Co 2(CO)6配合的炔丙基阳离子的新型高温反应,以自发和立体选择性(90–97%d,l-)方式发生,并产生自由基二聚体d,l- 3,4-二芳基-1,5- hexadiynes(2,11,14),进行说明。尽管据称有热不稳定性和必需的Co 2(CO)6的微妙性质,-与炔丙基阳离子复合时,反应温度可从20°C升高到147°C,反应时间从660分钟缩短至不到1分钟。同位素富集实验检测到(MS TOF / ESI / APCI)多达八个13个CO配体掺入金属核中,表明沿反应坐标方向单个电子,簇到簇的还原。发现反应动力学对取代基(H,OMe)的电子性质和芳环外围上的取代图案(0-,4-,3,4,5-)敏感。计算数据表明,单个电子从金属团簇到π键结合的炔丙基部分的自发转移取决于芳族化合物上的负电荷C1个′碳原子,位于阳离子中心的α处。反磁性物质(炔丙基阳离子)自发转化为顺磁性对应物(炔丙基自由基)
1,3-Steric Induction in Intermolecular Radical Reactions Mediated by a Co<sub>2</sub>(CO)<sub>6</sub>−Metal Core
作者:Gagik G. Melikyan、Ryan Spencer、Aaron Rowe
DOI:10.1021/om100472y
日期:2010.8.23
Diastereoselectivity of propargyl coupling reactions can be controlled by using the bulkiness of a gamma-substituent as a stereochemical tool. This 1,3-steric induction was observed with gamma-t-Bu and gamma-Me3Si groups, both favoring a d,l-configuration of the head-to-head coupling products, 3,4-disubstituted 1,5-alkadiynes (d,l- 95-100%). X-ray crystallography analysis suggests that the most favorable orientation of converging propargyl radicals is the one in which the bulky gamma-substituents are positioned anti to each other. Overall, the synthetic strategy of employing a Me3Si auxiliary group involves five steps and affords, with 28-33% overall yields, pure d,l-3,4-diaryl-1,5-hexadiynes, otherwise hardly accessible.