Metal-Free Functionalization of Azine N -Oxides with Electrophilic Reagents
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Metal-free functionalization of azine N-oxides with electrophilic reagents Sergey V. Baykov1, Vadim P. Boyarskiy1* 1
Saint Petersburg State University, 7/9 Universitetskaya Embankment, Saint Petersburg 199034, Russia; е-mail: [email protected] Submitted March 10, 2020 Accepted April 16, 2020
Translated from Khimiya Geterotsiklicheskikh Soedinenii, 2020, 56(7), 814–823
The review is devoted to the reactions of azine N-oxides with electrophilic reagents, which make it possible to functionalize the azine fragment with the formation of a new C–X bond (X = C, N, O, Hal, S, P) without the participation of transition metal complexes. The use of azine N-oxides as starting compounds is often associated with the employment of electrophilic reagents or media. The reaction of electrophiles with azine N-oxides allows one to purposefully and selectively introduce substituents at positions 2, 3, or 4 of the azine fragment (relative to the nitrogen atom), depending on the choice of the reagent or the reaction conditions. The review considers the reactions of intramolecular nucleophilic substitution with preliminary generation of azine N-oxide adducts with electrophilic reagents, SEAr reactions, and photocatalytic reactions. Original research published over the past 5 years is covered. Keywords: azine N-oxides, electrophilic reagents, catalysis by Brønsted acids, C–H functionalization.
Azine N-oxides, the most notable of which is pyridine N-oxide, are a class of heterocycles with interesting chemical properties. On the one hand, pyridine N-oxide is an extremely electron-deficient heterocycle. The acceptor properties of the pyridine π-system in it are aggravated by the positive charge on the nitrogen atom, which further lowers the energies of both occupied and empty π-orbitals. At a first glance, this should prevent any reaction with electrophilic reagents from occurring. But, on the other hand, the electron pair of a negatively charged oxygen atom effectively interacts with the heteroaromatic system (Fig. 1). In this case, the negative charge of the oxygen atom is delocalized in the pyridine ring, being distributed mainly over positions 2, 4, and 6, and making these positions more nucleophilic than in pyridine and reactive in electrophilic aromatic substitution reactions. In addition, the negatively charged oxygen atom gives the N-oxide basic properties (although they are several orders of magnitude lower than that of pyridine itself) and the ability to react with electrophiles. Therefore, in general, azine N-oxides are nucleophilic rather than electrophilic substrates. Azine N-oxides are widely used in organic chemistry.1 Moreover, the number of studies devoted to heterocyclic N-oxides has been increasing in recent years like an 0009-3122/20/56(7)-0814©2020 Springer Science+Business Media, LLC
avalanche, which requires a separate consideration of various aspects of the chemistry of these compounds. They can act as oxidizing agents2 or organocatalysts.3 Relatively recently, their reactivity in a transition metal-catalyzed C–H
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