Geometric Configurations Steric Effects

The basic concept was proposed by Baladin [1] asserting that the adsorption and the reaction activity depend on the attraction of molecules by surface atoms. Therefore, it depends on the steric effects or relates the spatial arrangement of atoms in a molecule and of surface atoms, especially, as it affects chemical reactions. This is called multiplet theory, which assumes congruence between the molecule and the surface atoms. 

The classical example is the reaction of ethanol on alumina, occurring dehydration and dehydrogenation reactions of ethanol, which depends on the interaction between the molecule and the surface atoms [2]. Besides the steric coincidence, it requires also equal energy levels or similar bonding forces. That depends on the spatial configuration. Generically, atoms of a surface flat (111) which can be represented as follows (Eig. 5.1). 

The steric configuration of an adsorbate allows us to calculate the distance between atoms of the surface and of the adsorbed molecules and to determine the 

Imagine the steric planar configuration of toluene and propylene in separate, and the configurations of o-propyl-benzene and p-propyl-benzene as the main products, one as desired and the other one as xmdesired [3]. Thus, 

Assuming different planar configurations and adsorption forms. There are three possibilities (Fig. 5.2). 

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The first two examples show that the geometrical configuration of the parent olefin is maintained. Also the orientation of the ethoxy group is syn to the adjacent substituent in the products. The cycloadduct of an unsymmetrical olefin has a substituent vicinal to the ethoxy group (head-to-head adduct). The addition reaction is subject to steric effects trans-2-butene reacts about twice as fast as cis-2-butene. 

Johnson rearrangement of allylic alccdiols (274) and (277) led to the enantiomeric 7,8-unsaturated esters (276) and (279), respectively (Scheme 48). Both transition states (275) and (278) favor a pseudo-equatorial position of the benzyloxymethylene substituent the newly formed chiral center is obtained in very hi optical purity throu the chirality transfer process. As a consequence of this geometrical preference, secondary allylic alcohols invariably provide predominantly ( )-configured double bonds upon thermal Gaisen rearrangement The ( -selectivity usually increases with the steric bulk of the C-2 substituent, an effect which was rationalized by a pseudo-1,3-diaxial interaction in the transition state leading to the (Z)-alkene (280 Figure S). ... 

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