Department of Chemistry, Beijing Normal University, Beijing 100875, People's Republic of China.
J Org Chem. 2010 Mar 5;75(5):1630-6. doi: 10.1021/jo902593s.
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-biradical, four stable isomers of (1)Xi (i = 1-4) were determined in the lowest singlet state. Because of the relatively high barrier (approximately 30 kcal x mol(-1)) on the isomerization pathways, the thermal equilibrium is not established among the four singlet isomers, which is different from the situation for the triplet 1,4-biradical. In this case, the subsequent reactions proceed mainly from the (1)X2 and (1)X3 isomers that correspond to (3)BR2 and (3)BR3. There is only one predominant pathway from the (1)X2 isomer, namely, the reversed-H shift to the initial reactant of o-acetylphenylacetic acid. However, several possible pathways exist for the (1)X3 deactivation: intramolecular cyclization, unimolecular decarboxylation, and the parent acid-catalyzed bimolecular decarboxylation. The unimolecular decarboxylation is not in competition with the cyclization. But the cyclization reaction is prevented by the parent acid-catalyzed bimolecular decarboxylation, which is responsible for the products of CO(2) and o-acyltoluene observed experimentally. The (1)X3/(1)X2 ratio is nearly equal to that for (3)BR3/(3)BR2, which indicates that the decarboxylation reaction has a quantum yield close to 0.5. The o-acyltoluene product was isolated, and its yield was experimentally estimated to be in the 50% range.
在本工作中，我们使用完全活性空间自洽场和密度泛函理论研究了邻乙酰基苯乙酸的近紫外光诱导脱羧反应。结果表明，在大约 300nm 的光照射下，邻乙酰基苯乙酸处于 S(1)((1)npi*)态，随后通过 S(1)/T(2)/T(1)三表面交叉迅速松弛并有效地发生系间窜跃至 T(1)((3)npi*)态。1,5-H 迁移在 T(1)途径上生成三重态 1,4-双自由基的势垒为 1.9 kcal/mol，这与实验中推断的邻烷氧基苯基酮的约 10(10)s(-1)的速率常数很好地一致。由于自旋守恒三重态途径的相对高的势垒或高的吸热性，随后的反应发生的可能性很小。结果，系间窜跃至最低单线态先于随后的反应发生。对于三重态 1,4-双自由基，发现四个异构体（3）BRi（i=1-4）是稳定的。计算的能量间隙表明（3）BR3/(3)BR2 的比例接近于 1:1，在热平衡时（3）BR1 和（3）BR4 的种群小于 1%。与三重态 1,4-双自由基一样，在最低单线态中确定了四个稳定的（1）Xi（i=1-4）异构体。由于异构体化途径上的相对高的势垒（约 30kcal/mol），四个单线态异构体之间没有建立热平衡，这与三重态 1,4-双自由基的情况不同。在这种情况下，随后的反应主要从（1）X2 和（1）X3 异构体进行，其对应于（3）BR2 和（3）BR3。从（1）X2 异构体只有一条主要途径，即向初始反应物邻乙酰基苯乙酸的反向-H 迁移。然而，（1）X3 的失活存在几种可能的途径：分子内环化、单分子脱羧和双亲酸催化的双分子脱羧。单分子脱羧与环化不竞争。但是，环化反应被双亲酸催化的双分子脱羧所阻止，这就是实验中观察到的 CO(2)和邻酰基甲苯产物的原因。（1）X3/(1)X2 的比例几乎等于（3）BR3/(3)BR2，这表明脱羧反应的量子产率接近 0.5。分离出邻酰基甲苯产物，并通过实验估计其产率在 50%范围内。
