Aspects of Phosphaallene Chemistry: Heat-Induced Formation of 1,2-Dihydrophosphetes by Intramolecular Nucleophilic Aromatic Substitution and Photochemical Generation of Tricyclic Phosphiranes.

3H-Phosphaallenes are accessible on a new and facile route and show a fascinating chemical behavior. The thermally induced rearrangement of Mes*P═C═C(H)R' (R' = tBu, Ad) afforded by C-H activation, isobutene elimination, and C-C and P-H bond formation bicyclic 1-benzo-dihydrophosphetes (2) with PC3 heterocycles. DFT calculations suggest a mechanism with intramolecular nucleophilic aromatic substitution and replacement of an alkyl group by the nucleophilic α-C atom of the phosphaallene. These bicycles formed W(CO)5 complexes (3) or afforded 1,2-dihydrophosphetes with P-bound alkenyl groups by catalyst-free hydrophosphination of alkynes (4 and 5). The resulting bulky phosphines formed complexes with IrCp*Cl2, RuCl2, AuCl, or CuO3SCF3. The Ru atom is coordinated by the P atom and a phenyl group. Irradiation of TripP═C═C(H)tBu led by the insertion of the central C atom of the P═C═C group into the α-C-H bond of an iPr substituent and by C-C and P-C bond formation to a new isomer of phosphaallenes, 10, which features a strained PC2 heterocycle. It formed adducts with M(CO)5 (M = Cr, Mo, W) and AuCl and reacted with SO2Cl2 by cleavage of one of the phosphirane P-C bonds to yield PC4 or PC5 heterocycles. Hydrolysis yielded a PC5 compound with a P(O)Cl group.

3H-Phosphaallenes Revisited: Facile Synthesis by Hydroalumination of Alkynylphosphines and ß-Elimination, Stability and Trapping of Transient Species by Coordination to Transition Metal Atoms.

A facile method for the efficient synthesis of 3H-phosphaallenes, R-P=C=C(H)-R', is presented, which comprises treatment of dialkynylphosphines with dialkylaluminium hydrides (hydroalumination) and elimination of aluminium alkynides from intermediate alkenyl-alkynylphosphines. The stability of the phosphaallenes depends on steric shielding by the substituents at phosphorus (aryl or CH(SiMe3 )2 groups). Only supermesityl compounds are persistent at room temperature in solution. This simple method starting with easily accessible dialkynylphosphines and commercially available aluminium hydrides (HAlEt2 , HAliBu2 ) allows the generation of transient species, which were trapped by coordination to transition metals. The η1 -coordination via a P-W bond was observed for tungsten, while the side-on coordination via the P=C bond resulted with platinum. Decomposition of the mesityl derivative yielded an unprecedented product, which may be formed by 1,3-H shift to the P atom, hydrophosphination of the P=C bond of a second phosphaallene and formation of a P-P bond.

A P-H Functionalized Al/P Frustrated Lewis Pair: Substrate Activation and Selective Hydrogen Transfer.

Hydroalumination of an alkynylphosphine gave an unprecedented P-H functionalized frustrated Lewis pair (FLP). The reactive P-H group does not influence the typical FLP properties, but the activation of substrates follows a new reaction pattern involving hydrogen transfer to yield unusual compounds with phosphaurea, iminophosphine, or phosphanyltriazene structural motifs.

Hydroalumination of alkynyl-aminophosphines as a promising tool for the synthesis of unusual phosphines: P-N bond activation, a transient phosphaallene, a zwitterionic AlP2C2 heterocycle and a masked Al/P-based frustrated Lewis pair.

Treatment of the new alkynyl-chlorophosphine, Mes-P(Cl)-C ≡ C-CMe3, with LiNR2 afforded various unprecedented aminophosphines, Mes-P(NR2)-C ≡ C-CMe3, which showed a fascinating diversity in their reactivity towards H-Al(t)Bu2. NMe2 and NEt2 derivatives yielded the hydroalumination products Mes-P(NR2)-C(Al(t)Bu2) = C(H)-CMe3 which have an Al-N and an activated P-N bond. Elimination of aluminium amide yielded the transient 3H-phosphaallene, Mes-P = C = C(H)-CMe3, which finally afforded a five-membered AlP2C2 heterocycle with an Al-P bond and two exocyclic C = C bonds. This heterocycle is directly formed with the sterically shielded (i)Pr2N- and dimethylpiperidinophosphines. A unique R2AlH adduct resulted from the NPh2 and N(SiMe3)2 substituted phosphines. It may be viewed as an Al/P-based frustrated Lewis pair (FLP) which coordinates an R2AlH moiety. The heterocyclic AlP2C2 compound is formed in the final step of this reaction. Hydroalumination with Et2AlH yielded [Mes-P(H)-C(AlEt2) = C(H)-CMe3]2 which features an Al2P2C2 heterocycle and two Al-P bonds. This dimer resembles the class of hidden or masked FLPs which show a reactivity similar to uncoordinated FLPs. Its unique structural motif is a P-H bond which may result in a new type of FLP chemistry.

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.