Homogeneous conditions

SCHEME 13.15 Homogeneous hydrogenation of arenes with ti -C3H5Co[P(OMe)Jj (34). 

SCHEME 13.16 Mechanism of the homogeneous hydrogenation of arenes with ri -CjHjCoPCOMefjjj (34). 

A variety of PAHs were also homogeneously hydrogenated in a highly regioselective manner but lacking stereoselectivity in the presence of Co fCOlg and synthesis gas (CO+H ) at elevated temperature. For instance, 9,10-dimethylanthracene (44) led to nearly equal amounts of cis- and 

SCHEME 13.17 Proposed mechanismforthe homogeneous hydrogenation of 9,10-dimethylanthracene with 

SCHEME 13.18 Homogeneous hydrogenation of arenes with niobium and tantalum compounds 47 and 48. 

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Table 2.10 shows the effect of substituents on the endo-exo ratio. Under homogeneous conditions there is hardly any substituent effect on the selectivity. Consequently the substituents must have equal effects on the Gibbs energies of the endo and the exo activated complex. 

In summary, the groups of Espenson and Loh observe catalysis of Diels-Alder reactions involving monodentate reactants by Lewis acids in water. If their observations reflect Lewis-acid catalysis, involvirg coordination and concomitant activation of the dienophile, we would conclude that Lewis-acid catalysis in water need not suffer from a limitation to chelating reactants. This conclusion contradicts our observations which have invariably stressed the importance of a chelating potential of the dienophile. Hence it was decided to investigate the effect of indium trichloride and methylrhenium trioxide under homogeneous conditions. 

Clearly, under homogeneous conditions, no significant catalytic effect is observed. Note that we have... 

An even less complicated reaction vessel may be used for reactions in liquid amtnonia which produce only a small amount of "heat" over a relatively long period and which proceed under homogeneous conditions. The conversion can then be performed in a one-necked flask with a stopper + gas outlet or small hole. 

The Smith-Ewart kinetics described assume homogeneous conditions within the particle. An alternative view, where monomer polymerizes only on the surface of the particle, has been put forth (35) and supported (36). The nature of the intraparticle reaction environment remains an important question. 

These values assume chlorination in carbon tetrachloride solution under homogeneous conditions favoring random distribution of chlorine atoms along the chain. Viscous reaction conditions, faster chlorine addition rates, lower temperature conditions, etc, can lead to higher AH at equivalent chlorine levels because of more blocky chlorine distribution on the polymer chain. 

These values are given for polymers of narrow molecular-weight distribution, with number-average molecular weights (M ) of about 20,000 prior to chlorination. Chlorination reactions are carried out under homogeneous conditions in CCl solutions at temperatures between 90 and 110°C with viscosities at about 5 Pa (50 P). 

A subsequent study, using clearly homogeneous conditions, of the dedeutera-tion of [2-2H]-4-nitro-, -chloro-, and -methyl-phenols by aqueous sulphuric acid showed that the rate coefficients clearly increased with increasing acidity (Table 120) and plots of log rate coefficient versus the acidity function — H0 were linear,... 

Partial reduction of alkynes to Z-alkenes is an important synthetic application of selective hydrogenation catalysts. The transformation can be carried out under heterogeneous or homogeneous conditions. Among heterogeneous catalysts, the one that... 

The chloromethylated polymers are very reactive substrates for nucleophilic attach further elaboration can be accomplished under homogeneous conditions In aprotlc solvents, or under heterogeneous conditions In the presence of phase transfer catalysts. The following examples are representative of approaches to functionalized condensation polymers via chloromethylated Intermediates. 

The most successful approach to producing an aminomethyl derivative was the Gabriel synthesis. A phthalimide substituent can be introduced by Sn2 displacement of the chloride on 17 with potassium phthalimide under homogeneous conditions in DMF. The reaction is quantitative in all D.F. ranges and the phthalimldo-methyl intermediates, 18, are quite soluble in organic solvents. 

M Jumaa, BW Muller. The effect of oil components and homogenization conditions on the physicochemical properties and stability of parenteral fat emulsions. Int J Pharm 163(1—2) 81—89, 1998. 

Methanol played a role not only on selectivity but also on activity. With the methanol-lean solution the conversion was higher than with the commercial, methanol-rich solution. This confirms that methanol competes with formaldehyde for adsorption on active sites in the zeolite cavities. The ratio between isomers was not very different from that achieved at pH 2 under homogeneous conditions. 

Assuming that the reaction occurs under homogeneous conditions, it appears quite clear that in the case of sufficiently high length of kinetic chains, the primary appearance of free radicals by the rate w (Scheme 2) is very soon replaced by the decomposition of hydroperoxides and the process has the character of a "closed loop" [68]. 

It is to be noted that the reactions mentioned so far were performed under homogeneous conditions and, in most cases, using polar solvents, which are efficient absorbers of MW energy. Rate enhancements were attributed to the superheating of the solvent due to the elevated pressures generated in the closed vessels. 

Substantial MW rate enhancements have been reported in the Biginelli synthesis of dihydropyrimidines [50, 51] under homogeneous conditions. The synthesis involves a one-pot cyclocondensation of a /i-ketoester with an aryl aldehyde and urea or thiourea in the presence of a catalytic amount of HC1 in ethanol solution. An example of this synthesis is shown in Scheme 4.17. 

The rates of certain reactions of polymers have been reported to be enhanced by MW under homogeneous conditions at atmospheric pressure. Lewis et al. [64] performed kinetic studies on the imidization of the polymer BDTA-DDS polyamic acid 46 in N-methylpyrrolidone (NMP) giving the polymer 47 (Scheme 4.24) and showed that the apparent activation energy was reduced from 105 kj mol-1 under conventional heating to 55 kj mol-1 under MW heating. Rate enhancements (kMw/kthermai)... 

However, more significant modifications in selectivity, which have useful applications in synthesis, have been reported in a number of other reactions performed under the action of MW irradiation [9, 44, 70]. Many of these reactions were performed under heterogeneous and/or solvent-free conditions, and only those performed under homogeneous conditions will be discussed here. 

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