Aluminacyclopropene: Syntheses, characterization, and reactivity toward terminal alkynes

Reactions of LAI with ethyne, mono- and disubstituted alkynes, and diyne to aluminacyclopropene LAI[eta(2)-C-2(R-1)(R-2)] ((L = HC[(CMe)(NAr)](2), Ar = 2,6-/Pr2C6H3); R-1 = R-2 = H, (1); R-1 = H, R-2 = Ph, (2); R-1 = R-2 = Me, (3); R-1 = SiMe3, R-2 = C equivalent to 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 degrees C. Compound 4 can be alternatively prepared by the substitution reaction of LAI[eta(2)-C-2(SiMe3)(2)] with Me3SiC equivalent to CC equivalent to CSiMe3 or by the reductive coupling reaction of LAII(2) with potassium in the presence of Me3SiC equivalent to CC equivalent to CSiMe3. The reaction of LAI with excess C2H2 and PhC equivalent to CH (< 1:2) afforded the respective alkenylalkynylaluminum compounds LAI(CH=CH2)(C equivalent to CH) (5) and LAI(CH=CHPh)(C equivalent to CPh) (6). The reaction of LAI (eta(2)-C2Ph2) with C2H2 and PhC equivalent to CH yielded LAI(CPh=CHPh)(C equivalent to CH) (7) and LAI(CPh=CHPh)(C equivalent to 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(l) center and the C equivalent to C unit needs almost no activation energy. Within the AlC2 ring the computational Al-C bond order of ca. 1 suggests an Al-C a bond and therefore less pi electron delocalization over the AlC2 ring. The computed Al-eta(2)-C-2 bond dissociation energies (155-82.6 kJ/mol) indicate a remarkable reactivity of aluminacyclopropene species. Finally, the H-1 NMR spectroscopy monitored reaction of LAI(eta(2)-C2Ph2) and PhC equivalent to CH in toluene-d(8) may reveal an acetylenic hydrogen migration process.