trans-2,5-Dialkylpyrrolidinyl-containing phosphinamines. Synthetic and mechanistic studies in Pd-catalysed asymmetric allylic alkylation

The preparation of new phosphinamine ligands possessing an enantiopure trans-2,5-dialkylpyrrolidinyl unit linked by a rigid o-phenylene bridge to a diphenylphosphine is described. Only that route forming the trans-2,5-dialkylpyrrolidine in the final step from (2-aminophenyl)diphenylphosphine proved successful. The cyclocondensation proceeded in 48% and 27% yields, respectively, for the dimethyl- and diethyl-analogues. Their palladium complexes were prepared and applied to the test of enantioselective alkylation of 1,3-diphenyl-2-propenyl acetate with dimethyl malonate in high chemical yields but with moderate enantioselectivities of up to 34% ee.¹H NMR spectra of the η³-allyl Pd complexes of four trans-2,5-dialkylpyrrolidine-containing ligands were analysed in an attempt to explain the results obtained. In the cases of the 1,3-diphenylallyl complexes, two diastereomers were observed for all four ligands and their configurations were assigned with the aid of COSY and NOESY experiments. The catalytic results obtained are best interpreted by the reaction proceeding with nucleophilic attack on the allyl trans to the phosphorus donor atom of the major diastereomer.     

1-Phosphino-2-sulfenylferrocenes as planar chiral ligands in enantioselective palladium-catalyzed allylic substitutions

The synthesis of a wide structural variety of enantiopure 1-phosphino-2-sulfenylferrocene ligands 1 possessing exclusively planar chirality is described. In the case of the readily available tert-butylsulfenyl derivatives very high enantioselectivities were obtained in the palladium-catalyzed allylic substitution of 1,3-diphenyl-2-propenyl acetate with dimethyl malonate (ee's up to 97%) and nitrogen nucleophiles (ee's up to 99.5%). Palladium complexes of these ferrocenes were characterized by NMR and X-ray diffraction, revealing the P,S-bidentate character of the ligands 1 and the formation of a single epimer on the stereogenic sulfur atom resulting from the complexation with palladium. A model justifying the observed asymmetric induction exerted by this novel family of chiral ferrocenes, supported by solution NMR studies on a palladium allylic complex, is discussed.     

Highly enantioselective Pd-catalyzed allylic alkylation using new chiral ferrocenylphosphinoimidazolidine ligands

New ferrocenylphosphinoimidazolidines containing central chirality and planar chirality were found to act as highly effective chiral ligands in Pd-catalyzed asymmetric allylic alkylation of 1,3-diphenyl-2-propenyl acetate with dimethyl malonate.     

The application of Pd-complexes of trans-2,5-dialkylpyrrolidinylbenzyldiphenylphosphines to enantioselective allylic alkylation

The preparation of cationic palladium (η³-C₃H₅) complexes 6 and 7, possessing an enantiopure trans-2,5-dialkylpyrrolidinyl unit, is described. These complexes were applied to the enantioselective alkylation of the test substrate, 1,3-diphenylpropenyl acetate 8, with dimethyl malonate in good to high conversions with enantioselectivities of up to 90% ee for the 2,5-diethylpyrrolidinyl substituted complex 7. Attempts to optimise reaction conditions included variation of solvent and temperature and the source of the malonate nucleophile.     

New dihydroxy bis(oxazoline) ligands for the palladium-catalyzed asymmetric allylic alkylation: experimental investigations of the origin of the reversal of the enantioselectivity

The origin of the reversal of the enantioselectivity in the palladium-catalyzed allylic alkylation of rac-1,3-diphenyl-2-propenyl acetate with dimethyl malonate anion using chiral dihydroxy bis(oxazoline) "BO" ligands derived from (1S,2S)-(+)-2-amino-1-phenyl-1,3-propanediol was investigated. To determine the structural effects of the dihydroxy BO ligand on this unique phenomenon, new homochiral dihydroxy BO ligands were prepared from L-threonine and L-serine and were assessed in the transformation. The results obtained with these novel BO ligands, compared with the one obtained by using the dihydroxy BO ligands derived from (1S,2S)-(+)-2-amino-1-phenyl-1,3-propanediol, reveal that the reversal in the enantioselectivity observed with the dihydroxy BO ligand depends on the structure of the ligand. The effect of different bases used to generate the dimethyl malonate anion was also examined. The results are discussed in terms of the interaction of one hydroxy group in the intermediate pi-allyl palladium complex with the dimethyl malonate anion.     

Palladium-catalyzed asymmetric allylic substitution using planar chiral hydrazone ligands

Palladium-catalyzed asymmetric allylic alkylation of 1,3-diphenyl-2-propenyl acetate (3) with a dimethyl malonate–BSA–LiOAc system has been successfully carried out in the presence of planar chiral phosphine-hydrazone ligands such as 2a in good yields with good enantioselectivities (up to 96% ee).     

