Dienes, hydrosilation

The hydrosilation of conjugated dienes is catalyzed by transition metal complexes34, and has been developed into a useful asymmetric synthesis of allylsilanes35,36. 

Efficient asymmetric transfer is also observed for 1,2-benzenediol induced reactions of a-sub-stituted (Z)- and ( ,)-allyl(trialkoxy)silanes prepared by hydrosilation of conjugated dienes, although in this case the electrophile attacks the C-C double bond of the allylsilane syn with... 

Platinum(O) phosphine complexes undergo a variety of oxidative addition reactions with compounds containing Group 14 elements. These reactions are of widespread interest because similar processes are probably involved in the catalysis by platinum complexes of reactions such as the hydrosilation of alkenes and the disilylation of dienes and alkenes. 

It has been shown that hydrosilylation may not perform as ideally as is required when preparing co-olefinic silicone compounds from organic a,co-dienes and hydrosil(ox)anes isomerization is a concern and the chemical equivalence of the double bonds requires a large excess of the diene compound to achieve essentially monohydrosilylation. Further side reactions are discussed by Torres et al [9],... 

Every example with a cyclohexene has proved hydrosilation to be very slow and difficult, but this difficulty was not experienced with cyclopen-tene structures by Petrov et al. (27). A number of chlorosilanes were added to bicyclo-(2,2,l)-hept-5-ene, bicyclo-(2,2, l)-hepta-2,5-diene, 2-methyl- and 2-vinylbicyclo-(2,2,l)-hept-5-ene, and dicyclopentadiene under mild conditions. Particularly surprising was the example of Eq. 

The activity of Ziegler-type systems such as M(acac) -AlEt3 (M = Cr, Mn, Fe, Co, or Ni acac = acetylacetonate) was examined with 1-olefins and triethyl- or triethoxysilanes (55). Systems with nickel or cobalt showed low activity for hydrosilation but isomerized the olefin and were reduced to the metal. The study was extended to dienes and acetylenes (56). Isoprene gave the same products with these catalysts as are made with chloroplatinic acid. Penta-1,3-diene with Pt gave l-methylbut-2-en-ylsilanes. The Ziegler catalysts gave mainly penta-2-enylsilanes... 

With dienes and acetylenes, only M = Fe, Co or Ni showed activity. The Fe and Co systems gave many side products. Therefore, the Ni catalyst was studied in greater detail. Bis-(7r-cycloocta-l,5-diene)nickel(0) was equivalent to the Ziegler catalyst in hydrosilation of penta-1,3-diene. 

These Ziegler catalysts were inactive for hydrosilation of cyclopenta-diene, c/s,c/s-cycloocta-l,5-diene, or norbornadiene. 

Recently, various rhodium carbene complexes were investigated as catalysts for hydrosilation of olefins, acetylenes, and dienes to see whether carbene ligands modify catalytic activity. All reactions were... 

The hydrosilation of butadiene, isoprene and other simple conjugated dienes with platinum catalysts has not been intensively studied. It usually occurs by 1,4 addition and only at elevated temperatures (10). Although palladium catalysts are practically ineffective to hydrosilate an alkene, they are very active under mild conditions with conjugated dienes to give two principal products, sometimes in 100% yields. With triethoxy silane, butadiene, and PdCl2(PhC=N)2 (5.5 x 103 moles/mole) at 22°C for 24... 

Hydrosilation of an alkenylsilane in the presence of a catalyst provides a route to five-, six- and seven-membered rings but is of little use for the smaller ones. Tin-substituted cyclohexa-2,5-dienes result directly from the a,[Pg.575]

Silacyclohex-2-enes are conveniently made through the formation of l-trichlorosilyl-5-chloro-l-pentene by hydrosilation, followed by CyClization using magnesium (Scheme 230) (65JOM(4)284). The Diels-Alder addition of silenes to 1,3-dienes provides a useful route to the isomeric silacyclohex-3-enes (Scheme 231) (79CR529). 

