Murphy JJ, Bastida D, Paria S, Fagnoni M, Melchiorre P * Barcelona Institute of Science
and Technology, Tarragona and Catalan Institution for Research and Advanced Studies,
Barcelona, Spain; University of Pavia, Italy
Asymmetric Catalytic Formation of Quaternary Carbons by Iminium Ion Trapping of Radicals.
Nature 2016;
532: 218-222
Key words
radical conjugated addition - quaternary carbon stereocenter - iminium ion trapping
- enantioselectivity - enones - photocatalysis
Significance
Melchiorre and co-workers report the first example of a radical conjugated addition
to β,β-disubstituted cyclic enones by a combination of photoredox and asymmetric organocatalysis
to give quaternary stereocenters with high enantioselectivity. This method exploits
iminium ion catalysis, previously applied only in polar chemistry, within the field
of radical reactivity.
Comment
The chiral organocatalyst has been elegantly designed with a redox-active carbazole
moiety that drives the formation of the iminium ion I. This reaction is followed by stereoselective trapping of the photochemically generated,
carbon-centered radical R•. The new radical species II leads, through an electron-relay mechanism, to III and, after tautomerization and reduction, the product is formed with high enantioselectivity.
