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)center dot AlH3 (NHC=Me(2)Im (1), Me(2)ImMe (2), iPr(2)Im (3 and [D-3]-3), iPr(2)Im(Me) (4), Dipp(2)Im (10); Im=imidazolin-2-ylidene, Dipp=2,6-diisopropylphenyl) and (NHC)center dot AliBu(2)H (NHC=iPr(2)Im (11), Dipp(2)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(2)Im)center dot AlH3 (3) was treated with a second equivalent of the carbene iPr(2)Im to give (iPr(2)Im)center dot AlH(RER-iPr(2)ImH(2)) (6). In 6, {(iPr(2)Im}AlH} is inserted into the NHC ring. In contrast, ring opening was observed with the sterically more demanding Dipp2Im with the formation of (iPr(2)Im)center dot AlH2(ROR-Dipp(2)ImH(2))H2Al center dot(iPr(2)Im) (9). In 9, two {(iPr(2)Im)center dot AlH2} moieties stabilize the ring-opened Dipp(2)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(2)Im)center dot AliBu(2)H (11) and (Dipp(2)Im)center dot AliBu(2)H (12). The adducts (NHC)center dot AlH3 and (iPr(2)Im)center dot AliBu(2)H reacted with cAAC(Me) by insertion of the carbene carbon atom into the Al-H bond to give (NHC)center dot AlH2/iBu(2)(cAAC(Me)H) (13-18) instead of ligand substitution, ring-expansion, or ring-opened products.