Selectivity Factor 1 - Electron Density

Just as different substituents are able to stabilise carbocations by releasing electrons towards them, substituents can release electrons into double bonds or draw them out from it. Electron donating groups such as alkyl groups and ethers will increase the electron density of double bonds to which they are attached. Therefore, in hydrocarbons the more heavily substituted double bonds will be more electron rich. So, if there are two or more double bonds in a molecule, electron deficient reagents such as ozone or peracids will preferentially attack the more/most electron rich olefin. 

Limonene (5.29) provides us with an example of this type of selectivity. The endocyclic double bond of limonene is trisubstituted and is therefore richer in electrons than the disubstituted olefin in the isopropenyl group. 

Ozone will therefore selectively cleave the ring double bond leaving the other untouched, provided of course, that no more than one molar equivalent of ozone is used. Similarly, one molar equivalent of m-chloroperbenzoic acid will selectively give only limonene 1,2-epoxide 

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The inner cavity of carbon nanotubes stimulated some research on utilization of the so-called confinement effect [33]. It was observed that catalyst particles selectively deposited inside or outside of the CNT host (Fig. 15.7) in some cases provide different catalytic properties. Explanations range from an electronic origin due to the partial sp3 character of basal plane carbon atoms, which results in a higher n-electron density on the outer than on the inner CNT surface (Fig. 15.4(b)) [34], to an increased pressure of the reactants in nanosized pores [35]. Exemplarily for inside CNT deposited catalyst particles, Bao et al. observed a superior performance of Rh/Mn/Li/Fe nanoparticles in the ethanol production from syngas [36], whereas the opposite trend was found for an Ru catalyst in ammonia decomposition [37]. Considering the substantial volume shrinkage and expansion, respectively, in these two reactions, such results may indeed indicate an increased pressure as the key factor for catalytic performance. However, the activity of a Ru catalyst deposited on the outside wall of CNTs is also more active in the synthesis of ammonia, which in this case is explained by electronic properties [34]. 

DEN in high selectivity, the transethylation was not explained by the shape-selective catalysis, either. The key factor for the high selectivity is the steric hindrance and high electron density at the substitution site of naphthalene. 

Both the conversion to aldehydes and the selectivity to normal aldehydes observed in the hydroformylation of 1-hexene by these complexes were markedly ligand dependent. A linear relationship between the electron density on the nitrogen atom and the normal/branched aldehyde ratio was found, indicating that for these aminophosphines the ratio is largely controlled by electronic factors.332... 

Similar geometric optimization has been reported for bicyclo[3.2.2]nona-6,8-diene (BND). The double bond situated in the opposite direction to the methylene group was found to be more exo-pyramidalized than the other double bond and the electron density (qi, HOMO) of the former double bond in HOMO of the molecule higher than that of the latter double bond. The exo and endo faces of exo-pyramidalized double bonds proved not to be equal and the electron density was found to be higher on the endo faces. The endo molecular complexes with bromine have been found by the HF/321G method to be more stable than their exo congeners this was attributed to electronic and steric factors. As a result, endo-facial stereoselectivity of bromination ( ) predominates.21 A related theoretical study of facial selectivity and regioselectivity of the electrophilic addition of chlorine to exo-tricyclo[4.2.1.02,5]nona-3,7-diene (exo-TND) has also been reported.22... 

Another important type of complex is formed between metal ions and cyclic organic compounds, known as macrocycles. These molecules contain nine or more atoms in the cycle and include at least three heteroatoms, usually oxygen, nitrogen, or sulfur. Crown ethers such as 18-crown-6 and dibenzo-18-crown-6 are examples of organic macrocycles. Some macrocyclic compounds form three-dimensional cavities that can just accommodate appropriately sized metal ions. Ligands known as cryptands are examples. Selectivity occurs to a large extent because of the size and shape of the cycle or cavity relative to that of the metal, although the nature of the heteroatoms and their electron densities, the compatibility of the donor atoms with the metal, and several other factors also play important roles. 

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