Chromium-salen complexes, asymmetric

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]. 

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). 

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

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. 

Almost 10 years after Aggarwal s report, Rawal and coworkers [22] reported the use of chiral chromium-salen complexes to catalyze enantioselective Nazarov cyclizations. While other asymmetric Nazarov reactions had been reported since Aggarwal s publication, the chromium-salen complex 86 was the first to cat-alytically perform the cydization of unactivated divinyl ketones 84 to give cyclic ketones 85 in high diastereoselectivity and enantioselectivity (Scheme 3.19). 

Schaus et al.41 have also reported an asymmetric hetero Diels-Alder reaction of Danishefsky s diene 10042 with aldehyde 101 catalyzed by chromium(III) complex 99 bearing a similar chiral salen ligand. Product 102 is obtained in moderate to good yield and stereoselectivity (Scheme 5-31 and Table 5-5). 

Chiral (salen)chromium(III) complexes catalyse the asymmetric allylation of a range of aldehyde types, using allylstannane reagents.171... 

Schaus, S. E., Brnalt, J., Jacobsen, E. N. Asymmetric Hetero-Diels-Alder Reactions Catalyzed by Chiral (Salen)Chromium(lll) Complexes. 

Related epoxidations of olefins with PhIO in the presence of Salen and related complexes of chromium(III), manganese(III) and cobalt(III) have been reported by Kochi and coworkers [58]. The use of nickel(II) Salen in conjunction with NaOCl was also described [59]. More recently, these systems formed the basis for the development, by Jacobsen and coworkers [60], of chiral manganese(III) Salen complexes for the enantioselective epoxidation of prochiral olefins by ArlO or NaOCl. Similarly, asymmetric epoxidations with moderate to good... 

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]. 

Another synthesis that targeted the Blechert indohzidine (—)-402, reported by Kwon et al., was part of a more comprehensive investigation of asymmetric iodocyclizations catalyzed by Salen complexes, notably the chromium(III) complex R,R)-SSS. In the example of interest, the azido alcohol 559 was transformed into the 2-iodoalkyl tetrahydrofuran 560 in 86% yield and 90% ee when treated with A/-chlorosuccinimide and iodine in the presence of the catalyst (Scheme 75). Reduction of the azide with tin(II) chloride was followed by cyclization under basic conditions and... 

The chiral diol 17 derived from tartaric acid is exploited in the titanium-catalyzed asymmetric pinacol coupling in the presence of Zn and MesSiCl to give the corresponding diol in 11-71 ee % [44], The chiral salen ligands 18-20 are used in the titanium-catalyzed enantioselective coupling reaction, which achieves the higher selectivity [45-47]. The chromium complex with TBOxH (21) efficiently catalyzes the asymmetric coupling reaction of both aromatic and aliphatic aldehydes [48]. 

Shortly after our initial report, Doyle and Jacobsen disclosed the use of prochiral tin enolates in a salen-chromium complex-catalyzed asymmetric alkylation reaction [45, 46], It is noteworthy that this reaction was the first example of an asymmetric metal-catalyzed a-alkylation of ketone enolates that utilizes a variety of alkyl halides and is not limited to allylic electrophiles. While this mechanistically interesting reaction does not proceed via a metal 7i-allyl complex, it is a rare example of a catalytic reaction that provides access to enantioenriched a-quatemary ketones. 

The base-catalysed hydrophosphonylation of aldehydes or imines (Pudovik reaction) [58] as a convenient method was widely used for the synthesis of 1-hydrox-yalkylphosphonates. Since the pioneering work of Shibuya [50] and Spilling [51] was reported, much attention has been devoted to developing enantioselective catalysts for the synthesis of chiral 1 -hydroxy alkylphosphonates. Chiral aluminium complexes were shown to be more effective chiral catalysts [59-62]. Based on the success of using A1 (salen) and A1 (salcyen) as asymmetric catalysts, Al-Schiff base complexes [63, 64] have been developed to catalyze the asymmetric addition reaction of dial-kylphosphonates and aldehydes. Tridentate Schiff base metal complexes, such as vanadium, chromium, and iron [65], have been successfully applied in many asymmetric synthetic reactions. We noticed that Al(III) complexes could catalyse the asymmetric Pudovik reaction and these ligands could be easily synthesized [66-70]. 

In the 15 years since Fiirstner et al. developed the first NHK reaction using catalytic quantities of chromium, the subsequent stereoselective, catalytic variant has become a valuable tool in catalytic asymmetric synthesis. Many studies on the NHK reaction have focused on the application of ligands that have proven successful in a range of other catalytic asymmetric processes. For example, the salen- and oxazoline-derived ligands that have been applied have advanced the scope of the NHK reaction in terms of reactivity and asymmetric induction in a wide array of carbon-carbon forming processes. These processes now include allylation, crotylation, vinylation, methallylation, aUenylation, homoallenylation, and propargylation. Xia and Yamamoto s TBOxCr(III)Cl complex has afforded some of the highest levels of enantioselectivity in a series of allylation, crotylation, aUenylation, and more recent dienylation and alkynylation studies. 

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