Promoted reactions

Several types of Pd-catalyzed or -promoted reactions of conjugated dienes via TT-allylpalladium complexes are known. The Pd(II)-promoted oxidative difunctionalization reactions of conjugated dienes with various nucleophiles is treated in Chapter 3, Section 4, and Pd(0)-catalyzed addition reactions of conjugated dienes to aryl and alkenyl halides in this chapter. Section 1.1.1. Other Pd(0)-catalyzed reactions of conjugated dienes are treated in this section. 

The lower temperatures and reduced degree of oxygen starvation in LPO (vs VPO) generally reduce carbon monoxide production markedly by promoting reaction 18 and suppressing reaction 21. As a consequence, acids, from further oxidation of aldehydes, are usually the main products. 

Metal Ion-Promoted Reactions of Thiols. Metal ion-promoted reactions of thiols have been reviewed (53). The bulk of the coverage concerns metal ion promoted aspects of sulfur chemistry. The main topics of interest are the formation of sulfenamides, sulfides, and disulfides using metal-mediated reactions. 

Furan and thiophene undergo addition reactions with carbenes. Thus cyclopropane derivatives are obtained from these heterocycles on copper(I) bromide-catalyzed reaction with diazomethane and light-promoted reaction with diazoacetic acid ester (Scheme 41). The copper-catalyzed reaction of pyrrole with diazoacetic acid ester, however, gives a 2-substituted product (Scheme 42). 

This process has many similarities to the Phillips process and is based on the use of a supported transition metal oxide in combination with a promoter. Reaction temperatures are of the order of 230-270°C and pressures are 40-80 atm. Molybdenum oxide is a catalyst that figures in the literature and promoters include sodium and calcium as either metals or as hydrides. The reaction is carried out in a hydrocarbon solvent. 

The Friedel-Crafts reaction is a very important method for introducing alkyl substituents on an aromatic ring. It involves generation of a carbocation or related electrophilic species. The most common method of generating these electrophiles involves reaction between an alkyl halide and a Lewis acid. The usual Friedel-Crafts catalyst for preparative work is AICI3, but other Lewis acids such as SbFj, TiC, SnCl4, and BF3 can also promote reaction. Alternative routes to alkylating ecies include protonation of alcohols and alkenes. 

This process is used to treat gas streams containing high concentrations of H2S. The chemistry of the units involves partial oxidation of hydrogen sulfide to sulfur dioxide and the catalytically promoted reaction of hh.S and SO2 to produce elemental sulfur. The reactions are staged and arc. is lollows ... 

A catalyst is a substance that increases the rate of a reaction, other than by a medium effect, regardless of the ultimate fate of this substance. For example, in hydroxide-catalyzed ester hydrolysis the catalyst OH is consumed by reaction with the product acid some writers, therefore, call this a hydroxide-promoted reaction, because the catalyst is not regenerated, although the essential chemical event is a catalysis. 

The mechanism of the light-promoted reaction involves a series of electron and proton transfers from water to Q. 

The effectiveness of nitromethane can be attributed to its high dielectric constant, at least in part, which tends to promote reactions which involve electron-rich intermediates. It may also result from the low solubility of the indole products in nitromethane since the indoles precipitate out of the reaction mixture in many cases. ... 

Alternatively, cyclization has been accomplished using various acids which dramatically altered the selectivity in the cyclization, a cyclization that failed under thermal conditions as in the preparation of 59 and 60. Often, the yield of the acid-promoted reaction was lower than the thermally-promoted cyclization. Of course, the temperature in which acid-promoted reactions were conducted was much lower than the normal 250 °C required for thermal cyclization. The selectivity was altered when an acid was used as in 58 —> 59 and 60, suggesting a change in mechanism. 

Benzo-3-thiatricyclo[4.1.0.02-7]heptene (I, X = H) is a valene -type valence isomer of 2 and 3 however, 1 isomerizes to 3 in photochemical and silver(I)-promoted reactions.75 However. when 1 is heated in carbon tetrachloride at 150°C 3 is not produced but instead the isomeric 1,2-disubstituted 2a,7b-dihydrobenzo[f>]cyclobuta[r/]thiophene. When a rhodium(I)... 

The diastereofacial selectivity of Lewis acid promoted reactions of allylsilancs with chiral aldehydes has been thoroughly investigated58. Aldehydes with alkyl substituted a-stereogenic centers react with a mild preference for the formation of Cram products, this preference being enhanced by the use of boron trifluoride-diethyl ether complex as catalyst58. 

