新型手性N,P配体的合成及其在烯丙基化中的应用

以手性5,6,7,8-四氢-8-喹啉醇为原料合成了新型手性N,P配体, 并将该配体应用于钯催化的1,3-二苯基-2-烯丙基乙酸酯的不对称烯丙基化反应, 得到100%的取代产物, 对映体过量达到73% ee.     

Screening of a modular sugar-based phosphite-oxazoline ligand library in asymmetric Pd-catalyzed heck reactions

We have synthesised a library of phosphite-oxazoline ligands derived from readily available D-glucosamine. These ligands have been successfully screened in the palladium-catalysed Heck reaction of several substrates with high regio- (up to 99 %) and enantioselectivities (ee's up to 99 %) as well as with improved activities under standard thermal conditions. The results indicate that the catalytic performance is highly affected by the oxazoline and biarylphosphite substituents and the axial chirality of the biaryl moiety of the ligand. The Heck reactions were also performed under microwave irradiation conditions, allowing a considerably shorter reaction time (full conversion in minutes) maintaining the excellent regio- and enantioselectivities.     

Synthesis and application of chiral N-heterocyclic carbene-oxazoline ligands: iridium-catalyzed enantioselective hydrogenation

Two libraries of enantiomerically pure imidazolium salts bearing an oxazoline unit were synthesized. Deprotonation of the imidazolium salts and complexation of the resulting oxazoline-carbene ligands to iridium(I) was achieved in one step by mixing the imidazolium salts with NaOtBu and [(eta(4)-cod)IrCl](2) in THF at room temperature. The air-stable complexes were purified by flash chromatography. All complexes were analyzed by two-dimensional (2D) NMR methods and one compound from each family was characterized by X-ray structure analysis. The two libraries of iridium complexes were successfully tested in the asymmetric hydrogenation of unfunctionalized and functionalized olefins. Enantioselectivities of up to 90 % ee were obtained with trans-alpha-methylstilbene. Upon complexation of imidazolium salt 15 p with R(1) = phenyl, C-H bond activation of the phenyl ring gave rise to iridium(III) complex 17, which was fully characterized by NMR spectroscopy and X-ray structure analysis. Complex 17 proved to be catalytically inactive in the hydrogenation.     

Asymmetric Hydrogenation of α,β‐Unsaturated Carboxylic Esters with Chiral Iridium N,P Ligand Complexes

Enantioselective conjugate reduction of a wide range of α,β‐unsaturated carboxylic esters was achieved using chiral Ir N,P complexes as hydrogenation catalysts. Depending on the substitution pattern of the substrate, different ligands perform best. α,β‐Unsaturated carboxylic esters substituted at the α position are less problematic substrates than originally anticipated and in some cases α‐substituted substrates actually reacted with higher enantioselectivity than their β‐substituted analogues. The resulting saturated esters with a stereogenic center in the α or β position were obtained in high enantiomeric purity.     

Double stereoselective coordination of chiral N,S ligands: Synthesis,coordination chemistry and utilization in Pd‐catalyzed allylic alkylation

A series of chiral pentane‐2,4‐diyl‐based thioether‐amine ligands [ 4 and 5 ; (R,S)‐ and (S,S)‐R¹SCH(CH₃)CH₂CH(CH₃)NHR², respectively, where 4a R¹ = iPr, R² = Ph; 4b R¹ = tBu, R² = Ph; 4c R¹ = 1‐Ad, R² = Ph; 5a R¹ = iPr, R² = Ph; 5b R¹ = tBu, R² = Ph; 5c R¹ = 1‐Ad, R² = Ph; 5d R¹ = iPr, R² = 4‐MeOC₆H₄; 5e R¹ = iPr, R² = 4‐MeC₆H₄; 5f R¹ = iPr, R² = 3,5‐Me₂C₆H₃] with stereogenic S‐ and N‐donor atoms has been prepared starting from cyclic sulfates via optically pure γ‐aminoalcohol or 2,4‐dimethylazetidine intermediates. The synthesis of the novel diastereomerically related ligand sets 4 and 5 was accomplished starting from the same source of chirality. The modular ligand structure and the novel synthetic strategies developed for their synthesis allowed the easy modification of the ligands’ (i) S‐ and (ii) N‐substituents, as well as (iii) the relative stereochemistry within the ligand backbone. Six‐membered [Pd(N,S)Cl₂]‐type chelate complexes of the diastereomerically related ligands 4a and 5a were synthesized and characterized by X‐ray crystallography in the solid phase, by density functional theory calculations and in solution by NMR spectroscopy. The coordination of 5a resulted in the formation of a single chair conformation by the stereospecific locking of both stereolabile (N and S) donor atoms. In contrast, compound 4a forms rapidly equilibrating palladium species due to the fast inversion of the sulfur donor. Ligands with stereochemically fixed donor atoms provided robust and efficient catalytic systems that can be effectively applied in alkylene carbonates as green reaction media. Remarkably, the phosphine‐free catalysts are air‐stable, and at room temperature in the presence of moisture gave excellent ee’s (up to 93%) in asymmetric allylation processes thanks to the double stereoselective coordination.     

