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 Mes2PC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 Mes2PC­(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
HAltBu2. 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.