Aluminacyclopropene: Syntheses, characterization, and reactivity toward terminal alkynes

Reactions of LAl with ethyne, mono- and disubstituted alkynes, and diyne to aluminacyclopropene LAl[η²-C₂(R¹)(R ²)] ((L = HC[(CMe)(NAr)]₂, Ar = 2,6-iPr₂C ₆H₃); R¹ = R² = H, (1); R ¹ = H, R² = Ph, (2); R¹ = R² = Me, (3); R¹ = SiMe₃, R² = C≡CSiMe ₃, (4)) are reported. Compounds 1 and 2 were obtained in equimolar quantities of the starting materials at low temperature. The amount of C ₂H₂ was controlled by removing an excess of C ₂H₂ in the range from -78 to -50 °C. Compound 4 can be alternatively prepared by the substitution reaction of LAl[η²- C₂(SiMe₃)₂] with Me ₃SiC≡CC≡CSiMe₃ or by the reductive coupling reaction of LAlI₂ with potassium in the presence of Me ₃SiC≡CC≡CSiMe₃. The reaction of LAl with excess C₂H₂ and PhC≡CH (<1:2) afforded the respective alkenylalkynylaluminum compounds LAl(CH=CH₂)(C≡CH) (5) and LAl(CH=CHPh)(C≡CPh) (6). The reaction of LAl(η²- C₂Ph₂) with C₂H₂ and PhC≡CH yielded LAl(CPh=CHPh)(C≡CH) (7) and LAl(CPh=CHPh)(C≡CPh) (8), respectively. Rationally, the formation of 5 (or 6) may proceed through the corresponding precursor 1 (or 2). The theoretical studies based on DFT calculations show that an interaction between the Al(I) center and the C≡C unit needs almost no activation energy. Within the AlC₂ ring the computational Al-C bond order of ca. 1 suggests an Al-C σ bond and therefore less π electron delocalization over the AlC₂ ring. The computed Al-η²-C₂ bond dissociation energies (155-82.6 kJ/mol) indicate a remarkable reactivity of aluminacyclopropene species. Finally, the ¹H NMR spectroscopy monitored reaction of LAl(η²-C₂Ph₂) and PhC≡CH in toluene-d₈ may reveal an acetylenic hydrogen migration process.