Catalytic, highly enantio- and diastereoselective pinacol coupling reaction with a new tethered bis(8-quinolinolato) ligand

A new class of chiral ligand, tethered bis(8-quinolinol) (TBOxH), is developed. Its chromium complex, TBOxCrCl (3 mol %), effectively catalyzes the pinacol coupling reaction of aromatic aldehydes at room temperature with high yield (up to 94%), high diastereoselectivity (up to dl:meso = 98:2), and high enantioselectivity (up to 98%). The scope of the present method turns out not to be limited to aromatic aldehyde derivatives, as cyclohexanecarboxaldehyde undergoes pinacolization as well (44% yield, dl:meso = 93:7, 84% ee). The method provides an efficient access to enantioenriched 1,2-diols.     

Vanadium-catalyzed pinacol coupling reaction in water

A catalytic pinacol coupling using water as a solvent was performed by a catalytic amount of vanadium(III) chloride and metallic Al as a co-reductant. A combination forms a binary catalytic system, being in sharp contrast to the reaction in organic solvent, which requires a chlorosilane as an additive. Various aromatic aldehydes underwent the reductive coupling to give the corresponding 1,2-diols in moderate to good yields.     

Electroreductive crossed pinacol coupling of aromatic ketones with aliphatic ketones and aldehydes

The intermolecular crossed pinacol coupling of aromatic ketones with aliphatic aldehydes and ketones was effected by electroreduction in the presence of chlorotrimethylsilane. The best result was obtained using a Pb cathode in Bu₄NPF₆/THF. The electroreduction of aromatic 1,4-, 1,5-, and 1,6-diketones under the same conditions gave four-, five-, and six-membered 1,2-diols with trans-stereoselectivity, while the reduction of these diketones with TiCl₄-Zn produced the cis-isomers of the same intramolecular crossed pinacol coupling products predominantly.     

Samarium mediated pinacol coupling of carbonyl compounds in the presence of a catalytic amount of iodine under aqueous media

In the presence of a catalytic amount of iodine, metallic samarium mediated pinacol coupling of carbonyl compounds has been carried out under saturated aqueous NH4Cl-THF conditions at room temperature. It gives corresponding 1,2-diols in good yields.     

Enantioselective pinacol coupling reaction of aromatic aldehydes catalyzed by chiral vanadium complexes

The asymmetric pinacol coupling of aromatic aldehydes by chiral salan–vanadium complexes as effective catalysts is reported. Chiral 1,2-diols were obtained with high diastereoselectivities (up to 90/10) and moderate to high enantioselectivities (up to 82% ee). The possible mechanism of the pinacol coupling reaction is also discussed.     

Pentamethylcyclopentadienide in organic synthesis: nucleophilic addition of lithium pentamethylcyclopentadienide to carbonyl compounds and carbon-carbon bond cleavage of the adducts yielding the parent carbonyl compounds

Lithium pentamethylcyclopentadienide (C₅Me₅Li, Cp*Li) reacted with aromatic aldehyde to provide the corresponding carbinol in excellent yield. The carbinol returns to the parent aldehyde and pentamethylcyclopentadiene upon exposure to acid or due to heating. Chlorodimethylaluminum is essential as an additive to attain the nucleophilic addition of Cp*Li to aliphatic aldehyde. The carbinol derived from aliphatic aldehyde returns to the parent aldehyde and pentamethylcyclopentadiene by the action of a catalytic amount of 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ). The reversible addition/elimination of the Cp* group can represent a protection of aldehyde. Mechanistic details of the carbon-carbon bond cleavage are also disclosed.     

An efficient and versatile approach for optical resolution of C2-symmetric axially chiral biaryl dials. Synthesis of enantiopure biaryl-derived cyclic trans-1,2-diols

An efficient and practical method for optical resolution of axially chiral biaryl dials using enantiomeric tert-butanesulfinamide as resolving agent was developed. The approach offers a very convenient and straightforward access to versatile enantiomerically pure C2-symmetric biaryl dials in good yields. With the obtained axially chiral dials, stereoselective synthesis of a series of cyclic trans-1,2-diols in optically pure form was investigated.     

Diastereoselective synthesis of cyclopentanediols by InCl 3/Al mediated intramolecular pinacol coupling reaction in aqueous media

A "green" and practical intramolecular pinacol coupling reaction promoted by InCl 3/Al catalysts in aqueous media has been developed. Under mild conditions, a novel class of polysubstituted cyclopentane-1,2-diols have been obtained with excellent diastereoselectivity.     

Chromium-catalyzed pinacol coupling of benzaldehyde in water

The pinacol coupling of benzaldehyde (0.25 M or 1.25 M) in water was catalyzed by 5-25 mol % CrCl₂ in the presence of Zn-dust or Al-dust at 20 °C or 60 °C. In all cases at most 50% of the pinacol coupling product, 1,2-diphenyl-1,2-ethanediol, was obtained with the major product, benzyl alcohol, being formed by a competitive 2e⁻ reduction of the carbonyl. The dl- to meso-diastereoselectivity of the pinacol products ranged from 0.6:1 to nearly 1:1.     

