Tetraaryl-1,3-dioxolane-4,5-dimethanol Research Articles

Dual control of stereoregularity and molecular weight in cationic polymerization of vinyl ether by tunable TADDOLs/TiCl4 initiating systems

The stereoregularity and molecular weight of poly(isobutyl vinyl ether) was simultaneously controlled by TADDOL/TiCl4 initiating systems with various aryl substituents on TADDOL.

Homochiral Porous Metal-Organic Polyhedra with Multiple Kinds of Vertices.

Metal-organic polyhedra featuring non-Archimedean/Platonic architectures with multiple kinds of vertices have aroused great attention for their fascinating structures and properties but are yet challenging to achieve. Here, we report a combinatorial strategy to make such nonclassic polyhedral cages by combining kinetically labile metal ions with non-planar organic linkers instead of the usual only inert metal centers and planar ligands. This facilitates the synthesis of an enantiopure twisted tetra(3-pyridyl)-based TADDOL (TADDOL = tetraaryl-1,3-dioxolane-4,5-dimethanol) ligand (L) capable of binding Ni(II) ions to produce a regular convex cage, Ni6L8, with two mixed metal/organic vertices and three rarely reported concave cages Ni14L8, Ni18L12, and Ni24L16 with three or four mixed vertices. Each of the cages has an amphiphilic cavity decorated with chiral dihydroxyl functionalities and packs into a three-dimensional structure. The enantioselective adsorption and separation performances of the cages are strongly dependent on their pore structure features. Particularly, Ni14L8 and Ni18L12 with wide openings can be solid adsorbents for the adsorptive and solid-phase extractive separation of a variety of racemic spirodiols with up to 98% ee, whereas Ni6L8 and Ni24L16 with smaller pore apertures cannot adsorb the racemates. The combination of single-crystal X-ray diffraction analysis of the host-guest adduct and GCMC simulation indicates that the enantiospecific recognition capabilities originate from the well-organized chiral inner sphere as well as multiple interactions within the chiral microenvironment. This work therefore provides an attractive strategy for the rational design of polyhedral cages, showing geometrically fascinating structures with properties different from those of classic assemblies.

Enantioselective Intramolecular Reductive Heck Reaction with a Palladium/Monodentate Phosphoramidite Catalyst

AbstractA palladium‐catalyzed enantioselective reductive Heck reaction of enones using monodentate phosphoramidite ligands is described. α,α,α’,α’‐tetraaryl‐1,3‐dioxolane‐4,5‐dimethanol (TADDOL)‐based phosphoramidites with palladium(II) acetate, and N‐methyl dicyclohexylamine as reducing agent gives the reductive Heck product in high yields and enantioselectivities of up to 90 %. The solvent plays an important role and in diethyl carbonate, the chemo‐ and enantioselectivity appeared to be the highest.

Insights into the asymmetric heterogeneous catalysis in porous organic polymers: constructing a TADDOL-embedded chiral catalyst for studying the structure-activity relationship.

Construction of porous organic polymers (POPs) as asymmetric catalysts remains as an important but challenging task. Herein, we exploit the "bottom-up" strategy to facilely synthesize an α,α,α',α'-tetraaryl-1,3-dioxolane-4,5-dimethanol (TADDOL)-based chiral porous polymer (TADDOL-CPP) for highly efficient asymmetric catalysis. Constructed through the covalent linkages among the three-dimensional rigid monomers, TADDOL-CPP possesses hierarchical porous structure, high Brunauer-Emmett-Teller (BET) surface area, together with abundant and uniformly-distributed chiral sites. In the presence of [Ti(OiPr)4], TADDOL-CPP acts as a highly efficient and recyclable catalyst in the asymmetric addition of diethylzinc (Et2Zn) to aromatic aldehydes. Based on the direct observation of the key intermediates, the reaction mechanism has been revealed by solid-state (13)C magic-angle spinning (MAS) NMR spectroscopy. In combination with the catalytic testing results, characterization on the working catalyst provides further information for understanding the structure-activity relationship. We suggest that the catalytic activity of TADDOL-CPP is largely affected by the structural rigidity, cooperative catalysis, local chiral environment, and hierarchical porous framework. We expect that the information obtained herein will benefit to the designed synthesis of robust POP catalysts toward practical applications.

