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1.
Treatment of internal alkynes with sodium methoxide in the presence of Pd(OAc)2 and PPh3 in methanol for 48 h gave the reduction products, alkenes or alkanes in good chemical yields. This reaction proceeds through a palladium methanolate complex, followed by β-hydride elimination and reductive elimination.  相似文献   

2.
Hydroxylamine derivatives of (S)-prolinol have been generated using a Cope elimination. These undergo reverse-Cope elimination onto a pendant double bond to give morpholine N-oxides containing three contiguous chiral centres.  相似文献   

3.
Abstract

Two novel chiral thiophosphoramides and two chiral phosphoramidites were synthesized starting from (S)-α-phenylethylamine and (R)-(+) or (S)-(?)-1,1′-Bi-2-naphthol (BINOL), respectively, and their application in combination with Lewis acid as cocatalysts in asymmetric Morita–Baylis–Hillman (MBH) reaction was investigated. Dramatic rate acceleration (the corresponding adducts were obtained in fair to excellent chemical yield within 15 min–5 h) was observed in these chiral phosphorus reagents/Lewis acid cocatalyzed MBH reaction between 4-nitrobenzaldehyde and activated alkenes, and in one case, moderate enantioselectivity was achieved (the corresponding adduct's ee value is 44%).  相似文献   

4.
The development of catalytic processes that effect enantioselective bond formation under mild conditions is an important and challenging task in modern chemical synthesis. In this connection, chiral C2-symmetric ansa-metallocenes (bridged metallocenes) have found notable applications as catalysts. This article discusses the chemistry of this class of chiral metallocene complexes with regard to their utility in catalytic and enantioselective C? C and C? H bond formation reactions. In addition, where applicable, a brief comparison with other related catalytic enantioselective processes is offered. Many of the reactions effected with high levels of enantioselectivity by catalytic amounts of these complexes are of great significance to the preparation of new materials and in the synthesis of therapeutic agents. For example, zirconocene complexes readily catalyze the enantioselective addition of alkylmagnesium halides to alkenes, and cationic zirconocene complexes may promote the highly stereoregulated copolymerization of terminal alkenes. Furthermore, the related chiral titanocenes are involved in an impressive range of useful asymmetric catalytic reactions, including the enantioselective hydrogenation of olefins and reduction of imines or ketones. This review attempts to bring together the practical aspects of the use of [(ebthi)M] complexes of Group 4 transition metals (catalyst synthesis and resolution), outline the manner in which the C2-symmetric chiral ligands are believed to initiate stereoselective bond formation, and highlight the aspects of this chemistry that are less well understood and require further research.  相似文献   

5.
Abstract

The use of π-electron-deficient aryl sulfones, especially 3,5-bis(trifluoromethyl) phenyl alkyl sulfones (BTFP-sulfones) as soft nucleophiles, as caboxylic acid protecting group and in Julia–Kocienski olefination reactions is described. In the case of α-(arylsulfonyl)acetates dialkylation, reactions are performed under phase-transfer analysis (PTC) conditions using K 2 CO 3 as base. Esters derived from 2-(arylsulfonyl)ethanol can be deprotected using aqueous NaHCO 3 . Alkyl BTFP sulfones are coupled with carbonyl compounds using KOH or P4-t-Bu as bases to give the corresponding alkenes after Smiles rearrangement.  相似文献   

6.
The mechanism of formation of alkene stereoisomers in the catalytic olefination reaction of carbonyl compounds was studied. 4-Chlorobenzaldehyde hydrazone 1 stereoselectively reacts with a number of F-, Cl-, Br-, and I-containing polyhaloalkanes in the presence of catalytic amounts of CuCl to give -substituted styrenes 2 with the more thermodynamically stable alkene isomer being the major product. A model for the formation of the stereoisomers of alkenes 2 in the olefination reaction is proposed. Stereoselectivity of the reaction is determined by elimination of copper(ii) halides from the lowest-lying conformers of organocopper intermediates II. According to quantum-chemical calculations, the elimination should involve the staggered conformations with antiperiplanar arrangement of C—Hal and C—Cu bonds and proceed by the E2 anti-elimination mechanism. The results of quantum-chemical calculations are in good agreement with the experimental E/Z alkene isomer ratios.  相似文献   

