Phase-transfer reaction catalyzed

Phase-Transfer Reactions Catalyzed by Metal Complexes... 

H. Alper, Phase transfer reactions catalyzed by metal complexes, in Phase Transfer Catalysis New Chemistry, Catalysts, and Applications (Ed. Ch. M. Starks), ACS Symposium Ser. No. 326, American Chemical Society, Washington, DC, USA, 1987, Chapter 2, p. 8. 

Shan, Y., R. H. Kang, and W. Li, Investigation of Phase-Transfer Reactions Catalyzed byPoly(ethylene glycol) Bound toMacrop-orous Polystyrene Supports, //tti. Eng. Chem Res., 7B, 1289 (1989). 

The predicted metabolites are also the starting point for the phase II metabolic prediction, to find where glucuronidation could occur. All the probable metabolites obtained from CYP metabolism reactions are submitted to a possible phase II reaction catalyzed by UGTs, using the UGT structure(s) as a template. The accessibility component is computed in the UGTcavity to prioritize glucuronic acid transfer. The final metabolite structures are then reported in graphical output or saved to a file. 

As is the case in all other quinine-catalyzed reactions, the quininium-salt-catalyzed phase-transfer reactions are subject to strong solvent effects (Table 8) (81). The fact that, in the presence of water, polar solvents lower the e.e., whereas apolar solvents raise the e.e., indicates that these are true phase-transfer reactions in which the ion pairs within the organic layer are responsible for the asymmetric induction. 

In 2000, Benaglia and coworkers reported preparation of MeO-PEG supported quaternary ammonium salt (10) and examined the catalytic efficiency in a series of phase-transfer reactions (Fig. 5.3) [69]. The reactions occurred at lower temperatures and with shorter reaction times than with comparable insoluble 2% cross-linked polystyrene-supported quaternary ammonium salts, although yields varied with respect to classical solution phase quaternary ammonium salt catalyzed reactions. It was observed that yields dropped with a shorter linker, and that PEG alone was not responsible for the extent of phase-transfer catalysis. While the catalyst was recovered in good yield by precipitation, it contained an undetermined amount of sodium hydroxide, although the presence of this byproduct was found to have no effect on the recyclability of the catalyst... 

Polymer-supported crown ethers and cryptands were found to catalyze liquid-liquid phase transfer reactions in 1976 55). Several reports have been published on the synthesis and catalytic activity of polymer-supported multidentate macrocycles. However, few studies on mechanisms of catalysis by polymer-supported macrocycles have been carried out, and all of the experimental parameters that affect catalytic activity under triphase conditions are not known at this time. Polymer-supported macrocycle... 

The idea that transition-metal complexes can act as PT agents was presented in some early studies of metal-complex-catalyzed reactions under PTC conditions. Lately, several approaches to the design of bifunctional catalysts have been made. Organometallic complexes capable of effecting phase transfer reactions were prepared for the first time in 1982 [192, 193]. Tertiary phosphines containing polyether substituents react with Pd(PhCN)2Cl2 to afford complexes capable of catalyzing the reduction of bromobenzene with NaH suspended in toluene. 

In the case of another biphasic variant, namely phase-transfer-catalyzed reactions (cf. Section 4.6.1 e.g., [38]), mass transfer rates of ionic intermediates between the aqueous and organic phases, their phase and partition equilibria, as well as the reaction rate in the organic phase, have to be analyzed to model the overall reaction rate. Finally, the dynamics of liquid-liquid phase transfer reactions, which are a part of multiphase reactions as shown above, can be described by a new phase-plane model [39] based on the two-film theory. 

Benzotrifluoride 1 is a colorless, low viscosity liquid. Its polarity is intermediate between those of dichloromethane and ethyl acetate, and it is miscible with common organic solvents and able to dissolve many organic compounds. Benzotrifluoride is used in a wide variety of reactions including radical reactions, oxidations and reductions, phase-transfer reactions, transition metal catalyzed processes and Lewis acid reachons. Ogawa and Curran reported that dichloromethane (CH2CI2) is replaceable by BTF in many instances [8]. 

More widely applicable are base-catalyzed phase-transfer reactions using aqueous concentrated or solid NaOH, KOH, K2CO3, NaH, etc. These include... 

