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Hydroalumination of aryldialkynylphosphines RP(C≡C-(t)Bu)(2) (R = Ph, Mes) with equimolar quantities of diethylaluminum hydride afforded mixed alkenyl-alkynyl cyclic dimers in which the dative aluminum-phosphorus bonds are geminal to the exocyclic alkenyl groups. Addition of triethylaluminum to isolated 1 (R = Ph) or to the in situ generated species (R = Mes) caused diethylaluminum ethynide elimination to yield the arylethylphosphorus dimers 2 and 3. These possess a chair-like Al(2)C(2)P(2) heterocycle with intermolecular Al-P interactions. The boat conformation (4) was obtained by the reaction of (t)Bu-P(C≡C-(t)Bu)(2) with di(tert-butyl)aluminum hydride. Despite being dimeric, 2 behaves as a frustrated Lewis pair and activates small molecules. The reaction with carbon dioxide gave cis/trans isomeric AlPC(2)O heterocycles that differ only by the configuration of the exocyclic alkenyl unit. Four isomers resulted from the reaction with phenyl isocyanate. This is caused by cis/trans isomerization of the initial C=O adduct and subsequent rearrangement to the AlPC(2)N heterocycle, being the C=N adduct.

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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 HAlR 2 (R = Me, iBu, Ph) afforded unique adducts in which an HAlR 2 molecule was coordinated by the Al/P FLP Mes 2 PC(CHPh)AlR 2 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 2 PCCPh yielded a hydride adduct with HAlNp 2 and an alkyne adduct with HAltBu 2 . The latter compound resulted from triple-bond activation and had a five-membered AlPC 3 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.

Treatment of aryl-di(tert-butylethynyl)phosphines Aryl-P(CtC-CMe 3 ) 2 [Aryl = C 6 H 5 (4) 2,4,6-Me 3 C 6 H 2 (5)] with dialkylaluminum hydrides, R 2 Al-H (R = CMe 3 , CH 2 CMe 3 ), afforded heterocyclic, zwitterionic compounds 6-9 by hydroalumination and release of the corresponding dimeric tertbutylethynyl-dialkylaluminum derivatives, [R 2 Al-CtC-CMe 3 ] 2 (10 and 11). The molecular structures of 6-8 reveal a P 2 C 2 Al ring with a tetracoordinated aluminum atom, two exocyclic CdC double bonds, a tricoordinated phosphorus atom, and a phosphonium cation carrying an aryl and an alkynyl group. The CtC triple bond of 7 was inserted into the Al-P bonds of the heterocycles upon warming to give the bicyclic compound 13. A similar compound ( 12) was directly formed when the bisalkyne 4 was treated with dineopentylaluminum hydride. Their molecular structures show the annulated four-(AlC 2 P) and five-membered heterocycles (P 2 C 3 ) to contain three different CdC double bonds. The sterically most shielded monocyclic compound (8; Aryl = 2,4,5-Me 3 C 6 H 2 ; R = CMe 3 ) decomposed upon warming. DFT calculations were used to examine these reactions.