Alkynyl functionalized Al/P-based frustrated Lewis pairs - aluminium alkynide elimination and evidence for the formation of 3H-phosphaallenes [R-P=C=C(H)-(t)Bu].

Hydroalumination of dialkynylphosphines, aryl-P(C≡C-(t)Bu)2 (aryl = 2,4,6-Me3C6H2 (1), Ph (2)), with the bulky dialkylaluminium hydride H-Al[CH(SiMe3)2]2 afforded mixed alkenyl-alkynyl phosphines (3 and 4) with aluminium atoms in geminal positions to phosphorus. These compounds contain coordinatively unsaturated aluminium and phosphorus atoms and may be applicable as alkynyl functionalised frustrated Lewis pairs. Their unique constitution with dialkylaluminium and alkynyl groups in close proximity favours unusual secondary reactions with the elimination of an aluminium alkynide and formation of reactive 3H-phosphaallenes, aryl-P=C=C(H)-(t)Bu (5 and 6), which depending on the steric shielding by the aryl groups, oligomerize at room temperature over days or weeks. The mesityl derivative 5 is formed in a very selective reaction, but the relatively unstable phenyl compound 6 was only detected in a mixture of several components. One of these (7) was isolated as orange-red crystals and identified as a heterocyclic zwitterionic compound with a central AlC2P2 ring. One of its phosphorus atoms is bonded to a terminal alkynyl group.

Alkynide and acetonitrile activation by strained AlPC2 heterocycles.

Bicyclic compounds with annulated four- (AlPC(2)) and five-membered (P(2)C(3)) rings activate and capture Al-alkynides and acetonitrile at room temperature to afford bicyclic compounds with unique six-membered Al(2)PC(3) and AlNPC(3) heterocycles.

Dimeric aluminum-phosphorus compounds as masked frustrated Lewis pairs for small molecule activation.

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.