Showing papers on "Lewis acids and bases published in 1998"

Heterogeneous Catalysts for Liquid-Phase Oxidations: Philosophers' Stones or Trojan Horses?

Abstract: Introduction In the early seventies one of us1 was involved in the development of the heterogeneous Ti(IV)/SiO2 catalyst which forms the basis of the Shell process for the epoxidation of propylene with ethylbenzene hydroperoxide (reaction 1).2 Halcon3 and ARCO4,5 workers had previously found, independently, that soluble compounds of early transition metals, e.g., Mo, W, Ti, and V, catalyze reaction 1. The mechanism of catalysis involves withdrawal of electrons from a coordinated alkylperoxo moiety, thereby increasing the electrophilic character of the peroxidic oxygens, i.e., the metal ion acts as a Lewis acid. Hence, effective catalysts are both a strong Lewis acid and a weak oxidant in their highest oxidation state. The latter criterion is necessary in order to minimize competing one-electron oxidation of the ROO ligand leading to homolytic decomposition of ROOH (see Scheme 1). These criteria are best met by molybdenum(VI), and soluble molybdenum compounds exhibit the best combination of activity and selectivity.6,7 Soluble titanium(IV) compounds, on the other hand, are rather mediocre catalysts for reaction 1. In contrast, Ti(IV)/SiO2 exhibits selectivities comparable to homogeneous molybdenum and (for a heterogeneous catalyst) high activities.8 The superior catalytic activity of Ti(IV)/SiO2 was attributed to both an increase in Lewis acidity of the Ti(IV), owing to electron withdrawal by silanoxy ligands, and to site isolation of discrete Ti(IV) centers in the silica lattice preventing oligomerization to unreactive μ-oxo species (which readily occurs with soluble Ti(IV) compounds). Furthermore, it was demonstrated that only the combination of titanium(IV) with silica affords a stable heterogeneous catalyst; all other combinations, e.g., Mo(VI), W(VI), V(V), etc., on silica, gave rapid leaching of the metal ion. One property which soluble Ti(IV) compounds and Ti(IV)/SiO2 share is a marked sensitivity toward deactivation by strongly coordinating ligands, especially water.9 For this reason Ti(IV)/ SiO2 is an ineffective catalyst for epoxidations with aqueous hydrogen peroxide. Hence the appearance in the mid-eighties of Enichem patents10 describing the remarkable catalytic activity of titanium(IV) silicalite (generally known as TS-1) in, inter alia, the selective epoxidation of olefins under very mild conditions with 30% aqueous hydrogen peroxide (Figure 1) was greeted with some scepticism. Thus, two materials, Ti(IV)/SiO2 and TS-1, having roughly the same elemental composition, i.e., 2% Ti in SiO2, exhibited totally different catalytic properties. Initial attempts by various groups to reproduce the Enichem results were largely unsuccessful. However, once it became clear that certain parameters in the synthesis

A Novel Class of Ruthenium Catalysts for Olefin Metathesis.

Abstract: The different nature of carbene ligands is clearly demonstrated by the first ruthenium-based complexes 1, which contain both alkylidene and N-heterocyclic carbene moieties. The latter exhibit a pronounced Lewis base behavior. Moreover, this difference forms the basis of the high catalytic activities of these compounds in olefin metathesis reactions.

Lewis Acid Catalysts Stable in Water. Correlation between Catalytic Activity in Water and Hydrolysis Constants and Exchange Rate Constants for Substitution of Inner-Sphere Water Ligands

