Norbornene complexes, ring-opening metathesis

In the case of other Group 6 metals, the polymerization of olefins has attracted little attention. Some molybdenum(VI) and tungsten(VI) complexes containing bulky imido- and alkoxo-ligands have been mainly used for metathesis reactions and the ring-opening metathesis polymerization (ROMP) of norbornene or related olefins [266-268]. Tris(butadiene) complexes of molybdenum ) and tungsten(O) are air-stable and sublimable above 100°C [269,270]. At elevated temperature, they showed catalytic activity for the polymerization of ethylene [271]. 

Ring-Opening Metathesis Polymerization (ROMP) of Norbornene by a Group VIII Carbene Complex in Protic Media, S.T. Nguyen, L.K. Johnson, R.H. Grubbs, et al, J. Am. Chem. Soc. 1992, 114, 3974-3975. 

With the discovery of ruthenium carbene complexes as highly effective catalysts for olefin metathesis under mild reaction conditions [233,234], the scope of ring-opening metathesis polymerization could be extended to include functionalized and sensitive monomers. The resulting (soluble) polymers have been used as supports for simple synthetic transformations [235-237]. Insoluble polymers have been prepared by ringopening metathesis copolymerization of norbornene with l,4,4a,5,8,8a-hexahydro-1,4,5,8-exo-endo-dimethanonaphthalene. These polymers have been used as supports for ruthenium carbene complexes [238]. 

Rhenium complex (37) has been used in the ring-opening metathesis polymerization of strained alkenes such as norbornene.37 The alkenes of the polymer backbone are predominantly Z, the polymer exhibiting high molecular weight and poly-dispersity. 

Ring-opening metathesis of the substrate 5-vinylnorborn-2-ene, on the other hand, could not be reversed under those experimental conditions. As had been suggested much earlier [133-135], strain release in the substrate is not the principal determinant in the several orders-of-magnitude difference in apparent ROMP reactivity of cyclohexene vs norbornene. Instead, the easy intramolecular 7i-complexation and concomitant ring-closing metathesis are the controlling factors. 

Complex 17 shows versatile reactivity in both catalytic and stoichiometric reactions. It is a catalyst for olefin metathesis, in which a metallacyclobutane is proposed to be a key intermediate [52]. Grubbs et al. successfully isolated titanacyclobutanes 18 by treatment of 17 with alkenes in the presence of a Lewis base such as dimethylaminopy-ridine [56,57]. When cyclic olefins such as norbornene are treated with a catalytic amount of 18, ring opening metathesis polymerization occurs [58]. 

The ring opening metathesis polymerization (ROMP) of norbornene by the OsO. complex has been reported recently [261]. The key intermediate is considered to be an oxaosmacyclobutane whose C—C and Os—O bonds can open to form an osmium alkylidene species (Scheme 10.30). 

Matyjaszewski et al. demonstrated that living ring opening metathesis polymerization (ROMP) could also be combined with ATRP to produce novel block copolymers [292]. ROMP of norbornene (NB) and dicyclopentadiene (CPD) were performed using an Mo-alkylidene complex, followed by reaction with p-(bro-momethyl) benzaldehyde to generate a benzyl bromide terminated polymer capable of being used as a macroinitiator for ATRP (Scheme 41). 

This reactivity was confirmed by mass-selecting 11 in a first mass-selection stage and subsequent reaction with 1-butene. The only metathesis product observed was 12. With cyclobutene or norbornene as substrates, the complexes 13 and 14 were formed predominantly as a result of ring-opening metathesis (ROM). 

Mashima, K. Kaidzu, M. Tanaka, Y. Nakayama, Y Nakamura, A. Hamilton, J. G Rooney, J. J. Control of stereoselectivity in the ring-opening metathesis polymerization of norbornene by the auxiliary ligands butadiene and o-xylylene in well-defined pentamethylcyclopentadiene tantalum carbene complexes. Organometallics 1998,17,4183 195. 

We have reported that norbornene monomers functionalized with arene complexes also undergo ring-opening metathesis polymerization to produce the corresponding cyclopentadienyliron-coordinated polynorbomenes. Scheme 13 shows the synthesis of polymers containing aromatic ether sidechains functionalized with organoiron moieties. ... 

Instead of a conventional polymerization reaction, Demonceau et al. [65] observed ring-opening metathesis polymerization of norbornene, which was initialized by ethyl diazoacetate or trimethyl-silyldiazomethane (Scheme 22.18). The used ruthenium complexes 62 and 63 gave moderate yields of the polymer (Figure 22.22, Table 22.27). 

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