Chiral Co salen complex

Asymmetric Ring Opening of Some Terminal Epoxides Catalyzed by Dimeric Type Novel Chiral Co(Salen) Complexes... 

Jacobsen et al. reported enhanced catalytic activity by cooperative effects in the asymmetric ring opening (ARO) of epoxides.[38] Chiral Co-salen complexes (Figure 4.27) were used, which were bound to different generations of commercial PAMAM dendrimers. As a direct consequence of the second-order kinetic dependence on the [Co(salen)] complex concentration of the hydrolytic kinetic resolution (HKR), reduction of the catalyst loading using monomeric catalyst leads to a sharp decrease in overall reaction rate. 

Covalent attachment chiral Co(salen) complexes to polystyrene and silica gave efficient and highly enantioselective catalysts for the hydrolytic kinetic resolution (HKR) of terminal epoxides, including epichlorohydrin. These systems provide practical solutions to difficulties with the isolation of reaction products from the HKR. Removal of the supported catalyst by filtration and repeated recycling was demonstrated with no loss of reactivity or enantioselectivity. The immobilised catalysts have been adapted to a... 

Scheme 7. Resin captured synthesis of polystyrene-bound chiral Co(salen) complex 36.

Cavazzini, M. Quid, S. Pozzi, G. (2002) Hydrolytic kinetic resolution of terminal epoxides eatalyzed by fluorous chiral Co(salen) complexes. Tetrahedron 58 3943-3949. 

Annis DA and Jacobsen EN. Polymer-supported chiral Co(salen) complexes Synthetic apphcations and mechanistic investigations in the hydrolytic kinetic resolution of terminal epoxides. J Am Chem Soc 1999 121(17) 4147-4154. 

Kim, G.-J., Lee, H., Kim, S.-J. Catalytic activity and recyclability of new enantioselective chiral Co-salen complexes in the hydrolytic kinetic resolution of epichlorohydrin. Tetrahedron Lett. 2003, 44, 5005-5008. 

Besides biomimetic complexes, Jacobsen described particularly efficient bis (chromium-salen) catalyst 9 for the asymmetric ring-opening reaction of epoxides with azide (Scheme 9) [42]. The efficiency of this class of catalysts is attributed to a cooperative mechanism, both substrates being activated toward each other by their respective chromium atom. Of note, a less pronounced cooperative effect was initially demonstrated in an intermolecular manner using monomeric Cr(N3)-salen catalyst [43]. Jacobsen also showed that an analogous cooperative mechanism takes place using polymer-supported chiral Co(salen) complexes for the hydrolytic kinetic resolution of terminal epoxides [44, 45]. 

Hydrolytic reactions In a continuous flow process, polystyrene- and silica-bound chiral Co(salen) complexes were applied as efficient and highly enantioselective catalysts for the hydrolytic KR of terminal epoxides [40]. 

Silica bound chiral Co-salen complex (36) was synthesized and adapted to a continuous-flow reaction. The optical kinetic resolution of racemic epoxide (38) was successful to yield the desired triol (39) in good yield (36% conversion) and high enantiomeric excess (Scheme 7.31) [127]. A PASSflow microreactor consisting of Co-salen monolith (37) was used for the dynamic kinetic resolution of epibromohydrin (40). Three runs performed on the 1-mmol scale were completed and afforded (J )-(41) in 76-87% yield with constant enantiomeric purity of 91-93% ee [128]. 

The asymmetric ring opening (ARO) of racemic terminal epoxides with H2O via hydrolytic kinetic resolution provides an efficient synthetic route to prepare optically pure terminal epoxides. The dimeric type chiral Co(salen)AlX3 complex has great potential to catalyze HKR of terminal epoxides in a highly reactive and enantioselective manner in comparison to their monomeric analogy. 

