Carbon polymerization with alkynes

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. 

Cationic zirconocene species efficiently activate alkenes toward carbon—carbon bond formation via carbometalation, as has been demonstrated in studies of alkene polymerization. Today, some zirconocene catalysts are available that allow single additions of metal-alkyls (mainly aluminum-alkyls) to alkenes or alkynes, thereby forming stable alkyl- or alkenyl-metals that do not undergo any further oligomerization. On the other hand, carbozirconation with Cp2ZrRCl in the presence of stoichiometric or catalytic amounts of activators has also been realized. 

Use of Co2(CO)8 in reactions involving 1,2-propadienes remains for the most part unexplored. It has been reported that terminal 1,2-propadienes react with Co2(CO)8 to form unidentified complexes, and that excess 1,2-propadiene is polymerized concurrently [30]. It has also been reported by Nakamura that a novel dimeric complex 54, in which a carbonyl ligand is connected to the central carbon of 1,2-propadiene, is produced by the reaction of 1,2-propadiene itself with Co2(CO)8 (Scheme 23) [31]. However, unlike the well-known chemistry of alkyne-Co2(CO)6 complexes, these 1,2-propadiene-cobalt carbonyl complexes have rarely been applied in synthetic reactions, probably due to their high activity in catalyzing the polymerization of 1,2-propadienes [32]. 

For olefins, cyclic, or better hi- or tricyclic ring structures with large ring strain (norborn-2-enes or norbornadienes for instance) are required. Alternatively, 1-alkynes can be used. In this case, the term 1-alkyne polymerization applies. This reaction proceeds via a- or j6-insertion of the alkyne into the metal-carbon double bond (Scheme 1). Both insertion mechanisms lead to a conjugated polymer. With a few exceptions [1-3], polymerizations based on a-insertion are the preferred ones, since they offer better control over molecular weights due to favorable values of kj/kp (ki, kp = rate constants of initiation and propagation, respectively). 

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... 

Alkyne polymerization in organic media has been reviewed [131]. A large variety of catalysts has been reported to polymerize alkynes in organic media. Similar to the polymerization of olefins, early transition metal as well as late transition metal catalysts are effective for this polymerization. Depending on the nature of the metal, two different mechanisms of polymerization have been suggested polymerization via a metal alkyl intermediate, or via a metal carbene (Scheme 7.9). With metal alkyl complexes, polymerization proceeds via migratory insertion of the alkyne into the metal-carbon bond [path (a) in Scheme 7.9] whereas with metal carbenes the mechanism is equivalent to that of metathesis [path (b)]. 

---