Treatment of Terminal Alkynes R-CC-H with Dialkylaluminum Hydrides: Hydroalumination versus Deprotonation

Dialkylaluminum hydrides R2Al-H (R = Me, Et, iBu, CH(2)tBu, tBu), reacted with tert-butylethyne, H-C C-CMe3, by hydroalumination and formation of the corresponding vinylic compounds, R2Al-C(H)=C(H)-CMe3. The products are dimeric in the solid state with the negatively charged vinylic carbon atoms in the bridging positions and adopt usually a centrosymmetric structure with the C=C double bonds on different sides of the central Al2C2 heterocycle. Only with R = Me the C-2v structure with the cis arrangement of the alkenyl groups was found. Complicated mixtures of isomeric substances were detected in solution. Quantum-chemical calculations revealed a considerable charge separation in the C=C double bonds of the dimeric formula units. The alpha-carbon atoms coordinated by both aluminum atoms show high negative NBO charges of about -1.0, while the beta-carbon atoms are almost uncharged (0.0 to -0.1). The rotational barriers decreased upon dimerization, Hence, the formation Of Such dimers may be the key step in the well-known cis/trans isomerization process of hydroalumination and hydrogallation products. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)