C2- and Cs-symmetric diamidodichlorotitanium complexes: Syntheses, structures and 1-hexene polymerization catalysis

The complexes l-[CMe₂{CHMeN(2-Prⁱ-C₆H₄)}₂TiCl₂] (2) and (3), C₂- and C_s-symmetric analogues, respectively, of McConville’s C_2v hexene polymerization precatalyst (1), were prepared by high-dilution salt-elimination from the lithium amides and characterized spectroscopically and crystallographically. Complex 2, though less active than 1, was a highly active catalyst of polymerization of 1-hexene when activated by MAO. Complex 3 was inactive under similar conditions. NMR analysis confirmed that there were more mmmm pentads in polymer produced by 2 than in the statistically atactic material produced by 1, though the isotacticity index was not high. The results are interpreted in terms of an isotactic/atactic block structure, caused by syn/anti fluxion of the two 2-isopropylphenyl rings in 2. Kinetic profiles and polydispersities were consistent with a slow initiation step involving monomer, followed by rapid propagation, with some chain transfer to aluminium and only a small extent of β-hydride elimination. The rubber-like polymers were indistinguishable by thermal analysis from those prepared by Ziegler-Natta catalyst systems.     

Synthesis of cis bis-β-lactams via Staudinger cycloaddition reaction using C2-symmetric 1,2-diamines

An efficient stereoselective synthesis of bis-β-lactams via cycloaddition reaction (Staudinger reaction) of ketenes with bisimines derived from C₂-symmetric 1, 2-diamines is described. The reaction provided diastereomeric mixture of meso and C₂-symmetric cis-bis-β-lactams with higher selectivity for meso-bis-β-lactams.     

Synthesis and diastereoselective Diels-Alder reactions of homochiral C2-symmetric butane-1,2-diacetal-based 1,3-dienes

The C₂-symmetric, butane diacetal (BDA) auxiliary-based dienes 2 and 3 are described, which display moderate to excellent diastereoselectivities in Diels-Alder reactions with a range of dienophiles under thermal and Lewis acid-catalysed conditions.     

A novel and efficient synthesis of chiral C2-symmetric 1,4-diamines

A novel and efficient method for synthesis of (R,R)- and (S,S)-C₂-symmetric 1,4-diamines was established. The key steps are a combination of Pinacol Coupling and Corey-Winter olefination.     

Synthesis of C1-symmetric chiral tripodal oxazolines through an oxazoline exchange reaction with amino alcohols

Various C₁-symmetric chiral tripodal tris(oxazolines) with two different oxazoline units were synthesized from chiral C₃-symmetric tris(oxazolines) through an oxazoline exchange reaction with amino alcohols in the presence of zinc chloride. Evaluation of the new oxazolines as chiral molecular receptor showed that some of the receptors have chiral discrimination ability.     

Synthesis and structure of C2-symmetric N-heterocyclic carbene complexes of palladium

The first 9- and 11-membered cis and trans C₂-symmetric benzimidazol-2-ylidene palladium(II) complexes based on a trans-2,2-dimethyl-1,3-dioxalane backbone were synthesised and the configuration of the complexes was elucidated via NMR and X-ray crystallography.     

Synthesis of C2-symmetric chiral crown ethers by lipase-catalyzed reactions

Kinetic resolution of a racemic mixture of C₂-symmetric 18-crown-6 diols (rac-1a) and 15-crown-5 diol (rac-1c) was achieved by lipase-catalyzed acetylation. The enantiomeric excess of the chiral crown diols (95% ee and 82% ee) was determined by ¹H NMR spectroscopy, using (R)-(+)-1-(1-naphthyl)ethylammonium hydrochloride as a shift reagent. The C₂-symmetric chiral 15-crown-5 diol (>95% ee) was also obtained by kinetic resolution of the racemic diacetate (rac-2c) using lipase-catalyzed solvolysis.     

