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

Uhl W., Keweloh L., Hepp A., Stegemann F., Layh M., Bergander K.

Abstract

The P–H functionalized Al/P-based frustrated Lewis pair (FLP) Mes(H)P–C[Al{CH(SiMe3)2}2]=C(H)–tBu (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 NH3 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.