应用量子化学反应性指数研究频哪醇重排反应

1,2-二醇类化合物的频哪醇重排反应是经典的化学重排反应。无环的或环状的1,2-二醇类化合物在质子酸或路易斯酸的作用下会重排发生取代基的1,2-迁移,其产物往往伴随不同基团的迁移而生。但是,主产物取决于二羟基相连接的碳原子上取代基的空间架构和性质。本文采用净亲电指数(Multiphilic index)和Wiberg键级等量子化学反应性指数来定量描述1,2-二醇类化合物的频哪醇重排反应机理,并依此作为预测频哪醇反应主产物的依据。     

Asymmetric pinacol coupling of aromatic aldehydes with TiCl2/enantiopure amine or hydrazine reagents

Asymmetric pinacol coupling of aromatic aldehydes under homogeneous conditions with TiCl₂ in the presence of enantiopure amines or hydrazines afforded 1,2-diols in moderate to excellent yields with good dl-diastereoselectivities and enantioselectivities in the range of 6–65% ee. A non-linear temperature effect (‘principle of isoinversion’) has been examined.     

Enantioselective Pinacol Coupling of Aromatic Aldehydes Induced by Chiral Titanium Complexes

The reductive coupling of carbonyl compounds catalyzed by the transition metal is an important group of carbon-carbon bond forming reaction. The pinacol coupling reaction of aldehydes is one of the most efficient methods for synthesis of vicinal diols1. L…     

Reactivity of Zirconocene Azametallacyclobutenes: Insertion of Aldehydes, Carbon Monoxide, and Formation of alpha,beta-Unsaturated Imines. Formation and Trapping of [Cp(2)Zr=O] in a [4 + 2] Retrocycloaddition(1)

Azametallacyclobutene Cp(2)ZrN-t-BuCEt=CEt (1) underwent an insertion reaction with CO to form the acyl complex 2 (Cp(2)Zr(N-t-BuCEtCEtCO), 67% yield). The addition of acetone to azametallacyclobutene 3 (Cp(2)Zr(NArCMeCPh), Ar = 2,6-dimethylphenyl) yielded the N-bonded enamine and O-bonded enolate complex of zirconocene 4 (Cp(2)Zr(NArCMeCPhH)(OCMeCH(2)), 76% yield). The addition of aldehydes RCOH to metallacycle 3 resulted in the insertion of the aldehyde into the Zr-C bond to form complexes Cp(2)Zr(NArCMeCPhCRHO) (8a) and Cp(2)Zr(NArCMeCPhC(i-Pr)HO (9) in 85% (R = Ph) and 73% yields, respectively. Similarly, treatment of metallacycle 10 (Cp(2)Zr(NArCEtCEt)) with benzaldehyde yielded the insertion product 11 (Cp(2)Zr(NArCEtCEtCPhHO)) in 56% isolated yield. The structure of complex 11 was confirmed by an X-ray crystallographic study. Heating the insertion products 8a and 9 led to elimination of the alpha,beta-unsaturated imines 13 and 14a (ArN=CMeCPh=CRH) in 53% and 72% yields, respectively, and the formation of oxozirconocene oligomer (Cp(2)ZrO)(n)(). The oxozirconocene monomer was trapped by dimethylzirconocene, preventing the formation of oligomer and resulting in the isolation of product 15. A kinetic study of this retrocycloaddition produced the following activation parameters: DeltaH() = 26.5 kcal/mol, DeltaS() = 3.48 eu. A Hammett sigma/rho study showed that electron-donating groups alpha to the metallacycle oxygen accelerate the retrocycloaddition (rho = -0.8).     

Samarium diiodide-catalyzed diastereoselective pinacol couplings

A complex of samarium diiodide (SmI(2)) with tetraglyme catalyzes the intermolecular pinacol coupling of aromatic or aliphatic aldehydes at loadings of 10 mol % in the presence of Me(2)SiCl(2) and Mg. Diastereoselectivity of up to 95/5 (+/-/meso) has been achieved for aliphatic aldehydes and up to 19/81 (+/-/meso) for aromatic aldehydes. De values of up to 99% have been achieved in intramolecular pinacol coupling reactions using the SmI(2)/tetraglyme/Mg/Me(2)SiCl(2) catalytic system.     

Allylic-type diindium reagents. Reactivity toward electrophiles and cascade coupling reactions with imines

The allylic-type diindium reagents A and B were prepared from 3-bromo-1-iodopropene (1a) and 4-bromo-2-iodobut-2-ene (1b), respectively, and their reactions with electrophiles were investigated. The diindium reagents A and B were initially reacted with imines and the resulting vinylindium compounds were then treated with organic halides in the presence of Pd(PPh(3))(4) to give linear N-aryl and N-tosyl homoallylic amines. Diindium A is stable in a small amount of water in solvent, whereas B is easily protonated to give a crotylindium reagent. The reaction of B with benzaldehyde gives mainly the 1,3- and 1,5-diols via a spontaneous coupling with two molecules of the aldehyde, in contrast to A, which reacts with one molecule of carbonyl compounds to give the vinylindium compounds.     