Abstract: Today’s environmental concerns demand clean reaction processes that do not use harmful organic solvents. 1 Water is no doubt the most environmentally friendly solvent; however, its use in organic reaction processes is rather limited because many organic materials do not dissolve in water, and therefore in most cases reactions proceed sluggishly. 2 In addition, many reactive intermediates and catalysts are decomposed by water. This is the case for Lewis acid catalyzed reactions, which are of great current interest because of the unique reactivities and selectivities they can achieve and for the mild conditions used. 3 Lewis acids have been believed to be unstable in water and therefore unusable in aqueous solution. On the other hand, we have recently found water-stable Lewis acids, lanthanide trifluoromethanesulfonates (lanthanide triflates), which can be used in several carbon -carbon bond-forming reactions in aqueous media. 4 The stability and catalytic activity of lanthanide triflates in water were ascribed to their large ionic radii and an equilibrium between the Lewis acids and water. We have now clarified that some metal salts other than lanthanides are also stable Lewis acids in water and work as catalysts. 5 In addition, common characteristics, a certain range of hydrolysis constants, and a high order of exchange rate constants for substitution of inner-sphere water ligands (water exchange rate constant (WERC)) have been found among these water-stable Lewis acids. We screened group 1 -15 metal chlorides in a model reaction of benzaldehyde with ( Z)-1-phenyl-1-(trimethylsiloxy)propene (the Mukaiyama aldol reaction) (Table 1). 6 The reaction is suitable for testing catalytic ability of the metal chlorides as Lewis acid catalysts in aqueous media, because the silyl enol ether is watersensitive (especially under acidic conditions) and if the Lewis acids hydrolyze in water, the enol ether decomposes rapidly and the desired reaction proceeds no further. In the first screening, the chloride salts of Fe(II), Cu(II), Zn(II), Cd(II), In(III), and Pb(II) as well as the rare earths (Sc(III), Y(III), Ln(III)) gave promising yields. When the chloride salts of B(III), Si(IV), P(III), P(IV), Ti(IV), V(III), Ge(IV), Zr(IV), Nb(V), Mo(V), Sn(IV), Sb(V), Hf(IV), Ta(V), W(VI), Re(V), and Tl(III) were used, decomposition of the silyl enol ether occurred rapidly and no aldol adduct was obtained. This is because hydrolysis of such metal chlorides is very fast and the silyl enol ethers were protonated then hydrolyzed to afford the corresponding ketone. On the other hand, no product or only a trace amount of the product was detected using the metal chloride salts of Li(I), Na(I), Mg(II), Al(III), K(I), Ca(II), Cr(III), Mn(II), Co(II), Ni(II), Ga(III), Ru(III), Rh(III), Pd(II), Ag(I), Ba(II), Os(III), Ir(III), Pt(II), Au(I), Hg(II), and Bi(III). Some of these salts are stable in water, but have low catalytic ability. After the first screening, a second test was performed for the more promising metals. This test was carried out using the same aldol reaction and the corresponding metal perchlorates or trifluoromethanesulfonates (triflates) (Table 1). 7

IR spectroscopy of small and weakly interacting molecular probes for acidic and basic zeolites

Abstract: The application of small and weakly interacting probe molecules for the characterization of acidic and basic properties by FTIR spectroscopy is exemplified by using H- and alkali cation-exchanged zeolites as typical solid Bronsted and Lewis acids and Lewis bases. Criteria for the selection of probe molecules are given. Bronsted acidity can be characterized by the H-bonding method when CO and N2 are used as molecular probes. Quantum chemical calculations are shown to provide important additional information on the electronic nature of the adsorption interaction and the vibrational behaviour of the probe molecule. Lewis acidity dominates in cation-exchanged zeolites for small cations (Li+, Na+) whereas basic properties develop with increasing cation radius. CO, CO2, N2 and CH4 interact with cation centers, the interaction energy decreasing with increasing cation radius. CO at very low equilibrium pressures permits a siting of Na+, and the Al distribution in six-rings (SII-sites) can be probed. CH4 interacts with cations in the M+···H3CH configuration having C3v symmetry. CH-acids such as Cl3CH(D), acetylene and methylacetylene, are shown to be potentially suitable probe molecules for basic properties using the H-bonding method. All three molecules undergo Oz2−···H–C H-bonding and the induced red-shift of the C–H stretching frequency permits a ranking of the base strength of a given series of materials.

Platinum- and Acid-Catalyzed Enyne Metathesis Reactions: Mechanistic Studies and Applications to the Syntheses of Streptorubin B and Metacycloprodigiosin

Abstract: Formal total syntheses of the antibiotics metacycloprodigiosin (2) and streptorubin B (3) are described, which are known to exhibit promising immunomodulating properties. The key step en route to their meta-bridged pyrrole core structures 5 and 7, respectively, consists of a metathesis reaction of electron-deficient enynes catalyzed by either platinum halides, hard Lewis acids, or HBF4. This transformation expands the pre-existing cycloalkene of the substrates by two C atoms, forges the bicyclic pyrrolophane structure of the targets, and simultaneously forms a bridgehead alkene function. The products of this skeletal rearrangement are converted into the targets by a sequence comprising (i) a stepwise reduction of their enone entity to the corresponding saturated alcohols and (ii) an aromatization of the N-tosylated dihydropyrroles 20 and 34 thus obtained via elimination of potassium sulfinate on exposure to KAPA (potassium 3-aminopropylamide). A careful analysis of the minor byproducts formed in the enyne...

