Aromaticity Switching Via Rearrangement of a Pyrrolyl-Substituted Phosphenium Cation

Area of Science:

• Organometallic Chemistry
• Aromaticity Studies
• Phosphorus Chemistry

Background:

• Phosphenium cations are Lewis acidic species with a vacant p-orbital.
• Aromaticity is a key concept in understanding the stability and reactivity of cyclic systems.
• Tautomerism involving aromaticity switching is a rare but significant chemical phenomenon.

Purpose of the Study:

• To synthesize and characterize a pyrrolyl-substituted bicyclic phosphenium cation.
• To investigate the spontaneous rearrangement and aromaticity switching in this novel compound.
• To elucidate the driving forces behind the observed rearrangement and its implications for phosphenium cation stabilization.

Main Methods:

• In situ synthesis and observation of the phosphenium cation.
• Nucleus-independent chemical shift (NICS) calculations to assess aromaticity.
• Reaction of the cation with chloride ions to probe reversibility.

Main Results:

• A pyrrolyl-substituted bicyclic phosphenium cation was successfully synthesized.
• The cation underwent spontaneous rearrangement, switching aromaticity between the [C4N] and [C2N2P]+ rings.
• NICS calculations confirmed aromatic character in both ring systems, with dearomatization of the adjacent ring.
• Addition of chloride ions reversed the aromatic switching, yielding a chlorophosphine.

Conclusions:

• Aromaticity switching via rearrangement is a rare phenomenon, exemplified here in a bicyclic phosphenium cation.
• The rearrangement is driven by enhanced stabilization of the Lewis acidic phosphenium cation through π-donation.
• This study demonstrates tautomerism across both rings, offering new insights into the chemistry of phosphorus cations.