Chiral Bis(N‐sulfonylamino)phosphine‐ and TADDOL‐Phosphite‐Oxazoline Ligands: Synthesis and Application in Asymmetric Catalysis

A series of N,P‐ligands has been prepared, containing a chiral oxazoline ring and as a second chiral unit a bis(N‐sulfonylamino)phosphine group embedded in a diazaphospholidine ring or a cyclic phosphite group derived from TADDOL. These modular ligands are readily synthesized from chiral amino alcohols and chiral 1,2‐diamines or TADDOLs. Palladium and iridium complexes derived from these ligands were found to be efficient catalysts for enantioselective allylic alkylation and olefin hydrogenation, respectively.     

Practical Synthesis of Chiral N,N′‐Bis(2′‐pyridinecarboxamide)‐1,2‐cyclohexane Ligands

A simple and scalable procedure for the preparation of chiral ligands 1 and 2 from trans‐1,2‐diaminocyclohexane and 2‐picolinic acid is described.     

Homogeneous Hydrogenation of Tri‐ and Tetrasubstituted Olefins: Comparison of Iridium‐Phospinooxazoline [Ir‐PHOX] Complexes and Crabtree Catalysts with Hexafluorophosphate (PF6) and Tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate (BArF) as Counterions

Four iridium complexes with achiral phosphino‐oxazoline (PHOX) ligands were readily prepared in two steps starting from commercially available phenyloxazolines. The air‐stable complexes with tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate (BAr_F) as counterion showed high reactivity in the hydrogenation of a range of tri‐ and tetrasubstituted olefins. The best results were obtained with an iridium complex ( 11 ) derived from a dicyclohexylphosphino‐oxazoline ligand bearing no additional substituents in the oxazoline ring. With several substrates, which gave only low conversion with the Crabtree catalyst, [Ir(Py)(PCy₃)(COD)]PF₆, full conversion was observed. The productivity of the Crabtree catalyst could be strongly increased by replacing the hexafluorophosphate anion with tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate. In one case, in the hydrogenation of a tetraalkyl‐substituted CC bond, [Ir(Py)(PCy₃)(COD)]BAr_F gave higher conversion than catalyst 11 . However, with several other substrates complex 11 proved to be superior.     

Chiral Tetraaza Ligands in Asymmetric Catalysis: Recent Progress

Nitrogen‐containing ligands have become very popular in asymmetric catalysis, due to their robustness and availability in comparison to phosphines. Nitrogen‐containing ligands are used in homogeneous catalysis, but they are suitable for heterogeneous processes, too. In this article we focus on chiral ligands with four nitrogen atoms and summarize recent progress achieved with these compounds in asymmetric catalysis.     

Ferrocene‐Based Chiral Phosphine‐Triazines: A New Family of Highly Efficient P,N Ligands for Asymmetric Catalysis

The presence of the additional heterocyclic nitrogen atoms in chiral P,N ligands has an important influence on the asymmetric catalysis, and a clear trend was observed in the present research that the enantioselectivity and reactivity were significantly increased by raising the number of heterocyclic nitrogen atoms in these P,N ligands. Through finely tuning the number of heterocyclic nitrogen atoms, a new family of ferrocene‐based chiral phosphine‐triazine ligands with three heterocyclic nitrogen atoms has been developed and successfully applied in Pd‐catalyzed asymmetric allylic substitution. Up to 99% ee with 99% yield of allylic alkylation products and 94% ee of allylic amination products have been obtained by the use of ligand (R_c,S_p)‐ 1f with a 4,6‐diphenoxy‐1,3,5‐triazine moiety.     

A New Chiral P,N‐Ligand Derived from 1‐Phenylphospholane‐2‐carboxylic Acid (Phenyl‐P‐proline) for Palladium‐Catalyzed Asymmetric Allylic Substitution Reactions

New types of P,N‐ligands, cis‐ and trans‐ 3 , containing a tetrahydroisoquinoline skeleton as an N‐donor were synthesized from (1R,2S)‐1‐phenylphospholane‐2‐carboxylic acid (phenyl‐P‐proline, 1 ). The cis isomer, cis‐ 3 , was found to act as an excellent ligand in palladium‐catalyzed asymmetric allylic substitution reactions. The reactions of 1,3‐diphenyl‐2‐propenyl acetate ( 5 ) with several nucleophiles in the presence of [Pd(π‐allyl)Cl]₂, cis‐ 3 (Pd : ligand=1 : 2), and a base afforded the desired products in high yields with high enantioselectivity. It was suggested that these ligands did not serve as P,N‐bidentate ligands but as P‐monodentate ligands in these reactions.     

