Phase transfer hydrogenation

In general, cydohexenones fail to give optically active products under phase-transfer, hydrogen peroxide, benzylquininium chloride, and basic reaction conditions (77). In fact, it appears that the starting material decomposes, since only water-soluble products are formed. Of several cydohexenones tried, only one, namely 4,4-diphenylcyclohexenone, furnished an optically active epoxide. 

Iridium-monotosylated ethylenediamine [Ts(en)] and Ir-CF3Ts(en) are highly active and chemoselective catalysts for the aqueous-phase transfer hydrogenation of aldehydes using sodium formate as the hydrogen donor.376... 

Wismeijer et al. studied the liquid phase transfer hydrogenation of 4-tert-butylcyclohexanone by 2-propanol at 83°C over activated y-Al203 as the catalyst [4]. The activity of the catalyst was found to increase with increasing activation temperature. Selective poisoning experiments indicated that coordinatively unsaturated Al " surface ions (Lewis acid sites), formed upon dehydroxylation, were essential for catalytic activity. During reaction the catalyst was found to become conditioned by irreversible alcoholysis of the initial active sites, producing less-active sites. The reaction mechanism, however, remained essentially the same as indicated by the constant ratio of c/s//rans-4-/er/-butylcyclohexanol (9/91). 

The alkene is allowed to react at low temperatures with a mixture of aqueous hydrogen peroxide, base, and a co-solvent to give a low conversion of the alkene (29). These conditions permit reaction of the water-insoluble alkene and minimise the subsequent ionic reactions of the epoxide product. Phase-transfer techniques have been employed (30). A variation of this scheme using a peroxycarbimic acid has been reported (31). 

Metallacarboranes. These are used in homogeneous catalysis (222), including hydrogenation, hydrosilylation, isomerization, hydrosilanolysis, phase transfer, bum rate modifiers in gun and rocket propellants, neutron capture therapy (254), medical imaging (255), processing of radioactive waste (192), analytical reagents, and as ceramic precursors. 

AcCl, NaOH, dioxane, Bu4N HSO, 25°, 30 min, 90% yield. Phase-transfer catalysis with tetra-n-butylammionium hydrogen sulfate effects acylation of sterically hindered phenols and selective acylation of a phenol in the presence of an aliphatic secondary alcohol. 

Electronegatively substituted acetylenes, such as dimethyl acetylenedicar-boxylate, do not react under normal conditions but will add the elements of hydrogen fluoride by reaction with fluoride ion (e g, CsF or tetraalkylammonium dihydrogen trifluoride) and a proton source under phase-transfer conditions [49, 50] (equation 8)... 

Fluonnated allylic ethers are prepared under phase-transfer catalysis (PTC) in the presence of tetrabutylammonium hydrogen sulfate (TBAH) fJ] (equation 2)... 

Other PK variations include microwave conditions, solid-phase synthesis, and the fixation of atmospheric nitrogen as the nitrogen source (27—>28). Hexamethyldisilazane (HMDS) is also an excellent ammonia equivalent in the PK synthesis. For example, 2,5-hexanedione and HMDS on alumina gives 2,5-dimethylpyrrole in 81% yield at room temperature. Ammonium formate can be used as a nitrogen source in the PK synthesis of pyrroles from l,4-diaryl-2-butene-l,4-diones under Pd-catalyzed transfer hydrogenation conditions. 

The highest yields in the Ciamician-Dennstedt reaction have been achieved using phase transfer catalysts (Table 8.3.1). In the reaction, the pyrrole or indole and a phase transfer catalyst (PTC, in this case benzyltriethylammonium chloride) are dissolved in chloroform and aqueous sodium hydroxide is added. Yields are typically in the 40s to 60s (rather than in the 20s for a typical Ciamician-Dennstedt reaction). More recently, yields as high as 80% have been reported using tetra-n-butylammonium hydrogen sulphate as the phase transfer catalyst. ... 

Epoxidation systems based on molybdenum and tungsten catalysts have been extensively studied for more than 40 years. The typical catalysts - MoVI-oxo or WVI-oxo species - do, however, behave rather differently, depending on whether anionic or neutral complexes are employed. Whereas the anionic catalysts, especially the use of tungstates under phase-transfer conditions, are able to activate aqueous hydrogen peroxide efficiently for the formation of epoxides, neutral molybdenum or tungsten complexes do react with hydrogen peroxide, but better selectivities are often achieved with organic hydroperoxides (e.g., TBHP) as terminal oxidants [44, 45],... 

Nylon-6,6, 2, 136, 530. See also PA-6,6 acid-catalyzed hydrolysis of, 568 acidolysis of, 568 alkaline hydrolysis of, 568-569 ammonolysis of, 555, 570 chemistry and catalysis of, 546 creation of, 1 hydrogen bonding in, 539 hydrolysis of, 531, 544, 552-555 phase-transfer-catalyzed alkaline hydrolysis of, 569-570... 

Amides are very weak nucleophiles, far too weak to attack alkyl halides, so they must first be converted to their conjugate bases. By this method, unsubstituted amides can be converted to N-substituted, or N-substituted to N,N-disubstituted, amides. Esters of sulfuric or sulfonic acids can also be substrates. Tertiary substrates give elimination. O-Alkylation is at times a side reaction. Both amides and sulfonamides have been alkylated under phase-transfer conditions. Lactams can be alkylated using similar procedures. Ethyl pyroglutamate (5-carboethoxy 2-pyrrolidinone) and related lactams were converted to N-alkyl derivatives via treatment with NaH (short contact time) followed by addition of the halide. 2-Pyrrolidinone derivatives can be alkylated using a similar procedure. Lactams can be reductively alkylated using aldehydes under catalytic hydrogenation... 

The reaction of an a-halo sulfone with a base to give an alkene is called the Ramberg-Bdcklund reaction. The reaction is quite general for a-halo sulfones with an (x hydrogen, despite the unreactive nature of a-halo sulfones in normal 8 2 reactions (p. 437). Halogen reactivity is in the order I>Br>Cl. Phase-transfer catalysis has been used. In general, mixtures of cis and trans isomers are obtained, but usually the less stable cis isomer predominates. The mechanism involves formation of an episulfone, and then elimination of SO9. There is much evidence for... 

The scope of reactions involving hydrogen peroxide and PTC is large, and some idea of the versatility can be found from Table 4.2. A relatively new combined oxidation/phase transfer catalyst for alkene epoxidation is based on MeRe03 in conjunction with 4-substituted pyridines (e.g. 4-methoxy pyridine), the resulting complex accomplishing both catalytic roles. 

Table 4.2 Examples of phase transfer catalysed hydrogen peroxide reactions...

Heteropoly acids can be synergistically combined with phase-transfer catalysis in the so-called Ishii-Venturello chemistry for oxidation reactions such as oxidation of alcohols, allyl alcohols, alkenes, alkynes, P-unsaturated acids, vic-diols, phenol, and amines with hydrogen peroxide (Mizuno et al., 1994). Recent examples include the epoxidations of alkyl undecylenates (Yadav and Satoskar, 1997) and. styrene (Yadav and Pujari, 2000). 

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