Reactions in Organic Media

Most organic solvents are less polar than water, so that if intimate ion pairs of glycosyl cations and anions are too unstable to exist in water, a fortiori they have no real existence in organic solvents and mechanistic proposals which invoke them are simply in error. Even the reality of the ion-molecule complexes shown to exist in water must be questioned. Nonetheless, many literature sources draw intimate ion pairs and even solvent-equilibrated oxocarbenium ions, in the absence of any direct evidence. 

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Jaeger and co-workers studied the regioselectivity of the reaction of a surfactant diene with a surfactant dienophile in micellar media" ". The orientational effects in the aggregates could result in an increase in the regjoselectivity in aqueous solutions of these compounds as compared to the reaction in organic media. 

Interestingly, at very low concentrations of micellised Qi(DS)2, the rate of the reaction of 5.1a with 5.2 was observed to be zero-order in 5.1 a and only depending on the concentration of Cu(DS)2 and 5.2. This is akin to the turn-over and saturation kinetics exhibited by enzymes. The acceleration relative to the reaction in organic media in the absence of catalyst, also approaches enzyme-like magnitudes compared to the process in acetonitrile (Chapter 2), Cu(DS)2 micelles accelerate the Diels-Alder reaction between 5.1a and 5.2 by a factor of 1.8710 . This extremely high catalytic efficiency shows how a combination of a beneficial aqueous solvent effect, Lewis-acid catalysis and micellar catalysis can lead to tremendous accelerations. 

Homolytic scission of the 0-0 bond of hydrogen peroxide may be effected by heat or UV irradiation.245 The thermal reaction requires relatively high temperatures (>90 Photolytic initiation generally employs 254 nm light. Reactions in organic media require a polar cosolvent (e.g. an alcohol). 

The performance of organic reactions in organized media, e.g. by zeolite confinement, and the use of zeolites as selective and green catalysts for organic transformations, have been popularized in recent years. The main advantage of the zeolites to be tested as media or catalysts for carrying out organic reactions is the so-called shape selectivity . ... 

Koskinen, A.M.P. and Klibanov, A.M. Eds. (1996) Enzymatic reactions in organic media. Blackie Academic and Professional, London. 

Sheldon, R.A. (1996) Large-scale enzymatic conversions in non-aqueous media. Enzymatic Reactions in Organic Media, A.M.P.Koskinen and A.M.Klibanov. London Chapman Hall. 

Ideally, thermodynamic activities of the reactants should be used in the equation, but since concentrations are normally easier to measure these are often used instead. The use of the activity of water (which can be measured fairly easily) and the concentrations of the other reactants has been recommended for studies of enzyme catalyzed reactions in organic media (Hailing, 1984). In order to increase the synthesis of the ester, the water concentration (or activity) should be reduced. This can be achieved by replacing part of the water with a water miscible solvent. 

The reaction conditions have a great influence on the enzyme-catalysed reactions in organic media and determine the reactions yield and selectivity. 

When it is important to control the water activity in a reactor, a water activity sensor is quite useful. The sensor should ideally measure the water activity in the liquid reaction medium. However, the sensors available are designed for gas phase measurements, and, provided there is effective enough equilibration between the liquid and gaseous phases, they can be used to control the water activity in the reactor. If the measured water activity is above the set point, drying is initiated, for example, by passing dry air through the reactor. On the other hand, if the water activity is too low, water can be added, either as liquid water or as humid air. Automatically controlled systems of this kind have been successfully used to monitor and control enzymatic reactions in organic media [13, 14]. 

Figure 2.1 Enantioselectivity of enzyme-catalyzed reactions in organic media plotted as a function of the logP-value of the solvent. Data obtained from the literature 1. (open circles) [81], 2. (open diamonds) [17], 3. (closed triangles) [19], 4. (closed circles) [20], 5. (open squares) [82], 6. (closed squares)...

Z. Yang, A. J. Russell, Fundamentals of non-aqueous enzymology. In Enzymatic Reactions in Organic Media, A. M. P. Koskinen, A. M. Klibanov, Eds., Blackie Academic and Professional London,... 

We illustrate in this section with a number of examples how the presence or absence of free volume within a reaction cavity determines the feasibility of a reaction in organized media. Presence of free volume alone may not be sufficient to effect a reaction within a reaction cavity. Its location and its directionality (presence of free volume in the critical dimension) are extremely important, as revealed by a number of examples discussed in Section D. 

VIII. NORRISH II REACTIONS IN ORGANIZED MEDIA A. Neat Crystalline Phases... 

Willner, I. Laane, C. Otvos, J.W. Calvin, M. in "Inorganic Reactions in Organized Media" Holt, S.L., Ed. Advances in Chemistry Series, No. 177, American Chemical Society,... 

Koskinen A.M.P. and Klibanov A.M. (Eds.), Enzymatic Reactions in Organic Media, Springer, Berlin, 1995. 

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