An interesting and stereoselective synthesis of 1,3-diols has been developed which is based on Lewis acid promoted reactions of /f-(2-propenylsilyloxy (aldehydes. Using titanium(IV) chloride intramolecular allyl transfer takes place to give predominantly Ag/r-l,3-diols, whereas anti-1,3-diols, formed via an / / /-molecular process, are obtained using tin(IV) chloride or boron trifluoride diethyl ether complex71. 

Lewis acids, particularly the boron trifluroride diethyl ether complex, are used to promote the reaction between allyl(trialkyl)- and allyl(triaryl)stannanes and aldehydes and ketones52-54. The mechanism of these Lewis acid promoted reactions may involve coordination of the Lewis acid to the carbonyl compound so increasing its reactivity towards nucleophilic attack, or in situ transmetalation of the allyl(trialkyl)stannane by the Lewis acid to generate a more reactive allylmetal reagent. Which pathway operates in any particular case depends on the order of mixing of the reagents, the Lewis acid, temperature, solvent etc.55- 58. 

The stereoselectivity of Lewis acid promoted reactions between 2-butenylstannanes and aldehydes has been widely studied, and several very useful procedures for stereoselective synthesis have been developed. In particular syn-products are formed stereoselectively in reactions between trialkyl- and triaryl(2-butenyl)stannanes, and aldehydes induced by boron trifluoride-diethyl ether complex, irrespective of the stannane geometry66. 

H. Kunz in Selectivity in Levis Acid Promoted Reaction, D. Schinzer, Ed., p 189, Kluwer, Amsterdam 1989. 

While general agreement has been reached concerning the catalytic behaviour of the product metal in promoting reaction, other aspects of the rate process have been less satisfactorily characterized these include the changes which precede nucleus formation, the distribution of such sites and development of the reaction interface. 

Metal-ion-promoted reactions of organo-sulphur compounds. D. P. N. Satchell, Chem. Soc. Rev., 1977,6, 345-371 (65). 

The cyclization of aryl 3-chloropropyl sulfones by potassium t-butoxide in t-butyl alcohol at 30 °C (equation 20) has a p value of 2.32 for substituents in Ar202. This is considered by Bird and Stirling to indicate the formation of an intermediate carbanion which is essentially in equilibrium with the reactants. A recent review by Stirling203 deals with structure-reactivity aspects of many sulfonyl promoted reactions of this type. 

In view of the above physical meaning of A it is clear why A can approach infinite values when Na+ is used as the sacrificial promoter (e.g. when using j "-Al203 as the solid electrolyte) to promote reactions such as CO oxidation (Fig. 4.15) or NO reduction by H2 (Fig. 4.17). In this case Na on the catalyst surface is not consumed by a catalytic reaction and the only way it can be lost from the surface is via evaporation. Evaporation is very slow below 400°C (see Chapter 9) so A can approach infinite values. 

When first discovered in the eighties as a pronounced apparent violation of Faraday s law it looked like a phenomenon of limited importance, praised however already by several leading electrochemists and surface scientists including Bockris21 and Pritchard.22 The subsequent involvement of the groups of Comninellis, Haller, Lambert, Sobyanin, Anastasijevic, Smotkin and others and the continuous discovery of new electrochemically promoted reactions broadened substantially the horizons of electrochemical promotion as it became obvious that the phenomenon was not limited to any particular electrolyte, conductive catalyst or type of reaction. 

Proton conductors ammonia synthesis, 468 conductivity, 93 ethylene oxidation, 470 hydrogen oxidation, 457 list of electrochemically promoted reactions, 146... 

Enzymes are biological catalysts that function by modifying substrate molecules to promote reaction. 

The results of reactions with and without MW irradiation are reported in Table 4.11. The reaction yields are comparable, but the reaction times of the irradiated reactions are considerably reduced. The alumina does not give acceptable results. The same reactions were carried out in nitrobenzene as solvent and under free-solvent conditions with and without MW irradiation. The results are reported in Table 4.12. In this case too, the only significant difference is the reaction time, so that the authors [41] concluded that MW-promoted reactions proceed like the thermal reactions except for a much higher reaction rate. 

N-protected aminoacids 14 behave as carboxyl partners in Ag20 promoted reactions of 2-bromo-isobutyramides or -propanamides affording, respectively, esters of 2-hydroxy-isobutyramides (ref. 20a) or lactamides 15 (ref. 20b). While we plan further research on stereochemical and other aspects of these reactions, model units obtained from 2-bromo-isobutyramides have been used in conformational studies of depsi-peptides (ref. 21). 

The Lewis acid promoted reactions of styrene with 7V-phenylsulfonyl-l,4-benzoquinone monoimine to the 2-aryIbenzofuranoid ring system have been reported previously. This offers a new route to henzofinanoid neoUgnans (( )-hcarin B, eupomatenoid-1, eupomatenoid-12) (Scheme 25, <96TL6969>). 

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