Phase transfer catalysed reduction

Soliddiquid phase-transfer catalysed reduction using lithium aluminium hydride... 

PHASE-TRANSFER CATALYSED REDUCTIONS WITH SODIUM... 

Ru(PPh3)3Cl2 is a good catalyst for the phase-transfer-catalysed reduction of nitro compounds to amines by synthesis gas (1 1 CO/H2, note that water is also present in the reaction mixture) at room temperature and atmospheric CO pressure [34] (eq. 7) ... 

Thus it was concluded that [HFe3(CO)9(NPh)] is not a kinetically relevant intermediate in the phase-transfer-catalysed reduction of ArN02 by Fe3(CO)i2, although it is possible that it is involved in a secondary pathway. 

Finally, in a very recent study [254], one of us has investigated the identity of the active species in the phase-transfer catalysed reduction of nitroarenes to anilines by Fe3(CO)i2 and in the analogous reaction promoted by fluoride. As earlier mentioned, [HFe3(CO)u] was proposed to be formed under the phase-transfer conditions and to be the active species. However, this is surprising. 

Trisulphonated triphenylphosphine (TPPTS) and a mixture of tetra-, penta-and hexasulphonated 2,2 -bis(diphenylphosphinomethylene)binaphthyl (BINAS) have also been used in conjunction with PdCb for the reduction of several nitroarenes in a biphasie xylene/H20 medium [82], but the experimental conditions (especially the pressure) are very foreing (120 bar, 100 °C), the catalytic ratio is very poor (40, for a reaction run for 20 h), and the selectivity (best 85 %, but around 50 % in other cases) is also much lower than for most of the catalytic systems discussed in this chapter, making the present catalytic system unattractive (Note that the claims in the paper that other systems are unselective for the nitro group and that this is the first system in which a water soluble catalyst was used are unbased. Apart from the selectivity problem, which is dealt in detail in the introduction, several catalytic systems have been previously mentioned which work under phase-transfer-catalysed conditions, under which anionic water-soluble complexes are surely formed). 

Ketones are reduced to alcohols in the phase-transfer catalysed H-transfer reduction with isopropanol, or better PhCH2CH20H, in the presence of Fe3(C0)i2 Fe(C0)5 is far less active. Mono-, di- and trinuclear iron hydride carbonyl anions are generated in situ. a-Trimethylsilylketones can be prepared via Rh catalysed oxidations with butenones (eqn.9). Azobenzene is isomerised and reduced to o-phenylenediamines by a RuCl3/PPh3/C0/Li0Ac system in secondary alcohols. By contrast, n-butanol leads to formation of benzimidazoles (Scheme 3). 

L. Horner and W. Brich. Studies on the occurrence of hydrogen transfer, 50. Electroreduction and phase transfer-catalysed boron hydride reduction of prochiral ketones. Annalen, 1978, 710. 

Very low asymmetric inductions have also been observed in the phase transfer borohydride reduction of ketones catalysed by quaternary ammonium derivatives (21) of ephedrine. 

A mamber of successful kinetic racemate resolutions have been claimed in displacement reactions on W ethyl-2-bromopropionate7 f [reaction 5]. Again the catalysts used were based on amines [I] and [II]. These results and those of asymmetric sodixam boro-hydride reductions of ketones under phase transfer catalysed conditions probably require further investigation before a clear picture emerges. 

Further work has been published concerning the use of N-methylephedrinium salts ° and other chiral ammonium salts in the phase-transfer catalysed borohydride reduction of prochiral ketones. 

Reductive dimerization of substituted pyridines to yield bipyridyls by zinc is catalysed by nickel salts under phase-transfer catalytic conditions [25]. 

