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Relevant academic research and scientific papers

Phosphorus-Based Organocatalysis for the Dehydrative Cyclization of N-(2-Hydroxyethyl)amides into 2-Oxazolines

A metal-free, biomimetic catalytic protocol for the cyclization of N-(2-hydroxyethyl)amides to the corresponding 2-oxazolines (4,5-dihydrooxazoles), promoted by the 1,3,5,2,4,6-triazatriphosphorine (TAP)-derived organocatalyst tris(o-phenylenedioxy)cyclotriphosphazene (TAP-1) has been developed. This approach requires less precatalyst compared to the reported relevant systems, with respect to the phosphorus atom (the maximum turnover number (TON) ～30), and exhibits a broader substrate scope and higher functional-group tolerance, providing the functionalized 2-oxazolines with retention of the configuration at the C(4) stereogenic center of the 2-oxazolines. Widely accessible β-amino alcohols can be used in this approach, and the cyclization of N-(2-hydroxyethyl)amides provides the desired 2-oxazolines in up to 99% yield. The mechanism of the reaction was studied by monitoring the reaction using spectral and analytical methods, whereby an 18O-labeling experiment furnished valuable insights. The initial step involves a stoichiometric reaction between the substrate and TAP-1, which leads to the in situ generation of the catalyst, a catechol cyclic phosphate, as well as to a pyrocatechol phosphate and two possible active intermediates. The dehydrative cyclization was also successfully conducted on the gram scale.

Rhodium(III)-Catalyzed Asymmetric Borylative Cyclization of Cyclohexadienone-Containing 1,6-Dienes: An Experimental and DFT Study

Because of the inherent difficulty in differentiating two olefins, the development of metal-catalyzed asymmetric cyclization of 1,6-dienes remains challenging. Herein, we describe the first rhodium(III)-catalyzed asymmetric borylative cyclization of cyclohexadienone-tethered mono-, 1,1-di-, and (E)-1,2-disubstituted alkenes (1,6-dienes), affording optically pure cis-bicyclic skeletons bearing three or four contiguous stereocenters with high yields (25-93%), and excellent diastereoselectivities (>20:1 dr) and enantioselectivities (90-99% ee). This mild catalytic approach is generally compatible with a wide range of functional groups, which allows several facile conversions of the cyclization products. Furthermore, on the basis of our SAESI-MS experiment and computational study, a Rh(I)/(III) catalytic cycle is proposed in this tandem reaction, and the Rh(I) active species catalyzes the overall transformation via sequential oxidative addition of B2pin2, olefin insertion, cyclizing conjugate addition, and reductive elimination. The irreversible conjugate addition determines the overall regioselectivity of borylative cyclization, and the ring strain favors the formation of 5,6-bicyclic structure. This highlights the control of ring strain in diene cyclizations, which provides a useful basis for future reaction designs.

Surfactant-Assisted Nanocrystalline Zinc Coordination Polymers: Controlled Particle Sizes and Synergistic Effects in Catalysis

Different morphologies and particle sizes of two crystalline zinc-based coordination polymers (CPs), [Zn(pytz)H2O]n (1; H2pytz=2,6-bis(tetrazole)pyridine) and [Zn2(pytz)24H2O] (2), from the bulk scale to the nanoscale, could be obtained under solvothermal conditions with different surfactants (polyvinylpyrrolidone (PVP) or polyethylene glycol (PEG) 2000) as templates. PVP and PEG 2000 could act as capping and structure-directing agents, respectively, to influence the growth of crystalline particles and control their sizes. CP 1 exhibits a two-dimensional framework with window-like units and 2 shows a bimetallic structure. Nanocrystalline 1 and 2 were used as heterogeneous catalysts to study how adjacent catalytic active sites synergistically effected their catalytic reactivities in the direct catalytic conversion of aromatic dinitriles into oxazolines. The results showed that 1 produced bis-oxazolines as the sole products, whereas 2 gave the mono-oxazolines as the major products under the same reaction conditions.

