Exploring photoinduced decarboxylation mechanism of o-acetylphenylacetic acid from the combined CASSCF and DFT studies

The Journal of Organic Chemistry
Lina Ding, Weihai Fang

Abstract

In the present work, we studied the near-UV photoinduced decarboxylation of o-acetylphenylacetic acid with complete active space self-consistent field and density functional theory. It was found that irradiation at approximately 300 nm results in o-acetylphenylacetic acid in the S(1)((1)npi*) state, which is followed by a rapid relaxation and efficient intersystem crossing to the T(1)((3)npi*) state via the S(1)/T(2)/T(1) three-surface intersection. The 1,5-H shift has a barrier of 1.9 kcal x mol(-1) on the T(1) pathway to the triplet 1,4-biradical, which is in good agreement with a rate constant of about 10(10) s(-1) inferred experimentally for o-alkylphenyl ketones. The subsequent reactions occur with little probability from the triplet 1,4-biradical, due to relatively high barrier or high endothermicity for the spin-conservation triplet pathways. As a result, intersystem crossing to the lowest singlet state takes place prior to the subsequent reactions. Four isomers, (3)BRi (i = 1-4), were found to be stable for the triplet 1,4-biradical. The calculated energy gap indicates that the (3)BR3/(3)BR2 ratio is close to 1:1, and populations of (3)BR1 and (3)BR4 are less than 1% at thermal equilibrium. Like the triplet 1,4-biradica...Continue Reading

References

Sep 1, 1989·Photochemistry and Photobiology·L L CostanzoM Fama
Feb 1, 2002·Organic Letters·Martin Sobczak, Peter J Wagner
Mar 12, 2004·Annual Review of Physical Chemistry·M W Schmidt, M S Gordon
Mar 22, 2007·Journal of Medicinal Chemistry·Klefah A K MusaLeif A Eriksson
Dec 3, 2009·The Journal of Organic Chemistry·Lina DingWei-Hai Fang

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