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44949-AC1
Development of Photochemically Removable Groups

Mark G. Steinmetz, Marquette University

Photochemical electrocyclic ring closure reactions which potentially produce zwitterionic intermediates are of interest, because they have a prospect for expelling leaving group anions.   Such photoreactivity would provide a basis for designing new classes of photochemically removable groups.  An important advantage of this approach is that flexibility may be possible in the choice of chromophore (and wavelength) for effecting the electrocyclic ring closure step in the photoreaction.   The approach is exemplified, in part, by the photochemical elimination reactions of the unsaturated anilides 1a,b, which, as our studies have recently found, undergo photoelimination of a variety of carboxylate leaving groups and phenolate (Scheme 1) to produce carboxylic acids and phenol along with comparable amounts of the elimination coproduct 3a,b.  The photoelimination products are accompanied by the cyclized product, 4a,b, which retains the leaving group.  This cyclized photoproduct can be considered to arise

 via an intramolecular suprafacial 1,5-H shift, a process that competes with elimination in the potential intermediate, 2a,b.  In support of the 1,5-H shift pathway, no deuterium is incorporated from D₂O into 4a,b via an enol formed upon protonation under buffered conditions (pD 7).  Of the two potential pathways for product formation, the elimination pathway generally predominates for carboxylate leaving groups,  whereas the 1,5-H shift pathway is almost exclusively observed with the poor leaving group, LG^- = OH.  As expected, only the 1,5-H shift pathway is observed for compounds 5a,b, which have no leaving group and are being studied for comparison to 1a,b.   The photochemistry of 1a is effected with 310 nm light, and quantum yields for total product formation are in the range 0.03-0.05 in 50% aqueous buffer in CH₃CN as solvent (pD 7).

Total quantum yields are insensitive to leaving group ability for all of the carboxylates and the phenolate as leaving groups.  The fact that comparable quantum efficiencies are observed for 1a and 5a further suggests that the electrocyclization step to form intermediate 2a is irreversible.   For the benzoyl derivative 1b 365 nm light can be used for photolysis, and quantum yields are 0.06-0.07 in 50% aqueous buffer in CH₃CN as solvent (pD 7).  Quantum yield determinations show that the photoreactivity of 1b is not quenched by 0.01 M sodium 2-naphthalene sulfonate or by air.   Laser flash photolysis studies of 5b show a transient absorption at 530 nm with lifetime of 322 ns in buffer that is thought to be due to the triplet excited state.  This transient absorption is quenched at nearly a diffusion controlled rate by sodium 2-naphthalene sulfonate.  Based upon these results, our preliminary conclusion, somewhat surprisingly, is that the photochemistry of 1b likely arises from the singlet excited state.  A second tentative conclusion is that the singlet excited state electrocylic ring closure reactions are relatively inefficient, possibly due to the relatively short lifetimes of the reactive excited states or because the above studies have used compounds with strongly electron withdrawing substituents attached to the aromatic ring.  Work is underway to determine electrocyclization efficiencies with Y = electron releasing substituents attached to the arene and examine more generally substituent effects on excited state electrocylizations of related arenes.   Additionally, the role of excited state lifetimes on electrocyclization efficiencies is being studied.

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