Catalytic asymmetric heterogeneous aziridination of styrene using CuHY: effect of nitrene donor on enantioselectivity

Abstract

The copper-catalysed aziridination of styrene with copper-exchanged zeolite Y (CuHY) and copper() triflate (trifluoromethanesulfonate) (Cu(OTf ) 2 ) as catalysts is described in detail. Two nitrene donors, [N-( p-tolylsulfonyl)imino)]phenyliodinane (PhI᎐ ᎐ NTs) and [N-( p-nitrophenylsulfonyl)imino]phenyliodinane (PhI᎐ ᎐ NNs) are compared. Modification of the catalyst with bis(oxazolines) affords enantioselective catalysts and a range of chiral bis-(oxazolines) has been studied. The ratio of nitrene donor to styrene is shown to be an important factor controlling both the yield and ee of aziridine formed. The best results are obtained with PhI᎐ ᎐ NNs; ee, ≥ 90%, together with high yields (≥ 85%), can readily be achieved with this nitrene donor using acetonitrile as solvent. Addition of the nitrene donor over a period of time, rather than all at the start of the reaction, is shown to enhance the yield of the aziridine but the ee is significantly decreased for both the homogeneous and the heterogeneous catalysts. Experiments in which the breakdown products of the nitrene donor, iodobenzene and the corresponding sulfonamide, are added at the start of the reaction show that a complex interplay exists at the copper active site between the reactants, products, chiral modifier and the solvent. However, the heterogeneous catalyst, CuHY, is found to give enhanced enantioselection for a range of bis(oxazolines) compared to the homogeneous catalyst, and the effect is considered to be due to the confinement of the catalyst within the micropores of the zeolite.

Bis(oxazoline)-copper complexes, immobilized by electrostatic interactions, as catalysts for enantioselective aziridination

“…Sulfur analysis of the residue after exchange shows that part of the triflate remains on the solid, probably due to substitution of only one of the triflate counter-ions of 4-Cu(OTf) 2 with the anionic centers of the solid. Several authors have described that the reaction results are strongly dependent on the reaction conditions, 4,6,12 in particular the relative amounts of styrene, nitrene precursor, Cu salt and chiral ligand. Therefore we tried to optimize the results in homogeneous phase before applying the immobilized catalysts.…”

Heterogeneous Enantioselective Catalyzed Carbonyl‐ and Imino‐Ene Reactions using Copper Bis(Oxazoline) Zeolite Y

“…95 %, can be designed on this basis. [7] In addition, we have shown that the copper bis(oxazoline) complex, when constrained within the zeolite structure, [8] gives substantially higher enantioselection than the homogeneous catalysts. [7,9] Similar effects have been noted for other immobilized catalysts.…”

“…[7] In addition, we have shown that the copper bis(oxazoline) complex, when constrained within the zeolite structure, [8] gives substantially higher enantioselection than the homogeneous catalysts. [7,9] Similar effects have been noted for other immobilized catalysts.[10] We have now found that bis(oxazoline)-modified CuH zeolite Y catalysts are extremely effective catalysts for asymmetric carbonyl-and imino-ene reactions and, furthermore, these catalysts can be readily recovered and reused without loss of catalyst performance.The reaction of ethyl glyoxylate with a range of alkenes has been investigated using the immobilized copper-zeolite Y (Cu-HY) bis(oxazoline) catalyst and the results are shown in Table 1. In the absence of the bis(oxazoline), negligible reaction was observed.…”

“…This is considered to be due to the confinement of the heterogeneous catalyst within the zeolite pores, and we have also observed this in our previous studies on aziridination. [7] Following the reaction of methyl pyruvate and a-methylstyrene (Scheme 1), the catalyst was recovered, dried, and reused giving the same results. The aqueous phase from the crude-product washing step was analyzed, and it was determined that only approximately 1 % of the copper added with the heterogeneous catalyst leached, under the reaction conditions, into the reaction mixture.…”

Tethering of Dinuclear Complexes to SBA‐15 and Their Application in CO2 Hydrogenation

“…In general, this "heterogenisation" has taken place using differential catalyst solubility approaches, [24][25] as well as through the use of non-covalent, [26][27] or covalent, [27][28][29][30][31][32][33] interactions between the molecular catalyst and the surface of the solid material either following modification of the surface of the solid or the catalytic material or both.…”