Cyclotrimerization catalysis

Curing agents and cyclotrimerization catalysis should be properly chosen for the epoxide-BPA/DC systems. As an example, a mixture consisting of Zn octoate, 2-ethylimidazole and triethylenediamine (DABCO) can be mentioned [79]. 

As noted above, the Pd(lll) model catalyst is covered by vinylidene species during cyclotrimerization catalysis in the presence of acetylene alone. An infrared interrogation of a Pd(lll) model catalyst during acetylene hydrogenation reveals the presence of some ethylidyne in addition to vinyhdene. This suggests that... 

Rhodium catalysis in an aqueous-organic biphasic system was highly effective for intramolecular [2+2+2] cyclotrimerization. It has been shown that the use of a biphasic system could control the concentration of an organic hydrophobic substrate in the aqueous phase, thus increasing the reaction selectivity. The intramolecular cyclization for... 

Controlling Factors in Homo neous Tiansistion-Metal Catalysis Table 2.3-1. Cyclotrimerization of butadiene by means of Ni-, Ct- or Ti-catalysts ... 

The kinetics of catalysis of cyclotrimerization was studied on the model system phenyl isocyanate/ace-tonitrile (solvent). Acetonitrile (AN, 99.64%, from Vinstron Corp.) was purified by refluxing with phosphorus pentoxide (5 g/1), then with calcium hydride (2 g/1) followed by distillation under nitrogen. Phenyl isocyanate was obtained from the Upjohn Company with a purity of 99.5%, and was purified by distillation. Tolylene diisocyanates (2,4 and 80/20 2,4/2,6 isomers) were obtained from the Mobay Chemical Co., and were purified by distillation. Cyclic sulfonium zwitterions (SZ) were obtained from the Dow Chemical Co. 

Cyclotrimerization of nitriles with heteroatomic substituents are also facile and important reactions. Melamine is formed on heating cyanamide above its melting point (equation 74) (59HC(l3)l, p. 309). Substituted cyanamides react to give either the expected 1,3,5-triazine or the isomer (142). The 1,3,5-triazine is the preferred product at high temperatures, under acid catalysis, and for cyanamides with bulky substituents, whilst the isomer is favoured by basic conditions and low temperatures (Scheme 80). The mechanism of formation of (142) has been proposed (Scheme 81) (78RCR975). 

Table XVI is a summary of typical results observed for cyclotrimerization of propionaldehyde to give 2,4,6-triethyl-1,3,5-trioxane (176). In catalysis by H3PMo12O40, the reaction mixture separates into two phases during the course of the batch reaction. The products are present in the upper layer and the catalyst in the lower layer, so that the catalyst solution can be used repeatedly without a catalyst isolation step. Selectivities exceeding 97% and turnovers exceeding 300 moles of product per mole of catalyst have been obtained.

Similar cyclotrimerizations were accomplished with irradiation in 3 hours, but in lower yield <95CC(2)179>. Rhodium catalysis yielded pyridines without irradiation in slightly longer times, but the unsymmetrical alkynes used led to isomeric products, such as (2) and (3), (Scheme 2) <95JO(488)47>. 

Reinhard, S., Soba, P., Rominger, F., and Bluemel, J. (2003) New silica-immobilized nickel catalysts for cyclotrimerizations of acetylenes. Advanced Synthesis and Catalysis, 345, 589-602. 

Related co-cyclotrimerizations of two alkyne molecules with limited isocyanates have also been achieved using cobalt and nickel catalysts. With respect to intramolecular versions, two examples of the cobalt(I)-catalyzed cycloaddition of a,m-diynes with isocyanates have been reported to afford bicyclic pyri-dones only in low yields, although 2,3-dihydro-5(lff)-indolizinones were successfully obtained from isocyanatoalkynes and several silylalkynes with the same cobalt catalysis [19]. On the other hand, the ruthenium catalysis using Cp RuCl(cod) as a precatalyst effectively catalyzed the cycloaddition of 1,6-diynes 21 with 4 equiv. of isocyanates in refluxing 1,2-dichloroethane to afford bicyclic pyridones 25 in 58-93% yield (Eq. 12) [20]. In this case,both aryl and aliphatic isocyanates can be widely employed. 

