Selective Reactions of Alkenes
Diimide (H-N=N-H) is an effective reagent for reducing alkenes. Yasushi Imada and Takeshi Naota of Osaka University have devised (J. Am. Chem. Soc. 2005, 127, 14544. ) a flavin-catalyzed protocol for diimide generation by O2 oxidation of hydrazine. The selectivity of diimide as a reductant is illustrated by the smooth conversion of 1 to 2. Because these are oxidizing conditions, 1 can alternatively be converted directly to 3, by running the reaction in trifluoroethanol.
Many polyoxometalates have been developed that catalyze the hydrogen peroxide epoxidation of alkenes. Usually, these are simple electrophilic processes. Noritaka Mizuno of the University of Tokyo has now reported (Angew. Chem. Int. Ed. 2005, 44, 5136. ) a vanadium catalyst that gives preferential epoxidation of the less-substituted alkene.
The most general procedure for catalytic enantioselective epoxidation is that developed by Yian Shi of Colorado State University, using the Oxone-generated dioxiranes from 7 and 8. Ketone 7 usually is used at 30% catalyst loading, while the more robust ketone 8 serves effectively at 8-10% loading. A. Vidal-Ferran of the Institute of Chemical Research of Catalonia has developed (J. Org. Chem. 2005, 70, 10143. ) an efficient and scalable procedure for the preparation of 7 and of 8 from fructose.
Terminal vinyl groups are particularly good ligands for transition metals, and so can often react selectively even in the presence of more electron rich alkenes. Ronny Neumann of the Weizmann Institute has found (Org. Lett. 2005, 7, 5039. ) a Ru catalyst/H2O2 combination that selective cleaves terminal vinyl groups to the corresponding aldehydes. Note that the conditions are mild enough that the other alkene is not brought into conjugation.
Yung-Son Hon of National Chung Cheng University, Taiwan, who more than ten years ago introduced the reduction of ozonides with Et3N, has now (Tetrahedron Lett. 2005, 46, 1365. ) described the facile conversion of α-silyloxy aldehydes to the corresponding α-silyloxy ketones. The rearrangement works equally well with α-acyloxy aldehydes. The net transformation is the one-carbon homologation of the starting aldehyde.
In another illustration of the selective reactivity of terminal vinyl groups, Masanobu Uchiyama, also of the University of Tokyo, has found (J. Am. Chem. Soc. 2005, 127, 13116. ) that with the silyl zincate, Ti-catalyzed Heck type addition proceeds smoothly. Allyl silanes are useful nucleophiles for organic synthesis.
Terminal vinyl groups can also be converted into alkenyl nucleophiles. Timothy F. Jamison of MIT has shown (J. Am. Chem. Soc. 2005, 127, 14194. ) that in the presence of silyl triflates, Ni(cod)2 catalyzes the addition of a terminal vinyl group to an alkene. Again, an internal alkene does not interfere.