7.
A combination of an in situ generated chiral CuI/DTBM‐MeO‐BIPHEP catalyst system and EtOK enabled the enantioselective SN2′‐type allylic cross‐coupling between alkylborane reagents and γ,γ‐disubstituted primary allyl chlorides with enantiocontrol at a useful level. The reaction generates a stereogenic quaternary carbon center having three sp3‐alkyl groups and a vinyl group. This protocol allowed the use of terminal alkenes as nucleophile precursors, thus representing a formal reductive allylic cross‐coupling of terminal alkenes. A reaction pathway involving addition/elimination of a neutral alkylcopper(I) species with the allyl chloride substrate is proposed.  相似文献   

8.
Nickel PyBox catalysts promote nucleophilic cyclopropanation reactions using CH2Cl2 as a methylene source and Mn as a stoichiometric reductant. The substrate scope includes a broad range of alkenes bearing electron-withdrawing substituents, including esters, amides, ketones, nitriles, sulfones, phosphonate esters, trifluoromethyl groups, and electron-deficient arenes. Enantioselective cyclopropanations of α,β-unsaturated esters have been developed using chiral PyBox ligands. Mechanistic studies suggest the intermediacy of a (PyBox)Ni=CH2 species, which adds to the alkene by a stepwise [2+2]-cycloaddition/C−C reductive elimination mechanism. DFT models provide a rationale for the nucleophilic character of the nickel carbene and the sense of enantioinduction.  相似文献   

9.
Tri‐O‐acetyl‐d‐glucal (1) was converted via Ferrier type II rearrangement with high α‐selectivity to 2,3‐unsaturated methyl glycosides 2a and 2b using ferric chloride as the catalyst. Palladium induced allylic substitution with sodium tert‐butylacetoacetate as a nucleophile leads to C‐4 branched sugars. Subsequent hydrogenation followed by treatment with trifluoroacetic acid affords the highly functionalized chiral cyclopentene derivative 5a as a versatile chiral building block for cyclopentanoids.  相似文献   

10.
Palladium-promoted alkylation of alkenes using chiral sulfoxide-containing carbanions and chiral lithiated oxazolines results in asymmetric induction (AI) ranging from 3–5% (1,5 induction), 20–40% (1,3 induction) to 44–52% (1,4 induction). No general trend allowing predictions of results was found. With 1-hexene, attack at C(1) is almost exclusive but propene gives a mixture of attack at C(1) and C(2). The use of a chiral ligand together with malonate anion also leads to some asymmetric induction (ca. 20%).  相似文献   

11.
Abstract

Whereas arylmetaphosphates (ArOPO2) tend to cyclise by intramolecular C-H insertion (phosphorylation) when generated in the gas phase, their alkyl-substituted counterparts exhibit non-electrophilic behaviour and give rise to alkenes by an unusual 1,2-methyl shift induced by a combination of hydrogen abstraction with concomitant elimination of metaphosphoric acid (HOPO2).  相似文献   

12.

Using chiral nonracemic electrophilic-organoselenium-reagents-asymmetric alkoxy-, hydroxy-, azido- and amido-selenenylation of alkenes were effected with high diastereoselectivity. These reagents have also been employed in catalytic amounts to promote one-pot selenenylation-deselenenylation processes. The asymmetric cyclization of properly substituted alkenes diastereoselectively afforded lactons, tetrahydrofurans oxazolines, thiazolines, pyrrolidines, isoxazolidines, 1,2-oxazines, and cyclic nitrones. Enantiopure dioxane, morpholine, tetrahydrofuran, oxazolidin-2-one and aziridine derivatives were prepared from alkenes, PhSeX, and optically active nucleophiles or substrates.  相似文献   

13.
The copper-catalyzed enantioselective radical difunctionalization of alkenes from readily available alkyl halides and organophosphorus reagents possessing a P−H bond provides an appealing approach for the synthesis of α-chiral alkyl phosphorus compounds. The major challenge arises from the easy generation of a P-centered radical from the P−H-type reagent and its facile addition to the terminal side of alkenes, leading to reverse chemoselectivity. We herein disclose a radical 1,2-carbophosphonylation of styrenes in a highly chemo- and enantioselective manner. The key to the success lies in not only the implementation of dialkyl phosphites with a strong bond dissociation energy to promote the desired chemoselectivity but also the utilization of an anionic chiral N,N,N-ligand to forge the chiral C(sp3)−P bond. The developed Cu/N,N,N-ligand catalyst has enriched our library of single-electron transfer catalysts in the enantioselective radical transformations.  相似文献   

