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In the production of aromatic N-oxides using the oxidation of N-containing heterocyclic aromatic substrates with H 2 O 2 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.

The immobilisation of enantioselective homogeneous catalysts is described using an approach in which cations are immobilised by ion-exchange within a microporous (zeolite Y) or a mesoporous material (Al-MCM-41). The catalysts are used under nonaqueous conditions so that cation leaching is minimised during the catalytic reaction. The cations can be modified using chiral ligands to form an enantioselective catalyst that is wholly heterogeneous and can be readily reused. Using this approach it is shown that copper-exchanged zeolite Y is a highly active catalyst for the aziridination of alkenes. Modification using bis(oxazolines) leads to the formation of an enantioselective aziridination catalyst. Using a similar approach, manganese-exchanged Al-MCM-41 modified with a chiral salen ligand is found to be an effective enantioselective heterogeneous epoxidation catalyst for cis-stilbene. The approach is also used to show that immobilisation of cobalt-exchanged Al-MCM-41 can also give some activity for the enantio selective epoxidation of cis-stilbene. However, in this case, although only low enantioselection is observed, the immobilised catalyst gives higher enantioselection than the non-immobilised homogeneous catalyst.

Copper-exchanged zeolite Y is a highly active catalyst for the aziridination of alkenes. Modification using bis(oxazolines) leads to the formation of an enantioselective aziridination catalyst. Using a similar approach, manganese-exchanged MCM-41 modified with a chiral salen ligand is found to be an effective enantioselective heterogeneous epoxidation catalyst for cis-stilbene.

Manganese-exchanged Al-MCM-41 modified by the chiral salen ligand [(R,R)-(-)-N,N'-bis(3,5-di-tert-butylsalicylidene)cyclohexane-1,2-diamine] can be used as an enantioselective heterogeneous epoxidation catalyst using iodosyl benzene as oxygen donor. Epoxidation of (Z)- and (E)-stilbene is studied in detail and experiments are described that demonstrate that the reaction is wholly catalysed heterogeneously. Similar enantioselectivity is observed for the oxidation of (Z)-stilbene to the (E)-epoxide using homogeneous (77.5% ee) or heterogeneous (70% ee) catalysts. The effect of temperature, solvents and donor ligands on the yield and enantioselection are discussed.

Manganese-exchanged Al-MCM-41 modified by the chiral salen ligand [(R,R)-(-)-N,N'-bis(3,5-di-tert-butylsalicylidene)cyclohexane-1,2-diamine] 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.

Copper-exchanged zeolite Y (CuHY) is found to be a highly effective heterogeneous catalyst for the aziridination of alkenes using (N-(p-tolylsulfonyl)imino)phenyliodinane (PhI=NTs) as the nitrogen source:[GRAPHICS]Exchange of zeolite Y with other cations (Ag+, Co2+, Fe3+, Mg2+, Ni2+, Zn2+) was found to be ineffective and the yield of the aziridine was lower than that obtained if no catalyst was present. This is considered to be due to the ability of these metals to catalyze the breakdown of the PhI=NTs reagent into iodobenzene and toluene sulfonamide. Modification of the CuHY catalyst with bis(oxazolines) leads to the preparation of the first heterogeneous enantioselective aziridination catalyst and the results showing the effect of temperature and modifier concentration are described and discussed. The optimum reaction conditions for the aziridination of styrene are found to be using acetonitrile solvent at -10 degrees C with a Cu2+: bis(oxazoline) ratio of 2:1, and under these conditions, e.e. of 34-35% have been observed. The catalyst can be recovered and reused without significant loss of catalyst performance. (C) 1999 Elsevier Science B.V. All rights reserved.

Copper-exchanged zeolite Y (CuHY) is found to be a highly effective heterogeneous catalyst for the aziridination of alkenes using [N-(p-tolylsulfonyl)imino]phenyliodinane (PhI=NTs) as the nitrogen source. Exchange of zeolite Y with other cations (Ag+, Co2+, Fe3+, Mg2+, Ni2+, Zn2+) was found to be ineffective. This is considered to be due to the ability of these metals to catalyse the breakdown of the PhI=NTs reagent into iodobenzene and toluene sulfonamide. Modification of the CuHY catalyst with bis(oxazolines) leads to preparation of the first heterogeneous enantioselective aziridination catalyst and the results showing the effect of temperature and modifier concentration are described and discussed. A pyridine-bridged bis(oxazoline) was observed to give the highest enantioselectivity of 61% ee for the aziridination of styrene using acetonitrile as solvent and at -10 degrees C.

Copper-exchanged zeolite Y is a highly active catalyst for the aziridination of alkenes; modification using bis(oxazolines) leads to preparation of the first heterogeneous enantioselective aziridination catalyst.