Cyclodextrin modified catalysts

Reaction conditions 0.1 g of the zeolite Y modified catalyst, tested in a conventional glass microreactor with racemic butan-2-ol (7.35 x 10" mol h-1), prevaporized in a nitrogen diluent (6.2 -6.7 x 10" mol h-1). Products were analyzed using on-line GC with a 40m capillary y- cyclodextrin colimm with trifluoroacetyl stationary phase, temperature programmed from 25-70 "C with a split ratio of 120 1. 

Table 1.1 shows a selection of catalysts developed in 2006-2007, which operate under biphasic aqueous conditions. For previous literature, readers are directed to an excellent review by List and co-workers.In terms of catalyst performance, since 2006 most of the best achievements have been reported for (9-modified Z/Y///A-4-hydroxy-i -prolinc derivatives 3-5, including the inclusion complex 6 of a proline derivative and a P-cyclodextrin, or catalysts consisting of chiral amines 7 and 8 or (thio)amides 9-14 derived from L-proline. The O-protected serine 15a, threonine 15b and threonine amide 16 complete the list of catalysts. 

Complexes with modified cyclodextrins were initially used as models of hydrolytic enzymes [20,181,197,198]. Then, Breslow and Kato studied such complexes in biomimetic oxidation and epoxidation. Reetz and some other researchers investigated them in hydroformylation. We examined cyclodextrin-based catalysts in hydroxylation and Wacker oxidation. Note that some of these studies demonstrated a substantial increase in the activity of the catalysts in comparison with their analogues prepared as ordinary mixtures, which was due to the cooperative substrate binding. 

A palladium complex with cyclodextrin modified with propionitrile and benzoylnitrile groups 73-74 was active in Wacker oxidation of higher 1-alkenes (Experiment 11-4, Section 11.7), and its activity was much higher than the activity of a catalj ic system prepared as a mixture of cyclodextrin and the palladium complex owing to the cooperative substrate binding and to the increase in the stability constant of the catalyst-substrate complex. As in hydroformylation, the catalyst was more active in the reaction with an aromatic substrate, styrene, than with linear alkenes [59,210-211], The catalyst activity depended on the 1-alkene chain length and was maximum for 1-heptene. 

Chiral recognition of A-[Co(phen)3]3+ has been observed in a modified /3-cyclodextrin.772 Chiral discrimination has also been seen in photoinduced energy transfer from luminescent chiral lanthanoid complexes773 to [Co(phen)3]3+ and between photoexcited [Ru(bpy)3]2+ and [Co(phen)3]3+ co-adsorbed on smectite clays.774 The [Co(bpy)3]3+ ion has been incorporated into clays to generate ordered assemblies and also functional catalysts. When adsorbed onto hectorite, [Co(bpy)3]3+ catalyzes the reduction of nitrobenzene to aniline.775 The ability of [Co(phen)3]3+ to bind to DNA has been intensively studied, and discussion of this feature is deferred until Section 6.1.3.1.4. 

In another example, undecanal was hydrogenated to undecanol with a water-soluble catalyst in the presence of chemically modified yS-cyclodextrins, which facilitated the mass transfer between the aqueous and the organic phase [134]. Hydrogenation of cinnamaldehyde with very high (99%) selectivity to cinnamyl alcohol was also performed in water scC02 biphasic systems [135] which al-... 

P-Cyclodextrin was modified by attaching 2-(diphenylphosphinoethyl)-thio- (127) and 2-bis(diphenylphosphinoethyl)amino- (126) moieties at the C-6 position [8-11]. The resulting macroligands were reacted with [ RhCl(NBD) 2] to provide the corresponding cationic rhodium-bisphosphine complexes. These catalysts showed pronounced selectivity due to complexation of the substrate by the CD unit adjacent to the catalyticaUy active metal center. For example, in competitive hydrogenation of similarly substituted terminal olefins (Scheme 10.4), 4-phenyl-but-l-ene was... 

Decene was hydrocarboxylated with a [PdClaj/TPPTS catalyst in acidic aqueous solutions (pH adjusted to 1.8) in the presence of various chemically modified cyclodextrins (Scheme 10.11) [18]. As in most cases, the best results were obtained with DiOMe-P-CD. In an interesting series of reactions 1-decene was hydrocarboxylated in 50 50 mixtures with other compounds. Although all additives decreased somewhat the rate of 1-decene hydroformylation, the order of this inhibitory effect was 1,3,5-trimethylbenzene < cumene < undecanoic acid, which corresponds to the order of the increasing stability of the inclusion complexes of additives with p-CD, at least for 1,3,5-trimethylbenzene (60 M ) and cumene (1200 M ). These results clearly show the possible effect of competition of the various components in the reaction mixture for the cyclodextrin. 

