Wittig olefination reaction catalyzed by a polyphosphamide: effect of nanoporosity and mechanistic insight

Phosphamides are traditionally recognized as flame retardants and chemotherapeutic agents owing to notable thermo-chemical stability. Due to their partial structural similarity with phosphine oxides, they can serve as versatile Lewis bases to catalyse diverse organic transformations including phosphine-based reactions. Herein, a polyphosphamide (b-PPA) is synthesized via condensation of phenylphosphonic dichloride and p-phenylenediamine spacer, which has a molecular weight of ∼11 kDa and a mesoporous network with a 19 m² g^-1 specific surface area. The presence of a redox-active -{P^V(O)NH}- linkage in the symmetric repeating unit of b-PPA with an average phosphorus content of 0.0076 mmol% gives a scope to perform catalytic Wittig olefination reaction (WOR) with p-CN benzaldehyde and ethyl bromoacetate, delivering a TON of 711. Under optimised conditions, the b-PPA shows a diverse substrate scope for WOR with dominant E selectivity of the synthesized olefins, and the TON reported is much larger than the previous report with an analogous polyphosphamide (t-PPA) catalyst containing a triazole spacer. Despite a comparable phosphorus content in both b-PPA and t-PPA, the higher microporosity in b-PPA results in higher conversion and TON for WOR. Hammett analysis, in situ IR spectroscopy, and theoretical modelling of the WOR infer that the formation of a four-membered oxaphosphatene ring via the nucleophilic attack of the Wittig ylide to the benzaldehyde is the rate-limiting step. The heterogeneous nature of the b-PPA allows us to recover and recycle the catalyst to yield a cumulative TON of 1783 with minimum active site loss. This study showcases the potential catalytic application of a nanoporous polyphosphamide, providing a scope to explore further emerging phosphorus-mediated organo-catalysis with improved performance compared to homogeneous and non-porous heterogeneous analogues.