Chromium salen, with epoxide

Ghromium complexes have been found to promote the co-polymerization of epoxides with G02- Recently, Darensbourg et al. have demonstrated that the chromium-salen complexes, remarkably more stable to the air and moisture than zinc-based co-polymerization catalysts, are effective catalysts for the co-polymerization of GHO with G02. " Under the condition of 5.9MPa GO2 pressure at 80 °G, complex 35a transforms GHO to the completely alternating co-polymer with a TON of 250 mol (mol of Zn) and a TOF of 10 mol (mol of Zn) h along with a small amount of eyelie earbonate production (Table 7). 

Table 7 Alternating co-polymerization of epoxide with CO2 catalyzed by chromium-salen complexes...

Bruns and Haufe have described the first examples of a transition metal complex mediated asymmetric ring opening (ARO) of both meso- and racemic epoxides via formal hydro-fluorination [23]. Initial attempts with chiral Euln complexes led to very low asymmetric induction. Opening of cyclohexene oxide 30 with potassium hydrogendifluoride in the presence of 18-crown-6 and a stoichiometric amount of Jacobsens chiral chromium salen complex 29 [24a] finally yielded two products 31 and 32 in a 89 11 ratio and 92% combined yield, the desired product 31 being formed with 55% ee. Limiting 29 to a catalytic amount of 10 mol% led to an increase in the ratio of 31, however, with the enantiomeric excess dropping to 11% (Scheme 5). 

A similar approach to the hydrolytic methodology uses a chromium-salen complex to open an epoxide with trimethylsilyl azide, as illustrated by the synthesis of the antihypertensive agent, (S)-propranolol (10) (Scheme 9.15).118,119... 

Scheme 22. Asymmetric amplification in the opening of meso epoxides with TMSN3 catalyzed by a chiral chromium-salen complex.66...

Chromium-salen complexes have been used for the reaction between styrene epoxide and scC02 in [C4Ciim][PF6], as illustrated in Scheme 9.15.1601 At low catalyst concentrations, 1-phenyl-1,2-ethanediol was detected as a by-product while at a catalyst loading of ca. 0.35 mol%, 100% selectivity was obtained. Recycling of the catalyst was possible, but the ionic liquid phase needed to be exhaustively purified with volatile organic solvents prior to its reuse. 

Chromium(salen) catalysts are excellent reagents for the desymmetrization of OT to-epoxides. Thus, tfr-stilbene oxide is converted to the (3, 3 )-aminoalcohol in the presence of catalytic quantities of chromium-salen complex in methylene chloride solution open to the atmosphere. The addition of small quantities of triethylamine was found to dramatically increase enantioselectivities (by almost 25%). This catalytic system also promotes an efficient aminolytic kinetic resolution (AKR) of racemic epoxides with 2-type symmetry (Equation 18) <20040L2173, 1999TL7303>. W fo-Epoxides can be opened with aromatic amines in water in the presence of 1 mol% of an Sc(ni) catalyst ligated to 1.2mol% of a chiral bipyridine ligand <2005OL4593>. 

Punniyamurthy T, Velusamy S, Iqbal J. Recent advances in transition metal catalyzed oxidation of organic substrates with molecular oxygen. Chem. Rev. 2005 105 2329-2363. McGarrigle EM, GUheany DG. Chromium- and manganese-salen promoted epoxidation of alkenes. Chem. Rev. 2005 105 1563-1602. 

Apart from the commonly used NaOCl, urea—H2O2 has been used/ With this reaction, simple alkenes can be epoxi-dized with high enantioselectivity. The mechanism of this reaction has been examined.Radical intermediates have been suggested for this reaction, polymer-bound Mn -salen complex, in conjunction with NaOCl, has been used for asymmetric epoxidation. Chromium-salen complexes and ruthenium-salen complexes have been used for epoxidation. Manganese porphyrin complexes have also been used. Cobalt complexes give similar results. A related epoxidation reaction used an iron complex with molecular oxygen and isopropanal. Nonracemic epoxides can be prepared from racemic epoxides with salen-cobalt(II) catalysts following a modified procedure for kinetic resolution. 

