Liquid Phase-transfer Conditions Research Articles

Synthesis of α-d-galactose-based azacrown ethers and their application as enantioselective catalysts in Michael reactions

Crown ethers derived fromd-galactose generated good to excellent enantioselectivities in a few Michael reactions under solid–liquid phase transfer conditions.

Stereoselective Synthesis of Unsaturated and Functionalized l-NHBoc Amino Acids, Using Wittig Reaction under Mild Phase-Transfer Conditions

The stereoselective synthesis of a new amino acid phosphonium salt was described by quaternization of melting triphenylphosphine with the γ-iodo NHBoc-amino ester, derived from L-aspartic acid. The deprotection of the carboxylic acid function to afford the phosphonium salt with a free carboxylic acid group was achieved by a palladium-catalyzed desallylation reaction. This phosphonium salt was used in the Wittig reaction with aromatic or aliphatic aldehydes and trifluoroacetophenone, under solid-liquid phase-transfer conditions in chlorobenzene and in the presence of K(3)PO(4) as weak base, to afford the corresponding unsaturated amino acids without racemization. Thus, the reaction with substituted aldehydes allows to graft various functionalized groups on the lateral chain of the amino acid, such as trifluoromethyl, cyano, nitro, ferrocenyl, boronato, or azido. In addition, the reaction of the amino acid Wittig reagent with α,β-unsaturated aldehydes leads to amino acids bearing a diene on the lateral chain. Finally, this amino acid phosphonium salt appears to be a new powerful tool for the preparation of unsaturated and non-proteinogenic α-amino acids, directly usable for the synthesis of customized peptides.

A highly selective monoalkylation of tert-butyl 3,4-dihydroxy-4-phenylpiperidine-1-carboxylate under phase transfer conditions

Liquid–liquid phase transfer conditions have been found to provide highly selective monoalkylation of tert-butyl 3,4-dihydroxy-4-phenylpiperidine-1-carboxylate, a transformation for which many other common alkylation protocols proved inadequate. The high preference for 3- O-alkylation in the phase transfer alkylation is emphasised by the absence of diether formation even in the presence of a large excess of reagents.

Simultaneous C- and N-Alkylation of 2-Oxo-4,6-diaryl-1,2,3,4-tetrahydropyridine-3-carbonitrile Under Solid–Liquid Phase-Transfer Conditions

The alkylation of 2-oxo-4,6-diaryl-1,2,3,4-tetrahydropyridine-3-carbonitrile 1 has been carried out using different alkyl/arylating agents in solid–liquid phase-transfer catalysis conditions. The aim was to study the effect of steric hindrance offered by the aryl group in the sixth position of the pyridine ring on the ambient N- vs. O-alkylation ratio. Simultaneous C- and N-alkylation was encountered and confirmed by x-ray crystallography. Our study to gain exclusive regiocontrol for simultaneous alkylation was carried out. An alternative route for C‒C bond formation was also established by the removal of the cyano functionality.

Asymmetric catalysis of carbon–carbon bond forming reactions using amino acid-derived C1-symmetrical salen ligands

Four amino acids (alanine, valine, phenylalanine and phenylglycine) have been converted into C 1-symmetrical salen ligands and complexed to titanium, vanadium, copper and cobalt. The resulting complexes have been used as asymmetric catalysts for asymmetric cyanohydrin synthesis, asymmetric Strecker reactions, asymmetric synthesis of α-methyl amino acids and asymmetric Darzens condensations. Satisfactory levels of asymmetric induction were obtained from reactions in which the (salen)metal complex acts as a chiral Lewis acid, but low levels of asymmetric induction were obtained from reactions carried out under solid–liquid phase-transfer conditions.

Highly Enantioselective Monoalkylation of p‐Chlorobenzaldehyde Imine of Glycine tert‐Butyl Ester under Mild Phase‐Transfer Conditions.

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Highly enantioselective monoalkylation of p-chlorobenzaldehyde imine of glycine tert-butyl ester under mild phase-transfer conditions

The selective monoalkylation of glycine tert-butyl ester aldimine Schiff base 3 has been realized in high chemical yield with excellent enantioselectivity under mild liquid–liquid phase-transfer conditions by the use of binaphthyl-derived chiral quaternary ammonium bromides 7 and 8 as catalysts. This achievement demonstrates that 3 can be used as a cost-effective substrate for the preparation of optically active α-alkyl-α-amino acid derivatives by chiral phase-transfer catalysis.