Preparation and characterization of some structural variants of Cp*Ti(1,2-propandiolato) complexes

1,2-Propandiol reacts with Cp*Ti(CH₃)₃ by rapid liberation of methane to yield a dimetallic complex 6 of the net composition (Cp*Ti)₂(1,2-propandiolato)₃. The X-ray crystal structure analysis revealed an unsymmetrical bridging between the [Cp*Ti(1,2-propandiolato)] and [Cp*Ti(1,2-propandiolato)₂] subunits. Cp*TiCl₃ reacts with 1,2-propandiol in a 1:1 stoichiometry in the presence of excess pyridine by replacement of two chlorides by a 1,2-propandiolato ligand. The resulting product was isolated as a dimer 8 and characterized by X-ray diffraction. It exhibits a central Ti₂O₂ ring that was formed by bridging between the two [Cp*TiCl(1,2-propandiolato)] subunits using the oxygen atoms of the primary end of the ligand. From the reaction mixture a more complicated condensation product 9 was isolated in a small yield that contains two [Cp*TiCl(1,2-propandiolato)] units connected in a similar way by a Cp*-free [Ti(1,2-propandiolato)₂] moiety as revealed by its X-ray crystal structure analysis. Complex [Cp*TiCl(1,2-propandiolato)]₂ (8) gives an active catalyst for the syndiotactic polymerization of styrene upon treatment with excess methylalumoxane in toluene solution.     

Enantiomerically Pure Cyclic trans-1,2-Diols,Diamines, and Amino Alcohols by Intramolecular Pinacol Coupling of Planar Chiral Mono-Cr(CO)3 Complexes of Biaryls

Without any formation of stereoisomers , the intramolecular pinacol cyclization of 1 —planar chiral mono-Cr(CO)₃ complexes of 1,1′-biphenyls with carbonyl functionalities at the 2- and 2′-positions—with samarium diiodide gives cyclic trans-1,2-diols 2 . Upon exposure to sunlight, the chromium-complexed diols 2 produce optically pure chromium-free trans-diols 3 . Similarly, the corresponding enantiomerically pure trans-1,2-diamines and amino alcohols are obtained from the planar chiral chromium complexes of biphenyls with diimino or keto-imino functionalities. R¹=H, OMe; R²=H, Me; R³=H, Me.     

Selective monobenzoylation of 1,2- and 1,3-diols catalyzed by Me2SnCl2 in water (organic solvent free) under mild conditions

We have developed an efficient method for selective monobenzoylation of 1,2- and 1,3-diols in water catalyzed by Me(2)SnCl(2). Treatment of 1,2- and 1,3-diols with benzoyl chlorides, DMT-MM, and potassium carbonate in the presence of a catalytic amount of Me(2)SnCl(2) and DMAP in water at room temperature gave monobenzoates in up to 97% yield.     

Aluminum enolates via retroaldol reaction: catalytic tandem aldol-transfer—Tischtschenko reaction of aldehydes with aldol adducts of ketones to ketones

Bidentate aluminum chelates derived from biphenol, binaphthol and catechol were found to be efficient catalysts for aldol-transfer reactions of ketone to ketone aldol adducts with aliphatic or aromatic aldehydes giving rise to the formation of aldol adducts of ketones to the aldehydes. In the presence of an excess of an aliphatic aldehyde, a catalytic tandem aldol-transfer—Tischtschenko reaction is observed. The tandem reaction produces monoesters of 1,3-diols with high anti selectivity and with modest to good chemical yield. 1,2-Unsaturated aldehydes are less reactive in the aldol-transfer reaction and require 2-4 times higher load of the catalyst to be used than aliphatic and aromatic aldehydes. Poor diastereoselectivity was observed in the formation of α-substituted aldols and 2-substituted monoesters of anti-1,3-diols indicating that the aldol-transfer reaction is not diastereoselective with the catalysts studied. The utility of the highly 1,3-anti selective formation of diolmonoesters was found to be limited by acyl migration.     

Studies on the Conversions of Diols and Cyclic Ethers. Part 48. Dehydration of alcohols and diols on the action of dimethylsulfoxide

The transformations of 13 alcohols and 13 diols in the presence of a small amount dimethylsulfoxide (1/16 mol) were studied. Relationships were found between the type of the hydroxy compound and the selectivity of the transformation, and conclusions were drawn regarding the transformation mechanism. The ether formation observed with certain alcohols proceeds via a carbenium cation. The reaction conditions applied were found suitable for inducing water elimination from the ditertiary 1,2- and 1,3-diols (pinacol rearrangement, 1,2-elimination). From the 1,4- and 1,5-diols the corresponding oxacycloalkanes can be obtained in good yield. Cyclodehydration occurs by intramolecular nucleophilic substitution, via a concerted mechanism. The effect of DMSO is exerted directly, and protoncatalysis occurs simultaneously.