In the chromium series, the reactive (salcn)chromium(V) oxo complex was isolated and X-ray characterized [3]. To explain why ( )-alkcncs arc epoxidized more effectively with the chromium-salen systems, the authors [ IS] proposed a bent geometry of the (salen)chromium(V) oxo intermediate however, this hypothesis has been neither confirmed nor denied. Our contributions to the discussion of the mechanism (namely, insight into the catalytic cycle) arc presented in this Chapter. (Salcn)chromium complexes arc known to catalyze other types of reactions (see (1,26] and references therein). 

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]. 

Umani-Ronchi adapted the Furstner protocol to achieve the first catalytic, enantioselective variant of this reaction. The chiral chromium salen complex was prepared from the in situ reduction of the anhydrous CrCb to CrCl2 with an excess of manganese metal, followed by complexation with the salen ligand 8 in the presence of catalytic triethylamine." Then the addition of allylic chloride (9) to aldehydes 10 to give the allylic alcohols 11 in moderate yields and in up to 95% ee. The same groups employed the same conditions for the addition of 2-butenyl bromides to aldehydes to achieve up to 83 17 syn/anti of allylic alcohol products and for the addition of 1,3-dichloropropene to aromatic aldehydes to obtain the syn chlorohydrin adduct in modest yield which were further converted to optically active vinyl epoxides. The [Cr(salen)]-catalyzed addition of propargyl halides to aromatic aldehydes allowed the synthesis of enantiomerically enriched homopropargyl alcohols in moderate yields with up to 56% ee. ... 

Figure 7.2 Biologically interesting molecules prepared by (salen)chromium-catalyzed ARO of epoxides with azide.

Abstract This chapter focuses on well-defined metal complexes that serve as homogeneous catalysts for the production of polycarbonates from epoxides or oxetanes and carbon dioxide. Emphasis is placed on the use of salen metal complexes, mainly derived from the transition metals chromium and cobalt, in the presence of onium salts as catalysts for the coupling of carbon dioxide with these cyclic ethers. Special considerations are given to the mechanistic pathways involved in these processes for the production of these important polymeric materials. 

Jacobsen and coworkers discovered that chiral salicylimidato transition metal complexes activate epoxides in a stereoselective manner. The published mechanism indicates that one Cr° (salen)-N3 with (/ ,/ )-cyclohexyl backbone acts as Lewis acid and coordinates to the oxygen of PO, while a second catalyst molecule transfers the azide to the activated epoxide and thus opens the ring. The coplanar arrangement of the two chromium complexes prefers one enantiomer of PO and so induces stereochemical information [99,100, 121-129]. (cf. also Sect. 8.3) (Fig. 42). 

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. ... 

An aggravating phenomenon associated with the (salen)Mn complexes is that the epoxidation of /ram-olefins proceeds typically with low ee s. Remarkably, however, the analogous chromium complexes (e.g., 14) catalyze such epoxidations with greater selectivity than for the corresponding d.v-olefins under the same conditions. Here the mechanism is presumed to involve an electrophilic process, which is supported by the fact that only electron-rich alkenes are effectively epoxidized. In the case of ram-l.l-methy 1-styrcnc (15), enantioselectivities of ca. 80% are observed [95TL7739],... 

Tridentate Schiff base chromium(III) complexes were identified as the optimal catalysts for the enantioselective ring opening of meso-aziridines by TMSN3.51 Indeed, preliminary studies have shown that, although the (salen)chromium complexes catalyzed the reaction to some extent, they consistently led to low enantioselectivities (<14% ee). It was rationalized that the diminished reactivity and selectivity of the salen complexes with aziridines compared to epoxides was a result of the steric hindrance created by the /V-substituent of the coordinated aziridine. As expected, improved results were observed using tridentate ligands on the chromium center because they offer a less-hindered coordination environment (Figure 17.7).51... 

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