Supported polyperoxometallates: Highly selective catalyst for oxidation of alcohols to aldehydes

A new highly selective heterogeneous catalyst is developed from phosphomolybdic acid for the oxidation of alcohols by supporting polyoxomolybdate anions on vanadium–aluminum mixed oxide. The Keggin anion was found to be stable on the surface of vanadium–aluminum mixed oxide unlike alumina. It was further observed to undergo degradation with peroxide, to form surface bound polyperoxomolybdate species. In order to examine its usefulness, the catalyst was screened for the oxidation of variety of alcohols, using hydrogen peroxide under solid–liquid–liquid phase transfer conditions to yield corresponding aldehydes with almost 100% selectivity. In all the substrates examined, no traceable amount of over oxidised product was detected.

Alkylation des Esters Sulfamiques par le Bromure de 4-Fluorobenzyle

The action of 4-fluorobenzyl bromide on sulfamic esters (R-O-SO 2 -NH 2 ) in liquid–liquid phase transfer conditions allows, depending on the nature of R, the preparation of the N-dialkyled products or the corresponding ethers by scission of the O–SO 2 bond.

Chiral designer phase-transfer catalysts for practical asymmetric synthesis

Abstract A series of C 2-symmetric, spiro-type chiral quaternary ammonium bromides have been designed as new, purely synthetic chiral phase-transfer catalysts, and readily prepared from commercially available optically pure (R)- or (S)-1,1'-bi-2-naphthol as a basic chiral unit. The structures of the assembled N-spiro chiral quaternary ammonium bromides were unequivocally determined by single-crystal X-ray diffraction analysis. The reactivity and selectivity of these chiral ammonium bromides as chiral phase-transfer catalysts have been evaluated in the asymmetric alkylation of N-(diphenylmethylene)glycine tert-butyl ester under mild liquid–liquid phase-transfer conditions, and the optimization of the reaction variables (solvent, base, and temperature) has also been conducted. Furthermore, the scope and limitations of other asymmetric transformations have been thoroughly investigated with a variety of substrates, in which the advantage of the unique N-spiro structure of our chiral phase-transfer catalysts and dramatic effect of the steric as well as the electronic properties of the aromatic substituents on the 3,3'-position of one binaphthyl moiety have been particularly emphasized.

Synthesis and evaluation of optically pure dioxolanes as inhibitors of hepatitis C virus RNA replication

A series of optically pure 1,3-dioxolane nucleoside mimics was synthesized by a synthetic route that allowed incorporation of a 5R-methyl substituent from commercially available starting materials. The pyrrolo[2,3-d]pyrimidine heterocycle was chosen as a substitute for the purine derivative. Coupling of the pyrrolo[2,3-d]pyrimidine and the dioxolane was performed under solid–liquid phase transfer conditions. The ability to inhibit HCV RNA replication was assessed in a cell based subgenomic replicon assay. None of the described compounds displayed significant anti-HCV activity.

Design of N‐Spiro C2‐Symmetric Chiral Quaternary Ammonium Bromides as Novel Chiral Phase‐Transfer Catalysts: Synthesis and Application to Practical Asymmetric Synthesis of α‐Amino Acids.

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Sterically Hindered Ketiminium Salts: a New Generation of Phase-Transfer Catalysts

Sterically hindered hydrazone 2 undergoes an unpredicted N–N bond scission via a methiodination sequence leading to iminium salt 5. This latter compound was found to be remarkably stable under basic and acidic conditions and it proved to be able to promote C-alkylation of the glycine derived imine 7 under classical catalytic liquid–liquid phase-transfer conditions. Both the reaction cleavage and the catalytic potential are discussed.

Aromatic nucleophilic fluorination with KF catalyzed by polymer-supported phosphonium salts under solid–solid–liquid phase transfer conditions

Fluorination of 2,4-dinitrochlorobenzene (DNCB) with solid KF was conducted in the presence of polymer-supported tetraphenylphosphonium chloride catalyst under solid–solid–liquid (S–S–L) phase transfer conditions. The triphase fluorination obeyed zero-order kinetics with respect to DNCB concentration and first-order kinetics with respect to the catalyst concentration. The apparent activation energy for the fluorination was estimated as about 40 kJ/mol. On the basis of these results, the ion exchange reaction between the Cl − ion of the supported catalyst and the F − ion of KF was concluded to be rate-determining in this S–S–L triphase fluorination. The addition effect of water on the triphase fluorination was discussed by means of both the prompted dissolution of solid KF and the decreased reactivity of the F − ion, induced by increased hydration.

