Brpnsted parameters

An archetypal example of TS imbalance is the nitroalkane anomaly. The rate (kj) of deprotonation of arylnitroalkanes is more sensitive to substituents on the aryl ring than the equilibrium constant (ATj = fc j/kj), as reflected in an unusually large value of the Brpnsted parameter, ocn (equation 5). In addition, )3 > 1.0 for ki and )3 < 0 for k-i. These data have been interpreted as showing that... 

Fig. 23.3. Schematic illustrations of the effect of changing the relative energy of reactants and products, A(A q,), in a reaction series (a) Only changes in the series, G is a constant in the series, (b) A jp changes as a result of a change of G. Only in part (a) the Brpnsted parameter (a) is meaningful as a progress parameter . In any event, the cause of the barrier is G.

The detection of Brpnsted acid sites, SiO(H)Al, is the most recent achievement of 170 NMR of zeolites [119-121]. High magnetic fields and double resonance techniques have allowed the observation of this important species in zeolite HY [120]. Chemical shifts of 21 and 24 ppm have been reported for zeolite HY for the Brpnsted sites in the supercage and sodalite cage, respectively [119]. Quadrupole interaction parameters are Cq = 6.0 and 6.2 MHz and r] = 1.0 and 0.9, respectively. Signal enhancement by 1H-170 cross-polarization has also permitted the detection of the acid sites in zeolite ZSM-5 [119], where they exist with lower abundance than in HY. 

As pointed out above, values of KTS are obtainable from rate data without making any assumptions about the reaction mechanism. Therefore, one may use KTs and its variation with structure as a criterion of mechanism, in the same way that physical organic chemists use variations in other kinetic parameters (Brpnsted plots, Hammett plots, etc.). For present purposes, the value of Kts can be useful for differentiating between the modes of binding in the S CD complex and the TS-CD transition state, between different modes of transition state binding, and hence between different types of catalysis (Tee, 1989). 

Hydrogen bond formation between dissimilar molecules is an example of adduct formation, since the hydrogen atom that is bonded to an electronegative atom, such as oxygen or nitrogen, is a typical acceptor atom. The ability of molecules to donate a hydrogen bond is measured by their Taft-Kamlet solvatochromic parameter, a, (or a . for the monomer of self-associating solutes) (see Table 2.3). This is also a measure of their acidity (in the Lewis sense, see later, or the Brpnsted sense, if pro tic). Acetic acid, for instance, has a = 1.12, compared with 0.61 for phenol. However, this parameter is not necessarily correlated with the acid dissociation constant in aqueous solutions. 

It is accepted that the acmal nucleophile in the reactions of oximes with OPs is the oximate anion, Pyr+-CH=N-0 , and the availability of the unshared electrons on the a-N neighboring atom enhances reactions that involve nucleophilic displacements at tetravalent OP compounds (known also as the a-effect). In view of the fact that the concentration of the oximate ion depends on the oxime s pATa and on the reaction pH, and since the pKs also reflects the affinity of the oximate ion for the electrophile, such as tetra valent OP, the theoretical relationship between the pATa and the nucleophilicity parameter was analyzed by Wilson and Froede . They proposed that for each type of OP, at a given pH, there is an optimum pK value of an oxime nucleophile that will provide a maximal reaction rate. The dissociation constants of potent reactivators, such as 38-43 (with pA a values of 7.0-8.5), are close to this optimum pK, and can be calculated, at pH = 7.4, from pKg = — log[l//3 — 1] -h 7.4, where is the OP electrophile susceptibility factor, known as the Brpnsted coefficient. If the above relationship holds also for the reactivation kinetics of the tetravalent OP-AChE conjugate (see equation 20), it would be important to estimate the magnitude of the effect of changes in oxime pX a on the rate of reactivation, and to address two questions (a) How do changes in the dissociation constants of oximes affect the rate of reactivation (b) What is the impact of the /3 value, that ranges from 0.1 to 0.9 for the various OPs, on the relationship between the pKg, and the rate of reactivation To this end, Table 3 summarizes some theoretical calculations for the pK. ... 

Following up on our idea, we initially focused on the exploration of appropriate Brpnsted acids and the examination of reaction parameters, such as catalyst loading, hydride source, temperature, solvent and concentration. While the use of various Brpnsted acids with different acidities resulted in the desired products, the application of DPP again showed the best reactivities and yielded a diverse set of tetrahydroquinolines with different substituents in 2-, 3- or 4-position. 

Hence, the initial reactions were conducted using BINOL-phosphate 5 in combination with various achiral Brpnsted acids, including pro-tonated pyridine derivatives, alcohols and acids. Best enantioselectiv-ities were observed with the addition of carbonic acids, phenol and hexafluoro isopropanol which provided the isoquinuclidines 28 with up to 88% ee. Further explorations concentrated on varying the reaction parameters including different protected imines, catalyst load-... 

Due to the substitutional disorder of the T-atoms, local geometrical parameters of the Brpnsted sites are not accessible from diffraction studies, the diffraction pattern reflects only an average geometry. If the concentration of the T-atom substitution is small, the average geometry can be quite close to the structure of an ideal all-silica structure and the distortion becomes more and more pronounced with an increasing amount of A1 (in zeolites) or of Si (in SAPOs). The most affected parameters are the T-0 distances and the Si-O-T bond angles. 

It is important to underline that, even if the Brpnsted site itself has the same constitution (Si-O(H)-Al) in alumino-silicates and silico-alumino-posphates, the typical geometrical parameters of the local structure are significantly dif-... 