Use of Lewis Acids in Free Radical Reactions.

Abstract: The neglect of free radicals over the past few years has been overcome, and they are no longer only considered as interesting reactive intermediates with limited synthetic potential. New opportunities are opened up by performing radical reactions in the presence of Lewis acids. Rate enhancement of radical addition to olefins as well as stereochemical control of such reactions can be achieved in a unique manner. Recent examples of enantioselective radical reactions and perspectives for applications in catalysis are discussed.

Enantioselective Total Synthesis of (−)-Taxol

Abstract: Enantioselective total synthesis of taxol has been accomplished. Coupling reaction of the optically pure A-ring hydroxy aldehyde with the aromatic C-ring fragment followed by Lewis acid mediated eight-membered B-ring cyclization gave the desired ABC endo-tricarbocycle. The C-ring moiety of this product was reduced under Birch conditions to the cyclohexadiene derivative, which was oxygenated by singlet oxygen from the convex β-face to give the C4β,C7β-diol stereoselectively. For introduction of the C19-methyl, the cyclopropyl ketone was prepared via cyclopropanation of the C-ring allylic alcohol or conjugate addition of a cyano group to the C-ring enone. Reductive cleavage of the cyclopropane ring followed by isomerization of the resulting enol to the corresponding ketone gave the crucial synthetic intermediate containing the C19-methyl group. Regioselective transformation of three hydroxyl groups of this intermediate, conversion of the C4-carbonyl group to the allyl chloride, and introduction of the C10-o...

Catalytic, Enantioselective Alkylation of α-Imino Esters Using Late Transition Metal Phosphine Complexes as Catalysts

Abstract: Over the past several years, highly effective methods for enantioselective aldol additions catalyzed by Lewis acids have been developed.1 Analogous alkylations of imines, however, have not been nearly as well studied nor as successful.2 R-Imino esters are almost unstudied in Lewis acid-catalyzed reactions,3 but are especially attractive imine substrates for the efficient syntheses of natural product precursors,4 pharmaceutically active compounds,5 and nonnatural amino acids;6 the last category has recently received much attention as peptidomimetics7 and in sitedirected mutagenesis studies.8 In a recent report, we demonstrated that select late transition metals can catalyze the cis-trans isomerization of prolyl peptides through simultaneous coordination of the metal to the amide nitrogen (Na) and the side chain carbonyl group (Figure 1, a).9 Catalysis fails to occur on simple amides that do not contain an additional binding site. These results prompted us to investigate whether analogous coordination of a transition metal to the nitrogen of a functionalized imine and a chelating carbonyl group could activate the substrate toward a highly enantioselective addition of nucleophiles (Figure 1, b). From our point of view, activated R-imino esters 1 seemed ideal substrates for Lewis acid catalyzed asymmetric alkylations for several reasons: (1) alkylation occurs readily at the imine carbon with a variety of nucleophiles, (2) the electron-withdrawing R-ester group provides additional activation of the imino group to nucleophilic attack, (3) the imine N and carbonyl O can form a stable five-membered chelate ring with a chiral Lewis acid catalyst (eq 1),10 providing additional rigidity to an activated complex and potentially enhanced product selectivity, and (4) alkylation of imine derivatives 1 with enol silane nucleophiles can lead to substituted γ-oxo R-amino acids (aspartic acid analogues) that comprise a class of interesting and useful biologically active natural compounds.11 We report herein a means to alkylate R-imino esters enantioselectively in up to 98% ee and in high chemical yields with enol silanes using chiral catalytic late transition metal phosphine complexes selected from Ag(I), Cu(I), Ni(II), and Pd(II) (eq 2).

Indium(III) Chloride-Promoted Rearrangement of Epoxides: A Selective Synthesis of Substituted Benzylic Aldehydes and Ketones

Abstract: A simple and efficient procedure for the rearrangement of substituted epoxides catalyzed by InCl3 has been developed. Aryl-substituted epoxides isomerize with complete regioselectivity to form a single carbonyl compound via cleavage of the benzylic C−O bond. The reactions are simple, fast, and high yielding. This procedure is very mild compared to those catalyzed with BF3 and other Lewis acids and compatible with several acid-sensitive functionalities. This protocol provides a highly selective synthesis of substituted benzylic aldehydes and ketones. However, rearrangement of alkyl-substituted epoxides is not very selective.

