Exploratory studies of photocyclization mechanisms of diaryl derivatives via o-dibenzoquinonemethide intermediates




Huang, Cai-Gu

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Three classes of new photoreactions were discovered and their reaction mechanisms investigated. These reactions are related in that the same critical o-quinonemethide intermediate is involved in the mechanism of each reaction. The first reaction is the photocyclization of 2-(2’-hydroxyphenyl)benzyl alcohol (1) and derivatives to 6H-dibenzo[b,d]pyrans (e.g., 7 from 1), with excellent chemical (>95%) and quantum yields (Φp = 0.50 for 7 from 1 in basic solution). Results from investigations of structure-reactivity, pH-effects and fluorescence data suggest that, in neutral solution, the primary photochemical step involves ionization of the phenol moiety to phenolate ion in S1, which is probably concerted with twisting of the phenyl rings, to give a more planar species in S,. The subsequent dehydroxylation step of the benzyl alcohol moiety, to give o-quinonemethide 20, is initiated by a charge transfer from phenolate to the adjacent phenyl ring. The thermal ring closure of 20 competes with nucleophilic solvent (e.g., H20 or MeOH) capture, to give observed pyran 7 and alcohol 1 (by H20) and 5 (by MeOH), respectively. In moderately strong acidic media, acid-catalyzed photosolvolysis occurs, to give carbocation 21 which can also cyclize to afford pyran 7. The second photoreaction is the photocyclization reaction of 2- phenoxybenzyl alcohols 22 and 23 to give dibenzo[b,d]pyrans 7 and 29, respectively, in aqueous solution. The primary photochemical step is believed to involve initial aryl-O bond homolysis followed by rearrangement to give alcohol 1, which cyclizes to observed pyran 7 upon secondary photolysis. The meta-substituted isomer 24 did not produce cyclized photoproducts, but instead gave isomeric hydroxybiphenyls which are also derived from initial aryl-O bond homolysis followed by simple radical recombination. The photocyclization appears to be general for the ortho-phenoxybenzyl alcohol system, in which an appropriate assembly of phenoxy and hydroxymethyl (CH2OH) functional groups is a necessary requirement. In acidic solution, a competing proton-assisted photosolvolysis reaction, via heterolysis of the benzylic C-OH bond, takes place for all these compounds, to give carbocation intermediates which were subsequently trapped by the solvent. The last reaction is the photoisomerization of xanthene (26) to pyran 7 (~70% yield and Φp ~ 0.0035 in aqueous solution). In addition to 7, 2- benzylphenol (40) (Φ = 0.001), 9,9’-bixanthyl (41) (Φ < 0.001) and alcohol 1 (Φ < 0.001) were also observed as minor products in the reaction. The photoisomerization is again initiated by aryl-O bond homolysis in S1, to give a singlet phenyl/phenoxy biradical 48 which undergoes a radical ipso-attack on the adjacent phenyl ring, followed by rearrangement to afford o-quinonemethide 20, which cyclizes to form pyran 7 in competition with nucleophilic solvent capture to give 5 (by MeOH). Xanthene derivative 42 also photoisomerizes to the corresponding pyran derivative 29, which was obtained in much lower yield due to secondary photochemistry of 29.



Organic photochemistry, Excited state chemistry