Gadolinium Photocatalysis: Dearomative [2+2] Cycloaddition/Ring-Expansion Sequence with Indoles

Area of Science:

• Photocatalysis
• Synthetic Organic Chemistry
• Lanthanide Chemistry

Background:

• Lanthanide photocatalysts are underutilized in synthetic chemistry compared to transition metals.
• Developing efficient photocatalysts for complex organic transformations is crucial.

Purpose of the Study:

• To introduce Gadolinium(III) as a photocatalyst for indole functionalization.
• To develop a regioselective intermolecular [2+2] photocycloaddition/ring-expansion sequence.
• To provide divergent access to cyclopenta[b]indoles and indolines.

Main Methods:

• Visible-violet-light-induced reaction using Gadolinium(III) triflate (Gd(OTf)3).
• Intermolecular [2+2] photocycloaddition followed by a ring-expansion cascade.
• Mechanistic studies involving excited state intermediates.

Main Results:

• Achieved highly regioselective synthesis of cyclopenta[b]indoles and indolines (>95:5 r.r.).
• Demonstrated broad substrate scope with 59 examples and good functional group tolerance.
• Showcased facile scale-up under mild, direct visible-light conditions.

Conclusions:

• Gd(OTf)3 is an effective and accessible photocatalyst for indole transformations.
• The reaction proceeds via a dearomatization cascade initiated by excited state Gd(III).
• This work expands the utility of lanthanides in photocatalytic synthetic chemistry.