Study on the Regioselectivie of Grignard Reagent Addition Reaction of Maleopimaric Acid Trimethyl Ester

Two regioselectivite adducts (compounds 3 and 4) were firstly synthesized from the Grignard reagent and maleopimaric acid trimethyl ester (2). The structures of all compounds were characterized by elemental analyses, NMR and MS, and stero-structures of 3 were further determined by COSY, HMQC and HMBC. The spatial structure of 3 was further confirmed by X-ray single crystal diffraction analysis method. Owing to the addition reaction selectivity of 2 was dependent on the steric hindrance of three methyl groups, the TADDOL (tetraaryl-1,3-dioxolane-4,5-dimethanols) product was prevented because of the formation of lactone 3, but the stereo-structure of the rosin-ring did not changed. Products of 3 and 4 are expected to apply as chiral derivatization reagents. Keywords maleopimaric acid trimethyl ester; grignard reagent; 2D NMR; regioselectivity

Rhodium‐Catalyzed Enantioselective Intramolecular [4+2] Cycloaddition using a Chiral Phosphine‐Phosphite Ligand: Importance of Microwave‐Assisted Catalyst Conditioning

AbstractThe use of modular α,α,α′,α′‐tetraaryl‐1,3‐dioxolane‐4,5‐dimethanol (TADDOL)‐ and 1,1′‐bi‐2‐naphthol (BINOL)‐derived phosphine‐phosphite ligands (L2*) in the asymmetric rhodium‐catalyzed intramolecular [4+2] cycloaddition (“neutral” Diels–Alder reaction) of (E,E)‐1,6,8‐decatriene derivatives (including a 4‐oxa and a 4‐aza analogue) was investigated. Initial screening of a small ligand library led to the identification of a most promising, TADDOL‐derived ligand bearing a phenyl group adjacent to the phosphite moiety at the arene backbone. In the course of further optimization studies, the formation of a new, more selective catalyst species during the reaction time was observed. By irradiating the pre‐catalyst with microwaves prior to substrate addition high enantioselectivities (up to 93% ee) were achieved. The new cyclization protocol was successfully applied to all three substrates investigated to give the bicyclic products in good yield and selectivity. 31P NMR and ESI‐MS measurements indicated the formation of a [Rh(L2*)2]+ species as the more selective (pre‐) catalyst.

ChemInform Abstract: Large-Scale Preparation of α,α,α′,α′-Tetraaryl- 1,3-dioxolane-4,5-dimethanols (TADDOLs): Versatile Auxiliaries for EPC- Synthesis and Their Solid-State Structure.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: α,α,α′,α′-Tetraaryl-1,3-dioxolane-4,5- dimethanols (TADDOLs) for Resolutions of Alcohols and as Chiral Solvating Agents in NMR Spectroscopy.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Experimental and Theoretical Studies on the Hydrogen-Bond-Promoted Enantioselective Hetero-Diels−Alder Reaction of Danishefsky's Diene with Benzaldehyde

The enantioselective hetero-Diels-Alder (HDA) reaction of Danishefsky's diene with benzaldehyde has been achieved catalytically by a series of alpha,alpha,alpha',alpha'-tetraaryl-1,3-dioxolane-4,5-dimethanol (TADDOL) derivatives through hydrogen-bonding activation, affording 2-phenyl-2,3-dihydro-4H-pyran-4-one after trifluoroacetic acid workup in moderate yield and good enantioselectivity. The alpha,alpha'-aryl substituents in the TADDOL molecules are found to exert a significant impact on both the activity and the enantioselectivity of the catalysis. In combination with the experimental investigations, the mechanism of the present reaction has also been studied theoretically using the ONIOM (B3LYP/6-31G:PM3) method with trans-1,3-dimethoxy-1,3-butadiene as the model for Danishefsky's diene. In agreement with the experimental findings, the calculation results indicate that this TADDOL-catalyzed HDA reaction proceeds via a concerted mechanism through an asynchronous and zwitterionic transition structure (TS). The carbonyl group of benzaldehyde is activated by forming an intermolecular hydrogen bond with one of the hydroxy groups of TADDOL. Meanwhile, the intramolecular hydrogen bond between the two hydroxy groups in TADDOL is found to facilitate the intermolecular hydrogen bonding with benzaldehyde. The sense of asymmetric induction is well-rationalized by the analysis of the relative energies of TADDOL-complexed TSs, while the different stereocontrol capabilities exhibited by TADDOLs in the reaction can be qualitatively established on the basis of the structural features of their corresponding TSs.

