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Cr-salen complexes

Table 1.12 Asymmetric Diels-Alder reactions catalyzed by the Cr-salen complex 15 [21]... Table 1.12 Asymmetric Diels-Alder reactions catalyzed by the Cr-salen complex 15 [21]...
Scheme 5.13 Electropolymerised Cr-salen-complexes for hetero-Diels-Alder reactions. Scheme 5.13 Electropolymerised Cr-salen-complexes for hetero-Diels-Alder reactions.
Scheme 24. Synthesis of chiral polymeric Cr(salen complex 52. Scheme 24. Synthesis of chiral polymeric Cr(salen complex 52.
Table 6 Stirred batch reactor ARO of epoxides catalyzed by a dimeric (R,R)-Cr(III)(salen) complex 65 immobilized in a supported ionic liquid phase compared to the dimeric complex impregnated on silica 64-silica and the reported homogeneous reactions with the monomeric Cr(salen) complex 63... Table 6 Stirred batch reactor ARO of epoxides catalyzed by a dimeric (R,R)-Cr(III)(salen) complex 65 immobilized in a supported ionic liquid phase compared to the dimeric complex impregnated on silica 64-silica and the reported homogeneous reactions with the monomeric Cr(salen) complex 63...
Schon, E. Zhang, X. Zhou, Z. Chisholm, M. H. Chen, P. Gas-Phase and Solution-Phase Polymerization of Epoxides by Cr(salen) Complexes Evidence for a Dinuclear Cationic Mechanism. Inorg. Chem. 2004, 43, 7278-7280. [Pg.679]

Motivated by these remarkable results, the first successful experiment to produce enantiomerically emiched (3-BL resulted when the chloride species of this chiral Cr salen) complex was reacted with Na[Co(CO)4l and applied in ring-expansion a low but reproducible excess of 6% ee was obtained [62]. [Pg.83]

Metallosalen complex [salen = N, A-ethylenebis(salicyldeneaminato)] has a structure similar to metalloporphyrin, and these two complexes catalyze the epoxidation of olefins. For example, Kochi et al. have found that metallosalen complexes such as (salen )manganese(III) [25] and (salen)chromium(IIl) complexes [26] (hereafter referred to as Mn- and Cr-salen complexes, respectively) serve as catalysts for the epoxidation of unfunctionalized olefins by using iodosylbenzene [25] or sodium hypochlorite [27], In particular, cationic Mn-salen complex is a good catalyst for epoxidation of unfunctionalized olefins, which proceeds through an oxo(salen)manganese(V) species (Scheme 6B.14) [25,28], The presence of oxo-Mn(V)-salen... [Pg.295]

Cr3+ chelated in planar salen-type ligands is a catalyst for olefin epoxi-dation with single oxygen donors such as PhlO. A Cr(V)=0(salen)+ compound transfers the active oxygen atom to the olefin (69). Cr remains firmly bound by the ligand throughout the catalytic cycle, and this may offer an opportunity to immobilize a Cr epoxidation catalyst. However, in a report on immobilization of such a Cr(salen)+ complex in Al-containing MCM-41, it was stated that the complex is simply physisorbed on the support (70) it is doubtful whether this provides a stable link. Moreover, the relevance of Cr(III)(salen)+ as an oxidation catalyst is limited since other metallosalen complexes are far more effective. [Pg.13]

MTO [methyltrioxorhenium(VII), cf. Chapter 3.3.13] can be used as a catalyst for the epoxidation of olefins with urea hydroperoxide in [EMIMJBF4 [19]. The activity is reported to be comparable with the reaction in organic solvents but side reactions are suppressed. The use of an ionic liquid as a co-solvent in CH2CI2 for the enantioselective Mn-salen complex-catalyzed epoxidation of olefins with Na(OCl) was reported to result in enhanced reaction rates at no loss of enantioselectivity [20]. Cr-salen complexes can further be used for the asymmetric kinetic resolution of epoxides by ring-opening with azide [21]. [Pg.641]

