Chiral Titanium Catalysts Research Articles

Readily Available Highly Active [Ti]-Adamantyl-BINOL Catalysts for the Enantioselective Alkylation of Aldehydes.

A series of enantiopure (R)-adamantyl 1,1′-binaphthalene-2,2′-diols were successfully synthesized in a straightforward one-step reaction from commercial products with excellent overall yields. The activity of chiral titanium catalyst derived from (R)-3,6,6′-tri(adamantan-1-yl)-[1,1′-binaphthalene]-2,2′-diol ligand, (R)-(2)-Ti, in the enantioselective asymmetric alkylation of aldehydes is significantly enhanced (up to 98% yield and 99% ee).

Application of a Heterogeneous Chiral Titanium Catalyst Derived from Silica-Supported 3-Aryl H8-BINOL to Enantioselective Alkylation and Arylation of Aldehydes.

A 3-aryl H8-BINOL was grafted on the surface of silica gel using a hydrosilane derivative as a precursor, and the resulting silica-supported ligand (6 mol %) was employed in the enantioselective alkylation and arylation of aldehydes in the presence of Ti(OiPr)4. The reactions using Et2Zn, Et3B, and aryl Grignard reagents all afforded the corresponding adducts in high enantioselectivities and yields. The silica-immobilized titanium catalyst could be reused up to 14 times without appreciable deterioration of the activity.

ChemInform Abstract: Enhancement of the Catalytic Activity of Chiral H8‐BINOL Titanium Complexes by Introduction of Sterically Demanding Groups at the 3‐Position.

AbstractIt is demonstrated that the activity of chiral titanium catalysts derived from H8‐BINOL ligands in the enantioselective arylation of aldehydes is significantly enhanced by an increase of the size of the substituent at the 3‐position.

Enhancement of the Catalytic Activity of Chiral H8 -BINOL Titanium Complexes by Introduction of Sterically Demanding Groups at the 3-Position.

The activity of chiral titanium catalysts derived from H8 -BINOL ligands in the enantioselective arylation of an aldehyde with PhTi(OiPr)3 is significantly enhanced by an increase of the size of the substituent at the 3-position. High enantioselectivity (> 90 % ee) can be obtained even at a substrate/catalyst ratio (S/C) of 800 for DTBP-H8 -BINOL (DTBP=3,5-di-tert-butylphenyl) and DAP-H8 -BINOL (DAP=3,5-di(9-anthraceny)phenyl). These titanium catalysts are successfully applied to the enantioselective arylation and heteroarylation of aldehydes at a S/C ratio of 400 by using organotitanium reagents generated in situ from bromide precursors. The remarkable weakening of the intramolecular aggregation of the two -Ti(OiPr)3 units in a DPP-H8 -BINOL (DPP=3,5-diphenylpheny)-derived bis-titanium complex is revealed by X-ray and variable-temperature (VT)-NMR studies. Based on these observations, a catalytic cycle, involving the rate-limiting aryl group transfer followed by aldehyde complexation and enantioselective arylation, is proposed to account for the high activity of the 3-substituted H8 -BINOL catalyst system.

ChemInform Abstract: Development of Highly Active Chiral Titanium Catalysts for the Enantioselective Addition of Various Organometallic Reagents to Aldehydes

AbstractReview: [56 refs.

Development of Highly Active Chiral Titanium Catalysts for the Enantioselective Addition of Various Organometallic Reagents to Aldehydes.

The catalytic enantioselective carbonyl addition reaction has long attracted the interest of chemists because of its synthetic importance. Although many highly enantioselective reactions have been developed, with few exceptions the reactions are carried out at relatively high catalyst loadings, making them less practical for scale-up applications. In addition, organometallic reagents employed as carbon nucleophiles have been limited to those with relatively low reactivity, such as diorganozincs and arylboronic acids. In an effort to enhance the practicality, a highly active and enantioselective chiral titanium catalyst system was recently developed in our laboratory, enabling the enantioselective carbonyl addition reaction to aldehydes using various organometallic reagents (RM; M = MgX, Li, BY2 , ZnX, AlMe2 ) at lower catalyst loadings (≤5 mol %).