Synthesis of a C3-symmetric macrocycle with alternating sugar amino acid and tyrosine residues

A C₃-symmetric macrocycle with alternating sugar amino acid and tyrosine residues was synthesized in seven steps from tyrosine tert-butyl ester and a sugar amino acid precursor derived from D-glucosamine. An Fmoc-protected D-glucosamine derivative was oxidized at C-6 to give the sugar amino acid, which was immediately coupled to tyrosine tert-butyl ester to produce an orthogonally protected building block. This building block was subsequently elongated to the trimer via the dimer, and finally cyclized to give the C₃-symmetric macrocycle.     

Recognition of dihydroxynaphthalenes by a C2-symmetric host

A functionalized C₂-symmetric host (2) shows high affinity and substrate selectivity for dihydroxynaphthalenes.     

C2-Symmetric bipyrrolidines as organocatalysts for asymmetric Diels-Alder reactions

A new class of C₂-symmetric 3,3′-dialkoxy-2,2′-bipyrrolidines have been designed and developed for asymmetric organocatalytic Diels-Alder reactions of α,β-unsaturated aldehydes. The bipyrrolidines combined with HClO₄ were found to be effective organocatalysts for enantioselective Diels-Alder reactions. The catalysis mode has been demonstrated by NMR and X-ray crystallographic studies for diiminium intermediate.     

Copper(II)-containing C2-symmetric bistetracarboline amides in enantioselective Henry reactions

A series of new C₂-symmetric chiral diamides were synthesized from l-tryptophan and used as chiral ligands chelated with Cu(II) in enantioselective Henry reactions. Ligand 4a with CuCl₂·2H₂O (5%) showed effective catalytic efficiency in Henry reaction. High yields (up to 99%) and excellent enantioselectivities (up to 98%) were achieved for both aromatic and aliphatic aldehyde substrates.     

Synthesis and MMA polymerization of chiral ansa-zirconocene ester enolate complexes with C2- and Cs-ligation

A series of chiral ansa-zirconocene ester enolate complexes incorporating C₂- or C_s-symmetric ligands, including neutral rac-(EBI)ZrCl[OC(OⁱPr)CMe₂] (1), rac-(EBI)Zr(OTf)[OC(OⁱPr)CMe₂] (2), rac-(EBI)Zr(OTf)[OC(OMe)C(Me)CH₂C(Me₂)C(OⁱPr)O] (3), [Me₂C(Cp)(Flu)]ZrMe[OC(OⁱPr)CMe₂] (4), and cationic [Me₂C(Cp)(Flu)]Zr⁺(THF)[OC(OⁱPr)CMe₂][MeB(C₆F₅)₃]⁻ (5), have been synthesized. Within the neutral C₂-ligated zirconocene ester enolate series, the chloride derivative 1 is inactive toward any methyl methacrylate (MMA) additions, the methyl derivative rac-(EBI)ZrMe[OC(OⁱPr)CMe₂] adds cleanly only 1 equiv. of MMA, and the triflate derivative 2 can add either 1 equiv. of MMA to form the single-MMA-addition product 3 or multiple equivalents of MMA to form P(MMA). Unlike the C_s-ligated methyl cation [Me₂C(Cp)(Flu)ZrMe]⁺, which is inactive for MMA polymerization under various conditions, the C_s-ligated ester enolate cation 5 is moderately active for polymerization of MMA and N,N-dimethylacrylamide at ambient temperature; the resulting P(MMA) has a high molecular weight of M_n = 388 000 Da but a low syndiotacticity of [rr] = 64%, and the polymerization conforms to a chain-end control mechanism.     

Synthesis and applications of C2-symmetric guanidine bases

The preparation of the tetracyclic C₂-symmetric guanidinium salts 5 and 11-13 is reported together with their application to enantioselective transformations.     

A simple, novel and convenient method for the synthesis of 1-aminophosphinic acids: synthesis of a novel C2-symmetric phosphinic acid pseudodipeptide

A simple, efficient and novel method has been developed for the synthesis of 1-aminophosphinic acids from simple starting materials. Treatment of aromatic aldehydes with ammonia and hypophosphorus acid gives novel C₂-symmetric 1-aminoarylmethylphosphinic acids. The synthesis of novel C₂-symmetric phosphinic acid pseudodipeptides is also discussed.     

