Phenylacetylenes, trimerization

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). 

Trimethylsilyl groups in these compounds are cleaved with hydrochloric acid giving the first cobalt-cyclobutadiene complexes with unsubstituted positions in the four-membered ring. The protecting role of SiMe3 groups (see also Section III, A) is important here, since phenylacetylene does not give cyclobutadiene complexes under similar conditions, but trimerizes instead. 

Acetylene reacts rapidly with B2C14 to form 1 1 and 1 2 addition products.478 The transformation of phenylacetylene is much slower and only the monoaddition product is formed.479 2-Butyne, however, undergoes trimerization to give hexam-ethylbenzene when less than a stoichiometric amount of the reagent is used,475 but reacts with an equimolar amount of B2C14 to yield the monoadduct.480... 

The first active catalyst system found was prepared by reaction of nickel ace-tylacetonate with organoaluminum compounds in the presence of phenylacetylene. A dark red solution was obtained which reacted at 80° C. under pressure with butadiene to about 24% cyclo-octadiene, 8% vinylcyclohexene, and 63% all-tmns-cyclododecatriene. The component which stabilizes the reduced nickel was then changed systematically to discover the possibility of directing the synthesis at will in the direction of a trimerization or dimerization. Today we can synthesize cyclo-octadiene in yields of 95% or cyclododecatriene in similarly good yields only by altering the electron-donor molecules used in preparing the catalyst. 

Phenylacetylene is trimerized at 371 K by head-to-tail coupling around the mixed-metal cluster [Mo2Ru(//3-S)Cp2(CO)7], It is believed that the dimolybdenum unit serves as the alkyne oligomerization site but the chemistry overall is dependent on the entire cluster functioning as a unitJ - ... 

The action of the complex 570 on various alkynes gives diverse products ethyl propiolate affords a mixture of all three possible triethyl benzenetricarboxylates, phenylacetylene yields mainly the trans-QnyviQ 571 3-hexyne behaves similarly, but 1-octyne isomerizes to 2-octyne . The palladacyclopentadiene 572 catalyses the co-trimerization of hex-3-yne... 

Soluble polyphenylenes with a branched structure (36) were prepared (equation 34) from diethynylbenzene and phenylacetylene by trimerization of the acetylene group in the presence of Ziegler catalysts . The oligomers (called H-resin ) can be cross-linked on heating to yield thermally and chemically stable resins. ... 

In 2010, Buchmeiser [56] developed a similar system that capitalized on the thermally reversible carboxylation [11] of NHCs (Scheme 31.13, inset). By employing the NHC-CO2 adduct (which essentially is a protected NHC), the reaction conditions did not have to be stringently air- and moisture-free to prevent NHC decomposition. Synthesis of the norbornene-functionalized monomer 37 allowed the molybdenum-catalyzed ROMP with l,4,4a,5,8,8a-hexahydro-l,4,5,8-exo-ewdo-dimethanonaphthalene (a ditopic norbornene) to produce crossHnked polymer 38 with pendant CO2-masked NHCs (Scheme 31.13). Upon heating in the presence of Rh, Ir, or Pd species, the NHC-metal-functionalized polymers 39 were formed and found to contain >20mol% metal, as determined with inductively coupled plasma optical emission spectrometry (ICP-OES). The C02-masked NHC material was found to catalyze the carboxylation of carbonyl compounds and the trimerization of isocyanates upon thermal deprotection (i.e., decarboxylation). Moreover, the NHC-metal-crosslinked materials were found to catalyze Heck reactions, transfer hydrogenations, and also the polymerization of phenylacetylene (M = 8.4 kDa, PDI = 2.45, as determined with GPC in DMF against PS standards). This modular system provides an array of options for catalysis from simple modifications of polymer-supported, C02-masked NHCs. 

Polymer-supported, carbon dioxide-protected NHCs were also prepared for catalysis reactions [150]. These supports were prepared using DMN-H6 with 3-(bicyclo[2.2.1]hept-2-ene-5-ylmethyl)-l-(2-propyl)-3,4,5,6-tetrahydropyrimidin-l-ium-2-carboxylate and a Schrock catalyst. They were later used for the trimerization reaction of isocyanates and for the cyanosilylation of carbonyl-containing compounds. Various metals could be immobilized through the NHC hgands including rhodium(I), iridium(l), and palladium(II). The resulting monoUths polymerized phenylacetylene and were successful catalysts for Heck-type couplings [150]. 

The actual mechanism of the thermally induced acetylene reaction has not been determined and was first proposed as a simple acetylenic trimerization. It is our opinion, however, that the terminal acetylene groups can simultaneously react via a number of alternate routes in addition to trimerization. Reaction of phenylacetylene and the model com-... 

The assumption that 1,3,5-triphenylbenzene is formed via trimerization of phenylacetylene has not been confirmed. Also, it cannot be formed as a result of autocondensation of oxime or ketone. Hence it follows that 1,3,5-triphenylbenzene is a minor product of the synthesis of pyrrole from acetophenone oxime and aphenylcetylene. 

The model reaction between bromo benzene and phenylacetylene gives information about the chain limiting reactions and defect structures in the polymer (76). The dehalogenation can be neglected in contrast to die Heck reaction. Diin and enin formation as well as trimerization are demonstrated in the model reaction. The trimerization and the formation of enin can be suppressed quantitatively. The formation of diin occurs in die range < 0,5 %. It is not clear if the diin is formed during the reaction or on working up. 

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