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Monday, March 24, 2008
Douglass F. Taber
University of Delaware

Reduction, Oxidation and Homologation of Alkenes

Alkenes are usually reduced by catalytic hydrogenation. Diimide reduction is a mild and neutral alternative. Keith R. Buszek, now at the University of Missouri, Kansas City, has shown (J. Org. Chem. 2007, 72, 3125. ) that the reduction can conveniently be carried out on resin-bound alkenes, using 2-NBSH (o-nitrobenzenesulfonylhydrazide) with Et3N for convenient room temperature diimide generation.

Ozone can be difficult to dispense accurately on small scale. Masahito Ochiai of the University of Tokushima has uncovered (J. Am. Chem. Soc. 2007, 129, 2772. ) an alternative, using acid-promoted Ph-I=O. Isolated alkenes also work well.

MCPBA is the reagent most commonly used for alkene epoxidation. Payne oxidation (H2O2 / CH3CN) is a convenient and inexpensive alternative. In the course of a study of the enantioselective enzymatic hydrolysis of 6, Takeshi Sugai of Keio University has described (Tetrahedron Lett. 2007, 48, 979. ) a practical procedure for multigram Payne epoxidation of 5.

Several procedures have been put forward for functionalizing terminal alkenes, exemplified by 7. Stefan Grimme and Armido Studer of the Universität Münster have developed (J. Am. Chem. Soc. 2007, 129, 4498. ) a free radical alkene amination, represented by the conversion of 7 to 9. Tehshik P. Yoon of the University of Wisconsin has found (J. Am. Chem. Soc. 2007, 129, 1866. ) that Cu catalyzes the addition of oxaziridines such as 10 to alkenes, to make 11. Shinji Nakamura of the University of Tokyo and Masanobu Uchiyama of the University of Tokyo and RIKEN have established (J. Am. Chem. Soc. 2007, 129, 28. ) that the anion from Cu promoted addition of the silyl zinc reagent to alkenes is long-lived enough to be trapped by electrophiles, including H+ to give 12. Hideki Yorimitsu and Koichiro Oshima of Kyoto University have developed (J. Am. Chem. Soc. 2007, 129, 6094. ) a complementary transformation, Ni-catalyzed addition of 13 to give 14. The conversion of 7 to 15 reported (Org. Lett. 2007, 9, 53. ) by Li-Biao Han of the National Institute of Advanced Industrial Science and Technology, Tsukuba, is likely also a free-radical process.

Several procedures have also been developed for homologating terminal alkenes, again exemplified by 7. The homologation can be either branching, as with 18 [John F. Hartwig, University of Illinois (J. Am. Chem. Soc. 2007, 129, 6690. )], 20 [Timothy F. Jamison, MIT (Org. Lett. 2007, 9, 875. )], and 21 [Timothy F. Jamison, (Angew. Chem. Int. Ed. 2007, 46, 782. )], or linear, as with 22 [Maurice Brookhart, University of North Carolina (J. Am. Chem. Soc. 2007, 129, 2082. )], 24 [David R. Liu, Harvard (J. Am. Chem. Soc. 2007, 129, 2230. )] and 26 [Samir Z. Zard, Ecole Polytechnique (Org. Lett. 2007, 9, 1773. )]. The Liu procedure can also be used to prepare 1,5-dicarbonyl compounds. The Zard protocol is particularly noteworthy, as the product chloroketone 26 can be carried on to the xanthate, which can be added to another alkene, so 25 serves as a linchpin reagent.

D. F. Taber, Org. Chem. Highlights 2008, March 24.