Enantioselective Photochemical Organocascade Catalysis

Abstract

Reported herein is a photochemical cascade process that combines the excited‐state and ground‐state reactivity of chiral organocatalytic intermediates. This strategy directly converts racemic cyclopropanols and α,β‐unsaturated aldehydes into stereochemically dense cyclopentanols with exquisite stereoselectivity. Mechanistic investigations have enabled elucidating the origin of the stereoconvergence, which is governed by a kinetic resolution process.

Photo‐Organocatalytic Enantioselective Radical Cascade Enabled by Single‐Electron Transfer Activation of Allenes

“…As for the scope of the allenic acid substrates 1, we observed an improved enantioselectivity when bulkier 4 a-d). As a limitation of the system, and in consonance with our previous studies, [12,13] aliphatic enals were not suitable substrates as the reactivity was completely inhibited. The tetracyclic adduct 4 g, bearing two spiro quaternary carbons, could also be successfully synthesised.…”

“…A competing pathway available to the allene cation radical 1 * + is the deprotonation at the terminal carbon, which leads to a reactive propargyl radical B (path ii in Scheme 1a). [13] We envisioned that the photochemistry of iminium ions could be used to activate allenes 1 and obtain reactive allene cation radicals 1 * + upon SET oxidation (Scheme 1b). [10,11] Recently, our laboratories identified a new photochemical catalytic mode of substrate activation that exploits the excited-state reactivity of chiral iminium ions C to enable asymmetric transformations previously inaccessible via ground-state organocatalytic pathways.…”

Chromophoraktivierung von α,β‐ungesättigten Carbonylverbindungen und ihre Anwendung in enantioselektiven Photoreaktionen

“…Mit einem chiralen C 2 -symmetrischen Pyrrolidin als Auxiliar wurde bis zu 82 % ee bei 40 %Umsatz erzielt. [65] [62] Mit dem Pyrrolidin 54 (TDS = Thexyldimethylsilyl) als Katalysator und verschiedenen Silanen 53 gelang die enantioselektive Alkylierung der Aldehyde 51 zu den Produkten der allgemeinen Struktur 55.D ie vergleichsweise hohe Oxidationskraft des photoangeregten Iminiumions 56 mit einem Redoxpotential E 1/2 (56 + */57C) %+ 2.4 V(gegen Ag/Ag + in MeCN) ermçglicht nach Ansicht der Autoren einen Einelektronentransfer vom Silan, das nach Abspaltung der Silylgruppe (formal als Kation) ein Radikal generiert, das mit dem Radikal 57 kombiniert.…”

Photo‐Organocatalytic Enantioselective Radical Cascade Reactions of Unactivated Olefins

“…We wondered if this new photochemical activation mode could be used to expand the synthetic potential of enantioselective radical cascades beyond the reactivity constraints of formal cycloaddition chemistry . In an initial attempt, we combined the excited‐state iminium ion chemistry with the ground‐state reactivity of enamines to access cyclopentanols, from cyclopropanols, with high stereoselectivity (Figure b) . However, while the first carbon–carbon bond‐forming step in this cascade sequence is a radical process, the subsequent carbon–carbon bond formation follows a two‐electron pathway.…”