New Catalytic Concepts for the Asymmetric Aldol Reaction

Abstract: Mukaiyamas classic catalytic asymmetric al- dol reaction with enolsilanes has been improved upon over the years; now new approaches to the reaction have been developed. Those reviewed here are the use of new transition metal based catalysts for the aldol reaction using silyl enolates, the use of other O-silylated nucleo- philes, tin-modified or even unmodified ketones as substrates, and the use of chiral nonmetallic Lewis acids or Lewis bases as catalysts.

Rational Design of Benzyl-Type Protecting Groups Allows Sequential Deprotection of Hydroxyl Groups by Catalytic Hydrogenolysis

Abstract: Benzyl protection of a hydroxyl group is one of the most frequently used procedures in synthesis because of the mild conditions involved in its removal by catalytic hydrogenolysis.1-3 The synthesis of polyhydroxylated compounds often requires orthogonal protecting strategies to distinguish between hydroxyl groups. It would be highly desirable to develop a range of benzyl-type protecting groups with different reactivities that can be sequentially removed via catalytic hydrogenolysis. This requires a detailed understanding of the mechanism of the cleavage of the benzyl oxygen bond by the palladium hydrogen species. Recently, we have determined the amphipolar nature of the palladium hydrogen bond (modes a, Mδ+ Hδ-, or b, MδHδ+) in both homogeneous4 and heterogeneous5 hydrogenation of alkenes. This has led us to test whether the electronic properties of the aromatic group can influence the rate of cleavage, which should in turn guide the development of hydroxyl protecting groups with different reactivities. The results in Table 1 show that the rate of debenzylation can be dramatically affected by the electronic properties of the aromatic ring. The substitution of the electron-withdrawing trifluoromethyl group onto the aromatic ring severely retards debenzylation under 1 atm of hydrogen. In contrast, there is considerable acceleration by electrondonating substituents, which suggests that the benzylic carbon bears a partial positive charge in the transition state. The hydrogenolysis of benzyl alcohols carried out in acetic acid has shown that protonation of the hydroxyl group is essential for the cleavage of the carbon-oxygen bond.6 Under the neutral conditions in our study, the reaction may occur by protonation of the benzyl oxygen atom, through the operation of mode b, MδHδ+, to give a positively charged benzylic carbon. Alternativly, it is possible that palladium could act as a Lewis acid and coordinate to the benzyl oxygen atom to promote the same electron-deficient transition state (mode a, Mδ+ Hδ-). The large difference in reactivity within this range of substituted benzyl groups suggests that they can be sequentially deprotected, therefore proving useful in multistep synthesis. To test the synthetic application of these groups, competition experiments were conducted on model systems with two differently substituted benzyl groups attached to ethanediol (Scheme 1a). Surprisingly, the benzyl group was cleaved first in competition with any of the substituted benzyl groups. This phenomenon has been observed with the 4-methoxybenzyl group (PMB); however, no explanation was proposed.7,8 The results with the linker experiments (Scheme 1a) seem to contradict those obtained when only one benzyl group is involved (Table 1). Surface scientists have determined that the aromatic ring lies flat on the metal surface for optimal coordination.9,10 It is possible that substitution on the aromatic ring could have an adverse steric effect that would interfere with the planar geometry required for effective binding and thus reduce its affinity for the metal surface. The linker experiments show that the limited number of active sites on the palladium surface could lead to a competition for adsorption sites between substituted and unsubstituted benzyl groups. This may explain why the least substituted benzyl group, although not electronically favored, can still be preferentially cleaved. It is clear that for the rational design of selective benzyl type protecting groups both electronic factors and adsorption must be taken into account. For synthetic purposes, it would be desirable to find a more labile group than the benzyl group for protection of the hydroxyl functionality. We anticipated that the 2-naphthylmethyl (NAP) group would fulfill these criteria: it is electron rich and should have a (1) Greene, T. W.; Wuts, P. G. M. In Protective Groups in Organic Synthesis; John Wiley & Sons, Inc.: New York, 1991. (2) (a) Czernecki, S.; Georgoulis, C.; Provelenghiou, C. Tetrahedron Lett. 1976, 3535. (b) Iverson T.; Bundle K. R. J. Chem. Soc., Chem. Commun., 1981, 1240. (3) Czech, B. P.; Bartsch, R. A. J. Org. Chem. 1984, 49, 4076. (4) Yu, J.; Spencer, J. B. J. Am. Chem. Soc. 1997, 119, 5257. (5) Yu, J.; Spencer, J. B. J. Org. Chem. 1997, 62, 8618. (6) Kieboom, A. P. G.; De Kreuk, J. F.; Van Berkum, H. J. Catal. 1971, 20, 58. (7) Srikrishna, A.; Viswajanani, J. A.; Sattigeri, J. A.; Vijaykumar, D. J. Org. Chem. 1995, 60, 5961. (8) Sajiki, H.; Kuno, H.; Hirota, K. Tetrahedron Lett. 1997, 38, 399. (9) Lin, R. F.; Koestner, R. J.; Van Hove, M. A.; Somorjai, G. A. Surf. Sci. 1983, 161. (10) Held, G.; Bessent, M. P.; Titmuss, S.; King, D. A. J. Chem. Phys. 1996, 11305. Table 1a

