Ring-opening metathesis enynes

In recent years, in addition to the ring-opening metathesis (ROM) and ring-closing metathesis (RCM), the enyne metathesis and the cross-metathesis (CM) have... 

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]. 

The olefin cross metathesis (CM) can be described as the intermolecular metathesis of alkylidene fragments between two different olefins [133]. It can be farther divided into three main subtypes cross metathesis, ring opening cross metathesis (ROCM) and enyne cross metathesis (ECM) (Scheme 3.9). 

Trost and Tanoury found an interesting skeletal reorganization of enynes using a palladium catalyst.In this reaction, the second product is derived from a metathesis reaction (Equation (5)). It was speculated that the reaction would proceed by oxidative cyclization of enynes with the palladium complex followed by reductive elimination and then ring opening. To confirm this reaction mechanism, they obtained a compound having a cyclobutene ring, which was considered to be formed by the reductive elimination (Equation (6)). 

Kitamura, T., Mori, M. Ruthenium-catalyzed ring-opening and ring-closing enyne metathesis. Org. Lett. 2001, 3, 1161-1163. 

Shortly after the discovery of enyne metathesis, Trost began developing cycloisomerization reactions of enynes using Pd(ll) and Pt(ll) metallacyclic catalysts (429-433), which are mechanistically divergent from the metal-carbene reactions. The first of these metal catalyzed cycloisomerization reactions of 1,6-enynes appeared in 1985 (434). The reaction mechanism is proposed to involve initial enyne n complexation of the metal catalyst, which in this case is a cyclometalated Pd(II) cyclopentadiene, followed by oxidative cyclometala-tion of the enyne to form a tetradentate, putative Pd(IV) intermediate [Scheme 42(a)]. Subsequent reductive elimination of the cyclometalated catalyst releases a cyclobutene that rings opens to the 1,3-diene product. Although this scheme represents the fundamental mechanism for enyne metathesis and is useful in the synthesis of complex 1,3-cyclic dienes [Scheme 42(fe)], variations in the reaction pathway due to selective n complexation or alternative cyclobutene reactivity (e.g., isomerization, p-hydride elimination, path 2, Scheme 40) leads to variability in the reaction products. Strong evidence for intermediacy of cyclobutene species derives from the stereospecificity of the reaction. Alkene... 

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... 

Totally intramolecular enyne metathesis Diels-Alder sequences have been demonstrated (e.g., the conversion of 244 into 246). Enyne RCM of substrate 247 affords stereochemically pure 248 due to kinetic resolution during metathesis.A process equivalent to enyne metathesis that proceeds through a non-carbene mechanism has been demonstrated using platinum chloride as a catalyst. In addition to routine observations, some other interesting side-reactions have been noted (Scheme 29). Competitive cross-metathesis with ethylene and intramolecular enyne metathesis were observed in the treatment of substrate 249 with catalyst 16 and ethylene.Cyclopropane ring opening was observed in the attempted enyne metathesis of substrate 253, resulting in alkylidenecyclopentene 254. 

Coupling an ROM event with enyne metathesis opens up further possibilities for RRM. As already discussed for reactions involving only olefins, norbomenes are valuable strained cycloolefins for the ring-opening step [22], Treatment of the norbomene derivative 123 with the first-generation Grubbs catalyst 1 under ethylene atmosphere induced a smooth domino ROM/enyne RCM/diene RCM process with quantitative formation of the tricyclic compound 124 (Scheme 2.45) [22b]. 

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