Reactivity of a P–H Functionalized Al/P‐Based Frustrated Lewis Pair – Hydrophosphination versus Classic Adduct Formation

摘要

> The P–H functionalized Al/P‐based frustrated Lewis pair (FLP) Mes(H)P–C[Al{CH(SiMe ₃ ) ₂ } ₂ ]=C(H)– t Bu ( 3 ) has the typical functionality of FLPs with Lewis acidic and basic centers in a single molecule and is able to transfer the P bound H atom to activated substrates. Nevertheless, 3 reacted with NH ₃ only by adduct formation with the N atom coordinated to Al ( 5 ). The relatively low basicity of the P atom prevented the formation of a hydrogen bridge between P and Al. Similarly, benzil and 3 afforded a simple adduct ( 6 ) with only one C=O group bonded to P and Al. The second C=O group and the P–H moiety were unaffected. 6 has two stereogenic centers, and the resulting diastereomers were identified by crystal structure determinations. Migration of the P bound H atom was only observed for substrates containing basic N atoms. Nitriles gave heterocyclic iminophosphine adducts ( 7 ) in which the N–H groups were coordinated to the Al atoms. An isocyanate and two isothiocyanates afforded adducts via Al– X and P–C bond formation ( X = O, S), and phosphaurea derivatives ( 8a to 8c ) resulted from H shift to the exocyclic N atoms. A carbodiimide reacted with 3 by the formation of a phosphaguanidine ( 8d ) with the imine N atom coordinated to Al. The latter reactions proceed under mild conditions and represent formally the hydrophosphination of unsaturated substrates.