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.     

Ring opening of epoxides with alcohols using Fe(Cp)2BF4 as catalyst

Fe(Cp)₂BF₄ is an efficient catalyst for the alcoholysis of aromatic, aliphatic, and cyclic epoxides giving excellent yields of the corresponding β-alkoxy alcohols under ambient conditions. The methanolysis of styrene oxide using Fe(Cp)₂BF₄ as a catalyst (5 mol %) gave excellent yield of 2-methoxy-2-phenylethanol with complete regio-selectivity. The ring opening of cyclic epoxides gave 77–97% yields of trans-β-methoxy alcohols, in 0.5–6 h. The use of 1,2-epoxyhexane and 1,2-epoxydodecane as substrates gave both regioisomers in excellent yields. The first order rate of reaction with respect to catalyst was observed for the kinetics of ring opening of 1,2-epoxyhexane with methanol.     

(1R,2R)-DPEN-derived triazolium salts for enantioselective oxodiene Diels-Alder reactions

Enantioselective oxodiene Diels-Alder reactions catalyzed by (1R,2R)-DPEN-derived triazolium salts were realized successfully. With 0.5 mol % of (1R,2R)-DPEN-derived triazolium salt C and 150 mol % of Et₃N, the reactions of various α-chloroaldehydes (α-bromoaldehyde) with substituted enones led to 3,4-dihydropyridinones and their derivatives in good yields, diastereoselectivities, and enantioselectivities (up to 97% ee).     

Fe2(SO4)3·xH2O-catalyzed per-O-acetylation of sugars compatible with acid-labile protecting groups adopted in carbohydrate chemistry

Fully acetylated saccharides are inexpensive and very useful starting materials for the synthesis of many naturally occurring glycosides, oligosaccharides, and glycoconjugates. Ferric sulfate hydrate (Fe₂(SO₄)₃·xH₂O) was found to be a valuable Lewis acid promoter in the per-O-acetylation reaction of saccharides with acetic anhydride in 100% of conversion rate and 88-99% yields. Interestingly, the procedure is perfectly compatible with the presence of a variety of acid-labile protecting groups, such as isopropylidene, benzylidene, trityl, and TBDMS groups. The reactions were simply performed by stirring the mixture of a sugar with a slight excessive acetic anhydride in the presence of 2.0 mol % of Fe₂(SO₄)₃·xH₂O at rt and the pure products were obtained by a simple dilution of the reaction mixture with dichloromethane and washings with aqueous Na₂CO₃.     

Structural transformation and oxidation of Sr2MnO3.5+x determined by in-situ neutron powder diffraction

The oxidation of the n = 1 Ruddlesden-Popper phase, Sr₂MnO_3.5+x, where 0 ≤ x ≤ 0.5 has been investigated using a combination of in-situ diffraction techniques. In agreement with previous reports the room temperature structure of Sr₂MnO_3.5+x was determined to be monoclinic crystallising in space group P2₁/c. On heating in air the material undergoes rapid oxidation at a relatively modest temperature, ∼275 °C. The oxidation process is coincident with a significant change in the structure, with the material now adopting a tetragonal I4/mmm structure. In the oxygen deficient phase where x > 0 the Mn coordination is square pyramidal, with a sixth partially occupied oxygen position giving rise to octahedral coordination. Oxidation of Sr₂MnO_3.5+x results in the filling of the partially occupied O4 positions and a resulting increase in symmetry, with the Mn coordination now adopting solely a distorted octahedral environment.     

Ferric sulfate [Fe2(SO4)3·xH2O]: an efficient heterogeneous catalyst for the synthesis of tetrahydroquinoline derivatives using Povarov reaction

Fe₂(SO₄)₃·xH₂O can be used as an efficient and reusable catalyst for the synthesis of pyrano- and furanotetrahydroquinolines via one-pot three-component Povarov reaction involving aromatic aldehydes, aromatic amines, and cyclic enol ethers. The catalyst is recyclable, economically viable, and environmentally benign. This protocol provides good yields and diastereoselectivity as well as applicability on a wide range of substrates.     

Rapid access to homoallylic alcohols via Pd(OAc)2 catalyzed Barbier type allylation in presence of DMAP

DMAP was found to accelerate significantly the rate of Pd(OAc)₂ catalyzed Barbier type allylation of carbonyl compounds by allylbromide using SnCl₂·2H₂O as reducing agent. Both aldehyde as well as ketones produced excellent yields within a short reaction time in the presence of 3 mol % of Pd(OAc)₂ and 12 mol % of DMAP at room temperature. Aldehydes could be allylated within 5–10 min whereas, in case of ketones, the reaction completes in 45–120 min.     