The first reports of a reaction of an amine with an aldehyde by Schiff [584] led to the establishment of a large class of ligands called Schiff bases. Among the most important of the Schiff bases are the tetradentate salen ligands (N,N -bis(salicy-laldehydo)ethylenediamine), which were studied extensively by Kochi and coworkers, who observed their high potential in chemoselective catalytic epoxidation reactions [585]. The best known method to epoxidize unfunctionalized olefins enantioselectively is the Jacobsen-Katsuki epoxidation reported independently by these researchers in 1990 [220,221]. In this method [515,586-589], optically active Mn salen) compounds are used as catalysts, with usually PhlO or NaOCl as the terminal oxygen sources, and with a O=Mn (salen) species as the active [590,591] oxidant [586-594]. Despite the undisputed synthetic value of this method, the mechanism by which the reaction occurs is still the subject of considerable research [514,586,591]. The subject has been covered in a recent extensive review [595], which also discusses the less-studied Cr (salen) complexes, which can display different, and thus useful selectivity [596]. Computational and H NMR studies have related observed epoxide enantioselectivities... [Pg.66]

The asymmetric ring-opening (ARO) reachon of epoxides by trimethylsilyl azide (TMSN3) catalyzed by the chiral Cr(salen) complex has been recognized as an attrachve approach to the synthesis of ophcally enriched P-amino alcohols [47]. In parhcular, the chiral Cr(salen) catalyst 34 exhibits remarkable stabihty under catalyhc conditions, which allows its repeated recycling. Jacobsen and co workers reported that this reachon could be run without solvents, and that the catalyst could be recycled several hmes without any loss of activity or enanhoselectivity... [Pg.252]

The ARO reachon of epoxides with TMSN3 in the SILP phase using dimeric Cr(salen) complex 35 (Figure 7.10) has been developed as one of the robust... [Pg.253]

Table 1. Recycling of Cr(salen) complex 1 in the ARO with TMSNj... Table 1. Recycling of Cr(salen) complex 1 in the ARO with TMSNj...
The addition of other sulfur nucleophiles was reported by Jacobsen to be catalyzed by the same Cr(salen) complex 1 initially reported for the ARO with TMSNj. Benzyl mercaptan afforded the ring-opened hydroxy sulfides in excellent yield and 59-70% ee [17]. The moderate levels of enantioselectivity were improved by use of the dithiol 5, which afforded mixtures of bishydroxy sulfides in which the ee of the chiral product 6 was substantially enriched (Scheme 5). The sulfide products could be easily elaborated into the free thiols by reductive de-benzylation, providing access to the jl-silyloxy thiol 8 in optically pure form. [Pg.1240]

A difficult challenge in developing ARO reactions with carbon nucleophiles is identifying a reagent that is sufficiently reactive to open epoxides but at the same time innocuous to chiral metal catalysts. A recent contribution by Crotti clearly illustrates this dehcate reactivity balance. The lithium enolate of acetophenone added in the presence of 20 mol % of the chiral Cr(salen) complex 1 to cyclohexene oxide in very low yield but in 84% ee (Scheme 10) [23]. That less than one turnover of the catalyst was observed strongly suggests that the lithium enolate and the Schiff base catalyst are not compatible under the reaction conditions. [Pg.1243]

Opening of epoxides and aziridines. An observation that concerns the enantioselective opening of meyo-epoxides by SiC in the presence of 31 indicates a beneficial effect of the o-methoxy group to form an octahedral complex. " p-Titanoxy radical intermediates are involved when using 32 to open epoxides. A chiral Cr-salen complex has been used to mediate epoxide opening with KHFi. Although the ee values are moderate, this is the first report of fluorohydrin formation by such a method. " ... [Pg.102]

In 1995, Jacobsen [13] reported a further advancement in the nucleophilic opening of meso epoxides. This discovery, which makes use of Cr-Salen catalysts (Scheme 3), is significant for a number of reasons. Firstly, the catalyst delivers high yields and enantioselec-tivities using only a 2 mol% loading at 0 °C with trimethylsilyl azide as the nucleophile. Secondly, kinetic investigations [14] reveal that the Cr-Salen complex 4 has an unexpected dual role in the process. Cr-Salen complexes, e. g. 4 and 5, can act as Lewis acids (cf. Fig. 2, pathway a). In addition, the azide is transferred in situ from Si to Cr giving com-... [Pg.63]

