Métodos de Evaluación

N N R1 H O O O R2 H + N N R1 H O O O R2 H

! This recognition motif was first described[189-191] _{by Hamilton and co-workers in their}

attempt to develop receptors for biscarboxylic acids. Analysis of the complexed structures by x-ray crystallography showed that the association does not involve proton transfer from the carboxylic acid to the pyridine. Furthermore, the 6-methyl substituent was attached to avoid self-association between the amidopyridine moieties.

! Since our group has successfully incorporated this recognition motif into a wide

range of replicating structures, no structural variants were synthesised and probed. Only recently, work on the development of self-replicating rotaxane structures led to the replacement of the original 6-methyl substituent with a 4,6-dimethyl moiety for steric

reasons.[192] _{Preliminary results showed a positive effect of the additional methyl group on}

the solubility of the investigated compounds. Moreover, controlling the electron density on the pyridine ring by varying the substituent pattern should allow a fine-tuning of the strength of the recognition event. An exhaustive investigation of such variation on the efficiency of self-replicating and AB systems can be found in chapter 4 of this thesis.

! The choice of solvent is closely connected with the employed recognition unit. Polar

solvents which act as good hydrogen bond donor/acceptor favour the solvation of the recognition sites and disrupt the binding. For this reason, non-polar solvents that do not participate in hydrogen bonding, such as chloroform or dichloromethane, must be chosen.

2.2 Template geometry restraints

Template molecule T has to possess the appropriate geometry to align A and B in a fashion

which brings their reactive sites into close proximity. Ideally, template molecules are of rigid and linear structures with their recognition units easily accessible to A and B. In the case of too much structural flexibility, the peril of forming a catalytically inert [A·B] complex is increased. However, since rigid structures, e.g. highly conjugated aromatic systems, tend to be less soluble than compounds with more conformational freedom, e.g. alkyl chains, it is often not straightforward to strike a balance between both requirements. Detailed investigations of how small changes in geometry can affect drastically the efficiency of self- replicating systems or how minor changes can turn a self-replicating system into an AB system can be found in chapter 3 of this thesis.

2.3 The chemical reaction suitable for self-replication

The reaction between A and B to from template T must have certain properties in order to

spectroscopy. Ideally, the reaction should not require the addition of a catalyst or working under inert conditions. The formation of the product should furthermore be irreversible to

facilitate kinetic analysis. Systems based on a reversible Diels-Alder reaction[141,193] _{and the}

reversible formation of an imine[185] _{have been investigated and the increasing complexity}

arising through the reversibility of the process was verified. Additionally, the chosen reaction should proceed with high (regio-)selectivity without producing any side-products. Moreover, the reactive sites should be fully orthogonal to the chosen recognition sites. The rate of reaction should allow its observation over the course of several hours without having to work at extreme temperatures. Synthetic access to the reagents should be straightforward

and for the analysis of the reaction kinetics by 1_{H NMR spectroscopy, starting materials and}

product should produce a defined set of peaks which can be integrated over time.

! The 1,3-dipolar cycloaddition between a maleimide and a nitrone was found to be a

suitable candidate for incorporation in self-replicating structures. The formation of the isoxazolidine product proceeds through a concerted mechanism in which the bonds between the diene and the dienophile are created simultaneously. The rate of the reaction has been found to be virtually insensitive towards addition of acid or base catalysts. As a consequence of the asymmetric geometry of the nitrone structure, two racemic diastereoisomeric products can be formed which are distinguished by the relative position of protons on the bicyclic ring

system. In case of the cis-isomer, all protons are on the same side with respect to the

isoxazolidine ring, whereas for the trans-isomer, the two hydrogens on the ring junction are

opposed to the remaining third proton. In the 1_{H NMR spectra, each diastereoisomer}

therefore shows a characteristic pattern for these protons.

! In the absence of any recognition effects, the cycloaddition between an N-aryl nitrone

and a maleimide proceeds through a simple bimolecular pathway with low selectivity, usually leading to a trans/cis ratio between 1.4:1 and 3.9:1 depending on the electronic nature of the substituents on the two reagents. This indifference towards electronic modification was also found in a screening of the reaction between an acylic N-phenyl nitrone and a set of N-aryl maleimides bearing various electronic substituents.[194] _{The analysis of the final}

stereochemistry resulted in modest trans/cis ratios in the narrow range of 1.6:1 (for strongly electron donating substituents) to 1:1.8 (for strongly electron withdrawing substituents). Compared to the selectivity found in other [4+2] cycloadditions such as the Diels-Alder

reaction, these trans/cis ratio are considerably low. This stems from the fact that the frontier

orbital orientation of the two reagent does not allow for efficient secondary interactions, thus leaving steric and/or aromatic stacking interactions (ᴨ-ᴨ interactions) of the substituents as

nitrone and a maleimide can be steered by both HOMO-LUMO interactions since the frontier molecule orbital (FMO) energies are similar. In this balanced situation, the influence of electron-donating or electron-withdrawing substituents on the reagents can alter the relative FMO energies and tip the scales in favour of one reaction type.

Scheme 2.2 1,3-dipolar cycloaddition reaction between nitrone 61 and maleimide 62 proceeding