Cyclotrimerization Of Isocyanates Research Articles

Catalysts for Isocyanate Cyclotrimerization

AbstractThe formation of isocyanurate via cyclotrimerization of isocyanates is widely reported to provide a variety of polyurethane materials with improved chemical and physical properties such as weatherability, mechanical properties, thermal stability and flame retardancy. The demand for development of effective and selective catalysts for cyclotrimerization of isocyanates has been increasing. This review comprehensively summarizes catalysts for the cyclotrimerization of isocyanates that have been reported in peer‐reviewed publications and provides a valuable guideline for choosing suitable catalysts to match specific requirements. The catalysts are categorized into two main classes: catalysts operating via a Lewis basic cyclotrimerization mechanism and metal‐containing catalysts. Catalyst structures, reaction conditions, reaction time, catalytic effectivity as well as types of isocyanates whose trimerization is catalyzed are described in detail. In addition, featuring the findings and viewpoints from mechanistic studies, this review aims to stimulate the design and development of new, more efficient catalysts, and to guide further study of the trimerization mechanism with different classes of catalysts.

Synergistic Catalysis by Brønsted Acid/Carbodicarbene Mimicking Frustrated Lewis Pair‐Like Reactivity

AbstractCarbodicarbene (CDC), unique carbenic entities bearing two lone pairs of electrons are well‐known for their strong Lewis basicity. We demonstrate herein, upon introducing a weak Brønsted acid benzyl alcohol (BnOH) as a co‐modulator, CDC is remolded into a Frustrated Lewis Pair (FLP)‐like reactivity. DFT calculation and experimental evidence show BnOH loosely interacting with the binding pocket of CDC via H‐bonding and π‐π stacking. Four distinct reactions in nature were deployed to demonstrate the viability of proof‐of‐concept as synergistic FLP/Modulator (CDC/BnOH), demonstrating enhanced catalytic reactivity in cyclotrimerization of isocyanate, polymerization process for L‐lactide (LA), methyl methacrylate (MMA) and dehydrosilylation of alcohols. Importantly, the catalytic reactivity of carbodicarbene is uniquely distinct from conventional NHC which relies on only single chemical feature of nucleophilicity. This finding also provides a new spin in diversifying FLP reactivity with co‐modulator or co‐catalyst.

Highly efficient cyclotrimerization of isocyanates using N-heterocyclic olefins under bulk conditions.

With a catalyst loading as low as 0.005%, high to excellent yields of isocyanurates could be achieved from N-heterocyclic olefin mediated organocatalytic cyclotrimerization of a wide range of isocyanates under bulk conditions. Experimental details coupled with structural characterization of the key intermediates led to comprehensive mechanistic studies of cyclotrimerization.

Yttrium dialkyl supported by a silaamidinate ligand: synthesis, structure and catalysis on cyclotrimerization of isocyanates.

A sterically demanding silaamidine (ArN = Si(L)NHAr) ligand was synthesized and employed for the preparation of a yttrium dialkyl complex, which catalytically enabled the cyclotrimerization of isocyanate with high activity and excellent functional group tolerance.

Potassium complexes containing bidentate pyrrole ligands: synthesis, structures, and catalytic activity for the cyclotrimerization of isocyanates.

Bidentate pyrrolyl ligands, 2-(t-butyliminomethyl)pyrrole and 2-(t-butylaminomethyl)pyrrole, reacted with KH to give complexes [C4H3N(2-CH[double bond, length as m-dash]NtBu)K(THF)]n (1) and [C4H3N(2-CH2NHtBu)K]n (2), respectively. Each has been characterized by C, H and N microanalysis, NMR spectroscopy, and single crystal X-ray structural analysis. The X-ray structure of complex 1 (n≥ 1) demonstrated that it exists as a 1D zig-zag coordination polymer in the solid state. Conversely, the structure of complex 2 (n≥ 1) showed that it is a 2D supramolecular network. They proved to be an effective class of catalysts for cyclotrimerization of isocyanate in excellent yield under mild conditions.

Lithium complex of 2-amino-functionalized benzoylpyrrole: Synthesis, structure, and catalytic activity for the cyclotrimerization of isocyanates

Lithium complex stabilised by 2-amino-functionalized benzoylpyrrole was synthesized, and its structural features were provided. The molecular structure shows a novel tetrameric cage structure, which includes a eight-membered (LiN)4 ring and a eight-membered (LiO)4 ring with the coordinating pyrrolyl ligands. It was proved to an efficient catalyst for the cyclotrimerisation of isocyanates in good to excellent yields under mild conditions.

