Urea as a Redox-Active Directing Group under Asymmetric Photocatalysis of Iridium-Chiral Borate Ion Pairs

The
development of a photoinduced, highly diastereo- and enantioselective
[3 + 2]-cycloaddition of N-cyclopropylurea with α-alkylstyrenes
is reported. This asymmetric radical cycloaddition relies on the strategic
placement of urea on cyclopropylamine as a redox-active directing
group (DG) with anion-binding ability and the use of an ion pair,
comprising an iridium polypyridyl complex and a weakly coordinating
chiral borate ion, as a photocatalyst. The structure of the anion
component of the catalyst governs reactivity, and pertinent structural
modification of the borate ion enables high levels of catalytic activity
and stereocontrol. This system tolerates a range of α-alkylstyrenes
and hence offers rapid access to various aminocyclopentanes with contiguous
tertiary and quaternary stereocenters, as the urea DG is readily removable.