Synthesis and catalytic applications of new chiral ferrocenyl P,O ligands

A new method to synthesize both enantiomers of 2-diphenylphosphino-ferrocenecarboxaldehyde with the phosphine group protected as a thiophosphine group was developed. These aldehydes react with 1,2 or 1,3 diols to give, in good yields, new chiral phosphine-acetals. For unsymmetrical (R)-1,3-butanediol, the new asymmetrical acetalic carbon is asymmetric and its configuration was completely controlled by the chirality of the diol. These new P,O ligands were tested in the palladium-catalyzed asymmetric allylic substitution of 1,3-diphenylprop-2-enylacetate. Good yields and enantioselectivities up to 77% were observed. The catalytic performances of two diastereoisomeric ligands with opposite configurations in planar chirality only proved to be significantly different, showing a strong influence of both planar and central chiralities.     

Atypical and Asymmetric 1,3-P,N Ligands: Synthesis,Coordination and Catalytic Performance of Cycloiminophosphanes

Novel seven-membered cyclic imine-based 1,3-P,N ligands were obtained by capturing a Beckmann nitrilium ion intermediate generated in situ from cyclohexanone with benzotriazole, and then displacing it by a secondary phosphane under triflic acid promotion. These “cycloiminophosphanes” possess flexible non-isomerizable tetrahydroazepine rings with a high basicity; this sets them apart from previously reported iminophophanes. The donor strength of the ligands was investigated by using their P-κ¹- and P,N-κ²-tungsten(0) carbonyl complexes, by determining the IR frequency of the trans-CO ligands. Complexes with [RhCp*Cl₂]₂ demonstrated the hemilability of the ligands, giving a dynamic equilibrium of κ¹ and κ² species; treatment with AgOTf gives full conversion to the κ² complex. The potential for catalysis was shown in the Ru^II-catalyzed, solvent-free hydration of benzonitrile and the Ru^II- and Ir^I-catalyzed transfer hydrogenation of cyclohexanone in isopropanol. Finally, to enable access to asymmetric catalysts, chiral cycloiminophosphanes were prepared from l -menthone, as well as their P,N-κ²-Rh^III and a P-κ¹-Ru^II complexes.     

Rh-catalyzed asymmetric hydrogenation of prochiral olefins with a dynamic library of chiral TROPOS phosphorus ligands

A library of 19 chiral tropos phosphorus ligands, based on a flexible (tropos) biphenol unit and a chiral P-bound alcohol (11 phosphites) or secondary amine (8 phosphoramidites), was synthesized. These ligands were screened, individually and as a combination of two, in the rhodium-catalyzed asymmetric hydrogenation of dehydro-alpha-amino acids, dehydro-beta-amino acids, enamides and dimethyl itaconate. ee values up to 98% were obtained for the dehydro-alpha-amino acids, by using the best combination of ligands, a phosphite [4-P(O)2O] and a phosphoramidite [13-P(O)2N]. Kinetic studies of the reactions with the single ligands and with the combination of phosphite [4-P(O)2O] and phosphoramidite [13-P(O)2N] have shown that the phosphite, despite being less enantioselective, promotes the hydrogenation of methyl 2-acetamidoacrylate and methyl 2-acetamidocinnamate faster than the mixture of the same phosphite with the phosphoramidite, while the phosphoramidite alone is much less active. In this way, the reaction was optimized by lowering the phosphite/phosphoramidite ratio (the best ratio is 0.25 equiv phosphite/1.75 equiv phosphoramidite) with a resulting improvement of the product enantiomeric excess. A simple mathematical model for a better understanding of the variation of the enantiomeric excess with the phosphite/phosphoramidite ratio is also presented.     

