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42667-B4
Mechanisms of Nucleophilic Aromatic Photosubstitution Reactions

Gene G. Wubbels, University of Nebraska at Kearney

We continued to be attracted by puzzling reports of nucleophilic aromatic photosubstitution mechanisms.  P. Klan and coworkers in Photochem. Photobiol. Sci., 2002, 1, 1012-16, reported that the regioselectivity of photosubstitution of 4-nitroanisole by hydroxide ion varied hugely with temperature in alcohol solvents.  The ratio of nitrite to methoxy displacement was claimed to vary from 99:1 at -20 ^oC to 1:1 at 78 ^oC.  Our investigation used DMSO-water solvents to avoid protolysis, and we found no appreciable change from the reported nitrite to methoxy displacement ratio of 4:1 between temperatures of –5 ^oC and 70 ^oC.  Moreover, we found that solutions of 0.20 M NaOH in anhydrous ethanol actually contain almost no hydroxide ion, protolysis converting over 95% of the hydroxide ion to ethoxide ion.  NMR and GC-MS analysis showed that the photoreactions in NaOH/ethanol gave almost none of the claimed products of photosubstitution by hydroxide ion (4-methoxyphenol and 4-nitrophenol), and large amounts of photoreduction and photosubstitution by ethoxide ion, as would be expected.  These results show that there is no unusual temperature effect on regioselectivity in this case. 

We also were re-attracted to the strange photochemistry of 4-O₂N – C₆H₄ – OCH₂CH₂ – NH₂. We had earlier reported on the intramolecular photoadduct formation of this molecule in alkaline water solutions, and the remarkable onset of photo-Smiles rearrangement of this molecule at very high hydroxide ion concentrations.  We found evidence by NMR that, at buffered pH's close to that of the ammonio group pK_a of about 9.2, the excited molecule indeed forms a photoadduct by attack of the amino nitrogen at a ring carbon meta to nitro, but that the anion formed on the ring undergoes protonation (deuteration) to give all three of the dihydrobenzenes possible (C₂-C₃, C₂-C₅, and C₂-C₁ adducts).  The adducts are thermally stable and assignable.  Two of the adducts undergo thermal deprotonation to make nitronate ions that can be detected and assigned.  UV-vis spectra of photoreactions confirmed the NMR findings.  We also found that the ratios of the dihydrobenzenes vary greatly with the pH.  We conclude provisionally that at lower pH, the protonation of the ring anion is intramolecular from the ammonium nitrogen of the initial adduct, while at higher pH, protonation is from the solvent. 

            We were skeptical about the many published claims that photosubstitution by secondary amines of a para-to-nitro alkoxy group on a nitrophenyl ether proceeded by initial electron transfer followed by coupling of the geminate radicals (at the amino nitrogen) to make a sigma complex preceding the product (an S_N(ET)Ar* mechanism).  We tested the extension of this mechanistic postulate to intramolecular cases of the type:  3 or 4-NO₂ – C₆H₄ – OCH₂CH₂ – NHCH₃.  The radical postulate predicts that the para isomer should undergo photo-Smiles rearrangement while the meta isomer should form a stable Meisenheimer adduct (a nitronate ion).  Our preliminary results indicate the opposite, namely, that the para isomer forms a stable Meisenheimer photo-adduct, while the meta isomer undergoes normal photo-Smiles rearrangement.  This suggests that these and the reported intermolecular cases actually proceed by heterolytic attack of the internal nucleophile at the position meta to nitro (S_N2Ar*). 

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