NCN-pincer metal complexes (Ti, Cr, V, Zr, Hf, and Nb) of the phebox ligand (S,S)-2,6-bis(4′-isopropyl-2′-oxazolinyl)phenyl

Reaction of (S,S)-2,6-bis(4′-isopropyl-2′-oxazolinyl) phenyllithium (i-Pr-Phebox-Li) (2a) with 4,4′-bis[P-(chlorogold(I)) diphenylphosphino]biphenyl [(dppbp)(AuCl)2] (5) afforded the new, bimetallic gold complex 4,4′-bis[P-(η1-C-i-Pr

A facile and efficient synthesis of bis(oxazoline)s

Thiophene-2,5-dicarboxylic acid, benzene-1,3-dicarboxylic acid, or furan-2,5-di-carboxylic acid, respectively, reacted with various β-amino alcohols in toluene under reflux within 24 h, to form nine bis(oxazoline)s (1-3) in good yields through water deprivation via a one-pot reaction. The synthetic method is facile and efficient and deserves great application potentials in the research and development in the area of bis(oxazoline)s.

Asymmetric conjugate reduction of α,β-unsaturated ketones and esters with chiral rhodium(2,6-bisoxazolinylphenyl) catalysts

New asymmetric conjugate reduction of β,β-disubstituted α,β-unsaturated ketones and esters was accomplished with alkoxylhydrosilanes in the presence of chiral rhodium(2,6-bisoxazolinylphenyl) complexes in high yields and high enantioselectivity. (E)-4-Phenyl-3-penten-2- one and (E)-4-phenyl-4-isopropyl-3-penten-2-one were readily reduced at 60°C in 95% ee and 98 % ee, respectively, by 1 mol % of catalyst loading. (EtO) 2MeSiH proved to be the best hydrogen donor of choice. tert-Butyl (E)-β-methylcinnamate and β-isopropylcinnamate could also be reduced to the corresponding dihydrocinnamate derivatives up to 98% ee.

Asymmetric conjugate reduction of α,β-unsaturated esters with chiral rhodium(bisoxazolinylphenyl) catalysts

Chiral rhodium(bisoxazolinylphenyl) complexes reduced α,β- unsaturated esters in high enantioselectivity up to 97-98% ee in the combination of alkoxyhydrosilanes.

Preparation of chiral phosphorus, sulfur and selenium containing 2-aryloxazolines

A series of enantiomerically pure 2-[2-(diarylphosphino)aryl]-oxazolines was prepared from commercially available or synthetic amino alcohols. For oxazoline formation three procedures were employed: (i) one pot condensation with a 2-halobenzoic acid (ii) ZnCl2 catalyzed condensation with a 2-halobenzonitrile, and (iii) a three step sequence via a 2-halobenzamide and a tosylate or chloride. Phosphinooxazolines containing stereogenic phosphorous were prepared by either diastereoselective nucleophilic substitution of halogenide of Ar1Ar2PCl or by nucleophilic aromatic substitution with LiPAr1Ar2. In addition, sulfur and selenium analogs were prepared.

Synthesis of Optically Active Bis(2-oxazolines): Crystal Structure of a 1,2-Bis(2-oxazolinyl)benzene * ZnCl2 Complex

Dinitriles (5-7, 12, 13) react with enantiomerically pure β-amino alcohols (8-11, 17) under zinc chloride catalysis to give optically active C2-symmetric bis(oxazolines). 1,2-Bis(2-oxazolin-2-yl)benzenes 1a-e are obtained under mild reaction conditions. 1H-NMR spectroscopy indicates the formation of 1:1 complexes 23 of these compounds with ZnCl2.The energy required for a conformational interconversion of zinc dichloride complex 23e was determined by variable-temperature 1H-NMR studies.An X-ray structure analysis was performed with the substituted zinc dichloride complex 23a.