Reactivity of NHC Alane Adducts towards N-Heterocyclic Carbenes and Cyclic (Alkyl)(amino)carbenes: Ring Expansion, Ring Opening, and Al-H Bond Activation.

The synthesis of mono-NHC alane adducts of the type (NHC)⋅AlH (NHC=Me Im (1), Me Im (2), iPr Im (3 and [D ]-3), iPr Im (4), Dipp Im (10); Im=imidazolin-2-ylidene, Dipp=2,6-diisopropylphenyl) and (NHC)⋅AliBu H (NHC=iPr Im (11), Dipp Im (12)) as well as their reactivity towards different types of carbenes is presented. Although the mono-NHC adducts remained stable at elevated temperatures, ring expansion occurred when (iPr Im)⋅AlH (3) was treated with a second equivalent of the carbene iPr Im to give (iPr Im)⋅AlH(RER-iPr ImH ) (6). In 6, {(iPr Im}AlH} is inserted into the NHC ring. In contrast, ring opening was observed with the sterically more demanding Dipp Im with the formation of (iPr Im)⋅AlH (ROR-Dipp ImH )H Al⋅(iPr Im) (9). In 9, two {(iPr Im)⋅AlH } moieties stabilize the ring-opened Dipp Im. If two hydridic sites are blocked, the adducts are stable with respect to further ring expansion or ring opening, as exemplified by the adducts (iPr Im)⋅AliBu H (11) and (Dipp Im)⋅AliBu H (12). The adducts (NHC)⋅AlH and (iPr Im)⋅AliBu H reacted with cAAC by insertion of the carbene carbon atom into the Al-H bond to give (NHC)⋅AlH /iBu (cAAC H) (13-18) instead of ligand substitution, ring-expansion, or ring-opened products.