Ring-expansion metathesis

This reaction was first reported by Grubbs and his group [44], An ROM/CM sequence between cyclooctene and one of the unsaturated ketones of 66 is followed by 

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REMP, the acronym for ring expansion metathesis polymerisation is a special case of ROMP, where the growing polymer chain stays attached to the catalyst at both ends nntil a macrocycle is released. This requires that the active carbene be tethered... 

A system obeying such an equilibration scheme has been proposed by Grubbs and coworkers.It is the ring-expansion metathesis polymerization of cyclic olefins catalysed by a cyclic mthenium complex. The synthesis of cyclic polyoctenamers using ris-cyclooctene as the monomer is illustrated in Scheme 5. 

Boydston AJ, Xia Y, Komfield JA, Gorodetskaya lA, Grubbs RH. Cycbc mtheruum-alkybdene catalysts for ring-expansion metathesis polymerization. J Am Chem Soc. 2008 130 12775-12782. 

Xia Y, Boydston AJ, Yao Y, et al. Ring-expansion metathesis polymerization catalyst-dependent polymerization profiles. J Am Chem Soc. 2009 131 2670-2677. 

Scheme 12.16 Olefin ring-expansion metathesis of cy-clooctene using a cyclic Grubbs catalyst to produce cyclic poly(cyclooctenamer) [56].

Figure 12.1 Alternative ring-expansion metathesis catalysts employed during optimization studies [58].

Another most recent and successful example concerns the ring-expansion metathesis polymerization (REMP) of cycloolefins using cyclic carbene-ruthenium complex. " In this reaction, the cyclic olefin coordinates onto the ruthenium active center before insertion into the cyclic carbene ring. 

Cyclic bottlebrush polymers are uniquely accessible in high yields through ring-expansion metathesis polymerization (REMP) (Figure 8.14). For example, the alkyne-containing macrocycle 27 was modified with PEG substituents to generate cyclic bottlebrush copolymer 28, which exists as a toroid [141]. To date, REMP has yielded bottlebrushes that serve as proof of concept, and it will be interesting to see whether this toroidal shape makes a difference in their function. 

Boydston AJ, Holcombe TW, Unmh DA, Frechet JMJ, Gmbbs RH (2009) A direct route to cyclic organic nanostmctures via ring-expansion metathesis polymerization of a dendronized macromonomers. J Am Chem Soc 131 5388-5389... 

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