Substrate-Dependent Two-State Reactivity in Iron-Catalyzed Alkene [2+2] Cycloaddition Reactions

Area of Science:

• Organometallic Chemistry
• Catalysis
• Reaction Mechanisms

Background:

• Iron-catalyzed alkene [2+2] cycloadditions offer a stepwise alternative to kinetically hindered concerted reactions.
• The fundamental reactivity paradigms governing these iron-catalyzed processes remain largely undefined.

Purpose of the Study:

• To elucidate the substrate-dependent reactivity scenarios in iron-catalyzed alkene [2+2] cycloaddition reactions.
• To clarify the role of redox-active ligands and electronic states in the catalytic cycle.

Main Methods:

• High-level combined CASPT2/DFT computational modeling.
• Analysis of key carbon-carbon coupling steps in iron catalysis.
• Investigation of metallacyclic intermediates and reductive elimination pathways.

Main Results:

• An unprecedented substrate-dependent two-state reactivity scenario was identified for the key C-C coupling step.
• Mono-olefins and mono-olefin/1,3-diene substrates exhibit distinct reactivity paradigms.
• Redox-active ligands promote a Fe(III)/Fe(I) reductive elimination pathway, more accessible than Fe(II)/Fe(0).

Conclusions:

• The study clarifies the mechanism of iron-catalyzed [2+2] cycloadditions, highlighting substrate influence on reactivity.
• Enhancing spin state transition efficiency offers a strategy to boost reactivity in cross cycloadditions.
• Ab initio multi-reference methods are effective for complex open-shell iron catalysis.