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 3 (NHC=Me 2 Im ( 1 ), Me 2 Im Me ( 2 ), i Pr 2 Im ( 3 and [D 3 ]‐3 ), i Pr 2 Im Me ( 4 ), Dipp 2 Im ( 10 ); Im=imidazolin‐2‐ylidene, Dipp=2,6‐diisopropylphenyl) and (NHC) ⋅ Al i Bu 2 H (NHC= i Pr 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 ( i Pr 2 Im) ⋅ AlH 3 ( 3 ) was treated with a second equivalent of the carbene i Pr 2 Im to give ( i Pr 2 Im) ⋅ AlH(RER‐ i Pr 2 ImH 2 ) ( 6 ). In 6 , {( i Pr 2 Im}AlH} is inserted into the NHC ring. In contrast, ring opening was observed with the sterically more demanding Dipp 2 Im with the formation of ( i Pr 2 Im) ⋅ AlH 2 (ROR‐Dipp 2 ImH 2 )H 2 Al ⋅ ( i Pr 2 Im) ( 9 ). In 9 , two {( i Pr 2 Im) ⋅ AlH 2 } 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 ( i Pr 2 Im) ⋅ Al i Bu 2 H ( 11 ) and (Dipp 2 Im) ⋅ Al i Bu 2 H ( 12 ). The adducts (NHC) ⋅ AlH 3 and ( i Pr 2 Im) ⋅ Al i Bu 2 H reacted with cAAC Me by insertion of the carbene carbon atom into the Al−H bond to give (NHC) ⋅ AlH 2 / i Bu 2 (cAAC Me H) ( 13 – 18 ) instead of ligand substitution, ring‐expansion, or ring‐opened products.