Metal-salt-based promoters

Often the requisite THF oxonium ion is generated m situ by using a combination of reagents based on the Meerwein syntheses of trialkyl oxonium salts (150). These combinations include epichlorohydrin or a reactive haUde with a Lewis acid, a reactive hahde with a metal salt, or sometimes just a Lewis acid alone. The epoxide portion is often referred to as a promoter. 

Extensive research has been conducted on catalysts that promote the methane—sulfur reaction to carbon disulfide. Data are pubhshed for sihca gel (49), alurnina-based materials (50—59), magnesia (60,61), charcoal (62), various metal compounds (63,64), and metal salts, oxides, or sulfides (65—71). Eor a sihca gel catalyst the rate constant for temperatures of 500—700°C and various space velocities is (72)... 

Sulfonium ylides generated through base-promoted deprotonation of sulfonium salt have been extensively studied. The reaction of sulfides with a diazo carbonyl compound in the presence of a transition metal catalyst is an alternative approach to obtain sulfonium ylides. Sulfonium ylides are more stable than the corresponding oxonium ylides. Stable sulfonium ylides generated by the reaction of an Rh(ii) carbene complex with thiophene have been reported (Figure 5). ... 

The number of solvents that have been used in SrnI reactions is somewhat limited in scope, but this causes no practical difficulties. Characteristics that are required of a solvent for use in SrnI reactions are that it should dissolve both the organic substrate and the ionic alkali metal salt (M+Nu ), not have hydrogen atoms that can be readily abstracted by aryl radicals (c/. equation 13), not have protons which can be ionized by the bases (e.g. Nth- or Bu O" ions), or the basic nucleophiles (Nu ) and radical ions (RX -or RNu- ) involved in the reaction, and not undergo electron transfer reactions with the various intermediates in the reaction. In addition to these characteristics, the solvent should not absorb significantly in the wavelength range normally used in photostimulated processes (300-400 nm), should not react with solvated electrons and/or alkali metals in reactions stimulated by these species, and should not undergo reduction at the potentials employed in electrochemically promoted reactions, but should be sufficiently polar to facilitate electron transfer processes. 

These salts can be used for the synthesis of both transition metal and main group element thionyl imides by metathetical reactions, for example, Cp2Ti(NSO)2 and Ph3 As(NSO) (x = 1, 2), respectively. " The M NSO group invariably adopts a cis geometry in these derivatives. A characteristic reaction of thionyl imides is the thermal or base-promoted elimination of SO2 to give the corresponding sulftir dimide. ... 

There are several ways to synthesize the homoleptic bis(phthalocyaninato) rare earth complexes RE(III)(Pc02, Figure 4.35. The simplest way is to heat a mixture of a metal salt and phthalonitrile in a ratio of approximately 1 8 to 280-290 °C followed by chromatographic separation of the product [57]. Alternatively, this cyclic tetramerization method towards RE(III)(Pc02 could be conducted with an organic base such as l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a promoter [58]. In addition, a ligand condensation method also plays an important role in the synthesis of homoleptic bis(phthalocyaninato) rare earth complexes. This procedure is based on the reaction of H2PC or Li2(Pc ) with rare earth salts in a solvent with a... 

Both one-component and two-component silicone sealants contain polydimethyl siloxane as the base polymer along with fillers such as calcium carbonate and/or fumed silica fillers, plasticisers (silicone oil) and a variety of cross-linking agents and adhesion promoters. Two-component sealants utilise a catalyst such as dibutyl tin dilaurate, alkyl silicate esters and metallic salts (Maslow, 1982). 

