Phenoxaphosphonium Mixed Ylides in Ring Expansion Reaction

Area of Science:

• Organic Chemistry
• Organophosphorus Chemistry
• Heterocyclic Chemistry

Background:

• Mixed phosphonium-iodonium ylides are versatile synthetic intermediates.
• Phenoxaphosphonium compounds containing six-membered rings present unique structural features.
• Accessing complex heterocyclic phosphorus compounds often requires novel synthetic strategies.

Purpose of the Study:

• To investigate the reactivity of mixed phosphonium-iodonium ylides with a phenoxaphosphonium core.
• To explore the potential for phosphacycle rearrangement and phosphorus oxidation.
• To synthesize and characterize novel dibenzooxaphosphepine oxides.

Main Methods:

• Reaction of six-membered phenoxaphosphonium ylides with aqueous tetrafluoroboronic acid.
• Isolation and purification of the rearranged products.
• Characterization of the resulting dibenzooxaphosphepine oxides using spectroscopic methods.
• Comparative hydrolysis study of a five-membered dibenzophosphole ylide.

Main Results:

• Aqueous tetrafluoroboronic acid treatment induced phosphacycle expansion and phosphorus oxidation in phenoxaphosphonium ylides.
• Three examples of difficult-to-access dibenzo[b,f][1,4]oxaphosphepine oxides were synthesized in excellent yields (up to 95%).
• The products were obtained as mixtures of stereoisomers.
• Hydrolysis of a five-membered ylide predominantly preserved the phosphole system, with minor cycle expansion.

Conclusions:

• The reaction provides an efficient route to dibenzooxaphosphepine oxides via ylide rearrangement.
• Phosphacycle expansion is a key transformation under these conditions.
• The study highlights the differential reactivity of five- and six-membered ylides.