Asymmetric activation of chiral alkoxyzinc catalysts by chiral nitrogen activators for dialkylzinc addition to aldehydes: super high-throughput screening of combinatorial libraries of chiral ligands and activators by HPLC-CD/UV and HPLC-OR/RIU systems

Concentration-Independent Stereodynamic g-Probe for Chiroptical Enantiomeric Excess Determination

Enantiomeric excess (ee) determination is crucial in many aspects of science, from synthesis to materials. Within this subject, coupling molecular sensors with chiroptical techniques is a straightforward approach to the stereochemical analysis of chiral molecules, especially in terms of process immediacy and labor. Stereodynamic probes typically consist of racemic mixtures of rapidly interconverting enantiomeric conformers able to recognize a chiral analyte and greatly amplify its chiroptical readout. A great number of sensors have been developed, but their activity is generally restricted to one or a few classes of chemicals, and the analysis outcome relies on precise knowledge of the probe and analyte concentrations. This aspect in particular limits the potential practical applications. Here we report an oxo-vanadium(V) aminotriphenolate complex that was found to act as a concentration-independent stereodynamic sensor for a wide range of compounds. The bare complex is CD-silent, but coordination of an enantioenriched substrate immediately gives rise to intense Cotton effects in the visible region. Furthermore, a geometry change during the substrate-complex interaction leads to a marked optical response, as witnessed by a strong red-shift of the probe absorption bands, thus allowing the generation of dichroic signals in an "interference-free" area of the spectrum. This peculiarity allows for a linear correlation at high wavelengths between the ee of the analyte and anisotropy g-factor. This parameter derives from the differential circularly polarized light absorption of the sample but is independent of concentration. The newly developed sensor based on a simple coordination process has an unprecedented general character in terms of substrate scope and employment.

Mini-ISES identifies promising carbafructopyranose-based salens for asymmetric catalysis: Tuning ligand shape via the anomeric effect

This study introduces new methods of screening for and tuning chiral space and in so doing identifies a promising set of chiral ligands for asymmetric synthesis. The carbafructopyranosyl-1,2-diamine(s) and salens constructed therefrom are particularly compelling. It is shown that by removing the native anomeric effect in this ligand family, one can tune chiral ligand shape and improve chiral bias. This concept is demonstrated by a combination of (i) x-ray crystallographic structure determination, (ii) assessment of catalytic performance, and (iii) consideration of the anomeric effect and its underlying dipolar basis. The title ligands were identified by a new mini version of the in situ enzymatic screening (ISES) procedure through which catalyst-ligand combinations are screened in parallel, and information on relative rate and enantioselectivity is obtained in real time, without the need to quench reactions or draw aliquots. Mini-ISES brings the technique into the nanomole regime (200 to 350 nmol catalyst/20 μml organic volume) commensurate with emerging trends in reaction development/process chemistry. The best-performing β-d-carbafructopyranosyl-1,2-diamine-derived salen ligand discovered here outperforms the best known organometallic and enzymatic catalysts for the hydrolytic kinetic resolution of 3-phenylpropylene oxide, one of several substrates examined for which the ligand is "matched." This ligand scaffold defines a new swath of chiral space, and anomeric effect tunability defines a new concept in shaping that chiral space. Both this ligand set and the anomeric shape-tuning concept are expected to find broad application, given the value of chiral 1,2-diamines and salens constructed from these in asymmetric catalysis.

Fluorous "racemic" mixture synthesis: polysaccharide-based chiral columns for simultaneous demix and enantioseparation of racemic fluorous tagged compounds

The proof of concept experiments of fluorous "racemic" mixture synthesis (FRMS) is shown using polysaccharide-based chiral stationary phases. The mixture of racemic O-benzoylmandelate derivatives bearing different lengths of fluorous cleavable tags undergoes sequential reactions to provide individual derivatives as well as their enantiomers resolved on polysaccharide-based chiral HPLC columns (DAICEL CHIRALCEL and CHIRALPAK series).