ChemInform Abstract: Enantioselective Reduction of Ketones Induced by a C2‐Symmetrical Chiral Hydroxyamide/Titanium(IV) Complex.

AbstractAn in situ generated chiral titanium catalyst and catecholborane as reducing agent are employed for the reduction of ketones to give the corresponding alcohols with up to < 99% e.e.

Titanium Catalyzed Enantioselective Oxidation of Thioethers with Hydrogen Peroxide

Catalytic asymmetric oxidation of prochiral sulfides has been a challenging goal for years, which reflects the importance of sulfinyl group as a chiral controller in many organic transformations, as well as the existence of natural and synthetic biologically active products that need a sulfinyl group with a defined stereoconfiguration. 30 years ago, owing to the milestone discoveries of Kagan and Modena, titanium-dialkyltartrate/alkyl hydroperoxide catalyst systems emerged as the protagonists in this area. In the last decade, various types of novel titanium based catalyst systems were developed, exploiting ‘green’ hydrogen peroxide as the terminal oxidant. This mini-review is aimed at discussing recent advances in enantioselective sulfoxidations with H2O2 in the presence of new generation chiral titanium catalysts, demonstrating high reactivity, efficiency, and enantioselectivity, which are likely to contribute to preparative syntheses of biologically active sulfoxides in near future. Peculiarities of their behavior and mechanisms of their catalytic action are discussed. Their viability is analyzed from the industrial and green chemistry perspectives. Keywords: Asymmetric synthesis, homogeneous catalysis, hydrogen peroxide, kinetic resolution, mechanism, sulfoxides, titanium.

The Effect of the Fluorine Substitution on the Enantioselective Oxidation of Sulfides with Chiral Titanium Catalysts: A Combined Computational and Experimental Investigation

AbstractThe results of a combined computational‐experimental study of the oxidation of various fluorinated aryl benzyl sulfides using tert‐butyl hydroperoxide (TBHP) in the presence of a complex of titanium and (S,S)‐hydrobenzoin are presented. As observed in previous studies for other aryl benzyl sulfides, the reaction leads to enantiopure sulfoxides (ee>98 %) in good isolated yields (81–96 %) except the case of pentafluorobenzyl pentafluorophenyl sulfide for which a lower ee (61 %) is observed. DFT computations on a model‐system formed by the substrate, the oxidant TBHP and the [(S,S)‐hydrobenzoin]2‐Ti complex satisfactorily explain this unexpected item. The enantioselectivity is governed by the relative energy of the two diastereomeric octahedral complexes that form if TBHP approaches the initial complex between substrate and [(S,S)‐hydrobenzoin]2‐Ti before the oxygen transfer. For pentafluorobenzyl pentafluorophenyl sulfide, the two octahedral complexes are almost degenerate and, thus, they form in similar amounts. As the two corresponding diastereomeric transition states are similar in energy, the probability to follow one or the other diastereomeric reaction channel becomes comparable, which leads to the lower enantiomeric excess experimentally observed. Our computations indicate that the particular “folded conformation”, adopted by the substrate only if both phenyl rings are fluorinated, is the key factor that determines the near degeneracy of the two diastereomeric octahedral complexes.

A Study of Factors Affecting Enantioselectivity in the Oxidation of Aryl Benzyl Sulfides in the Presence of Chiral Titanium Catalysts

AbstractA series of experiments was performed to test a theoretical model that we have recently proposed to explain the highly enantioselective oxidation of aryl benzyl sulfides with tert‐butyl hydroperoxide in the presence of a complex between titanium and (S,S)‐ or (R,R)‐hydrobenzoin. The studied variations involved the sulfides, the ligands, and the order of addition of reactants. The reaction path predicted by our model was confirmed in every experiment. In particular, aryl benzyl sulfides behaved as the ideal substrate for this type of asymmetric oxidation, which yields synthetically useful, enantiopure aryl benzyl sulfoxides in a straightforward manner.

ChemInform Abstract: Asymmetric Catalysis of the Friedel-Crafts Reaction with Fluoral by Chiral Binaphthol-Derived Titanium Catalysts Through Asymmetric Activation.

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: Synthetic Studies on Nogarol Anthracyclines. Enantioselective Total Synthesis of an Aminohydroxy Epoxybenzoxocin.

