Direct photolysis of phenacyl protecting groups studied by laser flash photolysis: An excited state hydrogen atom abstraction pathway leads to formation of carboxylic acids and acetophenone
Direct photolysis of phenacyl protecting groups studied by laser flash photolysis: An excited state hydrogen atom abstraction pathway leads to formation of carboxylic acids and acetophenone
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DOI：
10.1021/ja971431t
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发表时间：
1998-04-01
影响因子：
15
通讯作者：
Falvey, DE
中科院分区：
文献类型：
--
作者：
Banerjee, A;Falvey, DE
光化学可移除保护基(PRPG)是可以共价连接到分子的官能团上的分子实体，目的是使其在某些特定的反应条件下保持惰性。1当受保护的化合物暴露在光中时，PRPG被释放。这种保护基团不仅可用于多步合成，还可用于生物物理研究，在生物物理研究中，它们通常被称为“笼式分子”或“光触发剂”。3、4几个实验室的研究主要集中在各种芳香族R-酮基团作为PRPG。5被研究的最早的R-酮衍生物是那些基于苯酰基(即，苯甲酰甲基，PhCOCH2)生色团的衍生物。Sheehan6和其他7，8报道了4-甲氧基苯乙酸酯在1，4-二氧六环溶液中的光解反应，得到了相应的酸和4-甲氧基苯乙酮的良好产率。这些工作人员提出了一种激发态CO键均解产生苯酰基和酰氧基的机制(方案1)。虽然从酰氧基(RCO2·)中消除二氧化碳是一个非常快的过程，9特别是当产物(R·)是共振稳定的物种时，10假设H原子的提取与脱羧基竞争发生。在研究苯乙酸苯酯的光诱导电子转移行为的过程中，我们有机会在CH3CN中对苯乙酸苯酯进行了直接照射。假设机理在方案1中，该化合物的直接均解将导致苯基乙酰氧基(PhCH2CO2·)。Pinock11估计该物种的寿命为200ps。因此，我们预计快速脱羧会得到苯基自由基(PhCH2·)，它会发生二聚反应，得到稳定的联苄基(1，2-二苯乙烷)。与我们的预期相反，我们发现苯乙酸酯的光解没有产生可检测到的联苄基(通过高效液相分析得到0.5%)，相反，苯乙酮和苯乙酸的产率很高(式1)。12同样，当苯乙酸苯酯(2-甲氧基苯基)乙酸酯在类似的光解条件下进行光解时，检测到苯乙酮和相应的酸。
Photochemically removable protecting groups (PRPGs) are molecular entities that can be covalently attached to a functional group of a molecule for the purposes of rendering it inert to some particular set of reaction conditions. 1 The PRPG is then released when the protected compound is exposed to light. Such protecting groups find uses not only in multistep syntheses2 but also in biophysical studies where they are often called “caged molecules” or “phototriggers”. 3, 4Investigations in several laboratories have focused on various aromatic R-keto groups as PRPGs. 5 Among the earliest R-keto derivatives to be studied were those based on the phenacyl (ie, the benzoylmethyl, PhCOCH2) chromophore. Sheehan6 and others7, 8 reported that photolysis of 4-methoxyphenacyl esters in 1, 4-dioxane solutions gave good yields of the corresponding acids along with 4-methoxyacetophenone. These workers proposed a mechanism involving excited-state CO bond homolysis giving phenacyl radical along with an acyloxy radical (Scheme 1). Although elimination of CO2 from acyloxy radicals (RCO2•) is an extremely fast process, 9 especially when the product (R•) is a resonance-stabilized species, 10 it was assumed that abstraction of an H-atom was occurring competitively with decarboxylation. In the course of our studies on the photoinduced electrontransfer behavior of phenacyl esters, we had occasion to undertake direct irradiation of phenacyl phenylacetate 1 in CH3CN. Assuming the mechanism in Scheme 1, direct homolysis of this compound would lead to phenylacetoxy radical (PhCH2CO2•). Pincock11 has estimated the lifetime of this species to be 200 ps. Consequently, we anticipated rapid decarboxylation would give the benzyl radical (PhCH2•) which would dimerize to give bibenzyl (1, 2-diphenylethane) as a stable product. Contrary to our expectations, we found that photolysis of phenacyl ester gave no detectable bibenzyl (< 0.5% by HPLC analysis), but instead high yields of acetophenone and phenylacetic acid (eq 1). 12 Again, when phenacyl (2-methoxyphenyl) acetate was subjected to similar photolysis conditions, acetophenone and the corresponding acid were detected as products.