Chain-Amplified Photochemical Fragmentation of N-Alkoxypyridinium Salts: Proposed Reaction of Alkoxyl Radicals with Pyridine Bases To Give Pyridinyl Radicals

Photoinitiated Multicomponent Anti-Markovnikov Alkoxylation over Graphene Oxide

The development of graphene oxide–based heterogeneous materials with an economical and environmentally–friendly manner has the potential to facilitate many important organic transformations but proves to have few relevant reported reactions. Herein, we explore the synergistic role of catalytic systems driven by graphene oxide and visible light that form nucleophilic alkoxyl radical intermediates, which enable an anti-Markovnikov addition exclusively to the terminal alkenes, and then the produced benzyl radicals are subsequently added with N–methylquinoxalones. This photoinduced cascade radical difunctionalization of olefins offers a concise and applicable protocol for constructing alkoxyl–substituted N–methylquinoxalones.

Direct Experimental Evidence for Alkoxyl Radicals Reacting as Hydrogen Atom Donors toward Pyridines

Nanosecond transient absorption spectroscopy was used to generate ethoxyl radicals and demonstrate that they react with 2,6-lutidine and 4-phenylpyridine to give the corresponding N-hydropyridinyl radicals-products of a novel hydrogen atom transfer from the alkoxyl radical to the nitrogen atom of the substituted pyridines. Nanosecond kinetics show that both reactions are rapid (k ∼ 10⁷ M^-1 s^-1) in acetonitrile at room temperature. Rate constants measured for reaction of the ethoxyl vs. d₅-ethoxyl radical with 2,6-lutidine and 4-phenylpyridine show that both reactions exhibit primary H/D kinetic isotope effects for the hydrogen (deuterium) atom transfer reactions.

Photocatalyzed ortho-Alkylation of Pyridine N-Oxides through Alkene Cleavage

A photocatalyzed reaction of pyridine N-oxides with alkenes gives ortho-alkylated pyridines with cleavage of the carbon-carbon double bond. Benzyl and secondary alkyl groups are incorporated at the ortho position of pyridines in one pot.

N-alkoxyheterocycles as irreversible photooxidants

Irreversible photooxidation based on N-O bond fragmentation is demonstrated for N-methoxyheterocycles in both the singlet and triplet excited state manifolds. The energetic requirements for bond fragmentation are studied in detail. Bond fragmentation in the excited singlet manifold is possible for ππ* singlet states with energies significantly larger than the N-O bond dissociation energy of ca 55 kcal mol(-1). For the nπ* triplet states, N-O bond fragmentation does not occur in the excited state for orbital overlap and energetic reasons. Irreversible photooxidation occurs in the singlet states by bond fragmentation followed by electron transfer. Irreversible photooxidation occurs in the triplet states via bimolecular electron transfer to the donor followed by bond fragmentation. Using these two sensitization schemes, donors can be irreversibly oxidized with oxidation potentials ranging from ca 1.6-2.2 V vs SCE. The corresponding N-ethylheterocycles are characterized as conventional reversible photooxidants in their triplet states. The utility of these sensitizers is demonstrated by irreversibly generating the guanosine radical cation in buffered aqueous solution.