A highly electron-deficient carbon-oxygen double bond can also participate in the co-cyclotrimerization with alkynes under the ruthenium catalysis. The cycloaddition of commercially available diethyl ketomalonate with the diynes 21 proceeded at 90 °C in the presence of 5-10 mol % Cp RuCl(cod). The expected fused 2ff-pyrans 27, however, underwent thermal electrocyclic ringopening to produce cyclopentene derivatives 28 (Eq. 14) [23]. 

The NiY zeolite was also shown to be active for the cyclotrimerization of propyne with 1,2,4-trimethylbenzene being the main product. The activities of the above-mentioned transition metal ions for acetylene trimerization are not so surprising since simple salts and complexes of these metals have been known for some time to catalyze this reaction (161, 162). However, the tetramer, cyclooctatetraene, is the principal product in homogeneous catalysis, particularly when simple salts such as nickel formate and acetate are used as catalysts (161). The predominance of the trimer product, benzene, for the zeolite Y catalysts might be indicative of a stereoselective effect on product distribution, possibly due to the spatial restrictions imposed on the reaction transition-state complex inside the zeolite cages. 

The complexes 222-226 exhibited a high catalytic activity on the cyclotrimeriza-tion of aromatic isocyanates to produce triaryl isocyanurates (Scheme 83). They are the first reported Cp-free rare-earth metal complexes showing high activity and selectivity on the cyclotrimerization of aryl isocyanates. For comparison, the starting trisamide complex [Yb N(SiMe3)2 3( T-Cl)Li(THF)3] was studied for the catalysis and showed a catalytic activity comparable with those of the new complexes 222-226. All the complexes showed no catalytic activity on the cyclotrimerization of 4-nitrophenylisocyanate and exhibited a relatively low catalytic activity on the cyclotrimerization of aliphatic isocyanates [167]. 

Alkynes can also serve as substrates in [3 -I- 2] cycloadditions to MCP, as exemplified by the reaction of but-2-yne under nickel/phosphite catalysis to provide l,2-dimethyl-4-methylene-cyclopent-l-ene (2). 2 However, alkyne oligomerization, specifically cyclotrimerization, cannot be avoided with alkyl-substituted alkynes. When alkynes with electron-withdrawing substituents are employed, cyclotrimerization becomes the exclusive reaction. 

Another important factor in catalysis is the selectivity of a catalytic reaction. So far, however, information on the atom-by-atom evolution of this astonishing catalytic selectivity is still lacking. In this example, we illustrate such a size-dependent selectivity with the polymerization of acetylene on palladium nanocatalysts [46]. This reaction over supported Pd particles reveals a direct correspondence between reactivities observed on model systems and the behavior of industrial catalysts under working conditions [66]. In ultra-high vacuum (UHV) [67] as well as under high pressure, large palladium particles of typically thousands of atoms show an increased selectivity for the formation of benzene with increasing particle size [66]. In contrast, small palladium particles of typically hundreds of atoms are less selective for the cyclotrimerization, and catalyze butadiene and butene as additional products [66]. 

Due to the hyperconjugative effect of the alkyl groups, which change the electrophilic character of the cyanide carbon atom, the addition of another cyanide molecule in the presence of an acid is hindered."9,142 Furthermore, the formation of stable salt adducts as intermediates during acid catalysis prevents the cyclotrimerization.143 144... 