Hydrosilation of 1,5- and 1,6-dienes, catalyzed by a chiral neodymium complex, leads to intramolecular bond formation and results in the formation of silylated methylcy-clopentanes (equation 147)538. This reaction is in stark contrast to the reaction catalyzed by group VHI complexes, which gives a range of silanes but no carbocyclic compounds539,540. 

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. 

The hydrosilation of 1,3-dienes with Speier s catalyst often gives product mixtures of 1,2 and 1,4-addition, with the latter usually being the main product. In contrast to this, the palladium-catalyzed reaction with trichlorosilane gives the 1,4-addition product exclusively. If trimethylsilane is used, an adduct with two equivalents of diene can be obtained with an excellent yield (equation 13). ... 

Cyclic dienes are hydrosilated by 1,4-addition using Pd or Ni catalysts (equation 14). 

Palladium-catalyzed asymmetric hydrosilation of cychc and linear 1,3-dienes proceeds as 1,4-addition with moderate to good enantioselectivities. a-Ketoesters snch as pyruvates can be hydrosilated with Rh(I)-(-t-)-DIOP catalysts to yield lactates with much better optical purity than simpler ketones (equation 22). This effect is likely to be cansed by a ligand fimction of the ester moiety in the transition state. 

Asymmetric Intramolecular Hydrosilation. Intramolecular hydrosilation of allylic alcohols followed by oxidation is a convenient method for the stereoselective preparation of 1,3-diols. An enantioselective version is achieved by use of diene-free BINAP-Rh+ (eq 6). Both silyl ethers derived from cinnamyl alcohol and its cis isomer give (iJ)-l-phenylpropane-l,3-diol in high ee regardless of alkene geometry. 

Hydrosilation. The second most important method for preparing or-ganosilanes on a large scale is the addition of hydrosilanes across a carbon-carbon double bond. The reaction is quite general and applies to a wide variety of substituted alkenes, dienes, and alkynes. 

Other Rh catalysts were also employed for hydrosilation of a,p-unsaturated carbonyl compounds and unsaturated nitriles. Thus, Rh(acac)2 and a tetrakis( jL-acetato)dirhodium cluster were used as catalysts in the hydrosilation of a,P-unsaturated aldehydes. These reactions, however, are not chemoselective, as alkynes, conjugated dienes and alkenes are also hydrosilylated, and allylic heterosubstituents are reduc-tively cleaved. 

Selective reduction of pregna-14,16-dien-20-ones to pregn-14-en-20-ones is achieved via hydrosilation with tetramethyldisiloxane and catalytic amounts of chloroplatinic acid (Scheme 67). ° a,p-Unsaturated esters are also reduced to the corresponding saturated esters under these conditions. ... 

Qualitative photochemical studies concentrate on diene complexes because of their greater thermal inertness and ease of characterization. Formation of olefin complexes is induced in situ either by photochemical or thermal means and their presence determined by spectroscopy. The photocatalyzed hydrogenation and hydrosilation of 1,3-di-enes the photocatalyzed valence isomerization of norbomadiene to quadricyc-lane and the cis trans photoisomerization of coordinated olefins are potentially usefulHowever, these transformations are not photosubstitution reactions and are not discussed here the reader should consult ref. 1 and references cited therein. Photolysis of olefin complexes leads to olefin loss with high quantum efficiency unless the olefin is a chelating di- or polyene where, as with most chelating ligands, other reactions occur. 

Hydrosilations of dienes and alkynes have been studied, and in both cases single and double silylations are possible. For 1,3-dienes, both 1,2 and 1,4 addition products can be observed8,195 213 271-272. A Ziegler-type catalyst composed of nickel(II) pentadienoate and triethylaluminum is active in hydrosilylation of terminal alkynes, but this reaction is accompanied by oxidative dimerization of the alkyne (equation 101). The major product in most cases was the head-to-head dimer273. 

Photochemical studies of olefin complexes are more concentrated on diene compounds because they are more stable. Photochemical reactions of hydrogenation, isomerization, and hydrosilation of olefins were carried out. ... 

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