Ring-opening cross-metathesis reaction

The ring-opening cross-metathesis reaction is similar to the acyclic cross-metathesis reaction discussed above, except that one of the acyclic alkenes is replaced with a strained cyclic alkene (Scheme 5). 

In 1995 the first examples of ring-opening cross-metathesis reactions for the preparation of functionalised monomeric products using the Grubbs ruthenium vinylalkylidene catalyst 4 were published by Snapper and co-workers [47]. Reaction of a variety of symmetrical cyclobutenes with simple terminal alkenes... 

A subsequent publication by Blechert and co-workers demonstrated that the molybdenum alkylidene 3 and the ruthenium benzylidene 17 were also active catalysts for ring-opening cross-metathesis reactions [50]. Norbornene and 7-oxanorbornene derivatives underwent selective ring-opening cross-metathesis with a variety of terminal acyclic alkenes including acrylonitrile, an allylsilane, an allyl stannane and allyl cyanide (for example Eq. 34). 

Facile, regioselective ring opening-cross-metathesis reactions between unsymmet-rical norbornene derivatives and electron-rich alkenes in the presence of the second-generation Grubbs catalyst have been reported to generate highly substituted furans and pyrroles.114... 

The water-soluble complex [RuCl(triphenylphosphine monosulfonate) (=C=CHPh)][Na]2 (L3) made possible the following cascade ring-opening/ cross-metathesis reaction (Eq.9) [72]. 

For a related discussion for ring-opening cross metathesis reactions, see Bencze,... 

Fig. 40 Z- and enantioselective ring-opening/cross-metathesis reactions by Hoveyda et al.

The report by Basset and co-workers on the metathesis of sulphur-containing alkenes using a tungsten alkylidene complex, mentioned previously for the acyclic cross-metathesis reaction (see Sect. 2.2), also contained early examples of ring-opening cross-metathesis of functionalised alkenes [20]. Allyl methyl sulphide was reacted with norbornene in the presence of the tungsten catalyst 5, to yield the desired ring-opened diene 35 (Eq. 29). 

Successful ring-opening cross-metathesis with symmetrical internal acyclic alkenes was, however, achieved by Blechert and Schneider [49]. Reaction of a variety of functionalised norbornene derivatives with fraws-hex-3-ene in the presence of the ruthenium vinylalkylidene catalyst 4 yielded the ring-opened products as predominantly trans-trans isomers (for example Eq. 33). 

One special case of cross metathesis is ring-opening cross metathesis. When strained, cyclic alkenes (but not cyclopropenes [818]) are treated with a catalytically active carbene complex in the presence of an alkene, no ROMP but only the formation of monomeric cross-metathesis product is observed [818,937], The reaction, which works best with terminal alkenes, must be interrupted when the strained cycloalkene is consumed, to avoid further equilibration. As illustrated by the examples in Table 3.22, high yields and regioselectivities can be achieved with this interesting methodology. 

Ring-opening cross metathesis. Significant reactions belonging to this category are the formation of the cyclic acetals of l,4-alkadien-2-ones from cyclopropenone acetals and 1-alkenes, and the assemblage of 1,2-dialkenylcyclobutanes en route to complex 1,5-cyclooctadienes. 

Further studies of the regioselective tandem ring opening/cross metathesis of 2-emlo-substituted 7-oxanorbomenes with electron-rich olefins, a process first described in 2004, were reported. Reaction of the 2-exo isomer, like that of the 2-endo isomer, was also found to... 

Fig. 4.13. Examples of enyne metathesis reactions, ring opening-cross metathesis cascades, and other metathesis sequences.

More complex small molecules can also be made by metathesis cascades and tandem reaction sequences involving olehn metathesis components [41], The examples illustrated in Fig. 4.13 include inter- and intramolecular enyne metathesis between an olefin and an alkyne [42], ring-opening cross metathesis to form new substituted acyclic olefins [43], ring-opening ring-closing sequences... 

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