Kinetic selectivity in the N-alkylation of 2-pyridyl porphyrins: a facile approach to the alphaalphabetabeta scaffold.
meso-Tetrakis(2-pyridyl)-porphyrin (2-PyP) was tetra-N-alkylated with three different alpha-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 alphaalphabetabeta atropisomer. With alpha-bromo-N-n-butylacetamide, the corresponding alphaalphabetabeta 2-tetrakis (N-n-butylacetamido)-pyridyl porphyrin (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.
Datta
Ankona
A
Department of Chemistry, Princeton University, Princeton, NJ 08544, USA.
Quintavalla
Suzanne M
SM
Groves
John T
JT
D000478:Alkylation; D002851:Chromatography, High Pressure Liquid; D006358:Hot Temperature; D007202:Indicators and Reagents; D007700:Kinetics; D008958:Models, Molecular; D008968:Molecular Conformation; D011166:Porphyrins / Q000138:chemical synthesis*; D011725:Pyridines / Q000138:chemical synthesis*; D021241:Spectrometry, Mass, Electrospray Ionization; D013237:Stereoisomerism
D007202:Indicators and Reagents; D011166:Porphyrins; D011725:Pyridines