Moorthy Jarugu Narasimha, Samanta Subhas, Koner Apurba L, Saha Satyajit, Nau Werner M
Department of Chemistry, Indian Institute of Technology, Kanpur 208 016, India.
J Am Chem Soc. 2008 Oct 15;130(41):13608-17. doi: 10.1021/ja8032179. Epub 2008 Sep 17.
The photoreactivity and nanosecond transient phenomena have been investigated for a rationally designed set of ketones 4-9 in order to gain comprehensive insights concerning the influence of intramolecular hydrogen bonding on (i) the lifetimes of triplet 1,4-biradicals and (ii) the partitioning of the latter between cyclization and elimination. Comparisons of the photochemical results and lifetime data for the biradicals of ketones 6 versus 8 and 7 versus 9 revealed a remarkable influence of hydrogen bonding when superimposed upon steric factors: while 6 and 7 yielded cyclobutanols in poor yields, cyclization was found to be overwhelmingly predominant for 8-anti and moderately so for 9-anti, with a high stereoselectivity in the formation of cyclobutanols (>95% for 8-anti). The diastereochemistry in the case of 8 permitted the occurrence of fragmentation or cyclization almost exclusively (>90% cyclization for 8-anti and >75% elimination for 8-syn). Significantly, the intramolecular hydrogen bonding in the biradicals of 8 and 9 was found to reverse their partitioning between cyclization and elimination compared with the behavior of the biradicals of ketones 3; the ketones 8-anti and 9-anti underwent cyclization in benzene, predominantly leading to cyclobutanols with syn stereochemistry between the C2 and C3 substituents. In accordance with photoproduct profiles, an unprecedented approximately 2-fold difference in the lifetimes of the intermediate diastereomeric triplet biradicals of ketones 8 in nonpolar solvents (e.g., tau(syn) = 123 ns and tau(anti) = 235 ns in cyclohexane) was observed via nanosecond laser flash photolysis, while no such difference in lifetimes was found for the triplet biradicals of acetoxy ketones 9. The intriguing diastereodifferentiation in the lifetimes of the diastereomeric triplet 1,4-biradicals of 8 and the product profiles of ketones 6, 7, and 9 are best reconciled via a unified mechanistic picture in which superposition of steric factors over varying magnitudes of O-H...O hydrogen bonding selectively facilitates a particular pathway. In particular, the diastereodifferentiation in the photochemical outcomes for the diastereomers of ketone 8 and in the lifetimes of their triplet biradicals can be understood on the basis of rapid deactivation of the 8-syn triplet biradical via fragmentation and slow cyclization of the 8-anti triplet biradical from chair- and twist-boat-like hydrogen-bonded conformations, respectively. The photolysis in polar aprotic solvents such as DMSO and pyridine was found to reverse the chemoselectivity, yielding reactivity paralleling that of ketones 3, for which the steric factors between the C2 and C3 substituents control the photochemical outcome.
为了全面了解分子内氢键对（i）三重态1,4 - 双自由基寿命以及（ii）后者在环化和消除之间的分配的影响，对一组经过合理设计的酮4 - 9的光反应性和纳秒瞬态现象进行了研究。酮6与8以及7与9的双自由基的光化学结果和寿命数据的比较表明，当与空间因素叠加时，氢键具有显著影响：虽然6和7生成环丁醇的产率较低，但发现8 - 反式异构体的环化占绝对主导，9 - 反式异构体的环化也较为显著，形成环丁醇时具有高立体选择性（8 - 反式异构体> 95%）。8的非对映化学使得几乎仅发生碎片化或环化（8 - 反式异构体环化> 90%，8 - 顺式异构体消除> 75%）。值得注意的是，与酮3的双自由基行为相比，发现8和9的双自由基中的分子内氢键改变了它们在环化和消除之间的分配；8 - 反式异构体和9 - 反式异构体在苯中发生环化，主要生成C2和C3取代基之间具有顺式立体化学的环丁醇。根据光产物分布，通过纳秒激光闪光光解观察到，在非极性溶剂中（例如，在环己烷中，8的中间非对映三重态双自由基的寿命存在前所未有的约2倍差异，τ(syn) = 123 ns，τ(anti) = 235 ns），而对于乙酰氧基酮9的三重态双自由基，未发现寿命存在这种差异。8的非对映三重态1,4 - 双自由基寿命以及酮6、7和9的产物分布中有趣的非对映体区分，最好通过一个统一的机理图景来解释，即空间因素在不同程度的O - H...O氢键上的叠加选择性地促进了特定途径。特别是，基于8 - 顺式三重态双自由基通过碎片化快速失活以及8 - 反式三重态双自由基分别从椅式和扭曲船式氢键构象缓慢环化，可以理解酮8的非对映异构体在光化学结果以及它们的三重态双自由基寿命方面的非对映体区分。发现在极性非质子溶剂如DMSO和吡啶中的光解会逆转化学选择性，产生与酮3相似的反应性，对于酮3，C2和C3取代基之间的空间因素控制光化学结果。
