Chemistry and Chemical Biology ETDs

Publication Date

8-29-1975

Abstract

The purpose of this investigation was to study the reaction of 2-aminopyridine with picryl halides with the goal of providing evidence in support of a mechanism accounting for the formation of the observed products. Nuclear magnetic resonance spectroscopy was employed to measure the relative amounts of the various picryl protons in the product mixture, which were assumed to be proportional to the mole percent of the corresponding compounds. 2-Aminopyridine reacted with picryl chloride by a nucleophilic bimolecular substitution reaction to give 2-(N-picrylamino)pyridine as the major product; however, the major product of the reaction of 2-amino­pyridine with picryl fluoride is 1-picryl-2-(N-picrylimino)-1,2-dihydro­pyridine. The possibility of a common intermediate, 1-picryl-2-imino- 1,2-dihydropyridine, undergoing an amidine (Dimroth) rearrangement to 2-(N-picrylamino)pyridine was eliminated by using 2-aminopyridine-1-15N in the reactions. No migration of the 15N from the ring to the exocyclic nitrogen was observed. Kinetic data indicated that the reaction of 2-aminopyridine with picryl fluoride involved two sequential bimolecular reactions instead of a single termolecular reaction between one mole of 2-aminopyridine and two moles of picryl fluoride. Initially, picrylation occurs at the ring nitrogen with the formation of 2-imino-1-picryl-1,2-dihydropyridine that is not isolated, but reacts rapidly with another mole of picryl halide to give 1-picryl-2-(N-picrylimino)-1,2-dihydropyridine, the kinetically controlled product. The latter is catalytically converted into 2-(N­picrylamino)pyridine if 2-aminopyridine is present in excess. Anions of weak acids were found to be effective catalysts but neutral and strong bases would not catalyze the reaction. A mechanism for the overall conversion of 1-picryl-2-(N-picrylimino)-1,2-dihydropyridine into 2-(N-picrylamino)pyridine shows as a first step the reaction of 2-aminopyridine with hydrogen fluoride to give fluoride ions. A nucleophilic displacement of the 1-picryl group of 1-picryl-2-(N-picrylinidno)-1,2-dihydropyridine by fluoride ions forms an anion which abstracts a proton to give 2-(N-picrylamino)pyridine. Picryl fluoride, which is also regenerated in the process, reacts with 2-aminopyridine to form more of the kinetically controlled product. A studly of the aprotic solvent used in this conversion showed that their effectiveness, DMF < DMSO < HMPA, is directly related to their relative ability to act as hydrogen bond acceptors and to stabilize covalent Meisenheimer type intermediates. The initial ratios of products in various solvents confirmed spectroscopic studies that indicate that the ring nitrogen atom of 2-amino­pyridine is more nucleophilic than the exocyclic nitrogen atom. This justifies the predominance of the 1-picryl-2-(N-picrylimino)-1,2-dihydro­pyridine when picryl fluoride is the picrylating agent. However, with picryl chlolride and picryl bromide steric factors are encountered that force initial attack by the exocyclic amino nitrogen leading to 2-(N­picrylaminol)pyridine as the major product.

Project Sponsors

The U. S. Atomic Energy Commission through the University of California, Los Alamos Scientific Laboratory

Language

English

Document Type

Dissertation

Degree Name

Chemistry

Level of Degree

Doctoral

Department Name

Department of Chemistry and Chemical Biology

First Committee Member (Chair)

Eleftherios Paul Papadopoulos

Second Committee Member

Michael D. Coburn

Third Committee Member

Guido Herman Daub

Fourth Committee Member

Cary Jacks Morrow

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