Synthesis and Structure of an Amino-Linked N-Heterocyclic Carbene and the Reactivity of its Aluminum Adduct

Treatment of [HC{(MES)N(CHCH)N(CH2CH2NHtBu}]Br·HBr (1) with an excess of NaOH afforded the organic-soluble imidazolium salt 2, [HC{(MES)N(CHCH)N(CH2CH2NHtBu}]Br, the structure of which was confirmed by X-ray crystallography. Deprotonation of 2 by NaN(SiMe3)2 yielded the thermally unstable amino-linked free carbene, 3. The molecular structure of 3 was determined by single-crystal X-ray analysis, revealing an unexpectedly close intermolecular contact associated with the carbene and amine through an N−H···C interaction. In contrast to compound 3, the reaction of 2 with LiN(SiMe3)2 gave the much more stable LiBr carbene adduct 3-LiBr, which serves as an effective carbene transfer agent for organoaluminum compounds to give the corresponding AlR3-NHC (4a, Me; 4b, Et) in high yield. X-ray diffraction studies of 4a and 4b confirmed the formation of monomeric distorted tetrahedral Al species, in which the NHC binds via conventional σ-donation of the lone pair to the electrophilic metal center. The reaction of benzaldehyde with 4a resulted in the quantitative formation of a zwitterionic species consisting of a distorted pseudotetrahedral aluminate center covalently linked to imidazolium, 5. The product 5 resulted from insertion of the carbonyl moiety into the Al-carbene group. A similar reactivity with benzaldehyde was observed in compound 6, which was independently synthesized from addition of 1,3-bis(mesityl)imidazol-2-ylidene to AlMe3.