Activation of Isocyanates and Carbon Dioxide by a Monomeric Aluminium Hydrazide as an Active Lewis Pair

The monomeric aluminium hydrazide H10C5NN(AltBu2)Ad (4; Ad=adamantyl, NC5H10=piperidinyl) was obtained in high yield by hydroalumination of the corresponding hydrazone derivative 1. Compound 4 has a strained AlN2 heterocycle formed by a donor–acceptor bond between the β‐nitrogen atom of the hydrazide group and the aluminium atom. Opening of this bond resulted in the formation of an active Lewis pair that was able to cooperatively activate carbon dioxide or isocyanates. Insertion of the heterocumulenes into the AlN bond selectively afforded a carbamate and two urea derivatives in high yield. In the first step, phenyl isocyanate gave the adduct 6, which has the oxygen atom coordinated to the aluminium atom and its central carbon atom bound to the nitrogen atom of the piperidine moiety. Adduct 6 represents a reasonable intermediate state for these activation processes. The applicability of hydroaluminated compounds, such as 4, in organic synthesis was demonstrated by the reaction with an imidoyl chloride, which gave the corresponding amidrazone derivative 9. Activation and insertion: A monomeric dialkylaluminium hydrazide was obtained by hydroalumination of a sterically encumbered hydrazone with di(tert‐butyl)aluminium hydride. The ring strain in the three‐membered AlN2 heterocycle favours the activation of small molecules and facilitates highly selective insertion into AlN bonds to yield carbamate and urea derivatives (see scheme).