Acid-catalyzed reactions, transition states

The si facial preference displayed by the reagent is enhanced in reactions proceeding through Lewis acid-catalyzed open transition states.Thus, when reacted with the ketene silyl acetal shown (eq 2) under zinc iodide catalysis, a 96 4 ratio of products was obtained. The corresponding uncatalyzed reaction led to an 85 15 mixture of the same products in similar yield. ... 

There has been little study of the stereoselectivity of the reaction under acidic conditions. In the absence of a coordinating Lewis acid, there is no preference for a cyclic transition state. When regioisomeric enols are possible, acid-catalyzed reactions tend to proceed through the more substituted of the enols. This reflects the predominance of this enol. (See Section 7.2.)... 

Parallel reactions give rate equations having sums of rate terms. Each term provides the transition state composition for a reaction path. Eor example, some acid-catalyzed reactions have the rate equation... 

Nitrones are a rather polarized 1,3-dipoles so that the transition structure of their cydoaddition reactions to alkenes activated by an electron-withdrawing substituent would involve some asynchronous nature with respect to the newly forming bonds, especially so in the Lewis acid-catalyzed reactions. Therefore, the transition structures for the catalyzed nitrone cydoaddition reactions were estimated on the basis of ab-initio calculations using the 3-21G basis set. A model reaction indudes the interaction between CH2=NH(0) and acrolein in the presence or absence of BH3 as an acid catalyst (Scheme 7.30). Both the catalyzed and uncatalyzed reactions have only one transition state in each case, indicating that the reactions are both concerted. However, the synchronous nature between the newly forming 01-C5 and C3-C4 bonds in the transition structure TS-J of the catalyzed reaction is rather different from that in the uncatalyzed reaction TS-K. For example, the bond lengths and bond orders in the uncatalyzed reaction are 1.93 A and 0.37 for the 01-C5 bond and 2.47 A and 0.19 for the C3-C4 bond, while those in... 

The Lewis acid-catalyzed reaction of nitrone 21 with ethyl vinyl ether 22 (Scheme 8.8) was also investigated for BH3 and AlMe3 coordinated to 21 [32]. The presence of BH3 decreases the activation energy for the formation of 23 by 3.1 and 4.5 kcal mol to 9.6 kcal mol for the exoselective reaction and 11.6 kcal-mol for the endo-selective reaction, respectively, while the activation energy for the formation of 24 increases by >1.4 kcal mol , compared to those for the uncatalyzed reaction. The transition-state structure for the BH3-exo-selective 1,3-dipolar cycloaddition reaction of nitrone 21 with ethyl vinyl ether 22 is shown in Fig. 8.19. 

HS may alter the reactivities of bound substrates in a way similar to that of anionic surfactants (inhibiting base-catalyzed and accelerating acid-catalyzed reactions). These effects were attributed to electrostatic stabilization of the transition state for the acid catalysis in which the substrate becomes more positively charged, and to destabilization of the transition state for base-catalyzed hydrolysis in which the substrate becomes more negatively charged. 

The highly ordered cyclic transition state of the Diels-Alder reaction permits design of reaction parameters which lead to a preference between the transition states leading to diastereomeric or enantiomeric adducts. (See Part A, Section 2.3, to review the principles of diastereoselectivity and enantioselectivity.) One way to achieve this is to install a chiral auxiliary.56 The cycloaddition proceeds to give two diastereomeric products which can be separated and purified. Because of the lower temperature required and the greater stereoselectivity observed in Lewis acid-catalyzed reactions, the best enantioselectivity is often observed in catalyzed reactions. Chiral esters and amides of acrylic acid are particularly useftd because the chiral auxiliary can be easily recovered upon hydrolysis of the adduct to give the enantiomerically pure carboxylic acid. 

This review starts with an introduction to the principles and techniques of solid-state NMR spectroscopy and the description of the most important experimental approaches for NMR investigations of solid catalysts in the working state (Sections II and III). Section IV is a summary of experimental approaches to the characterization of transition states of acid-catalyzed reactions under batch reaction conditions. 

IV. Study of Transition States of Acid-Catalyzed Reactions... 

Some time ago it was suggested by Zucker and Hammett (1939) that the behavior of the rates of acid-catalyzed reactions in moderately concentrated mineral acid (1-10m) could be used as a criterion for the degree of hydration of transition states. More recently this suggestion has been elaborated by Bunnett (1961). An acidity function, H0, analogous to pH, was defined (Hammett, 1940) by equations (19) and (20) where HI+ and I are the protonated and unprotonated forms of an... 

Nucleophilic participation of HMPT has been reported in dipolar aprotic solvents. Formation of a sulfonium salt has been observed in an acid-catalyzed reaction in DMSO. Another research group has examined the nucleophilic splitting of stereoisomeric 3-methoxycyclopentene and hexene oxides. Detailed studies have been performed on the geometry of the transition state in the reactions of cyclopentene and cyclohexene oxides, and their competitive opening by charged and neutral nucleophiles in acidic aqueous methanol. Other authors have dealt with the opening of epoxynorbornenes and the nature of the inter-... 

The most important consequence of restricted transition state selectivity is that ZSM-5 and many other medium-pore zeolites deactivate much slower than most other crystalline and amorphous catalysts. The difference is not trivial. In most acid catalyzed reactions large-pore zeolites deactivate within minutes or in hours, whereas the activity of ZSM-5 ranges from weeks to years. Most of the coke in large-pore zeolites is formed within the pores. In ZSM-5 most of the coke is deposited on the outer surface of the crystals like an eggshell over an egg [23] because coke precursors cannot form in the pores of pentasil molecular sieves. The resistance of ZSM-5 to coking makes a number of industrial processes economical. 

The specific acid-catalyzed reaction behaves in many respects like the water reaction. Thus, unusually large negative entropies of activation are found for all but those substrates which react by the acylium ion path (Schaleger and Long, 1963). At least two water molecules are involved in the transition state, the presence of at least five exchangeable hydrogens in the transition state, is deduced from solvent isotope effects in mixed water-heavy water (Salomaa et al., 1964). From the dependence of both acid-catalyzed exchange and hydrolysis in concentrated sulfuric acid on the activity of water. Lane (1964) concluded that two water molecules must be involved in the transition state. Except for the extra proton, the transition states for the acid-catalyzed and water reactions are very probably similar. 

Zucker-Hammett hypothesis This hypothesis states that, if in an acid-catalyzed reaction, lg k] (first-order rate constant of the reaction) is linear in H0 (Hammett acidity function), water is not involved in the transition state of the rate-controlling STEP. However, if lg k, is linear in lg[H+], then water is involved. This has been shown to be incorrect by Hammett. 

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