A Modular Furanoside Thioether‐Phosphite/Phosphinite/ Phosphine Ligand Library for Asymmetric Iridium‐Catalyzed Hydrogenation of Minimally Functionalized Olefins: Scope and Limitations

A highly modular furanoside thioether‐phosphite/phosphinite/phosphine ligand library has been synthesized for the iridium‐catalyzed asymmetric hydrogenation of minimally functionalized olefins. These ligands can be prepared efficiently from easily accessible D ‐(+)‐xylose. We found that their effectiveness at transferring the chiral information in the product can be tuned by correctly choosing the ligand components. Enantioselectivities were therefore excellent (ees up to 99%) in a wide range of E‐ and Z‐trisubstituted alkenes using 5‐deoxyribofuranoside thioether‐phosphite ligands. It should be pointed out that these catalysts are also very tolerant to the presence of a neighbouring polar group. For 1,1‐disubstituted substrates, both enantiomers of the hydrogenation product can be obtained in high enantioselectivities simply by changing the configuration of the biaryl phosphite moiety. The asymmetric hydrogenation was also performed using propylene carbonate as solvent, which allowed the iridium catalysts to be reused while maintaining the excellent enantioselectivities.     

Threonine‐Derived Phosphinite‐Oxazoline Ligands for the Ir‐Catalyzed Enantioselective Hydrogenation

A series of chiral phosphinite‐oxazolines was synthesized in four steps starting from carboxylic acids and threonine methyl ester. In the asymmetric hydrogenation of a number of alkenes, iridium complexes of these ligands induced significantly higher enantioselectivities than the corresponding serine‐derived complexes. Enantiomeric excesses of 89 to 99% were obtained for unfunctionalized alkenes with turnover numbers of up to 5000.     

Highly Enantioselective Hydrogenation of Itaconic Acid Derivatives with a Chiral Bisphospholane‐Rh(I) Catalyst

The new – commercially in multi‐kg quantities available – chiral bisphospholane ligand, catASium^® M, has been successfully used in the Rh(I)‐catalysed enantioselective hydrogenation of itaconic acid derivatives. Chiral ß‐substituted succinic acid derivatives were produced in good to excellent enantioselectivities. Turnover frequencies by up to 40,000 h⁻¹ have been achieved.     

Highly Efficient and Enantioselective Iridium‐Catalyzed Asymmetric Hydrogenation of N‐Arylimines

A catalytic method employing the cationic iridium‐(S_c,R_p)‐DuanPhos [(1R,1′R,2S,2′S)‐2,2′‐di‐tert‐butyl‐2,2′,3,3′‐tetrahydro‐1H,1′H‐1,1′‐biisophosphindole] complex and BARF {tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate} counterion effectively catalyzes the enantioselective hydrogenation of acyclic N‐arylimines with high turnover numbers (up to 10,000 TON) and excellent enantioselectivities (up to 98% ee), achieving the practical synthesis of chiral secondary amines.     

Optically Pure 1,2‐Bis[(o‐alkylphenyl)phenylphosphino]ethanes and Their Use in Rhodium‐Catalyzed Asymmetric Hydrogenations of α‐(Acylamino)acrylic Derivatives

Optically pure (S,S)‐1,2‐bis[(o‐alkylphenyl)phenylphosphino]ethanes 1a–d were prepared in four steps from phenyldichlorophosphine via phosphine‐boranes as the intermediates. The rhodium complexes 5a–d of these diphosphines were used for the asymmetric hydrogenations of α‐(acylamino)acrylic derivatives including β‐disubstituted derivatives. Markedly high enantioselectivity (78–>99%) was observed for the reduction of β‐monosubstituted derivatives. β‐Disubstituted derivatives were also reduced in considerably high enantioselectivity (up to 90%). The single crystal X‐ray analysis of the rhodium complex 5c of (S,S)‐1,2‐bis[phenyl(5′,6′,7′,8′‐tetrahydronaphthyl)phosphino]ethane ( 1c) revealed its δ‐type structure with face orientation of the two tetrahydronaphthyl groups and edge orientation of the two phenyl groups. This conformation corresponds to that of the rhodium complex of 1,2‐bis[(o‐methoxyphenyl)phenylphosphino]ethane (DIPAMP); the rhodium complex of (R,R)‐DIPAMP, whose chirality at phosphorus is opposite that of 5c , exhibits a λ‐type structure with the face orientation of the two o‐methoxyphenyl groups and the edge orientation of the two phenyl groups. The conformational similarity of these rhodium complexes as well as the stereochemical outcome in the asymmetric hydrogenations means that the coordinative interaction of the methoxy group of DIPAMP with rhodium metal is not the main factor that affects asymmetric induction.     

New Chiral Diphosphine Ligands Designed to have a Narrow Dihedral Angle in the Biaryl Backbone

A series of novel optically active diphosphine ligands, (4,4′‐bi‐1,3‐benzodioxole)‐5,5′‐diylbis(diarylphosphine)s ( 6 ), which are called SEGPHOS, has been designed and synthesized with dihedral angles in the Ru complexes being less than that in the corresponding BINAP‐Ru complex. The stereorecognition abilities of SEGPHOS‐Ru complex catalysts in the asymmetric catalytic hydrogenation of a wide variety of carbonyl compounds are superior to those observed with BINAP‐Ru complex catalysts.     