Enantioselective addition of diethylzinc to aromatic aldehydes catalyzed by Ti(IV) complexes of C2-symmetrical chiral BINOL derivatives

Enantioselective addition of diethylzinc to a series of aromatic aldehydes is developed using new chiral C₂-symmetric ligand (S)-2,2′-(1,1′-binaphthyl-2,2′-diylbis(oxy))bis(methylene)bis(4-nitrophenol) (S)-2b. The catalytic system employing 10 mol % of (S)-2b and 120 mol % of Ti(OiPr)₄ was found to promote the addition of diethylzinc to a wide range of aromatic aldehydes with electron-donating and electron-withdrawing substituents, giving up to 89% ee and up to 95% yield of the corresponding secondary alcohol under mild conditions.     

Synthesis of C2-symmetric trans-2,6-diarylpiperidinones via aryl cuprate addition: an unexpected stereochemical outcome

An efficient procedure for the preparation of trans-2,6-diaryl piperidinones has been developed. Addition of aryl Grignard reagents to 2-aryl dihydropyridones under catalytic copper promoted conditions generates the trans isomer exclusively, an unprecedented stereochemical event. The X-ray structures of both starting material and product have been solved and shed light on the steric constraints and substrate geometry leading to the observed product. The reaction conditions tolerate a variety of aromatic nucleophiles to generate C₂-symmetric products in good overall yields.     

The design and synthesis of a new potentially C3-symmetric ferrocenylphosphine

A new potentially C₃-symmetric phosphine ligand ‘manphos’ has been obtained and fully characterized. The ligand which is a tri-ferrocenyl-tetra-phosphine is obtained in a simple and effective two step synthesis starting from 1,1′-dibromoferrocene via the intermediate compound tris-(1′-bromoferrocenyl)phosphine or alternatively via 1'-bromo-1-diphenylphosphinoferrocene. The iso-propylphosphino-analogue of manphos, tris-(1'-diisopropylphosphinoferrocenenyl)phosphine, has also been obtained, in addition to several functionalised derivatives of triferrocenylphosphine where the ferrocene rings have been substituted in the 1′-position.     

Synthesis of C3-Symmetric star-shaped molecules containing 1,3-azoles via hetero-aryl Heck coupling

We have demonstrated a useful synthetic strategy to assemble star-shaped C₃-symmetric molecules containing 1,3-azole moieties at the periphery. To generate these C₃-symmetric heterocycles, we have employed the Pd/Cu-based coupling reactions. To this end, we have used benzoxazole, benzothiazole, and benzimidazole as coupling partners to generate the corresponding hetero-aryl Heck coupling products.     

Development of a new class of C1-symmetric bisphosphine ligands for rhodium-catalyzed asymmetric hydrogenation

A new class of C₁-symmetric bisphosphine ligands with three hindered quadrants have been obtained through facile synthesis from chiral BINOL derivatives. Their rhodium complexes have exhibited high enantioselectivities (up to 98% ee) in the asymmetric hydrogenation of various unsaturated prochiral olefins, providing an efficient catalytic system for the enantioselective synthesis of chiral amino acids and amines.     

A new C2-symmetric azolium compound for Cu-catalyzed asymmetric conjugate addition of R2Zn to cyclic enone

A new chiral N-heterocyclic carbene (NHC) ligand was designed. Thus, an efficient synthetic route to C₂-symmetric bis(hydroxyamide)-functionalized benzimidazolium salts from chiral β-amino alcohols was developed. The combination of Cu(OTf)₂ and the chiral azolium compound efficiently promoted the conjugate addition reaction of cyclic enone with dialkylzinc to give the corresponding adduct in good yield. Among a series of chiral NHC proligands, the functionalized benzimidazolium chloride possessing a tert-butyl group as a stereodirecting group was found to be the best choice of ligand. Under optimized reaction conditions, an excellent enantioselectivity (96% ee) was realized by allowing 2-cyclohepten-1-one to react with Bu₂Zn at room temperature.