Free-Radical Telomerization of Chiral Acrylamides: Control of Stereochemistry in Additions and Halogen-Atom Transfer

The free-radical reactions of carbon radicals substituted α to a chiral pyrrolidine amide have been studied. The pyrrolidine used was 2,5-dimethylpyrrolidine, available as either the R,R or S,S enantiomer from d- or l-alanine, and the radicals were generated (1) by tin radical abstraction of the halogen from the 2-iodobutyramide of the pyrrolidine, (2) by decomposition of a pyrrolidine amide substituted Barton ester 4, and (3) by radical addition to the acrylamide of the pyrrolidine 3. Addition of chiral α amide radicals to alkenes occurs with a selectivity of ∼15:1 at room temperature, while bromine-and iodine-atom transfer to these radicals occurs with a selectivity of 10:1 at room temperature. Telomerization of the acrylamide 3 is achieved by photolysis of BrCCl3 in the presence of acrylamide. Telomers are readily formed and the lower telomers, n = 1 to 5, were isolated and characterized. Halogens were removed from the telomers by reduction with tributyltin hydride and in this way, the major diastereomers of the n = 2 and n = 3 telomers were converted to known compounds. Chain-transfer constants for the BrCCl3 telomerization were determined and they range from 0.3 to 0.5 for Cn = C2 to C5. An analysis of the chiral amide auxiliary is presented that may prove useful in the consideration of other auxiliary groups. © 1991, American Chemical Society. All rights reserved.

Duke Scholars

Author Andrew T. McPhail Chemistry