| Abstract
| - Methacrylanilides, ArN(CH3)COC(CH2LG)CH2, with allylic leaving groups (LG− = BocAla, PhCO2−, PhCH2CO2−, PhO−) undergo photochemical electrocyclic ring closure to produce a zwitterionic intermediate. Further reaction of the intermediate results in expulsion of the leaving group to give an α-methylene lactam as the major product. In addition, a lactam product that retains the leaving group is formed via a 1,5-H shift in the intermediate. Elimination of the leaving group is generally preferred, even for LG− = PhO−, although in benzene as the solvent the lactam retaining the phenolate group becomes the sole photoproduct. The electrocyclic ring closure occurs in the singlet excited-state for the para-COPh-substituted anilide derivative and is not quenched by 0.15 M piperylene or 0.01 M sodium 2-naphthalenesulfonate (2-NPS) as triplet quenchers. Comparable concentrations of 2-NPS strongly quench the transient absorption of the triplet excited state observed at 450−700 nm according to laser flash photolysis experiments. In aqueous media, quantum yields for total products are insensitive to leaving group ability, and Φtot(para-CO2CH3) = 0.04−0.06 at 310 nm and Φtot(para-COPh) = 0.08−0.1 at 365 nm, for which Φisc = 0.15.
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