Salt promoters

Pd(II) salts promote the carbonylation of organomercury compounds. Reaction of phenylmercury chloride and PdCh under CO pressure affords benzophenone (429)[387]. Both esters and ketones are obtained by the carbonylation of furylmercury(Il) chloride in alcohol[388]. Although the yields are not satisfactory, esters are obtained by the carbonylation of aryl- and alkylmercuryfll) chlorides[389,390]. One-pot catalytic carbonylation of thiophene, furan, and pyrrole (430) takes place at the 2-position via mercuration and transmetallation by the use of PdCb, Hg(N03), and CuCl2[391]. 

Quaternary ammonium salts compounds of the type R4N" X find application m a technique called phase transfer catalysis A small amount of a quaternary ammonium salt promotes the transfer of an anion from aqueous solution where it is highly solvated to an organic solvent where it is much less solvated and much more reactive... 

Reactions. Catalytic amounts of arsenic, antimony, or tin salts promote the formation of pentathionate (16) ... 

Molten salts promote rapid corrosion of many constmctional materials at relatively low temperatures. Low-melting-point salts include sodium salts from saline atmospheres, fireside ash, silicate insulation, contaminants in the feed, etc. Corrosion rates of several mm/year can be observed at temperatures as low as 520°C. High chromium- and nickel-containing alloys up to 50% Cr/50% Ni are employed. 

The presence of Cu(I) salts promotes intermolecular photocycloaddition of simple alkenes. Copper(I) triflate is especially effective.182 It is believed that the photoreactive species is a 2 1 alkene Cu(I) complex in which the two alkene molecules are brought together prior to photoexcitation.183... 

Diphenylsilane catalyzed by various salts promotes the 1,2-reduction of cinnamaldehyde.318 Cesium fluoride catalysis is particularly effective.320 It is possible to stop these reactions at the silyl ether stage.73,320 The 1,2-reduction of citral is accomplished in high yield with diphenylsilane and Wilkinson s catalyst (Eq. 262) 435 Interestingly, the trialkylsilanes, ethyldimethylsilane and triethylsilane, give high yields of the 1,4-reduction product whereas diisopropylsilane gives a 1 1 mixture of 1,2- and 1,4-reduction (Eq. 263)435... 

In an extension beyond hetaryl onium salt promoted hemiacetal activation, Ishido and coworkers have reported the carbodiimide activation of hemiacetals [141]. In the method (Scheme 3.13), the hemiacetal donor 1 is treated with a carbodiimide electrophile 83 and copper(I) chloride to provide glycosyl isourea intermediate 85. Highly susceptible to hydrolysis, the isourea 85 was not isolated but could be detected by 13C NMR and IR spectroscopy [142,143], Accordingly, the reaction between intermediate 85 and the glycosyl acceptor (NuH) provides glycoside product 3, along with urea by-product 84. 

Representative Procedure for Trityl Salt Promoted Clycosylation with Glycosyl Phenyl Carbonate Donors [360]... 

Effect of Added Salt. It 1s well known that the presence of salt promotes dissolution (1.31. For a fixed NaOH concentration, the dissolution rates of both p-Cl-PHMP and PBPh-1 were found to increase... 

One approach which enables lower water concentrations to be used for rhodium-catalysed methanol carbonylation is the addition of iodide salts, especially lithium iodide, as exemplified by the Hoechst-Celanese Acid Optimisation (AO) technology [30]. Iodide salt promoters allow carbonylation rates to be achieved at low (< 4 M) [H2O] that are comparable with those in the conventional Monsanto process (where [H20] > 10 M) while maintaining catalyst stability. In the absence of an iodide salt promoter, lowering the water concentration would result in a decrease in the proportion of Rh existing as [Rh(CO)2l2] . However, in the iodide-promoted process, a higher concentration of methyl acetate is also employed, which reacts with the other components as shown in Eqs. 3, 7 and 8 ... 

Bulky quaternary ammonium salts promote the ruthenium-catalysed oxidation of anilines by hydrogen peroxide to nitrobenzenes [25]. In the absence of the ammonium salt, the major product is the azoxybenzene, whereas lower molecular weight tetra-alkylammonium salts produce a mixture of products. 

