Cobaloxime π-Cation Steric and Stereoelectronic Effects: The Amazing Effect of a Single Methyl Group Adjacent to the Site of Reaction
作者:Yoshiyuki Nishikubo、Bruce P. Branchaud
DOI:10.1021/ja993556z
日期:1999.12.1
inclusion of an additional methyl group in going from 9 to 10 completely turns off an otherwise very facile reaction, presumably due to severe steric effects of the py(dmgH)2Co moiety of the cobaloxime which forces the acyclic chain of 10 to adopt a conformation which is stereoelectronically nonproductive for cobaloxime π-cation formation.
通常,β-羟基烷基钴肟对酸非常敏感,容易形成瞬态钴肟 π-阳离子,这些阳离子可以被亲核试剂捕获或发生不可逆的烯烃损失。令人惊讶的是,发现 2,3-二羟基-3,7-二甲基-6-辛烯基钴肟 (1) 对酸处理稳定,包括强酸樟脑磺酸的酸处理。对 2,3-二羟基丙基钴肟 (8)、2,3-二羟基丁基钴肟 (9) 和 2,3-二羟基-3-甲基丁基钴肟 (10) 的酸稳定性进行了比较研究,结果表明 8 和 9表现出正常的酸敏感性,而 10 对酸稳定。在从 9 到 10 的过程中包含一个额外的甲基完全关闭了原本非常容易的反应,