Stereoselective hydrosilylation

The very first example of the catalytic reductive cyclization of an acetylenic aldehyde involves the use of a late transition metal catalyst. Exposure of alkynal 78a to a catalytic amount of Rh2Co2(CO)12 in the presence of Et3SiH induces highly stereoselective hydrosilylation-cyclization to provide the allylic alcohol 78b.1 8 This rhodium-based catalytic system is applicable to the cyclization of terminal alkynes to form five-membered rings, thus complementing the scope of the titanocene-catalyzed reaction (Scheme 54). 

SCHEME 13. The Ojima-Crabtree mechanism for stereoselective hydrosilylation of 1-alkynes... 

Once a chiral silane is synthesized, intramolecular reaction such as the Heck reaction and the radical cyclization reaction with transfer of chirality from the silicon center can be pursued. Other reactions of interest include the stereoselective hydrosilylation of aldehydes and ketones. Our method could be used to tether chiral silanes to solid support for uses in solid support... 

Cyclic seven-membered vinyl silanes 161 were obtained by regio- and stereoselective hydrosilylation of internal alkynes catalyzed by the ruthenium complex [Cp Ru(MeCN)3]PF6, as shown in Equation (33) <2005JA10028>. Hydrosilylation of 2,2-divinyladamantane with bis(hydrosilane) species 162 in the presence of Zeise s dimer [Pt2Cl4(CH2CH2)2] gave the disilacyclic 163 in high yields (Equation 34) <19980M4267>. 

Stereoselective hydrosilylation of conjugated dienes gives chiral monoaddition products, sometimes as a mixture of regioisomers. The yield and enantiomeric excess of the products depend on the nature of the chiral organometallic catalyst used. 

Application of various chiral palladium complexes to the stereoselective hydrosilylation of dienes results in the formation of 3-silylcvcloalkenes irrespective of the isomer of the diene used (in the case of cyclohexadienes). When complexes containing menthyldiphenylphosphine (MDPP) and neomenthyldiphenylphosphine (NMDPP) are used, the chiral products with an excess of the (S)-enantiomer are always formed1011. 

When [(/ )-(S)-PPFA]PdCl2 is used in the stereoselective hydrosilylation of 1-aryl-1,3-butadienes with trichlorosilane, the chiral (Z)-l-aryl-l-trichlorosilyl-2-butene 17 and the regiomeric 1-aryl-3-trichlorosily 1-1-butene 18 are formed. Subsequent reaction with methyllithium produces the corresponding trimethylsilyl derivatives13. 

Stereoselective hydrosilylation of 1,3-cyclopentadiene giving (f )-3-trimethylsilyl-1 -cydopen-tene [(/ )-13] can be achieved using ferrocenylphosphine-palladium complexes as catalysts15. 

Stereoselective hydrosilylation of alkadienes is also brought about using chiral ferrocenylphosphine-palladium complexes15. 

Transition metal complexes, particularly those of Group VIII, are popular catalysts. Of these, Pt and Rh are employed most frequently. A Cr catalyst is a unique catalyst for stereoselective hydrosilylation of 1,3-dienes. Catalysts based on Fe, Os and Ir are rarely used. 

Stereoselective hydrosilylation of terpene ketones such as camphor and menthone catalyzed by RhCI(PPh3)3 followed by hydrolysis exhibits significant differences in stereochemistry from other reductions using metal hydrides123,115. The bulkiness of hydrosilanes exerts a remarkable influence on the stereochemical course of the reaction, i.e., a bulky hydrosilane favors the production of the more stable alcohols (equation 61). 

On using chiral ketones, an asymmetric induction takes place at the silicon center156 even with an achiral catalyst such as RhCl(PPh3)3, which is an application of the stereoselective hydrosilylation of cyclic terpene ketones (see Section IV.C). Double asymmetric induction is found to be effective also in this case, which brings about a high optical yield156 (equation 88). 

Japanese scientists succeeded in stereoselective hydrosilylation of substituted cyclohexanones catalyzed by RhCKPPhsjs and a silica-linked... 

In 1999 [22] and 2001 [23], Matsumura and co-workers reported the first examples of stereoselective hydrosilylation with HSiCla and (5)-proline derivatives as effective activators. These works can be considered as milestones for the asymmetric reduction of ketones and imines using HSiCla as reducing agent and paved the road to the synthesis of other related systems. Since then, considerable efforts have been devoted to the development of efficient catalysts for the reduction of carbon-nitrogen double bonds, and remarkable progress has been made. 

Ruthenium-based catalysts allow stereoselective hydrosilylation of alkynes giving /3-(Z)-alkenylsilanes or a-vinylsilanes as predominant products, respectively. [ Ru(p-cymene)Cl2 2], [RuCl(SiMe2Ph)(CO)(Pi-Pr3)2], and [RuHCl(CO)(Pi-Prs)2] have been recently demonstrated as active catalysts for the efficient (Z)-selective hydrosilylation of a wide range of functionalized alkynes (90-92), whereas Grubbs first-generation catalyst or cationic cyclopentadienyl ruthe-nium(II) complex [ Cp RuGVIeCNlalPFe] affords selectively a-vinylsilanes (93-95). 

Several independent protocols using a combination of transition metal-catalyzed stereoselective hydrosilylation, such as palladium-catalyzed crosscoupling sequence leading to stereodefined r-conjugated alkene derivatives, have been successfully developed in the last decade (4). Alkenylsilanes or siloxanes, prepared via platinum or rhodium complex-catalyzed intermolecular hydrosilylation of terminal alkynes have been highly stereospecifically cross-coupled with aryl and alkenyl halides to give unsymmetrical stilbenes, alkenylbenzenes, and conjugated dienes (Scheme 24) (4). 

Types of reaction O-Si bond formation Reaction conditions Toluene, room temperature Synthetic strategy Chemo- and stereoselective hydrosilylation Catalyst lndium(lll) chloride (hiCls)... 

The 6-deoxy- -L-altropyranose derivative (48) was obtained by stereoselective hydrosilylation (Me PhSiH -[Rh(norbornadien)Cl]2) of the 5,6 ene (49) followed by protodesilylation (Bu iNF). The stereocontrol led synthesis of 3-deoxy-l,2-0-isopropylidene-8-L-threo-pentofuranose (50) from diacetoneqlucose involved reduction of the known enal (51) first with sodium borohydride then with nickel boride. Only moderate stereoselectivity was attained in the reduction [with LiAlH, ... 

Stereoselective hydrosilylations of alkynes have been explored using a wide variety of rhodium and ruthenium catalysts. ... 

Rhodium-catalyzed Additions. The title compound has been a test reagent in several rhodium-catalyzed additions to the triple bond. For example, a rhodium-catalyzed silylcarbamoylation in the presence of a secondary amine afforded the silylated product in reasonable yield (eq 40). In the presence of a CO atmosphere and with 5% rhodium on alumina as catalyst, stereoselective hydrosilylation has been reported (eq 41). ... 

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