Electroreduction of aliphatic amides in the presence of chlorotrimethylsilane gives coupling products and this reaction is useful for the synthesis of a-amino ketones (Scheme 22) [41]. In this reaction, the formation of an Mg salt promotes the coupling of two anion radical centers. 

The combination of the electrogenerated chloro cation [Cl] with diphenyldise-lenide is a typical example of the functionalization of an olefin. The electrochemical oxyselenation-deselenation of (1) to (2) proceeds in an MeOH-NaCl-(Pt) system (Scheme 1) [32]. The bromide salt-promoted oxyselenation of olefins is discussed in Sect. 15.2.2. Penicillin (3) can be converted into the oxazoline-azetidinone (4) by a chloride salt-promoted paired reaction in an MeOH-t-BuOH(5 l)-MgCl2-(Pt) system at —40°C in 74 93% yield (Scheme 2) [33, 34]. This conversion probably involves an initial attack of an... 

The estimation of the conditions, suppressing the silyl-mediated catalysis and preferable for the carbenium-promoting catalysis, is of significant importance to introduce the chiral information in the product. Since it was observed that a carbe-nium salt promoted the reaction and thus provided the enantioselectivity in the outcome, the rigid conformation and the enhanced reactivity of the carbocation may be the key requirement for the productive enantioselective carbenium catalysis in the aldol-type additions. 

Effect of 1 salts Promoter/stabiliser Promoter/stabiliser Poison... 

I, 5-dithioniabicyclo[3.3.0]octane bis(trifluoromethanesulfonate) 7 in acetonitrile at 50°C. The salt promotes pinacol coupling of aromatic ketones even at —40 °C. In addition, the diastereoselectivity dl meso) of the coupling reaction of acetophenone in acetonitrile at —40°G is 94 6. "... 

Since the crown ethers are very effective complexing agents, the amount of free M+ in solution, as in (33)—(36), is expected to be small the crown ether competes very well with Rh and X for M +. Indeed, it is found that the addition of excess salt causes a much lower degree of rate inhibition in [18]-crown-6 as compared to some other solvents. For example, Fig. 10 illustrates the differences between [18]-crown-6 and tetraglyme as the level of salt promoter is increased. The capability of using an excess of salt reduces the criticality of precisely controlling the salt concentration and is beneficial for the stability of the catalyst (92). 

Studies of ruthenium-catalyzed reactions in carboxylic acid solvents have been reported by Knifton (171,172), but most of these experiments contain added salt promoters which greatly modify the catalytic behavior. These experiments will be considered in Section V, along with other Lewis base-promoted ruthenium systems. 

Reactions of ruthenium catalyst precursors in carboxylic acid solvents with various salt promoters have also been described (170-172, 197) (Table XV, Expt. 7). For example, in acetic acid solvent containing acetate salts of quaternary phosphonium or cesium cations, ruthenium catalysts are reported to produce methyl acetate and smaller quantities of ethyl acetate and glycol acetates (170-172). Most of these reactions also include halide ions the ruthenium catalyst precursor is almost invariably RuC13 H20. The carboxylic acid is not a necessary component in these salt-promoted reactions as shown above, nonreactive solvents containing salt promoters also allow production of ethylene glycol with similar or better rates and selectivities. The addition of a rhodium cocatalyst to salt-promoted ruthenium catalyst solutions in carboxylic acid solvents has been reported to increase the selectivity to the ethylene glycol product (198). 

Ruthenium-Catalyzed CO Hydrogenation with Potassium Salt Promoters h... 

The observed ratio of [HRu3(CO)n] to [Ru(CO)3I3] in solutions after catalysis is sometimes found to vary from the 2 1 ratio shown by (59). This may be expected if acids or bases (e.g., a basic solvent) are involved in oxidation or reduction processes, which can interconvert the two such equilibria can change with pressure (193). Nevertheless, these two species are normally observed to be stable under catalytic conditions, and a combination of the two is found to provide the optimum catalytic rates (e.g., see Fig. 21). Catalyst solutions derived from nonhalide salt promoters are presumed to contain [HRu3(CO)n] and an oxidized ruthenium species analogous to [Ru(CO)3I3]-, although no detailed studies of such systems have been reported. 

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