Aryl stacking

Experiments with quinoxaline complex 23 and phenazine complex 24 established that additional binding interactions were available in the form of aryl-aryl stacking between aromatic subunits in the components. In the case of quinoxaline this accounts for about 1.6 kcal or a factor of 15 in Ka. In 24, these attractive forces are partially offset by steric effects introduced by the remote ring as shown. 

Among the uses of other amino acids, L-tryptophan gives high ees and des in direct aldols of cyclic ketones and benzaldehydes in water 118 hydrophobic and aryl-stacking effects have been considered in explaining the selectivities. 

Aryl stacking effects in the transition state appear to play a role in the enantioselective addition of dimethylzmc to benzaldehyde when an A-benzylmandelamide-Ti(IV) complex is used as catalyst.235... 

Figure 1 Energetics of aryl stacking and hydrogen bonding in the complexation of 9-ethyladenine (3)...

Series of molecules such as (2a)-(2d) in Figure 1 allowed us to evaluate the energetics of aryl stacking versus hydrogen bonding in the complexation of 9-ethyladenine (3). From here, replicating molecules were just an idea away. 

One of the earliest examples of a polymer imprinted with a phosphate derivative came from Turkewitsch et al., who constructed a MIP for the selective recognition of cAMP in aqueous media (Fig. 3) [117]. The custom-designed fimctional monomer 1 was used to target the nucleotide via both aryl stacking and electrostatic interactions between the positively-charged nitrogen and the phosphate diester anion (Fig. 4). Rather cleverly, this func-... 

Tridentate chiral Schiff bases derived from (- -)-Q -pinene (123) catalyse enantioselective addition of diethylzinc to aryl aldehydes. Steric, aryl stacking, and electronic effects are significant in the transition state, the latter being probed by a Hammett plot of ee versus for a series of benzaldehydes. Complexes of the Schiff base (123) with titanium(IV) catalyse enantioselective cyanation. ... 

Figure 5 Examples of self-complementary molecules that yield dimeric assemblies (a) Cartoon representation of velcrand dimers that assemble through CH-pi, aryl-aryl stacking, and entropic effects (b) Rebek s softbaU dimCTs assembled through H-bonding (c) Nolte s molecular clips assanble into dimers through aryl-stacking and entropic effects. ...

Such an expression allows comparison of molecular recognition forces with the interactions encoded in lipophilicity. This is done in Figure 1, where the polar component of lipophilicity is seen to correspond to ion—dipole bonds, hydrogen bonds, orientation forces, and induction forces, whereas the hydrophobic component corresponds to dispersion forces and hydrophobic interactions. Only a limited number of recognition forces cannot find expression in lipophilicity as conventionally measured, namely, ionic bonds, charge transfer interactions, and aryl/aryl stacking interactions. ... 

---