Al/P-Based Frustrated Lewis Pairs: Limitations of
Their Synthesis by Hydroalumination and Formation of Dialkylaluminum
Hydride Adducts
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Abstract
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<sub>2</sub>PCCPh with small dialkyl- or diarylaluminum
hydrides HAlR<sub>2</sub> (R = Me, <i>i</i>Bu, Ph) afforded
unique adducts in which an HAlR<sub>2</sub> molecule was coordinated
by the Al/P FLP Mes<sub>2</sub>PC(CHPh)AlR<sub>2</sub> 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 Ph<sub>2</sub>PCCPh yielded
a hydride adduct with HAlNp<sub>2</sub> and an alkyne adduct with
HAl<i>t</i>Bu<sub>2</sub>. The latter compound resulted
from triple-bond activation and had a five-membered AlPC<sub>3</sub> heterocycle in which a CC 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