Developments in Alkene and Alkyne Metathesis
Jon D. Rainier of the University of Utah has put forward (J. Am. Chem. Soc. 2007, 129, 12604. ) an elegant alternative to Ru-catalyzed alkene metathesis, demonstrating that an ω-alkenyl ester such as 1 will cyclize to the enol ether 2 under Tebbe conditions.
The particular reactivity of free alcohols in Ru-catalyzed alkene metathesis is underscored by the observation (Tetrahedron Lett. 2007, 48, 6905. ) by Javed Iqbal of Dr. Reddy’s Laboratories, Ltd., Miyapur that attempted metathesis of the ether 4a failed, but metathesis of the diol 4b proceeded efficiently. Kazunori Koide of the University of Pittsburgh has demonstrated (Org. Lett. 2007, 9, 5235. ) that the yields of cross-metathesis with an alkenyl alcohol could be enhanced by binding it to a trityl resin. He observed that the Grela catalyst 8 was particularly effective in this application.
Residual Ru species do not interfere with some subsequent transformations. Rodrigo B. Andrade of Temple University has demonstrated (Tetrahedron Lett. 2007, 48, 5367. ) that metathesis with an α,β-unsaturated aldehyde such as 11 can be followed directly by phosphonate condensation to give the doubly-homologated product 12. Philip J. Parsons of the University of Sussex has found (Org. Lett. 2007, 9, 2613. ) that the nitro functional group is compatible with the Ru catalyst. The product nitro alkene 15 could be cyclized (intramolecular Michael addition) to the cyclopentane 16, or (intramolecular dipolar cycloaddition) to the cyclopentane 17.
There has been much interest in carrying out the several alkene metathesis transformations (cross metathesis, ring closing metathesis, ring-opening metathesis polymerization) in water. Robert H. Grubbs of the California Institute of Technology has designed (Angew. Chem. Int. Ed. 2007, 46, 5152. ) the ammonium salt 18 for this purpose, and Karol Grela of the Polish Academy of Sciences in Warsaw and Marc Mauduit of ENSC Rennes have jointly (Chem. Commun. 2007, 3771. ) put forward the pyridinium salt 19. Remarkably, Ronald T. Raines of the University of Wisconsin has shown (Org. Lett. 2007, 9, 4885. ) that the Hoveyda catalyst 20 is sufficiently stable in aqueous acetone and aqueous DME to function efficiently.
Although alkyne metathesis is now well developed as a synthetic method, it has been limited to internal alkynes. For this reason, methyl alkynes are often specially prepared to set the stage for C-C triple bond formation by metathesis. André Mortreux of ENSC-Lille has now designed (Adv. Synth. Catal. 2007, 349, 2259. ) a binuclear catalyst 22 that works well with terminal alkynes such as 21. What remains to be determined is the functional group compatibility of 22.