Palladium(0)-Catalyzed Trimerization of Arylisocyanates into 1,3,5-Triarylisocyanurates in the Presence of Diimines: A Nonintuitive Mechanism

Preparation and Characterization of Novel Poly(thiourethane)-Poly(isocyanurate) Covalent Adaptable Networks: Effect of the Catalysts

Poly(thiourethane)-based covalent adaptable networks are synthesized by reacting a trimer of hexamethylene diisocyanate (Desmodur N3300) containing isocyanurate groups in its structure with 1,6-hexanedithiol. The catalysts evaluated for this process include dibutyltin dilaurate (DBTDL), lanthanum triflate (La(OTf)3), and a thermal precursor of 1,8-diazabicyclo[5.4.0]undec-7-ene (BGDBU). The use of DBTDL results in the initiation of curing upon mixing, while the other two catalysts exhibit a latency period in the reactive mixture, with curing starting at about 90 °C. Notably, the use of the lanthanum salt produces an additional minor exothermic reaction at 80 °C. This phenomenon corresponds to the trimerization of isocyanates rending isocyanurates, leaving a portion of unreacted thiols. Materials prepared with BGDBU or La(OTf)3 present shorter relaxation times than those prepared with DBTDL. Nevertheless, the materials containing the lanthanum salt do not reach complete relaxation, likely due to the reinforcement of the permanent network through increased isocyanurate content. The formation of isocyanurates produces a stoichiometric imbalance, leaving unreacted thiols. This transforms the exchange process into a dual mechanism involving a dissociative process of thiourethanes to isocyanate and thiol, along with an interchange through thiol attacking the thiourethane group. The materials exhibit good recyclability and self-healing characteristics.

Strongly Polarized Iridiumδ--Aluminumδ+ Pairs: Unconventional Reactivity Patterns Including CO2 Cooperative Reductive Cleavage

The iridium tetrahydride complex Cp*IrH₄ reacts with a range of isobutylaluminum derivatives of general formula Al(iBu)_x(OAr)_3-x (x = 1, 2) to give the unusual iridium aluminum species [Cp*IrH₃Al(ⁱBu)(OAr)] (1) via a reductive elimination route. The Lewis acidity of the Al atom in complex 1 is confirmed by the coordination of pyridine, leading to the adduct [Cp*IrH₃Al(ⁱBu)(OAr)(Py)] (2). Spectroscopic, crystallographic, and computational data support the description of these heterobimetallic complexes 1 and 2 as featuring strongly polarized Al(III)^δ+-Ir(III)^δ- interactions. Reactivity studies demonstrate that the binding of a Lewis base to Al does not quench the reactivity of the Ir-Al motif and that both species 1 and 2 promote the cooperative reductive cleavage of a range of heteroallenes. Specifically, complex 2 promotes the decarbonylation of CO₂ and AdNCO, leading to CO (trapped as Cp*IrH₂(CO)) and the alkylaluminum oxo ([(iBu)(OAr)Al(Py)]₂(μ-O) (3)) and ureate ({Al(OAr)(ⁱBu)[κ²-(N,O)AdNC(O)NHAd]} (4)) species, respectively. The bridged amidinate species Cp*IrH₂(μ-CyNC(H)NCy)Al(ⁱBu)(OAr) (5) is formed in the reaction of 2 with dicyclohexylcarbodiimine. Mechanistic investigations via DFT support cooperative heterobimetallic bond activation processes.

Mechanistic Investigation into the Acetate-Initiated Catalytic Trimerization of Aliphatic Isocyanates: A Bicyclic Ride

The acetate-initiated aliphatic isocyanate trimerization to isocyanurate was investigated by state-of-the-art analytical and computational methods. Although the common cyclotrimerization mechanism assumes the consecutive addition of three equivalents of isocyanate to acetate prior to product formation, we found that the underlying mechanism is more complex. In this work, we demonstrate that the product, in fact, is formed via the connection of two unexpected catalytic cycles, with acetate being only the precatalyst. The initial discovery of a precatalyst activation by quantum chemical computations and the resulting first catalysis cycle were corroborated by mass spectrometric and NMR experiments, thereby additionally revealing a catalyst migration to the second catalytic cycle. These results were further confirmed by computations, completing the full mechanistic understanding of this catalytic system. Identification of a side product with undesired properties for final coating applications allows for process optimization in the chemical industry.

