Cyclotrimerizations benzene ring

Structural analysis of the homopolymers by spectroscopic methods confirmed that the diynes had undergone [2 + 2 + 2] polycyclotrimerizations by forming new benzene rings from their acetylenic triple bonds. The ratio of the 1,2,4- to 1,3,5-isomers of the trisubstituted benzene rings was estimated to be 2.2 1. Careful evaluation of the 111 NMR spectra unveiled that the number of terminal triple bonds in the final hb-PAs was much smaller than that in an ideal hyperbranched structure produced by the diyne polycyclotrimer-ization. This result suggests that intra-sphere ring formation might have been involved in the cyclotrimerization polymerization. 

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. ... 

Transition-metal-catalyzed [2 + 2 + 2]-cycloaddition of alkynes, so-called cyclotrimerization, has been widely investigated as a powerful tool to construct benzene rings.117 Cyclotrimerization of the amino-dialkynes and dialkynyl ethers 109 with alkynes provides various kinds of benzene-fused aza- and oxa-heterocyclic compounds 111 (Scheme 39, route a).118 The key intermediate for the [2 + 2 + 2]-cycloaddition is the metallacyclopentadienes 110, as men-... 

Thes-triazine ring is similar to the benzene ring in terms of being highly resonance stabilized, and, therefore, thermally stable. This property was first exploited with the melamine-formaldehyde resins (see also Section 28.3.2), whereby preformed triazine structures are polycondensed. But the triazine ring can also be formed in situ, as, for example, in the cyclotrimerization of nitriles or primary or secondary biscyanamides in ketones or lower alcohols ... 

The reactions that yield benzene rings can be categorized further into the following types according to the substrates involved (1) intermolecular cycloaddition of three alkynes (cyclotrimerization), (2) partially intramolecular cycloaddition ofdiynes with alkynes, and (3) fully intramolecular cyclotrimerization of triynes. In the next section, the synthetic routes to benzene derivatives using ruthenium-catalyzed cycloaddition are surveyed according to these classifications. Classic examples of [2 + 2 + 2] alkyne cycloadditions using stoichiometric ruthenium mediators are included since they provide useful information on the further development of ruthenium catalysis. 

Acetylene cyclotrimerization is a well-established method for the effective transformation of triple bonds to benzene rings. Acetylenic polymerization has emerged... 

Cyclotrimerization of alkynes into the benzene ring has found numerous applications in the syntheses of natural compounds such as protoberberine skeleton [84], precursors of aromatic steroids [85], and tetracyclic terpenes [86]. A typical example is the short synthesis of estrone 147, in which the whole steroid framework 149 was assembled from the starting diyne 148 in one step (Scheme 65) [87]. 

It is possible to carry out the [2+2+2] cyclotrimerization reaction in a regioselective manner by using a partially or completely intramolecular approach. Rhodium-catalyzed intramolecular cyclotrimerization of 1,6,11-triynes, which construct fused 5-6-5 ring-systems, has been studied extensively [33-36]. Cyclization of 1,6,11-triyne 47 catalyzed by RhCl(PPh3)3, gives the tricyclic benzene 48 in good yield (Eq. 14) [33a]. 

Rh(l)-catalyzed [2-I-2-I-2] cyclotrimerization of 1,6-diynes (e.g., 1391 and 1394) with monoynes (e.g., 1392) in combination with stereospecific Ag(i)-catalyzed aldimine (metallo)azomethine ylide — cycloaddition cascades affords rapid access to complex heterocyclic benzene derivatives 1393 and 1395 in one-pot processes with the generation of five new bonds, four stereocenters and three rings (Schemes 266 and 267) <2000T8967>. 

Among the metallacycloalkenes with different ring size, metallacyclopro-penes and -pentadienes are to be mentioned because they are involved as intermediates in the cyclotrimerization of alkynes to benzene derivatives, and in this section attention is mainly directed to such species. Cotrimeriza-tions also are possible with alkyne-like systems. 

Intermediates containing metallapentadiene rings are frequently involved in metal-catalysed cyclotrimerization of acetylenes to benzenes, a fact illustrated by the facile catalysis of the cyclotrimerization of PhC2Ph by the nickelole complex (35). ... 

Since Reppe s discovery of the cyclotrimerization of acetylene to benzene in the presence of nickel carbonyl-phosphine complexes, the use of nickel catalysts in many organic transformations has become popular. Transition metal complex catalysis provides many elegant entries to carbon-carbon bond-forming reactions in organic synthesis. One notable example is carbocyclic ring expansion mediated by nickel(O) complexes. ... 

Assembly of carbon-rich molecules by means of acetylene chemistry is also of note prior to 1900. Although isolated from petroleum products in 1825 by Faraday [la], the first total synthesis of benzene by Berthelot in 1867 via cyclotrimerization of acetylene [11] set a precedent for future thermal trimerization reactions, a key procedure to produce uniquely substituted arene rings [le]. The first ethynyl ho-... 

---