Ahlke Hayen et.al.: 1,3-Stereoinduction in Radical Reactions: Radical
Additions to Dialkyl 2-Alkyl-4-methyleneglutarates
- Author(s): Ahlke
Hayen, Rainer Koch,
Wolfgang Saak, Detlef Haase, Juergen O. Metzger
- Title: 1,3-Stereoinduction in Radical Reactions: Radical Additions to
Dialkyl 2-Alkyl-4-methyleneglutarates
- Date of Upload: 2000
- Article: J. Am.
Chem. Soc. 2000, 122, 12458-12468, Contribution from the Department of
Chemistry, UniVersity of Oldenburg, P.O. Box 2503, 26111 Oldenburg,
Germany
- Keywords: radical reactions, selectivity, stereoselectivity,
enentioselectivity,
- Abstract:
- Tin hydride-mediated radical additions to a series of
R-methylene-glutarates 1, furnishing 2,4-dialkylsubstituted glutarates 3 are
reported. The diastereoselectivity of hydrogen transfer to the intermediate
adduct radicals 2, possessing a stereogenic center in c-position, was
disappointing in the temperature range of -78 to 80 °C. However, the reactions
proved to be able to proceed with excellent 1,3-diastereoselectivities under
chelation-controlled conditions, depending on the steric impacts of 2- and
4-alkyl substituents as well as on the ester-alkyl moiety and choice of Lewis
acid. Using MgBr2aOEt2 as additive, syn-selectivities of 98:2 were achieved
upon initial tert-butyl radical addition at -78 °C. High
anti-diastereoselectivities were observed in the MgBr2aOEt2-controlled pathway
at 70 °C when smaller alkyl radicals such as cyclohexyl, ethyl, and methyl
were applied. Interesting and uncommon temperature dependences were observed
in the temperature range of -78 to 100 °C, revealing strong entropic effects
in the transition states. A model that accounts for the opposed stereochemical
outcomes under chelation-controlled conditions is presented.
- URL: http://www.chemie.uni-oldenburg.de/oc/metzger/publikationen/pdf/89.pdf
(application/pdf)
This shadow-file was created by My Meta Maker
1.6.1. Last Update: Wed Jun 16 12:31:18 2004