How Dihalogens Catalyze Michael Addition Reactions

Area of Science:

• Computational Chemistry
• Organic Reaction Mechanisms
• Catalysis

Background:

• The aza-Michael addition is a crucial carbon-nitrogen bond-forming reaction in organic synthesis.
• Dihalogen molecules are known to influence organic reactions, but their precise catalytic role in aza-Michael additions requires detailed investigation.

Purpose of the Study:

• To quantum chemically investigate the catalytic effect of various dihalogen molecules (F2, Cl2, Br2, I2) on the aza-Michael addition of pyrrolidine and methyl acrylate.
• To elucidate the underlying mechanism responsible for the observed reactivity trends in dihalogen-catalyzed aza-Michael additions.

Main Methods:

• Relativistic density functional theory (DFT) and coupled-cluster theory were employed for high-level quantum chemical calculations.
• Activation strain and bonding analyses were performed to understand the electronic and steric factors governing the reaction.

Main Results:

• Activation barriers for the aza-Michael addition systematically decrease with increasing atomic weight of the dihalogen, from 9.4 kcal/mol for F2 to 5.7 kcal/mol for I2.
• The primary catalytic effect stems from a reduction in Pauli repulsion between the pyrrolidine's lone pair and the methyl acrylate's pi-electron system.
• Contrary to common assumptions, enhanced donor-acceptor (HOMO-LUMO) interactions do not drive the catalytic activity.

Conclusions:

• Dihalogens, particularly heavier ones like I2, effectively catalyze aza-Michael additions by diminishing Pauli repulsion.
• This study reveals an unexpected mechanism for dihalogen catalysis, shifting focus from electronic interactions to steric repulsion effects.
• The findings provide new insights into designing catalysts for C-N bond formation via Michael addition reactions.