Pd(OAc)2 in combination with NaHCOs as a base in DMSO as solvent catalyzed the aerobic oxidation of primary and secondary allylic and benzylic alcohols to the corresponding aldehydes and ketones, respectively, in fairly good yields . Similarly, PdClj, in combination with sodium carbonate and a tetraalkylammonium salt, Adogen 464, as a phase transfer catalyst, catalyzed the aerobic oxidation of alcohols, e.g. 1,4- and 1,5-diols afforded the corresponding lactones (Reaction 15) . ... 

Phase transfer processes rely on the catalytic effect of quaternary onium or crown type compounds to solubilize in organic solutions otherwise insoluble anionic nucleophiles and bases. The solubility of the ion pairs depends on lipophilic solvation of the ammonium or phosphonium cations or crown ether complexes and the associated anions (except for small amounts of water) are relatively less solvated. Because the anions are remote from the cationic charge and are relatively solvation free they are quite reactive. Their increased reactivity and solubility in nonpolar media allows numerous reactions to be conducted in organic solvents at or near room temperature. Both liquid-liquid and solid-liquid phase transfer processes are known the former ordinarily utilize quaternary ion catalysts whereas the latter have ordinarily utilized crowns or cryptates. Crowns and cryptates can be used in liquid-liquid processes, but fewer successful examples of quaternary ion catalysis of solid-liquid processes are available. In most of the cases where amines are reported to catalyze phase transfer reactions, in situ quat formation has either been demonstrated or can be presumed. 

The reaction of trimethylsulfonium iodide with benzaldehyde under basic phase transfer conditions catalyzed by chiral quaternary ammonium salts such as (-)-N,N-dimethyl-ephedrinium bromide has been reported to yield styrene oxide in high optical purity [19], which may be somewhat overestimated [20]. 

Phase-transfer catalysis, also often referred to as ion pair partition" is a novel synthetic technique which has been the subject of much interest in recent years not only in the field of organic synthesis but also in polymer chemistry. The term "phase-transfer catalysis" was first introduced in 1971 by Stark > who studied kinetics in detail and the mechanism of reactions which are catalyzed by small amounts of onium salts such as quaternary ammonium or phosphonium compounds. Brandstrbm and Makosza also made major Initial contributions in the understanding of such reactions and the application thereof in various synthetic reactions. A generally accepted phase-transfer reaction scheme is shown in... 

The base-catalyzed Michael—type addition of active hydrogen compounds to activated double bonds is generally performed under homogeneous conditions and lays therefore at the borderline of the scope of this paper, even if it has been sometimes carried out under typical phase transfer conditions. However, considering that the catalysts promoting phase transfer reactions, i.e. ammonium salts, amines and crown ethers, are generally active in the Michael addition, the reported reactions will be discussed here in some details also for the reasons mentioned in the introduction. 

P-diketone. Under similar phase-transfer reaction conditions, compounds 48 were obtained with better results [50b,c] when catalyzed by chiral sprro-ammonium salt 49 (Ar = S.S-ICFjIjQH,]. 

Many catalysts can be used tetrabutylammonium halides, tetrabutylammonium hydroxide, tetrabutylammonium hydrogen sulfate, tetrabutylphosphonium bromide, 18-crown-6 ether, and cryptand[2.2.2]. There have been few studies on the influence of the catalyst on the reactions. However, Nishibuko et al carried out an excellent study on the influence of experimental conditions on phase transfer catalyzed polymer modification they showed that the nature of the catalyst and the type of phase transfer reaction (solid-liquid, liquid-liquid), as well as the polarity of the solvent are very important parameters. The purity of the system must be carefully controlled thus, the presence of traces of water may have a great influence on the conversion and the occurrence of side reactions. 

A large number of highly enantioselective transformations catalyzed by chiral phase-transfer catalysts have been developed over the past decades, and some of these reactions have been applied to the asymmetric synthesis of natural products and compounds showing bioactivity. Furthermore, the practicability of asymmetric phase-transfer reactions in the large-scale preparation of drugs and relative intermediates has widely been recognized in both industry and academia. 

Nucleophilic Reactions. Useful nucleophilic substitutions of halothiophenes are readily achieved in copper-mediated reactions. Of particular note is the ready conversion of 3-bromoderivatives to the corresponding 3-chloroderivatives with copper(I)chloride in hot /V, /V- dim ethyl form am i de (26). High yields of alkoxythiophenes are obtained from bromo- and iodothiophenes on reaction with sodium alkoxide in the appropriate alcohol, and catalyzed by copper(II) oxide, a trace of potassium iodide, and in more recent years a phase-transfer catalyst (27). 

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