Cyclotrimerization of phenylacetylene

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). 

Ali A, Malan PP, Singleton E, Meijboom R. Fulvene-ruthenium and Cp-mthenium complexes via [2 + 2 + 1] cyclotrimerization of phenylacetylene with [RuCl(Tp) (1,5-... 

Figure 7 Cyclotrimerization of phenylacetylene (14) catalyzed by TaBrs and the stereochemical structures of 1,3,5- and 1,2,4- isomers of the residtant cycUc products of...

Reaction of the isomers IV with phenylacetylene yields a range of complexes, of which Os3(CO)7L3 is one. This is related to the complex Os3(CO)7[C2(CsHs)2]3, obtained in the corresponding diphenylacetylene reaction and shown to be an intermediate in the cyclotrimerization of diphenylacetylene to hexaphenylbenzene 124). The structure of this complex is discussed in Section III,F. 

Supercritical water has the potential to be an interesting solvent system for organotransition metal systems. An example of this potential is found with the complex (7r- cp)Co(CO)2, which in organic solvents catalyzes the cyclotrimerization of hexyne-1, phenylacetylene, and butyne-2. At I40°C in aqueous media these reactions show poor selectivity to benzenes, but at 374°C in supercritical water the selectivity is again high (139). 

The catalytic cyclotrimerization of alkynes to give substituted benzenes was investigated in supercritical water by the groups of Parsons [53a] and Dinjus [53b] using [CpCo(CO>2] as the catalyst. The reaction proceeded smoothly with 1-alkynes and phenylacetylene at T - 374 °C and p = 250 bar using substrate to catalyst ratios up to 35 1. The ratios of isomeric trisubstituted ben-... 

Cyclotrimerization side products in Glaser coupling of phenylacetylene. 

The versatihty of the cyclotrimerization and the easy access to substituted phenylacetylene by modem coupling methods offers a wide range of variously decorated HBCs. An intriguing example is shown in Scheme 20. Permethoxylated hexa-pen-hexabenzocoronene (permethoxylated HBC) 90, which results in a double concave conformation, was easily synthesized via cyclotrimerization of a hexamethoxysubstituted phenylacetylene 88 followed by the usual cyclodehydrogenation reaction [50]. The non-planarity of the system is explained by the steric... 

Like in the case of cyclotrimerization of the monoyne (phenylacetylene), the diyne polycyclotrimerizations should give rise to the formation of 1,3,5- and 1,2,4-substituted phenylene structures in the polymers. This is confirmed by the... 

In 1988, Linstrumelle and Huynh used an all-palladium route to construct PAM 4 [21]. Reaction of 1,2-dibromobenzene with 2-methyl-3-butyn-2-ol in triethylamine at 60 °C afforded the monosubstituted product in 63 % yield along with 3% of the disubstituted material (Scheme 6). Alcohol 15 was then treated with aqueous sodium hydroxide and tetrakis(triphenylphosphine)palladium-copper(I) iodide catalysts under phase-transfer conditions, generating the terminal phenylacetylene in situ, which cyclotrimerized in 36% yield. Although there was no mention of the formation of higher cyclooligomers, it is likely that this reaction did produce these larger species, as is typically seen in Stephens-Castro coupling reactions [22]. 

A Ni(dppe)Br2-Zn system effectively catalyzes co-cydotrimerization of an allene with a propiolate. The reaction is highly regio- and chemoselective to afford a poly-substituted benzene derivative in good yield. (Scheme 16.82) [92], From the observation that no desired [2 + 2 + 2] product is obtained for the reaction of 1-hexyne and phenylacetylene with w-butylallene under similar conditions, the presence of an electron-withdrawing C02Me group in the alkyne moiety is essential for the success of the present [2 + 2 + 2]-co-cyclotrimerization. 

Reports on ruthenium catalytic activity focus more on mechanistic consideration of the prototypical phenylacetylene dimerization than in establishing its synthetic applicability. It is not unusual that changing the alkyne substituents results in reversed selectivity (i.e. R = Ph or SiMe3 gave ( )- or (Z)- isomers, respectively) [27]. Competitive alkyne cyclotrimerization (R = COOMe) [27] or butatriene formation (R= CH2Ph, Bu) [10, 21] have occasionally been reported as possible drawbacks in enyne synthesis. The operating mechanism restricts the reaction to terminal alkynes. 

As a rule, each phenylacetylene derivative was evaluated at an initial concentration of 125 mM reactions using phenylacetylene alone were conducted at concentrations of 125, 250, and 500 mM in order to establish the effect of eth myl concentration on the measured rate. Because more than one material is produced in even the simplest cyclotrimerization reaction, all reactions were followed only by measuring the disappearance of the starting material(s). Although attempts were made to fit the resulting data into the expected second- or third-order kinetics plots, it was finally concluded that the reactions were better described as zero-order. Accordingly, data were plotted on linear concentration and time scales. 

Because preparative cyclotrimerization reactions are usually conducted at high concentration, the initial, faster rates in this study were considered more important. For each run, the rates of disappearance of the substituted aryl acetylene and phenylacetylene, along with the reaction ratio, are listed in Table I. 

Because the exact mechanism of the cyclotrimerization reaction is not adequately understood, it is useless to conjecture on exactly how the substituent influences the reaction rate. However, it is useful to know that spectroscopic data correlates with the observed rates and this may prove advantageous in the prediction of cyclotrimerization rates for other substituted phenylacetylenes. 

The first group of catalysts is just MoCls and WC16. These metal chlorides polymerize various monosubstituted acetylenes. Table 4 demonstrates that WC16 and MoCl5 are specifically effective for phenylacetylene polymerization among various transition metal chlorides. It is noted that NbCl5 and TaCl5 selectively cyclotrimerize phenylacetylene, and that other metal chlorides hardly induce any reactions. 

A related structure is the complex Pd(dppf)( j -CH2=CsPh3) the triphenylfulvalene ligand arises, in this case, from phenylacetylene cyclotrimerization in the presence of PdCl2(dppf). These molecules are reminiscent of the complex Pd(PPh3)2( -CH2=CsHMe4), also synthesized by a sophisticated protocol, for which the structure is similar with 1.424 A and 10.81° for the same structural parameters. ... 

Importantly, the catalytic mixture IX is also able to cyclotrimerize monosubsti-tuted alkynes such as phenylacetylene with excellent regiocontrol, and to cocyclize diynes with alkynes in good yields (Scheme 1.9). The latter category has recently been exploited for the synthesis of substituted anthracenes, pentaphenes, and trinaph-thylenes [14d], as well as for quick access to diverse polymerizable molecules [15]. It is noteworthy that the reaction times can be decreased further by adding a silver salt such as AgOTf or AgSbFe [14c]. 

HIr(cod)[l,l-bis(diphenylphosphino)methane] (DPPM) has been reported as a regioselective cyclotrimerization catalyst for phenylacetylene. The reaction of p-substituted aryl alkyne gave 1,2,4-triarylbenzene exclusively [17]. 

Intermediates of the reaction of eq 42 are not normally observed, but some examples of complexes formed from octacar-bonyldicobalt and two alkyne moieties are known, e.g. from cy-clooctyne, giving a product which promotes the cyclotrimeriza-tion of cyclooctyne (eq 43), and also from MeN(CMe2C2H)2. The product of the latter reaction has been treated with phenylacetylene, forming an arene system (eq 44). Thus, both of these examples show that this type of bis(alkyne)cobalt complex can also be on the cyclotrimerization pathway. 

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