Novel planar chiral phosphite: Preparation and use in the synthesis of Pd(II) complexes and in Pd-catalyzed allylic alkylation

The planar chiral diaryl phosphorimidite ligand containing additional C-stereocenters and neutral and cationic palladium(II) chelates with this ligand, cis-[Pd(η²-P,N)Cl₂] and [Pd(Allyl)(η²-P,N)]BF₄, were synthesized for the first time. The possibility of using these compounds in asymmetric allylic alkylation of 1,3-diphenylallyl acetate with dimethyl malonate in an optical yield of up to 73% was demonstrated.     

Complexing and catalytic properties of easily available chiral iminophosphite based on biphenyl-2,2"-diol

A chiral iminophosphite ligand based on biphenyl-2,2"-diol and its chelates with rhodium(i) and palladium(ii) were synthesized for the first time. Successful use of these compounds in asymmetric allylic substitution was demonstrated. The enantioselectivity of palladium-catalyzed alkylation of methyl pent-3-en-2-yl carbonate with dimethyl malonate reached 70%, that in the alkylation of 1,3-diphenylallyl acetate with dimethyl malonate was 88%. In the rhodium-catalyzed sulfonylation of 1,3-diphenylallyl acetate with sodium p-toluenesulfinite, the enantioselectivity was 56%.     

Palladium-catalyzed allylic substitution assisted by 1,3,2-diazaphospholidine derivatives of (<Emphasis Type="Italic">R,R</Emphasis>)-<Emphasis Type="Italic">N</Emphasis>-naphthyltartarimide

P,P-Bidentate diamidophosphite ligands containing the (3R,4R)-3,4-dioxy-1-(1-naphthyl)pyrrolidine-2,5-dione framework and 1,3,2-diazaphospholidine rings with the stereogenic P atoms were obtained. The use of these ligands provides up to 85% ee in Pd-catalyzed asymmetric amination of (E)-1,3-diphenylallyl acetate and up to 95% ee in its asymmetric alkylation with dimethyl malonate.     

Cymantrene-based iminodiamidophosphites: the first phosphite-type ligands with planar chirality

Optically active phosphite-type ligands with planar chirality have been synthesized for the first time. The new cymantrene-based P,N-iminodiamidophosphites demonstrated high enantioselectivity in the Pd-catalyzed allylic substitution of 1,3-diphenylallyl acetate with dimethyl malonate (up to 94% ee), pyrrolidine (up to 78% ee) and sodium para-toluene sulfinate (up to 84% ee).     

Role of planar chirality of S,N- and P,N-ferrocene ligands in palladium-catalyzed allylic substitutions

Palladium-catalyzed asymmetric allylic substitutions using thioether and phosphino derivatives of ferrocenyloxazoline as ligands have been investigated with a focus on studying the role of planar chirality. In allylic alkylation, up to 98% ee and 95% ee were achieved with S,N- and P,N-ligands, respectively. In allylic amination, 97% ee was realized with P,N-ligands in the presence of TBAF. Several palladium allylic complexes were characterized by X-ray diffraction and/or solution NMR. Thioether derivatives of ferrocenyloxazolines with only planar chirality showed lower enantioselectivity in the allylic alkylation except 5c because of the formation of a new chirality on sulfur atom during the coordination of sulfur with palladium. On the other hand, in the planar chiral P,N-ligands without central chirality, (Sp)-11a-c there was no such disturbance and comparatively higher enantioselectivity in both palladium-catalyzed allylic alkylation and amination was provided.     

Readily available phosphine–imine ligands from α-phenylethylamine for highly efficient Pd-catalyzed asymmetric allylic alkylation

A series of novel chiral phosphine–imine ligands have been prepared by a two-step transformation from chiral α-phenylethylamine. The resulting chiral ligands were found to be effective for the palladium-catalyzed asymmetric allylic alkylation of 1,3-diphenylprop-2-en-1-yl pivalate with dimethyl malonate, in which up to 94% ee and 99% conversions were obtained. The results demonstrate that the chirality resided on the chelate ring of P–Pd–N complex is more effective for the transfer of the stereochemical information by comparison with the result obtained by Hashimoto and coworkers’ phosphine–imine ligand, in which the chirality lay in the outside of P–Pd–N chelate ring. The effect of solvent, base and substitutent in phosphine–imine ligand on this catalytic reaction is also described.     

Synthesis of new fluorous modular chiral ligand derivatives from amino alcohols and application in enantioselective carbon–carbon bond-forming alkylation reactions

N-Trifluoracyl β-chalcogeno amides and N-perfluoracyl β-thio amide ligands were prepared by a simple and efficient reaction sequence. These new ligands were evaluated in palladium-catalyzed alkylation of rac-(E)-1,3-diphenyl-2-propenyl acetate in the presence of dimethyl malonate and an enantioselectivity of up to 99% was obtained. After catalysis, the fluorous ligand can be easily recovered by liquid–liquid extraction and reused without loss in the activity.     