Macrocyclic complexes containing a platinacycle or palladacycle composed of an isocyanate dimer unit: Reactivity towards isocyanides and cyclotrimerization of isocyanates

The reactions of [Pt(styrene)(PMe3)2] with 2equiv. of alkyl or aryl isocyanate affored five-membered platinacycles, cis-[Pt{–N(R)C(O)N(R)C(O)–}(PMe3)2] (R=CH2C6H5, p-ClC6H4, p-OMeC6H4). These complexes are the first examples of platinacycles containing an isocyanate dimer unit. When the five-membered bis(phosphine) platinacycles or palladacycles were treated with 2equiv. of elemental sulfur, 16-membered cyclic products as an assembly of four platinacycles or palladacycles, [M(PR3){–N(R)C(O)N(R)C(O)–}]4, were readily obtained. These cyclic tetramers were cleaved using tert-butyl isocyanide (CN–tbutyl, 4equiv.), affording the corresponding monomeric complexes, [M(PR3)(CN–tbutyl){–N(R)C(O)N(R)C(O)–}] (M=Pt, Pd). An unusual cyclotrimerization of organic isocyanates catalyzed by zerovalent Pt complexes or five-membered platinacycles was observed. In addition, the direct cyclotrimerization of alkyl or aryl isocyanates using dialkyl Pt(II) or Pd(II) complexes was investigated. The cross cyclotrimerization of an aryl isocyanate and its derivative using a zerovalent Pd complex was also investigated.

Surface functionalization of polypropylene‐bearing isocyanate groups in solid state and their cyclotrimerization with diisocyanates

ABSTRACTSolid‐state graft polymerization of 3‐isopropenyl‐α,α′‐dimethylbenzene isocyanate (TMI) onto the surface of polypropylene beads was carried out in a triethylborane/oxygen redox system. Chemical structures were characterized using attenuated total reflectance–Fourier transform infrared spectroscopy. Results showed that TMI was successfully grafted because of the appearance of an NCO absorption peak at 2255 cm−1. The emergence of oxygen and nitrogen elements detected by EDS and XPS also demonstrated the existence of isocyanate group on PP‐grafted. The grafting ratio of TMI to polypropylene was examined using 9‐(methylamino‐methyl)anthracene (MAMA) as an intermediate substance. The fluorescent property of MAMA before and after reaction was characterized to guarantee interaction between MAMA and isocyanate. Thermal properties were examined using differential scanning calorimetry–thermogravimetric analysis. Results indicated that melting temperature (Tm) of pure PP was 168oC, while the PP‐grafted decreased to 164oC. Meanwhile, decomposition temperature (Td) decreased with increased grafting ratio for about 8 to 15oC; however, when styrene was introduced, Tm increased probably because of the stabilizing effect on macromolecular radicals and the suppression effect on chain degradation. Besides, the cyclotrimerization of isocyanates on the grafted polymer chain was further conducted to prepare thermally stable isocyanurate composite materials, remedying the Td loss of PP‐g‐TMI by improving for 10oC appropriately. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42186.

Cyclic tetramers of a five-membered palladacycle based on a head-to-tail-linked isocyanate dimer and their reactivity in cyclotrimerization of isocyanates.

Reactions of [Pd(styrene)(PR3)2], generated from trans-[PdEt2(PR3)2] and styrene, with 2 equiv. of benzyl isocyanate in THF at room-temperature afforded unusual cyclic Pd-tetramers of five-membered rings consisting of organic isocyanate dimers and palladium, [Pd(PR3){-C(O)N(R)C(O)N(R)-}]4 (PR3 = PMe3, ; PR3 = PMe2Ph, ). Additionally, a cyclic trimer, (RNCO)3, (R = benzyl) was produced as a catalytic product. Treatment of the cyclic tetramer () with 4 equiv. of chelated phosphine, such as (1,2-bis(diethylphosphino)ethane) (DEPE) or (1,2-bis(dimethylphosphino)ethane) (DMPE), readily caused conversion to a metallacyclic cis-form, [Pd{N(R)C(O)N(R)C(O)}(P ∼ P)] (P ∼ P = DEPE, ; P ∼ P = DMPE, ) in quantitative yields. In contrast, reactions of Pd(0)-PR3 with 2 equiv. of Ar-NCO (Ar = Ph, p-tolyl, p-ClC6H4) afforded metallacyclic complexes having a dimeric isocyanato moiety, cis-[Pd{C(O)N(Ar)-C(O)N(Ar)}(PR3)2] (PR3 = PMe3 Ar = C6H5, ; p-MeC6H4, ; p-Cl-C6H4, ; PR3 = PMe2Ph, Ar = p-Cl-C6H4, ). Treatment of the palladacyclic complex () with an equimolar amount of chelated phosphine such as DEPE readily caused conversion to a palladacyclic cis-form, [Pd{N(Ar)C(O)N(Ar)C(O)}(DEPE)], in quantitative yield. The catalytic cyclotrimerization of benzyl isocyanate to [Pd(styrene)(PMe3)2] was achieved by varying the molar ratio of R-NCO (R = benzyl). In addition, catalytic cyclotrimerization was performed from the five-membered palladacyclic complexes or the Pd(0)-PR3 complex with excess Ar-NCO.

