Selective Reactions of Alkenes
Gentle tapping of a terminal alkene specifically to the corresponding methyl-substituted internal alkene is a potentially value-adding transformation, as illustrated by the conversion of the inexpensive allylated ketone 1 to the propenylated ketone 2, that would be difficult to prepare by other means. The procedure developed (Org. Lett. 2006, 8, 5481. ) by Stephen Hanessian of the Université de Montréal is particularly appealing, as many labs already have the second-generation Grubbs catalyst. The mildness of the protocol is underscored by the observation that the isomerization stops at 2, not going on to the much more stable 3.
Allylic oxidation, to convert an alkene into the enone, has usually been effected using stoichiometric chromium. Tony K. M. Shing of the Chinese University of Hong Kong has described (Org. Lett. 2006, 8, 3149. ) a promising alternative, using catalytic Mn(OAc)3 with t-butylhydroperoxide.
Chuan He of the University of Chicago has shown (Org. Lett. 2006, 8, 4175. ) that triflic acid is an effective catalyst for alkene hydroamination and hydroetherification. The reaction works both intermolecularly (illustrated, 6 → 7) and intramolecularly.
The catalytic epoxidation of a terminal alkene with high ee has been a long-sought goal. Giorgio Strukul of the Università di Venezia recently (J. Am. Chem. Soc. 2006, 128, 14006. ) reported a solution to this problem, using a chiral Pt cation catalyst and hydrogen peroxide. If an epoxide of even higher ee were required, it would be possible to polish these products following the Jacobsen procedure, selectively hydrolyzing away the minor enantiomer.
Subrata Ghosh of the Indian Association for the Cultivation of Science, Kolkota has observed (Org. Lett. 2006, 8, 3781. ) remarkable regioselectivity in the Ti(III)-mediated reduction of the epoxide 10. This selectivity may be due to steric interactions between the bulky titanocene and the endo silyloxymethyl group. The overall conversion of the alkene precursor to 11 is stereochemically complementary to the results expected from hydroboration.
Much progress has been reported on the specific homologation of terminal alkenes. Kay Severin of the École Polytechnique Fédéral de Lausanne has developed (J. Am. Chem. Soc. 2006, 128, 7440. ) Ru catalysts for the free radical homologation of alkenes, with CCl4, to give 13, or with p-tosyl chloride. Howard Alper of the University of Ottawa has optimized (Org. Lett. 2006, 8, 6143. ) conditions for converting alkenes to esters such as 14. Marc L. Snapper of Boston College has found (Org. Lett. 2006, 8, 2603. ) that alkene metathesis to form secondary allylic alcohols can be followed by alkene migration, to give ketones such as 15. Andrew S. Weller and Michael C. Willis of the University of Bath (Angew. Chem. Int. Ed. 2006, 45, 7618. ) and Chul-Ho Jun of Yonsei University, Seoul (Org. Lett. 2006, 8, 2937. ) have improved procedures for alkene hydroacylation, giving 16 and 17 respectively. Andreas Kirschning of the Universität Hannover has shown (Org. Lett. 2006, 8, 135. ) that Heck conditions work well to couple a simple terminal alkene with an iodoalkene, to give dienes such as 18. Finally, the C-H activation procedure developed (J. Am. Chem. Soc. 2006, 128, 5604. ) by Robert G. Bergman and Jonathan A. Ellman of the University of California, Berkeley leads initially to the Z aldehyde 19. This can be equilibrated under mild conditions to the more stable E isomer 20.