Treatment of Terminal Alkynes R–C≡C–H with Dialkylaluminum Hydrides: Hydroalumination versus Deprotonation

Abstract

Dialkylaluminum hydrides R2Al–H (R = Me, Et, iBu, CH2tBu, 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 C2v 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 α‐carbon atoms coordinated by both aluminum atoms show high negative NBO charges of about –1.0, while the β‐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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Aluminum Hydride Catalyzed Hydroboration of Alkynes

“…Yang, Parameswaran, and Roesky have reported the catalytic hydroboration of alkynes by a N,N'-bis-2,6-diisopropylphenyl diketiminate (NacNac)-supported aluminum dihydride. Treatment of the isolated alkenyl aluminum species 5 [22] with HBpin formed the borylated alkene 2c (Scheme 4a). Ther ate-determining step in this mechanism was predicted computationally to be protonation of the borylated alkenyl group at the aluminum center by the incoming terminal alkyne,w ith an activation barrier of 45.3 kcal mol À1 .…”

Aluminium Functionalized Germanes: Intramolecular Activation of Ge–H Bonds, Formation of a Dihydrogen Bond and Facile Hydrogermylation of Unsaturated Substrates

“…The formation of the products was evident from the disappearance of the Ge–H signal at about δ = 4.5 in the 1 H NMR spectra and the appearance of characteristic downfield signals for Ph 2 C( H )O at δ = 6.09 and C( H )=O, C( H )=N and CH=C H at δ > 7. The trans ‐configuration of the alkene in 10 is confirmed by the large 3 J HH coupling constant of 19.8 Hz , . Further characteristic signals are found in the 13 C NMR spectra with δ = 83.2 [Ph 2 C (H)O] (upfield to Ph 2 CO) and δ = 173.7, 156.1 and 134.2/148.9 [N‐ C (H)=O, N= C (H) and C H= C H] (downfield relative to the starting materials).…”

Hydroaluminierung von Silicium‐zentrierten Dialkinen – Synthese gemischt‐substituierter Alkenyl‐alkinylsilane

“…Im ersten Fall verhindert die sterische Abschirmung den Substituentenaustausch, im zweiten Fall bilden sich bevorzugt eindimensional unendliche Koordinationspolymere mit ausschließlich vinylischen Kohlenstoffatomen in den verbrückenden Positionen. Die Produkte der einfachen Addition mit intakten AlR 2 ‐Gruppen gebunden an die Alkenyleinheiten entstehen auch mit terminalen Alkinen (H–C≡C–R) 6 und mit Trimethylsilylethinylbenzol‐Derivaten [7, 8]. In Abhängigkeit von der sterischen Abschirmung treten mit den Trimethylsilylverbindungen spontane cis / trans ‐Umlagerungen ein, die im Einklang mit quanten‐chemischen Rechnungen einer intermolekularen Aktivierung bedürfen 8.…”