Cyclo-oligomerization of isocyanates with Na(PH2) or Na(OCP) as “P−” anion sources

We show that the 2-phosphaethynolate anion, OCP-, is a simple and efficient catalyst for the cyclotrimerization of isocyanates. This process proceeds step-wise and involves five-membered heterocycles, namely 1,4,2-diazaphospholidine-3,5-dionide anions and spiro-phosphoranides as detectable intermediates, both of which were also found to be involved in the catalytic conversion. These species can be considered as adducts of a phosphide anion with two and four isocyanate molecules, respectively, demonstrating that the OCP- anion acts as a formal "P-" source. The interconversion between these anionic species was found to be reversible, allowing them to serve as reservoirs for unique phosphorus-based living-catalysts for isocyanate trimerization.

Tridentate pyrrolylzinc compounds: Synthesis, structures, reactivities and catalytic cyclotrimerization reaction of isocyanate

The reactions of ZnEt2 with NNN-tridentate pincer type pyrrolyl ligands, 2,5-bis((dimethylamino)methylene)-1H-pyrrole (HL1), 2,5-bis((pyrrolidin-1-yl)methylene)-1H-pyrrole (HL2) and 2,5-bis((piperidino)methylene)-1H-pyrrole (HL3), afford zinc ethyl compounds [C4H2N(2,5-CH2NMe2)2]ZnC2H5 (1), [C4H2N(2,5-CH2N(CH2)4)2]ZnC2H5 (2), and [C4H2N(2,5-CH2N(CH2)5)2]ZnC2H5 (3) in high yield. Subjecting 1, 2, 3 with one equivalent of tert-butylphenol in THF generated {[C4H2N(2,5-CH2NMe2)2]Zn(O–C6H4-4-tBu)}2 (4), {[C4H2N(2,5-CH2N(CH2)4)2]Zn(O–C6H4-4-tBu)}2 (5), {[C4H2N(2,5-CH2N(CH2)5)2]Zn(O–C6H4-4-tBu)}2 (6), respectively. Each has been characterized by satisfactory C, H and N microanalysis, NMR spectroscopy at ambient temperature, and single crystal X-ray structural analysis. The compounds Et6Zn6O2[C4H2N(2,5-CH2N(CH2)4)2]2 (7) and Et6Zn6O2[C4H2N(2,5-CH2N(CH2)5)2]2 (8) obtained accidentally from zinc ethyl compounds with traces of water. Each of compound 1–6 has been used as initiator for cyclotrimerization of isocyanate, and the organozinc compound 3 shows moderate reactivity toward the cyclotrimerization of isocyanate in Et2O solvent under mild conditions.

ChemInform Abstract: An Expeditious Method for the Selective Cyclotrimerization of Isocyanates Initiated by TDAE.

Abstract We developed a rapid and green synthesis of various isocyanurates by cyclotrimerization of isocyanates using TDAE (tetrakis(dimethylamino)ethylene). TDAE displays excellent performance in catalytic quantities, affording the corresponding trimer of isocyanates very rapidly, under air and at room temperature in good to excellent yields.

Lithium, magnesium, zinc complexes supported by tridentate pincer type pyrrolyl ligands: Synthesis, crystal structures and catalytic activities for the cyclotrimerization of isocyanates

Lithium, magnesium, zinc complexes incorporating substituted symmetrical tridentate pyrrolyl ligands were synthesized conveniently and their application for the cyclotrimerization of isocyanate to corresponding isocyanurate has been investigated. The reaction of 1 equivalent of 2,5-bis(pyrrolidinomethyl)pyrrole {C4H2NH(2,5-CH2NC4H8)2} with n-BuLi generated [Li{C4H2N(2,5-CH2NC4H8)2}]2 (1) in diethyl ether. Treatment of 2 equivalents of C4H2NH(2,5-CH2NC4H8)2 with MgBu2 or ZnEt2 in THF afforded [Mg(THF){C4H2N(2,5-CH2NC4H8)2}2] (2) or [Zn{C4H2N(2,5-CH2NC4H8)2}2] (3) in high yield, respectively. Compounds 1, 2 and 3 were characterized by 1H and 13C NMR spectra and their molecular structures were determined by single crystal X-ray diffraction. Furthermore, compounds 1, 2 and 3 were proved to be moderate to good initiators for cyclotrimerization of isocyanate to the corresponding isocyanurate in diethyl ether.

