Bhogala Balakrishna R, Captain Burjor, Ramamurthy Vaidhyanathan
Department of Chemistry, University of Miami, Coral Gables, FL.
Photochem Photobiol. 2015 May-Jun;91(3):696-704. doi: 10.1111/php.12353. Epub 2014 Nov 7.
Photodimerization of cocrystals of four bispyridylethylenes and two stilbazoles with urea as a template in the solid state has been investigated following our success with thiourea. Four investigated olefins photodimerized quantitatively to a single dimer in the crystalline state only. The reactivity of urea-olefin crystals is understood on the basis of their packing arrangements in the crystalline state. In reactive crystals the adjacent reactive molecules are within 4.2 Å and parallel, whereas the unreactive ones have their adjacent molecules are farther than 4.6Å and nonparallel. Thus, with the knowledge of crystal packing the reactivity of urea-olefin crystals is predictable on the basis of Schmidt's topochemical postulates. The templating property of urea, similar to thiourea, derives from its ability to form hydrogen bonds with itself and the guest olefins. Despite the similarities in molecular structures of urea and thiourea their subtle electronic properties, yet to be fully understood, affect the crystal packing and consequently their reactivity in the crystalline state. Further work is needed to fully exploit the templating properties of urea.
继我们在硫脲方面取得成功之后，对四种双吡啶乙烯和两种二苯乙烯基苯与尿素作为模板形成的共晶体在固态下的光二聚反应进行了研究。所研究的四种烯烃仅在结晶状态下定量光二聚为单一二聚体。尿素 - 烯烃晶体的反应活性是基于它们在结晶状态下的堆积排列来理解的。在反应性晶体中，相邻的反应性分子间距在4.2 Å以内且相互平行，而无反应性的晶体中相邻分子间距大于4.6 Å且不平行。因此，基于施密特的拓扑化学假设，了解晶体堆积情况后，尿素 - 烯烃晶体的反应活性是可预测的。尿素的模板性质与硫脲类似，源于其自身以及与客体烯烃形成氢键的能力。尽管尿素和硫脲的分子结构相似，但它们微妙的电子性质（尚未完全理解）影响晶体堆积，进而影响它们在结晶状态下的反应活性。需要进一步开展工作以充分利用尿素的模板性质。
