Schiffbase catalysts

Carriera and coworkers employed titanium SchifFbase catalysts such as (7.189) (see Section 7.1) in an ene reaction of 2-methoxypropene (7.190) with various aldehydes (7.01) (not especially reactive ones). The overall synthetic strategy of using 2-methoxypropene (7.190) has significant merit in comparison with aldol reactions, because it is a cheap starting material, and means that a silyl enol ether does not have to be prepared prior to an aldol reaction. The Schiff base chromium complexes such as (7.192) developed by Jacobsen and coworkers is effective with both aliphatic and aromatic aldehydes providing up to 96% ee in the reaction of the latter type of substrate with 2-methoxypropene (7.190). This complex also catalyses the ene reaction with 2-silyloxypropene with high ee. ... 

The catalysts 4a-d show no activity at room temperature but are stable for months in solution. We tested their activity in the ring-opening metathesis polymerization (ROMP) of 1,5-cyclooctadiene (COD) at 90°C in toluene-d8 (Figure 2). The Schiffbase catalysts 4a-d and 5c are clearly less active than... 

Pd NPs are commonly used in oxidations of alcohols and can be incorporated into an The morphology of the particles can be suited to different apphcations, e.g., flower-like particles, due to their concave tetrahedral subunits, exhibited a high electrocatalytic activity toward ethanol and methanol oxidation compared with that of the commercial Pd black catalyst.A Pd complex containing triphenylphosphine and a Schiffbase catalyst was used " for the study of the solvent efiect in carbonylation of primary and secondary alcohols to aldehydes and ketones, in the presence of NaOCl as an oxidant. By kinetic study of different proportions between the imidazolium-based IL ([C2mim][PF6]), it was shown that the acceleration of the reaction depends on the mixing proportion and that the best ratio was 1 1. 

One of the earliest examples of such catalysis was demonstrated in 1966 by the Japanese chemist Hitosi Nozaki, who reacted styrene and ethyl diazoacetate in the presence of a chiral Schiffbase-Cu11 complex [72-74], Although the initial enantios-electivity was modest (<10% ee), the principle was proven. Some years later, the companies Sumitomo and Merck used similar copper catalysts for asymmetric cyclopropanation on a multikilogram scale, in the production of various insecticides and antibiotics [75]. One of Nozaki s PhD students at that time was Rioji Noyori, who later developed the BINAP asymmetric hydrogenation catalysts for which he received the 2001 Nobel Prize in Chemistry [7[. 

To a solution of benzaldehyde, N-(tert-butoxycarbonyl)imine (10.7 mg, 52.3 pmol), tert-butyl glycinate benzophenone Schiffbase (14.8 mg, 50.1 pmol) and catalyst (5.0 mg, 5.0 pmol) in fluorobenzene (0.5 mL) was added Cs2C03 (32.6 mg, 100.1 pmol) at -30 °C. After stirring for 19 h, the reaction was quenched by the addition of water. The water layer was extracted three times with EtOAc. The combined organic layer was dried over MgS04 and concentrated. The residue was purified by column chromatography on silica gel (hexane ether, 10 1 as eluent) to give the product (24.5 mg, 98%). 

Mechanistically, the antibody aldolases resemble natural class I aldolase enzymes (Scheme 4.7) [52]. In the first step of a condensation reaction, the s-amino group of the catalytic lysine reacts with a ketone to form a Schiffbase. Deprotonation of this species yields a nucleophilic enamine, which condenses with electrophilic aldehydes in a second step to form a new carbon-carbon bond. Subsequent hydrolysis of the Schiffbase releases product and regenerates the active catalyst. 

The basic approach to prepare Co(II)-complexes of salen (N,lSr-bis(salicylidene)ethylene-diamine)-type molecules is the flexible ligand method [9]. In this process the Schiffbase ligand can diffuse by twisting into the zeolite where it becomes too large to exit by complexation with the cobalt ion. The flexible ligand method, however, was not usefiil for the preparation of Co-salophen/ zeolite catalyst, because the product was inactive in the oxidation reactions. The salophen molecule does not seem to be flexible enough and can not get into the zeolite to produce the suitable complex in the supercage. 

