Felkin规则和Cram规则 不对称诱导

费尔金-安过渡态模型（Felkin-Anh模型）是一个用来解释α-手性酮亲核加成反应中产物的立体选择性现象的理论模型。是对克莱姆模型（Cram规则）的发展。

历史

该模型由费尔金在1960年代提出。1976年经安和埃森斯坦修正后得名，故有时也称费尔金-安-埃塞斯坦模型。费尔金是法国化学家，安是一个姓阮（Nguy?n Tr?ng ánh/Anh，阮仲映/英/婴？）的旅法越南化学家。

模型图示

如图纽曼投影式所示。亲核试剂从左侧进攻羰基。手性碳上的最大基团与羰基碳氧键保持垂直。基团大小由CPI规则确定。

Felkin规则和Cram规则说的是同一件事，Felkin规则是Cram规则的发展，更准确。适用条件是C=O的a-C是手型的。

Cram I：如果在 C=O 的α-C 联有三个体积不同的基团，就会造成羰基平面两侧的空间阻碍不同，给亲核试剂进攻羰基创造了空间上的选择性，我们用 L、M、S 分别表示α-C 上体积大、中、小的三个基团，规则如图所示。

Cram II ：规则二适用于当α-C上有-OH，-NHR之类的基团从而和羰基氧形成氢键的情况。本情况下，应该取重叠式构象为最稳构象，亲核试剂从S侧进攻。

Cram法则

Felkin-Ahn and Chelation Control

In Felkin-Ahn model, a nucleophile comes from the least hindered side. The best way to do Felkin-Ahn model is to draw a newmen projection. Then have the nucleophile attack from the smallest group. Felkin-Ahn model example:

Here, the model shows that the nucleophile prefers to attack from the least hindered side.

Chelation Control:

In Chelation Control there is always a lewis base or lewis acid is present. Example: Lewis Bases are OR', NR2' or SR' and lewis acids are Li+, MgX+, Zn+2. Since lewis base is present the double bonded oxygen and lewis base form a ring with lewis acid. Chelation Control example:

Next slide shows that when a Lewis-base or Lewis-Acid is present, Chelation Control gives the major product, by forming a ring in transition state and Felkin-Ahn models fails.

In General: How to choose between Felkin-Ahn and Chelation Control:

Asymmetric induction (also enantioinduction) in stereochemistry describes the preferential formation in a chemical reaction of one enantiomer or diastereoisomer over the other as a result of the influence of a chiral feature present in the substrate, reagent, catalyst or environment.[1] Asymmetric induction is a key element in asymmetric synthesis.

Asymmetric induction was introduced by Hermann Emil Fischer based on his work on carbohydrates.[2] Several types of induction exist.

Internal asymmetric induction makes use of a chiral center bound to the reactive center through a covalent bond and remains so during the reaction. The starting material is often derived from chiral pool synthesis. In relayed asymmetric induction the chiral information is introduced in a separate step and removed again in a separate chemical reaction. Special synthons are called chiral auxiliaries. In external asymmetric induction chiral information is introduced in the transition state through a