Chromium salen catalysts

Enantioselective addition of allylstannane to straight-chain aldehydes has been achieved using a chromium-salen catalyst.186... 

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>. 

Aziridines can be converted to cyclic carbamates (oxazohdinones) by heating with carbon dioxide and a chromium-salen catalyst/ The reaction of aziridines with Lil, and then CO2 also generates oxazolidinonesJ ... 

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]. 

Chiral salen chromium and cobalt complexes have been shown by Jacobsen et al. to catalyze an enantioselective cycloaddition reaction of carbonyl compounds with dienes [22]. The cycloaddition reaction of different aldehydes 1 containing aromatic, aliphatic, and conjugated substituents with Danishefsky s diene 2a catalyzed by the chiral salen-chromium(III) complexes 14a,b proceeds in up to 98% yield and with moderate to high ee (Scheme 4.14). It was found that the presence of oven-dried powdered 4 A molecular sieves led to increased yield and enantioselectivity. The lowest ee (62% ee, catalyst 14b) was obtained for hexanal and the highest (93% ee, catalyst 14a) was obtained for cyclohexyl aldehyde. The mechanism of the cycloaddition reaction was investigated in terms of a traditional cycloaddition, or formation of the cycloaddition product via a Mukaiyama aldol-reaction path. In the presence of the chiral salen-chromium(III) catalyst system NMR spectroscopy of the crude reaction mixture of the reaction of benzaldehyde with Danishefsky s diene revealed the exclusive presence of the cycloaddition-pathway product. The Mukaiyama aldol condensation product was prepared independently and subjected to the conditions of the chiral salen-chromium(III)-catalyzed reactions. No detectable cycloaddition product could be observed. These results point towards a [2-i-4]-cydoaddition mechanism. 

The second order in catalyst points to the involvement of two chromium salen molecules in the transition state complex. Therefore several dimeric species were synthesised with suitable linkers the dimeric catalysts gave reaction rates that were one or two orders of magnitude higher than that of the monomeric catalyst. Trimeric species gave still higher reaction rates The... 

We have utilized somewhat less-effective optional approaches to copolymer purification with attendant catalyst recovery. One of these methods involved the replacement of the f-butyl substituents on the 5-position of the phenolate ligands with poly(isobutylene) (PIB) groups, as illustrated in Fig. 14 [39]. Importantly, this chromium(III) catalyst exhibited nearly identical activity as its 3,5-di-t-butyl analog for the copolymerization of cyclohexene oxide and carbon dioxide. The PIB substituents on the (salen)CrCl catalysts provide high solubility in heptanes once the copolymer is separated from the metal center by a weak acid. 

Other studies on the chromium(in)-salen catalysts of type 10 have shown that the 3 -substituent (i.e., R) exhibits relatively little sensitivity with regard to chiral induction, in stark contrast to the analogous Mn-salen complexes, in which the 3 -position must bear a sterically bulky group for acceptable enantiomeric excesses. Thus, the 3 -chloro catalyst with a triphenyl-... 

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). 

Mirkin and coworkers reported on catalytic molecular tweezers used in the asymmetric ring opening of cyclohexene oxide. In this case the early transition metal is the catalyst and rhodium functions as the structural inductor metal. The catalyst consists of two chromium salen complexes, the reaction is known to be bimetallic, and a switchable rhodium complex, using carbon monoxide as the switch. Indeed, when the salens are forced in dose proximity in the absence of CO the rate is twice as high and the effect is reversible [77]. 

The fixation of C02 into a three-membered ring has also been promoted, under relatively severe conditions, by other catalytic systems such as tetrapheny-lantimony halides (333 K, 5 MPa) [68e] or the p-methoxyphenol/DMAP system (DMAP = 4,6-(dimethylamino)pyridine 393 K, 3.6MPa, 48h) [68f] or, under much milder conditions, by alkali [68g-k] or tetralkylammonium halides [68j], or by (Salen)Cr(III)(DMAP) [681]. It is worth noting that, with 2-alkyl or 2-aryl substituted aziridines, alkali or tetralkylammonium halides catalyze the formation of the 4-substituted regioisomer as the main or unique product, whereas the chromium(III) catalyst promoted the preferential conversion to the 5-subshtuted regioisomer with high selectivity and yield. 

Gibson and workers [27] at BP have reported a high-throughput approach directed towards the optimization of chromium polyethylene catalysts. The study aimed to improve the performance of a previously discovered chromium catalyst. In this study, the ca. 200-member hemi-salen ligand library was complexed in situ to a soluble chromium precursor [p-tolylCrCl2(thf)3], followed by addition of 180 equiv. of MAO, and exposure to 1 atm of ethylene for 15 min. Relative activities were determined from the polyethylene yields, and a new highly active chromium catalyst was uncovered. The Coates group has reported a clever approach to the... 

Jacobsen s cobalt and chromium salen complexes 69 and 70 have proven extremely successful in the enantioselective ring opening of meso-epoxides (and kinetic resolution of racemic epoxides). Recent accounts of these most efficient and practical catalysts can be found elsewhere [71-73]. 

Fig. 18 Representative examples of homogeneous catalysts for CO epoxide copolymerization Inoue s tetraphenylporphyrin aluminium chloride complex (a), zinc hw-2,6-fluorophenoxide complex (b), zinc (3-diiminate complex (c) and chromium-salen complex (d)...

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. 

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. 

This is all very well but what about using dienes which are more typically electron rich (with one oxygen substituent instead of two) 255 in combination with normal aliphatic aldehydes 256 A catalyst that could achieve this would be very useful. One solution is a modification of the chromium salen complex which replaces half the salen with an adamantyl group and the other half with d.v-aminoindanol 209. The synthesis of this complex 258 is straightforward since commercially available compounds 257 and 209 are combined in high-yielding reactions and the complex itself is impressively enantioselective.58 The hexafluoroantimonate catalyst 260 was more enantioselective than the corresponding chloride 259. 

While the a-allylation of enolates occurs with high ee using palladium catalysts (see Section 10.2) there have been few reports on the enantioselective metal-catalysed enolate alkylation. The best results to date have been achieved by Doyle and lacobsen using the chromium(salen) complex (12.42) in the alkylation of cyclic tin enolates with a range of alkyl halides, including propargyl and benzylic hahdes... 

Scheme 2.47 Enantioselective Henry reactions with the use of chiral potassium(i)-his(chromium(iii)-salen) catalyst hearing an oligo(ethylene glycol) linkage.

The oxidation of alcohols to carbonyl compounds is a fundamental reaction that has synthetic and chemical importance. Using chromium-based catalysts, researchers have developed several catalysts that have impacted alcohol oxidation reactions. Recently, homogeneous catalysts have had problems with catalyst/product separation and suffer from poor catalyst recyclability. Therefore, the quest for a resolution to this problem has led researchers to scaffold salen complexes onto a silica-based material such as MCM-41. Zhou et al. used an ion-exchangeable, layered polysiloxane support to immobili.se their sulfonato-(salen)Cr(m) complex. They reacted benzyl alcohol, cyclo-hexanol and -hexanol with hydrogen peroxide as oxidant in an ionic liquid at 40 °C. Several ionic liquids were investigated [BMImX (BMIm = 1-n-butyl-3-methylimidazolium X =PF6, BF4, NOs")] and compared for each substrate. 

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