Metal-catalyzed alkyne polymerization

The addition of transition metal alkyls to alkynes is less common and often less facile than the corresponding metal-hydride additions, although this reaction is probably k r to many metal-catalyzed alkyne polymerizations (see below). In one case where an alkyne adduct has been established, i.e., the reaction of L2PtClMe with electrophilic alkynes, cts-M—C addition is observed (Scheme 4-24) [94]. 

In addition to the reactions discussed above, there are still other alkyne reactions carried out in aqueous media. Examples include the Pseudomonas cepacia lipase-catalyzed hydrolysis of propargylic acetate in an acetone-water solvent system,137 the ruthenium-catalyzed cycloisomerization-oxidation of propargyl alcohols in DMF-water,138 an intramolecular allylindination of terminal alkyne in THF-water,139 and alkyne polymerization catalyzed by late-transition metals.140... 

In 1954, Ziegler and coworkers [11,12] reported that traces of nickel salts dramatically alter the course of the growth reaction of ethylene with trialkylalanes, the Aufbau process. Instead of the low molecular weight polyethylene which was expected, the only product was butene. This observation culminated in Ziegler s discovery of transition metals that were highly effective in polymerizing ethylene, an accomplishment for which he later shared the Nobel Prize. It also opened the door to transition metal catalyzed hydroalumination reactions. In 1968, Eisch and Foxton showed that addition of nickel(II) salts increased the rate of the hydroalumination of alkynes by approximately 100-fold [13]. The active catalyst was believed to be a nickel(O) species. 

More recently, a Pd(II) salt was shown to catalyze the 1,2-insertion polymerization of a 7-oxanorbornadiene derivative (Fig. 10-16) [50]. The resulting saturated polymer, when heated, gives polyacetylene via a retro-Diels-Alder reaction. (This reaction is reminiscent of the Durham route to polyacetylene discussed below). One advantage of this technique over other routes is that it employs a late transition metal polymerization catalyst. Catalysts using later transition metals tend to be less oxophilic than the d° early transition metal complexes typically used for alkene and alkyne polymerizations [109,110]. Whereas tungsten alkylidene catalysts must be handled under dry anaerobic conditions, the Pd(II)-catalyzed reaction of water-insoluble monomers may be run as an aqueous emulson polymerization. 

The coordinated alkene or alkyne ligand can be attacked by other alkene or alkyne molecule to accomplish some metal-catalyzed synthetically useful transformations. Typical examples include dimerization and polymerization of alkenes catalyzed by highly electrophilic cations [PdL2(MeCN)2] + (L = MeCN, PR3) (e.g. Scheme 8.35) [57], and Cope rearrangement of 1,5-hexadiene derivatives catalyzed by PdCl2 (Scheme 8.36) [58], It was proposed that the key step in these reactions was the C-C bond formation via attack of the external alkene at the alkene carbon which was made highly electron-dehcientby coordination to Pd(II) ion. 

The insertions of alkynes into metal-carbon bonds are thermodynamically more favored than the insertions of olefins into metal-carbon bonds because the cleavage of one carbon-carbon TT-bond in an alk3me requires less energy than the cleavage of the C-C n-bond in an olefin and the sp -C-M bond in the product of alkyne insertion is stronger than the sp -C-M bond in the product of alkene insertion. The insertions of alkynes into the vinyl complexes that result from alkyne insertion are also favored thermodynamically. Thus, multiple insertions of alkynes to form polyacetylenes, just like the multiple insertions of alkenes to form polyolefins, are knoivn. Because of the conducting properties of polyacetylenes, the transition-metal-catalyzed polymerization of alkynes to form polyacetylenes has been studied. ... 

The insertion of alkynes into Pd-C or Pd-ER bonds (ER = SiR3, SnRs, BR2, etc.) of palladium(ll) derivatives is an important reaction involved in the mechanism of alkyne polymerization and in the numerous Pd-catalyzed additions to alkynes. It must occur by previous coordination of the alkyne to the metal, but the intermediate Pd( 7 -alkyne) complex is usually not detected either in catalytic studies or in examples of stoichiometric reactions reported in recent years,and those studies will not be collected here. Double insertion of alkynes into Pd-C bonds leads sometimes to <7-77 -enyl derivatives and some examples will be given in Section 8.06.6.5. Reviews are available dealing with palladium-catalyzed processes of interest where addition of two different groups to alkynes with Pd(ll) complexes takes place. 

Thankfully, numerous experts have kindly agreed to provide up-to-date chapters on several aspects in the area, thereby contributing to a collection of recent developments. In the first chapter, Ryohei Kakuchi discusses the use of metal-catalyzed MCRs for the synthesis of polymers. Special attention is laid to Cu-catalyzed developments. In the second chapter, Rongrong Hu and Ben Zhong Tang summarize developments of multi-component polymerizations of alkynes and... 

In the Sect. 2.1, a variety of transition-metal-catalyzed hydrothiolation reactiOTis of alkynes with thiols have been described. Transition metal catalysts are clearly useful for the regio- and/or stereoselective addition of thiols to carbon-carbon triple bonds. In contrast, the transition-metal-catalyzed addition of thiols to carbon-carbon double bonds such as alkenes has not been developed hitherto, except for Lewis acid-catalyzed hydrothiolation [8, 67-72]. This is most probably due to the lower coordination ability of alkenes compared with alkynes, which may permit the polymerization of metal sulfide complexes, resulting in inactivation of them as key catalysts for hydrothiolation. 

A simple alternative to the Lindlar reduction process has been recognized.95 The palladium(II) acetate-catalyzed polymerization of triethoxysilane in water produces finely divided palladium metal dispersed on a polysiloxane matrix with concomitant hydrogen evolution. Addition of (EtO)3SiH to the mixture of an alkyne and all other necessary constituents allows facile and selective reduction of the alkyne without an external hydrogen source ... 

Metal acetylacetonates quench triplet species generated by flash photolysis of aromatic ketones and hydrocarbons.330-333 More recently, these reactions have been studied from a synthetic standpoint. Triplet state benzophenone sensitizes photoreduction of Cu(MeCOCHCOMe)2 by alcohols to give black, presumably polymeric, [Cu(MeCOCHCOMe)] . This reacts with Lewis bases to provide complexes of the type CuL2(MeCOCHCOMe) (L = bipyridyl/2, ethylenediamine/2, carbon monoxide, Ph3P). Disubstituted alkynes yield Cu(C2 R2 XMeCOCHCOMe) but terminal alkynes form CuQR acetylides.334 The bipyridyl complex of copper(I) acetylacetonate catalyzes the reduction of oxygen to water and the oxidation of primary and secondary alcohols to aldehydes and ketones.335... 

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