Trimerisation reactions

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. 

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). 

In the presence of a nickel catalyst electron-deficient allynes are prone to homo- and/or crosscyclotrimerisation reactions. Hence, it does not seem easy to carry out allynylcyanation of allynes using a allynyl cyanide. However, in the presence of a Lewis acid (BPhs) the trimerisation reaction can be successfully suppressed and nickel-catalysed allynylcyanation of alkynes achieved. In addition, bully silyl substituted alkynyl cyanides exhibit better reactivity in the allynylcyanation reactions than other allynyl cyanides the former require lower catalyst loadings and give higher product yields. ... 

Secondly, whilst both R NBr and EMI are effective catalysts for the trimerisation reaction, EMI is the more effective. In fact, it is so effective that the reaction was probably partially completed before the DSC thermograms at zero time were obtained. This may explain the discrepancies in the kinetic parameters for the reaction given in Table 7, The heat of formation of the trimer of MDI for the R NBr catalysed reaction yields a value of 155,6... 

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

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. 

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. 

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],... 

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). 

We are mainly concerned here with general processes but two specific reactions are of some interest. Fragmentation occurs in the pyrolysis of CFBrs, giving perfluorobenzene (Figure 9.3) [3-5] this was probably the first synthesis of this compound, although it was not reported for some time. Trimerisation of hexafluoro-2-butyne leads to hexakis(trifluoro-methyl)benzene [6,7], areaction that was referred to in Chapter 7, Section lllB, Subsection 1. 

Several macrocyclic compounds have been prepared using Wittig reactions including tetraepoxyannulenes, which utilized the ylide derived from phos-phonium salt (57), and rotaxanes, molecules in which one or more macrocyclic components are trapped around a rod section of a dumbbell-shaped molecule, obtained by treating terephthaldehyde derivatives with dibenzylic bis(triphenyl-phosphonium)-stoppered [2]rotaxane (58). Aromatic triesters have been obtained by the (ethoxycarbonylmethylene)triphenylphosphorane-catalysedcyclo-trimerisation of ethyl propynoate. ... 

The chapter ends with a rather miscellaneous collection of reactions such as one example of an electrocyclic ring closure 19. There are more important examples of ring closing olefin metathesis (RCM) and other organometallic reactions such as the Pauson-Khand reaction and co-trimerisation. 

In the same vein but more in the style of Vollhardt s co-trimerisation is a remarkable series of reactions that combine three alkynes. One is propargyl alcohol and the other two are tethered by a sulfonamide.36 In the simple case 245, trimerisation with the Grubbs catalyst (see above) gives one positional isomer 244 of a benzo-pyrroline while Wilkinson s catalyst [(Ph3P)3RhCl] gives the other possible isomer 246. 

Triazines are usually most easily obtained by substitution reactions on 2,4,6-trichloro-l,3,5-triazine, but the ring system can also be synthesised by cyclocondensation reactions. Trimerisation of nitriles (a common industrial method) or imidates gives symmetrically substituted compounds mono-substituted-... 

Isocyanates have some important reactions without the participation of active hydrogen compounds. These reactions, of real importance in polyurethane chemistry are dimerisation, trimerisation, formation of carbodiimides and reaction with epoxides and cyclic anhydrides. 

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