Chiral Phosphaalkene−Oxazoline Ligands for the Palladium-Catalyzed Asymmetric Allylic Alkylation

Exploiting the Chiral Ligands of Bis(imidazolinyl)- and Bis(oxazolinyl)thiophenes-Synthesis and Application in Cu-Catalyzed Friedel-Crafts Asymmetric Alkylation

Five new C₂-symmetric chiral ligands of 2,5-bis(imidazolinyl)thiophene (L1–L3) and 2,5-bis(oxazolinyl)thiophene (L4 and L5) were synthesized from thiophene-2,5-dicarboxylic acid (1) with enantiopure amino alcohols (4a–c) in excellent optical purity and chemical yield. The utility of these new chiral ligands for Friedel–Crafts asymmetric alkylation was explored. Subsequently, the optimized tridentate ligand L5 and Cu(OTf)₂ catalyst (15 mol%) in toluene for 48 h promoted Friedel–Crafts asymmetric alkylation in moderate to good yields (up to 76%) and with good enantioselectivity (up to 81% ee). The bis(oxazolinyl)thiophene ligands were more potent than bis(imidazolinyl)thiophene analogues for the asymmetric induction of the Friedel–Crafts asymmetric alkylation.

Recent Advances in Enantioselective Pd-Catalyzed Allylic Substitution: From Design to Applications

This Review compiles the evolution, mechanistic understanding, and more recent advances in enantioselective Pd-catalyzed allylic substitution and decarboxylative and oxidative allylic substitutions. For each reaction, the catalytic data, as well as examples of their application to the synthesis of more complex molecules, are collected. Sections in which we discuss key mechanistic aspects for high selectivity and a comparison with other metals (with advantages and disadvantages) are also included. For Pd-catalyzed asymmetric allylic substitution, the catalytic data are grouped according to the type of nucleophile employed. Because of the prominent position of the use of stabilized carbon nucleophiles and heteronucleophiles, many chiral ligands have been developed. To better compare the results, they are presented grouped by ligand types. Pd-catalyzed asymmetric decarboxylative reactions are mainly promoted by PHOX or Trost ligands, which justifies organizing this section in chronological order. For asymmetric oxidative allylic substitution the results are grouped according to the type of nucleophile used.

A readily accessible and modular carbohydrate-derived thioether/selenoether-phosphite ligand library for Pd-catalyzed asymmetric allylic substitutions

A large library of thioether/selenoether-phosphite ligands have been tested in the Pd-catalyzed asymmetric allylic substitution reaction. The presented ligands are derived from cheap and available carbohydrates and they are air-stable solids and easy to handle. Their highly modular nature has made it possible to achieve excellent enantioselectivities in the substitution of a range of hindered and unhindered substrates (ees up to 99% and 91%, respectively). In addition, twelve C-, N- and O-nucleophiles can be efficiently introduced, independently of their nature. Among the whole library, ligands that contain an additional chiral centre in the alkyl backbone chain next to the phosphite group and an enantiopure biaryl phosphite group provided the best enantioselectivities. In general, there is a cooperative effect between these two chiral elements, and therefore, a matched combination between them is necessary to achieve the highest enantioselectivities. However, in the case of cyclic substrates, this cooperative effect is less pronounced and advantageously, both enantiomers of the product can be obtained by setting up the desired configuration of the biaryl phosphite group. Studies of the key Pd-π-allyl intermediates allowed us to better understand the enantioselectivities obtained experimentally.

1,1'-Binaphthyl-substituted diphosphene: synthesis, structures, and chiral optical properties

1,1'-Binaphthyl-substituted diphosphene 1 possessing a P[double bond, length as m-dash]P double bond and an axially chiral 1,1'-binaphthyl group was synthesized and fully characterized. Diphosphene 1 was prepared as an optically active form and thus is the first example of a chiral diphosphene. The CD spectra of 1 showed apparent circular dichroism in the longer wavelength region, caused by the P[double bond, length as m-dash]P moiety.

