A Rationalization of Orientation in Nucleophilic Aromatic Photosubstitution

Abstract

Consideration of the relationship between the mechanisms and orientation effects of the nitro group in nucleophilic photosubstitutions of nitroaromatics leads to the proposal of two rules, making use of frontier molecular orbital theory, for interpreting the regioselectivities and determining the reaction mechanisms. The first rule is a summary of previously proposed interpretations, concerning photoreactions accompanying the direct addition of a nucleophile to an excited nitroaromatic substrate. In this case, the regioselectivity is governed by the HOMO of the substrate. The second rule regards reactions involving photo-induced electron transfer, where regioselectivity is LUMO-controlled. The application of these rules is successfully demonstrated for various types of compounds.

Nucleophilic photosubstitutions of o-methoxynitrobenzenes

“…The whole area of methoxynitroaromatic compounds and their photochemistry has attracted new interest with the realization that nucleophilic photosubstitution reactions of these compounds can occur by two distinct pathways: a direct SN2 Ar* photosubstitution and a route initiated by electron transfer from the "nucleophile" to the nitro compound (4,5). Substitutions tneta to the nitro group and those at the ipso position are SN2 Ar" processes, and appear to be initiated by attraction of the incoming nucleophile to locations where the excited state HOMO electron density is low.…”

“…Thus 3-nitroveratrole afforded 2-methoxy-3-nitrobenzonitrile 13 in high yield, and with quantum yield (4) close to 0.1, while o-nitroanisole yielded 4-methoxy-3-nitrobenzonitrile 14, formal substitution of H-having occurred. This behaviour is similar to that previously observed for the photoreaction of p-nitroanisole with this nucleophile (16).…”

Synthesis of nitropolychlorinated dibenzo-p-dioxins (NPCDDs) and their photochemical reaction with nucleophiles

“…Nucleophilic aromatic photosubstitutions have been extensively examined for nearly 40 years, with nitrophenyl ethers being among the most thoroughly studied (8-1 1). Two major reaction mechanisms have been distinguished: the S,2Ar* mechanism involving direct formation of a u-complex by attack of the nucleophile at a ring position meta to NO,; and an electron transfer reaction that is possible when the "nucleophile" is relatively easily oxidized, and which normally leads to substitution para to the nitro group (12)(13)(14)(15)(16)(17). In the present work we have explored the feasibility of inducing the S,2Ar* reaction between NPCDD congeners and the poorly oxidizable nucleophiles CN-and EtO-, with a view to identifying the products and elucidating the reaction mechanisms.…”

Comparison of photoreactions of flutamide in acetonitrile and 2-propanol solvents in the absence of cage-forming compounds

“…Aromatic nitro compounds are known to undergo various photoreactions, such as photosubstitution, photoredox reactions, photodissociation and photo-induced nitro-nitrite rearrangement [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. Photosubstitution and photoredox reactions have been well investigated [1][2][3][4][5][6][7][8][9][10][11], but photodissociation and nitro-nitrite rearrangement have not yet been fully elucidated [10][11][12][13][14][15][16][17].…”

“…Photosubstitution and photoredox reactions have been well investigated [1][2][3][4][5][6][7][8][9][10][11], but photodissociation and nitro-nitrite rearrangement have not yet been fully elucidated [10][11][12][13][14][15][16][17]. Nitro-nitrite rearrangement and photodissociation have low reaction efficiencies, and the short-lived intermediates of these reactions are difficult to observe.…”