Transition-state molecular shape

Up to this point, we have emphasized the stereochemical properties of molecules as objects, without concern for processes which affect the molecular shape. The term dynamic stereochemistry applies to die topology of processes which effect a structural change. The cases that are most important in organic chemistry are chemical reactions, conformational changes, and noncovalent complex formation. In order to understand the stereochemical aspects of a dynamic process, it is essential not only that the stereochemical relationship between starting and product states be established, but also that the spatial features of proposed intermediates and transition states must account for the observed stereochemical transformations. 

Because the pore dimensions in narrow pore zeolites such as ZSM-22 are of molecular order, hydrocarbon conversion on such zeolites is affected by the geometry of the pores and the hydrocarbons. Acid sites can be situated at different locations in the zeolite framework, each with their specific shape-selective effects. On ZSM-22 bridge, pore mouth and micropore acid sites occur (see Fig. 2). The shape-selective effects observed on ZSM-22 are mainly caused by conversion at the pore mouth sites. These effects are accounted for in the hydrocracking kinetics in the physisorption, protonation and transition state formation [12]. 

Ad(ii) On catalysts with pores and cavities of molecular dimensions, exemplified by mordenite and ZSM-5, shape selectivity provides constraints of the transition state on the S 2 path in either preventing axial attack as that of methyl oxonium by isobutanol in mordenite that has to "turn the comer" when switching the direction of fli t through the main channel to the perpendicular attack of methyl oxonium in the side-pocket, or singling out a selective approach from several possible ones as in the chiral inversion in ethanol/2-pentanol coupling in HZSM-5 (14). Both of these types of spatial constraints result in superior selectivities to similar reactions in solutions. 

Conclusive evidence has been presented that surface-catalyzed coupling of alcohols to ethers proceeds predominantly the S 2 pathway, in which product composition, oxygen retention, and chiral inversion is controlled 1 "competitive double parkir of reactant alcohols or by transition state shape selectivity. These two features afforded by the use of solid add catalysts result in selectivities that are superior to solution reactions. High resolution XPS data demonstrate that Brpnsted add centers activate the alcohols for ether synthesis over sulfonic add resins, and the reaction conditions in zeolites indicate that Brpnsted adds are active centers therein, too. Two different shape-selectivity effects on the alcohol coupling pathway were observed herein transition-state constraint in HZSM-5 and reactant approach constraint in H-mordenite. None of these effects is a molecular sieving of the reactant molecules in the main zeolite channels, as both methanol and isobutanol have dimensions smaller than the main channel diameters in ZSM-S and mordenite. 

Conducting reactions in nanospace where the dimensions of the reaction vessel are comparable to those of the reactants provides a new tool that can be used to control the selectivity of chemical transformations.1 This dimensional aspect of nano-vessels has been referred to as shape selectivity.2 The effect of spatial confinement can potentially be exerted at all points on the reaction surface but its influence on three stationary points along the reaction coordinate (reactants, transition states, and products) deserve special attention.3,4 (1) Molecular sieving of the reactants, excluding substrates of the incorrect dimension from the reaction site can occur (reactant selectivity). (2) Enzyme-like size selection or shape stabilization of transition states can dramatically influence reaction pathways (transition state selectivity). (3) Finally, products can be selectively retained that are too large to be removed via the nano-vessel openings/pores (product selectivity). 

Spatioselectivity or Transition State Selectivity. TSS occurs when the formation of reaction intermediates (and/or transition states) is sterically limited by the space available near the active sites. Contrary to molecular sieving, TSS does not depend on the size of pore openings, but depends on the size and shape... 

I. A property present in a molecular entity (or a transition state) if the energy of that entity or state is enhanced due to unfavorable bond lengths, bond angles, or dihedral angles relative to some appropriate standard. It is the standard enthalpy of a structure relative to a strainless structure (real or hypothetical). 2. The change of volume or shape of a body, or portion of a body, due to the influence of one or more applied forces. 

Zeolite catalysed alkylation of polynuclear aromatics is considered to be simultaneously governed by several mechanisms. To achieve highly shape-selective catalysis, it is essential that the pore size precisely corresponds to the molecular dimensions of reactants and products, and to the transition state of the reaction intermediates. 

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