Table 3.3 Electrochemical ligand parameters of the five species which may be involved in nitrile hydrolysis (substrates, products, intermediates). Whereas nitriles are not tightly bound, hydroxide binds to either metal center more strongly than both carboxamide and the corresponding base, acylamidate. Here, however, relative Brpnsted acidities of water and carboxamides must be taken into account. Water can only be transferred to a nitiile before being deprotonated at Zn(II) but not at Co(II) which means that this is not the actual mechanism of biochemical nitiile hydrolysis. Negative values of -log are given in brackets because they correspond to unstable complexes which hardly persist in water...

The adsorption of ammonia, pyridine, trimethylamine, and triethylamine on silica and silica-alumina was studied microcalorimetrically by Cardona-Martinez and Dumesic (18, 105). The calorimetric results of this study were correlated successfully in terms of Drago parameters for each catalyst. These parameters describe well the acidic properties of silica and the strongest sites (Lewis acid sites) on silica-alumina and may allow the prediction of heats of adsorption for a wide range of basic molecules with known Drago parameters on these sites. Parameters to describe the strength of the Brpnsted sites could not be determined because the contribution from these sites could not be studied independently. 

In an attempt to avoid the limitations of the one-parameter equations, Edwards (19, 20) in 1954 added polarizability (as measured by the oxidation potential of oxidative dimerization of nucleophiles, E°) as a second parameter to get a modified Brpnsted equation ... 

A major contribution of Ritchie s has been his observation that a large number of nucleophiles show a constant selectivity toward a variety of electrophiles. In LFER terms, the reactivity of the nucleophile can be given by a single parameter with no selectivity coefficient (such as Brpnsted (5 or Swain-Scott s) (26b) ... 

It was soon realized, however, that this simple definition of nucleophilicity would not suffice. Shortly afterward, Edwards (6) attempted to define relative nucleophilicities in terms of two parameters, H (basicity) and Eox (oxidation potential), using the (variable) coefficients a and to relate these properties to changes in the electrophile (equation 5). Later, Edwards and Pearson substituted a polarizibility parameter, P, for Eox. In essence, equation 5 is a Brpnsted equation with a second parameter added. 

From the results summarized in Table I, apparently the Brpnsted relationship will hold for all combinations of nucleophiles and electrophiles. Because, as pointed out previously, the Hammett equation is really a special case of the Brpnsted relationship, all the legion of nucleophile-electrophile, rate-equilibrium Hammett correlations that have been studied also fall under the scope of the Brpnsted relationship. For example, nucleophilicities of ArO , ArS , ArC(CN)2 , and the other families listed in footnote c of Table I have generally been correlated by the Hammett equation, where the acidities of benzoic acids in water are used as a model for substituent interactions with the reaction site (a), and the variable parameter p is used to define the sensitivity of the rate constants to these substituent effects. The Brpnsted equation (equation 3) offers a much more precise relationship of the same kind, because this equation does not depend on an arbitrary model and allows rate and equilibrium constants to be measured in the same solvent. Furthermore, the Brpnsted relationship is also applicable to families of aliphatic bases such as carboxylate ions (GCH2C02 ), alkoxide ions (GCH20 ), and amines (GCH2NH2). In addition, other correlations of a kinetic parameter (log fc, AGf, Ea, etc.) can be included with various thermodynamic parameters (pKfl, AG°, Eox, etc.) under the Brpnsted label. 

According to my treatment, the magnitude of the a effect increases with the extent of bond formation in the transition state as represented by the Brpnsted, Pnuc, parameter. This finding is in accord with experimental observations, and Aubort and Hudson (34) showed some time ago (Table II) that the a effect for p-nitrophenyl acetate (Pnuc - 0.8) is much greater than that for an alkyl bromide (Pnuc O. 3). Dixon and Bruice (35) showed a similar effect for the reaction of hydrazines with a wide range of electrophiles. 

At the time of the study by Dessy et al. (7), little was known about the Brpnsted basicity of MLn in a quantitative sense. Therefore, what role, if any, was played by the parameter H in the Edwards equation was unclear. Recently, a number of pKa values were measured for transition metal hydrides, both neutral, HMLn, and cationic, HMLn+. The solvents used were chiefly methanol (12) and acetonitrile (13, J. R. Norton and J. Sullivan, personal communication). 

As explained earlier, Figures 1 and 2 will not be greatly changed if data in solution are used. Individual bases will move up or down, parallel to the lines shown. Therefore, under the normal conditions for nucleophilic substitution, ease of oxidation (E0 ) and Brpnsted basicity are not independent parameters for bases where the donor atom is a second-row element. Either parameter may be used as a measure of nucleophilic reactivity. 

Stereochemical and kinetic analyses of the Brpnsted acid-catalysed intramolecular hydroamination/deuterioamination of the electronically non-activated cyclic alkene (13) with a neighbouring sulfonamide nucleophile have been found to proceed as an anh-addition (>90%) across the C=C bond to produce (15). No loss of the label was observed by and NMR (nuclear magnetic resonance) spectroscopies and mass spectrometry (MS). The reaction follows the second-order kinetic law rate = 2 [TfOH] [13] with the activation parameters being = 9.1 0.5 kcal moP and = -35 5 cal moP An inverse a-secondary kinetic isotope effect of d/ h = (1-15 0.03), observed for (13) deuteration at C(2), indicates a partial CN bond formation in the transition state (14). The results are consistent with a mechanism involving concerted, intermolecular proton transfer from an N-protonated sulfonamide to the alkenyl C(3) position coupled with an intramolecular anti-addition by the sulfonamide group. ... 

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