Martha Berros

Flat versus twisted rotamers of 2,4-­disubstituted thiazoles: the effect of intermolecular hydrogen bonds

In the title compounds, 2-amino-4-(2-chloro-4,5-dimethoxyphenyl)-1,3-thiazole, C(11)H(11)ClN(2)O(2)S, (I), and 4-(2-chloro-4,5-dimethoxyphenyl)-2-methyl-1,3-thiazole, C(12)H(12)ClNO(2)S, (II), the dihedral angles between the thiazole moiety and the chloroaryl group are 51.61 (10) and 8.44 (14), respectively. This difference is a consequence of intermolecular hydrogen bonds forcing the stabilization of a twisted rotamer in (I). Substitution of the amino function by a methyl group precludes these contacts, giving a flat rotamer in (II).

4-(4-bromophenyl)-2-methyl-1,3-thiazole.

In the structure of the title compound, C(10)H(8)BrNS, the dihedral angles between the planes of the thiazole and aryl rings, viz. 4.2 (6) and 7.5 (6) degrees for the two independent molecules, are consistent with insignificant molecular perturbation by the weak intermolecular contacts. The molecules are close to being related by a non-crystallographic inversion centre, with C-H.pi and pi-pi intermolecular interactions observed.

Impedimento eléctrico y otros factores en la nitración de la 2-aminopiridina

The different isomer yields observed in many aromatic electrophilic substitution reactions can be explained by steric hindrance. However, this is not the case when there are drastic differences in the reaction yields of the isomeric products obtained. This is generally due to the presence of other factors, for instance, electric rejection between two positive charges in the reaction stage. Thus, a very important point to bear in mind is electric hindrance, a new theoretical concept. We have taken as an example 2-aminopyridine nitration. We provide an extended theory on this subject, which is in accordance with the observed regiochemistry and with the reaction yields of the isomeric products obtained. Dipole moments were also taken into account. We discuss too the 2-nitraminopyridine rearrangement in acidic medium. The theoretical discussion is also in agreement with reported trans-nitration experimental results. Our proposals were also contrasted with the findings from thermolysis and photolysis carried out with 2- nitraminopyridine.