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

Abstract

Treatment of aminoalkynes (ynamines), Me3Si-CC-NR2 [NR2 = N(CH2CH2)2NMe, N(CHMe-CH2)2CH2], with dialkylaluminum hydrides, H-AlR′2 (R′ = CH2 tBu, 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.

Dehydrochlorination of Carboxylic Acid Chlorides by an Aluminum/Phosphorus‐Based Frustrated Lewis Pair and the Formation of Ketene Adducts

“…This property sheds an interesting light on the general character of frustrated Lewis pairs in terms of intra-and intermolecular activation and dual catalysis. In this view, the dehydrochlorination reactions reported here have similarity to earlier work of our groups, where we reported that an N/Al-Lewis pair is able to deprotonate even phenylacetylene, [20] a reaction which is absolutely inconceivable for tertiary amines.…”

Frustrated Lewis Pair Chemistry with Magnesium Lewis Acids

“…Although the majority of systems is limited to P/B or N/B pairs, the use of Al‐, C‐, or Si‐based Lewis acids emerged. Especially FLPs with Al Lewis acids have been able to add unique metal‐based reactivities to the growing repertoire: for example, the complexation of alkali metal hydrides, stoichiometric and catalytic dehydrocoupling of amine–boranes, activation of alkenes,, alkynes,, carbodiimides, isocyanates, hydrogen, CO 2 , and other carbonyl groups . Our expertise in alkaline earth metal chemistry stimulated us to expand the scope of Lewis acids with group 2 metal Lewis acids.…”

Molecular Structures of Dimeric and Trimeric Dineopentylaluminum Hydride and Trimeric Diphenylaluminum Hydride

“…[10] It is an important starting compound in hydroalumination reactions [1,11] and has an interme-diate steric shielding between that of bis(tert-butyl)aluminum hydride and the commercially available diisobutylaluminum hydride which is a standard reagent in organic transformations. Its molecular structure in the solid state has not yet been reported.…”