Asymmetric Homogeneous Hydrogenation in Flow using a Tube‐in‐Tube Reactor

In this update, the asymmetric homogeneous hydrogenation of a number of trisubstituted olefins utilizing the recently developed tube‐in‐tube gas‐liquid flow reactor is described. A number of chiral iridium‐ and rhodium‐based catalysts and other parameters such as pressure, solvent, temperature and catalyst loading were screened. The advantage of the flow set‐up for rapid screening and optimization of reaction parameters is illustrated. Furthermore, a comparative study using batch conditions aided in the optimization of the flow reaction set‐up. The set‐up was further modified to recycle the catalyst which prolonged catalytic activity.     

A Phosphite‐Pyridine/Iridium Complex Library as Highly Selective Catalysts for the Hydrogenation of Minimally Functionalized Olefins

A modular library of readily available phosphite‐pyridine ligands has been successfully applied for the first time in the iridium‐catalyzed asymmetric hydrogenation of a broad range of minimally functionalized olefins. The modular ligand design has been shown to be crucial in finding highly selective catalytic systems for each substrate. Excellent enantioselectivities (ees up to 99%) have therefore been obtained in a wide range of E‐ and Z‐trisubstituted alkenes, including more demanding triaryl‐substituted olefins and dihydronaphthalenes. This good performance extends to the very challenging class of terminal disubstituted olefins, and to olefins containing neighbouring polar groups (ees up to 99%). Both enantiomers of the reduction product can be obtained in excellent enantioselectivities by simply changing the configuration of the carbon next to the phosphite moiety. The hydrogenations were also performed using propylene carbonate as solvent, which allowed the iridium catalyst to be reused and maintained the excellent enantioselectivities.

     

Highly Efficient Asymmetric Hydrogenation of Alkyl Aryl Ketones Catalyzed by Iridium Complexes with Chiral Planar Ferrocenyl Phosphino-Thioether Ligands

Iridium complexes of planar-chiral ferrocenyl phosphine-thioether ligands were tested in the hydrogenation of simple ketones. Optimization of the conditions led to a highly active catalytic system with turnover numbers up to 915 and turnover frequencies up to ca. 250 h⁻¹. Furthermore, very high enantioselectivities (up to >99 %) together with complete conversions were obtained for the asymmetric hydrogenation of various acetophenones at 10 °C.     

MOP‐Type Binaphthyl Phosphite and Diamidophosphite Ligands and Their Application in Catalytic Asymmetric Transformations

Monodentate phosphite and diamidophosphite ligands have been developed based on O‐methyl‐BINOL. These chiral ligands are easy to prepare from readily accessible phosphorylating reagents – (S_a or R_a)‐2‐chlorodinaphtho[2,1‐d:1′,2′‐f][1,3,2]dioxaphosphepine and (2R,5S)‐2‐chloro‐3‐phenyl‐1,3‐diaza‐2‐phosphabicyclo[3.3.0]octane. The new ligands have demonstrated excellent enantioselectivity in the palladium‐catalysed allylic substitution reactions of (E)‐1,3‐diphenylallyl acetate with sodium p‐toluenesulfinate (up to 99 % ee), pyrrolidine (up to 97 % ee), dipropylamine (up to 95 % ee) and dimethyl malonate (up to 99 % ee). In the palladium‐catalysed deracemization of ethyl (E)‐1,3‐diphenylallyl carbonate, up to 96 % enantioselectivity has been achieved. The diamidophosphite ligands have exhibited very good enantioselectivity in the Rh‐catalysed asymmetric hydrogenation of dimethyl itaconate (up to 90 % ee).     

Asymmetric Synthesis of the Roche Ester and its Derivatives by Rhodium‐INDOLPHOS‐Catalyzed Hydrogenation

(S)‐3‐Hydroxy‐2‐methylpropionate, known as the Roche ester, and several of its derivatives were successfully synthesized through asymmetric rhodium‐catalyzed hydrogenation, using INDOLPHOS (diisopropyl{1‐[(S)‐3,5‐dioxa‐4‐phosphacyclohepta[2,1‐a;3,4‐a′]dinaphthalen‐4‐yl]‐3‐methyl‐2‐indolyl}phosphine) as the chiral ligand, in excellent yield and the highest ee reported up to now (TOF over 5500 h⁻¹ at 25 °C; up to 98% ee at −40 °C).     

Asymmetric Mukaiyama‐Aldol Reaction in Aqueous Media Promoted by Zinc‐Based Chiral Lewis Acids

Asymmetric aldol reactions in aqueous media have been realized by using zinc‐based chiral Lewis acids. The aldol products have been obtained with high yield, diastereocontrol and a good level of enantioselectivity. The reactivity of both acetophenone and propiophenone enol ether surrogates was tested with a range of aldehydes.