Hydrosilylation of 1,3-Dienes

The hydrosilylation of dienes can also form different regioisomeric products, depending on the identity of the silane and catalyst. In principle, the hydrosilylation of dienes could be used as a route to form allylsilane reagents. However, these reactions are complex because allylic and homoallylic silanes can both be produced, and products from two hydrosilylations of the diene, or products from a combination of hydrosilylation and oligomerization of the diene, can be formed. 

Specific examples of the hydrosilylation of dienes are shown in Equations 16.25-16.28. The hydrosilylation of butadiene with Speier s catalyst gives a mixture of mono- and diaddition products, but the hydrosilylation of isoprene forms predominantly 3-methyl-2-butenylsilane, as shown in Equation 16.25. In contrast, triphenylphospliine complexes of palladium generate 2-butenylsilane from a single 1,4-addition of silane to butadiene  

Ligand L Catalyst (mol %) HSiXj Temp (X) Time (h) Yield (%) % EE Cf39 Ref. 

43a SiRs = SiMeCl2 43b SiRs = SiCIs 43c SiRs = SiF2Ptl 

46a SiRg = SiClg 47a SiRg = SiClg 46b SiRg = SiFPhg 

Intramolecular asymmetric hydrosilylation-oxidation of (alkenyloxy) hydrosilanes provides an efficient method for the preparation of optically active polyols from al-lylic alcohols. Cyclization of silyl ethers 54 of a meso-type allyUc alcohol in the pres- 

43a SiRg = SiMeClg 43b SiRg = SiClg 43c SiRg = SiFgPh 

Ugand L Catalyst (mol%) HSiXj Temp rc) Time h) Yield (%) %EE Cf43 Rtf. 

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Hydrosilylation of dienes accompanied by cyclization is emerging as a potential route to the synthesis of functionalized carbocycles. However, the utility of cycliza-tion/hydrosilylation has been Umited because of the absence of an asymmetric protocol. One example of asymmetric cycUzation/hydrosilylation has been reported very recently using a chiral pyridine-oxazoUne ligand instead of 1,10-phenanthroline of the cationic palladium complex (53) [60]. As shown in Scheme 3-21, the pyridine-oxazoUne Ugand is more effective than the bisoxazoUne ligand in this asymmetric cyclization/hydrosilylation of a 1,6-diene. 

Cyclization/Hydrosilylation of Dienes 11.11.4.1 Lanthanide Metallocene Catalysts... 

Dienes are less reactive toward transition metals than enynes and diynes, and perhaps for this reason, the development of effective catalyst systems for the cyclization/hydrosilylation of dienes lagged behind development of the corresponding procedures for enynes and diynes. The transition metal-catalyzed cyclization/hydrosilylation of dienes was first demonstrated by Tanaka and co-workers in 1994. Reaction of 1,5-hexadiene with phenyl-silane catalyzed by the highly electrophilic neodymium metallocene complex Cp 2NdCH(SiMe2)3 (1 mol%) in benzene at room temperature for 3 h led to 5- ///76 -cyclization and isolation of (cyclopentylmethyl)phenylsilane in 84% yield (Equation (15)). In comparison, neodymium-catalyzed reaction of 1,6-heptadiene with phenylsilane led to 5- X(9-cyclization to form (2-methylcyclopentylmethyl)phenylsilane in 54% yield as an 85 15 mixture of trans. cis isomers (Equation (16)). 

Guo X, Farwaha R, Rempel G L. (1990) Catalytic hydrosilylation of diene-based polymers. 1. Hydrosilylation of Polybutadiene. Macromolecules 23 5047-5054... 

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. 

The sequential cyclization/silylation reactions of 1,5-dienes and 1,6-dienes are catalyzed by Cp 2YMe(THF). The reaction tolerates a number of functional groups and proceeds with good yields and diastereoselectivities to give phenylsilane products which can be converted easily to synthetically more versatile alcohols (Scheme 276). The hydrosilylation of dienes is also effectively catalyzed by the neodymium alkyl complex Cp 2NdCH(SiMe3)2. 

More substituted double bonds are less reactive, and therefore, regioselective hydrosilylations of dienes are possible (8) and (9) [45], Yttrocene catalysts are less reactive in case of 1,1-disubstituted alkenes however, the larger samarocene is better suitable to catalyze this transformation with excellent diastereoselectivity (10) [47],... 

The main synthetic routes to allylsilanes are Grignard reactions, reductive silylation, and Wittig-type reactions with halogenosilanes and hydrosilylation of dienes. 

Hydrosilylation of alkenes.1 This lanthanide is an effective catalyst for hydrosilyla-tion of mono- and 1,1-disubstituted alkenes at 20° in CpHr, or C6H5CH3. Hydrosilylation of dienes with a terminal and an internal olefin is highly selective. 

The text of a lecture on the benzophenone-sensitized c/j-Zraws-photochemistry of penta-1,3-diene and hexa-2,4-diene has been published.80 A quantum-chain process is described. Wrighton and Schroeder61 have reported the photochemical hydrosilylation of dienes by irradiation in the presence of Cr(CO) and a silane. The following example is typical ... 

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