Cycloolefin complexes

Although many stable cycloolefin complexes of the group VIB metals have been reported (47, 268), it is only recently that tt complexes of the simple monoolefins such as ethylene have been isolated (148, 195, 203, 572). 

Various metal (Fe, Co, Ni, etc.) cycloolefin complexes have been obtained using this procedure. They are formed in very high purity. 

Table 9. Thermodynamic data for copper(I) cycloolefin complex formation at 30 °C in 1M Li CIO4...

Nickel(O) forms a n-complex with three butadiene molecules at low temperature. This complex rearranges spontaneously at 0 °C to afford a bisallylic system, from which a large number of interesting olefins can be obtained. The scheme given below and the example of the synthesis of the odorous compound muscone (R. Baker, 1972, 1974 A.P. Kozikowski, 1976) indicate the variability of such rearrangements (P. Heimbach, 1970). Nowadays many rather complicated cycloolefins are synthesized on a large scale by such reactions and should be kept in mind as possible starting materials, e.g. after ozonolysis. 

Metallocene Catalysts. Polymerization of cycloolefins with Kaminsky catalysts (combinations of metallocenes and methylaluminoxane) produces polymers with a completely different stmcture. The reactions proceeds via the double-bond opening in cycloolefins and the formation of C—C bonds between adjacent rings (31,32). If the metallocene complexes contain bridged and substituted cyclopentadienyl rings, such as ethylene(hisindenyl)zirconium dichloride, the polymers are stereoregular and have the i j -diisotactic stmcture. 

The first documented example of the living ROMP of a cycloolefin was the polymerization of norbornene using titanacyclobutane complexes such as (207) 510-512 Subsequent studies described the synthesis of di- and tri-block copolymers of norbornenes and dicyclopentadiene.513 However, functionalized monomers are generally incompatible with the highly electrophilic d° metal center. 

As pointed out in an earlier section, Katz and co-workers demonstrated that Casey s complex, (CO)5W=CPh2, could also be employed to initiate the metathesis of a-olefins (26) as well as the polymerization of certain cycloolefins (27, 63). 

Annulation of tricarbonyl cyclobutadiene iron to tropone and tropylium ion to give complexes 282 and 285 is achieved by Wittig cycloolefination of dialdehyde 281 with biphosphonium salts, 280 and 283, respectively (Scheme 71 77JA513 78AJC1607). 

For example, it is possible to synthesize isotactic as well as syndiotactic polypropylene in high configurational purity and high yields. The same holds for syndiotactic polystyrene. Furthermore, metallocene catalysts open the possibility to absolutely new homopolymers and copolymers like, e.g., cycloolefin copolymers (COG) and even (co)polymers of polar monomers.The simplest metallocene catalyst consists of two components. The first one is a n-complex (the actual metallocene) that can be bridged via a group X and therefore can become chiral ... 

Ring-opening metathesis polymerization of cycloolefins,93-99 a reaction of significant practical importance (see Section 12.3), is catalyzed by a number of well-defined transition-metal complexes. Alkylidene and metallacyclobutane... 

Coordination polymerisation via re complexes comprises polymerisation and copolymerisation processes with transition metal-based catalysts of unsaturated hydrocarbon monomers such as olefins [11-19], vinylaromatic monomers such as styrene [13, 20, 21], conjugated dienes [22-29], cycloolefins [30-39] and alkynes [39-45]. The coordination polymerisation of olefins concerns mostly ethylene, propylene and higher a-olefins [46], although polymerisation of cumulated diolefins (allenes) [47, 48], isomerisation 2, co-polymerisation of a-olefins [49], isomerisation 1,2-polymerisation of /i-olcfins [50, 51] and cyclopolymerisation of non-conjugated a, eo-diolefins [52, 53] are also included among coordination polymerisations involving re complex formation. 

The same group of coordination polymerisations in which alkene undergoes re complex formation with the metal atom includes the copolymerisation of ethylene, a-olefins, cycloolefins and styrene with carbon monoxide in the presence of transition metal-based catalysts [54-58], In this case, however, the carbon monoxide comonomer is complexed with the transition metal via the carbon atom. Coordination bond formation involves the overlapping of the carbon monoxide weakly antibonding and localised mostly at the carbon atom a orbital (electron pair at the carbon atom) with the unoccupied hybridised metal orbitals and the overlapping of the filled metal dz orbitals with the carbon monoxide re -antibonding orbital (re-donor re bond) [59], The carbon monoxide coordination with the transition metal is shown in Figure 2.2. 

Ever since their original discoveries, Ziegler Natta catalysts and Phillips catalysts have been used for both the homopolymerisation and the copolymerisation of olefins. Moreover, Ziegler-Natta catalysts also allowed the copolymerisation of olefins with vinylaromatic monomers, conjugated dienes and cycloolefins. Other coordination catalysts such as group 8 metal compounds, especially cationic Pd(II) complexes, enabled the alternating copolymerisation of olefins and carbon monoxide [2,29,30,37,43,46,241,448 450],... 

Preferred olefins in the polymerisation are one or more of ethylene, propylene, 1-butene, 2-butene, 1-hexene, 1-octene, 1-pentene, 1-tetradecene, norbornene and cyclopentene, with ethylene, propylene and cyclopentene. Other monomers that may be used with these catalysts (when it is a Pd(II) complex) to form copolymers with olefins and selected cycloolefins are carbon monoxide (CO) and vinyl ketones of the general formula H2C=CHC(0)R. Carbon monoxide forms alternating copolymers with the various olefins and cycloolefins. 

Analogously to ethylene-carbon monoxide copolymers, alternating copolymers between cycloolefins such as norbornene and carbon monoxide have been synthesised using cationic Pd(II) complexes modified by phosphorus ligands such as [Pd(MeCN)n(PPh)4 J[BF4]2( = 1,2,3) [27]. General requirements for the... 

Alternating copolymers of cycloolefins and carbon monoxide have also been obtained in the case of cyclopentene the copolymerisations were run using cationic Pd(II) complexes modified by an achiral l,3-bis(diphenylphosphino)-propane ligand or a chiral 2,4-bis(diphenylphosphino)pentane ligand. 

The current general understanding of the mechanism operating in cycloolefin metathesis polymerisation leads us towards the acceptance of the structure of active sites in systems with catalysts belonging to the aforesaid three major groups as one that alternates between metal carbene complexes and metallacyclobutanes. 

Although this view is oversimplified and borderline metal carbene complexes have been isolated, this approach is convenient for discussing the activity of metal carbene species in the ring-opening metathesis polymerisation of cycloolefins. Calculations have predicted [81,82] and recent results have shown [83] that, in some systems, metal alkylidene reactivity is competitive with metal carbene reactivity, i.e. olefin metathesis is competitive with olefin cyclopropanation. 

Electrophilic metal carbene complexes such as (CO)5W=C(Ph)OMe generally exhibit poor activity as catalysts for metathesis polymerisation, and higher temperatures are required to bring about the polymerisation of high-strained cycloolefins such as norbornene or cyclobutene [84,85], However, their activity can be enhanced by the addition of a Lewis acid such as TiCL into the polymerisation system [86]. Electrophilic complexes such as (CO)5W=CPh2 also generally exhibit poor activity but they are more active than those mentioned above and enable the polymerisation of various cycloolefins [87,88],... 

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