Frederick E. Hancock

Oxidation of crotyl alcohol using Ti-β and Ti-MCM-41 catalysts

A comparative study of the oxidation of the crotyl alcohol using hydrogen peroxide and tert-butyl hydroperoxide as oxidants with TS-1,Ti-beta, Ti-Al beta, Ti-MCM-41, Ti-Al-MCM-41 and Ti-grafted-MCM-41 as catalysts is described and discussed. With hydrogen peroxide as oxidant, significant Ti-leaching is observed with all the catalysts except TS-I (Ti-Al beta > Ti-grafted-MCM-41 > Ti-MCM-41 > Ti beta >Ti-Al-MCM-41 much greater than TS-I). For Ti-Al beta, Ti-grafted- MCM-41 and Ti-Al-MCM-41, initial heterogeneously catalysed formation of the epoxide was observed. However, the formation of a Ti-species in solution is shown to contribute to competing homogeneously catalysed formation of ether diols and triol. Using tert-butyl hydroperoxide as oxidant the Ti-leaching was minimised and selective epoxide formation was observed with Ti-beta, Ti-Al beta and Ti-MCM-41 as heterogeneous catalysts, although, with Ti-Al beta, the ether diols and triol products dominated due to acid catalysed solvolysis of the epoxide

Heterogeneous Enantioselective Synthesis of a Dihydropyran Using Cu-Exchanged Microporous and Mesoporous Materials Modified by Bis(oxazoline)

The formation of dihydropyran from the Diels–Alder reaction between E-ethyl-2-oxo-3-pentenonate and vinyl ethyl ether is investigated using copper (II) bis(oxazoline) as catalyst. The homogeneously and heterogeneously catalyzed reactions are contrasted. Immobilization using mesoporous materials (Cu-MCM-41, Cu-AlSBA-15, Cu-MSU-2) and zeolite Y is found to produce an effective heterogeneous catalyst. Although the level of enantioselection is not high in this initial study, the CuH-zeolite Y/bis(oxazoline) catalyst gives the highest ee (41% ee), which is significantly higher than that observed for the Cu(OTf)2 homogeneous catalyst (20% ee) under comparable conditions. In addition, with the heterogeneously catalyzed reaction, the enantioselection changes from the initial 2R,4S product to the 2S,4R diastereoisomer. This behavior is not observed with the homogeneously catalyzed reaction, which always yields the 2R,4S product. These results are discussed in terms of the confinement of the catalyst complex within the pores of the heterogeneous catalyst.

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.

Heterogeneous aziridination of alkenes using Cu2+ exchanged zeolites

Abstract 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: Download full-size image Exchange of zeolite Y with other cations (Ag + , Co 2+ , Fe 3+ , Mg 2+ , Ni 2+ , Zn 2+ ) 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°C with a Cu 2+ : 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.

Catalytic asymmetric heterogeneous aziridination using CuHY/bis(oxazoline): effect of reaction conditions on enantioselectivity

The copper-catalyzed aziridination of styrene with copper-exchanged zeolite HY (CuHY) and copper(II) triflate (trifluoromethanesulfonate) (Cu(OTf)2) as catalysts is described using N-(p-tolylsulfonyl)imino]phenyliodinane (PhI=NTs) as the nitrene donor. The effects on the ee and yield of the aziridine when the catalyst is modified by the presence of a chiral bis(oxazoline) are investigated in detail. The heterogeneously catalyzed reaction under these conditions shows a slight, but significant, enhancement in ee with increasing conversion at 25 °C. This is not observed in the more rapid homogeneously catalyzed reaction under identical reaction conditions using PhINTs as the nitrene donor. The enhancement in ee is proposed to result from the preferential reaction of the (S)-aziridine with the Cu2+:bis(oxazoline) complex in the presence of PhI=NTs, leading to an enhancement of the (R)-aziridine in the remaining aziridine product.

Aziridination of styrene: comparison of [N-(p-tolylsulfonyl)imino]phenyliodinane and chloramine-T as nitrene donors

Abstract Chloramine-T and [ N -( p -tolylsulfonyl)imino]phenyliodinane (PhINTs) are contrasted as nitrene donors for the aziridination of styrene using copper(II) triflate, and copper-exchanged zeolite Y (CuHY) as catalysts. For both catalysts, PhINTs is found to give significantly higher yields of the aziridine both in the presence and absence of a chiral bis(oxazoline) modifier. In addition, chloramine-T is found to induce leaching of most of the Cu from CuHY, and with this nitrene donor CuHY does not function as a heterogeneous catalyst. In contrast, PhINTs causes negligible leaching of Cu from CuHY and, consequently, for the heterogeneous CuHY catalyst, PhINTs is the preferred nitrene donor. With chloramine-T, the beneficial effects observed on addition of copper powder are shown to be due to the copper powder acting as a reservoir for Cu 2+ in solution, since Cu 2+ in solution is rapidly deactivated by toluenesulfonamide, a degradation product of the nitrene donor.