Hydroamination reactions involving alkynes and enantiomerically pure chiral amines can produce novel chiral amine moieties after single pot reduction of the Schiffbase intermediate 82 (Scheme 11.27) [123]. Unfortunately, partial racemiza tion ofthe amine stereocenter was observed with many titanium based hydroamina tion catalysts, even in the absence of an alkyne substrate. No racemization was observed when the sterically hindered Cp 2TiMe2 or the constrained geometry catalyst Me2Si(C5Me4)(tBuN)Ti(NMe2)2 was used in the catalytic reaction. Also, the addition of pyridine suppressed the racemization mostly. 

Ooi and Maruoka developed an efficient phase transfer catalyst (46a-e), which consisted of chiral N-spiro ammonium salts with binaphthalene skeleton. 3,3 -(3,4,5-Trifluorophenyl)ammonium salt (46e) provided a perfect stereoselection in benzylation of benzophenone Schiffbase of glycine terf-butyl ester (47) (Scheme 5.13, Table 5.5) [19]. The perfect stereoselective alkylation is applicable for a variety of alkyl bromides in the presence of 1 mol% of the catalyst (46e). Not only monoalkylation but also the consecutive double alkylation of 49 was successful to give 50 in excellent enantioselectivities (Scheme 5.14) [20]. The protocol is useful for the enantioselective aldol reaction of 47 with aldehyde (51) [21] and a-imino ester [22], in which catalysts (46f) and (46g) were effective (Scheme 5.15) [23]. 

Rosseinsky and coworkers reported the postmodification of IRMOF-3 with sali-cylaldehyde by imine condensation, leading to the formation of the MOF-supported Schiffbase with 13% yield. The resulting salicylidene-functionalized IRMOF-3 was used as an N-O ligand for a vanadium oxide complex and characterized by liquid-state NMR and PXRD analyses. The obtained MOF-supported vanadium catalyst was found to be active for cyclohexene a-oxidation with tBuOOH, although both conversion and TOF were relatively low, a possible problem involving framework coUapse was identified [118]. 

The (Schiffbase)vanadium(V) complex 53 with tridentate imine auxiliaries acts as a catalyst for the oxidation of Br with tert-butyl hydroperoxide (TBHP) in nonaqueous solvents. The principal objective of this study performed by Hartung and coworkers [72] was associated with the search for an adequate combination of bromide source and primary oxidant that would reduce the inherent propensity of (SchifFbase) vanadium]V) complexes in the presence of TBHP to convert alkenols (54) directly into hydroxy-functionalized tetrahydrofurans (56) [73]. Reactivity-selectivity studies on the vanadium(V)-catalyzed oxidation of bromide were investigated through ESI (—)-MS" in order to interpret crucial steps in the process such as peroxide activation... 

In a related study, the same group reported a series of mixed-ligand din-uclear manganese(II) SchifFbase complexes as catalysts for the MW-assisted oxidation of alcohols (Scheme Acetophenone yield of 81% is... 

Representative chiral phase-transfer catalysts (Q X ) for the asymmetric alkylation of glycine Schiffbase 1 are summarized in Figure 14.2a [8-13]. These catalysts were also applied to other asymmetric alkylations with different nucleophiles (Figure 14.2b) [14—17]. 

Cinchona alkaloid thiourea catalyst 17 (Figure 37.2) was later employed by Gong el al. to induce the enantio- and diastereoselective 1,3-dipolar cycloaddition of azomethine ylides generated from Schiffbases and nitroalkenes [31]. This process provided straightforward access to chiral highly substituted pyrrolidines with good yields, moderate enantioselectivities (<63% ee), and excellent diastereoselectivities... 

Zhang Ml, Zhao PP, Leng Y, Chen GJ, Wang J, Huang J (2012) Schiffbase structured add-base cooperative dual sites in an ionic solid catalyst lead to efficient heterogeneous knoeve-nagel condensations. Chemistry 18 (40) 12773-12782. doi 10.1002/chem.201201338... 

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