Vinylidene complexes, alkyne polymerization

Metal allenylidene complexes (M=C=C=CR2) are organometallic species having a double bond betv een a metal and a carbon, such as metal carbenes (M=CR2), metal vinylidenes (M=C=CR2), and other metal cumulenylidenes like M=C=C= C=CR2 [1]. These metal-carbon double bonds are reactive enough to be employed for many organic transformations, both catalytically and stoichiometrically [1, 2]. Especially, the metathesis of alkenes via metal carbenes may be one ofthe most useful reactions in the field of recent organic synthesis [3], vhile metal vinylidenes are also revealed to be the important species in many organic syntheses such as alkyne polymerization and cycloaromatization [4, 5]. 

The tungsten carbene complex [(CO)5W=CHAr] (Ar = Ph, p-tol) reacts with 2-butyne to give stilbenes and [(CO)5W(MeC=CMe)]. Terminal alkynes, such as Bu C=CH, are however polymerized. Vinylidene complexes are proposed as intermediates since the acetylene-vinylidene rearrangement has ample precedence in stoichiometric reactions (Scheme 26). 

Abstract Several routes to access ruthenium (Ru)-vinylidene complexes are described, the majority of which employ alkynes and a Ru source as the starting materials. The successful application of Ru-vinylidenes as efficient pre-catalysts for the synthesis of carbocyclic and heterocyclic compoimds by ring-closing metathesis (RCM) of a, -dienes, and in the synthesis of polymers by ring-opening metathesis polymerization (ROMP) of cyclooctene, norbomene, 5-substituted norbomene, and dicyclopentadiene is fully illustrated. Relevant aspects concerning the activity and selectivity of this type of Ru complexes in metathesis reactions are highlighted. 

Another class of transition metal complexes studied by our group consists of alkyne and vinylidene complexes of tungsten and molybdenum in high oxidation states. Alkyne complexes are interesting from a theoretical point of view because they may be considered to be either side-on coordinated ligand complexes or metallacyclopropenes. They are also important as possible intermediates in the cyclooligomerization and polymerization of alkynes. o " Scheme 3 shows the two types of alkyne complex that were studied. 

Metal vinylidenes easily form by isomerization of metal-alkynecomplexes, which is important for instance in alkyne polymerization by the metathesis mechanism. Synthesis of Grubbs ruthenium vinylidene complexes [Ru(PR3)2Cl2(=C=CHPh)] originally proceeded by reaction of the ruthenium precursor with diphenyl-cyclopropene, but a more modern method leading to the parent vinylidene complex involves benzylidene exchange for the vinylidene in the metathesis with 1,3-butadiene ... 

Finally, Katz and subsequently Rooney (386-388) showed that (CO)sW [C(OMe)Ph] catalyzes the polymerization of alkynes via carbene-alkyne and purported metallacyclobutene intermediates (Scheme 34). Later computational work by Hofmann (388b) called into question the formation of the latter species, suggesting that the reaction proceeds directly via the vinyl-carbene. Formation of the alkyne-carbene intermediate by low-temperature photolysis of (CO)5W[C—(OMe)Ph] in the presence of alkynes leads to polymerization upon warming. The use of a pre-formed carbene complex is not required since active polymerization catalysts can be formed by photolysis of W(CO)5 in the presence of terminal alkynes in hydrocarbon solutions. A key step in catalyst generation is rearrangement of a coordinated alkyne to a vinylidene ligand (389). 

The above shown polymer photocrosslinks when irradiated in the presence of small amounts of W(CO)6 using a light beam at 320-390 nm. UV photolysis of W(CO)6 in the presence of an alkyne results in the formation of a ri -alkyne tungsten pentacarbonyl complex, [(r -RC=CR )W(CO)5], that rearranges to a vinylidene derivative [R(H)C=C=W(CO)5] and is an intermediate in the initiation step in the polymerization of alkynes. 

Some general reviews relating to the chemistry of Ru/Os-r hydrocarbon complexes appear in the literature the reactivity of Ru-H bonds with alkenes and alkynes/ aspects of ruthenium/osmium vinylidene/allenylidene/cumul-enylidene complexes,equilibria of M-R/M=CR2/M=CR complexes, the organometallic chemistry of metal porphyrin complexes, and the reactions of [Os(P Pr3)2(CO)HGl], ruthenium pyrazoly I borate complexes,and metallabenzynes. Other reviews relate more to applications of some of the complexes outlined in this chapter. See, for example, metal vinylidenes in catalysis,the development of Grubbs-type alkene metathesis catalysts, applications of ruthenium/osmium carbene complexes in metathesis polymerization, and the role of Ru /V-hetero-cyclic carbene complexes in metathesis polymerization. ... 

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