Silica gel supported TaBr5: new catalyst for the facile and rapid cyclization of 2′-aminochalcones to the corresponding 2-aryl-2,3-dihydroquinolin-4(1H)-ones under solvent-free conditions

Silica gel supported TaBr₅ (5-10 mol %) is a new solid-support catalyst that can be used under solvent-free conditions for the facile and efficient isomerization of 2′-aminochalcones to the corresponding 2-aryl-2,3-dihydroquinolin-4(1H)-ones. The catalyst is easily prepared, stable and employed under environmentally friendly conditions.     

Platinum(II) complex-catalyzed enantioselective aldol reaction with ketene silyl acetals in DMF at room temperature

[{(R)-binap}Pt(μ-OH)]₂2X is a weak Lewis acid, which can catalyze the enantioselective aldol reaction of aldehydes with ketene silyl acetals in DMF at room temperature. The platinum(II) complex-catalyzed the enantioselective aldol reaction of aldehydes with 1-methoxy-2-methyl-(1-trimethylsilyloxy)propene gave the corresponding aldols in high yields with enantioselectivity up to 92%. With 5 mol % loading of the complexes, the enantioselective aldol reaction of aldehydes with 1-benzyloxy-1-(trimethylsilyloxy)propene smoothly proceeded in DMF containing 10% HMPA as to predominantly give anti-propionates with enantioselectivity up to 89%, irrespective of the silyl nucleophile geometry.     

Catalytic enantioselective allylation of aldehydes using β-amido functionalized allylstannanes with chiral In(OTf)3/i-Pr-pybox complexes

The enantioselective allylation of aldehydes using a variety of β-amido functionalized allyltributylstannanes proceeded smoothly with good to high yields and enantioselectivities in the presence of 10 mol % of a chiral catalytic complex prepared from In(OTf)₃ and 2,6-bis[(S)-4-isopropyloxazolin-2-yl]pyridine {(S)-i-Pr-pybox}, providing the corresponding chiral γ-hydroxy amides.     

Chiral 2-C-methylene glycosides and carbohydrate-derived pyrano[2,3-b][1]benzopyrans: synthesis via InCl3 catalyzed stereoselective Ferrier rearrangement of 2-C-acetoxymethyl glycal derivatives

2-C-Acetoxymethyl glycal derivatives react with aliphatic alcohols in the presence of InCl₃ (30 mol %) to furnish the corresponding 2-C-methylene glycosides in excellent yields and with exclusive α-selectivity except for the methyl 2-C-methylene glycosides, which are formed in ∼2:1 anomeric ratio in favour of the α-anomer. The reaction of 2-C-acetoxyglycals with phenols, however, produces the corresponding chiral carbohydrate-derived pyranobenzopyran derivatives via initial Ferrier rearrangement followed by tandem cyclization in excellent yields and moderate to high stereoselectivities in favour of the corresponding 10a-R-pyrano[2,3-b][1]benzopyran derivatives.     

Structural studies of cerium tantalates

Structural data obtained from neutron diffraction studies of some cerium tantalate phases are presented, including the first report of the high temperature structure of a CeTaO₄ phase, Ce_0.85TaO_3.84 deduced from in situ data recorded at 900°C in vacuum. It was found that this material transformed from the low temperature LaTaO₄ type phase to the orthorhombic A2₁am structure reported here, with a unit cell of a=5.64062(2) Å, b=14.81609(6) Å and c=3.93482(1) Å. This data agrees well with the previously proposed structural transformations.     

Selective cyclization of alkynols and alkynylamines catalyzed by potassium tert-butoxide

Potassium tert-butoxide (t-BuOK) was found to be an effective catalyst for the cyclization of aromatic alkynols and alkynylamines. In the presence of 10 mol % t-BuOK, a range of alkynols were converted to the corresponding exo-cyclic enol ethers as pure Z-stereoisomers with 100% selectivity and moderate to excellent yields. Moreover, the cyclization of alkynylamines was also achieved to afford indoles and isoindolin-1-ones in good yields.     

One-pot three components synthesis of O-acetylcyanohydrins with TMSCN, acetic anhydride and carbonyl compounds under solvent-free condition

One-pot three components synthesis of O-acetylcyanohydrins has been developed in the presence of B(C₆F₅)₃ as the catalyst. Variety of aldehydes or ketones reacts with TMSCN and acetic anhydride (Ac₂O) under the influence of 1 mol % of B(C₆F₅)₃ to give good to excellent yield of the products without solvent at rt.     

