Anilines self-association

Self-association of protic solvents, including anilines and aliphatic amines in apolar aprotic solvents, is an important process, because it involves a high percentage of stoichiometric amine as solute, depending on the solute structure, the solvent, the temperature and, obviously, the amine concentration. 

Self-association of anilines was suggested to be responsible for several anomalous kinetic behaviours in nucleophilic aromatic substitution reactions in apolar aprotic solvents40, but this conclusion was questioned41. 

The monomer/dimer equilibrium may be studied at very low concentrations. The KD value (equation 3) ranges from 0.44 M 1 at 10 °C to 0.26 M 1 at 70 °C, in a concentration range of aniline from 0.03 to 0.2 M. At concentration values of up to 4 M of aniline, the self-association involves monomer, dimer and tetramer species. The tetramer probably exist in a cyclic form. 

Self-association is a relevant property also for derivatives of aniline, such as anilides49 or Schiff bases50. In anilides, the usual hydrogen bonding interaction involves the NH group of one molecule and a C=0 group of a second molecule to produce a cyclic dimer 5. 

Studies on the association124 of aniline or A-methylaniline with benzene, A,A-dimethyl-aniline, pyridine and A,A-dimethylcyclohexylamine in forming 1 1 complexes in cyclohexane (taking into account the self association of the protic amine) reveal that formation of... 

The aniline-zinc porphyrin interaction has also been exploited to form dimers. Hunter (60) reported the dimerization of porphyrins functionalized at one meso position with ortho or meta aniline groups (47, 48, Fig. 15). Both compounds showed concentration-dependent H NMR spectra with large upfield shifts for the aniline protons. The dimerization constants are 160 and 1080 M-1 respectively for 47 and 48, and these values are an order of magnitude higher than the association constants of simple reference complexes (K — 10 and 130 M 1 respectively), which is indicative of cooperative self-assembly. The complexa-tion-induced changes in chemical shift were used to obtain three-dimensional structures of the dimers. 

Hunter (60) reported a self-assembled open polymer formed by a zinc porphyrin bearing one para-aniline substituent at the meso position. The ortho- and mela-analogs discussed above form closed dimers, but the geometry of the para-derivative precludes this, and polymerization is the only alternative (76, Fig. 31). Although the dilution experiments could be fitted to a non-cooperative polymerization model with a pairwise association constant (K = 190 M 1) practically identical to that found for simple aniline-zinc porphyrin complexes (K = 130 M 1), broadening of the 4H NMR spectrum at high concentrations is characteristic of oligomerization. 

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