Microsolvation Effects

The rate acceleration imposed by 0-cyclodextrin was explained in terms of a microsolvent effect 6> The inclusion of the substrate within the hydrophobic cavity of cyclodextrin simulates the changes in solvation which accompany the transfer of the substrate from water to an organic solvent. Uekama et al.109) have analyzed the substituent effect on the alkaline hydrolysis of 7-substituted coumarins (4) in the... 

In contrast to the reactions of the cycloamyloses with esters of carboxylic acids and organophosphorus compounds, the rate of an organic reaction may, in some cases, be modified simply by inclusion of the reactant within the cycloamylose cavity. Noncovalent catalysis may be attributed to either (1) a microsolvent effect derived from the relatively apolar properties of the microscopic cycloamylose cavity or (2) a conformational effect derived from the geometrical requirements of the inclusion process. Kinetically, noncovalent catalysis may be characterized in the same way as covalent catalysis that is, /c2 once again represents the rate of all productive processes that occur within the inclusion complex, and Kd represents the equilibrium constant for dissociation of the complex. 

The manifestation of noncovalent catalysis as a microsolvent effect is illustrated by cycloamylose-catalyzed decarboxylations of activated carboxylic acid anions. Anionic decarboxylations, as illustrated in scheme VII, are generally assumed to proceed by a rate-determining heterolytic... 

Cramer and Kampe (1965), in fact, proposed a specific interaction between the included substrate and the cycloamylose hydroxyl groups to explain accelerated rates of decarboxylation. In view of the more recent results, particularly the insensitivity of the rate accelerations to the structure of the substrate and the pH independence, a nonspecific microsolvent effect now seems more likely. 

An additional example of cycloamylose-induced catalysis which can probably be attributed to a microsolvent effect is the oxidation of a-hy-droxyketones to a-diketones (Scheme VIII). The rate of this oxidation is accelerated by factors ranging from 2.1 to 8.3 as the structure of the substrate is varied. As noted by Cramer (1953), these accelerations may be attributed to a cycloamylose-induced shift of the keto-enol equilibrium to the more reactive enol form. 

Hydrolyses of p-nitrophenyl and 2,4-dinitrophenyl sulfate are accelerated fourfold and eightfold, respectively, by cycloheptaamylose at pH 9.98 and 50.3° (Congdon and Bender, 1972). These accelerations have been attributed to stabilization of the transition state by delocalization of charge in the activated complex and have been interpreted as evidence for the induction of strain into the substrates upon inclusion within the cycloheptaamylose cavity. Alternatively, accelerated rates of hydrolysis of aryl sulfates may be derived from a microsolvent effect. A comparison of the effect of cycloheptaamylose with the effect of mixed 2-propanol-water solvents may be of considerable value in distinguishing between these possibilities. 

Recently, an example of cycloamylose-induced catalysis has been presented which may be attributed, in part, to a favorable conformational effect. The rates of decarboxylation of several unionized /3-keto acids are accelerated approximately six-fold by cycloheptaamylose (Table XV) (Straub and Bender, 1972). Unlike anionic decarboxylations, the rates of acidic decarboxylations are not highly solvent dependent. Relative to water, for example, the rate of decarboxylation of benzoylacetic acid is accelerated by a maximum of 2.5-fold in mixed 2-propanol-water solutions.6 Thus, if it is assumed that 2-propanol-water solutions accurately simulate the properties of the cycloamylose cavity, the observed rate accelerations cannot be attributed solely to a microsolvent effect. Since decarboxylations of unionized /3-keto acids proceed through a cyclic transition state (Scheme X), Straub and Bender suggested that an additional rate acceleration may be derived from preferential inclusion of the cyclic ground state conformer. This process effectively freezes the substrate in a reactive conformation and, in this case, complements the microsolvent effect. 