Enantioselective Diels-Alder reactions catalyzed by hydrogen bonding.

Like molecules of life (e.g., proteins and DNA), many pharmaceutical drugs are also asymmetric (chiral); they are not superimposable on their mirror images. One mirror image form (enantiomer) of a drug can have desirable activity, the other not. Consequently, the development of methods for the selective synthesis of one enantiomer is of great scientific and economic importance. We report here that a simple, commercially available chiral alcohol, alpha,alpha,alpha',alpha'-tetraaryl-1,3-dioxolane-4,5-dimethanol (TADDOL), catalyzes the all-carbon Diels-Alder reactions of aminosiloxydienes and substituted acroleins to afford the products in good yields and high enantioselectivities (up to 92% enantiomeric excess). It is remarkable that the reactions are promoted by hydrogen bonding, the ubiquitous "glue" that helps to keep water molecules together and holds up the 3D structures of proteins. Hydrogen bond catalysis is little used in chemical synthesis, wherein most reactions are promoted by complexes of Lewis acidic metal salts coordinated to chiral ligands. As it does for enzymes, hydrogen bonding not only organizes TADDOL into a well defined conformation, but, functioning as a Brønsted acid catalyst, it also activates the dienophile toward reaction with the diene. The gross structure of the TADDOL has been found to have a profound influence on both the rate and the enantioselectivity of the cycloadditions. These structure-function effects are rationalized by evaluating the conformation adopted by the TADDOLs in the crystal state. It is suggested that pi,pi-stacking plays an central role in the overall catalytic cycle, in particular, the enantioselective step.

Enantioselective reduction of C=O and C=N bonds by TADDOL-containing aluminum hydride reagents based on NaAlH4 and AlH3

Prochiral substrates (alkyl aryl ketones, cyclopropyl methyl ketone, 1-indanone, 1-tetralone, ethyl 2-oxo-4-phenylbutyrate, and N-(diphenylphosphinyl)acetophenoneimine) were subjected to asymmetric reduction with aluminum hydride reagents, which were prepared by modifications of NaAlH4 or AlH3 with chiral α,α,α",α"-tetraaryl-1,3-dioxolane-4,5-dimethanols (TADDOL). The effects of the nature of the substituents in TADDOL, the structure of the prochiral substrate, and the reaction conditions on the stereochemistry of reduction were investigated. The highest enantioselectivity (70—90% ee) was achieved upon reduction of alkyl aryl ketones and N-(diphenylphosphinyl)acetophenoneimine with NaAl(TADDOLate)H2 in THF or diglyme at a temperature from –70 to –20 °C. The mechanism of asymmetric induction in the reduction reactions of ketones with aluminum hydride reagents is discussed. The stereochemical results of reduction were explained by comparing three-dimensional models of the most probable transition states.

Building an Organic Zeolite from a Macrocyclic TADDOL Derivative or How to Teach an Old Dog New Tricks

Crystal structures of the macrocyclic TADDOL (,,,-tetraaryl-1,3-dioxolane-4,5-dimethanol) derivative A and its clathrate with diethyl ether are presented. Both the host and clathrate crystallize in the same space group (P212121) with very similar cell dimensions, i.e., they are isomorphous. At ambient temperature, diethyl ether escapes from A ¥E t2O, and clathrate single crystals are transformed into the guest-free form without breakdown of the crystal lattice. Conversely, crystalline A, when exposed to Et2O vapor, incorporates guest molecules in a ratio of up to ca. 1 : 1. This behavior, usually the hallmark of inorganic nanoporous materials, allows to describe this compound as an −organic zeolite×.

