Mechanistic Studies of Adamantylacetophenones with Competing Reaction Pathways in Solution and in the Crystalline Solid State
Mechanistic Studies of Adamantylacetophenones with Competing Reaction Pathways in Solution and in the Crystalline Solid State
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DOI：
10.1021/acs.joc.9b01720
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发表时间：
2019-09-06
影响因子：
3.6
通讯作者：
Garcia-Garibay, Miguel A.
中科院分区：
文献类型：
--
作者：
Hipwell, Vince M.;Garcia-Garibay, Miguel A.
晶体中的光化学反应发生在分子流动性高度受限的条件下，使得通常仅获得一种产物，即使在气相或溶液中可以观察到许多其它产物。选择在金刚烷基和邻位芳族基团上均具有γ-氢原子的一系列2-（1-金刚烷基）-O-烷基-苯乙酮，以确定是否可以设计和观察结晶状态下的竞争性Norrish II型反应途径。结果表明，二级金刚烷基和苄基氢之间的夺氢激发态竞争不仅受到相对键离解能的影响，而且还受到晶体中分子构象的影响。所得到的双自由基物种的后续命运是由自由基重组形成的光产物和反向氢原子转移再生的起始酮之间的竞争决定的。晶体信息，在溶液中的光产物分布，并在固态，和多个机械实验，包括瞬态吸收光谱在乙腈和纳米晶体悬浮在水中的结果，报告。结果表明，它是可能的工程师在晶体中的竞争反应，并考虑所有上述因素是必要的，以考虑所观察到的光产物的选择性。
Photochemical reactions in crystals occur under conditions of highly restricted molecular mobility such that only one product is generally obtained, even when there are many others that can be observed in the gas phase or in solution. A series of 2-(1-adamantyl)-o-alkyl-acetophenones with gamma-hydrogen atoms on both the adamantyl and ortho aromatic groups was selected to determine whether one can engineer and observe competing Norrish type II reaction pathways in the crystalline state. It was shown that excited state competition for hydrogen abstraction between secondary adamantyl and benzylic hydrogens is affected not only by the relative bond dissociation energies but also by the molecular conformation in the crystal. The subsequent fate of the resulting biradical species is determined by competition between radical recombination to form the photoproduct and reverse hydrogen atom transfer to regenerate the starting ketone. Crystallographic information, photoproduct distributions in solution and in the solid state, and the results of multiple mechanistic experiments, including transient absorption spectroscopy in acetonitrile and with nanocrystals suspended in water, are reported. The results demonstrate that it is possible to engineer competing reactions in crystals and that consideration of all of the aforementioned factors is necessary to account for the observed photoproduct selectivity.