Polyurethanes and Polyallophanates via Sequence-Selective Copolymerization of Epoxides and Isocyanates

Aryl isocyanates are introduced as comonomers for ring-opening copolymerization (ROCOP) with epoxides. Informed by studies of reaction kinetics, we show that divergent sequence selectivity for AB- and ABB-type copolymers can be achieved with a single dimagnesium catalyst. The resulting materials respectively constitute a new class of polyurethanes (PUs) and a new class of materials featuring an unprecedented backbone structure, the polyallophanates (PAs). The successful use of isocyanate comonomers in this way marks a new direction for the field of ROCOP while providing distinct opportunities for expansion of PU structural diversity. Specifically, the methodology reported herein delivers PUs featuring fully substituted (tertiary) carbamyl nitrogen atoms, a structural motif that is almost inaccessible via extant polymerization strategies. Thus, in one step from commercially available comonomers, our methodology expands the scope of ROCOP and gives access to diverse materials featuring both privileged (PU) and unexplored (PA) microstructures.

Thermal stabilities and conformational behaviors of isocyanurates and cyclotrimerization energies of isocyanates: a computational study

Isocyanurates are cyclic trimers of isocyanate molecules. They are generally known as highly thermostable compounds. However, it is interesting how the thermal stabilities of the isocyanurate molecules will be altered depending on the substituents of their three nitrogen atoms. We performed computational investigations on the thermochemical behaviors of isocyanurate molecules with various alkyl and phenyl substituents. The cyclotrimerization processes of isocyanates are highly exothermic. Our best estimate of the enthalpy change for the cyclotrimerization of methyl isocyanate into trimethyl isocyanurate was -66.4 kcal mol-1. Additional negative cyclotrimerization enthalpy changes were observed for n-alkyl-substituted isocyanates. This trend was enhanced with an extension of n-alkyl chains. Conversely, low negative cyclotrimerization enthalpy changes were shown for secondary and tertiary alkyl-substituted isocyanates. The n-alkyl-substituted isocyanurates were shown to be stabilized due to attractive dispersion interactions between the substituents. Meanwhile, the branched alkyl-substituted isocyanurates were destabilized due to the deformation of their isocyanurate rings. For various alkyl-substituted isocyanates, the sum of the deformation energy of the isocyanurate ring and the intramolecular inter-substituent nonbonding interaction energies was found to be linearly correlated with their cyclotrimerization energies. The cyclotrimerization energy for phenyl isocyanate was shown to have significantly deviated from the linear relationship observed for the alkyl-substituted isocyanurates. This is probably attributable to a remarkable change in the orbital resonance interactions during the cyclotrimerization of phenyl isocyanate to triphenyl isocyanurate.

Nickel-catalyzed reductive amidation of aryl-triazine ethers

The reaction of activated phenolic compounds, 2,4,6-triaryloxy-1,3,5-triazine (aryl-triazine ethers), with various isocyanates or carbodiimides in the presence of a nickel pre-catalyst resulted in the synthesis of aryl amides in good to excellent yields.

Intertwined chemistry of hydroxyl hydrogen-bond donors, epoxides and isocyanates in the organocatalytic synthesis of oxazolidinones versus isocyanurates: rational catalytic investigation and mechanistic understanding

The efficiency and chemoselectivity of the cycloaddition of isocyanates to epoxides to afford oxazolidinones were investigated using hydroxyl hydrogen-bond donors as organocatalysts.

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.

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]N^tBu)K(THF)]_n (1) and [C₄H₃N(2-CH₂NH^tBu)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.

