Strategies for Alkene Metathesis

Amir H. Hoveyda of Boston College has reported (J. Am. Chem. Soc. 2005,127, 8526) the development of a family of chiral Mo metathesis catalysts that convert prochiral dienes such as 5 and 8 

Several years ago, Professor Hoveyda designed the chelated Ru complex 12a as a versatile and stable metathesis catalyst. Dennis P. Curran of the University of Pittsburgh has now introduced (J. Org. Chem. 2005, 70, 1636) the fluorous-tagged Ru catalyst 12b. The fluorous tag allows the facile recovery of most of the active catalyst. The advantages of this are two-fold the valuable catalyst can be re-used, and there will potentially be less Ru contamination in the cyclized product. 

Congratulations to the recipients of the 2005 Nobel Prize in Chemistry The five Organic Highlights columns for the month of October will be devoted to organic synthesis applications of alkene metathesis. 

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This method is specific for metallacyclopentanes. The alkene-coupling process is favored by metal reduction. A typical synthetic strategy is the in situ reduction of a metal halide precursor in the presence of the alkene see, for example, the synthesis of 79 in Scheme 34.1 An alkylidene precursor may also lead to a metallacycle with elimination of the car-bene ligand as in the synthesis of 81, representing a deactivation pathway for alkene metathesis catalysts. Ilie two alkenes may be generated in situ in the coordination sphere by rearrangement processes, such as intramolecular hydrogen transfer from an alkyl-vinyl precursor. I ... 

Figure 3.38. Mechanism of olefin metathesis and strategies for the cleavage of alkenes from polymeric supports by olefin metathesis.

The main strategies used for the preparation of alkenes by cleavage from insoluble supports are (3-elimination and olefin metathesis (Figure 3.35). Because some of these strategies enable the preparation of pure alkenes, devoid of additional functional groups, the linkers are sometimes also called traceless linkers, although the C=C double bond reveals the original point of attachment to the support. 

For the cleavage of alkenes from a support by metathesis, several strategies can be envisaged. In most of the examples reported to date, ring-closing metathesis of resin-bound dienes has been used to release either a cycloalkene or an acyclic alkene into solution (Figure 3.38, Table 3.44). Further metathesis of the products in solution occurs only to a small extent when the initially released products are internal alkenes, because these normally react more slowly with the catalytically active carbene complex than terminal alkenes. If, however, terminal alkenes are to be prepared, selfmetathesis of the product (to yield ethene and a symmetrically disubstituted ethene) is likely to become a serious side reaction. This side reaction can be suppressed by conducting the metathesis reaction in the presence of ethene [782,783]. 

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