Synthesis of chiral iminoalkyl functionalised N-heterocyclic carbenes and their use in asymmetric catalysis

A library of new chiral iminoalkyl imidazolium salts has been synthesised from amino acids using a modular design approach. Deprotonation with silver oxide yields silver carbene transfer reagents, which can be used as ligand sources in asymmetric catalysis. Preliminary testing has shown that the ligands induce enantioselectivity in the palladium-catalysed allylic alkylation of 1,3-diphenylprop-3-enyl acetate with dimethyl malonate.     

新型手性硫醚-亚磷酰胺配体的设计与应用

根据模块组合法原理,设计并合成了一种具有亚磷酰胺结构的硫醚配体,该配体具有原料便宜易得、合成步骤简单、易于修饰和结构稳定等特点。以配体/钯络合物催化的丙二酸二甲酯与1,3-二苯基烯丙基醋酸酯的烯丙基化反应作为模板反应,探究了溶剂、碱对其立体控制的影响。随后探究了中心手性与轴手性的匹配/错配现象以及硫醚片段的空间位阻、电性对反应立体控制的作用。在最佳条件下,产物的对映选择性能够达到-76% ee。     

Palladium-catalyzed asymmetric allylic alkylation using (R)-2-(methoxymethyl)pyrrolidine-derived aminophosphine ligands

Palladium-catalyzed asymmetric allylic alkylation of 1,3-diphenyl-2-propenyl acetate 7a using a dimethyl malonate–BSA–LiOAc system has been successfully carried out in the presence of new chiral aminophosphine ligands such as 4 in good yields with good enantioselectivities (up to 96% e.e.).     

Simple d-glucosamine-based phosphine-imine and phosphine-amine ligands in Pd-catalyzed asymmetric allylic alkylations

A new family of phosphine-imine and phosphine-amine ligands based on d-glucosamine were synthesized in order to probe previous asymmetric allylic alkylation results with those of disaccharide ligands of the same class. In most cases, good-to-excellent activities and enantioselectivies were observed with these ligands with ee’s reaching up to 87% in the Pd-catalyzed allylic alkylation reaction of racemic (E)-1,3-diphenyl-2-propenyl acetate with dimethyl malonate as the nucleophile.     

Preparation of indole-phosphine oxazoline (IndPHOX) ligands and their application in allylic alkylation

Two classes of indole-phosphine oxazoline ligands have been prepared from readily available starting materials in good overall yields. These modular ligands include an indole skeleton with either a phosphine moiety or an oxazoline ring at the 2- or 3-position, respectively. The utility of these ligands was demonstrated in a catalytic asymmetric reaction: the palladium-catalyzed allylic alkylation of 1,3-diphenyl-2-propenyl acetate with dimethyl malonate was performed with enantioselectivities as high as 98%.     

Palladium-catalyzed asymmetric allylic alkylation using chiral aminophosphine ligands

Palladium-catalyzed asymmetric allylic alkylation of 1,3-diphenyl-2-propenyl acetate 18 with a dimethyl malonate–BSA–LiOAc system has been successfully carried out in the presence of new chiral aminophosphine ligands 1–5 in good yields with good enantioselectivities (up to 85% e.e.).     

Novel C2-symmetric planar chiral diphosphine ligands and their application in pd-catalyzed asymmetric allylic substitutions

Novel C(2)-symmetric diphosphine ligands possessing only the planar chirality of ruthenocene, 1,1'-bis(diphenylphosphino)-2,2'-disubstituted-ruthenocenes (4), were prepared. With this kind of ligands, excellent enantioselectivity and especially highly catalytic activity in palladium-catalyzed asymmetric allylic substitutions of rac-1,3-diphenyl-2-propenyl acetate (9) were observed, compared to their ferrocene analogues 1. Good enantioselectivity and highly catalytic activity were also obtained with 4 in palladium-catalyzed asymmetric allylic substitutions of cyclohexen-1-yl acetate (12). Further study on the effect of R in ester group on enantioselectivity of 4 showed an opposite trend compared with their ferrocene analogues 1 in asymmetric allylic substitutions. For ruthenocene ligands 4, the one with the smaller R in the ester group gave higher enantioselectivity for the palladium-catalyzed asymmetric allylic substitutions of 9, while a converse trend had been observed with 1. However, for the palladium-catalyzed asymmetric allylic substitutions of 12, ligand 4 with a larger R in the ester group resulted in somewhat higher enantioselectivity but still an opposite trend with ligand 1. The X-ray diffraction study of crystal structures of 4 and 1 with Pd(II) was carried out and showed that the enantioselectivity was correlated to the twist angle existing in the palladium complex.