Gallium "Shears" for C=N and C=O Bonds of Isocyanates

Digallane [L¹ Ga-GaL¹ ] (1, L¹ =dpp-bian=1,2-[(2,6-iPr₂ C₆ H₃ )NC]₂ C₁₂ H₆ ) reacts with RN=C=O (R=Ph or Tos) by [2+4] cycloaddition of the isocyanate C=N bonds across both of its C=C-N-Ga fragments to afford [L¹ (O=C-NR)Ga-Ga(RN-C=O)L¹ ] (R=Ph, 3; R=Tos, 4). The reactions with both isocyanates result in new C-C and N-Ga single bonds. In the case of allyl isocyanate, the [2+4] cycloaddition across one C=C-N-Ga fragment of 1 is accompanied by insertion of a second allyl isocyanate molecule into the Ga-N bond of the same fragment to afford compound [L¹ Ga-Ga(AllN- C=O)₂ L¹ ] (5) (All=allyl). In the presence of Na metal, the related digallane [L² Ga-GaL² ] (2; L² =dpp-dad=[(2,6-iPr₂ C₆ H₃ )NC(CH₃ )]₂ ) is converted into the gallium(I) carbene analogue [L² Ga:]^- (2 A), which undergoes a variety of reactions with isocyanate substrates. These include the cycloaddition of ethyl isocyanate to 2 A affording [Na₂ (THF)₅ ]{L² Ga[EtN-C(O)]₂ GaL² } (6), cleavage of the N=C bond with release of 1 equiv. of CO to give [Na(THF)₂ ]₂ [L² Ga(p-MeC₆ H₄ )(N-C(O))₂ -N(p-MeC₆ H₄ )]₂ (7), cleavage of the C=O bond to yield the di-O-bridged digallium compound [Na(THF)₃ ]₂ [L² Ga-(μ-O)₂ -GaL² ] (8), and generation of the further addition product [Na₂ (THF)₅ ][L² Ga(CyNCO₂ )]₂ (9). Complexes 3-9 have been characterized by NMR (¹ H, ¹³ C), IR spectroscopy, elemental analysis, and X-ray diffraction analysis. Their electronic structures have been examined by DFT calculations.

Aluminium-catalysed isocyanate trimerization, enhanced by exploiting a dynamic coordination sphere

Main-group metals are inherently labile, hindering their use in catalysis. We exploit this lability in the synthesis of isocyanurates. For the first time we report a highly active catalyst that trimerizes alkyl, allyl and aryl isocyanates, and di-isocyanates, with low catalyst loadings under mild conditions, using a hemi-labile aluminium-pyridyl-bis(iminophenolate) complex.

Reactivities of zero-valent group 10 complexes toward organic isocyanates: synthesis of metallacycles containing dimeric isocyanate units, isocyanate cyclotrimerization, and computational chemistry

Five-membered metallacycles involving dimeric isocyanate units converted into cyclic tetramers and isocyanurates, depending on the central metal or organic isocyanates, and their synthetic and theoretical studies were carried out.

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

A four-coordinate yttrium dialkyl complex with a sterically demanding silaamidinate ligand exhibited high activity and excellent functional group tolerance for the catalysis of isocyanate cyclotrimerization.

Highly bulky and stable geometry-constrained iminopyridines: Synthesis, structure and application in Pd-catalyzed Suzuki coupling of aryl chlorides

A series of bulky geometry-constrained iminopyridylpalladium chlorides were developed. The steric environment adjacent to the nitrogen atom in the pyridine rings and diimine parts enhanced the thermal stability of the palladium species. Bulkier groups at the imino group stabilized the palladium species and the corresponding palladium chlorides showed high activities in the coupling reaction of aryl chlorides.

Cyclotrimerisation of isocyanates catalysed by low-coordinate Mn(ii) and Fe(ii) m-terphenyl complexes

Isocyanurates can be synthesised through the cyclotrimerisation of primary isocyanates catalysed by low-coordinate manganese(ii) and iron(ii) m-terphenyl complexes under mild conditions.