A simple, efficient and highly selective deprotection of t-butyldimethylsilyl (TBDMS) ethers using silica supported sodium hydrogen sulfate as a heterogeneous catalyst

t-Butyldimethylsilyl (TBDMS) ethers have been efficiently and selectively deprotected using silica supported sodium hydrogen sulfate (NaHSO₄·SiO₂) as a heterogeneous catalyst at room temperature to regenerate the parent alcohols in high yields.     

A novel Prins-alkynylation reaction for the synthesis of 4-phenacyl tetrahydropyrans

Aldehyde, homoallylic alcohol, and alkyne undergo smooth Prins-type cyclization in the presence of BF₃·OEt₂/CuCl (10 mol % each) in dichloromethane under mild reaction conditions to afford 4-phenacyl tetrahydropyran derivatives in good yields. This method is highly stereoselective, affording cis-tetrahydropyrans exclusively. The salient features of this method are high conversions, mild reaction conditions, short reaction times, high selectivity, and operational simplicity.     

Direct Mannich reaction mediated by Fe(Cp)2PF6 under solvent-free conditions

Fe(Cp)₂PF₆ (5 mol %) efficiently catalyzed Mannich reaction of aldehydes, anilines, and ketones under solvent-free condition to give β-amino-ketones in high yield (up to 94%) within 30 min with anti-isomer in excess. Simple experimental conditions and product isolation procedure makes this protocol potential for the development of clean and environment-friendly strategy for the synthesis of β-amino-ketones.     

Structure and electron density analysis of electrochemically and chemically delithiated LiCoO2 single crystals

Single crystals of Li_0.68CoO₂, Li_0.48CoO₂, and Li_0.35CoO₂ were successfully synthesized for the first time by means of electrochemical and chemical delithiation processes using LiCoO₂ single crystals as a parent compound. A single-crystal X-ray diffraction study confirmed the trigonal R3¯m space group and the hexagonal lattice parameters a=2.8107(5) Å, c=14.2235(6) Å, and c/a=5.060 for Li_0.68CoO₂; a=2.8090(15) Å, c=14.3890(17) Å, and c/a=5.122 for Li_0.48CoO₂; and a=2.8070(12) Å, c=14.4359(14) Å, and c/a=5.143 for Li_0.35CoO₂. The crystal structures were refined to the conventional values R=1.99% and wR=1.88% for Li_0.68CoO₂; R=2.40% and wR=2.58% for Li_0.48CoO₂; and R=2.63% and wR=2.56% for Li_0.35CoO₂. The oxygen-oxygen contact distance in the CoO₆ octahedron was determined to be shortened by the delithiation from 2.6180(9) Å in LiCoO₂ to 2.5385(15) Å in Li_0.35CoO₂. The electron density distributions of these Li_xCoO₂ crystals were analyzed by the maximum entropy method (MEM) using the present single-crystal X-ray diffraction data at 300 K. From the results of the single-crystal MEM, strong covalent bonding was clearly visible between the Co and O atoms, while no bonding was found around the Li atoms in these compounds. The gradual decrease in the electron density at the Li site upon delithiation could be precisely analyzed.     

An efficient approach to construct 2-arylbenzo[b]furans from 2-methoxychalcone epoxides

An efficient and practical method for construction of 2-arylbenzo[b]furans from 2-methoxychalcone epoxides has been reported. Catalyzed by 2 mol % of BF₃·Et₂O, 2-methoxychalcone epoxides went through the Meerwein rearrangement, followed by deformylation in one-pot to successfully afforded 2-methoxydeoxybenzoins. Afterward, 2-arylbenzo[b]furans were obtained in high yields (87%–100%) via intermolecular cyclodehydration of 2-methoxydeoxybenzoins with 48% HBr. By utilization of this approach, the natural product stemofuran A and the key intermediate of eupomatenoid 6 have been synthesized conveniently.     

Osmium (VI) catalyzed chemoselective oxidation of allylic and benzylic alcohols

A mild and highly chemoselective approach to oxidation of allylic, electron rich/deficient benzylic, and heterocyclic alcohols employing catalytic quantities of K₂[OsO₂(OH)₄] (3 mol %) and chloramine-T (50 mol %) is described. The protocol offers short reaction times (25 min–2 h), controlled oxidation, and tolerance to a variety of substrates. A systematic mechanistic study based on the LC-ESI-MS/MS reveals the presence of imidotriooxoosmium species which further reacts with alcohol to give the oxidized product.