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== 立体選択性 ==
{{Main|{{仮リンク|不斉触媒反応|en|Asymmetric catalytic reduction}}}}
 
=== ジアステレオ選択的還元 ===
シクロヘキサノンの還元においては、ヒドリド源が[[シクロヘキサンの立体配座|アキシャル]]位を攻撃してエカトリアル位にヒドロキシ基がつくか、エカトリアル位を攻撃してアキシャル位にヒドロキシ基が結合したアルコールができる。アキシャル位への攻撃（赤で示した）ではヒドリドが[[ひずみ (化学)|1,3-ジアキシャル相互作用]]による反発を受ける。一方エカトリアル位の攻撃ではヒドリドは相互作用を受けることがないが、新しくできたアルコールが基質と水素原子が[[反応中間体]]が{{仮リンク|アルカンの立体化学|en|Alkane stereochemistry|label=ねじれひずみ}}を受ける（アキシャル位にヒドロキシ基が結合したアルコールの[[ニューマン投影法]]で下に示した）。
In the reduction of cyclohexanones, the hydride source can attack [[Axial bond|axially]] to produce an equatorial alcohol, or [[Equatorial bond|equatorially]] to produce an axial alcohol. In axial attack (shown in red), the hydride encounters [[Strain (chemistry)|1,3-diaxial strain]]. In equatorial attack (shown in blue), the hydride avoids the 1,3-diaxial interaction, but the substrate undergoes unfavorable [[Strain (chemistry)|torsional strain]] when the newly formed alcohol and added hydrogen atom eclipse each other in the reaction intermediate (as shown in the Newman projection for the axial alcohol).
[[Fileファイル:Axial vs equatorial.png|centre|frameless|316x316px]]
Large reducing agents, such as LiBH(Me<sub>2</sub>CHCHMe)<sub>3</sub>, are hindered by the 1,3-axial interactions and therefore attack equatorially.<ref name=":0"/> Small reducing agents, such as NaBH<sub>4</sub>, preferentially attack axially in order to avoid the eclipsing interactions, because the 1,3-diaxial interaction for small molecules is minimal; stereoelectronic reasons have also been cited for small reducing agents' axial preference.<ref>{{Cite journal|title = Theoretical evidence in support of the Anh?Eisenstein electronic model in controlling ?-facial stereoselectivity in nucleophilic additions to carbonyl compounds|url = http://xlink.rsc.org/?DOI=c39900000456|journal = Journal of the Chemical Society, Chemical Communications|volume = 0|issue = 6|doi = 10.1039/c39900000456|first = Stephen S.|last = Wong|first2 = Michael N.|last2 = Paddon-Row|pages=456–458|date = January 1990}}</ref> Making the substrate bulkier (and increasing 1,3-axial interactions), however, decreases the prevalence of axial attacks, even for small hydride donors.<ref>{{Cite journal|title = Lithium trisiamylborohydride. A new sterically hindered reagent for the reduction of cyclic ketones with exceptional stereoselectivity|journal = Journal of the American Chemical Society|date = 1976-05-01|issn = 0002-7863|pages = 3383–3384|volume = 98|issue = 11|doi = 10.1021/ja00427a061|first = S.|last = Krishnamurthy|first2 = Herbert C.|last2 = Brown}}</ref>
 
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