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（Nguyễn Trọng 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 preferen
tial 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 reactive carbonyl speciesasymmetric 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 catalyst of chiral ligand. This method of asymmetric synthesis is economically most desirable.
Contents
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∙ 1 Carbonyl 1,2 asymmetric induction
o 1.1 Cram's rule
o 1.2 Felkin model
o 1.3 Felkin-Anh model
o 1.4 Anti–Felkin selectivity
∙ 2 Carbonyl 1,3 asymmetric induction
o 2.1 Chelation model
o 2.2 Non-chelation model
o 2.3 Cram–Reetz model
o 2.4 Evans model
∙ 3 Carbonyl 1,2 and 1,3 asymmetric induction
∙ 4 Acyclic alkenes asymmetric induction
∙ 5 See also
∙ 6 References
∙ 7 External links
Carbonyl 1,2 asymmetric induction[edit]
Several models exist to describe chiral induction at carbonyl carbons during nucleophilic additions. These models are based on a combination of steric and electronic considerations and are often in conflict with each other. Models have been devised by Cram (1952), Cornforth (1959), Felkin (1969) and others.
Cram's rule[edit]
The Cram's rule of asymmetric induction developed by Donald J. Cram in 1952[3] is an early concept relating to the prediction of stereochemistry in certain acyclic systems. In full the rule is:
In certain non-catalytic reactions that diastereomer will predominate, which could be formed by the approach of the entering group from the least hindered side when the rotational conformation of the C-C bond is such that the double bond is flanked by the two least bulky groups attached to the adjacent asymmetric center.
The rule indicates that the presence of an asymmetric center in a molecule induces the formation of an asymmetric center adjacent to it based on steric hindrance.
In his 1952 publication Cram presented a large number of reactions described in the literature for which the conformation of the reaction products could be explained based on this rule and he also described an elaborate experiment (scheme 1) making his case.
The experiments involved two reactions. In experiment one 2-phenylpropionaldehyde (1, racemic but (R)-enantiomer shown) was reacted with the Grignard reagent of bromobenzene to 1,2-diphenyl-1-propanol (2) as a mixture of diastereomers, predominantly the threo isomer (see for explanation the Fischer projection).

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