Cyclotrimerizations substituted benzenes

Dimethyl acetylenedicarboxylate (DMAD) (125) is a very special alkyne and undergoes interesting cyclotrimerization and co-cyclization reactions of its own using the poorly soluble polymeric palladacyclopentadiene complex (TCPC) 75 and its diazadiene stabilized complex 123 as precursors of Pd(0) catalysts, Cyclotrimerization of DMAD is catalyzed by 123[60], In addition to the hexa-substituted benzene 126, the cyclooctatetraene derivative 127 was obtained by the co-cyclization of trimethylsilylpropargyl alcohol with an excess of DMAD (125)[6l], Co-cyclization is possible with various alkenes. The naphthalene-tetracarboxylate 129 was obtained by the reaction of methoxyallene (128) with an excess of DMAD using the catalyst 123[62],... 

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. 

Cyclotrimerization of acetylenes to form benzene derivatives is studied intensively mainly because of the challenge of chemo- and regioselectivities. A regiose-lective and highly chemoselective method for preparing substituted benzenes was described via PdCl2-catalyzed cyclotrimerization in the presence of CuCl2 537... 

The 1,3-diyne 160 undergoes an interesting regioselective cyclotrimerization to afford unsymmetrically 1,3,5-substituted benzene 162. In this reaction the enetriyne 161 is formed at first by the Pd-catalysed dimerization of 160, and the reaction... 

Particularly important advances have been made recently in the chemistry of M2(OR)6 compounds. Many examples of simple crosswise addition of a R C CR" molecule to M2(OR)6, in the presence of pyridine, have been observed. The products differ in the details of structure and composition depending on the nature of R, R, and R", and on whether M is Mo or W, but in each case there is a quasitetrahedral M2Q cluster in which an M=M bond (2.55 to 2.66 A in length) is present. These simple adducts will react further with alkynes to give products in which two or more alkyne molecules have been linked, as in (18-C-XXIII). Further reaction affords alkyne cyclotrimerization products (substituted benzenes). 

A recent review (190) lists some twenty alkynes that have been cyclotrimerized to substituted benzenes in the presence of carbonylcobalt catalysts. The catalysts that have been used in these reactions include... 

The involvement of metalocyclopentadiene intermediates in the cyclotrimerization of alkynes (Scheme 6) has been established for some metal systems 162, 198, 201-204). However, there is no evidence to indicate participation of cobaltocyclopentadiene species in cyclotrimerization reactions involving carbonylcobalt complexes. It is worth noting, however, that 1,4-addition of an alkyne to such an intermediate would lead to the formation of 1,2,4- and 1,3,5-substituted benzenes. Moreover, the 1,2,4-derivative would be favored statistically. This line of reasoning has led to the incorporation of a cobaltocyclopentadiene intermediate in the mechanism proposed 120) (Scheme 7) for the Co4(CO)ia-catalyzed cyclotrimerization of PhC=CH. 

Like simple alkynes, conjugated diynes undergo cyclotrimerization in the presence of transition metal catalysts, but whereas simple alkynes give symmetrically substituted benzenes almost exclusively, diynes give symmetrical (256) and unsymmetrical... 

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... 

In practice the alkyne cyclotrimerization to benzene and its derivatives may be performed using both homogeneous and heterogeneous catalysts. Most catalysts reported in the survey of the catalysts (cf. Section 3.3.8.2) may be applied, giving good yields, to the cyclotrimerization of unsymmetrically substituted terminal and also internal alkynes. As mentioned above, in the case of terminal alkynes... 

Cyclotrimerization of alkynes to substituted benzene derivatives CO2 Cobalt catalyst... 

In the cyclotrimerization of aikynes to give substituted benzenes few intermediates are isolated owing to their thermal lability or their high reactivity. Metallocyclopenta-diene derivatives are involved in these reactions ... 

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-... 

The 1,3,5-unsymmetrically substituted benzene 224 was prepared in 64% yield by intermolecular cyclotrimerization of 1,3-decadiyne (223). The reaction was surprisingly regioselective to give 224 as a single product [68]. 

The [2 + 2 + 2] cyclotrimerization of alkynes and its analogous arynes [134] leads directly to aromatic hydrocarbons and can be considered as special cases of [2 + 2 + 2] cycloaddition reactions of unsaturated compounds [135, 136]. Especially the catalyzed cyclotrimerization reactions are often highly regio- and stereoselective and have been established as valuable methods for the syntheses of highly substituted benzenes, biphenylenes, triphenylenes, and in the approach towards natural products. 

Cycloaddition of alkynes is an easy and efficient approach towards the synthesis of benzene derivatives [21]. However, fluorine-containing alkynes are rarely shown to undergo such reaction [22]. [RUj(CO)32], with phosphine or some phosphine derivative as additive, is able to cyclotrimer-ize the trifluoromethyl group substituted internal alkynes [23]. Particularly, when 2-(diphenylphosphino)benzonitrile (2-DPPBN) is used, the cyclotrimerized unsymmetrical benzene derivative forms in 77-92% yield with more than 98% regioselectivity depending upon the substituent in Ar... 

The [2+2+2] cycloaddition or cyclotrimerization is a very useful atom economical transformation, which allows the synthesis of substituted benzenes from monoalkynes. Various reviews have been published on this topic [1], Historically, it was discovered (or invented) by Reppe and Sweckendiek... 

Besides 6-azulenyl-substituted benzenes, 2-azulenyl-substituted benzenes 12a,b were later synthesized by the same research group from di(2-azulenyl) acetylene 13a,b using cobalt-mediated cyclotrimerization, as shown in Scheme 4.4 [13, 14]. Compound 12a exhibited a reversible reduction wave at —2.29 V versus Fc+ZFc, which involves multiple electron transfer in one step to produce highly charged anionic species, and two irreversible oxidation waves. However, the exact number of transferred electrons could not be determined from the analysis of the reduction wave in the cyclic voltammogram. With... 

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