Terminal alkenes, hydrosilation

The C—Si bond formed by the hydrosilation of alkene is a stable bond. Although it is difficult to convert the C—Si bond to other functional groups, it can be converted to alcohols by oxidation with MCPBA or H2O2. This reaction enhances the usefulness of hydrosilylation of alkenes [219], Combination of intramolecular hydrosilylation of allylic or homoallylic alcohols and the oxidation offers regio- and stereoselective preparation of diols [220], Internal alkenes are difficult to hydrosilylate without isomerization to terminal alkenes. However, intramolecular hydrosilation of internal alkenes can be carried out without isomerization. Intramolecular hydrosilylation of the silyl ether 572 of the homoallylic alcohol 571 afforded 573 regio- and stereoselectively, and the Prelog-Djerassi lactone 574 was prepared by applying this method. 

Some hydrometalation reactions have been shown to be catalyzed by zirconocene. For instance, CpiZrCf-catalyzed hydroaluminations of alkenes [238] and alkynes [239] with BU3AI have been observed (Scheme 8-34). With alkyl-substituted internal alkynes the process is complicated by double bond migration, and with terminal alkynes double hydrometalation is observed. The reaction with "PrjAl and Cp2ZrCl2 gives simultaneously hydrometalation and C-H activation. Cp2ZrCl2/ BuIi-cat-alyzed hydrosilation of acyclic alkenes [64, 240] was also reported to involve hydrogen transfer via hydrozirconation. 

Addition of hydrosilane to alkenes, dienes and alkynes is called hydrosilylation, or hydrosilation, and is a commercially important process for the production of many organosilicon compounds. As related reactions, silylformylation of alkynes is treated in Section 7.1.2, and the reduction of carbonyl compounds to alcohols by hydrosilylation is treated in Section 10.2. Compared with other hydrometallations discussed so far, hydrosilylation is sluggish and proceeds satisfactorily only in the presence of catalysts [214], Chloroplatinic acid is the most active catalyst and the hydrosilylation of alkenes catalysed by E PtCU is operated commercially [215]. Colloidal Pt is said to be an active catalytic species. Even the internal alkenes 558 can be hydrosilylated in the presence of a Pt catalyst with concomitant isomerization of the double bond from an internal to a terminal position to give terminal silylalkanes 559. The oxidative addition of hydrosilane to form R Si—Pt—H 560 is the first step of the hydrosilylation, and insertion of alkenes to the Pt—H bond gives 561, and the alkylsilane 562 is obtained by reductive elimination. 

Regiospecificity of the reaction is normally high but with some alkenes, such as styrene, considerable amount of the /3-isomer (38.5%) is produced exceptionally in addition to the a-isomer (58.7%). Interestingly, alkenes having an inner double bond undergo hydrosilation with isomerization to give exclusively terminal products, as shown in equation (24). 

Metal complex catalyzed hydrosilations usually give terminal silylation, even with internal alkenes. The additions occur in a cis fashion and the stereochemistry at silicon is retained. However, great variations in regioselectivity have been observed for the same reagents with different catalysts. No stereochemical generalizations can be made. The... 

Additions of Si-H bonds to alkynes occur under similar conditions and with the same catalysts as hydrosilation of alkenes. Free radical addition to terminal alkynes gives cis products by a stereospecific terminal trans addition . Supported platinum catalysts give trans products by a terminal cis addition . Chloroplatinic acid catalyzed additions to terminal alkynes give mixtures of trans-1-alkenylsilanes and trans-2-alkenylsilanes in a ratio ranging from 1 1 to 1 5 depending on the substituents on silicon . Addition of SiH2Cl2 to CH2=CHC(CH3)3 gives trans-1-alkenyl- and bis(trans-l-alkenyl)silane products, but no (2-alkenyl)silane °. Internal alkynes react more slowly than terminal alkynes, and even reactions catalyzed by chloroplatinic acid require heat. 

Functionalizations via Silyl Hydride Functionalization and Hydrosilation A new general functionalization method based on the combination of living anionic polymerization and hydrosilation chemistry has been developed as illustrated in Scheme 7.26 [281]. First, a living polymeric organolithium compound is quantitatively terminated with chlorodimethylsilane to prepare the corresponding co-silyl hydride-functionalized polymer. The resulting co-silyl hydride-functionalized polymer can then react with a variety of readily available substituted alkenes to obtain the desired chain-end functionalized polymers via efficient regioselective transition-metal-catalyzed hydrosilation reactions [282-284]. 

The hydrosilylation of alkenes (Equation 16.12) and alkynes (Equation 16.13), alternatively termed hydrosilation, is the addition of a silicon-hydrogen bond across the C-C TT-bond to form a new alkylsilane or vinylsilane. This reaction has been catalyzed by complexes containing many different metals, but is most commonly conducted with complexes of platinum, rhodium, and palladium. The hydrosilylation of alkenes t3q>ically forms terminal alkylsilanes as the major regioisomer, and the hydrosilylation of vinylarenes often generates the chiral branched alkylsilane. The hydrosilylation of alkynes has also been developed. As shown generally in Equation 16.13, these reactions can occur by either cis or trans addition, depending on the catalyst. In some cases, the reactions of silanes with olefins form vinylsilanes (called dehydrogenative silylation. Equation 16.14). The addition of an Si-Si bond of a disilane across an olefin has also been reported (Equation 16.15), and this reaction is called disilation of olefins. 

Unlike hydrosilation discussed in the preceding subsection, which requires catalysis in essentially all cases, hydrostannation can be observed typically at or above 50 °C under thermal conditions. Under such conditions, however, the reaction tends to be capricious and unpredictable. Thus, for example, the reaction of terminal alkynes with HSnMes, HSnBus, and other triorganylstannanes tends to produce mixtures of a-, cis-f -, and trans-j8-stannyl-substimted alkenes except for some special cases, such as those shown in Scheme 19.W7H49]... 

The reaction of terminal allyl alcohols proceeds in a 5-endo fashion to give five-membered ring compounds regioselectively and stereoselectively. Subsequent oxidation affords 2,3-5 y -l,3-diols preferentially, regardless of the nature of the catalyst (eq 1). The stereoselectivity increases with increased bulkiness of the al-lylic substituent and the nature of the alkene substituent (see below). 5-Exo type cyclization occurs with homoallyl alcohols to form five-membered heterocycles and 1,3-diols after oxidation. Two chiral centers are produced in this reaction. The 2,3-relationship (anti) is controlled by the allylic substituent, while the 3,4-relationship is determined by the stereochemistry of the alkene the hydrosilation occurs by cis addition of Si-H to the alkene (eq 2). ... 

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