Structure-reactivity relationships functional group effects

Electrophilic aromatic substitution is a situation in which it is useful to discuss TS structure in terms of a reaction intermediate. The ortho, para, and meta directing effects of aromatic substituents were among the first structure-reactivity relationships to be developed in organic chemistry. Certain functional groups activate aromatic rings toward substitution and direct the entering electrophile to the ortho and para positions, whereas others are deactivating and lead to substitution in the meta position. The bromination of methoxybenzene (anisole), benzene, and nitrobenzene can serve as examples for discussion. 

The transfer and the adsorption are strongly influenced by the size of the compound (surface and volume) and by the functional groups present in the molecule (alcohols, aldehydes, ketones, carboxylic acids, amines, mercaptans, halogenated molecules) inducing some polarization effects. Some studies have reported a quantitative structure activity relationship (QSAR) between molecular structure and adsorption parameters (adsorption capacities, energies) [32,37,78]. The reactivity of some compounds leads to oxidation at the adsorbent surface, which plays a catalytic role. Moreover, a mixture of molecules in air... 

In 2005, the Tsogoeva group examined the structure-activity relationships for different unmodified N-terminal primaiy amino dipeptides [e.g. H-Phe-His-OH, H-His-Phe-OH, H-Leu-Phe-OH, H-Leu-His-OH, H-His-Leu-OH, H-Lys-His-OH) in the known model reaction of acetone with 4-nitrobenzaldehyde.The reactivities and stereoselectivities were found to be dependent upon the intramolecular cooperation of side-chain functional groups and the presence of a suitable combination and sequence of amino acids. Interestingly, among the studied N-terminal primaiy amino dipeptides, H-Leu-His-OH (24) appeared to be the most effective (Scheme 13.25a). Furthermore, the synthetic scope of the selected peptide catalyst H-Leu-His-OH in aldol reaction of acetone with several aromatic aldehydes has been demonstrated. Good yields (up to 96%) and enantioselectivities (up to 76% ee) were obtained with electron-deficient aromatic aldehydes (Scheme 13.25a). 

---