Abstract
Development of a simple and easy handing process for preparation of multifunctional heterogenous catalysts and exploration of their applications in sequential organic transformation are of great significance in heterogeneous asymmetric catalysis. Herein, through the utilization of a anion–bonding strategy, we anchor conveniently both organic bases and chiral ruthenium complex into the nanopores of Me-FDU−12, fabricating a Lewis base/Ru bifunctional heterogeneous catalyst. As we envisaged, cyclic amine as a Lewis base promotes an intermolecular aza–Michael addition between enones and arylamines, affording γ-secondary amino ketones featuring with aryl motif, whereas ruthenium/diamine species as catalytic promoter boosts an asymmetric transfer hydrogenation of γ-secondary amino ketones to γ-secondary amino alcohols. As expected, both enhance synergistically the aza–Michael addition/asymmetric transfer hydrogenation one–pot enantioselective organic transformation, producing chiral γ-secondary amino alcohols with up to 98% enantioselectivity. Unique features, such as operationally simple one–step synthesis of heterogeneous catalyst, homo–like catalytic environment as well as green sustainable process make this heterogeneous catalyst an attracting in a practical preparation of optically pure pharmaceutical intermediates of antidepressants.
Highlights
With the extensive explorations of mesoporous silicas as supports in catalysis, various strategies have been employed to construct supported molecular catalysts(Song, 2005; Heitbaum et al, 2006; Yang et al, 2007; Margelefsky et al, 2008; Thomas and Raja, 2008; Bartók, 2010; Yu and He, 2012)
In the part of chiral ruthenium/diamine–complexes, besides the general carbon peaks around 70 ppm for the carbon atoms of –NCH groups and around 130 ppm for the carbon atoms of aromatic groups from TsDPEN, the characteristic peaks found at 108.4 and 101.5 ppm could be attributed to the carbon of mesitylene, while that at 23.3 ppm belonged to the carbon atoms of the CH3 groups of mesitylene
To confirm DABCO molecules and chiral ruthenium/diamine–complexes dual active centers immobilized via the BF−4 anions, the solid–state 19F MAS NMR spectrum of catalyst 1 was used to compare with those of two corresponding single immobilized DABCO molecules and chiral ruthenium/diamine–complexes on Me-FDU-12
Summary
With the extensive explorations of mesoporous silicas as supports in catalysis, various strategies have been employed to construct supported molecular catalysts(Song, 2005; Heitbaum et al, 2006; Yang et al, 2007; Margelefsky et al, 2008; Thomas and Raja, 2008; Bartók, 2010; Yu and He, 2012). We employ a large–pore size of FDU−12 as a support and assemble a Lewis base and a chiral ruthenium/diamine dual species within its regular nanopores via a BF−4 anion–bonding approach, fabricating a bifunctional heterogeneous catalyst.
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