Trimerization Cyclotrimerization

The formation of isocyanurates in the presence of polyols occurs via intermediate aHophanate formation, ie, the urethane group acts as a cocatalyst in the trimerization reaction. By combining cyclotrimerization with polyurethane formation, processibiUty is improved, and the friabiUty of the derived... 

Whereas cyclotrimerization of phenylacetylene with uncomplexed PdCl2 provides only low yields of the unsymmetrical trimer, and polymers, on treatment of 3-hexyne with Pd/C and Me3SiCl 14 hexaethylbenzene 2165 is obtained in quantitative yield [78] (Scheme 13.23). 

Transition metal-catalyzed intermolecular [2 + 2 + 2] cyclotrimerization of alkynes to benzene derivatives has been extensively studied. In this section, the focus is on the cyclo-trimerizations of the substrates bearing three independent unsaturated bond components. The key issue with this type of process usually involves the challenge of controlling regioselectivity [1—1]. However, 1,3,5-trisubstituted benzene 44 can be obtained as the sole product in good yield when 3-butyn-2-one 43 is used as the substrate for the cyclotrimerization catalyzed by Rh2(pfb)4 (pfb=perfluorobutyrate) in the presence of EtsSiH under a CO atmosphere (Eq. 11) [30]. 

Homogeneous nickel complexes proved to be versatile catalysts in dimerization and trimerization of dienes to yield different oligomeric products.46-55 Depending on the actual catalyst structure, nickel catalyzes the dimerization of 1,3-butadiene to yield isomeric octatrienes, and the cyclodimerization and cyclotrimerization to give 1,5-cyclooctadiene and all-trans-l,5,9-cyclododecatriene, respectively46 56 [Eq. (13.13)]. Ziegler-type complexes may be used to form cis,trans,trans-1,5,9-cyclododecatriene37,57 58 [Eq. (13.14)], which is an industrial intermediate ... 

Trimerization of imidates is a valuable route to 1,3,5-triazines. Imidates can be considered as activated nitriles and cyclotrimerize more readily. Most symmetrical 2,4,6-trialkyl-1,3,5-triazines are easily formed, although large alkyl substituents may give rise to steric hindrance (61JOC2778). Symmetrical isocyanurates (525) are readily available from isocyanates, RNCO catalysts include tertiary amines, phosphines and sodium methoxide. Aldehydes RCHO and ammonia give hexahydro-1,3,5-triazines (526), known as aldehyde ammonias (73JOC3288). 

Probably the cyclotrimerization of nitriles is the best known route to 1,3,5-triazines. The reaction has the obvious limitation that it is of value for preparing the symmetrical derivatives only. Nevertheless, many important triazines, such as cyanuric chloride, are made in this way. There are a number of other cyclotrimerization reactions which are also useful, in particular the trimerization of imidates. An easy route to 1,3,5-triazine from ammonium acetate has been developed. 

The enantiomeric purity of the 3-pinanecarbaldehyde corresponds to the a-pinane utilized (70-85%). Enantiomerically pure aldehyde can be obtained by the acid-catalyzed trimerization of the aldehyde, with only one enantiomer being preferentially cyclotrimerized to a crystalline compound.2311 Cleavage of the trimer results in enantiomerically pure aldehyde. If cobalt catalysts are employed in the cyclization, rearrangement to the bomane structure takes place (equation 9).25... 

In general the thermal reaction of allene gives a complex mixture of dimers, trimers, and higher oligomers including small amounts of spiro compounds (140). A highly selective dimerization to 1,2-dimethylene-cyclobutane is achieved by thermal reaction of dilute solutions Theoretically the process [2s + 2a] of allenes and related cumulenes may be facilitated by participation of the orthogonal pir-orbital in the addition 142). However, the concertedness of the cyclodimerization is still in dispute. Selective cyclotrimerization and pentamerization of allene have not... 

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. 

Co2(CO)8 and Hg[Co(CO)j2 (41), i7T-CMCo CO)2 191), Co CO)aNO 192), Co2(CO)8(RCaR ) (193), and Co4(CO)io(RC2R ) 48). The trimeri-zation reactions with symmetrically disubstituted alkynes, RC=CR, lead inevitably to hexasubstituted benzenes. Unsymmetrical alkynes RC=CR are trimerized generally to 1,2,4- rather than 1,3,5-trisubstituted benzenes. These cyclotrimerization reactions can be achieved readily by... 

