Mechanistic Models for LAH Reductions of Acetonitrile and Malononitrile. Aggregation Effects of Li⁺ and AlH₃ on Imide-Enamide Equilibria


The results are reported of an ab initio study of the addition of LiAlH4 to acetonitrile and malononitrile at the MP2(full)/6-311+G* level considering the effects of electron correlation at higher levels up to QCISD(T)/6-311++G(2df,2pd) and including ether solvation. All imide (RCH2CH═N-) and enamide (RCH-CH═NH ↔ RCH═CHN-H) adducts feature strong interactions between the organic anion and both Li+ and AlH3. The relative stabilities of the tautomeric LAH adducts are compared to the tautomer preference energies of the LiH adducts and of the hydride adducts of the nitriles. Alane affinities were determined for the lithium ion pairs formed by LiH addition to the nitriles. The results show that alane binding greatly affects the imide-enamide equilibria and that alane complexation might even provide a thermodynamic preference for the imide intermediate. While lithium enamides of malononitrile are much more stable than lithium imides, alane binding dramatically reduces the enamide preference so that both tautomers are present at equilibrium. Implications are discussed regarding to the propensity for multiple hydride reductions and with regard to the mechanism of reductive nitrile dimerization. A detailed mechanism is proposed for the formation of 2-aminonicotinonitrile (2ANN) in the LAH reduction of malononitrile.



Keywords and Phrases

Ab initio study; Enamides; Hydride reduction; Lithium imide; Lithium ions; Malononitriles; Mechanistic models; Organic anions; Relative stabilities; Strong interaction; Acetonitrile; Binding energy; Complexation; Cyanides; Ethers; Hydrides; Lithium; 2 aminonicotinonitrile; acetonitrile; aluminum lithium hydride; imide; malononitrile; unclassified drug; ab initio calculation; addition reaction; complex formation; electron; molecular interaction; reduction; solvation; tautomer

International Standard Serial Number (ISSN)

0022-3263; 1520-6904

Document Type

Article - Journal

Document Version


File Type





© 2013 American Chemical Society (ACS), All rights reserved.

Publication Date

01 Jan 2013