To a solution of hexa hydrated aluminum chloride (1.99g, 8.28 mmol, 1.0 equiv.) in tetrahydrofuran (25 mL), precatalyst (R,R)-salenCo (5.0 g, 8.28 mmol, 1.0 equiv. ) was added and stirred in at open atmosphere at room temperature. As soon as the chiral Co (salen) was added color of the solution changes from brick red to dark olive green. The mixture was stirred for 1 h. The resulting solution was concentrated imder reduced pressure. The crude solid was worked up with H2O and CH2CI2. Yield = 98-99 % as a dark green solid powder. The complex have been analyzed by A1 NMR with reference to [A1(D20)6] and... 

Fig. 1. UV-Vis absorption spectra of the precatalyst Scheme 2. Possible working model for the HKR chiral Co(salen) and monomer and dimer complex. of terminal epoxides catalyzed by C0-AIX3...

Reversal of the conformation of the chiral Mn-salen complex forces the substituents on the ethylenediamine moiety to take the disfavored axial position. This disfavored conformation (Fig. 4-8A) should be stabilized by the co-... 

Carbon dioxide is one of the most abundant carbon resources on earth. It reacts with an epoxide to give either a cyclic carbonate or a polycarbonate depending on the substrates and reaction conditions. Kinetic resolution of racemic propylene oxide is reported in the formation of both cyclic carbonate and polycarbonate. The fe ei value defined as ln[l-(conversion)(l+%ee)]/ln[l-(conversion)(l% ee)] reached 6.4 or 5.6 by using a Co(OTs)-salen complex with tetrabutylammonium chloride under neat propylene oxide or using a combination of a Co-salen complex and a chiral DMAP derivative in dichloromethane, respectively. 

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]. 

Enantiomer-differentiating co-polymerization of terminal epoxides is achieved by chiral chromium and cobalt complexes. Jacobsen etal. reported the co-polymerization of 1-hexene oxide with GO2 by using complex 35a. The reaction proceeds with kinetic resolution at 90% conversion, the unreacted epoxide is found to be enriched in the (i )-enantiomer of 90% ee. Detailed information about the resultant polymer, however, is not described. As discussed in the previous section, chiral cobalt-salen complex 34c co-polymerizes PO and GO2 (Table 3). When 34c with /r<3 / j--(li ,2i )-diaminocyclohexane backbone is applied to the co-polymerization, (A)-PO is consumed preferentially over (i )-enantiomer with a of 2.8 to give optically active PPG (Equation (8)). In a similar manner, a binary catalyst system, 34d/Bu4NGl, preferentially consumes (A)-PO over R)-PO with = 2.8-3.5. ... 

Attempts to aziridinate alkenes with iron catalysts in an asymmetric manner have met with only limited success to date [101], In an early report on the use of various chiral metal salen complexes, it was found that only the Mn complex catalyzed the reaction whereas all other metals investigated (Cr, Fe, Co, Ni etc.) gave only unwanted hydrolysis of the iminoiodinane to the corresponding sulfonamide and iodoben-zene [102], Later, Jacobsen and coworkers and Evans et al. achieved good results with chiral copper complexes [103]. 

In the dendritic [Co(salen)] complexes prepared by Breinbauer and Jacobsen the dendrimer again serves as - covalent - support material for the catalytic entities attached to the periphery [62]. These dendritic Jacobsen catalysts were obtained by reaction of the corresponding PAMAM dendrimers with active ester derivates of chiral ]Co(II)-(salen)] units according to standard peptide coupling methods. In hydrolytic kinetic resolution of vinylcyclohexane oxide the dendrimer 14 (Fig. 6.40) showed a dramatically increased reactivity compared to the commercially available monomeric Jacobsen catalyst [63-67]. Whereas the latter merely gave a conversion of less than 1% with an indeterminable ee, 14 afforded a conversion of 50% with an ee of 98 2. 

Sigman and Jacobsen reported the first example of a metal-catalyzed enantioselective Strecker-type reaction using a chiral Alnl-salen complex (salen = N,N -bis(salicyhdene)-ethylenediamine dianion) [4]. A variety of N-allylimines 4 were evaluated in the reaction catalyzed by complex 5 to give products 6, which were isolated as trifluoroacetamides in good yields and moderate-to-excellent enantioselectivities (Scheme 3). Substituted arylimines 4 were the best substrates, while alkyl-substituted imines afforded products with considerably lower ee values. Jacobsen and co-workers also reported that non-metal Schiff base catalysts 8 and 9 proved to be effective in the Strecker reaction of imines 7 with hydrogen cyanide to afford trifluoroacetamides 10 after reaction with trifluoroacetic anhydride, since the free amines were not stable to chromatography (Scheme 4) [5]. 