A chiral synthesis of the aminohydroxy expoxybenzoxocin 6 is described. Enantioselective Friedel-Crafts coupling using a chiral titanium catalyst was employed to produce the optically active atrolactic ester 16a from the phenol 11 and l-menthyl pyruvate (12). The phenolic group in 16a was protected as the benzyl ether and the t-alcohol functionality as the MEM ether to give 20, which after sequential reduction/oxidation provided the aldehyde 22. Addition of the acetylide anion of propargyl aldehyde diethyl acetal (23) to aldehyde 22, followed by oxidation of the resultant diastereoisomeric carbinols, gave the acetylenic ketone 24. Lindlar reduction of 24 afforded the trans-enone 26. Reaction of 26 with thiophenylate anion furnished 27, which was then cyclized to the alpha-methyl pyranoside 29. Oxidation of 29 to the sulfoxide and subsequent thermolysis afforded the hexenulose 30. Sequential epoxidation of 30, reduction of the keto epoxide 31, and reaction of the resultant epoxycarbinol 32 with dimethylamine produced the aminohydroxy pyranose 33a. Debenzylation of 33a to the phenol 33b, followed by intramolecular cyclization, completed the fabrication of the optically active aminohydroxy epoxybenzoxocin 6. The 17-step sequence from the phenol 11 to 6 was achieved in 22% overall yield.

ChemInform Abstract: Asymmetric Friedel-Crafts Reactions with Fluoral, Catalyzed by Chiral Binaphthol-Derived Titanium Catalysts

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.

A Combined Theoretical and Experimental Investigation on the Enantioselective Oxidation of Aryl Benzyl Sulfides in the Presence of a Chiral Titanium Catalyst

An experimental investigation of the enantioselective oxidation of aryl benzyl sulfides by tert-butyl hydroperoxide in the presence of a titanium/hydrobenzoin catalyst has shown that these sulfides are ideal substrates for this catalytic system, with negligible interference by the substituents on the aryl groups. A reaction mechanism based on DFT computations has been proposed. The DFT MPWB1K functional was used in the theoretical investigation to account for weak hydrogen-bonding and pi interactions. The computed reaction profile explains the experimentally observed enantioselectivity, which is determined by the thermodynamics of the first phase of the reaction. A detailed discussion of the hydrogen-bonding and pi interactions that drive the reaction along the observed stereochemical path is given.

Efficient bimetallic titanium catalyst for carbonyl-ene reaction

A new family of achiral 3,3′,5,5′-tetrasubstituted-2,2′,6,6′-tetrahydroxy biphenyl ligand 4 was developed. The axial chirality of the ligand could be induced by the chelation of 2,2′,6,6′-tetrahydroxy groups with ( R)-BINOL-Ti(O i Pr) 2 to form an axially chiral bimetallic titanium catalyst 9. Compared with ( R)-BINOL-Ti(O i Pr) 2 catalyst, this novel catalyst 9 exhibited excellent activity and enantioselectivity for the carbonyl-ene reaction of methylstyrene and ethyl glyoxylate. 3,3′,5,5′-Tetrasubstituted groups showed a remarkable effect on both enantioselectivity and yield. With 9d prepared from 3,3′,5,5′-tetramethyl-2,2′,6,6′-tetrahydroxy biphenyl 4d as the catalyst, the best result, up to 97.6% ee and 99% yield, was obtained. Additionally, the bimetallic catalyst 9 also showed better catalytic capability than the corresponding monometallic catalyst.

The First Asymmetric Addition of Organogallium to Aldehydes Catalyzed by Chiral Titanium Catalysts.

The addition of organogallium to aldehydes was realized with titanium tetrachloride as a Lewis acid catalyst. For the first time, the catalytic asymmetric addition of organogallium to aldehydes was investigated with chiral titanium complexes, which were formed from titanium tetrachloride and salan ligands, with mediocre to good chemical yields and enantioselectivities.

The first asymmetric addition of organogallium to aldehydes catalyzed by chiral titanium catalysts

The addition of organogallium to aldehydes was realized with titanium tetrachloride as a Lewis acid catalyst. For the first time, the catalytic asymmetric addition of organogallium to aldehydes was investigated with chiral titanium complexes, which were formed from titanium tetrachloride and salan ligands, with mediocre to good chemical yields and enantioselectivities.