Synthesis of highly rigid phosphine–oxazoline ligands for palladium-catalyzed asymmetric allylic alkylation

Rigid phosphine–oxazoline ligands with a spirocarbon stereogenic center were developed, which exhibited excellent catalytic performance for Pd-catalyzed allylic alkylation reactions.

Rational design of sulfoxide–phosphine ligands for Pd-catalyzed enantioselective allylic alkylation reactions

A new type of chiral sulfoxide–phosphine ligands have been developed by a rational combination of two privileged scaffolds and successfully applied to Pd-catalyzed enantioselective allylic alkylation reactions.

Diastereoselective synthesis and coordination chemistry of enantiopure keto-bis-(2-phosphametallocene)s

Enantiopure 2-(chlorocarbonyl)phosphametallocenes [MCp*(2-{COCl}-3,4-Me(2)-5-Ph-PC(4))] (M = Fe, Ru) react with phospholide anions to give 2-phosphametallocene-2'-acylphospholides K[MCp*(2-CO-2'-{3',4'-Me(2)-5'-PhPC(4)}-3,4-Me(2)-5-Ph-PC(4))] (M = Fe, Ru) and these have been converted into keto-bis-(2-phosphametallocene)s through reaction with [FeClCp*(tmeda)]; templation of this process with CuBr gives rise to the C(2)- (or pseudo-C(2)- when M = Ru) symmetric form of [{MCp*(3,4-Me(2)-5-Ph-PC(4))}(2)-2,2'-(CO)] (M = Fe, Ru; Fe, Fe) with high (>95%) diastereoselectivity. X-Ray structures of these ligands coordinated to [RuCp*Cl] and [PtCl(2)] centres show that the spatial orientation of the very flexible keto-bis-(2-phosphametallocene) structure is highly responsive to the coordination sphere of the chelated platinum or ruthenium centre.

Allylic alkylations catalyzed by palladium-bis(oxazoline) complexes derived from heteroarylidene malonate derivatives

A series of simple heteroarylidene malonate-type bis(oxazoline) ligands 4 and 5 were applied to the palladium-catalyzed allylic alkylation reaction, and the ligand 4a bearing a phenyl group afforded excellent enantioselectivity (up to 96% ee) for the allylic alkylation product. Other substrates were also examined, giving the allylic alkylated products in high yield but with poor ee values.

Synthesis and coordination behavior of Cu(I) bis(phosphaethenyl)pyridine complexes

Cu(I) complexes bearing BPEP as a PNP-pincer type phosphaalkene ligand undergo effective bonding interactions with SbF(6)(-) and PF(6)(-) as non-coordinating anions to give [Cu(SbF(6))(BPEP)] and [Cu(2)(BPEP)(2)(μ-PF(6))](+), respectively [BPEP = 2,6-bis(1-phenyl-2-phosphaethenyl)pyridine]. NMR and theoretical studies indicate a reduced anionic charge of the μ-PF(6) ligand, which is induced by the strong π-accepting ability of BPEP.

Highly enantioselective synthesis of α,α-dialkylmalonates by phase-transfer catalytic desymmetrization

A novel enantioselective synthetic method for the construction of a quaternary carbon center from malonates via phase-transfer catalytic (PTC) alkylation has been developed. The asymmetric α-alkylation of diphenylmethyl tert-butyl α-alkylmalonates with alkylating agents under phase-transfer catalysis conditions (aq 50% KOH, toluene, 0 °C) in the presence of (S,S)-3,4,5-trifluorophenyl-NAS bromide (8) as PTC catalyst afforded the corresponding α,α-dialkylmalonates in high chemical (up to 99%) and optical yields (up to 97% ee) which could be readily converted to versatile chiral intermediates. Notably, the direct double α-alkylations of diphenylmethyl tert-butyl malonate also provided the corresponding α,α-dialkylmalonates without loss of enantioselectivity. The synthetic potential of this method has been demonstrated by the preparation of α,α-dialkylamino acid and oxindole systems.