Page 191 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

Scheme 2.6. Examples of Stereoselective Reactions 171
A. Catalytic Hydrogenation. (See section 2.4.1.1) SECTION 2.4
Unfunctionalized alkene usually reacts by preferential syn delivery of hydrogen from the less hindered face of Stereoselective and
the double bond. The degree of stereoselectivity is dependent on the reactant structure, catalysts and reaction Stereospecific Reactions
conditions. Donor functional groups, particularly hydroxy and amino can be syn directive.
CH CH
1 a CH 3 Pt, H 2 3 + 3
CH CH 3
CH 3 racemic
3
70–85% 30 –15%
2 b
(CH 3 ) 2 CH CH 2 CH
Pt, H 2 (CH ) CH 3
3 2
OH OH 90%
(The same article also sites other examples with low stereoselectivity.)
A representative exception.
3 c CH 3 CH 3 CH 3
+
CH 3 CH 3 CH 3
25% racemic
75%
B. Hydride Reduction of Cyclic Ketones (see Section 2.4.1.2)
Unhindered cyclohexanones normally react with NaBH and LiAlH by preferential reagent approach from
4
4
the axial direction forming mainly the equatorial alcohol. The presence of axial subsituents or use of more
sterically demanding reagents, such as alkylborohydrides leads to selective equatorial approach and formation
of axial alcohols. Bicyclic ketones are generally reduced by hydride approach from the less hindered face
of the carbonyl group.
4 d
LiAlH 4
(CH ) C O (CH ) C OH + (CH ) C OH
3 3
3 3
3 3
8%
92%
[(isoamyl) BH] –
5 e (CH 3 ) 3 C O 3 (CH ) C OH
3 3
> 99%
NaBH 4
6 f +
OH
O
86% OH 14%
C. Nucleophilic Addition to Acyclic Ketones (see Section 2.4.1.3)
Adjacent stereocenters influence the mode of addition of nucleophiles such as hydrides and organometallic
reagents to acyclic ketones. The Felkin-Ahn transition state provides a predictive model that is general when
steric effects are dominant. Other factors must be considered when polar or chelating substituents are present.
2
7 g O C 2 H MgBr OH C 2 H 5 + C H 5 OH
5
Ph CH Ph CH Ph
CH 3 3 3
H H H
CH 3 88% CH 3 12% CH 3
8 h PhCH OCH O
PhCH 2 OCH 2 O PhCH OCHO 2 2
2
Zn(BH 4 ) 2
+
(CH 3 ) 2 CH
(CH 3 ) 2 CH CH 3
(CH 3 ) 2 CH CH 3 CH 3
O OH OH
4
96
(Continued)