The dialkylaluminum and dialkylgallium alkynides [R2E‐C≡C‐R′]2 (R = Me, CMe3; E = Al, Ga; R′ = Ph) containing C≡C triple bonds attached to their central aluminum or gallium atoms are easily obtained by the reactions of dialkylelement chlorides with lithium alkynides or by treatment of the corresponding alkyne R‐C≡C‐H with dialkylaluminum or dialkylgallium hydrides. The first reaction is favored by the precipitation of LiCl, the second one by the formation of elemental hydrogen. All products form dimers in which the carbanionic carbon atoms of the alkynido groups adopt bridging positions, but, interestingly, different types of molecular structures were observed depending on the steric demand of the substituents terminally attached to the aluminum or gallium atoms. The small methyl substituents gave structures in which the aluminum or gallium atoms seemed to be side‐on coordinated by the C≡C triple bonds of almost linear E‐C≡C groups. In contrast, the more bulky tert‐butyl groups forced an arrangement in which the C≡C triple bonds were perpendicular to the E‐E axis of the molecules. Different bonding modes result, which were analyzed by quantum‐chemical calculations.

Hydroalumination Reactions Employing the Bulky Dialkylaluminum Hydride [(Me3Si)2HC]2Al‐H ‐ Prevention of Secondary Reactions Usually, the hydroalumination of alkynes results in secondary reactions under release of the corresponding trialkylaluminum compounds. In contrast, the monoaddition products of R2Al‐H to C≡C triple bonds [R2Al‐(R′)C=C(R″)‐H] could be isolated in high yields now by employing the bulky dialkylaluminum hydride [(Me3Si)2HC]2Al‐H (1). This is shown here by two reactions starting with a monoalkyne (Me3C‐C≡C‐C6H5) and a dialkyne (Me3C‐C≡C‐C6H4‐C≡C‐CMe3). Both products have the aluminium atoms attached to those carbon atoms of the C=C double bonds which are in the α‐position with respect to the phenyl groups. The configuration of the alkenyl groups verifies the cis‐addition of the Al‐H bonds in all cases.

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Hydroalumination of H5C6-C≡N with di(tert-butyl)aluminum hydride 1or the corresponding diethyl compound 2yielded the products tBu2Al-N=C(C6H5)H 3and Et2Al-N=C(C6H5)H 4, respectively, both of which form dimers possessing Al2N2 heterocycles with two exocyclic C=N double bonds. NMR spectroscopic data indicate the occurrence of cis/trans isomers in solutions of compound 4. The dimerization of the imide via Al-N interactions was prevented by employing the hydride [(Me3Si)2HC]2AlH 6bearing the bulky bis(trimethylsilyl)methyl substituents. Its reaction with benzonitrile yielded the compound R2Al-N=C(C6H5)H(N≡C-C6H5) 7[R = CH(SiMe3)2], in which the coordinative saturation of the aluminum atoms was achieved by adduct formation with one molecule of the starting nitrile. In these cases the C≡N triple bond inserted completely into the Al-H bond of the hydride. In contrast, the reaction of tert-butyl isonitrile afforded the product tBu2Al-C(H)=NC6H5 8by the insertion of its terminal carbon atom into the Al-H bond. Hence, it has a geminal arrangement of the aluminum and hydrogen atoms. Dimerization of 8yielded a six-membered heterocycle. Hydroalumination does not occur upon treatment of the hydride 1with trimethylsilylnitrile. Instead, the Si-CN bond was cleaved, and the aluminum cyanide (tBu2Al-C≡N)4 9was isolated in a high yield.

Ein unerwartetes Produkt: Ein stabiles Butadien‐2‐yl‐Kation (rot im Bild) entsteht durch Hydroaluminierung von 1,4‐Di(tert‐butyl)butadiin mit Di(tert‐butyl)aluminiumhydrid. Seine Stabilisierung erfolgt insbesondere durch Hyperkonjugation mit einer Al‐C‐Bindung. Die Nucleophilie des kationischen Kohlenstoffatoms im Zwitterion reicht nicht aus, um das Hydridion aus der effektiven, chelatartigen Koordination durch beide Aluminiumatome zu lösen.