Ring opening alkene metathesis

E2S.il (a) Ring opening alkene metathesis polymerization. ROMP can result in reduced steric strain, thereby providing a thermodynamic driving force for the reaction. 

The resulting carbene complex 41b bears a hetero substituent and shows activity in the ring-opening/cross metathesis of strained bicyclic alkenes and... 

Fig. la—d Typical alkene metathesis reactions ring-closing (RCM) and ring-opening (ROM) metathesis (a), diene cross metathesis (CM, b), ROM-RCM (c), and ROM-double RCM (d) sequences (ring-rearrangement reactions, RRM)... 

The significant potential of the ruthenium complex 65 was further underlined in the catalytic asymmetric ring-opening/cross metathesis of the cyclic alkene 70 (Scheme 44). This transformation is catalyzed by 5% mol of 65 at room temperature, in air, and with undistilled and nondegassed THF to deliver the corresponding diene 71 in 96% ee and 66% isolated yield. In standard conditions (distilled and degassed THF), the alkene 70 reacts in 75 min to give the diene in 95% ee and 76% yield, with only 0.5 mol % of catalyst. 

Keywords Cross-metathesis, Ring-opening, Alkenes, Catalysis, Dienes... 

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). 

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). 

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... 

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). 

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). 

As another example, studies of the catalytic activity of the gas-phase ions [Ru-Cl(alkylidene)(diphosphane)]+ toward ring-opening olefin metathesis of norbor-nene show that an alkene group in the growing polymer chain reaches back to oc-... 

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. 

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... 

A ruthenium-catalyzed ring opening cross-metathesis of 8-oxabicyclo[3.2.1]oct-6-en-3-one 1049 with alkenes provides an efficient method for the preparation of substituted tetrahydropyran-4-ones 1050 (Equation 408, Table 50) <1999X8169, 20010L4275>. Similarly, ozone can be used to cleave the same ring system during the synthesis of chiral tetrahydropyran-4-ones <2006T257>. 

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. 

Disubstituted cyclopropenes undergo ring-opening cross-metathesis with certain alkenes under the influence of 6 to produce (3S)-1,4-alkadienes. ... 

Ring-opening cross-metathesis. The metathesis between a cyclobutene and a terminal alkene leads only to a cross product. This observation is helpful for the design of short syntheses of multifidene and viridiene. ... 

Application of RCM to acrylates derived from terpene aldehydes containing a remote double bond yielded the 6-substituted dihydropyran-2-ones <01TL6069> and oxabicyclo-[3.2.1]octane undergoes an efficient ring-opening cross-metathesis with electron-rich alkenes to give unsymmetrically 2,6-disubstituted tetrahydropyran-4-ones <01OL4275>. 

Scheme 7.3 Gibbs free-energy profile of the ring-opening cross-metathesis of norbornene with alkenes. The free energies were calculated at the B3LYP/LANL2DZ level of theory, and are in kcal mol [37].

Scheme 7.19 E- and Z-selective asymmetric ring-opening cross-metathesis of oxabicyclic alkenes with enol ethers and aryl olefins. From Ref [81].

As one of the earliest demonstrations of the synthetic utility of selective ring-opening cross-metathesis (ROCM) reactions. Snapper and co-workers reported the ROCM of substituted cyclobutene and terminal olefins to yield 1,5-diene products catalyzed by Grubbs first generation catalyst in 1995 (Fig. 20) [46]. Although the selectivity was only moderate (generally 2 1 Z E for six examples reported), the bias towards the formation of Z-alkene represents a rare example. A model was proposed to account for the selectivity, where steric interactions between the substrate and the phosphine ligand are minimized [47]. 

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