Controlled radical polymerization of methyl methacrylate initiated by an alkyl halide in the presence of the Wilkinson catalyst

Abstract: It is found that although the Wilkinson catalyst is less efficient than the Sawamoto and Matyjaszewski systems in terms of polymerization kinetics, it allows MMA to be polymerized in a living manner at a temperature as low as 60°C and in the absence of any Lewis acid

Metal- and Ligand-Accelerated Catalysis of the Baylis-Hillman Reaction.

Abstract: The Baylis−Hillman reaction, the coupling of an unsaturated carbonyl compound/nitrile with aldehydes, is a valuable reaction but is limited in its practicality by poor reaction rates. We have endeavored to accelerate the reaction using Lewis acids and found that while conventional Lewis acids gave reduced rates group III, and lanthanide triflates (5 mol %) gave increased rates. The optimum metal salts were La(OTf)3 and Sm(OTf)3, which gave rate accelerations (krel) of approximately 4.7 and 4.9, respectively, in reactions between tert-butyl acrylate and benzaldehyde when using stoichiometric amounts of DABCO. At low loadings of DABCO (up to 10 mol %), no reaction occurred due to association of DABCO with the metal. Use of additional ligands to displace the DABCO from the metal was studied, and the rate of reaction was found to increase further in most cases. Of the ligands tested, at 5 mol %, (+)-binol gave one of the largest rate accelerations (3.4-fold) and was studied in more detail. It was found that r...

The mechanism of catalysis and the inhibition of the Bacillus cereus zinc-dependent beta-lactamase.

Abstract: The plot of kcat/Km against pH for the Bacillus cereus 569/H beta-lactamase class B catalysed hydrolysis of benzylpenicillin and cephalosporin indicates that there are three catalytically important groups, two of pKa 5.6+/-0.2 and one of pKa 9.5+/-0.2. Below pH 5 there is an inverse second-order dependence of reactivity upon hydrogen ion concentration, indicative of the requirement of two basic residues for catalysis. These are assigned to zinc(II)-bound water and Asp-90, both with a pKa of 5.6+/-0.2. A thiol, N-(2'-mercaptoethyl)-2-phenylacetamide, is an inhibitor of the class B enzyme with a Ki of 70 microM. The pH-dependence of Ki shows similar pH inflections to those observed in the catalysed hydrolysis of substrates. The pH-independence of Ki between pH 6 and 9 indicates that the pKa of zinc(II)-bound water must be 5.6 and not the higher pKa of 9.5. The kinetic solvent isotope effect on kcat/Km is 1.3+/-0.5 and that on kcat is 1.5. There is no effect on reactivity by either added zinc(II) or methanol. The possible mechanisms of action for the class B beta-lactamase are discussed, and it is concluded that zinc(II) acts as a Lewis acid to stabilize the dianionic form of the tetrahedral intermediate and to provide a hydroxide-ion bound nucleophile, whereas the carboxylate anion of Asp-90 acts as a general base to form the dianion and also, presumably, as a general acid catalyst facilitating C-N bond fission.