A New Class of Modular P,N‐Ligand Library for Asymmetric Pd‐Catalyzed Allylic Substitution Reactions: A Study of the Key Pd–π‐Allyl Intermediates

A new class of modular P,N‐ligand library has been synthesized and screened in the Pd‐catalyzed allylic substitution reactions of several substrate types. These series of ligands can be prepared efficiently from easily accessible hydroxyl–oxazole/thiazole derivatives. Their modular nature enables the bridge length, the substituents at the heterocyclic ring and in the alkyl backbone chain, the configuration of the ligand backbone, and the substituents/configurations in the biaryl phosphite moiety to be easily and systematically varied. By carefully selecting the ligand components, therefore, high regio‐ and enantioselectivities (ee values up to 96 %) and good activities are achieved in a broad range of mono‐, di‐, and trisubstituted linear hindered and unhindered substrates and cyclic substrates. The NMR spectroscopic and DFT studies on the Pd–π‐allyl intermediates provide a deeper understanding of the effect of ligand parameters on the origin of enantioselectivity.     

Optimized Scalable Synthesis of Chiral Iridium Pyridyl-Phosphinite (Pyridophos) Catalysts

Iridium catalysts with chiral P,N ligands have greatly enhanced the scope of asymmetric olefin hydrogenation because they do not require a coordinating group near the C=C bond like Rh and Ru catalysts. Pyridophos ligands, possessing a conformationally restricted annulated pyridine framework linked to a phosphinite group, proved to be particularly effective, inducing high enantioselectivities in the hydrogenation of a remarkably broad range of substrates. Here we report the development of an efficient scalable synthesis for the two most versatile Ir-pyridophos catalysts, derived from 2-phenyl-8-hydroxy-5,6,7,8-tetrahydroquinoline or the analogue with a five-membered carbocyclic ring, respectively, by modification and optimization of the original synthetic route. The optimized route renders both catalysts readily accessible in multi-gram quantities in analytically pure form in overall yields of 26–37 %, starting from acetophenone and cyclopentanone or cyclohexanone, respectively. A major advantage of the new synthesis is the efficient and practical kinetic resolution of the late-stage pyridyl alcohol intermediates with commercial immobilized Candida antarctica lipase B, giving access to both enantiomers of these catalysts as essentially enantiopure compounds. The catalysts are obtained as crystalline solids, which are air-stable and can be stored for years at −20 °C without notable decomposition.     

Asymmetric Hydrogenation of α,β‐Unsaturated Nitriles with Base‐Activated Iridium N,P Ligand Complexes

Although many chiral catalysts are known that allow highly enantioselective hydrogenation of a wide range of olefins, no suitable catalysts for the asymmetric hydrogenation of α,β‐unsaturated nitriles have been reported so far. We have found that Ir N,P ligand complexes, which under normal conditions do not show any reactivity towards α,β‐unsaturated nitriles, become highly active catalysts upon addition of N,N‐diisopropylethylamine. The base‐activated catalysts enable conjugate reduction of α,β‐unsaturated nitriles with H₂ at low catalyst loadings, affording the corresponding saturated nitriles with high conversion and excellent enantioselectivity. In contrast, alkenes lacking a conjugated cyano group do not react under these conditions, making it possible to selectively reduce the conjugated C?C bond of an α,β‐unsaturated nitrile, while leaving other types of C?C bonds in the molecule intact.     

Chiral P,N‐ligands for the highly enantioselective addition of diethylzinc to aromatic aldehydes

A new sterically hindered chiral P,N‐ligand was synthesized and successfully applied to copper catalyzed asymmetric addition of diethylzinc to aromatic aldehydes. Various aromatic aldehydes can react smoothly to give the corresponding addition products with good to excellent enantioselectivities, which provides a readily accessible method for the preparation of chiral secondary alcohols.     

Extending the Substrate Scope of Bicyclic P‐Oxazoline/Thiazole Ligands for Ir‐Catalyzed Hydrogenation of Unfunctionalized Olefins by Introducing a Biaryl Phosphoroamidite Group

This study identifies a series of Ir‐bicyclic phosphoroamidite–oxazoline/thiazole catalytic systems that can hydrogenate a wide range of minimally functionalized olefins (including E‐ and Z‐tri‐ and disubstituted substrates, vinylsilanes, enol phosphinates, tri‐ and disubstituted alkenylboronic esters, and α,β‐unsaturated enones) in high enantioselectivities (ee values up to 99 %) and conversions. The design of the new phosphoroamidite–oxazoline/thiazole ligands derives from a previous successful generation of bicyclic N‐phosphane–oxazoline/thiazole ligands, by replacing the N‐phosphane group with a π‐acceptor biaryl phosphoroamidite moiety. A small but structurally important family of Ir‐phosphoroamidite–oxazoline/thiazole precatalysts has thus been synthesized by changing the nature of the N‐donor group (either oxazoline or thiazole) and the configuration at the biaryl phosphoroamidite moiety. The substitution of the N‐phosphane by a phosphoroamidite group in the bicyclic N‐phosphane–oxazoline/thiazole ligands extended the range of olefins that can be successfully hydrogenated.