Asymmetric addition of phenylboronic acid to cycloalkanones mediated by a Rh/phosphoramidite complex: a comparison between tropos versus nontropos behavior

Herein we report the asymmetric addition of phenylboronic acid to different cycloalkenones, mediated by a Rh(I) complex with the tropos phosphoramidite (S)-L₁ or the nontropos phosphoramidite (S)-L₂ ligand. Different values of enantiomeric purity of the 3-phenylcycloalkanone products have been obtained, mainly depending on the ring size of the substrates, with (S)-L₁ affording higher ee values than (S)-L₂. These results could be explained by reasoning that for the tropos phosphoramidite, a ‘chiral pocket’ is formed when the ligand links to the metal atom. By fitting in this pocket, the substrate can stereoselectively coordinate to the Rh(I) atom, undergoing attack of the phenyl ring from the phenylboronic acid to a preferred enantioface. This causes the high (up to 96%) values of enantioselectivity observed with some of the substrates. A ‘chiral pocket’ cannot be formed with the nontropos ligand (S)-L₂ and, as a consequence, the stereoselectivity is greatly reduced.     

Catalytic asymmetric synthesis of Leukotriene B4

Leukotriene B₄ 1 was prepared from two chiral synthons 8 and 14. The chiral secondary alcohols of 8 and 14 were constructed by BINOL/Ti(OiPr)₄ catalyzed enantioselective alkynylzinc addition to aldehydes.     

Chiral thiophosphoramide and selenophosphoramide ligands in the Cu(I)-promoted catalytic enantioselective 1,3-dipolar cycloaddition reactions of azomethine ylides and pyrrole-2,5-dione derivatives

Chiral C₂-symmetric diphenylthiophosphoramide ligand L1 prepared from C₂-symmetric (1S,2S)-(−)-1,2-diphenylethylenediamine was found to be a fairly effective chiral ligand for Cu(I)-promoted 1,3-dipolar cycloaddition of imines and pyrrole-2,5-dione derivatives to give the corresponding adducts in moderate enantioselectivities and good yields.     

A new chiral phosphine oxide ligand for enantioselective 1,3-dipolar cycloaddition reactions of azomethine ylides

A new set of phosphorus-containing chiral ligands has been synthesized and used with silver(I) for the catalytic asymmetric 1,3-dipolar cycloaddition reaction of azomethine ylides. One of these ligands (POFAM6) was found to produce an effective catalyst for the asymmetric 1,3-dipolar cycloaddition reaction of azomethine ylides with electron deficient dipolarophiles to form pyrrolidines in up to 97% yield and 89% ee.     

Chiral diphenylthiophosphoramides: a new class of chiral ligands for the silver(I)-promoted enantioselective allylation of aldehydes

Chiral C₂-symmetric diphenylthiophosphoramides 1 and 2 were prepared in high yields from the reaction of diphenylthiophosphinic chloride with (1R,2R)-(−)-1,2-diaminocyclohexane and (1R,2R)-(+)-1,2-diphenylethylenediamine, respectively. Another novel chiral ligand 4 was prepared from reaction of diphenylthiophosphinic chloride with (R)-(+)-1,1′-binaphthyl-2,2′-diamine using butyllithium as a base. They were used as catalytic chiral ligands in the silver(I)-promoted enantioselective allylation reaction of aldehydes with allyltributyltin.     

Dinuclear and polynuclear silver(I) saccharinate complexes with 1,3-diaminopropane and N-methylethylenediamine constructed from Ag?C interactions

New dinuclear and polynuclear Ag(I) complexes with the formula of [Ag₂(sac)₂(pen)₂] (1) and [Ag₂(sac)₂(nmen)]_n (2), (sac = saccharinate, pen = 1,3-diaminopropane, nmen = N-methylethylenediamine) have been synthesized and characterized by IR spectroscopy and thermal (TG/DTG, DTA) analysis. In addition, their structures were determined by single crystal X-ray diffraction technique. In 1, Ag(I) ions are doubly bridged by two pen ligands, besides pen ligands exhibit an interesting coordination mode by binding bridging ligand. Sac ligands connect to silver atom through its imino N atom. Furthermore, each Ag(I) ion exhibits a T-shaped coordination geometry. In 2, Ag(I) coordination environment is again T-shaped, including weak Ag-Ag bonds. The sac exhibits bidentate bringing mode, involving its imino nitrogen and carbonyl oxygen atoms, besides, bridging of Ag(I) centres by sac ligands results in argentophilic contacts. The polymeric units are assembled into two-dimensional networks by hydrogen bonds, C-H?π stacking interactions, weak Ag?C_sac (η²) and Ag?O interactions.     

