Allylsilanes

4.4.40.59 Method 59:
Allylation of Aldehydes and Ketones
Homoallylic alcohols are available by the reactions of allylsilanes with aldehydes or ketones
either byprior activation of the carbonyl functionalitywith a Lewis acid or byprior
activation of allylsilane itself with a Lewis base. In some exceptional cases, [42,43] no external
activation is required; for an illustrative example see Scheme 5. Use of chiral Lewis
acids [280] or chiral Lewis bases [281,282] allows asymmetric synthesis of homoallylic alcohols.
4.4.40.59.1 Variation 1:
By Lewis Acid Catalyzed Allylation
Lewis acid catalyzed allylation of aldehydes and ketones with allylsilanes can provide
homoallyl alcohols. [20,21,30] These reactions usuallyrequire stoichiometric amounts of
Lewis acid, although a catalytic version is also possible (Scheme 4). [40] Diastereocontrol in
the reactions of but-2-enylsilanes with aldehydes was shown in Scheme 2. Remarkably, a
single diastereomer of dienediol 231 is obtained bythe titanium tetrachloride catalyzed
reaction of an isomeric mixture of allylsilanes 230 with pentanal (Scheme 88). [283] Incidentally,
the pure E,E-isomer of 230 is available bythe synthetic method described in Section
4.4.40.21 (Scheme 36).
Scheme 88 Stereoselective Synthesis of 6,9-Divinyltetradecane-5,10-diol [283]
Me 3Si
SiMe 3
BuCHO, TiCl4
MeNO 2, CH 2Cl 2
69%
230 231
1,5-Asymmetric induction by a formal S E2 reaction of an allylsilane is also known. For example,
transmetalation of allylsilane 232 with tin(IV) chloride, followed bytreatment of
the resulting product with benzaldehyde provides the 1,5-anti- and 1,5-syn alkenols antiand
syn-233 (Scheme 89). [284] The correct time lag between the tin(IV) chloride and aldehyde
additions is critical for better yields and stereoselectivity in the synthesis of antiand
syn-233. Allylsilane 232 was prepared bythe methodologydescribed in Section
4.4.40.12.
Scheme 89 1,5-Asymmetric Induction in the Reaction of a 5-Alkoxyalk-2-enylsilane [284]
BnO
232
SiMe 3
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4.4.40 Allylsilanes 905
1. SnCl4, −78 oC, 2 h
2. PhCHO, −78 oC 73%
Bu
OH
OH
OH
OH
+
BnO Ph BnO Ph
anti-233
73:27
syn-233
Sometimes, the addition of an allylsilane to an aldehyde can be combined with a pericyclic
reaction to give polysubstituted cyclohexane derivatives, for example, the preparation
of 234 (Scheme 90). [285] This application in synthesis took advantage of the method
of preparation of isoprenylsilane 31 described in Section 4.4.40.7.1.
Sarkar, T. K., SOS, (2002) 4, 837. 2002 Georg Thieme Verlag KG
Bu
for references see p 920