Effect of preparation method on leaching of Ti from the redox molecular sieve TS-1

The leaching of Ti from the redox molecular sieve TS-1 when used as a catalyst for the oxidation of crotyl alcohol with hydrogen peroxide is described and discussed. The crystallisation time of the TS-1 is found to be the critical preparation parameter. TS-1 prepared using a 2 day crystallisation period leaches Ti, whereas TS-1 prepared using a 10 day crystallisation period is inert to leaching. Silanisation of the external surface of the 2 day preparation effectively stops the leaching of Ti, without significantly affecting the catalytic performance. Similarly, treatment of the 2 day TS-1 with sodium azide also minimises leaching of Ti but with a reduction in catalytic activity. The Ti leaching is shown to be caused by the reaction of a triol with TS-1 in the presence of hydrogen peroxide from the surface of the 2 day TS-1. A possible mechanism is proposed in which the triol by-product chelates the Ti, thereby breaking Ti–O–Si framework bonds and causing leaching to form a Ti species in solution. The reactivity of this Ti species in solution is modelled using the reaction of crotyl alcohol with hydrogen peroxide in the presence of titanyl acetylacetonate, and it is found that this effectively catalyses the formation of triol. Hence, it is concluded that, once initiated, Ti leaching will be catalysed by the reaction products of the solution Ti species that is formed.

Observation of the enhancement in enantioselectivity with conversion for the aziridination of styrene using copper bis(oxazoline) complexesElectronic supplementary information (ESI) available: further data. See http://www.rsc.org/suppdata/cc/b3/b309507j/

During the aziridination of styrene using copper bis(oxazoline) complexes the ee increases with conversion due to further reactions of the product.

Effect of structure of the redox molecular sieve TS-1 on the oxidation of phenol, crotyl alcohol and norbornylene.

A range of crystalline TS-1 samples with different morphologies as well as the corresponding TS-1 precursor structures have been synthesised using hydrothermal crystallisation. The materials have been characterised using powder X-ray diffraction, IR and Raman spectroscopy and electron microscopy. The materials were used as catalysts for the oxidation of crotyl alcohol, phenol and norbornylene and, in particular, the reactivity of the precursor structures was contrasted with crystalline TS-1. The oxidation of crotyl alcohol, selected as a relatively non-reactive substituted alkene, did not require the TS-1 structure for reactivity and TS-1 precursor structures are active, although crystalline TS-1 was found to be more reactive than the precursor structures. In contrast, phenol hydroxylation is only catalysed by crystalline TS-1. The reaction of phenol is observed to occur only on the exterior surface of large TS-1 crystallites. With smaller crystallites of TS-1, i.e. the size range of interest for catalysis, the rapid subsequent reaction of hydroquinone makes it difficult to determine whether reaction occurs solely on the exterior of the crystallites or at sites within the porous structure. Hence it is suggested that this reaction has limited scope as a probe reaction for the reactivity of sites within the crystallites. It is, however, feasible that phenol hydroxylation is a viable probe reaction for TS-1 type structural units. Norbornylene was studied as an example of a reactant too large to enter the internal pore structure of TS-1 and hence only reaction at pore mouths and external surface sites was possible. Larger TS-1 crystallites were more active for this substrate than suggested by surface area considerations. The results are discussed in terms of the selection of model reactions for the study of TS-1 catalysts.

Catalytic asymmetric heterogeneous aziridination of styrene using Cu2+-exchanged zeolite Y: effect of the counter-cation on enantioselectivity and on the reaction profile

Effective catalysts have Cu2+-exchange levels of ca. 40–60% of the maximum concentration for electroneutrality and, consequently, Cu-zeolite Y catalysts contain an additional counter-cation (H+, Li+, Na+, K+, Rb+, Cs+) whose effect is explored. With Li+, Na+, K+ counter-cations, the zeolite structure is not markedly affected but, with Rb+ and Cs+, there is some loss of crystallinity. Replacement of H+ by group I cations does not markedly influence the overall ee observed for aziridine indicating that the presence of protons in the Cu-HY catalysts are not detrimental to the reaction. Catalysts containing group I cations typically give decreased leaching of Cu2+ during the reaction. At low nitrene donor to styrene molar ratios (1∶1), replacement of H+ by group I cations leads to a small enhancement in the ee of aziridine, although the yield of aziridine formed is decreased under all reaction condition. At higher molar ratios of nitrene donor to styrene, the ee is suppressed, particularly with Rb and Cs. The effect of reaction time on aziridine yield reveals a reaction profile in which the reaction initially proceeds rapidly, then slows down prior to accelerating again in the latter part of the reaction. This effect is accentuated by increasing the size of the counter-cation. This reaction profile is also observed for the homogeneously catalysed pathway and, consequently, it cannot be due solely to a confinement effect within the zeolite pores. Over addition of reaction by-products (NsNH2, PhI) accentuates the shape of this reaction profile and the effect is discussed in terms of the interactions of such molecules at the active site.