Contra-Thermodynamic Positional Isomerization of Olefins

Area of Science:

• Organic Chemistry
• Photochemistry
• Catalysis

Background:

• Olefin isomerization is crucial in organic synthesis.
• Achieving contra-thermodynamic isomerization, especially for more substituted olefins, remains challenging.
• Existing methods often lack efficiency or substrate scope.

Purpose of the Study:

• To develop a light-driven method for contra-thermodynamic positional isomerization of olefins.
• To enable the synthesis of less thermodynamically stable alkene isomers.
• To expand the substrate scope for olefin isomerization reactions.

Main Methods:

• Stepwise proton-coupled electron transfer (PCET) activation of olefin substrates.
• Utilizing an excited-state oxidant and a Brønsted base.
• Employing a Cr(II) cocatalyst to form an allylchromium(III) intermediate.
• Regioselective protodemetalation using methanol.

Main Results:

• Successful light-driven, contra-thermodynamic positional isomerization of olefins.
• Formation and capture of an allylic radical intermediate.
• Highly regioselective synthesis of less thermodynamically stable alkene products.
• Demonstrated substrate scope including enol ethers, enamides, styrenes, 1,3-dienes, and tetrasubstituted alkyl olefins.
• Accumulation of the less substituted, less stable olefin isomer due to its inertness.

Conclusions:

• The developed method provides an efficient route to thermodynamically unfavorable olefin isomers.
• The photoredox-catalyzed approach offers a versatile platform for olefin functionalization.
• Mechanistic insights into the protodemetalation step were obtained.