Al/P-Based Frustrated Lewis Pairs: Limitations of Their Synthesis by Hydroalumination and Formation of Dialkylaluminum Hydride Adducts

Aluminum-phosphorus-based frustrated Lewis pairs (Al/P FLPs) are valuable reagents for the dipolar activation or coordination of small molecules or ionic compounds. They are accessible by hydroalumination of alkynylphosphines. However, as reported in this article, the application of this simple method for the synthesis of a broad variety of different compounds is limited to sterically shielded systems. Hydroalumination of Mes(2)PC CPh with small dialkyl- or diarylaluminum hydrides HAlR2 (R = Me, iBu, Ph) afforded unique adducts in which an HAlR2 molecule was coordinated by the Al/P FLP Mes(2)PC(=CHPh)AlR2 via an Al-P and an Al-H-Al 3c bond. A new Al/P FLP was obtained with equimolar quantities of dineopentylaluminum hydride. The less shielded alkynylphosphine Ph2PC CPh yielded a hydride adduct with HAlNp2 and an alkyne adduct with HAltBu(2). The latter compound resulted from triple-bond activation and had a five-membered AlPC3 heterocycle in which a C=C bond was bonded to the P and Al atoms of an Al/P FLP. Both compounds were isolated in high yields by application of the appropriate stoichiometric ratios of the starting materials.