Enantioselective Ag(I)-catalyzed [3+2] cycloaddition of azomethine ylides using a chiral ferrocene-based phosphine–phosphoramidite ligand having a stereogenic P-center

A series of chiral ferrocene-based phosphine–phosphoramidite ligands (PPFAPhos) have been employed in the Ag(I)-catalyzed asymmetric [3+2] cycloaddition of azomethine ylides with dimethyl maleate. The results showed that the ligand with a stereogenic P-center in the phosphino moiety displayed the best diastereo- and enantioselectivities, in which up to 99% enantiomeric excesses and 99/1 endo/exo selectivities have been achieved.     

Synthesis and X-ray structures of rhodium complexes with new chiral biaryl-based NHC-ligands

A new series of chiral NHC–rhodium complexes has been prepared from the reactions between [Rh(COD)Cl]₂, NaOAc, KI and dibenzimidazolium salt 4a or monobenzimidazolium salts 4b–d, which are derived from chiral 2,2′-diamino-6,6′-dimethyl-1,1′-biphenyl, 2,2′-diamino-1,1′-binaphthyl or 6,6′-dimethyl-2-amino-2′-hydroxy-1,1′-biphenyl. The steric and electronic effects of the ligand play an important role in the complex formation. For example, treatment of chiral monobenzimidazolium salt 4b (with a NMe₂ group) with 0.5 equiv of [Rh(COD)Cl]₂ in the presence of NaOAc and KI in CH₃CN at reflux gives a chiral Rh(I) complex 5b, while chiral monobenzimidazolium salt 4d (with a MeO group) affords a racemic Rh(I) complex 5d. Under similar reaction conditions, treatment of dibenzimidazolium salt 4a with 0.5 equiv of [Rh(COD)Cl]₂ in the presence of NaOAc and KI gives a racemic Rh(III) complex 5a, while the dibenzimidazolium salt [C₂₀H₁₂(C₇H₅N₂Me)₂]I₂ derived from chiral 2,2′-diamino-1,1′-binaphthyl affords a chiral Rh(III) complex [C₂₀H₁₂(C₇H₄N₂Me)₂]RhI₂(OAc). All compounds have been characterized by various spectroscopic techniques, and elemental analyses. The solid-state structures of the rhodium complexes have been further confirmed by X-ray diffraction analyses.     

A class of α-amino acids-derived multifunctional amidophosphane precatalysts: application to the highly enantio- and diastereoselective silver(I)-catalyzed 1,3-dipolar cycloaddition reaction

A class of multifunctional amidophosphanes derived from chiral α-amino acids have been developed with two amide bonds, a tertiary amine and a phosphine. In combination with Ag(I) salts, these amidophosphanes have been demonstrated as highly efficient multifunctional catalysts in the asymmetric 1,3-dipolar cycloaddition of azomethine ylides as well as the three-component reaction of the α-iminoesters in situ generated. Under optimal conditions, highly functionalized endo-8 pyrrolidines were obtained with good to excellent yields (up to 99% yield) and enantioselectivities (up to 98% ee).     

Nickel(I) complex as the final product of the sequence of spontaneous transformations in the system Ni(allyl)2-(2,6-diisopropylphenyl)diazabutadiene

A reaction of Ni(Allyl)₂ with bis(2,6-diisopropylphenyl)diazabutadiene gave an imino amide allyl nickel(II) complex (I). Complicated rearrangements of the imino amide ligand in the coordination sphere of complex I spontaneously yielded a paramagnetic Ni(I) ??-allyl complex as a final reaction product. The nickel complexes produced in this system were studied by EPR, IR, and 2D NMR spectroscopy and mass spectrometry. Structure I was examined by X-ray diffraction.     

Enantioselective 1,4-addition of arylboronic acids to α,β-unsaturated carbonyl compounds catalyzed by rhodium(I)-chiral phosphoramidite complexes

A chiral bidentate phosphoramidite (5a) was synthesized from Shibasaki’s linked-(R)-BINOL and P(NMe₂)₃ as a new ligand for rhodium(I)-catalyzed asymmetric 1,4-addition of arylboronic acids to α,β-unsaturated carbonyl compounds. The effects of 5a and Feringa’s monodentate phosphoramidite (4, R¹, R² = Et) on the yields and enantioselectivities were fully investigated. The reaction was significantly accelerated in the presence of a base such as KOH and Et₃N, allowing the reaction to be completed at the lower temperatures than 50 °C. The addition to cyclic enones such as 2-cyclopentenone, 2-cyclohexenone and 2-cycloheptenone at 50 °C in the presence of an [Rh(coe)₂Cl]₂-4 (R¹, R² = Et) complex resulted in enantioselectivities up to 98%, though it was less effective for acyclic enones (0–70% ee). On the other hand, a complex between [Rh(nbd)₂]BF₄ and 5a completed the addition to cyclic enones within 2 h at room temperature in the presence of Et₃N with 86–99% yields and 96–99.8% ee. This catalyst was also effective for acyclic enones, resulting in 62–98% yields and 66–94% ee. The 1,4-additions of arylboronic acids to unsaturated lactones and acyclic esters with rhodium(I)-phosphoramidites complexes were also investigated.     

