Generation of Weakly Bound Al–N Lewis Pairs by Hydroalumination of Ynamines and the Activation of Small Molecules: Phenylethyne and Dicyclohexylcarbodiimide

Treatment of aminoalkynes (ynamines), Me₃Si-CC-NR₂ [NR₂ = N­(CH₂CH₂)₂NMe, N­(CHMe-CH₂)₂CH₂], with dialkylaluminum hydrides, H-AlR′₂ (R′ = CH₂<i>t</i>Bu, <i>i</i>Bu, <i>t</i>Bu), afforded by hydroalumination the corresponding alkenyl derivatives R′₂Al-(Me₃Si)­CC­(H)-NR₂ (<b>3</b> to <b>6</b>) in which the opposite Lewis functionalities adopted a <i>cis</i>-arrangement with the aluminum and nitrogen atoms on the same side of the resulting CC double bonds. Intramolecular Al–N interactions gave four-membered AlC₂N heterocycles, but ring strain may cause relatively long Al–N distances of 2.07 to 2.15 Å. These compounds represent a new type of Lewis acid–base pairs, and we observed C–H bond activation by opening of the Al–N bond upon treatment with H-CC-C₆H₅. Compound <b>7</b> was isolated, which had the proton attached to nitrogen and the anionic ethynyl group bonded to aluminum. The formation of <b>7</b> is reversible in solution with an increasing dissociation into the starting compounds at elevated temperatures. Two different structural motifs were obtained upon reaction of the Lewis acid–base pairs with dicyclohexylcarbodiimide, which inserted selectively into the bond between the aluminum and the vinylic carbon atom. In the first compound (<b>8</b>) an amidinato ligand was formed, which coordinated the metal atom by both nitrogen atoms to give an AlCN₂ heterocycle. A six-membered heterocycle (<b>9</b>) resulted from the insertion of only one CN double bond of the carbodiimide into the Al–C­(vinyl) bond. <b>9</b> had an unchanged interaction of the metal atom with the α-nitrogen atom of the former ynamine. The latter compound may be considered an intermediate, and rearrangement to yield an amidinato compound analogous to <b>8</b> was indeed observed with extended reaction time