Kinetic selectivity in the N-alkylation of 2-pyridyl porphyrins: A facile approach to the ααββ scaffold

meso-Tetrakis(2-pyridyl)-porphyrin (2-PyP) was tetra-N-alkylated with three different α-bromoacetamides to generate a series of water-soluble N-alkylpyridinium porphyrins (1-3). The product mixtures showed a marked preference for the formation of the ααββ atropisomer. With α-bromo N-n-butylacetamide, the corresponding ααββ 2-tetrakis (N-n-butylacetamido)-pyridyl porhyrin (2-TnBuPyP, 3) was obtained in 69% isolated yield in a single step. Prolonged heating lead to equilibration of the rotational isomers for the less bulky alkyl groups, indicating that the observed preference is a kinetic effect. The intermediate products for the N-bornyl case were identified by LC/ESI-MS to deduce an explanation for the observed nonstatistical selectivity.