Imine hydrogenation with simple alkaline earth metal catalysts

Abstract

W ith molecular hydrogen being one of the cleanest reducing agents, catalytic hydrogenation using the more noble transition metals is among the most studied of all chemical processes 1 . Increasing social pressure towards a sustainable society, however, dictates replacement of costly, and often harmful, precious metals by more abundant first-row transition metals or even biocompatible redox inactive main group metals [2][3][4][5][6] . The alkaline earth metal calcium does not possess partially filled d orbitals for substrate activation, but has recently shown catalytic activities in the hydrogenation of C= C double bonds with molecular H 2 (ref. 7 ). Although restricted to conjugated C= C bonds, this example strikingly broke the dogma that transition metals are needed for alkene hydrogenation. This was followed by the development of metal-free frustrated Lewis pair (FLP) catalysts [8][9][10][11] and, most recently, cationic calcium hydride catalysts that are also able to hydrogenate unactivated alkenes 12 . Figure 1a shows a working hypothesis for styrene hydrogenation with a dibenzylcalcium catalyst (CaBn 2 ) 7 . The first step is the generation of a calcium hydride species, for which ample precedence exists [13][14][15][16][17] . Further reaction with H 2 may cause precipitation of insoluble (CaH 2 ) n , but catalyst loss is partly prevented by aggregation to soluble but undefined Ca x Bn y H z species. Despite a lack of d orbitals, alkene activation proceeds through a weak electrostatic calciumalkene interaction, recently shown to be of importance in calcium catalysis 18 . The benzylic calcium intermediate formed after insertion may, after successive styrene insertions, form polystyrene 19 , but high H 2 pressure (20-100 bar) can prevent this side reaction by promoting σ-bond metathesis. The latter step in the cycle is, like the initiation reaction, formally a deprotonation of H 2 by a resonance-stabilized benzylic carbanion. Considering the high pK a of H 2 (≈ 49) 20 , this reaction seemed questionable. Stoichiometric conversions of model systems, however, underscored the feasibility of this pathway 7 . Independent theoretical calculations illustrate that the final σ-bond metathesis step is indeed highly endergonic: Gibbs free energy of activation Δ G ‡ (60 °C, 20 bar) = 25.7 kcal mol −1 (ref. 21 ).As the highly atom-efficient catalytic reduction of imines by H 2 received much less attention than alkene or ketone hydrogenation [22][23][24] , it remained an important question whether calcium-catalysed hydrogenation can be extended to imine reduction. Current stateof-the-art imine hydrogenation catalysts can be divided into four categories that vary in terms of substrate activation and nucleophilic power ( Fig. 2a-d). Figure 2a shows organometallic metal hydrides that rely on hydride nucleophilicity. Apart from few early transition metal catalysts (Ti 25 , lanthanides 26 ), these are generally based on late transition metals (Rh, Ir) 22 . The aluminium hydride compound (iso-butyl) 2 AlH is an odd example of a ...

Metal⋅⋅⋅F−C Bonding in Low‐Coordinate Alkaline Earth Fluoroarylamides

“…[23] The interatomic distances to the terminal N C6F5 groups,a t2 .385(2) and 2.383(2) ,a re comparable to those in 4'.T aking the three Na toms into account, each calcium in 5 2 lies in ap erfect trigonal planar geometry (S q (Ca1) = 359.958 and S q (Ca2) = 359.908). [26,27] The centrosymmetric barium complex [Ba{m-N(C 6 F 5 ) 2 }-{N(C 6 F 5 ) 2 }·toluene] 2 (6 2 )i sd inuclear ( Figure 5). Complex 5 2 is ar are example of charge-neutral three-coordinate calcium complex free of coordinated solvent;o ther representative examplesi nclude [Ca{N(SiMe 3 ) 2 } 2 ] 2 and [{BDI DiPP }CaN(SiMe 3 ) 2 ]( 3) already mentioned, [6e,8,23] as well as [Ca(O-tBu 2 -2,6-C 6 H 3 ) 2 ] 2 , [24] the N-heterocyclicc arbene adducts [Ca{N(SiMe 3 ) 2 } 2 NHC aryl ] (aryl = mesitylo r2 ,6-iPr 2 C 6 H 3 ), [25] and the imine adduct [Ca{N(Si-Me 3 ) 2 } 2 PhCH=NtBu].…”

Barium‐Catalysed Dehydrocoupling of Hydrosilanes and Borinic Acids: A Mechanistic Insight

“…Along with other main‐block elements, the large alkaline earth (AE) metals calcium, strontium and barium have been attracting wide interest as viable alternatives to expensive, and often toxic, late transition metals, to devise molecular (pre)catalysts that are implemented in a variety of organic transformations . AE‐based catalytic systems show excellent overall performance in reactions such as the hydroelementations of unsaturated carbon=carbon, C=N or C=O bonds, as well as polymerisations and dehydrocouplings . Beyond their large size, the originality of AE catalysts often results from their highly electropositive nature, which increases upon descending group 2 .…”

LiAlH4: From Stoichiometric Reduction to Imine Hydrogenation Catalysis

“…[13][14][15] Recently, Cp*ZnH and iBu 2 AlH were found to be catalytically active in imine hydrogenation but harsh working conditions,including high H 2 pressure (70-100 bar), are needed. [21] This finding was rather unexpected since the catalyst initiation step (Scheme 1) is formally ad eprotonation of H 2 (pK a % 49), [22] thus giving the metal hydride catalyst and amine N''Hw ith am uch lower pK a (25.8). [19] Since the imine itself is aL ewis base,b ulky imines can be reduced by B(C 6 F 5 ) 3 as ac atalyst.…”

“…[19] Since the imine itself is aL ewis base,b ulky imines can be reduced by B(C 6 F 5 ) 3 as ac atalyst. [21] Given the simplicity of these group 2m etal catalysts,w e wondered whether LiAlH 4 ,w hich in imine reduction is normally added stoichiometrically,c ould be catalytically active.A lready in the 1960s,L iAlH 4 was used as ac atalyst for alkyne reduction, [24] however, the very harsh conditions needed (35 mol %c at, 190 8 8C, 80 bar H 2 )d id not encourage follow-up research. [21] This finding was rather unexpected since the catalyst initiation step (Scheme 1) is formally ad eprotonation of H 2 (pK a % 49), [22] thus giving the metal hydride catalyst and amine N''Hw ith am uch lower pK a (25.8).…”

“…[21] Theresults of catalytic runs with LiH or AlH 3 alone,orwith NaAlH 4 and NaBH 4 ,d iffer from the LiAlH 4 -catalyzed reaction. Va riation of substrates generally needs somewhat higher H 2 pressures (6-25 bar) and longer reaction times,t hus limiting its scope when compared to that of group 2metal catalysts.…”