C2-symmetric copper(ii) complexes as chiral lewis acids. enantioselective catalysis of the glyoxylate-ene reaction

Abstract: The development of enantioselective Lewis acid catalyzed carbonyl addition reactions of π-nucleophiles such as enolsilanes and allylstannanes is a topic of current interest.1 The extension of this general process to include simple olefinic nucleophiles via the carbonyl-ene reaction2 has important practical implications. In this context, Mikami and Nakai have reported a catalytic enantioselective ene reaction with glyoxylate esters;3 however, due to the limiting reactivity of the catalyst-glyoxylate complex,4 only nucleophilic 1,1-disubstituted olefins may be employed. We have recently reported that bidentate bis(oxazolinyl) (box) Cu(II) complexes 1-3 are effective enantioselective catalysts in Diels-Alder5 and aldol reactions6 with substrates that can participate in catalyst chelation. In this study, we demonstrate that

Million-fold acceleration of a Diels-Alder reaction due to combined Lewis acid and micellar catalysis in water

Abstract: The effect of micelles of sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), dodecyl heptaoxyethylene ether (C12E7), and copper and zinc didodecyl sulfate (M(DS)2) on the Diels−Alder reaction of 3-(para-substituted phenyl)-1-(2-pyridyl)-2-propen-1-ones 1a−g, containing neutral, cationic, or anionic substituents, with cyclopentadiene (2) has been studied. In the absence of catalytically active transition-metal ions, micelles invariably retard the reaction. This can be rationalized on the basis of different binding locations of the reaction partners in the micelle. These binding sites have been probed using solubilizate-induced aromatic shifts in the 1H NMR spectrum of the surfactant and paramagnetic counterion-induced relaxation enhancements of the 1H NMR signals of the solubilizate. In contrast to SDS, CTAB, and C12E7, Cu(DS)2 micelles catalyze the Diels−Alder reaction between 1 and 2 with extremely high efficiency, leading to rate enhancements up to 1.8 × 106 compared to the uncatalyzed...

Characterization of acid sites in zeolitic and other inorganic systems using solid-state 31P NMR of the probe molecule trimethylphosphine oxide

Abstract: The ability to determine the types and concentrations of acid sites in zeolites and fluid-catalytic cracking systems is important for an increased understanding of structure/performance relationships in these materials. Currently, a variety of thermal methods exist that allow quantitative measurement of the Bronsted acid site concentration. In addition, numerous spectroscopic methods using probe molecules are available for qualitative and quantitative detection of both Bronsted and Lewis acid sites. In studies utilizing solid-state NMR spectroscopy, probe molecules containing 31P nuclei present substantial advantages over probes isotopically enriched with 13C and 15N nuclei. These advantages include increased sensitivity and chemical shift dispersion. While a number of phosphorus-based experiments have probed the interaction of trimethylphosphine with solid acid catalysts, initial studies of the more stable trimethylphosphine oxide (TMPO) have only been reported on amorphous silica−alumina surfaces. We no...

Design of Brønsted Acid-Assisted Chiral Lewis Acid (BLA) Catalysts for Highly Enantioselective Diels−Alder Reactions

Abstract: Bronsted acid-assisted chiral Lewis acid (BLA) was highly effective as a chiral catalyst for the enantioselective Diels−Alder reaction of both α-substituted and α-unsubstituted α,β-enals with various dienes. Hydroxy groups in optically active binaphthol derivatives and boron reagents with electron-withdrawing substituents were used as Bronsted acids and Lewis acids, respectively. Intramolecular Bronsted acids in a chiral BLA catalyst played an important role in accelerating the rate of Diels−Alder reactions and in producing a high level of enantioselectivity. In particular, excellent enantioselectivity was achieved due to intramolecular hydrogen bonding interaction and attractive π−π donor−acceptor interaction in the transition-state assembly by hydroxy aromatic groups in a chiral BLA catalyst.

Catalytic, Enantioselective Synthesis of α-Aminonitriles with a Novel Zirconium Catalyst.

Abstract: Strecker reactions of aldimines with Bu3 SnCN in the presence of the novel chiral zirconium binuclear catalyst 1 provide α-aminonitriles in good yields and with high enantioselectivities. The reaction can be applied to a wide range of substrates. Since both enantiomers of the chiral sources are readily avaibable, both enantiomers of the α-aminonitriles are easily prepared according to this method. L=N-methylimidazole.

The First Enantioselective Aza-Diels-Alder Reactions of Imino Dienophiles on Use of a Chiral Zirconium Catalyst.

Abstract: Chiral dihydropyridone derivatives 3 are obtained in high yields and with good enantioselectivities by the title reaction of aldimines such as 1 with Danishefsky's diene 2. 6,6′-Dibromo-1,1′-binaphthol complexes with Group 4 metals serve as catalysts; zirconium proved to be especially effective.