TADDOLs under closer scrutiny – why bulky substituents make it all different

AbstractA systematic investigation of the rotational behavior of aryl substituents in α,α,α′,α′‐tetraaryl‐1,3‐dioxolane‐4,5‐dimethanols (TADDOLs) is presented. In the use as chiral ligands for enantioselective metal‐catalyzed reactions, a change from phenyl to bulkier substituents, e.g., 1‐naphthyl, gives rise to an astounding alteration of the selectivity, The possible existence of preferred rotamers of TADDOLs has so far not been given due attention, which encouraged us to look at the validity of the Knowles model, originally formulated for diaryl substituted bisphosphines. 1H‐NMR Investigations at various temperatures as well as X‐ray powder diffraction were employed to study the rotation in the case of tetra(1‐naphthyl) TADDOL 1. To support the interpretation of the experimental results, molecular mechanics, semiempirical, and ab initio calculations were performed. For comparison, the energy surface of tetraphenyl TADDOL 2 was calculated as well. Our results lead to the conclusion that for 1, only one major conformation is present in both solution and solid state, which determines the stereochemical outcome of the catalyzed reactions.

Polymer-Grafted Ti-TADDOL Complexes. Preparation and Use as Catalysts in Diels-Alder Reactions.

Different strategies have been evaluated for the preparation of polymers functionalized with chiral fragments containing the alpha,alpha,alpha',alpha'-tetraaryl-1,3-dioxolane-4,5-dimethanol (TADDOL) structure. Best results are obtained when TADDOL derivatives containing OH phenolic groups (17 and 18) are prepared in solution and then selectively anchored to chloromethylated polymers by reaction in THF containing NaH and small amounts of 18C6 and NBu(4)I. Resins 19 having the desired functionality have been prepared in this way. The procedure allows the efficient introduction of this functional group in PS-DVB polymers with different loading and cross-linking degrees, as well as on SMOP resins. These functionalized resins have been used to prepare Ti-TADDOL-supported complexes which have been tested as catalysts in the reaction of cyclopentadiene with 3-crotonoyl-1,3-oxazolidin-2-one. Although all the polymers obtained efficiently catalyze the reaction, only the one prepared from alpha,alpha,alpha',alpha'-tetra-3,5-dimethylphenyl-1,3-dioxolane-4,5-dimethanol induces asymmetry.

Polymer‐ and Dendrimer‐Bound Ti‐TADDOLates in Catalytic (and Stoichiometric) Enantioselective Reactions: Are pentacoordinate cationic Ti complexes the catalytically active species?

Abstractα,α,α′,α′‐Tetraaryl‐1,3‐dioxolane‐4,5‐dimethanols (TADDOLs), containing styryl groups either at C(2) of the heterocyclic ring or in the α‐position, were prepared in the usual way (18–22, 24, 25). These compounds were copolymerized with styrene and divinylbenzene in a suspension, yielding polymers (33–40, Scheme 3) as beads with a rather uniform particle‐size distribution (150–45 μm), swellable in common organic solvents. HOCH2‐ and BrCH2‐substituted TADDOLs were also prepared and used for attachement to Merrifield resin or to dendritic molecules (23, 26–32). The TADDOL moieties in these materials are accessible to form Ti (and Al) complexes (Scheme 4) which can be used as polymer‐ or dendrimer‐bound reagents (stoichiometric) or Lewis acids (catalytic). The reactions studied with these new chiral auxiliaries are: enantioselective nucleophilic additions to aldehydes (of R2Zn and RTi(OCHMe2)3; Scheme 5, Table 1) and to ketones (of LiAlH4, Table 2); enantioselective ring opening of meso‐anhydrides (Scheme 6); [4+2] and [3+2] cycloadditions of 3‐crotonyl‐1,3‐oxazolidin‐2‐one to cyclopentadiene and to (Z)‐N‐benzylidenephenylamine N‐oxide ( → 48, 49, Scheme 7, Tables 3, 4, and Fig. 5). The enantioselectivities reached with most of the polymer‐bound or dendritic TADDOL ligands were comparable or identical to those observed with the soluble analogs. The activity of the polymer‐bound Lewis acids was only slightly reduced as compared with that encountered under homogeneous conditions. Multiple use of the beads (up to 10 times), without decreased performance, has been demonstrated (Figs. 3 and 4). The poorer selectivity in the Diels‐Alder reaction (Scheme 7a), induced by the polymer‐bound Cl2Ti‐TADDOLate as compared to the soluble one, is taken as an opportunity to discuss the mechanism of this Lewis‐acid catalysis, and to propose a cationic, trigonal‐bipyramidal complex as the catalytically active species (Fig. 6). It is suggested that similar cations may be involved in other Ti‐TADDOLate‐mediated reactions as well.