The effects of acid and base catalysis on the cyclotrimerization of aromatic and heterocyclic nitriles are fundamentally different at high vs normal pressure in the latter case the reaction occurs in the presence of strong acids and bases. Thus, heating benzonitrile in a sealed tube at 360 °C for 11 hours gives only traces of 2,4,6-triphenyl-l,3,5-triazine. In contrast, at 30000-49000 atm and 300-500°C, benzonitrile is converted almost quantitatively into the triazine in 6-18 minutes. 

Best yields for these mixed cyclotrimerizations are obtained with imidates of lower aliphatic nitriles higher aliphatic derivatives give progressively lower yields and with aromatic and electron-withdrawing substituents the reaction is sluggish. Methyl isonicotinimidate (33) reacts with 2 equivalents of formimidamide hydrochloride under base catalysis to give 2-(4-pyridyl)-1,3,5-triazine (34).317... 

The first application of phosphinines in catalysis was reported by Zenneck et al. in 1996 in the case of t 6-Fe complexes [46, 98], It was shown that complex 73 could catalyse the cyclotrimerization of dimethyl acetylenedicarboxylate as well as the formation of pyridines from alkynes and nitriles. Importantly, the catalytic activity of this complex was found to be superior to that of the corresponding benzene [Fe(n6-C6FI6)(COD)] complex. Reactions of methylpropargyl ether with butyronitrile in the presence of complex 73 as catalyst in a ratio 620 2,720 1 afforded a mixture of functional benzenes and pyridines. Turn over numbers (TON) for the conversion into pyridines reached 160 and those for the formation of functional benzenes reached 326, thus corresponding to a chemoselectivity of 0.49 (Scheme 26). 

Unfortunately, all of the above-described synthetic approaches towards HBC (1) suffer from serious drawbacks, such as harsh reaction conditions, a complicated experimental work-up, and low yields. Furthermore, under aluminum(III) chloride catalysis, dealkylation, migration of the alkyl substituents - or even chlorination of the aromatic system- occurs, which clearly limits the accessibility of functionalized HBC derivatives for further investigations and applications. In order to overcome these problems, the weaker Lewis acid iron(III) chloride in nitromethane was used instead of AICI3, and the reaction conditions were carefully optimized [55, 56]. In this way, access was obtained to a multitude of HBC derivatives 8 and 9 with diverse substitution patterns and symmetries bearing solubilizing alkyl chains and halogen substituents, starting from functionalized hexaphenylbenzenes. The sixfold symmetric hexaphenylbenzenes 10 were synthesized by the Co2(CO)g-catalyzed cyclotrimerization of substituted diphenyl-acetylenes 11 (Scheme 13.4a) [57], whereas the intramolecular Diels-Alder reaction... 

The formation of isocyanurates in the presence of polyols occurs via intermediate allophanate formation, ie, the urethane group acts as a cocatalyst in the trimerization reaction. By combining cyclotrimerization with polyurethane formation, processibility is improved, and the friability of the derived foams is reduced. The trimerization reaction proceeds best at 90-100°C. These temperatures can be achieved using a heated conveyor or a RIM machine. The key to the formation of PUIR foams is catalysis. Strong bases, such as potassium acetate, potassium 2-ethylhexoate, and tertiary amine combinations, are the most useful trimerization catalyst. A review on the trimerization of isocyanates is available (104). 

The catalysis of the cyclotrimerization of acetylenes by transition metal complexes has been extensively studied (reviews Bird, 1967 Hoogzand and HUbel, 1968 Maitlis, 1971, 1973 Heck, 1974). Various mechanistic studies have been done on this type of reaction (Blomquist and Maitlis, 1962 Meriwether et al., 1962 Schrauzer, 1964 Collman et al., 1968 Whitesides and Ehmann, 1969 Yamazaki and Hagihara, 1970 Miiller and Beissner, 1973 Gardner et a/., 1973). The iridium-promoted trimerization of dimethyl acetylenedicarboxylate has been demonstrated to occur through an iridocycle as shown in Scheme 2 (Collman and Kang, 1967 Baddley and Tupper, 1974). 

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