Highly Active Superbulky Alkaline Earth Metal Amide Catalysts for Hydrogenation of Challenging Alkenes and Aromatic Rings

Abstract

Tw oseries of bulkyalkaline earth (Ae) metal amide complexes have been prepared:A e[N(TRIP) 2 ] 2 (1-Ae) and Ae[N(TRIP)(DIPP)] 2 (2-Ae) (Ae = Mg, Ca, Sr,B a; TRIP = SiiPr 3 ,D IPP = 2,6-diisopropylphenyl). While monomeric 1-Ca was already known, the new complexes have been structurally characterized.M onomers 1-Ae are highly linear while the monomers 2-Ae are slightly bent. The bulkier amide complexes 1-Ae are by far the most active catalysts in alkene hydrogenation with activities increasing from Mg to Ba. Catalyst 1-Ba can reduce internal alkenes like cyclohexene or 3-hexene and highly challenging substrates like 1-Me-cyclohexene or tetraphenylethylene.I ti sa lso active in arene hydrogenation reducing anthracene and naphthalene (even when substituted with an alkyl) as well as biphenyl. Benzene could be reduced to cyclohexane but full conversion was not reached. The first step in catalytic hydrogenation is formation of an (amide)AeH species,w hich can form larger aggregates. Increasing the bulk of the amide ligand decreases aggregate sizeb ut it is unclear what the true catalyst(s) is (are). DFT calculations suggest that amide bulk also has an oticeable influence on the thermodynamics for formation of the (amide)AeH species.C omplex 1-Ba is currently the most powerful Ae metal hydrogenation catalyst. Due to tremendously increased activities in comparison to those of previously reported catalysts,t he substrate scope in hydrogenation catalysis could be extended to challenging multi-substituted unactivated alkenes and even to arenes among which benzene. Scheme 4. Energy profiles (DH in kcal mol À1 )for a) the hydrogenation of ethylene by catalysts 1-Ca(orange), 1-Ba (black) and CaN'' 2 (red), and b) benzene hydrogenation by 1-Ba;B3PW91/def2tzvpp including correction for dispersion (GD3BJ) and solvent (PCM = benzene).

Metallic Barium: A Versatile and Efficient Hydrogenation Catalyst

“…As a first test case we chose the reduction of cyclohexene, a challenging substrate [12] that previously only could be converted by the hitherto most reactive group 2 metal catalyst with a superbulky amide ligand: Ba[N(Si i Pr 3 ) 2 ] 2 [5d] . Noticeably, with a catalyst loading of 5 mol % (H 2 12 bar, 80 °C) cyclohexene was fully reduced within 0.5 hour (Table 1, entry 1).…”

“…Under forced conditions (150 °C, 50 bar), benzene was quantitatively converted to cyclohexane (entry 1). Although a prolonged reaction time of six days is needed, it should be noted that with Ba[N(Si i Pr 3 ) 2 ] 2 after three days only 18 % conversion could be reached [5d] . Conjugated rings in naphthalene or biphenyl reacted smoothly (entries 2,3) but anthracene gave essentially no conversion (entry 4), also not under forced conditions (150 °C, 50 bar).…”

“…One disadvantage of the bulky Ba amide catalyst Ba[N(Si i Pr 3 ) 2 ] 2 is its inability to hydrogenate substrates that contain heteroatoms like O or N. This was attributed to substrate‐metal coordination, blocking potential coordinations sites for H 2 activation [5d] . Metallic Ba, however, is able to catalyze the hydrogenation of (iso)quinoline and pyridines (entries 10–13).…”

“…[9] These capping ligands solubilize the Ae hydride clusters but also prevent further aggregation to insoluble (AeH 2 ) n salts. It was found that increasing the size of the amide ligand from N(SiMe 3 ) 2 to N(Si i Pr 3 ) 2 led to a considerable boost in catalytic activity and broadened the substrate scope from alkenes to arenes [5d] . This has been rationalized by the fact that larger capping ligands like N(Si i Pr 3 ) 2 form smaller, more active, metal hydride clusters.…”

Contemporary Molecular Barium Chemistry

“…The Ba‐amide precatalyst has the advantage over the barium‐hydride cluster [Ba 7 H 7 {N(SiMe 3 ) 2 } 7 · (C 6 H 6 ) 2 ] of a much simpler synthesis, even if the catalytic results are slightly poorer. More recently, the bulkier amido precursors [Ba{N(Si i Pr 3 )(DiPP)} 2 ] and [Ba{N(Si i Pr 3 ) 2 } 2 ] were used to generate in situ “(amide)BaH” clusters for the hydrogenation of alkenes . The precatalyst [Ba{N(Si i Pr 3 ) 2 } 2 ] was found to be the most active, a result attributed to its increased steric bulk favoring smaller, more active agglomerates in solution.…”