Handbook of Functionalized Organometallics Applications in S

182
5 Polyfunctional Silicon Organometallics for Organic Synthesis
four contiguous chiral centers with fairly high diastereoselectivity. The strain on
silicon induced by the pinacol ligand is essential for the stereoselective aldol reaction;
no reaction takes place when pinacol is replaced by 2,4-dimethyl-2,4-pentanediol.
The stereochemical outcome of the reaction is explained in terms of a chairlike
six-membered transition state 55 for intramolecular crotylsilylation of b-siloxy
aldehydes.
Bis(allyl)homoallyloxysilanes 56a and 56b are designed for a tandem intramolecular
silylformylation±allylsilylation reaction, which has turned out to be an efficient
approach to construct polyol and polyketide frameworks [21]. For example,
heating a solution of 56 in benzene at 60 C in the presence of Rh(acac)(CO) 2
under CO atmosphere followed by the Tamao oxidation gives syn,syn-triols
59 stereoselectively via oxasilacyclopentanes 57 and 58 (Scheme 5.14). Bis(cis-crotyl)silane
56b is readily prepared by double Pd-catalyzed 1,4-hydrosilylation of 1,3butadiene
with dichlorosilane followed by reduction with LiAlH 4 and alcoholysis
with the corresponding homoallylic alcohol.
R
O Si H
56a (R = H)
56b (R = Me)
iPr
H
R
b a
R
O Si O
R
57
1) Rh(acac)(CO) 2 (3 mol%)
CO (900 or 1000 psi)
benzene, 60 ºC
2) H2O2, NaHCO3, MeOH
92 : 8 ds
H
path a
Scheme 5.14 Tandem silylformylation±allylation of alkenes.
iPr
OH OH OH
R
59a (R = H: 59% yield)
59b (R = Me: 67% yield)
[O]
O Si
R
O
The tandem silylformylation±allylation methodology is extended to remote 1,5stereocontrol
[22]. Thus, treatment of homopropargylic hydrosilyl ethers 60, produces,
in a manner similar to homoallylic hydrosilyl ethers 56, 3-silyl-1,5-diol 61
whose oxidation or protodesilylation/acetylation gives, respectively, 1,5-anti diol 62
or diacetate 63 with high diastereoselectivity (Scheme 5.15). The 1,5-anti selectivity
contrasts sharply to the one obtained in the tandem reaction of homoallylic silyl
ethers 56.
H
58
R