In new studies heteropoly acids as cocatalysts were found to be very effective in combination with oxygen in the oxidation of ethylene.1311 Addition of phosphomo-lybdic acid to a chloride ion-free Pd(II)-Cu(II) catalyst system results in a great increase in catalytic activity and selectivity.1312 Aerobic oxidation of terminal alkenes to methy ketones can be performed with Pd(OAc)21313 or soluble palladium complexes. Modified cyclodextrins accelerates reaction rates and enhance selectivities in two-phase systems under mild conditions.1315 1316... 

Cyclodextrins have been covalently modified for catalytic oxidation, such as compounds 57, 62-65 (Schemes 3.14 to 3.16) [44, 45]. Enantioselective epoxidation of styrene derivatives, and carene using 20-100 mol% of the CD-ketoester 57 has been achieved. The inclusion-complex formation was confirmed by aH NMR titration experiments, confirming the 1 1 substrate catalyst stoichiometry under the reaction conditions. In the oxidation of carene, NOE and ROESY experiments showed different behavior according to the size of the R group (Scheme 13.14). Evidence was found for the formation of inclusion complexes with compounds 58 and 59. On the other hand, compounds 60 and 61 proved to interact with the catalyst via a tail inclusion vide infra). The increased diastereoselectivity observed with compounds 58 and 59 might be explained by a closer proximity to the covalently linked dioxirane. 

In other reports, /i-cyclodextrins have been used to induce asymmetry in borohydride reduction of ketones, a diastereoselective reduction has been controlled by a n-allyltricarbonyliron lactone tether , a phosphinamide has been combined with a dioxaborolidine unit as an activated, directed catalyst for ketone reduction, reductive amination using benzylamine-cyanoborohydride converts 3-hydroxy ketones into syn-1,3-amino alcohols, l-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)propan-1 -one has been reduced diastereoselectively, and production of chiral alcohols via (i) Itsuno-Corey and Brown procedures and (ii) lithium aluminium hydride modified by chiral nucleophiles has been reviewed. 

If an achiral ferrocene derivative is converted to a chiral one by chiral reagents or catalysts, this may be called an asymmetric synthesis. All asymmetric syntheses of ferrocene derivatives known so far are reductions of ferrocenyl ketones or aldehydes to chiral secondary alcohols. Early attempts to reduce benzoylferrocene by the Clemmensen procedure in (5)-l-methoxy-2-methylbutane as chiral solvent led to complex mixtures of products with low enantiomeric excess [65]. With (25, 3R)-4-dimethylamino-l,2-diphenyl-3-methyl-2-butanol as chiral modifier for the LiAlH4 reducing agent, the desired alcohol was formed with 53% ee (Fig. 4-9 a) [66]. An even better chiral ligand for LiAlH4 is natural quinine, which allows enantioselective reduction of several ferrocenyl ketones with up to 80% ee [67]. Inclusion complexes of ferrocenyl ketones with cyclodextrins can be reduced by NaBH4 with up to 84% enantioselectivity (Fig. 4-9 b) [68 — 70]. 

The topic of molecular recognition should gain increased attention in catalyst design. For example, specific structural interactions of higher olefins (e. g., 1-de-cene) with chemically modified -cyclodextrins allow efficient hydroformylation in a two-phase aqueous system even though the olefin is completely insoluble in water at the same time, olefin isomerization at the rhodium catalyst is hampered [81]. 

Monflier, E. Versatile inverse phase transfer catalysts for the functionalization of substrates in aqueous-organic two-phase systems the chemically modified 3-cyclodextrins. Rec. Res. Dev. Org. Chem. 1998, 2, 623-635. 

Finally, extraction of the important reactive species can be executed in the opposite direction, from organic phase to water. This is called inverse phase-transfer catalysis. Catalysts for such processes are mostly cyclodextrins or modified derivatives thereof. Relatively few applications of this type of PTC have been published. Whereas the present section is concerned only with the organic phase as the location of the proper chemical reaction, important contributions of inverse PTC toward organometallic catalysis are detailed in Section 4.6.2. 

The best results in terms of activity have been obtained with cationic surfactants such as octadecyltrimethylammonium bromide. The normal to branched (njiso) aldehydes ratio was found to be very dependent on the nature of the surfactant. For example, methyl 9-decenoate hydroformylation gave methyl 11-formylunde-canoate with an n/iso aldehydes ratio of 6.1 1, 4.0 1, 2.3 1 and with anionic, amphophilic, and cationic surfactants, respectively. Interestingly, hydroformylation of this substrate has also been achieved successfully with inverse-phase transfer catalysts such as chemically modified /l-cyclodcxtrins. In this approach, the cyclodextrin forms an inclusion complex with methyl 9-decenoate and transfers the alkene into the aqueous phase. Under optimal conditions, the aldehydes are obtained in a 100% yield and in an n/iso aldehydes ratio of 2.3 1 [10]. 

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