3,3'-Diphosphoryl-1,1'-bi-2-naphthol-Zn(II) complexes as conjugate acid-base catalysts for enantioselective dialkylzinc addition to aldehydes

A highly enantioselective dialkylzinc (R(2)(2)Zn) addition to a series of aromatic, aliphatic, and heteroaromatic aldehydes (5) was developed based on conjugate Lewis acid-Lewis base catalysis. Bifunctional BINOL ligands bearing phosphine oxides [P(=O)R(2)] (7), phosphonates [P(=O)(OR)(2)] (8 and 9), or phosphoramides [P(=O)(NR(2))(2)] (10) at the 3,3'-positions were prepared by using a phospho-Fries rearrangement as a key step. The coordination of a NaphO-Zn(II)-R(2) center as a Lewis acid to a carbonyl group in a substrate and the activation of R(2)(2)Zn(II) with a phosphoryl group (P=O) as a Lewis base in the 3,3'-diphosphoryl-BINOL-Zn(II) catalyst could promote carbon-carbon bond formation with high enantioselectivities (up to >99% ee). Mechanistic studies were performed by X-ray analyses of a free ligand (7) and a tetranuclear Zn(II) cluster (21), a 31P NMR experiment on Zn(II) complexes, an absence of nonlinear effect between the ligand (7) and Et-adduct of benzaldehyde, and stoichiometric reactions with some chiral or achiral Zn(II) complexes to propose a transition-state assembly including monomeric active intermediates.

"Cassette" in situ enzymatic screening identifies complementary chiral scaffolds for hydrolytic kinetic resolution across a range of epoxides

‘Cassette’-ISES (In Situ Enzymatic Screening) Identifies Complementary Chiral Scaffolds for Hydrolytic Kinetic Resolution Across a Range of Epoxides

A new ‘Cassette’-In Situ Enzymatic Screen (ISES) for combinatorial catalysis is introduced. This allows the experimentalist to obtain an information-rich readout, in real time, providing an estimate of the sense and magnitude of enantioselectivity across more than one substrate. In its first iteration, the screen identified CoIII-salen catalysts with β-pinene- and α-naphthylalanine-derived chiral scaffolds with broad, yet complementary, substrate specificities.

Double-cuvette ISES: in situ estimation of enantioselectivity and relative rate for catalyst screening

Described is a new method for the screening of an array of catalysts, in situ, to estimate enantioselectivity and relative rates. We term this approach "double-cuvette ISES (in situ enzymatic screening)". The Co(III)-salen mediated hydrolytic kinetic resolution (HKR) of (+/-)-propylene oxide is used as a model reaction to demonstrate proof of principle. In two parallel cuvettes, a lower CHCl3-based organic layer is loaded with the epoxide and the chiral salen catalyst. Aqueous reporting layers, containing distinct "reporting enzymes" and their nicotinamide cofactors, are layered above the organic layers. The 1,2-propanediol enantiomers formed by the chiral catalyst diffuse into the aqueous layer and are oxidized there by the reporting enzymes at rates dependent upon the diol concentration, the R:S ratio of the diol, and the enantioselectivity of the reporting enzymes. A focused chiral salen library was constructed from seven chiral 1,2-diamines, derived from amino acid, terpenoid, and carbohydrates skeletons, and seven salicylaldehyde derivatives. Double-cuvette ISES identified a couple of interesting combinatorial hits in this salen array, wherein either the sense or magnitude of enantioselection for a given chiral diamine depends significantly upon the choice of "salicylaldehyde" partner. A comparison of predicted ee's and relative rates using this new screening tool with those independently measured is provided.

Chiral aminonaphthol-catalyzed enantioselective carbonyl addition of diethylzinc to aromatic aldehydes high-throughput screened by CD-HPLC analysis

Optically active aminonaphthols derivatives are obtained by condensation of 2-naphthol, substituted benzaldehyde, and (S)-methylbenzylamine under mild conditions, without side products. Their absolute configurations are determined by X-ray crystallographic analysis. The addition of diethylzinc to aromatic aldehydes is considerably accelerated by the presence of a catalytic amount of crystalline to give, after hydrolysis, the corresponding 1-phenylpropanol in good enantiomeric purity, as determined by CD-HPLC analysis as HTPS (high-throughput screening).

Concise Synthesis of Novel Practical Sulfamide−Amine Alcohols for the Enantioselective Addition of Diethylzinc to Aldehydes

Novel sulfamide-amine alcohol ligands were designed using a grafting strategy and synthesized from readily available starting materials via a simple, efficient method. The key features of these ligands for the asymmetric addition of diethylzinc to aldehydes included stability, enhanced effectiveness without using Ti(O(i)Pr)(4), suitability for a variety of aldehydes, the ability to operate at room temperature, and selectability to afford either absolute configuration products with enantiomeric excess up to >99%.

High-throughput methods for the development of new catalytic asymmetric reactions

Chiral, single enantiomer pharmaceuticals have become increasingly more important. Therefore, research aimed at providing new methods for their selective preparation has taken on an even greater importance. One of the most efficient strategies for the synthesis of non-racemic, chiral molecules is asymmetric catalysis. There are many variables involved in the discovery of a new catalytic asymmetric transformation; hence, methods for the rapid screening of large numbers of catalysts have been developed. Herein, these techniques and strategies for the rapid discovery of novel asymmetric catalysts are reviewed.

