Phase transfer catalysed reaction

Phase-transfer catalysed reactions. F. Montanari, D. Landini and F. Roll a, Top. Curr. Chem., 1982, 101,147-200 (295). 

A study of the anion displacement reactions of n-octyl methanesulfonate reveals striking differences in the reactivity of various anions under phase transfer and homogeneous conditions [43], In the phase transfer catalysed reactions the reactivities of the anions were found to decrease in the order ... 

For most phase-transfer catalysed reactions, the rate-determining step is the interaction of the reactive substrate with the anionic species in the organic phase and, compared with the corresponding interfacial reaction in the absence of the catalyst, rate enhancements of 107 are not uncommon. The virtual absence of water from the organic phase under strongly basic liquiddiquid or soliddiquid two-phase conditions allows for the formation of water-sensitive anions, such as carbanions (Chapter 6), and obviates the need for strictly anhydrous conditions and the use of bases such as sodium hydride or sodamide, etc. The phase-transfer catalytic process consequently has lower safety risks and is environmentally more friendly. 

In the following chapters, published procedures are given for a wide range of phase-transfer catalysed reactions. Particular catalysts, reaction conditions, and work-up procedures are recommended. In general, these are thought to be the optimum conditions but, where there are viable alternatives, they are indicated. 

The phase-transfer catalysed reaction of alkyl halides with potassium carbonate in dimethylacetamide, or a potassium carbonate/potassium hydrogen carbonate mixture in toluene, provides an excellent route to dialkyl carbonates without recourse to the use of phosgene [55, 56], An analogous reaction of acid chlorides with sodium hydrogen carbonate in benzene, or acetonitrile, produces anhydrides (3.3.29.B, >80%), although there is a tendency in acetonitrile for aliphatic acid chlorides to hydrolyse yielding the acids [57]. 

Cyclic trithiocarbonates are conveniently prepared by the phase-transfer catalysed reaction of 1,2- and 1,3-dichloro- and dibromoalkanes with sodium trithiocarbonate [47]. Diodoalkanes produce alkenes. 

The soliddiquid phase-transfer catalysed reaction of pyrrole with prop-2-ynyl bromide, in the presence of Aliquat, yields l-(prop-2-ynyl)pyrrole (20%), together... 

TV-Aminoethylazoles, which are frequently encountered as intermediates in the preparation of pharmacologically active compounds, are readily obtained by solid liquid phase-transfer catalysed reaction of the azole, diazole or triazole and their benzo derivatives with 2-chloroethylammonium chloride at reflux temperature using a procedure analogous to 5.3.3 [25]. 

The reaction of activated methylene groups with tosyl azide to yield the corresponding diazo derivatives proceeds in high yield [23]. The phase-transfer catalysed reaction is sensitive to the strength of base used the reaction of acetoacetic esters requires relatively mild conditions, otherwise diazoacetic esters are produced (Table 5.41). 

In one of the earliest studies of phase-transfer catalysed reactions, Makosza and his research group generated dichlorocarbene using a catalytic amount of benzyltriethyl-ammonium chloride in an aqueous sodium hydroxiderchloroform two-phase system [2, 3] and it has become the standard method for the phase-transfer catalytic production of dichlorocarbene and related dihalocarbenes. 

Phase-transfer catalysed reaction of dichlorocarbene generated from sodium trichloroacetate... 

Phase-transfer catalysed reaction of dibromocarbene generated from sodium tribromoacetate... 

The phase-transfer catalysed reaction of nickel tetracarbonyl with sodium hydroxide under carbon monoxide produces the nickel carbonyl dianions, Ni,(CO) 2- and Ni6(CO)162, which convert allyl chloride into a mixture of but-3-enoic and but-2-enoic acids [18]. However, in view of the high toxicity of the volatile nickel tetracarbonyl, the use of the nickel cyanide as a precursor for the carbonyl complexes is preferred. Pretreatment of the cyanide with carbon monoxide under basic conditions is thought to produce the tricarbonylnickel cyanide anion [19], as the active metal catalyst. Reaction with allyl halides, in a manner analogous to that outlined for the preparation of the arylacetic acids, produces the butenoic acids (Table 8.7). 

Although there are other convenient procedures for the conversion of sulphides into sulphoxides and sulphones, the phase-transfer catalysed reaction using Oxone has the advantage that the oxidation can be conducted in the presence of other readily oxidized groups, such as amines, alkenes, and hydroxyl groups, and acid-labile groups, such as esters and carbamates [6, 7], Hydrolysis of very acid-labile groups, such as ketals, can result in production of the keto sulphone. 