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

Cyclic tetramers of a five-membered palladacycle involving an isocyanate dimer were prepared, and catalytic cyclotrimerization from a palladium zero-compound or a single palladacycle with organic isocyanates was performed.

The formation of high-purity isocyanurate through proazaphosphatrane-catalysed isocyanate cyclo-trimerisation: computational insights

Polyurethane foams are widely used materials and control of their physical properties is a significant challenge. Management of cyclo-trimerisation during the polymerisation process is vital when tailoring the mechanical properties of the foam. Proazaphosphatranes are known to efficiently catalyse the cyclo-trimerisation of organic isocyanates, giving high purity isocyanurate with little uretdione by-product. The mechanism of this catalysis was previously unknown, although some zwitterionic intermediates have been identified spectroscopically. We have investigated a nucleophilic-catalysis reaction pathway involving sequential addition of methyl isocyanate to activated zwitterionic intermediates using density functional theory calculations. Evidence for significant transannulation by the proazaphosphatrane nitrogen was found for all intermediates, offering stabilisation of the phosphonium cation. Steric crowding at the proazaphosphatrane nucleophilic phosphorus gives rise to a preference for direct isocyanurate formation rather than via the uretdione, in sharp contrast to the uncatalysed system which has been found to preferentially proceed via the kinetic uretdione product. The investigations suggest the mechanism of proazaphosphatrane catalysed cyclo-oligomerisation does not proceed via the uretdione product, and hence why little of this impurity is observed experimentally.

Reactive N-protonated isocyanate species stabilized by bis(μ-hydroxo)divanadium(IV)-substituted polyoxometalate

O- or N-protonated? The bis(μ-hydroxo)divanadium(IV)-substituted γ-Keggin-type polyoxometalate (see picture, left) (TBA)(4)[γ-SiV(IV)(2)W(10)O(36)(μ-OH)(4)] (TBA = tetra(n-butyl)ammonium) was synthesized and characterized by X-ray crystallography. Its reaction with phenyl isocyanate gave (TBA)(4)[γ-SiV(IV)(2)W(10)O(38)(μ-OH)(2)(PhNHCO)(2)], which contains two N-protonated phenyl isocyanate species and catalyzes the cyclotrimerization of phenyl isocyanate.

Palladium-catalyzed cross-coupling of aryl chlorides and triflates with sodium cyanate: a practical synthesis of unsymmetrical ureas

An efficient method for palladium-catalyzed cross-coupling of aryl chlorides and triflates with sodium cyanate is reported. The protocol allows for the synthesis of unsymmetrical N,N'-di- and N,N,N'-trisubstituted ureas in one pot and is tolerant of a wide range of functional groups. Insight into the mechanism of aryl isocyanate formation was gleaned through studies of the transmetalation and reductive elimination steps of the reaction, including the first demonstration of reductive elimination from an arylpalladium isocyanate complex to produce an aryl isocyanate.

Isocyanate diinsertion into the N-H bond of the 2-pyridylamino ligand of organolanthanides

[Cp2LnNHPy]2 (Py = 2-pyridyl) (1a-e) react with phenyl isocyanate to form the N-H diinsertion products Cp2Ln[eta2:eta1-PyNCON(Ph)CONHPh](THF) (Ln = Yb (3a), Er (3b), Y (3c), Dy (3d), Gd (3e)). It has been proven that nPr2NH can abstract one PhNCO unit from 3c to form Cp2Y[eta3-OC(NHPh)NPy] (2c) and nPr2NHCONHPh (4), representing a rare example of selective release of a functional group of ligands in organolanthanide chemistry. Hydrolysis of 2c gives the organic nitrogen-containing product PyNHCONHPh (5). Moreover, 3c can also be obtained by the reaction of 2c with PhNCO. These results demonstrate that the diinsertion of PhN=C=O into the N-H bond of coordinated amino ligands might proceed in a stepwise manner. All the compounds were characterized by elemental analysis and spectroscopic properties. The structures of compounds 3a-e and 4 are also determined through X-ray single-crystal diffraction analysis.