Insight into the mechanism of the asymmetric addition of alkyl groups to aldehydes catalyzed by titanium-BINOLate species

The asymmetric addition of alkyl groups to aldehydes catalyzed by BINOLate-titanium complexes has become the testing grounds to evaluate the potential of new BINOL-based ligands. We have investigated the mechanism of this reaction and report our findings here. Model systems for the open form of the catalyst, (BINOLate)[Ti(O-i-Pr)(3)](2), based on mono-oxygen-alkylated BINOL ligands have been examined. Comparison of the reactivity and enantioselectivity of the mono-alkyl BINOL derivatives with those of BINOL indicate that the open form of the catalyst, (BINOLate)[Ti(O-i-Pr)(3)](2), is not active in the asymmetric addition reaction. Several BINOLate-titanium complexes have been synthesized and characterized by X-ray crystallography. These include the dinuclear (BINOLate)Ti(O-i-Pr)(2).Ti(O-i-Pr)(4), which contains a bridging naphtholate and isopropoxy group, trinuclear (BINOLate)Ti(O-i-Pr)(2).[Ti(O-i-Pr)(4)](2), and trimeric [(BINOL)Ti(O-i-Pr)(2)](3). The solid-state and solution structures reported here indicate that (BINOLate)Ti(O-i-Pr)(2) prefers to bind to titanium tetraisopropoxide rather than to itself, explaining why no nonlinear effects are observed in the catalytic reaction. Additionally, experimental evidence suggests that the BINOLate-titanium species responsible for the catalytic and stoichiometric asymmetric addition reactions are different, indicating that the proposed intermediate, (BINOLate)Ti(R)(aldehyde)(O-i-Pr), is not involved in either of these processes. Reactions were examined using different sources of the alkyl group [ZnMe(2) or MeTi(O-i-Pr)(3)]. Under similar conditions, it was found that the product ee's were the same, independent of whether ZnMe(2) or Me-Ti(O-i-Pr)(3) was used as the source of the alkyl groups. This indicates that the role of the dialkylzinc is not to add the alkyl group to the carbonyl but rather to transfer the alkyl group to titanium. On the basis of these results, we hypothesize that the intermediate in the asymmetric addition involves (BINOLate)Ti(O-i-Pr)(2)(aldehyde).MeTi(O-i-Pr)(3).

Emerging methods for the rapid determination of enantiomeric excess

Methods for enantiomeric excess determination using a variety of spectroscopic techniques are summarized. Particular attention is paid to techniques that have promise for application to problems of combinatorial catalyst discovery but have not yet been so employed.

Optimization of catalyst enantioselectivity and activity using achiral and meso ligands

The optimization of asymmetric catalysts for enantioselective synthesis has conventionally revolved around the synthesis and screening of enantiopure ligands. In contrast, we have optimized an asymmetric reaction by modification of a series of achiral ligands. Thus, employing (S)-3,3'-diphenyl BINOL [(S)-Ph(2)-BINOL] and a series of achiral diimine and diamine activators in the asymmetric addition of alkyl groups to benzaldehyde, we have observed enantiomeric excesses between 96% (R) and 75% (S) of 1-phenyl-1-propanol. Some of the ligands examined have low-energy chiral conformations that can contribute to the chiral environment of the catalyst. These include achiral diimine ligands with meso backbones that adopt chiral conformations, achiral diimine ligands with backbones that become axially chiral on coordination to metal centers, achiral diamine ligands that form stereocenters on coordination to metal centers, and achiral diamine ligands with pendant groups that have axially chiral conformations. Additionally, we have structurally characterized (Ph(2)-BINOLate)Zn(diimine) and (Ph(2)-BINOLate)Zn(diamine) complexes and studied their solution behavior.

A high-throughput screening protocol for fast evaluation of enantioselective catalysts

A new high-throughput screening protocol that allows fast evaluation of enantioselective catalysts has been developed. The usefulness of norephedrine-derived beta-amino alcohols as catalysts for the enantioselective alkylation of prochiral aldehydes has been determined by simultaneous screening of three representative substrates. GC analysis of the crude product mixture using a selectively modified cyclodextrin as the chiral stationary phase avoids time-consuming workup procedures. The chemical yield, enantioselectivity, substrate specificity, and catalytic activity of the chiral catalysts as well as the induced absolute configuration have been determined in a single screening experiment and two short GC runs.