Key to the success of this co-cyclotrimerization procedure is the selection of the appropriate monomers. A co-cyclotrimerization in which one monomer reacts much more rapidly than the other will result in a heterogeneous product as the monomer ratio changes. Moreover, if the mono-ethynyl capping agent reacts much more slowly than the multi-ethynyl monomer, gel formation can occur early in the reaction. Alternatively, if the mono-ethynyl material reacts much more rapidly, it can be exhausted early in the reaction, having produced a non-reactive trimer, and the multi-ethynyl monomers will gel later in the reaction. These problems can be avoided by using monomers of comparable reactivity or by adjusting the feed ratio to compensate for unequal reactivities. With either approach, it is necessary to determine the relative cyclotrimerization rates of each ethynyl monomer. In this paper, we report the initial results of our measurements of these rates. 

Cyclotrimerization has been achieved with a variety of metals. The products may contain the trimer as a flat 1,3,5-triphospha-benzene unit (63), or as a folded Dewar-benzene type ligand (64). Recently, the pentamerization of a phosphaaUcyne has also been reported. ... 

Aromatic compounds can be prepared by cyclotrimerization of alkynes or triynes. Cyclotrimerization is possible by heating to 450-600°C with no catalyst. The spontaneous (no catalyst) trimeiization of t-BuC=CF gave 1,2,3-tri-tert-butyl, 5,6-trifluorobenzene (220), the first time three adjacent tert-butyl groups had been put onto a benzene ring. The fact that this is a head-to-head joining allows formation of 220 from two alkynes. The fact that 219 (a Dewar benzene) was also isolated lends support to this scheme. Three equivalents of 3-hexyne trimerized to hexaethylbenzene at 200°C in the presence of Si2Cl6. ... 

In the course of the cyclotrimerization of alkynes with olefins, the cobalta-cyclopentene 70 appears. When it reacts with alkynes, PPhj is replaced by RC=CR, which is inserted into a Co—C bond with formation of the trimerization product 104 (Scheme 37). ... 

Alkyne cyclotrimerization occurs at various homogeneous and heterogeneous transition metal and Ziegler-type catalysts [7], Substituted benzenes have been prepared in the presence of iron, cobalt, and nickel carbonyls [8] as well as trialkyl- and triarylchromium compounds [9]. Bis(acrylonitrile)nickel [10] and bis(benzonitrile)palladium chloride [11] catalyze the cyclotrimerization of tolane to hexaphenylbenzene. NiCl2 reduced by NaBH4 has been utilized for the trimer-ization of 3-hexyne to hexaethylbenzene [12]. Ta2Cl6(tetrahydrothiophene)3 and Nb2Cl6(tetrahydrothiophene)3 as well as 7 -Ind-, and 77 -Ru-rhodium... 

From a mechanistic viewpoint, the formation of the cyclo-P6 ligand from P4 has not yet been completely clarified. Nevertheless, suggestive hypotheses based on the dimerization of P3 units or the trimerization of P2 units have been proposed [70,102,106]. The latter mechanism is particularly intriguing as it parallels the well-known metal-mediated cyclotrimerization of acetylenes in the coordination sphere of cobaltocenes [107]. 

In the cyclotrimerization of propynoic acid, trimellitic (l,2,4-fb,l l3((X)2H),) and trimesic acid (l,3,5-CgH3(CO2H)3) are obtained as the product of head-to-head or head-to-tail linkage, respectively. For the sake of comparison, reactions were conducted both in neat water and in neat THF. In all cases, the yield of the trimer was higher in the aqueous system, evidencing the high tolerance of the rhodium catalyst towards water. 

The cyclotrimerization of nitriles (R—CN) is a very common route to 1,3,5-triazi-nes.10-12,119-121 The reaction conditions and catalysts are dependent on the R part of the nitrile molecule several reaction mechanisms are involved in the trimerization reaction. Aliphatic and aromatic nitriles can trimerize upon application of high pressure and/or heat however, catalysts are used more frequently, e.g. hydrogen chloride, ehlorosulfonic acid, boron trifluoride, sodium metal, Grignard reagents and sodium triphenylmethanide. The same also applies to all other nitriles (a-atom not carbon). 

Considerable effort has been expended to synthesize trialkyl-substituted 1,3,5-triazines by cyclotrimerization of alkyl cyanides,119-127137 unfortunately with little success. The reactions require high pressures and high temperatures, and in many cases alcohols are used as solvents and catalysts.l3s 141 An intermediate aliphatic imidate 2 is postulated, which trimerizes to the substituted 1,3,5-triazine 3. 

Steric factors also have an influence on cyclotrimerization. Very bulky ori/io-substituted aryl cyanides require in some cases more vigorous reaction conditions compared to meta- and para-derivatives and the yields are lower. Anthracene-9-carbonitrile, adamantane-1-carboni-trile, and phenanthrene-9-carbonitrile do not undergo trimerization, even under 8000-17000atm at 150-240 °C.141... 

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