Preparation and Structural Analysis of Several New α,α,α′,α′‐Tetraaryl‐1,3‐dioxolane‐4,5‐dimethanols (TADDOL's) and TADDOL analogs, their evaluation as titanium ligands in the enantioselective addition of methyltitanium and diethylzinc reagents to benzaldehyde, and refinement of the mechanistic hypothesis

AbstractPreparation and screening of twenty new ligands, all analogs of α,α,α′,α′‐tetraaryl‐1,3‐dioxolane‐4,5‐dimethanol (TADDOL), for the Ti‐catalyzed asymmetric addition of methyltri(isopropoxy)titanium and diethylzinc to benzaldehyde are described. These ligands have the dioxolane ring of the TADDOL's replaced by cyclobutane, cyclopentane, cyclohexene, cyclohexane, bicyclo[2.2.1]heptene and ‐heptane and bicyclo[2.2.2]octene and ‐octane moieties; several have H‐atoms or alkyl groups in place of the aryl groups, and nine of them have C2 symmetry. X‐Ray crystallography and molecular mechanics are used to analyze the structure of the ligands, and two structural features appear to correlate with selectivity: (i) the torsion angle for the chelating O‐atom and the ortho‐C‐atom of the axial Ph group (a small, ca. 19°, angle is optimum, Fig.8) and (ii) the “degree of perpendicularity” of the axial Ph group (Fig. 9). Competition experiments indicate that TADDOL 1a catalyzes both the methyltitanium and diethylzinc additions ≥ 50 times faster than the related dioxolane analogs 12a, 12c, and 12e (Scheme 7), indicating that both axial and equatorial aryl groups (see Footnote 6) are necessary for ligand‐accelerated catalysis of these reactions. A refined mechanistic hypothesis is presented (Fig. 10) to explain the selectivities observed for these new ligands. Our analysis suggests that a combination of structural features appear necessary for good catalytic efficiency and high selectivity. These features, especially the rather subtle conformational effects, appear to be optimized (among the ligands tested) in the TADDOL's.

On the Mechanisms of Enantioselective Reactions Using α,α,α′,α′ ‐Tetraaryl‐1,3‐dioxolane‐4,5‐dimethanol(TADDOL)‐Derived Titanates: Differences between C2‐ and C1‐symmetrical TADDOLs – facts, implications and generalizations

AbstractThe titanates derived from α,α,α′,α′‐tetraaryl‐1,3‐dioxolane‐4,5‐dimethanols (TADDOLs, prepared from tartrate) act as catalysts for enantioselective additions of dialkylzinc compounds to aldehydes. For the standard reaction chosen for this investigation of the mechanism, the addition of diethylzinc to benzaldehyde, there is very little change of selectivity with different aryl substituents on the TADDOLate ligands (Tables 2–4, examples). With 0.02 to 0.2 equiv. of the chiral titanates, selectivities above 90% are observed only in the presence of excess tetraisopropyl titanate! According to NMR measurements (Fig. 2), the chiral bicyclic titanate and the achiral titanate do not react to give new species under these conditions. From experiments with different stoichiometries of the components, and with different achiral or chiral OR groups on the Ti‐atom of the seven‐membered ring titanate, it is concluded (i) that a single chiral titanate is involved in the product‐forming step, (ii) that the bulky TADDOLate ligand renders the Ti‐center catalytically more active than that of (i‐PrO)4Ti, due to fast dynamics of ligand exchange on the sterically hindered Ti‐center (Table 5, Fig. 3), and (iii) that the role of excess (i‐PrO)4Ti is to remove – by ligand exchange – the product alkoxides (R*O) from the catalytically active Ti‐center (Scheme 4, Table 6). Three new crystal structures of TADDOL derivatives (two clathrates with secondary amines, and a dimethyl ether) have been determined by X‐ray diffraction (Figs. 5–7), and are compared with those previously reported. The distances between the C(aryl)2O oxygen atoms in the C2‐ and C1‐symmetrical structures vary from 2.58 to 2.94 Å, depending upon the conformation of their dioxolane rings and the presence or absence of an intramolecular H‐bond (Fig. 8). A single‐crystal X‐ray structure of a spiro‐titanate, with two TADDOLate ligands on the Ti‐atom, is described (Fig. 9); it contains six different seven‐membered titanate‐ring conformations in the asymmetric unit (Fig. 10), which suggests a highly flexible solution structure. The structures of Ti TADDOLate complexes are compared with those of C2‐symmetrical Ru, Rh, and Pd disphosphine chelates (Table 7). A common topological model is presented for all nucleophilic additions to aldehydes involving Ti TADDOLates (Si attack with (R,R)‐derivatives, relative topicity unlike; Fig. 11). Possible structures of complexes containing bidentate substrates for Ti TADDOLate‐mediated ene reactions and cycloadditions are proposed (Fig. 12). A simple six‐membered ring chair‐type arrangement of the atoms involved can be used to describe the result of TADDOLate‐mediated nucleophilic additions to aldehydes and ketones, with Ti, Zr, Mg, or Al bearing the chiral ligand (Scheme 6). A proposal is also made for the geometry of the intermediate responsible for enantioselective hydrogenation of N‐(acetylamino)cinnamate catalyzed by Rh complexes containing C2‐symmetrical diphosphines (Fig. 13).