14.
Azaborolyl anions, the five‐membered BN heterocycles, have attracted a considerable attention due to their aromaticity and isoelectronic relationship with ubiquitous cyclopentadienyl ligands. Besides their syntheses and applications in the preparation of metal complexes, the other aspects of their chemistry have been virtually unexplored. Reduction of the azabutadienyl chelate boron dichloride [ArN?C(R)CH?C(R)]BCl2 ( 2 , Ar=2,6‐Me2C6H3, R=tBu) with two equivalents of potassium yielded the novel 2‐chloro‐azaborolyl anion [ArNC(R)CHC(R)BCl]K(thf) ( 3 ) as a stable product in good yield. Reaction of 3 with 1,3,4,5‐tetramethylimidazol‐2‐ylidene (NHC) yielded the first NHC–azaborole adduct with the elimination of KCl. The salt elimination reaction was also observed in the reactions with H2O and the organic azide ArN3, leading to the formation of an oxo‐bridged 3H‐1,2‐diazaborole and an intramolecular donor‐stabilized iminoborane, demonstrating that 3 is a source of the unexplored 1,2‐azaborole isosteric to cyclopentadienylidene.  相似文献   

15.
An enantiodivergent method for the synthesis of multiply substituted allenes is described. Highly enantioenriched, point‐chiral boronic esters were synthesized by homologation of α‐seleno alkenyl boronic esters with lithiated carbamates and eliminated to form axially chiral allene products. By employing either oxidative or alkylative conditions, both syn and anti elimination could be achieved with complete stereospecificity. The process enables the synthesis of either M or P allenes from a single isomer of a point‐chiral precursor and can be employed for the enantioselective assembly of di‐, tri‐, and tetrasubstituted allenes.  相似文献   

16.
The chemistry of frustrated Lewis pairs (FLPs) provides the most important approach for the metal‐free hydrogenation and hydrosilylations. Great progress has been achieved in this area for the past decade. Some promising results have also been obtained. This perspective article mainly focuses on the recent advances for the synthesis of chiral Lewis acidic boranes in category of three protocols, 1) hydroboration of chiral internal alkenes with Piers’ borane HB(C6F5)2; 2) in situ hydroboration of chiral alkenes or alkynes without any purification; 3) and substitution reaction of (C6F5)nBCl3–n with chiral organometallic reagents, as well as their applications in the metal‐free asymmetric hydrogenations and hydrosilylations.

  相似文献   


17.
Abstract

Reactions of α,β and β-phosphonyl radicals with alkenes leading to the formation of the P(C)n-C bonds (n=1 ÷ 3) and illustrated by a synthesis of Methylenomycin B are described. Desulfenylation and deselenylation reactions of α-heterosubstituted α-phosphoryl sulfides and selenides are also presented.  相似文献   

18.
Eight (biaryl)phosphite/pyridine ligands 12ad have been prepared by the modular functionalization of positions C-2 and C-3 of two d-glucopyranoside backbones. The chiral ligands were examined in the iridium-catalyzed asymmetric hydrogenation of poorly functionalized alkenes, as a function of the relative position of the coordinating groups and the geometric properties of the biaryl phosphite moieties. Enantiomeric excesses of up to 90% were achieved in the hydrogenation of E-2-(4-methoxyphenyl)-2-butene by using 1a and 1c, which seemingly combine the beneficial effect of the phosphite at the 2-position with the matching (Rax)-configuration of their encumbered biaryl substituents. The results of the hydrogenation of more challenging substrates, such as Z-trisubstituted alkenes, alkenes with a neighboring polar group or demanding 1,1-di-substituted alkenes, generally confirmed this trend, and in some significant cases, the chiral hydrogenated products were isolated with ees of 65–79%.  相似文献   

19.
A novel chiral calix[4](azoxa)crown-7 (9) has been synthesized and its metal ion recognition properties investigated. The starting reagents, chiral diamine 5 and calix[4]arene diacid chloride derivative 8, were prepared according to literature methods. 1H and 13C NMR data show that 9 exists in a cone conformation. In liquid–liquid extraction experiments, 9 exhibits selectivity for Li+ among the other alkali metals and a good extraction ability for transition metal cations, suggesting its potential use in different fields, such as a sensor for ions as well as for chiral molecules.  相似文献   

20.
New enantiomerically pure phenanthroline- and bipyridine-containing macrocycles have been synthesized by combination of the coordinating unit to inexpensive and readily available chiral templates. The catalytic properties of their Cu(I) complexes have been studied in the cyclopropanation of alkenes as test reaction. A simple structural modification of the chiral cavity allowed us to successfully control the trans or cis diastereoselectivity of the reaction.  相似文献   

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