Dramatically synergetic effect of carboxylic acid additive on tridentate titanium catalyzed enantioselective hetero-diels-alder reaction: additive acceleration and nonlinear effect.

This paper describes the successful development of a group of highly efficient chiral tridentate titanium catalysts for hetero-Diels-Alder reaction of Danishefsky's diene and a variety of aldehydes through ligand and additive diversity. Dramatically synergetic effect of carboxylic acid additives and influence of substituent in chiral Schiff base ligands on the enantioselectivities of the reaction are reported. It was found that a chiral drug Naproxen (A21) was a highly efficient additive for Ti-catalyzed HDA reaction, affording 2-substituted 2,3-dihydro-4H-pyran-4-one in up to 97 % ee and >99 % yields. Quantitative study of the effect of chiral carboxylic acid A21 revealed that the reaction could be accelerated by one order of magnitude. Another interesting feature of present catalytic system is the existence of significant positive nonlinear effect, which indicates that the heterochiral Schiff base-titanium complexes may have higher stability than their homochiral counterparts. As a result, the homochiral Schiff base-titanium complexes with higher ee than that of starting ligand will react with carboxylic acid additive to form the more active species and predominate the catalytic process. The isolation and characterization of stable heterochiral ((+/-)-L1)2Ti complex has also been successful, which strongly supported the explanation for positive nonlinear effect observed in the catalytic system.

Concept of improved rigidity: how to make enantioselective hydrophosphonylation of cyclic imines catalyzed by chiral heterobimetallic lanthanoid complexes almost perfect

The catalytic and enantioselective hydrophosphonylation of cyclic imines using cyclic phosphites is described for the first time. In contrast to the application of acyclic phosphites, significant improvements are presented arising from the concept of improved rigidity by utilization of cyclic phosphites in the lanthanoid BINOL complex catalyzed hydrophosphonylation of 3-thiazolines. Cyclic phosphites are shown to provide certain improvements within the catalytic cycle. Influence of parameters such as concentration of the catalyst and the phosphite on the catalysis is examined as well as the effects of the substituents on the starting material. The pharmacologically interesting thiazolidinyl phosphonates are synthesized in excellent optical purities of up to 99% ee and high chemical yields of up to 99%. The required amount of catalyst is reduced to 2.5 mol %. The highest efficiency of the reaction involving cyclic phosphites is achieved using the catalytic system "2.5 mol % (S)-YbPB/2.5 equiv phosphite/50 degrees C/48 h/THF-toluene (1:7)". On the basis of the results a refinement of the proposed catalytic cycle has been provided. For comparison cyclic phosphites were used in hydrophosphonylation with a chiral titanium catalyst.

Synthetic studies on nogarol anthracyclines. Enantioselective total synthesis of an aminohydroxy epoxybenzoxocin.

A chiral synthesis of the aminohydroxy expoxybenzoxocin 6 is described. Enantioselective Friedel-Crafts coupling using a chiral titanium catalyst was employed to produce the optically active atrolactic ester 16a from the phenol 11 and l-menthyl pyruvate (12). The phenolic group in 16a was protected as the benzyl ether and the t-alcohol functionality as the MEM ether to give 20, which after sequential reduction/oxidation provided the aldehyde 22. Addition of the acetylide anion of propargyl aldehyde diethyl acetal (23) to aldehyde 22, followed by oxidation of the resultant diastereoisomeric carbinols, gave the acetylenic ketone 24. Lindlar reduction of 24 afforded the trans-enone 26. Reaction of 26 with thiophenylate anion furnished 27, which was then cyclized to the alpha-methyl pyranoside 29. Oxidation of 29 to the sulfoxide and subsequent thermolysis afforded the hexenulose 30. Sequential epoxidation of 30, reduction of the keto epoxide 31, and reaction of the resultant epoxycarbinol 32 with dimethylamine produced the aminohydroxy pyranose 33a. Debenzylation of 33a to the phenol 33b, followed by intramolecular cyclization, completed the fabrication of the optically active aminohydroxy epoxybenzoxocin 6. The 17-step sequence from the phenol 11 to 6 was achieved in 22% overall yield.