The Phospha-Bora-Wittig Reaction.

Area of Science:

• Organophosphorus Chemistry
• Synthetic Organic Chemistry
• Main Group Chemistry

Background:

• The classical Wittig reaction is a cornerstone for synthesizing alkenes from carbonyl compounds.
• Phosphaalkenes, phosphorus analogues of alkenes, are valuable synthetic intermediates but their preparation can be challenging.
• Transient phosphaborenes represent a class of reactive species with potential in novel bond formations.

Purpose of the Study:

• To develop a new synthetic route for the direct preparation of phosphaalkenes.
• To explore the reactivity of transient phosphaborenes with carbonyl compounds.
• To investigate the mechanistic parallels between the novel phospha-bora-Wittig reaction and the classical Wittig reaction.

Main Methods:

• Reaction of transient phosphaborene (Mes*P═B-NR₂) with various carbonyl compounds (aldehydes, ketones, esters, amides).
• Isolation and characterization of the intermediate 1,2,3-phosphaboraoxetanes.
• Thermal or Lewis acid-promoted cycloreversion of intermediates to yield phosphaalkenes.
• Experimental studies and density functional theory (DFT) calculations to elucidate reaction mechanisms.

Main Results:

• The phospha-bora-Wittig reaction successfully yields phosphaalkenes from diverse carbonyl substrates.
• Transient phosphaborenes react with carbonyl compounds to form 1,2,3-phosphaboraoxetanes, analogous to Wittig oxaphosphetanes.
• Cycloreversion of these intermediates provides a viable route to phosphaalkenes.
• DFT studies and experimental data confirm significant mechanistic similarities to the classical Wittig reaction.

Conclusions:

• The phospha-bora-Wittig reaction offers a direct and versatile method for synthesizing phosphaalkenes.
• The reaction proceeds through 1,2,3-phosphaboraoxetane intermediates, mirroring the Wittig mechanism.
• This work expands the synthetic utility of phosphaborenes and provides new insights into phosphorus-based olefination reactions.