An efficient and highly stereoselective synthesis of trimethylsilylvinylcyclopropane derivatives via an organotelluronium salt: first example of catalytic Wittig-type cyclopropanation

A one-pot reaction of 3-trimethylsilylprop-2-enyl(diisobutyl)telluronium bromide 1 with α,β-unsaturated ketones gives, by way of Michael addition in the presence of caesium carbonate under solid–liquid phase-transfer conditions, trimethylsilylvinylcyclopropanes both in high to excellent yields and with high stereoselectivity. A similar high-yielding cyclopropanation occurs in THF/trace of water with diisobutyl telluride as catalyst in a one-pot reaction of (E)-3-bromo-1-trimethylsilylprop-1-ene with α,β-unsaturated ketones in the presence of caesium carbonate.

Mechanism and anion activation in solid–liquid phase-transfer reactions catalysed by cyclophosphazenic polypodands. Comparison with cyclic analogue crown ethers

A kinetic study of the nucleophilic substitution of the methanesulfonic group by anions (Cl–, Br–, I–, SCN–, C6H5O–, C6H5CH2COO–) catalysed by cyclophosphazenic polypodands has been performed under solid–liquid phase-transfer conditions. The results obtained show that the mechanism of PTC previously found for cyclic ligands (crown ethers, cryptands) also operates in the case of these open-chain ligands: the attack by the anionic nucleophile on the substrate occurs in the organic phase and is rate determining of the overall process. The nucleophilicity scales found (I– > C6H5O–≈ SCN– > C6H5CH2COO–; I– > Br– > Cl–) as well as the anion activation are comparable with those found for cyclic crown ethers. This indicates that the anionic reactive species involved are similar in the complexes of both ligands.

Cobalt Catalysed Carbonylation of Organic Halides under Solid-Liquid Phase Transfer Conditions

Abstract Organic halides are carbonylated under solid liquid phase transfer conditions using Co−(CO)4 as catalyst under CO (1 atmosphere) at 25°C to afford corresponding esters in good yields. No Carbonylation is effected in absence of phase transfer agent (PTA). The striking example is the carbonylation of ethyl haloacetate to diethyl malonate, an important organic intermediate. The parameters such as concentration of PTA, and catalyst and temperature are optimised.

A facile synthesis of α,β-unsaturated epoxides via allyldi-isobutyltelluronium bromide

In the presence of solid KOH, allyldi-isobutyltelluronium bromide 2 reacts directly with aromatic aldehydes at room temperature under solid–liquid phase-transfer conditions to afford α,β-unsaturated epoxides 3 in excellent yields.

Palladium-catalysed desulphonylative vinylation of arenesulphonyl chlorides under solid–liquid phase-transfer conditions

The reaction of arenesulphonyl chlorides (1a–n) with butyl acrylate (2a) in the presence of a catalytic amount of bis(benzonitrile)dichloropalladium [PdCl2(PhCN)2] under solid–liquid phase-transfer conditions gave the corresponding butyl (E)-3-arylprop-2-enoates (3a–n) in good yield. The alkenes (2b–g) could also be used in place of (2a).

Esterification of 1,4-dichlorobutane with sodium formate under solid–liquid phase transfer catalysis. A kinetic study

Kinetic data are reported regarding the esterification of 1,4-dichlorobutane with sodium formate catalyzed by quaternary ammonium salts as a model for reactions in series, under solid–liquid phase transfer conditions. The process was found to follow a consecutive first-order mechanism of the general type A → R → S. The reactivity of the quaternary ammonium salts with regard to the counteranion was Cl− > Br− > 1− > HSO4−. The reaction rate was linearly dependent on catalyst concentration up to 12 mol% of catalyst relative to the substrate. Above this concentration the rate was constant and independent of the amount of the catalyst. The activation energy of the two consecutive steps was found to be similar (21 kcal/mol). Therefore, the product distribution (R/S) is not appreciably affected by temperature. A mechanism termed "Thin aqueous boundary layer" is suggested for nucleophilic displacement reactions under solid–liquid phase transfer conditions. Keywords: phase transfer catalysis, series reactions, kinetic study.