Selective Reactions of Alkenes
Peter W. Roesky of the Freie Universitšt Berlin has devised (Chem. Commun. 2006, 874. ) a La catalyst that effects hydrosilylation of a terminal alkene 1 to 2. Sjoerd Harder of the Universitšt Duisberg-Essen has developed (Angew. Chem. Int. Ed. 2006, 45, 2741. ) alternative Ca and K catalysts for hydrosilylation. He notes that 3 is converted cleanly into 4, an outcome that is complementary to hydroboration. The product silanes are easily oxidized to the corresponding alcohols.
Another alternative to hydroboration is epoxidation followed by selective reduction of the more substituted end of the epoxide. Juan M. Cuerva and J. Enrique Oltra of Universidad de Granada and Diego J. CŠrdenas of the Universidad Autůnoma de Madrid have found (Angew. Chem. Int. Ed. 2006, 45, 5522. ) that the Cp2Ti reagent prepared by stirring Cp2TiCl2 with Mn dust in THF works well, and that water is a particularly effective additive. Note that both diastereomers of the epoxide 5 are converted to the same axial (equatorial H delivery) product 6.
Phosphonium salts, used to prepare alkenes, are commonly prepared by displacement of the corresponding halide. Masahiko Yamaguchi of Tohoku University has shown (J. Am. Chem. Soc. 2006, 128, 50. ) that phosphonium salts can also be prepared from alkenes (7 → 8). Both internal alkenes and terminal alkenes are converted into the terminal phosphonium salt.
Alkenes can also be homologated catalytically. Jonathan A. Ellman and Robert G. Bergman of the University of California at Berkeley have found (J. Org. Chem. 2006, 71, 1969. ) that Rh catalyzes the addition of NH-3,4-dihydroquinazoline 9 to terminal alkenes such as 1. The product 10 is valuable in itself, and it can also be hydrolyzed to the carboxylic acid, for a net one-carbon homologation.
Timothy F. Jamison of MIT has developed (J. Am. Chem. Soc. 2006, 128, 5362. ) Ni catalysts for the specific ene reaction of monosubstituted alkenes. Thus, with the Ni catalyst, the diene 11 reacts with benzaldehyde 12 to give 14. With a conventional Lewis acid catalyst, 11 is converted to 13. Both aromatic and t-alkyl aldehydes work well in the Ni reaction.
It has been known for some time that alkenes can be homologated via the corresponding organoboranes, but such procedures are in fact little used. Philippe Renaud of the Universitšt Bern has put forward (Angew. Chem. Int. Ed. 2006, 45, 5847. ) a simple protocol that makes it easier to carry out such alkene homologations. Dimethylacetamide-catalyzed hydroboration of 15 with catecholborane gave the initial borane 16, which was not isolated, but was carried on directly to 17 - 20.