Novel silver(I)-saccharinate complexes exhibiting Ag···π and C-H···Ag close interactions with a new coordination mode of saccharinate

Two novel silver(I) complexes, namely [Ag₂(μ₃-sac)₂(μ-nmpen)]_n (1) and [Ag(sac)(mpr)]₂ (2) (sac = saccharinate; nmpen = N-methyl-1,3-propanediamine; mpr = 2-methyl-1-pyrroline) have been synthesized and characterized by IR spectra, thermal (TG, DTG and DTA) analysis and single crystal X-ray diffraction techniques. Complexes crystallize in the monoclinic system with the space group C2/c and P2₁/c, respectively. In 1, Ag(I) ion exhibits a distorted tetrahedral geometry by tridentate μ₃-bridging sac and μ₂-bridging nmpen ligands. The sac ligand exhibits a new μ₃-coordination mode by means of μ₂-bridging O atom of sulfonyl group and N atom of imino group. Furthermore, complex 1 exhibits a two-dimensional polynuclear structure. In 2, the silver(I) ion is linearly coordinated by the N atoms of a sac and a mpr ligands, forming mononuclear species. The individual molecules are linked into dimers by Ag···C_sac (η¹) interactions between silver(I) ion and phenyl ring of the adjacent complex and these dimers are assembled into two-dimensional layered networks through weak Ag···Ag (3.507 Å), SO···Ag (2.961 Å) and π···π interactions. The most interesting structural features of complexes is the presence of obvious C-H···M hydrogen-bonding interactions between the Ag centers and H atoms of nmpen or mpr ligands.     

Rhodium complexes with chiral counterions: achiral catalysts in chiral matrices

The neutral complexes [Rh(I)(NBD)((1S)-10-camphorsulfonate)] (2) and [Rh(I)((R)-N-acetylphenylalanate)] (4) reacted with bis-(diphenylphosphino)ethane (dppe) to form the cationic Rh(I)(NBD)(dppe) complexes, 5 and 6, respectively, accompanied by their corresponding chiral counteranions. Analogously, 4 reacted with 4,4^′-dimethylbipyridine to yield complex 7. Complexes 5 and 6 disproportionated in aprotic solvents to form the corresponding bis-diphosphine complexes 8 and 9, respectively. 8 was characterized by an X-ray crystal structure analysis. In order to form achiral Rh(I) complexes bearing chiral countercations new sulfonated monophosphines 13-16 with chiral ammonium cations were synthesized. Tris-triphenylphosphinosulfonic acid (H₃TPPS, 11) was used to protonate chiral amines to yield chiral ammonium phosphines 14-16. Thallium-tris-triphenylphosphinosulfonate (Tl₃TPPS, 12) underwent metathesis with a chiral quartenary ammonium iodide to yield the proton free chiral ammonium phosphine 13. Phosphines 15 and 16 reacted with [Rh(NBD)₂]BF₄ to afford the highly charged chiral zwitterionic complexes [Rh(NBD)(TPPS)₂][(R)-N,N-dimethyl-1-(naphtyl)ethylammonium]₅ (17) and [Rh(NBD)(TPPS)₂][BF₄][(R)-N,N-dimethyl-phenethylammonium]₆ (18), respectively. Complexes 5, 6, and 18 were tested as precatalysts for the hydrogenation of de-hydro-N-acetylphenylalanine (19) and methyl-(Z)-(α)-acetoamidocinnamate (MAC, 20) under homogeneous and heterogeneous (silica-supported and self-supported) conditions. None of the reactions was enantioselective.     

New silver(I)–monophosphine complex derived from chiral Ar-BINMOL: synthesis and catalytic activity in asymmetric vinylogous Mannich reaction

A new family of C₂-axially chiral monophosphines (Ar-NNPs) from Ar-BINMOLs was developed for silver-catalyzed asymmetric vinylogous Mannich reaction of (furan-2-yloxy)trimethylsilane with aromatic aldimines. It was found that the enantioselective vinylogous Mannich-type reactions of trimethylsiloxyfuran with aldimines are catalyzed efficiently by silver(I) complexes of the Ar-BINMOL-derived chiral monophosphine. This procedure displays wide aldimine versatility, excellent yields (up to 99% isolated yields), moderate to good enantioselectivity (up to 78%ee) and exceptional diastereoselectivity (>99:1 dr) in most cases examined. The molecular structure of silver–monophosphine complex was confirmed by X-ray analysis and revealed that the benzyl group on chiral monophosphine provided dual function with weak silver–π/π–π stacking and steric repulsion to favour the diastereoselective Re-nucleophilic addition of siloxyfuran to imine.     