Enantioselective preparation of sec. Alcohols from aldehydes and dialkyl zinc compounds, generated in situ from Grignard reagents, using substoichiometric amounts of TADDOL-titanates

Using the Schlenk trick (precipitation of MgX 2 from ethereal solutions by the addition of 1,4-dioxane) mixtures of a Grignard reagent RMgX (X = Cl, Br, I) and 0.5 equiv. ZnCl 2 in Et 2O can be converted to zinc alkyls R 2Zn which in turn are added with enantio-selectivities of up to 99 : 1 to aliphatic and aromatic aldehydes in the presence of Ti(OCHMe 2) 4 and a chiral titanate derived from an α,α,α′,α′-tetraaryl-1,3-dioxolane-4,5-dimethanol (TADDOL). Grignard reagents containing remote double bonds, benzene rings, or acetal groups can also be employed. Different TADDOLs are compared with respect to their usefulness in this kind of enantioselective reaction.

α,α,α′,α′‐Tetraaryl‐1,3‐dioxolane‐4,5‐dimethanols (TADDOLs) for Resolutions of Alcohols and as Chiral Solvating Agents in NMR Spectroscopy

AbstractThe use of α,α,α′,α′ ‐tetraaryl‐1,3‐dioxolane‐4,5‐dimethanols ( = TADDOLs;1) as chiral NMR shift reagents (1H, 13C, 19F) is described. In many cases, the ratio of enantiomeric alcohols and amines can be determined under standard conditions of measurement (CDCl3 as solvent, room temperature). The preparation and use of a new type of TADDOL, the tetrakis(dimethylamino) derivative 1d, is described. Menthol, octan‐2‐ol, and oct‐1‐yn‐3‐ol are partially resolved by crystallization of clathrates with 1c and 1d.

Enantioselective preparation of sec. alcohols from aldehydes and dialkyl zinc compounds, generated in situ from Grignard reagents, using substoichiometric amounts of TADDOL-titanates

Using the Schlenk trick (precipitation of MgX 2 from ethereal solutions by the addition of 1,4-dioxane) mixtures of a Grignard reagent RMgX (X = Cl, Br, I) and 0.5 equiv. ZnCl 2 in Et 2 O can be converted to zinc alkyls R 2 Zn which in turn are added with enantio-selectivities of up to 99 : 1 to aliphatic and aromatic aldehydes in the presence of Ti(OCHMe 2 ) 4 and a chiral titanate derived from an α,α,α′,α′-tetraaryl-1,3-dioxolane-4,5-dimethanol (TADDOL). Grignard reagents containing remote double bonds, benzene rings, or acetal groups can also be employed. Different TADDOLs are compared with respect to their usefulness in this kind of enantioselective reaction. Primary alkyl MgX (X=Cl, Br, I) and ZnCl 2 give (RCH 2 ) 2 Zn reagents which can be added to aliphatic and aromatic aldehydes with high enantioselectivities in the presence of chiral titanates.