Epoxidation of crotyl alcohol using Ti-containing heterogeneous catalysts: Comments on the loss of Ti by leaching

The leaching of Ti from the redox molecular sieve TS-1 is compared and contrasted with Ti leaching from Ti-Al beta, Ti-MCM-41, and a Ti-xerogel under both batch and continuous flow reaction conditions for the oxidation of crotyl alcohol with hydrogen peroxide. The product distributions for the catalysts are similar, with epoxide being formed initially and, subsequently, the secondary reaction products of the trial and ether dials. Ti leaching is more pronounced under continuous flow conditions and the order of stability is TS-1 > Ti-Al beta > Ti-MCM-41 > Ti-xerogel, for both continuous flow and batch reaction conditions. TS-1 leaching is shown to be caused by the reaction of trial with TS-1 in the presence of hydrogen peroxide. A possible mechanism in which the triol byproduct chelates the Ti and breaks the Ti-O-Si framework bonds, leading in turn to the formation of a Ti species in solution, is proposed. The soluble Ti species is found to be an active homogeneous catalyst giving triol as the major product from crotyl alcohol. Under batch reaction conditions, it is possible that an equilibrium is established between the solution Ti species and Ti on the catalyst surface. This equilibrium is perturbed when continuous flow conditions are used, and this leads to increased Ti leaching. (C) 2001 academic Press

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

The copper-catalysed aziridination of styrene with copper-exchanged zeolite Y (CuHY) and copper(II) triflate (trifluoromethanesulfonate) (Cu(OTf)(2)) as catalysts is described in detail. Two nitrene donors, [N-(p-tolylsulfbnyl)-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 bean important factor controlling, both the yield and ee of aziridine formed. The best results are obtained with PhI=NNs;ee, greater than or equal to 90%, together with high yields (greater than or equal to 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

Catalytic asymmetric heterogeneous aziridination of styrene using CuHY/bis(oxazoline): comments on the factors controlling enantioselectivity

The copper-catalyzed aziridination of styrene is described using both heterogeneous, copper-exchanged zeolite HY, and homogeneous, copper (II) triflate catalysts using both [N-(p-tolylsulfonyl)imino]phenyliodinane (PhI=NTs) and [N-(p-nosylsulfonyl)imino]phenyliodinane (PhI=NNs) as nitrene donors. The key differences observed for the two catalysts when modified by chiral bis(oxazoline) ligands are discussed in detail. In particular, the heterogeneously catalyzed asymmetric reaction can give much higher enantioselection than the comparable homogeneously catalyzed reaction. The structure of the bis(oxazoline) ligand is the critical factor, and bis(oxazoline) ligands that are ineffective with the homogeneous catalysts are highly effective for the Cu2+ cation constrained within the zeolite micropores. The consequences of this observation for the design of chiral ligands for asymmetric heterogeneous catalysis are discussed. The effect of the degree of styrene conversion on the enantioselectivity is described in detail using PhI=NNs as a nitrene donor. The reaction shows a significant enhancement in ee with conversion at 25degreesC, and the possible origin of this effect is discussed

Heterogeneous aziridination of styrene using N-(p-nitrophenylsulfonyl)imino phenyliodinane as nitrene donor: influence of the reaction parameters on yield and enantioselectivity

The copper-catalysed aziridination of styrene with copper-exchanged zeolite Y (CuHY) and copper(II) triflate (Cu(OTf)(2)) as catalysts is described and discussed. In particular, the effects of reaction conditions on the yield and enantiomeric excess of the aziridine product are described using [N-(p-nitrophenyisulfonyl)imino]phenyliodinane (PhI=NNs) as nitrene donor. By careful control of the styrene:nitrene donor molar ratio and the solvent, an ee of 95% can be obtained for the heterogeneously catalysed bis(oxazoline)-modified zeolite CuHY The ee achieved with the zeolite immobilised catalyst is significantly higher than that achieved for the non-immobilised homogeneous catalyst under comparable reaction conditions. (C) 2002 Elsevier Science B.V. All rights reserved

N-oxidation of pyridines by hydrogen peroxide in the presence of TS-1

In the production of aromatic N-oxides using the oxidation of N-containing heterocyclic aromatic substrates with H2O2 as oxidant, the non-catalysed homogeneous oxidation is found to play an important part in the overall reaction. In addition, when TS-1 is used as a catalyst, there are many potential competitive interactions between the catalyst, the reactants and the products, which limit the effectiveness of the catalyst. It is concluded that the use of TS-1 and other microporous catalysts for the heterogeneous N-oxidation of pyridine and substituted pyridines needs to be interpreted with caution

Enantioselective epoxidation of (Z)-stilbene using a chiral Mn(III)-salen complex: effect of immobilisation on MCM-41 on product selectivity

Manganese-exchanged Al-MCM-41 modified by the chiral salen ligand [(R,R)-(-)-N,N'-bis(3,5-di-tert-butylsalicylidene)cyclohexane-1,2-diamin e] has been investigated as a heterogeneous catalyst for the enantioselective epoxidation of (Z)-stilbene using iodosylbenzene as oxygen donor, with particular interest in the effect of reaction conditions on the cis:trans ratio of the epoxide product. Immobilisation of the chiral Mn-salen complex in Al-MCM-41 increases the cis:trans ratio of the epoxide product when compared to the non-immobilised complex under the same conditions. Increasing the level of Mn-exchange in the Al-MCM-41 increases the amount of trans-epoxide, whereas increasing the iodosylbenzene:substrate ratio increases the amount of cis product formed. Increasing the reaction temperature also increases the amount of trans-epoxide for the homogeneous Mn-complex under the same conditions. A series of experiments is described in which the external ion-exchange sites on Al-MCM-41 are preferentially silanised, which enables the cis/trans selectivity for external and internal sites to be determined. Mn-salen immobilised on the external surface of Al-MCM-41 gives the same cis:trans ratio as that observed with the non-immobilised Mn-salen complex in solution, whereas Mn-salen immobilised within the pores gives the cis-epoxide preferentially. The enantioselection of the immobilised chiral Mn-salen complex is shown to decrease with reaction time at -10 degrees C, but the cis:trans epoxide ratio remains unchanged; whereas for the non-immobilised complex in solution the enantioselection is independent of reaction time. Iodobenzene, a decomposition product formed from iodosylbenzene, is found to act as a poison for the immobilised catalyst, leading to a slower reaction and lower enantioselection