Synthesis and property of polyoxazolidone having fluorene moiety by polyaddition of diisocyanate and diepoxide

ABSTRACTPolyoxazolidones having fluorenyl group were synthesized by polyaddition of 9,9‐diglycidyl fluorene with various diisocyanates. The polymer from 9,9‐diglycidyl fluorene and methylenediphenyl 4,4′‐diisocyanate was afforded in high yield although polydispersity of the polymer was found relatively broad. The IR spectrum of the obtained polymer showed two absorption in carbonyl region. One of them was assigned to the expected oxazolidone, while the other at 1710 cm−1 appeared due to a carbonyl group of the isocyanurate moieties produced by cyclotrimerization of isocyanate. It is assumed that the cyclotrimerization would cause the broad polydispersity caused by the branched structure formed by isocyanurate. The polymers obtained with three kinds of diisocyanates (methylenediphenyl 4,4'‐diisocyanate, 1,6‐hexamethylene diisocyanate, 1,4‐phenylene diisocyanate) showed high thermal stability, as their Td10 was depended on the structure of diisocyanate. All polymers had high transparency in a visible region. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 1755–1760

An expeditious method for the selective cyclotrimerization of isocyanates initiated by TDAE

We developed a rapid and green synthesis of various isocyanurates by cyclotrimerization of isocyanates using TDAE (tetrakis(dimethylamino)ethylene). TDAE displays excellent performance in catalytic quantities, affording the corresponding trimer of isocyanates very rapidly, under air and at room temperature in good to excellent yields.

ChemInform Abstract: Generation of Low‐Valent Alkoxy Niobium from Nb(OEt)5 and Grignard Reagents and Their Use as Catalysts in the Cyclotrimerization of Isocyanates.

AbstractFurthermore, the title reagents can be used for the allylation reaction of 4‐octyne.

Generation of low-valent alkoxy niobium from Nb(OEt)5 and Grignard reagents and their use as catalysts in the cyclotrimerization of isocyanates

A new highly active low-valent alkoxyniobium species has been developed following the reaction of Nb(OEt)5 with Grignard reagent such as i-PrMgCl or EtMgCl, with the material behaving as an efficient catalyst for cyclotrimerization reaction of isocyanates. In this reagent system, the existence of a low-valent niobium species was confirmed by the formation of the niobium–alkyne complex prepared from the Nb(OEt)5/Grignard reagent system and an alkyne. Furthermore, the hydrolysis and diallylation reactions of the niobium–alkyne complex provided further confirmation of its existence, with corresponding (Z)-alkene and diallylated products being isolated in good yields.

Bidentate pyrrolyl lithium complexes: Synthesis, crystal structure and catalytic activity for the cyclotrimerization of isocyanates

Three bidentate pyrrolyl lithium complexes were synthesized and their application for the cyclotrimerization of isocyanate to corresponding isocyanurate has been investigated and show high catalytic activities. All lithium complexes were characterized by NMR spectroscopy, elemental analysis and X-ray diffraction analysis.

Development of high‐performance networked polymers based on cyclotrimerization of isocyanates: Control of properties by addition of monoisocyanates

Highly heat-resistant networked polymers consisting of isocyanurate moieties have been developed based on cyclotrimerization of isocyanates. To the polymerization system using methylene diphenyl 4,4′-diisocyanate (MDI) as the main component, a series of aromatic monoisocyanates (p-tolylisocyanate, p-trifluoromethylphenylisocyanate, p-tert-butylphenylisocyanate, and p-butylphenylisocyanate) were added with varying feed ratio [MDI]0:[monoisocyanate]0 to achieve successful control of flexibility of thecorresponding networked polymers without serious deterioration of the thermal stability.

Development of high‐performance networked polymers consisting of isocyanurate structures based on selective cyclotrimerization of isocyanates

AbstractSelective and quantitative cyclotrimerization of p‐tolylisocyanate proceeded by using sodium p‐toluenesulfinate as a catalyst and 1,3‐dimethylimidazolidinone as a solvent. Exploitation of this system to the cyclotrimerization of methylene diphenyl 4,4′‐diisocyanate (MDI) permitted formation of the corresponding networked polymer, which was selectively consisted of isocyanurate moiety and thus exhibited excellent thermal stability. Utilization of phenyl isocyanate (PhNCO) as a comonomer with varying feed ratio [MDI]0/[PhNCO]0 allowed successful control of flexibility of the networked polymers, while retaining its high thermal stability. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011