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

Abstract

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 tBu2Al+ 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.

Poly(pyrazolyl)aluminate Complexes Containing Aluminum–Hydrogen Bonds

“…Pathways that lead to hydride and pyrazolate transfer with 2 presumably also afford the neutral species Al(Ph 2 pz) 3 and [AlH(Ph 2 pz) 2 ] n , but examples of these complexes have not been crystallized in pure form so far. However, Al(tBu 2 pz) 3 and Al hydride pyrazolates have been reported. The present work also implies that analogues of 1 – 4 containing 1,2,4-triazolyl and tetrazolyl groups should be easily prepared by protonolysis routes between LiAlH 4 and the hydrogen-substituted heterocycles because some of the corresponding B ligands are known. ,, Al-based ligands containing 1,2,4-triazolyl and tetrazolyl groups would have high nitrogen contents, should be highly endothermic, and would thus be relevant to ongoing efforts to construct energetic metal salts for a variety of applications …”

Aluminum, Gallium, and Indium Hydrazides and the Generation of Oligonuclear Element-Nitrogen Cages: Molecular Intermediates on the Way to Element Nitrides

“…The remarkable octagallium compound (GaMe) 8 [N(H)N(CMe 3 )] 4 O 4 ( 25 ; Figure ) was formed in a reproducible manner in 23% yield in the presence of stoichiometric quantities of water . Further 1,2-hydrazinediides have been obtained on other routes, , such as hydroalumination of 2,3-diazabutadienes , or a diazene, the reaction of hydrazides with element hydrides, ,,,, the treatment of element halides with dilithium bis(trimethylsilyl)hydrazide, or the reaction of a gallium(I) aryl compound with a diaryldiazene . These methods have considerable disadvantages such as the formation of inorganic salts as byproducts, the coordination of lithium cations to the hydrazide groups, , N–N bond cleavage reactions, , or limited accessibility of the starting materials.…”

“…Further 1,2-hydrazinediides have been obtained on other routes, , such as hydroalumination of 2,3-diazabutadienes , or a diazene, the reaction of hydrazides with element hydrides, ,,,, the treatment of element halides with dilithium bis(trimethylsilyl)hydrazide, or the reaction of a gallium(I) aryl compound with a diaryldiazene . These methods have considerable disadvantages such as the formation of inorganic salts as byproducts, the coordination of lithium cations to the hydrazide groups, , N–N bond cleavage reactions, , or limited accessibility of the starting materials. Hydroalumination of tetramethyl-2,3-diazabutadiene has been used for generation of a unique cage and an unprecedented tricyclic compound .…”

“…Hydroalumination of tetramethyl-2,3-diazabutadiene has been used for generation of a unique cage and an unprecedented tricyclic compound . The applicability of this method is, however, rather limited because, in most cases, only half of the CN double bonds are reduced to yield hydrazonides. , …”

Chemoselective Hydroalumination of 1-Aza-but-1-en-3-ynes (C-Iminoalkynes): Formation of Propargylamines by Imine Reduction and of 5-Aluminazoles and 1-Aza-butadienes by Anti-Michael Attack

“…Due to their conjugated triple and double bonds, 1-aza-but-1-en-3-ynes 1 form a highly interesting class of compounds with challenging regio- and chemoselectivities in hydroalumination reactions . The polar iminic double bond is expected to induce higher reactivity compared to that of but-1-en-3-ynes like 2 . Figure presents the NBO charges, based on B3LYP/def2TZVP calculations.…”