Bronsted relation derivations from

Related ammonium salts derived from amines, such as [CH3NH3]C1, [(CH3)2NH2]C1, and [(CH3)3NH]C1, also give acidic solutions because they too have cations with at least one dissociable proton. The pH of a solution that contains an acidic cation can be calculated by the standard procedure outlined in Figure 15.7. For a 0.10 M NH4C1 solution, the pH is 5.12. Although the reaction of a cation or anion of a salt with water to produce H30+ or OH - ions is sometimes called a salt hydrolysis reaction, there is no fundamental difference between a salt hydrolysis reaction and any other Bronsted-Lowry acid-base reaction. 

Although dimeric Sharpless ligands as catalysts showed impressive results in related organocatalytic transformations, they provided only limited success in asymmetric MBH reactions (Scheme 5.12) [70]. These compounds are bifunctional catalysts in the presence of acid additives one of the two amine function of the dimers forms a salt and serves as an effective Bronsted acid, while another tertiary amine of the catalyst acts as a nucleophile. Whereas salts derived from (DHQD)2PYR, or (DHQD)2PHAL afforded trace amounts of products in the addition of methyl acrylate 8a and electron-deficient aromatic aldehydes such as 27, (DHQD)2AQN, 56, mediated the same transformation in ee up to 77%, albeit in low yield. It should be noted that, without acid, the reaction afforded the opposite enantiomer in a slow conversion. 

In Ch. 19 Williams describes theoretical simulations of free-energy-relation-ships in proton transfer processes. Both linear and non-linear relations are observed, usually described in terms of Bronsted coefficients or Marcus intrinsic barriers. Derived from empirical data, the phenomenological parameters of themselves do not lead to satisfying explanations at a fundamental molecular level. Theoretical simulations can fill in this gap. 

A similar equation can be derived for acid catalysis by HX and HY. The expression in brackets in (124) represents the deviation from the simple Bronsted relation caused by the presence of the subsidiary equilibria in (122) if Xi = Xj no deviation will be observed, since we shall then have Kx = Ky = K. 2i deviation exists, it is clear that the separate values of Xi and X2 cannot be derived from observations on a single catalysed reaction, though in principle they can be obtained if the deviations are measured for two reactions with different values of p. However, a simpler result is obtained if, as is often the case, either x or X2 is close to zero. Thus if X2 = 0, corresponding to an anion which exists in a single form, (124) becomes... 

The over-bar notation for the Bronsted coefficient and its derivative, e.g. a , is introduced to distinguish it from the inverse coupling length used in Section 10.3) This result is closely related to, but more fundamentally based than the well-knoivn and ividely employed (cf Refs. [9c, 12[) Marcus relation [19]... 

So far our discussion of slow proton-transfers and Bronsted exponents has been a qualitative one, apart from the crude electrostatic model represented by Figures 15 and 16. Recently considerable use has been made of a general equation relating the rate of a reaction to its standard free energy change, which was first derived by Marcus for electron-transfer reactions, and later applied by him and by others to reactions involving the transfer of atoms or protons. For present purposes the Marcus equation can be written as ... 

More recently, several approaches to catalytic, enantioselective Nazarov cyclizations have been developed [28]. Trauner reported that substrate 266 underwent electrocyclic rearrangement, furnishing 268 in 94 % ee in the presence of the chiral Sc-PYBOX catalyst 267 (Equation 30) [134]. The enantio-discriminating step in this cyclization is believed to be protonation of the intermediate enolate that ensues from the Nazarov cyclization. Rueping has documented enantioselective cyclizations of related dienones in the presence of the chiral phosphoric acid derivative 270 (Equation 31) [135]. Chiral Bronsted acid catalysis thus effected the cyclization of 269 to give products 271 (87% ee) and 272 (95% ee). 

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