Catalysts dichloromethane-palladium

Chlorostannate ionic liquids have been used in hydroformylation reactions [23], Acidic [bmimjCl-SnCb and [l-butyl-4-methylpyridinium]Cl-SnCl2 were prepared from mixing the respective [cation]+ Cl with tin(II)chloride in a ratio of 100 104, much in the same way that the chloroaluminates are made (see Chapter 4). Both these chlorostannate ionic liquids melt below 25 °C. Addition of Pd(PPh3)2Cl2 to these chlorostannate ionic liquids leads to a reaction medium that catalyses the hydroformylation of alkenes such as methyl-3-pentenoate as shown in Scheme 8.9. The ionic liquid-palladium catalyst solution is more effective than the corresponding homogeneous dichloromethane-palladium catalyst solution. The product was readily separated from the ionic liquid by distillation under vacuum. This is an important reaction as it provides a clean route to adipic acid. 

Either protic (alcohols, preferentially methanol) or aprotic solvents (toluene, dichloromethane, THE) can be used, depending on the structure of the metal precursors that can generate the catalysts by a number of pathways. Metals other than palladium, for example nickel [4], can form active catalysts for alkene/CO copolymerisation, yet with largely lower productivities as compared to structurally similar palladium precursors [1]. For this reason, only Pd"-catalysed alkene/CO copolymerisation reactions are reviewed and commented in the present chapter. 

The palladium catalyst supported on the dendrimer with 24 phosphine end groups (2) was used in a CFMR. In the continuous process a solution of allyl trifluoroacetate and sodium diethyl 2-methylmalonate in THF (including -decane as an internal standard) was pumped through the reactor. Figure 4 shows the conversion as a function of the amount of substrate solution (expressed in reactor volumes) pumped through the reactor. The reaction started immediately after the addition of the catalyst, and the maximum conversion was reached after two reactor volumes had passed, whereupon a drop in conversion was observed. It was inferred from the retention of the dendrimer (99.7% in dichloromethane) that the decrease was not caused by dendrimer depletion, and it was therefore ascribed to the... 

The carbonylation of the 2-aminostyrene 187 in dichloromethane under a carbon monoxide and hydrogen atmosphere produces the lactam in the presence of a palladium acetate/tricyclohexylphosphine catalyst in excellent yield and selectivity (Equation 123) <1996JA4264>. 

By using this approach they have selectively resolved 5-vinyloxazolidinone (roc)-45 with a (R)-BINAP-palladium(O) based catalyst (Scheme 12). At best, the enantioselectivity was moderate up to 62% ee for (R)-47. The choice of solvent was also very important THF favoured the (S,S)-allylic phthalimide (S,S)-46 (5 1), whereas acetonitrile favoured the complementary phthalimide (R)-47 (2.4 l)(Scheme 12 entry 1 versus 3). Ideally, the selective removal of both enantiomers need to occur at an equal rate to satisfy this PKR approach. This relative rate was achieved by changing the solvent to dichloromethane (Scheme 12 entry 2) but sadly the overall enantiocontrol was much lower. Whereas, by changing the chiral ligand to (S,S)-DIOP the relative rate of removal of both enantiomers were near perfect (1 1.2) and equally the enantiomeric control in both allylic phthalimides (S,S)-46 and (R)-47 were very similar 50% ee and 46% ee respectively (Scheme 12 entries 4 and 5). 

The reaction products were isolated by distillation from the ionic liquid and all of the palladium catalysts could be reused several times before turnover frequencies noticeably deteriorated. However, once the ionic liquid catalyst solution was saturated with either NaBr or NaCl, it had to be regenerated by dissolving in chloroform or dichloromethane and filtering through celite. If this procedure was not followed then the precipitated sodium halide led to increased reaction times due to decreased diffusion of the substrate in the ionic liquid. 

A bimetallic catalyst prepared from BINOL and lithium aluminum hydride has been found to result in useful asymmetric induction in the Pudovik reaction [17]. The (f )-ALB catalyst 64 (10 mol %) facilitates the addition of dimethyl phosphite to a variety of electron-rich and electron-poor aryl aldehydes in high yield with induction in the range 71-90 % ee. The nature of the solvent is important in this reaction—the induction for addition to benzaldehyde dropped from 85 % ee to 65 % ee when the solvent was changed from toluene to dichloromethane. Aluminum seems to be a key to the success of this reaction, because reaction with benzaldehyde was not as successful with other bimetallic catalysts. BINOL catalysts with lanthanum and potassium gave only 2 % ee, a catalyst with lanthanum and sodium gave a low 32 % ee, and a catalyst with lanthanum and lithium gave only a 28 % ee [18]. Aliphatic aldehydes were not successfully hydrophosphonylated with dimethyl phosphite by catalyst 64 (Sch. 9). Induction was low (3-24 % ee) for unbranched and branched substrates. a,/3-Unsaturated aldehydes were, however, reported to work nearly as well as aryl aldehydes with four examples in the range 55-89 % ee. The failure of aliphatic aldehydes with this catalyst can be overcome by reduction of the product obtained from reactions with a,)3-unsaturated aldehydes. As illustrated by the reduction of 67 with palladium on carbon, this can be done without epimerization of the a-hydroxy phos-phonate. 

Tin Hydrides. Tributyltin hydride reduces aldehydes to primary alcohols by simply heating in methanol. °° A mixture of BusSnH and phenylboronic acid (p. 815) reduces aldehydes in dichloromethane. °° Reduction of ketones was achieved with Bu2SnH2 and a palladium catalyst. ° Using triaryltin hydrides with Bp3 OEt2, where aryl is 2,6-diphenylbenzyl, selective reduction of aliphatic aldehydes in the presence of a conjugated aldehyde was achieved. 

During the following 15 years, only small advances were achieved in increasing catalyst efficiencies. Independently, Fenton [9a] at Union Oil and Nozaki [9b] at Shell Development Company (USA) discovered several related palladium chlorides, palladium cyanides, and zero-valent palladium complexes as catalysts. Sen and co-workers [10] reported that cationic bis(triphenyl-phosphine)-palladium tetrafluoroborate complexes in aprotic solvents such as dichloromethane, produced ethylene/carbon monoxide copolymers under very mild conditions. The reaction rates were, however, very low, as were the molecular weights. 

Metal-based catalysts also were used for methane oxidation. Especially over metals such as platinum and palladium, trace amounts of methanol, formaldehyde, and formic acid can be found. Organic halides increased the yield of partial oxidation products and inhibited the complete combustion of methane [173]. Inhibition effects of dichloromethane was observed. Mann and Dosi [174] used a Pd/Al203 catalyst and found that the addition of halogen compounds reduced the conversion of methane in the following order ... 

Immobilized dicyclohexylphosphine ligand, 310, has been used as the starting point for the preparation of supported palladacycles 313 and 314. Simply stirring 310 and dimeric palladium complexes 311 and 312 for 1 h in dichloromethane gave 313 and 314, respectively (Scheme 104). They are active Suzuki coupling catalysts on the first use but cannot be recycled. 

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