REACTIONS IN NON-AQUEOUS MEDIA

Carbon monoxide is readily oxidized in the coordination sphere of a number of transition metal complexes. In many cases the product of reaction is a carbonate complex which is formed irreversibly, thus precluding the possibility of a catalytic transformation. In Section 5 the reaction between CO and platinum dioxygen complexes was shown to give carbonate complexes. The reaction between iridium, ruthenium and osmium carbonyl complexes and dioxygen to give coordinated carbonate was discussed in Section 6. 

The relative amounts of OC O, 0C 0 and 0C 0 in the carbon dioxide complex were found to be about 1,2, and 1, respectively while the ratio of 0PPh3 to 0PPh3 was 1 1. Such a distribution led Iwashita and Hayata [110] to propose a reaction mechanism in which coordinated O2 and two coordinated CO ligands form an intermediate having four equivalent oxygens and two equivalent carbon atoms. 

Since rhodium carbon dioxide complexes are not as stable as the corresponding carbonate complexes with respect to CO, catalysis is possible. For example thermal decomposition of the complex [Rh2(PPh3)3(C0)2(C02)] C6H6, begins at 90 °C and the complex gives up one mole of CO2 per rhodium atom [192]. Furthermore, treatment of a benzene solution of [Rh2(PPh3)3(C0)2(C02)2] with carbon monoxide gave a rhodium carbonyl complex and 2 moles of carbon dioxide, equation (148). 

It is interesting to note that passing air through a benzene solution of the dicarbon monoxide complex leads to the mono-carbon monoxide complex, [Rh2(PPh3)3(C0)2(C02)] reported by Iwashita and Hayata [110]. 

Shortly after the report of the reaction of a rhodium carbonyl complex with O2 to give coordinated CO2, the catalytic oxidation of CO to CO2 was reported in a similar system [193]. Kiji and Furukawa reported that both [RhCl(COXPPh3)2] and [RhCl(COXMe2SO)2] catalyzed the oxidation of CO in benzene, ethanol, or dimethylsulfoxide. However, the rate was slow and the catalytic efficiency was poor. Reactions catalyzed by [RhCl(COXMe2SO)2] in benzene gave approximately 11m moles of CO2 per m mole of metal complex. The authors describe the reaction as involving (1) coordination of molecular oxygen, (2) reaction with CO in the coordination sphere, and (3) displacement of the product by unreacted CO. 

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Tapia, O. and Andres, J. A simple protocol to help calculate saddle points. Transition state structures for the Meyer-Schuster reaction in non-aqueous media an ab initio MO study., Chem. Phys. Letters, 109 (1984), 471-477... 

Just as with hydrothermal synthesis, it is possible to carry out the reaction in non-aqueous media, it is possible to form micelles using alcohol as the polar phase. Using ethanol as the polar phase, it is possible to further tailor the solubihties of metal hydroxides. In the SrFei20i9 hexaferrite, simply coprecipitation strontium and iron to make the mixed metal hydroxide results in a mixed-oxide phase. Sr(OH)2 has such a high solubUity... 

In addition to hydrolyzing carboxylic ester bonds, lipases can catalyze a variety of esterification reactions in non-aqueous media. Industrial applications of lipases have focused on their enantioselectivity, regioselectively and substrate... 

High-pressure kinetics indicate the absence of solvent dynamical effects in outer-sphere bimolecular self-exchange reactions in non-aqueous media, even in the most favorable experimentally accessible case (the Fe(phen)j couple) - on the contrary, expectations based on TST alone are generally met, and AV is negative. 

Deetz JA, RozzeU J (1988) Enzyme-catalysed reactions in non-aqueous media. TIBTECH 6 15-19... 

Ha SH, Lan MN, Lee SH et al. (2007) Lipase-catalyzed biodiesel production from soybean oil in ionic liquids. Enzyme Microb Technol 41 480 83 Hari Krishna S, Karanth NG (2002) Lipases and lipase-catalyzed esterification reactions in non-aqueous media. Catal Rev 44(4) 499-591... 

Yadav GD, Devi KM (2004) Immobilized Upase-catalysed esterification and transesterification reactions in non-aqueous media for the synthesis of tetrahydrofurfuryl butyrate comparison and kinetic modeUng. Chem Eng Sd 59(2) 373-383... 

We have now moved to the area of coordination chemistry in which water is to be avoided as a solvent. This is a general area with many facets. Examples of the preparation of complexes by substitution reactions in non-aqueous media are the following. 