Phosphorus oxychloride is a suitable reagent for preparation of the symmetrically substituted phospho-triesters of type (RO)3PO. The preparation is easily achieved by treatment of phosphorus oxychloride with 3 equiv. of alcohols or their metal salts. The reaction is generally promoted by a base or acid. Titanium trichloride is a particularly effective catalyst for the reaction. Conversion of POCI3 to unsymmetri-cally substituted phosphotriesters is achievable with difficulty. Phosphorochloridates and phosphorodichloridates have been used for the preparation of mixed tertiary phosphoric esters of type (ROlmPOfOROn (ffi = 1, n = 2, or m = 2, n = 1) in a very wide variety. Reaction of phosphorus oxychloride and 1 or 2 equiv. of alcohols followed by hydrolysis forms phosphomonoesters or phosphodi-esters, respectively. The hydrolysis may be generally effected by dilute aqueous alkali. Some phosphoFodichlori te intermediates are easily hydrolyzed by water. For example, the phosphorylation of a ribonucleoside (1 equation 4) with phosphorus oxychloride in an aqueous pyridine-acetonitrile mixture furnishes the nucleoside S -monophosphate (2) in excellent yield. ... 

Treatment of the phosphino-iminium salts (169) with potassium hydroxide in THF affords the formylphosphines (170), which are remarkably stable in solution compared with the related phosphine oxides Phosphines bearing aminoyl radical substituents, e.g. (171), have also been prepared. The phospha[3]triangulane, (172), has been obtained from the reaction of bicyclo-propylidene with a metal complexed phenylphosphinidene. " The phosphino-trithiacyclophane (173) has been prepared by the base-promoted reaction of... 

Mukaiyama aldol reactions using a catalytic amount of a Lewis acidic metal salt afford silylated aldols (silyl ethers) as major products, but not free aldols (alcohols). Three mechanistic pathways which account for the formation of the silylated aldols are illustrated in Scheme 10.14. In a metal-catalyzed process the Lewis acidic metal catalyst is regenerated on silylation of the metal aldolate by intramolecular or intermolecular silicon transfer (paths a and b, respectively). If aldolate silylation is slow, a silicon-catalyzed process (path c) might effectively compete with the metal-catalyzed process. Carreira and Bosnich have concluded that some metal triflates serve as precursors of silyl triflates, which promote the aldol reaction as the actual catalysts, as shown in path c [46, 47]. Three similar pathways are possible in the triarylcarbenium ion-catalyzed reaction. According to Denmark et al. triarylcarbenium ions are the actual catalysts (path b) [48], whereas Bosnich has insisted that hydrolysis of the salts by a trace amount of water generates the silicon-based Lewis acids working as the actual catalysts (path c) [47]. Otera et al. have reported that 10-methylacridinium perchlorate is an efficient catalyst of the aldol reaction of ketene triethylsilyl acetals [49]. In this reaction, the perchlorate reacts smoothly with the acetals to produce the actual catalyst, triethylsilyl perchlorate. 

We recently disclosed that salts of alkali and alkaline earth metals effectively promote fhe aldol reaction of DMS enolates (Scheme 10.34) [101]. For example, fhe CaCl2-catalyzed reaction of propiophenone DMS enolate with aldehydes proceeds smoothly in DMF at 30 °C with high reaction efficiency. In the metal salt-catalyzed aldol reaction fhe counter anion of fhe metal salt plays a crucial role in rate acceleration. The activity of metal salt increases with increasing intrinsic nucleophilicity of fhe counter anion TfO metal salts work as Lewis bases to activate DMS enolates. To our surprise, the... 

The majority of the catalysts used in the work reported here were based on MgO or ZnO and were promoted by Li or Ba ions. They were synthesised by wet impregnation using an appropriate alkali-metal salt. The preparation of other samples are reported elsewhere (refs. 5,6). All the samples were calcined in air at 850°C prior to being tested. The reaction system made use of quartz fixed-bed reactors and gas analysis was carried out with gas chromatography (ref. 5). The catalyst (particle-sizes from 0.3 - 0.6 mm) was diluted with the same weight of quartz particles of the same size. The process conditions used are given with the results. One set of data were obtained with a recycle reactor for this, a... 

Concerning the cyanating agent, hydrogen cyanide and its alkaline metal salts are frequently used to effect cyanation in the presence of a catalytic or a stoichiometric amount of a base, such as amines, phosphines, and metal alkoxides. Acetone cyanohydrin, which serves as an alternative to hydrogen cyanide, also promotes the cyanation of carbonyl and imino compounds (transhydrocyanation) to yield the corresponding cyanohydrins or aminonitriles along with the formation of acetone. 

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