Stereoselective Control in Phase-transfer Catalysed Reactions... 

A degree of stereoselective control of the course of a reaction, which is absent or different from that prevalent when the reaction is conducted in the absence of quaternary ammonium salts, may be achieved under standard phase-transfer catalysed reaction conditions. The reactions, which are influenced most by the phase-transfer catalyst, are those involving anionic intermediates whose preferred conformations or configurations can be controlled by the cationic species across the interface of the two-phase system. For example, in the base-catalysed Darzens condensation of aromatic aldehydes with a-chloroacetonitriles to produce oxiranes (Section 6.3), the intermediate anion may adopt either of the two conformations, (la) or (lb) which are stabilized by interaction across the interface by the cations (Scheme 12.1) [1-4]. 

Jones Quaternary Ammonium Salts Their Use in Phase-Transfer Catalysed Reactions, 2001... 

An exciting addition to the armoury of asymmetric phase transfer catalysed reactions has been the oxidative cyclisation of 1,5-dienes (Scheme 13) [21]. This tandem reaction process leads to the formation of tetrahydrofurans such as 35 in a single step from the open chain dienes 34. The step which determines the sense of asymmetry is the initial attack of permanganate anion, and this chiral information is efficiently relayed in the cyclisation to give products with three new stereogenic centres. For example, oxidation of the di-enone 34 with potassium permanganate, catalysed by the salt 36, gave the tetrahydrofuran 35 in 72% ee. 

Phase transfer reactions have featured in several sections of this book, including epoxidation (Section 4.5), Darzens condensation (Section 7.5) and Wadsworth-Emmons reactions (Section 12.5). Another important aspect of phase-transfer catalysed reactions has been with alkylation reactions. The asymmetric alkylation of glycinate Schiff base (12.45) using N-benzylcinchoninium halides as catalysts is particularly noteworthy, since the products are readily converted into amino acids. Corey and coworkers have developed the original work. 

The modem methods for performing the GBH reaction are based on the thermal decomposition of certain aryltriazenes in the presence of trifluoroacetic acid [66,67], or on the phase transfer catalysed reaction of aryldiazonium tetrafluoroborate in the presence of potassium acetate [61], both methods in an excess of arene as the reactant and reaction solvent. The first method requires previous preparation of the aiyltriazene (VII) by diazotation of an aromatic amine in hydrochloric acid followed by addition of piperidine [66], other dialkylamine or 5-aminotetrazole [67]. For example, from the diazotized aniline solution 43, by addition of an equimolar amount of piperidine, the yellow-orange 1-phenyl-3,3-(pentanediyl)triazene (48) readily crystallize in 81% overall yield [66], Scheme 17. 

Many quaternary ammonium salts containing one or two large alkyl groups, such as cetyltrimethylammonium bromide, C16H33N (CH3)3Br, produce micelles as well as being phase transfer agents. Indeed, there are borderline cases where a particular quaternary ammonium salt may behave as both a surfactant and a phase transfer catalyst or as either one, depending on the particular reaction conditions. Starks (6) discusses further similarities and essential differences of the two phenomena. The most important difference is that whereas the rate of phase transfer catalysed reactions are directly... 

Typically, a phase transfer catalysed reaction will use less than 5 mol% of catalyst, above which quantity the cost effectiveness of the system decreases. Stoichiometric quantities of onium salts are used in the ion pair extraction technique, but this method can very often be replaced by PTC. 

C in the presence of CO, H2O, base and a phase transfer catalyst (particularly 18-crown-6, 87 turnovers). Aromatic nitro compounds are reduced under mild conditions in a genuine bimetallic phase transfer catalysed reaction (reaction 7) , or by a (PtClj (PPhs )2 ]/SnCli,/Et3N system under more forcing conditions. ... 

Jones Quaternary Ammonium Salts Their Use in Phase-Transfer Catalysed Reactions, 2001 Varvoglis Hypervalent Iodine in Organic Synthesis, 1997 Wakefield Organomagnesium Methods in Organic Synthesis, 1995... 

Phase-transfer-catalysed reactions of allenes with dicobalt octacarbonyl, methyl iodide, and carbon monoxide, at room temperature and atmospheric pressure, afford unsaturated hydroxy ketones and dienones. Although the yields are not high, the method constitutes a simple one-pot procedure for the synthesis of an interesting class of compounds not easily accessible by other means. 

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