Scheme 4. Kinetic resolution of terminal epoxides catalysed by chiral Cr-Salen complex 5. Scheme 4. Kinetic resolution of terminal epoxides catalysed by chiral Cr-Salen complex 5.
Figure H. X-Band EPR spectra (77 K) of frozen 0.05 M solutions of Cr (salcn) complexes (a) complex 11 in ObCb (b) complex 12 in DMF-toluene mixed solvent (c) complex 13 in DMF toluene mixed solvent. X-Band EPR spectra (77 K) of frozen 0.03 M solutions of Cr "(salen) complexes (d) complex 11 in CHjCb + pyridine (0.3. M) (e) complex 12 in DMF-iolucnc mixed solvent + pyridine (4,0 M) (0 complex 13 in DMF-iotuene mixed solvent t pyridine (3.3 M). Spectrometer frequency 9.3 GHz microwave power, 2(X) mW modulation frequcncy.lOO kHz modulation amplitude. 10 G gain, 1 10 ... Figure H. X-Band EPR spectra (77 K) of frozen 0.05 M solutions of Cr (salcn) complexes (a) complex 11 in ObCb (b) complex 12 in DMF-toluene mixed solvent (c) complex 13 in DMF toluene mixed solvent. X-Band EPR spectra (77 K) of frozen 0.03 M solutions of Cr "(salen) complexes (d) complex 11 in CHjCb + pyridine (0.3. M) (e) complex 12 in DMF-iolucnc mixed solvent + pyridine (4,0 M) (0 complex 13 in DMF-iotuene mixed solvent t pyridine (3.3 M). Spectrometer frequency 9.3 GHz microwave power, 2(X) mW modulation frequcncy.lOO kHz modulation amplitude. 10 G gain, 1 10 ...
Table 102 Some Synthetic Details and Properties of Porphyrin and Cr -Salen Complexes... Table 102 Some Synthetic Details and Properties of Porphyrin and Cr -Salen Complexes...
Following these results, Darensbourg et al. have continued the research and used other bifunctional Cr(salen) complexes as catalysts for polycarbonate synthesis. They observed that when a monofunctional Cr(salen) complex (5) was used to catalyze the reaction between epoxide and CO2, the product formed was cyclic carbonate. However, when a bifunctional Cr(salen) catalyst (6) was used, 79% selectivity towards the polycarbonate was obtained at 70 °C. The reason for this difference lies in the structure of the bifunctional catalyst, which provides steric hindrance in the epoxide ring-opening process to form the cyclic carbonate. Therefore, it can be inferred that spatial requirements in the active site of the metal catalyst determine the selectivity for the kinetic polymer product over the thermodynamically more stable cyclic carbonate product. [Pg.260]

We have attempted to synthesize the 6/6/11 triqrclic core (214) of manzamine B (Scheme 18)(Matsumura et al. impublished results). Asymmetric intermolecular Diels-Alder reaction of a formyl-dehydropiperidine derivative (217) and sUoxy-aminodiene (218) promoted by a chiral Cr-salen complex [104] gave a cycloadduct (216) with high enantio- and diastereose-lectivity to establish an all-czs stereochemistry. The 11-membered ring was constructed by RCM with modest selectivity. [Pg.230]

Kwiatkowski has reported a Cr-Salen complex (84) which at high pressures (10 kbar) produces the homoallylic alcohols in good yield and moderate enantioselectivities. ... [Pg.599]

In 1995, Katsuki and Yamashita [11a] found that Mg-salen complex and Co- or Cr-salen complexes served as Lewis acid catalysts for asymmetric Diels-Alder reactions and hetero Diels-Alder (HDA) reaction of Danishefsky s diene (l-methoxy-3-trimethylsilyloxy-l,3-butadiene) with aldehydes. Later on, they found that second-generation (R,S)-(ON )Ru-salen complex bearing chiral binaphthyl and ethylenediamine units also served as a catalyst for asymmetric HDA reaction between Danishefsky s diene and benzaldehyde [llbj. Recently, in 2001 they reported that (R,S)-Ru(II)-salen complex served as a good catalyst for... [Pg.336]


See other pages where Cr-salen complexes is mentioned: [Pg.323]    [Pg.327]    [Pg.281]    [Pg.782]    [Pg.313]    [Pg.241]    [Pg.118]    [Pg.161]    [Pg.207]    [Pg.211]    [Pg.192]    [Pg.253]    [Pg.1247]    [Pg.64]    [Pg.349]    [Pg.354]    [Pg.2736]    [Pg.169]    [Pg.170]    [Pg.154]    [Pg.259]    [Pg.37]    [Pg.616]    [Pg.468]   
See also in sourсe #XX -- [ Pg.281 ]




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