Alkyne Metathesis

During recent years, olefin metathesis has gained a position of increasing significance. In particular, ring-closing olefin metathesis (RCM) of dienes provides good access to various carbo- and heterocycles of ring sizes >4. Among the shortcomings that infringe upon the superb overall application profile of RCM in the synthesis of medium- and macrocyclic products, the lack of control over the stereo- chemistry of the newly formed double bond is most noteworthy. The products formed are usually obtained as mixtures of the (E)- and (Z)-isomer. This constitutes a significant drawback in target oriented synthesis, as can be seen from many examples reported in the literature.
To circumvent this problem, an indirect but stereoselective approach to macrocyclic (E)- and (Z)-alkenes has been proposed, which comprises a ring-closing metathesis reaction of diynes (RCAM) followed by semireduction of the resulting cycloalkyne product.

We have described an improved "instant" catalyst for ring-closing alkyne metathesis reaction. Catalyst formed in situ from molybdenum hexacarbonyl and 2-fluorophenol can be used without exclusion of air and moisture and shows high activity in metathesis of functionalized diynes.

The high activity of the 2-fluorophenol-based system can be clearly seen from the stereoselective synthesis of yuzu lactone, the minty, camphor-like odored macrolide isolated from the flesh and peel of the Japanese citrus tree Citrus junos Tanaka (Yuzu) [17].

With the help of some additives, this system can be also used in lower temperatures (80 °C) [27]. This improved catalysts has been used by Bunz et al. in synthesis of some twisted diphenylacetylenes via RCAM. Also in alkyne metathesis polymerization it has been recently found that "the most active and promising catalyst is the combination of acac₂MoO₂ with Grela's 2-fluorophenol".

· Brizius, G.; Billingsley, K.; Smith, M. D.; Bunz, U. H. F. Org. Lett. 2003, 5, 3951-3954. · Bly, R. K.; Dyke, K. M.; Bunz, U. H. F. J. Organomet. Chem. 2005, 690, 825-829.

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