Catalytic cycloadditions enantioselective reactions

The main strategy for catalytic enantioselective cycloaddition reactions of carbonyl compounds is the use of a chiral Lewis acid catalyst. This approach is probably the most efficient and economic way to effect an enantioselective reaction, because it allows the direct formation of chiral compounds from achiral substrates under mild conditions and requires a sub-stoichiometric amount of chiral material. 

Because ketones are generally less reactive than aldehydes, cycloaddition reaction of ketones should be expected to be more difficult to achieve. This is well reflected in the few reported catalytic enantioselective cycloaddition reactions of ketones compared with the many successful examples on the enantioselective reaction of aldehydes. Before our investigations of catalytic enantioselective cycloaddition reactions of activated ketones [43] there was probably only one example reported of such a reaction by Jankowski et al. using the menthoxyaluminum catalyst 34 and the chiral lanthanide catalyst 16, where the highest enantiomeric excess of the cycloaddition product 33 was 15% for the reaction of ketomalonate 32 with 1-methoxy-l,3-butadiene 5e catalyzed by 34, as outlined in Scheme 4.26 [16]. 

The first example of an enantioselective catalytic Diels-Alder reaction is the report in 1976 of cycloaddition between methyl acrylate and cyclopentadiene [3]. The catalyst was BF3 complexed to (-)-methyl menthyl ether. The enantiomeric excess is very small (3 %), and it seems unlikely that chiral catalysis occurred (e.g., the undissociated complex). 

In the last decade a variety of catalytic asymmetric DA reactions wifh alkenes has been developed most, however, involve the use of a cychc diene, particularly cyclopentadiene. There are a few examples of catalytic asymmetric reactions of siloxydienes with alkenes. Corey et al. have reported enantioselective cycloaddition of sil-oxydiene 106 to mefhacrolein as the key step of fhe asymmetric synfhesis of cassiol (Scheme 10.116) [314]. Recently, Rawal et al. have demonstrated that Cr(III)-salen complex 108a catalyzes the cycloaddition of l-amino-3-siloxy-l,3-dienes to a, -unsa-turated aldehydes wifh high enantioselectivity [315]. The highly functionalized cyclohexene products have been used for alkaloid synthesis. Ghosez et al. have introduced asymmetric DA reaction of siloxy-substituted azadienes 109 under catalysis by Cu(II)-box complex 70a [316]. 

In a simplified catalytic cycle, reversible coordination of the dienophile to the Lewis acid (LA) activates the substrate toward diene cycloaddition. In the catalyst turnover event, the Lewis acid-product complex dissociate to reveal the de-complexed cycloadduct and regenerated catalyst (Scheme 2). While this catalytic cycle neglects issues of product inhibition and nonproductive catalyst binding for dienophiles having more than one Lewis basic site, the gross features of this process are less convoluted than many other enantioselective reactions e.g., olefin dihydroxylation, aldol reactions), a fact which may provide insight as to why this process is frequently used as a test reaction for new Lewis acid catalysts. 

Keywords Catalytic antibody. Hapten, Enantiofacial, Enantioselective, Diels-Alder cycloaddition, Cationic reactions, Aldol condensation. Disfavored cyclization... 

The same research group has also demonstrated a catalytic enantioselective tandem carbonyl yhde formation-cycloaddition of the a-diazo-j8-ketoester 91 using 0.5 mol% of Rh2(R-DDBNP)4 95, as catalyst to afford the cycloadduct 93 in good yields (Scheme 28) with up to 90% ee [ 109]. A detailed study on enantioselective reaction using a series of dirhodium tetrakiscar-boxylate and tetrakisbinaphtholphosphate catalysts under different solvent conditions has been described [56]. These studies indicate that dirhodium tetrakisbinaphtholphosphate catalysts are superior to the more commonly used carboxylates and carboxamidates in asymmetric transformations. Typically, the reaction [58] of the nitrophenyl-substituted diazodione 96 and phenyl acetylene in the presence of the binaphthyl catalyst 95 at 0 °C afforded the cycloadduct 97 with 76% ee (Scheme 29). 

Tanaka et al. examined chiral Rh-catalyzed intramolecular cycloaddition using triynes with substituents at both ends and obtained [7] to [lOJmeto-cyclophanes with high enantioselectivity. This is the first example of catalytic and enantioselective synthesis of planar-chiral cyclophanes using the [2+ 2+ 2]-cycloaddition approach (Scheme 8.14) [11a]. The tether structure of the 1,6-diyene moiety of the triynes affected the stereoselectivity, and the reaction of ester-tethered triynes realized almost perfect enantioselectivity (Scheme 8.15) [11a]. 

We are now standing in the middle of the next step of the development of cycloaddition reactions - catalytic and catalytic enantioselective versions. The last two decades have been important in catalysis - how can we increase the reaction rate, and the chemo-, regio, diastereo-, and enantioselectivity of cycloaddition reactions. Metal catalysis can meet all these requirements ... 

Catalytic Enantioselective Cycloaddition Reactions of Carbonyl Compounds... 

This chapter will focus on the development of catalytic enantioselective cycloaddition reactions of carbonyl compounds with conjugated dienes (Scheme 4.1) [3]. 

To achieve catalytic enantioselective cycloaddition reactions of carbonyl compounds, coordination of a chiral Lewis acid to the carbonyl functionality is necessary. This coordination activates the substrate and provides the chiral environment that forces the approach of a diene to the substrate from the less sterically hindered face, introducing enantioselectivity into the reaction. 

The catalytic enantioselective cycloaddition reaction of carbonyl compounds with conjugated dienes has been in intensive development in recent years with the main focus on synthetic aspects the number of mechanistic studies has been limited. This chapter will focus on the development and understanding of cycloaddition reactions of carbonyl compounds with chiral Lewis acid catalysts for the preparation of optically active six-membered ring systems. 

Some of the developments of catalytic enantioselective cycloaddition reactions of carbonyl compounds have origin in Diels-Alder chemistry, where many of the catalysts have been applied. This is valid for catalysts which enable monodentate coordination of the carbonyl functionality, such as the chiral aluminum and boron complexes. New chiral catalysts for cycloaddition reactions of carbonyl compounds have, however, also been developed. 

Jacobsen et al. took an important step towards the development of a more general catalytic enantioselective cycloaddition reaction of carbonyl compounds by introducing chiral tridentate Schiff base chromium(III) complexes 15 (Scheme 4.15)... 

The interest in chiral titanium(IV) complexes as catalysts for reactions of carbonyl compounds has, e.g., been the application of BINOL-titanium(IV) complexes for ene reactions [8, 19]. In the field of catalytic enantioselective cycloaddition reactions, methyl glyoxylate 4b reacts with isoprene 5b catalyzed by BINOL-TiX2 20 to give the cycloaddition product 6c and the ene product 7b in 1 4 ratio enantio-selectivity is excellent - 97% ee for the cycloaddition product (Scheme 4.19) [28]. 

---