Dual activation in oxidative coupling of 2-naphthols catalyzed by chiral dinuclear vanadium complexes

An efficient enantioselective oxidative coupling of 2-naphthol derivatives based on a concept of dual activation catalysis is realized. A chiral dinuclear vanadium(IV) complex (R_a,S,S)-1e possessing (S)-tert-leucine moieties at the 3,3′-positions of the (R)-binaphthyl skeleton was developed, which was found to promote the oxidative coupling of 2-naphthol to afford (S)-BINOL with 91% ee. To verify the dual activation mechanism, mononuclear vanadium(IV) complex (S)-8 was also prepared. Kinetic analysis revealed that the reaction rate of oxidative coupling of 2-naphthol promoted by (R_a,S,S)-1e is 48.3 times faster than that of (S)-8. The two vanadium metals in the chiral complex activate two molecules of 2-naphthol simultaneously in an intramolecular manner coupling reaction, achieving a high reaction rate with high enantiocontrol. Reaction mechanisms of the oxidative coupling reaction promoted by either vanadium(IV) or vanadium(V) complexes are also described.     

The first example of a (1R,2S,5R)-(−)-menthyl chiral auxiliary in intramolecular nitrilimine cycloadditions: synthesis of enantiopure pyrazolo[1,5-a][4,1]benzoxazepines and pyrazolo[1,5-a][4,1]benzodiazepines

By using (1R,2S,5R)-(−)-menthyl as a chiral auxiliary, we have developed a synthesis of hydrazonoyl chlorides 2 and 8, treatment of which with silver carbonate promoted the in situ generation of the corresponding nitrilimines 3 and 9. The latter underwent intramolecular cycloaddition giving, respectively, enantiopure pyrazolo[1,5-a][4,1]benzoxazepines 4, 5 and pyrazolo[1,5-a][4,1]benzodiazepines 10.     

Bifunctional AgOAc/ThioClickFerrophos complex-catalyzed asymmetric 1,3-dipolar cycloaddition of azomethine ylides with aryl- and alkylidene malonates

AgOAc/ThioClickFerrophos complex-catalyzed 1,3-dipolar cycloaddition of glycine imino ester, the precursor of azomethine ylide, with aryl- and alkylidene malonates afforded the corresponding exo-cycloadducts, that is, proline ester derivatives in high yields with high enantiomeric excess (up to 99% ee). The reactions proceeded smoothly under base-free conditions demonstrating the bifunctional catalysis of the silver complex.     

Copolymerization of Ethylene with lO-Undecen-l-ol Using Highly Active Vanadium（III） Precatalysts Bearing Bis（imino）pyrrolyl Ligands

The copolymerizations of ethylene with 10-undecen-1-ol have been investigated using vanadium precatalysts, bis（imino）pyrrolyl vanadium（Ⅲ） complexes 1-3, 2,5-C4H2N（CH=NR）2VCl2（THF）2 [R = C6H5 （1）, 2,6-iPr2C6H3 （2）, C6F5 （3）], and the iminopyrrolyl and b-diketiminate ones for comparison. The polar monomer was pretreated by diethylaluminium chloride （present also as the cocatalyst） before the copolymerization. The monomer reactivity ratios were evaluated using the Fineman-Ross method. The ligand structure considerably influenced the catalytic activity and tolerance towards the polar monomer, the polar monomer incorporation and the molecular weights of the resultant copolymers. The bis（imino）pyrrolyl vanadium complexes exhibited promising catalytic performance for the copolymerization, and a high catalytic activity up to 3.84 kg/mmolv·h with a high comonomer incorporation of 14.0 mol% was achieved by complex 3 under mild conditions.     

Axially chiral phosphine–oxazoline ligands in the silver(I)-catalyzed asymmetric Mannich reaction of fluorinated aldimines with trimethylsiloxyfuran

Axially chiral phosphine–oxazoline ligand L7, prepared from (S)-binol, was found to be a fairly effective chiral ligand in the silver(I)-catalyzed asymmetric Mannich reaction of fluorinated aldimines with trimethylsiloxyfuran to give the corresponding adducts in up to 99% yield, over 20:1 dr and 81% ee.     

Synthesis and characterization of asymmetric NHC complexes

New chiral and non-chiral rhodium(I)–NHC complexes were synthesized. The first attempt by deprotonation of an imidazolinium salt with KOtBu and reaction with [Rh(COD)Cl]₂ leads to the corresponding rhodium(I) complex. Due to the basic conditions during the reaction a loss of chirality occurs. An alternative transmetallation reaction with a silver(I)–NHC complex yields the desired rhodium(I)–NHC complex under retention of chirality. Both Rh complexes were fully characterized by analytical methods.