Treatment of Tetramethylpyrazole and 3,5-Diphenylpyrazole with Dialkylaluminum Hydrides - Hydroalumination versus Deprotonation

Treatment of tetramethylpyrazole, N2C3Me4, with equimolar quantities of di(tert-butyl)aluminum hydride leads to the addition of an Al–H bond to one of the C=N double bonds. The dimeric product (1) contains a central six-membered Al2N4 ring in which two Bu2Al units are bridging two N2C3 heterocycles. In the zwitterionic, non-centrosymmetric compound one aluminum atom is coordinated by two imino nitrogen atoms, while the second one is bonded to two amide nitrogen atoms. No double hydroalumination occurs upon treatment of tetramethylpyrazole with two equivalents of the hydride. Instead, an adduct (2) of the monomeric hydroalumination product with di(tert-butyl)aluminum hydride was isolated in which the two aluminum atoms are connected by a 3c-2e Al–H–Al bond. A unique trinuclear compound (3) is obtained upon reaction of tetramethylpyrazole with an excess of the sterically less shielded diethylaluminum hydride. It contains two different N2C3 heterocycles: One still contains a C=N double bond similar to that in compounds 1 and 2, while the second one is completely reduced by double hydroalumination to give a saturated heterocycle. The two rings are bridged by three AlEt2 groups. Deprotonation results upon treatment of 3,5-diphenylpyrazole, N2C3H2(C6H5)2, with di(tert-butyl)aluminum hydride.