Catalytic Asymmetric Induction with Chiral Lewis Acids

Catalytic Asymmetric Induction with Chiral Lewis Acids 

149a Ar= phenyl, X =TfO 149b Ar= phenyl, X = Tf2N 149c Ar = 3,5-dimethylphenyl(mexyl), X = TfO 

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The choice of solvent has had little, if any, influence on the majority of Diels-Alder reactions.210,211 Although the addition of a Lewis acid might be expected to show more solvent dependence, generally there appears to be little effect on asymmetric induction.118129 However, a dramatic effect of solvent polarity has been observed for chiral metallocene triflate complexes.212 The use of polar solvents, such as nitromethane and nitropropane, leads to a significant improvement in the catalytic properties of a copper Lewis acid complex in the hetero Diels-Alder reaction of glyoxylate esters with dienes.213... 

Asymmetric catalytic hetero Diels-Alder reactions give access to synthetically important substituted heterocycles [45]. Asymmetric oxa Diels-Alder reactions involving aldehydes and ketones and catalyzed by chiral Lewis acid catalysts can be performed with a high degree of chiral induction [46]. The field is much less advanced that of the corresponding catalytic enantioselective aza Diels-Alder reactions. 

Mukaiyama aldol reactions are useful means of constructing complex molecules for the total synthesis of natural products. Although catalytic asymmetric Mukaiyama aldol reactions have been achieved by use of a variety of chiral Lewis acids [42], no report of the use of chiral lanthanide catalysts was available until recently, despite the potency of these catalysts. Shibasaki and co-workers reported the first examples of chiral induction with chiral lanthanide complexes (Sch. 7) [43]. Catalysts prepared from lanthanide triflates and a chiral sulfonamide ligand afforded the corresponding aldol products in moderate enantiomeric excess (up to 49% ee). 

Catalytic asymmetric induction in Diels-Alder reactions is somewhat more difTicult to analyze based on these models. The chiral Lewis acids shown in Figure 43 all promote asymmetric Diels-Alder cycloaddition with variable degrees of enantioselectivity. " ... 

Olefin hydrocyanation using palladium catalysts has been less well studied than with nickel. Nevertheless, zerovalent complexes of palladium, particulrly triarylphosphite complexes, hydrocyanate a wide range of olefins in useful yields (see Table 1). Early work reported the merit of excess phosphorus ligand to promote the reaction, and further paralleling the observations with nickel, Lewis acids have been used to improve catalytic activity. However, addition of ZnClj fails to improve nitrile product yield . Asymmetric induction in hydrocyanation results in optical yields of 30% in the synthesis of exo-2-cyanonorbomane using the chiral ligand DIOP, and studies on the stereochemistry of HCN and DCN addition to terminal alkenes and a substituted cyclohexene with the same catalyst have been reported. ... 

In all of the examples considered so far, the chiral element has been employed in stoichiometric quantities. Ultimately, it would be desirable to require only a small investment from the chirality pool. This is only possible if the chiral species responsible for enantioselectivity is catalytic. It is worth stating explicitly that, in order to achieve asymmetric induction with a chiral catalyst, the catalyzed reaction must proceed faster than the uncatalyzed reaction. One example of an asymmetric aldol addition that has been studied is variations of the Mukaiyama aldol reaction [110] whereby silyl enol ethers react with aldehydes with the aid of a chiral Lewis acid. These reactions proceed via open transition structures such as those shown in Figure... 

The earliest report of a reaction mediated by a chiral three coordinate aluminum species describes an asymmetric Meerwein-Poimdorf-Verley reduction of ketones with chiral aluminum alkoxides which resulted in low induction in the alcohol products [1]. Subsequent developments in the area were sparse until over a decade later when chiral aluminum Lewis acids began to be explored in polymerization reactions, with the first report describing the polymerization of benzofuran with catalysts prepared from and ethylaluminum dichloride and a variety of chiral compounds including /5-phenylalanine [2]. Curiously, these reports did not precipitate further studies at the time because the next development in the field did not occur until nearly two decades later when Hashimoto, Komeshima and Koga reported that a catalyst derived from ethylaluminum dichloride and menthol catalyzed the asymmetric Diels-Alder reaction shown in Sch. 1 [3,4]. This is especially curious because the discovery that a Diels-Alder reaction could be accelerated by aluminum chloride was known at the time the polymerization work appeared [5], Perhaps it was because of this long delay, that the report of this asymmetric catalytic Diels-Alder reaction was to become the inspiration for the dramatic increase in activity in this field that we have witnessed in the twenty years since its appearance. It is the intent of this review to present the development of the field of asymmetric catalytic synthesis with chiral aluminum Lewis acids that includes those reports that have appeared in the literature up to the end of 1998. This review will not cover polymerization reactions or supported reactions. The latter will appear in a separate chapter in this handbook. 

Some advancement has been described for the development of a catalytic enantioselective ester enolate Claisen rearrangement. The strong Lewis basic carboxylate functionality present in the Claisen product effectively coordinates with Lewis or Bronsted acids prohibiting the catalytic turnover. Corey has reported the use of stoichiometric bromoborane 175 to generate chiral boron enolate which undergoes [3,3]-sigmatropic rearrangement to yield 176. ° Kazmaier relied on excess quinidine 178 to provide for asymmetric induction in the conversion of 177 to amino acid 179. ... 

A one-pot titanium-catalyzed tandem sulfoxidation-kinetic resolution process was developed by Chan using TBHP as the oxidant This process combines asymmetric sulfoxidation (at 0°C) and kinetic resolution (at room temperature). Excellent enantiomeric excesses (up to >99.9%) and moderate to high chemical yields of sulfoxides were obtained [270] (Scheme 14.113). The effect of fluorine substitution at the backbone of BINOL on the catalytic activity in titanium-catalyzed sulfide oxidation with TBHP or cumyl hydrc en peroxide (CHP) was studied by Yudin [271]. Introduction of fluorines into the BINOL scaffold was found to increase the electrophilic character of the Lewis acidic titanium center of the catalyst The most intriguing difference between the FsBINOL and BINOL systems is the reversal in the sense of chiral induction upon fluorine substitution. A steroid-derived BINOL ligand has also been used for the same reaction [272]. 

As important hetero Diels-Alder reactions catalyzed by aluminum Lewis acid, two kind of reactions, namely, [4 + 2] cycloaddition of Danishefski s diene with carbonyls and [4 + 2] cycloaddition of nitroalkenes with electronic rich alkenes, have been well known. The former reaction provides highly functionalized pyrones. As the first example of pyrone synthesis through catalytic asymmetric hetero Diels-Alder reaction with chiral aluminum complexes, in 1987, Quimpere and Jankowski reported the reaction of oxomalonate with 1-methyl-1,3-butadiene using Koga s catalyst (48). However, the asymmetric induction and chemical yield were quite poor (Scheme 6.147) [175]. 

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