The report by Kochi and co-workers in 1986 that a (salen)manganese(lll) complex (Mn(salen) complex) was an efficient epoxidation catalyst for simple olefins <1986JA2309> quickly led to independent reports from the groups of Jacobsen <1990JA2801> and Katsuki <1990TL7345> that chiral Mn(salen) complexes could catalyze asymmetric epoxidation reactions. The reaction requires the use of a stoichiometric oxidant initially iodosylarenes were utilized, but it was quickly found that NaOCl was also successful. 

Kinetic resolution in the catalytic conversion of racemic chloro propanols to optically active epoxides has been achieved by use of a chiral Co(salen) type complex in combination with K2CO3. Although enantioselectivity was modest (< 35 % ee), this first use in asymmetric epoxide formation of the chiral ligand system that was later brought to fame through the Jacobsen-Katsuki asymmetric epoxidation is noteworthy [56,57]. When applied to the prochiral l,3-dichloro-2-propanol, asymmetric induction of up to ca. 60 % ee was achieved (Sch. 8) [58]. 

A chiral cobalt-salen complex bearing lil j serves as an active catalyst for the HKR of terminal epoxides [90]. The polymeric salen-Co complex 158 (Scheme 3.46) also showed a high enantioselectivity in the same reaction [91]. 

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... 

Jacobsen disclosed a chiral Co(salen) catalyst that promoted the hydrolytic kinetic resolution (HKR) of terminal epoxides [40]. Remarkably low levels of the Co(salen)OAc complex 12 effected enantioselective epoxide hydrolysis to afford mixtures of the unreacted epoxide and the ring-opened diol. Controlling the amount of water in the HKR allowed either of these chiral products to be generated in high enantiopurity (Tables 3 and 4) [41]. Significant differences in vola-... 

In particular, for the synthesis of optically pure chemicals, several immobilization techniques have been shown to give stable and active chiral heterogeneous catalysts. A step further has been carried out by Choi et al. [342] who immobilized chiral Co(III) complexes on ZSM-5/Anodisc membranes for the hydrolytic kinetic resolution of terminal epoxides. The salen catalyst, loaded into the macroporous matrix of Anodise by impregnation under vacuum, must exit near the interface of ZSM-5 film to contact with both biphasic reactants such as epoxides and water. Furthermore, the loading of chiral catalyst remains constant during reaction because it cannot diffuse into the pore channel of ZSM-5 crystals and is insoluble in water. The catalytic zeolite composite membrane obtained acts as liquid-liquid contactor, which combines the chemical reaction with the continuous extraction of products simultaneously (see Figure 11.28) the... 

In 1999, Jacobsen [210] prepared a polystyrene-supported Co-salen complex 332 by grafting a monophenol derivative 330 of an highly efficient chiral salen onto hydroxymethyl polystyrene beads (90 pm) derivatized as their /r-nitrophenyl carbonate 331 (Scheme 140). 

In 1994, Jacobsen and co-workers demonstrated that stereoseleetive oxidation of benzylic C—H bonds is possible utilizing readily available chiral Mn(salen) complexes.They studied the kinetic resolution of 1,2-dihy-dronaphthalene oxide via an asymmetric C—H bond hydroxylation reaction (Scheme 1.59). During the course of experiments on the asymmetric epoxidation of 1,2-dihydronaphthalene with C24, it was observed that the... 

Asymmetric Ru((II)-catalysed aziridination of terminal olefins has been achieved using Ru(CO)(salen) complex as chiral catalyst and sulfonyl azide (173) as nitrenoid source. " Under such reaction conditions, the desired aziridines have been obtained in high yields and excellent enantioselectivities (up to 99% ee). 

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