Ring-closing enyne metathesis reaction

Closely related to the ring-closing metathesis of enynes (Section 3.2.5.6), catalyzed by non-heteroatom-substituted carbene complexes, is the reaction of stoichiometric amounts of Fischer-type carbene complexes with enynes [266,308 -315] (for catalytic reactions, see [316]). In this reaction [2 + 2] cycloaddition of the carbene complex and the alkyne followed by [2 -t- 2] cycloreversion leads to the intermediate formation of a non-heteroatom-substituted, electrophilic carbene complex. This intermediate, unlike the corresponding nucleophilic carbene... 

Isotopic labelling experiments (2H and 13C) of Ru-catalysed enyne ring-closing metathesis reactions involving enynes (69) and (70) give results that are not compatible with the commonly proposed yne-then-ene mechanism and are explained by the alternative ene-then-yne mechanism.145... 

The synthesis of Az-carbacephems has been accomplished by ring-closing metathesis reaction of monocyclic diene- and enyne-/3-lactams <1998JOC7893, 1999J(PI) 1695>. The enyne metathesis of compounds 190 afforded bicycles 191 in good yields (Equation 13). 

In both reactions of scheme 10.51, the thermal instability of Ru(II)-catalyst systems during prolonged heating at 88 °C in ring-closing metathesis reactions of di-enyne and triyne substrates has been circumvented by use of a microwave-induced temperature increase to 160 °C when the reaction times were reduced to minutes (Scheme 10.51) [105]. 

The enyne cross metathesis was first developed in 1997 [170,171]. Compared to CM it benefits from its inherent cross-selectivity and in theory it is atom economical, though in reality the aUcene cross-partner is usually added in excess. The inabihty to control product stereochemistry of ECM reactions is the main weakness of the method. ECM reactions are often directly combined with other transformations like cyclopropanation [172], Diels-Alder reactions [173], cychsations [174] or ring closing metathesis [175]. 

Ring-closing metathesis of an enyne, which has double and triple bonds in the molecule, is a remarkable reaction which is useful in synthetic organic chemistry. In enyne metathesis, the double bond is cleaved and carbon-carbon bond formation occurs between the double and triple bonds. The cleaved alkylidene part is moved to the alkyne carbon. Thus, the cyclized compound formed in this reaction has a diene moiety [Eq. (6.77)]. The reaction is also called skeletal rearrangement and is induced by Pt, Pd, Ga, and Ru catalysts ... 

Enynes without the cycloalkene moiety can also react with electron-deficient alkenes by a cascade ring-closing metathesis-cross metathesis (RCM-CM) process [23] (Scheme 10). The use of Hoveyda s catalyst is necessary, not to stop the reaction at the RCM step, but to perform the subsequent CM step. Indeed, the organic product arising from the RCM is first formed and then reacts with the alkene in the presence of the ruthenium complex to give the CM reaction. 

A catalytic tandem cyclopropanation-ring-closing metathesis of dienyne 80 led to derivative 81 in good yield (Scheme 30 <2004JA9524>). For internal alkynes, carbene-mediated ring-closing enyne metathesis was observed. Less favorable alkyne binding leads to preferential reactions of the metal carbene with the 1-alkene moiety. 

Highly strained cyclic compounds containing an alkyne group can be prepared upon complexation to Co2(CO)6. The success of these reactions relies on the severely bent nature of the alkyne triple bond when complexed to cobalt. Cyclization of (137) to form bicyclo[7.3.0]tridecane (138), a calichiamycinone derivative has been reported (Scheme 204). The nine-membered enyne complex (140) was prepared by ring-closing metathesis of (139) (Scheme 205). A related reaction was used to give complex (141) (Scheme 206). It should be noted that no cyclization occurs without prior complexation to cobalt in the latter case. A complexation-initiated... 

The synthesis of heterocycles via the olefin metathesis reaction (RCM, ring closing metathesis, section 2) and via the cycloisomerization of dienes, diynes, and enynes (section 3) belongs to category 1... 

Z Intramolecular Reaction of 1,co-Dienes and Enynes Ring-Closing Metathesis (RCM)... 

A branching pathway based on the chemistry of Michael adducts (7) was developed by Porco (Scheme 27.2) [47]. Reduction of the nitro group triggered lactamization to yield y-lactams such as 8. In contrast, with appropriately positioned alkenyl and alkynyl substituents, cyclization via ring-closing metathesis or Pauson-Khand reaction was possible. With =allyl and R =C=CCH20Me, enyne metathesis yielded the cyclic diene 9. In contrast, with R =C=CH and R = allyl, a Pauson-Khand reaction allowed the remarkable bridged cyclopentenone 10 to be obtained. 

Many ruthenium-carbene five-membered ring compounds have recently been reported to show good activity for metathesis [51-71]. Many types of reactions, such as ring-closing metathesis, ring-opening metathesis, ctoss metathesis, enyne metathesis, and diyne metathesis, proceed with the help of these catalysts, as shown in Eqs. (8.15), (8.16), (8.17), (8.18), and (8.19) [58]. 

---