An additional example of a cycloamylose-induced rate acceleration which may be reasonably attributed to a conformational effect is the facilitation of the transfer of the trimethylacetyl group from the phenolic oxygen of 9 to the aliphatic oxygen of the adjacent hydroxymethyl group to form 10. This intramolecular transesterification is remarkably enhanced relative to a comparable intermolecular reaction,6 and occurs, at pH 7.0 and 25.5°, with a rate constant of 0.0352 sec-1 (Griffiths and Bender, 1972). An even larger rate enhancement is achieved upon inclusion of this material within the cyclohexaamylose cavity—fc2 = 0.16 sec-1. This fivefold acceleration cannot be satisfactorily explained either by a microsolvent effect which would be expected to depress the rate of the reaction or, at this pH, by covalent... 

The rate of decarboxylation of activated carboxylate anions [e.g. (10)], shows strong solvent dependence. It is not surprising, therefore, that these reactions have been used to probe the microsolvent effects of micelles and CDs (Fendler and Fendler, 1975). In particular, it was anticipated that complexation with a CD might result in catalysis by providing an environment for the reaction that is less polar than water. 

The stabilization of a substratS)(as the result of inclusion complex formation occurs noncova-lently and may be the result of either a microsolvent effect ora conformational effect (Szejtli, 1982). The kinetic method utilizes this reduction in rate of the reactioiScrfhen ligand is present to obtain information about the nature of the complex. The basic assumption is that the decreased reactivity is the sole result of complexation, the compleSdsfeing less reactive than frtafeThe kinetic scheme can be represented as Scheme 8.1 ... 

Theoretical studies of the microsolvation effect on SN2 reactions have also been reported by our coworkers and ourselves (Gonzalez-Lafont et al. 1991 Truhlar et al. 1992 Tucker and Truhlar 1990 Zhao et al. 1991b, 1992). Two approaches were used for interfacing electronic structure calculations with variational transitional state theory (VST) and tunneling calculations. We analyzed both the detailed dynamics of microsolvation and also its macroscopic consequences (rate coefficient values and kinetic isotope effects and their temperature... 

Microsolvent effects in the cyclodextrin cavity have also been observed in hypochlorite chlorination of acetophenone1029. Higher para selectivity has been observed in the bromination of acetanilide and benzanilide in presence of cyclodextrins or amylose1030 and in the anodic chlorination of anisole with cyclodextrin-modified electrode1031. 

Chaudhari, A. Lee, S.-L. A computational study of microsolvation effect on ethylene glycol by density functional method, J. Chem. Phys. 2004,120, 7464-7469. 

A visual pigment, rhodopsin, is mimicked by (29) [45]. (29) has an absorption maximum at 375 nm in its neutral form. However, the maximum shows a red shift of 100 - 120 nm on protonation of both the nitrogen atoms at low pH. The absorption spectrum of the diprotonated (29) is almost identical with that of native rhodopsin. Probably the retinal moiety of (29) is included in the cavity of cyclodextrin at low pH, resulting in the red shift due to a combination of an electrostatic effect and a microsolvent effect. 

The cavity of CyD is apolar, and thus any reactions, which proceed rapidly in apolar media, should be accelerated simply by a "microsolvent effect . A classic example of this effect of a CyD is decarboxylation of anions of activated acids (e.g. a[Pg.99]

CyDs accelerate or decelerate various reactions, ediibiting many kinetic features shown by enzyme reactions, i.e. catalyst-substrate complex formation, competitive inhibition, saturation, and stereospecific catalysis [67]. CyD-catalyzed reactions can generally be classified in the following three categories according to the type of stimulation (a) partidpation of the hydroxyl groups of CyDs (b) the microsolvent effect of the hydrophobic CyD cavity and (c) the conformational or steric effect of CyDs [67]. 

Mercier SR, Boyarkin OV, Kamariotis A, Guglielmi M, Tavernelli I, Cascella M, Rothlisberger U, Rizzo TR. (2006) Microsolvation effects on the excited-state dynamics of protonated Tryptophan. J. Am. Chem. Soc. 128 16938 16943. 

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