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)(R2)] ((L = HC[(CMe)(NAr)]2, Ar = 2,6-iPr2C6H3); R1 = R2 = H, (1); R1 = H, R2 = Ph, (2); R1
= R2 = Me, (3); R1 = SiMe3, R2 = C⋮CSiMe3, (4)) are reported. Compounds 1 and 2 were obtained in
equimolar quantities of the starting materials at low temperature. The amount of C2H2 was controlled by
removing an excess of C2H2 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 Me3SiC⋮CC⋮CSiMe3 or by the reductive coupling
reaction of LAlI2 with potassium in the presence of Me3SiC⋮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.