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′ = CH2tBu, iBu, tBu), afforded
by hydroalumination the corresponding alkenyl derivatives R′2Al-(Me3Si)­CC­(H)-NR2 (3 to 6) in which the opposite Lewis functionalities adopted
a cis-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 7 was isolated, which
had the proton attached to nitrogen and the anionic ethynyl group
bonded to aluminum. The formation of 7 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 (8) an
amidinato ligand was formed, which coordinated the metal atom by both
nitrogen atoms to give an AlCN2 heterocycle. A six-membered
heterocycle (9) resulted from the insertion of only one
CN double bond of the carbodiimide into the Al–C­(vinyl)
bond. 9 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 8 was indeed observed with extended
reaction time.