Trimerisation

The underlying reaction for polyisocyanurate formation is the trimerisation of an isocyanate under the influence of specific catalysts (Figure 27.9). 

Amongst the catalysts used or the polymerisation-trimerisation reactions are alkali metal phenolates, alcoholates and carboxylates and compounds containing o-(dimethylaminomethyl)phenol subgroups. Fluorocarbons such as trichloro-fluoromethanes are used as the sole blowing agents in the absence of any isocyanate-water reaction. 

Because of the high cross-link density of polyisocyanurates as prepared above, the resultant foams are brittle, so that there has been a move towards polyisocyanurate-polyurethane combinations. For example, isocyanurate-con-taining polyurethane foams have been prepared by trimerisation isocyanate-tipped TDI-based prepolymers. The isocyanurate trimerising reaction has also been carried out in the presence of polyols of molecular weight less than 300 to give foams by both one-shot and prepolymer methods. 

Besides trimerisation, leading to the production of polyisocyanurates, isocyanates can react with each other to form polycarbodi-imides with the simultaneous evolution of carbon dioxide ... 

Hendra and Loader (48) demonstrated that acetaldehyde trimerises on the surface to paraldehyde. 

Crude cyanogen chloride (endothermic, prepared from hydrogen cyanide and chlorine) may trimerise violently to cyanuric chloride, catalysed by traces of hydrogen chloride or ammonium chloride. 

The nickel-catalysed co-trimerisation of acetylenes with isocyanates gave good yields of uracils 46, depending on the isocyanate substituents, pyridones being unwanted by-products... 

Papanikolopoulou, K., Teixeira, S., Belrhali, H., Forsyth, V. T., Mitraki, A., and van Raaij, M.J. (2004b). Adenovirus fibre shaft sequences fold into the native triple beta-spiral fold when N-terminally fused to the bacteriophage T4 fibritin foldon trimerisation motif. / Mol. Biol. 342, 219-227. 

In further work on nylon [145], this trifunctional reactant was applied simultaneously with various nucleophilic dyes of the aminoalkyl type (Scheme 7.71). As in the case of the Basazol system on cellulose (Scheme 7.60), the intended formation of covalent dye-fibre linkages has to compete with side reactions, such as partial hydrolysis (Scheme 7.70), di- or trimerisation that may lead to less than optimum fastness, or substrate crosslinking that may adversely influence desirable fibre characteristics. 

Butadiene when trimerised over Ziegler-Natta catalyst to yields 1, 5, 9-cyclododecatriene. Hydrogenation form cyclododecane which yields dodecyl lactam. 

Another, highly selective oligomerisation reaction of ethene should be mentioned here, namely the trimerisation of ethene to give 1-hexene. Worldwide it is produced in a 0.5 Mt/y quantity and used as a comonomer for ethene polymerisation. The largest producer is BP with 40 % market share utilizing the Amoco process, formerly the Albemarle (Ethyl Corporation) process. About 25 % is made by Sasol in South Africa where it is distilled from the broad mixture of hydrocarbons obtained via the Fischer-Tropsch process, the conversion of syn-gas to fuel. The third important process has been developed by Phillips. 

Figure 9.9. Mechanism for the trimerisation of ethene using chromium catalysts...

Figure 9.11. Ligand for use as a chromium catalyst for the trimerisation of ethene...

The polymerisation reaction is a step growth polymerisation similar to a condensation polymerisation of amides or esters. The reaction starts with monomers, which dimerise, trimerise, etc. continuously maintaining a Flory-Schulz distribution. 

Hong and co-workers have described a formal [3-t-3] cycloaddition of a,P-unsaturated aldehydes using L-proline as the catalyst (Scheme 72) [225], Although the precise mechanism of this reaction is unclear a plausible explanation involves both iminium ion and enamine activation of the substrates and was exploited in the asymmetric synthesis of (-)-isopulegol hydrate 180 and (-)-cubebaol 181. This strategy has also been extended to the trimerisation of acrolein in the synthesis of montiporyne F [226],... 

Propionsdure. Nach Abb. 41 ist bei den Carbonsauren ein cyclisches Dimeres bevorzugt. Allerdings liegen weitere sechs freie Elektronen-paare sterisch leicht erreichbar. Trimerisation an diesen Stellen ist... 

An example of termination by formation cyclic trimers is reported in a recent publication by Shashoua et al. (12 a) describing anionic polymerization of mono-isonitriles to nylons-1. The trimerisation competes with further propagation steps and seems to be favored by higher temperature. 

The role of metallacycles, as intermediates in chromium-catalysed ethylene trim-erisation, is highlighted in this article. It is also shown that when 37 is treated with ethene in a protonolysis reaction, hexane and butane are liberated (1 3 ratio hexane butane). This observation gives strength to the argument that metallacycles are intermediates in chromium-catalysed ethylene trimerisation. Scheme 13 provides a simplified representation for chromium-based ethylene trimerisation. 

A rare example of a ferracycloheptane 108 was obtained as the product of the photochemical reaction of a Petitt s cyclobutadiene iron complex with dimethyl-maleate [Eq. (43)].118 The ferracycloheptane arises from the insertion of a maleate into each of two Fe-C bonds and might therefore be considered a special case of alkene trimerisation (vide infra). The cyclobutene fragment in the final metallacycle remains coordinated to iron, as established crystallographically (Fig. 34). 

Owing to the realisation of the importance of medium to large metallacycles in selective catalytic oligomerisation processes, particularly ethylene trimerisation and tetramerisation, we envisage that the field of metallacycloalkanes will be a significant growth area. Clearly new routes will have to be devised for the synthesis of metallacycloheptane, -octane and -nonanes so that these novel ring compounds can be efficiently prepared and studied. 

Enterobactin was originally isolated from bacterial cultures of Salmonella typhimurium or Escherichia coll by lengthy extraction and chromatographic purification procedures giving yields in the region of 15 mg of product per litre of nutrient solution. Understandably, the exacting nature of this procedure has led to the development of a number of laboratory procedures to produce enterobactin synthetically. The most efficient of these makes use of the template effect (Section 3.9.1) involving the trimerisation of bulky triphenylmethyl substituted L-serine derivative about a tin atom (Scheme 3.28). 

Cryptophane synthesis is accomplished by one of two methods. Initial procedures employed a covalent template effect in which one CTV-derived bowl preorganised the cyclisation of the second under high dilution conditions (Scheme 6.16a). More recently, a much more straightforward procedure has been developed in which cryptophanes are formed directly in a double trimerisation reaction (Scheme 6.16b). 

Treatment of acetone with ammonia and the mild dehydrating agent calcium chloride at room temperature gives the piperidone 72 in one step.11 Presumably the acetone dimerises and trimerises but the incipient polymerisation is nipped in the bud by the capture of one or more of these intermediates by ammonia and the formation of the only possible stable six-membered ring heterocycle 72. The yield (48%) seems poor but excess acetone is recovered in the isolation and the yield is 70% if that is taken into account. In any case, making 430 g of 72 from 1 kg acetone is a cheap process with so much added value . 

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