An unexpected 1,2-alkyl shift within a chelate bonded organoaluminium-enamine

The organozinc-enamines EtZn[R′NCHCHN(Et)R′] (R ^tBu (1a), ^tpent (2a),ⁱPr (3a)) react with Et₃Al via a transmetallation reaction to the corresponding organoaluminium-enamines Et₂Al[R′NCH CHN(Et)R′] (R′ ^tBu (1c), ^tpent (2c)) and Et₂Zn. Prior to the transmetallation reaction initial 1:1 zinc-enamine-trialkylaluminium adducts are formed. Compounds 1c and 2c are thermally unstable and rearrange via a 1,2-alkyl shift partly in combination with a hydrogen shift, to Et₂Al[R′NCHC(H)R′NEt] (R′ ^tBu (1d), ^tpent (2d)) (70%) and Et₂Al[R′NCH₂CR′NEt] (R′ ^tBu (1e), ^tpent (2e)) (30%). The driving force for the alkyl shift, which is restricted to tertiary alkyl groups, is release of steric strain in the five-membered NCCN aluminium chelate ring.