5-exo-trig Versus 6-endo-trig Cyclization of Alk-5-enoyl Radicals: The Role of One-Carbon Ring Expansion
作者:Chryssostomos Chatgilialoglu、Carla Ferreri、Marco Lucarini、Alessandro Venturini、Andreas A. Zavitsas
DOI:10.1002/chem.19970030309
日期:——
with the formation of the smaller ring. The stereoselectivity was higher in the 1,6-ring closure (70:30) than in the 1,5-ring closure (55:45), the trans isomer being predominant in both. For the onecarbon ring expansion studies, the radicals of interest were obtained by deoxygenation of suitable alcohols via the O-phenyl thiocarbonates with (TMS)3-SiH. The one-carbon ring expansion in the cyclopentanone
使Alk-5-烯酰基自由基以exo和endo模式环化,得到相应的环酮自由基,它们通过单碳环展开相关联。在 233–323 K 的温度范围内,由相应的苯硒酸酯与 Bu3SnH 反应产生的 2-甲基庚-5-烯酰基自由基的闭环测定相对动力学数据。转换为绝对速率提供了 5 的 Arrhenius 表达式-exo-trig 和 6-endotrig 环化。分别对 hex-5-enoyl 和 hept-5-enoyl 自由基进行从头算和半经验 (AM1) 计算,结果有助于合理化指数前因子和活化能。1,5 和 1,6 环闭合都通过较低能量的“椅子状”过渡态发生。观察到的高区域选择性是由于与较小环的形成相关的有利熵和焓因子。1,6-闭环 (70:30) 的立体选择性高于 1,5-闭环 (55:45),反式异构体在两者中均占优势。对于单碳环扩展研究,感兴趣的自由基是通过 O-苯基硫代碳酸酯与 (TMS)3-SiH