Reactivity and selectivity in the Wittig reaction: a computational study.

Area of Science:

• Organic Chemistry
• Computational Chemistry
• Reaction Mechanisms

Background:

• The Wittig reaction is a cornerstone of organic synthesis for alkene formation.
• Understanding the factors governing its stereoselectivity is crucial for synthetic control.
• Previous studies have proposed mechanisms, but detailed computational insights into transition state (TS) structures and selectivity determinants are needed.

Purpose of the Study:

• To investigate the salt-free Wittig reaction mechanism using DFT calculations.
• To elucidate the factors influencing E/Z selectivity in non-, semi-, and stabilized ylides.
• To clarify the role of ylide stabilization and phosphorus substituents on reaction outcomes.

Main Methods:

• Density Functional Theory (DFT) calculations were employed.
• Realistic systems with continuum solvation were modeled.
• Analysis of transition state (TS) geometries and energies was performed.

Main Results:

• DFT results strongly support the established Wittig reaction mechanism and align with experimental selectivities.
• E/Z selectivity is critically dependent on the energy of the elimination TS for non- and semi-stabilized ylides.
• TS structures are dictated by specific interactions: 1,2; 1,3; and C-H...O for non-/semi-stabilized ylides, and dipole-dipole interactions for stabilized ylides.

Conclusions:

• The study provides a refined understanding of Wittig reaction stereoselectivity.
• Ylide stabilization and phosphorus substituents significantly impact oxaphosphetane reversibility and TS geometry.
• These findings offer a robust foundation for designing novel ylides with predictable selectivity.