Hischer T, Steinsiek S, Ansorge-Schumacher MB. Use of polyvinyl alcohol cryogels for the compartmentation of biocatalyzed reactions in non-aqueous media. Biocatal. Biotransform. 2006 24 437-442. 

The Bronsted-Lowry theory of acids and bases referred to in Section 10.7 can be applied equally well to reactions occurring during acid-base titrations in non-aqueous solvents. This is because their approach considers an acid as any substance which will tend to donate a proton, and a base as a substance which will accept a proton. Substances which give poor end points due to being weak acids or bases in aqueous solution will frequently give far more satisfactory end points when titrations are carried out in non-aqueous media. An additional advantage is that many substances which are insoluble in water are sufficiently soluble in organic solvents to permit their titration in these non-aqueous media. 

Kinetics and mechanisms of substitution reactions of cobalt(III) frans-dioximes, in non-aqueous media. N. M. Samus and A. V. Ablov, Coord. Chem. Rev., 1979, 28,177-203 (47). 

The exchange reaction between Pb(lV) and PbiJI) in non-aqueous media... 

If in the quaternary ion a /3-hydrogen is not available, as in tetramethyl-, benzyltrimethyl- or phenyltrimethylammonium hydroxide, reaction 4.101 cannot occur, but reaction 4.102 still can this means that the quaternary ammonium ions without a /1-hydrogen are appreciably more stable, so that the type of base concerned is mainly the commercially available one (in alcoholic solution) (CH3)4NOH appears especially attractive, although the (CH3)4N salts are less soluble in non-aqueous media than, for instance, (C4H9)4N salts. 

The formation of a weakly basic tertiary amine in reaction 4.102 does not alter the titrant normality, but in the titration of an acid it may suppress the height of the titration curve on the basic side. In an extensive study of twelve quaternary ammonium titrants in non-aqueous media (mainly isopropyl alcohol), Harlow73 observed large differences in stability the presence of water had a profound stabilizing action but at the sacrifice of basic strength inert and basic solvents increased the rate of decomposition (see Fig. 4.18). 

Electrodes of the first kind have only limited application to titration in non-aqueous media a well-known example is the use of a silver electrode in the determination of sulphides and/or mercaptans in petroleum products by titration in methanol-benzene (1 1) with methanolic silver nitrate as titrant. As an indicator electrode of the second kind the antimony pH electrode (or antimony/antimony trioxide electrode) may be mentioned its standard potential value depends on proton solvation in the titration medium chosen cf., the equilibrium reaction on p. 46). 

Coulometry. Even in water, controlled potential or potentiostatic coulometry is a difficult and often time-consuming technique, as the analyte must participate in a direct electrode reaction. Therefore, in non-aqueous media there are only a few examples of its application, e.g., the potentiostatic coulometry of nitro and halogen compounds in methanol (99%) with graphical end-point prediction, as described by Ehlers and Sease153. 

N-AryInitrones (XIII) formed by oxidation of N-hydroxy-N-methyl arylamines, show high reactivity toward carbon-carbon and carbon-nitrogen double bonds in non-aqueous media (21,203) (Figure 10). Under physiological conditions, however, it appears that N-arylnitrones exist as protonated salts that readily hydrolyze to formaldehyde and a primary N-hydroxy arylamine and efforts to detect N-arylnitrone addition products in cellular lipid, protein or nucleic acids have not been successful (204). Nitroxide radicals derived from N-hydroxy-MAB have also been suggested as reactive intermediates (150), but their direct covalent reaction with nucleic acids has been excluded (21). 

The porphyrin complexes of manganese exist in a range of formal oxidation states of which the +3 state is most stable. Mn(m) porphyrins may be electrochemically oxidized in non-aqueous media to yield products which are probably Mn(m) cation radicals or di-cations (Kelly Kadish, 1982). The potentials at which these reactions occur are sensitive... 

Nitrosonium (NO+) is a strong oxidant and the reduction potential to NO has been measured in non-aqueous media (1.67 V vs. SCE in CH3CN), and estimated for water (Eq. (3)) (12,15). NO+ is subject to rapid hydrolysis to nitrite (2H+ + N02 ), and therefore if formed in biological media would be short-lived. However, other less water-sensitive chemical species can act as NO+ donors in reactions leading to the nitrosation of various substrates. For example, the reactions of certain metal nitrosyl complexes with nucleophiles such as R SH can lead to the transfer of NO+ as illustrated in Eq. (4). Such reactions will be discussed in greater detail below. 

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