Quaternary ammonium salts aid the transfer of the hypophosphite anion in the palladium-catalysed reduction of, for example, alkynes to alkenes, nitroarenes to aminoarenes, and in the hydrogenolysis of tetrazolyl aryl ethers to phenols [12-14], It has been demonstrated that the hydrogenolysis is ineffective when preformed tetra-n-butylammonium hypophosphite is employed in a dry homogenous organic solvent [13, 14], For optimum hydrogen transfer, the concentration of hypophosphite relative to the substrate must be controlled at a low level and this is most effectively accomplished with a two-phase system. 

Reductive metallation of aldehydes (but not ketones) by tri-n-butyl-(trimethyisilyl)stannane to yield a-hydroxystannanes is catalysed by tetra-n-butylammonium cyanide [15]. Other phase-transfer catalysts are not as effective and solvents, other than tetrahydrofuran, generally give poorer conversions. Use of a chiral catalyst induced 24% ee with 3-phenylpropanal. 

Figure 8 shows that the concentration of U(IV) in the TBP phase increases with illumination time as a result of photocatalysed reduction of U022+ originating from the aqueous phase. The concentration of U4+ in TBP continues to increase until it is equal to the original concentration of U(VI) in the aqueous phase. At the end of the period of illumination, the concentrations of both U(IV) and U(VI) in the aqueous phase are virtually zero, indicating that 100% of the U(VI) has been reduced to U(IV) and transferred from the aqueous phase to the non-aqueous phase. Figure 8 also compares this two-phase data with the comparable one-phase data of Fig. 7 and it can be seen that the rates of U(IV) evolution are virtually identical, suggesting that the semiconductor-photo catalysed reduction... 

A trinuclear cobalt(I) complex, PhCCo3(CO)9, can also catalyse the reduction of nitro compounds in the presence of hydroxide ion at room temperature under a normal pressure of CO [49]. Satisfactory results were obtained under phase transfer conditions. The catalyst and the aromatic nitro compounds were dissolved in benzene under carbon monoxide and an aqueous solution of sodium hydroxide containing cethyltrimethylammonium bromide was added. At a substrate/cat =10 ratio, ca. 60-80 % of amine was obtained in a 18 h reaction. The reaction also proceeded in a homogeneous phase (methanol-water, methanol, dioxane-water) but with lower conversions (less than 45 %). Cobalt complexes such as MeCCo3(CO)9 and MeCo(CO)4 were also active, but less effective. At the end of the reaction, the catalyst was recovered only in part (ca. 15 %). In the organic phase, an IR absorption at 1891 cm, attributable to [Co(CO)4] anion, was observed. Strangely enough, the preformed [Co(CO)4] anion has not been tested as catalyst. The active species was supposed to be the hydride cluster anion reported in Scheme 6. 

The easily prepared, stable solid reagent diphenylamine-borane (Ph2NH BH8) has been shown to be more reactive than aliphatic amine-boranes and almost as reactive as borane-THF for the reduction of ketones acids are also reduced to alcohols. Polyethylene glycols (PEG) catalyse the reduction of ketones by sodium borohydride under phase-transfer (PT) conditions, for example in solid-liquid PT with PEG as solvent. The solid zinc borohydride-dimethylformamide complex reduces aldehydes and ketones to alcohols, but only one hydrogen atom from each tetrahydridoborate unit is utilized. The different rates of reduction of various classes of ketone (saturated aliphatic faster than aromatic, and a -unsaturated very slow) suggest a possible selectivity between ketones. The corresponding cadmium complex, prepared in situ, reacts similarly. Lithium methylborohydride, LiMeBHj, prepared as shown in equation (1), where... 

Alk-2-enenitriles also result from the dehydrobromination of y-hromo-P-oxo-nitriles, alk-3-enenitriles from the palladium-catalysed reductive cleavage of 2-acyloxy-3-ethylenenitriles, 2-dimethylamino-alk-2-enenitriles from the reaction between aldehydes and diethyl 1-dimethylamino-l-cyanomethane-phosphonate under phase-transfer catalysis, 2-ureido-alk-2-enenitriles via... 

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