Cram规则和Felkin-Anh规则

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一、Cram规则：

此规则由Donald J. Cram在1952年提出【J. Am. Chem. Soc. 1952, 74, 5828; 5851】，解释和预测亲核试剂对α-手性醛酮加成的立体选择性。RL, RM, Rs 三个基团分别代表α-碳原子上连接的大、中、小( 体积) 原子( 基团)。Cram认为，进攻试剂Nu对羰基加成的第一步就是其金属部分先与羰基氧原子配位，加大了氧原子一端的体积，使之定位于RM和Rs之间，即反应物的优势构象为RL与羰基处于对位交叉式，加成的第二步即亲核试剂向Rs侧进攻最为有利，Cram规则。

规则特点：

1、α-手性碳上带有“RL, RM, Rs” 三个基团，在纽曼投影式中RL与羰基处于对位交叉式。

2、亲核试剂Nu从空间位阻更小的Rs一侧进攻。



二、Felkin-Anh规则：

此规则是对Cram规则的发展，1968年，Felkin提出了另一规则，而后Anh在量化计算和考虑Burgi-Dunitz轨道角的基础上修正了Felkin规则，后被称为Felkin-Anh规则【Top. Curr. Chem. 1980, 88, 145】。现在Felkin-Anh规则已经成为解释和预测亲核试剂对α-手性醛酮加成的立体选择性的最常用规则。

规则特点：

1、RL与羰基处于垂直交叉式。

2、亲核试剂从空间位阻更小的Rs一侧进攻。

3、亲核试剂从符合Burgi-Dunitz轨道角（107°）（点击查看详细介绍）的方向进攻。

根据Burgi-Dunitz轨道角亲核试剂从羰基107°方向进攻，上图构象刚好在Rs一侧进攻，此方向进攻得到的产物为主要产物。

此构象从Burgi-Dunitz轨道角方向进攻，处于RM位置进攻，位阻较大，此产物为次要产物。



三、Cram螯合规则：

α-手性碳上带有好的螯合基团（OR, NR2, SR）时，在好的螯合金属（Li+，MgX+, ZnCl2，TiCl4，MgCl2）存在下，作为Lewis酸的螯合金属和羰基络合的同时也和作为Lewis碱的螯合基团进行络合形成了一个环状结构。

规则特点：

1、螯合基团与羰基处于重叠交叉式，在螯合金属的络合下形成环状结构。

2、亲核试剂从空间位阻更小的Rs一侧进攻。

当Lewis酸和Lewis碱同时存在，满足Cram螯合规则条件时，会形成环状过渡态，Felkin-Anh规则不适用。



参考文献

一、《有机人名反应、试剂与规则》，黄培强等编著，P410-413。

二、https://www.bilibili.com/video/av201367616/


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