LiAlH₄: From Stoichiometric Reduction to Imine Hydrogenation Catalysis

Abstract

Dedicated to Professor Dietmar Stalke on the occasion of his 60th birthday.Abstract: Imine-to-amine conversion with catalytic instead of stoichiometric quantities of LiAlH 4 is demonstrated (85 8 8C, catalyst loading ! 2.5 mol %, pressure ! 1bar). The effects of temperature,p ressure,s olvent, and catalyst modifications,a s well as the substrate scope are discussed. Experimental investigations and preliminary DFT calculations suggest that the catalytically active species is generated in situ:L iAlH 4 + Ph(H)C=NtBu!LiAlH 2 [N(tBu)CH 2 Ph] 2 .Acooperative mechanism in whichL ia nd Al both play ap rominent role is proposed.

H2 Activation by Non‐Transition‐Metal Systems: Hydrogenation of Aldimines and Ketimines with LiN(SiMe3)2

“…In conclusion, LiN(SiMe 3 ) 2 hase merged as ar emarkably reactive agent for the H 2 activation coupled with imine hydrogenation. Our results, togetherw ith the recent studies of Alonso and Harder [10] and Stephan, [11] demonstrate considerable potential for group 1a nd 2m etal amides as hydrogenation catalysts. Compared to Group 2m etal amides, LiN(SiMe 3 ) 2 requires higher H 2 pressure (100 bar vs. 1-6 bar) butr eactions may be conducted at lower temperatures (50 8Ca nd in some cases even room temperature vs.8 0 8C).…”

“…[9] Duringt he final stages of our work presentedh erein, Alonso and Harder reported on the use of group 2m etal amides {Ca[N(SiMe 3 ) 2 ] 2 in particular} as catalysts for the hydrogenation of aldimines under mild conditions (6 bar of H 2 ,8 08C). [10] During the course of our work on H 2 activation with pyridinium salts and base, [4b,c] we studied the reductiono fi mine 1 (Scheme 2), which took place upon treatment with ap yridinium salt, LiN(SiMe 3 ) 2 ,a nd H 2 .W ef ound that the reaction did not proceed, as initially assumed, through hydridet ransfer from ad ihydropyridine generated from the pyridinium salt by deprotonationa nd subsequent addition of H 2 .I nstead, we ob-tained clear evidence for am echanism involving solely LiN-(SiMe 3 ) 2 and H 2 .C onsidering that such ap urely base-mediated hydrogenation of imines wasn ot knowna tt hat time, we decided to investigate this process in more detail. Our study shows that LiN(SiMe 3 ) 2 ,e ven in substoichiometric amounts, induces clean reduction of imines with H 2 as the reductant at moderate temperatures in the range 25-80 8C.…”

“…Our study shows that LiN(SiMe 3 ) 2 ,e ven in substoichiometric amounts, induces clean reduction of imines with H 2 as the reductant at moderate temperatures in the range 25-80 8C. Comparedt o the application range of Group 2m etal amides, [10] which is limited to aldimines, the reaction scope is remarkably broad including ketimines and even iminesb earing acidic protonsi n the a-position. [11] We initiallyf ocusedo ur study on the reductiono fa romatic aldimines prepared from benzaldehyde derivatives and aniline, cyclohexyl-, or benzylamine (Table1).…”

Heterometallic Mg−Ba Hydride Clusters in Hydrogenation Catalysis

“…Similar observations have been made for Li/Al hydride catalysts. [53][54][55] We herein present synthesis and structures of well-defined Mg/ Ba hydride clusters, which could be seen as potential intermediates during hydrogenation catalysis, and explore the effect of Mg/Ba metal-mixing in hydrogenation catalysis.…”

An Easy Approach to Obtain Alcohol-Amines by Reduction of Alcohol Functionalized Imines

“…The reduction of imines to isolate amines is a well-known field of study. In fact, it is one of the central reactions in organic chemistry, and the search for more efficient and practical synthetic methods for carrying out this reduction is a theme of constant interest [3][4][5]. Many reduction agents have been tested in order to produce the mentioned transformation, and H2 [6], silanes [7,8], boranes [9], and borohydrides [10] are maybe the most popular ones.…”