3).     

NMR study of O and N,O‐substituted 8‐quinolinol derivatives

The ¹H and ¹³C NMR spectral study of several biologically active derivatives of 8‐quinolinol have been made through extensive NMR studies including homodecoupling and 2D‐NMR experiments such as COSY‐45°, NOESY, and HeteroCOSY. Electron donating resonance and electron withdrawing inductive effect of several groups showed marked changes in chemical shifts of nuclei at the seventh positions of O‐substituted quinolinols (2–15). Although in N‐alkyl, 8‐alkoxyquinolinium halides (16–21), ring A rightly showed low frequency chemical shift values. Copyright © 2013 John Wiley & Sons, Ltd.     

Synthesis of triarylamine‐containing poly(arylene ether)s by nucleophilic aromatic substitution reaction

We report synthesis of a series of new triarylamine‐containing AB‐type monomers and their polymers via nucleophilic aromatic substitution (S_NAr) reaction. Monomers consisting of a hydroxyl group at the para position of the nitrogen group in one phenyl ring and a fluorine leaving group at the para position in another phenyl ring were synthesized via palladium‐catalyzed amination reaction. The fluorine leaving group was activated by trifluoromethyl group at the ortho position and an electron‐withdrawing group (EWG) introduced at the para position of the unsubstituted phenyl ring that enabled control over monomer reactivity. S_NAr reaction of the monomers successfully produced corresponding poly(arylene ether)s with pendant EWGs that exhibited good solubility and thermal stability. Optical and electrochemical properties of the polymers were also affected by incorporation of EWGs. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2692‐2702     

Chemoselectivity in the Reaction of 2‐Diazo‐3‐oxo‐3‐phenylpropanal with Aldehydes and Ketones

The chemoselectivity in the reaction of 2‐diazo‐3‐oxo‐3‐phenylpropanal ( 1 ) with aldehydes and ketones in the presence of Et₃N was investigated. The results indicate that 1 reacts with aromatic aldehydes with weak electron‐donating substituents and cyclic ketones under formation of 6‐phenyl‐4H‐1,3‐dioxin‐4‐one derivatives. However, it reacts with aromatic aldehydes with electron‐withdrawing substituents to yield 1,3‐diaryl‐3‐hydroxypropan‐1‐ones, accompanied by chalcone derivatives in some cases. It did not react with linear ketones, aliphatic aldehydes, and aromatic aldehydes with strong electron‐donating substituents. A mechanism for the formation of 1,3‐diaryl‐3‐hydroxypropan‐1‐ones and chalcone derivatives is proposed. We also tried to react 1 with other unsaturated compounds, including various olefins and nitriles, and cumulated unsaturated compounds, such as N,N′‐dialkylcarbodiimines, phenyl isocyanate, isothiocyanate, and CS₂. Only with N,N′‐dialkylcarbodiimines, the expected cycloaddition took place.     

Synthesis and Properties of 2‐Phenylbenzoxazole‐Based Luminophores for in situ Photopolymerized Liquid‐Crystal Films

We report the synthesis of a series of blue‐emitting 2‐phenylbenzoxazoles (PBOs) substituted at either the 5‐ or 6‐position of the benzoxazole ring and the para‐position of the phenyl substituent. The thermal and optical properties of the materials can be rationalized in terms of the position of the substituent at the benzoxazole moiety and the electron‐withdrawing or electron‐donating character of the substituents. From the results, we conclude that the combination of an electron‐donating substituent at the benzoxazole fragment and an electron‐withdrawing one at the phenyl fragment has a more marked effect on the electronic properties of the aromatic PBO core than other possibilities. This particular combination gives luminophores that are suitable for optical applications on the basis of their high emission efficiency and photostability. In view of that, oriented films were prepared by in situ polymerization of a mixture of a liquid crystalline direactive matrix containing 5% (w/w) of the luminophore. The films exhibit linearly polarized emission.     

A Novel Iron‐catalyzed One‐pot Synthesis of 3‐Amino‐1,2,4‐triazoles

A novel one‐pot synthesis of 3‐amino‐1,2,4‐triazole developed via iron (III) catalyzed route is reported. The new method is more efficient, simple, and convenient and presents a concise new strategy for the synthesis of 3‐amino‐1,2,4‐triazole derivatives. The iron (III) complex intermediate assisted in the intramolecular bond cyclization owing to its Lewis acidity or oxidizing properties. A series of aromatic nitriles bearing different electron‐donating and electron‐withdrawing groups substituted at para and/or ortho positions were also investigated. The position of the substituents affected the yield of the final compound, with the para‐substituted substrates giving relatively higher yields.     

Solid‐State Emissive Aroyl‐S,N‐Ketene Acetals with Tunable Aggregation‐Induced Emission Characteristics

N‐Benzyl aroyl‐S,N‐ketene acetals can be readily synthesized by condensation of aroyl chlorides and N‐benzyl 2‐methyl benzothiazolium salts in good to excellent yields, yielding a library of 35 chromophores with bright solid‐state emission and aggregation‐induced emission characteristics. Varying the substituent from electron‐donating to electron‐withdrawing enables the tuning of the solid‐state emission color from deep blue to red.     

Hydride transfer reactions via ion–neutral complex: fragmentation of protonated N‐benzylpiperidines and protonated N‐benzylpiperazines in mass spectrometry

An ion‐neutral complex (INC)‐mediated hydride transfer reaction was observed in the fragmentation of protonated N‐benzylpiperidines and protonated N‐benzylpiperazines in electrospray ionization mass spectrometry. Upon protonation at the nitrogen atom, these compounds initially dissociated to an INC consisting of [RC₆H₄CH₂]⁺ (R = substituent) and piperidine or piperazine. Although this INC was unstable, it did exist and was supported by both experiments and density functional theory (DFT) calculations. In the subsequent fragmentation, hydride transfer from the neutral partner to the cation species competed with the direct separation. The distribution of the two corresponding product ions was found to depend on the stabilization energy of this INC, and it was also approved by the study of substituent effects. For monosubstituted N‐benzylpiperidines, strong electron‐donating substituents favored the formation of [RC₆H₄CH₂]⁺, whereas strong electron‐withdrawing substituents favored the competing hydride transfer reaction leading to a loss of toluene. The logarithmic values of the abundance ratios of the two ions were well correlated with the nature of the substituents, or rather, the stabilization energy of this INC. Copyright © 2010 John Wiley & Sons, Ltd.     

Iminophosphine palladium(II) complexes: synthesis,characterization, and application in Heck cross‐coupling reaction of aryl bromides

Palladium(II) complexes containing phosphorus and nitrogen donor atoms (iminophosphine), dichlorido{N‐[2‐(diphenylphosphino)benzylidene]‐2‐trifluoromethylaniline}palladium(II) 1 , dichlorido{N‐[2‐(diphenylphosphino)benzylidene]‐3‐trifluoromethylaniline}palladium(II) 2 , dichlorido{N‐[2‐(diphenylphosphino)benzylidene]‐2‐methylaniline}palladium(II) 3 , dichlorido{N‐[2‐(diphenylphosphino)benzylidene]‐3‐methylaniline}palladium(II) 4 have been successfully synthesized and fully characterized by FT‐IR and NMR (¹H, ³¹P, ¹⁹F, and ¹³C) spectroscopy techniques. These complexes were first step tested in the reaction of bromobenzene and styrene to determine the optimal coupling reaction conditions and then successfully applied as catalysts for Heck cross‐coupling reactions of activated and deactivated aryl bromides with styrene derivatives and several acrylates. Copyright © 2014 John Wiley & Sons, Ltd.