Abstract
This work describes the synthesis of twenty N-mono- and N,N-di-substituted propenamides and rationalises reactivity based on conformation. The N substituents are the 5,6,7,8-tetrahydro-1-naphthalenyl or 2,6-dimethylphenyl, and the benzoyl groups. The compounds also have substituents to the C=C group with a range of inductive and mesomeric effects. Synthesis was achieved mostly by acylation of amines/amides using acyl chlorides in the presence of a base. Fourteen novel compounds were synthesised (and six more, previously noted in the literature, were fully characterised for the first time); melting points are reported for all and also full allocation of NMR peaks. The conformation of each compound was elucidated by NMR, crystallography (in the solid state) and computational studies (in vacuo). Computational studies were used to obtain energy profiles for rotation around the C-N bond. The energy minimum conformation, as well as the magnitude of the rotational barrier, in conjunction with crystallographic findings, allows conclusions about the conformation in solution. Thus the N-(5,6,7,8-tetrahydro-1-naphthalenyl)propenamides were found to prefer an aromatic ring planar to the amide group, whereas N-(2,6-diMePh)-, N-benzoyl-N-(2,6-diMePh)- and N-benzoyl-N-(5,6,7,8-tetrahydro-1-naphthalenyl)propenamides prefer a highly twisted one. The reactivity of the compounds towards bromination was assessed by kinetic studies of the reaction in methanol and by NMR product analysis of bromination in methanol and in chloroform. Although addition of bromine to alkenes is traditionally thought of as a fast reaction whereas bromination of benzene is slow, it was found here that when the conformation of the propenamides was planar allowing N-Ar conjugation, bromination of the aromatic ring occurred in preference to C=C addition, especially in the more polar methanol. Furthermore, substituents to the C2 of the propenoyl group were found to affect the rate of this aromatic bromination. When twisting of the aromatic ring minimised N-Ar conjugation, bromination was by addition to C=C, but the reaction was slow.