Alkenylsilanes preparation

Alkenylfluorosilanes readily couple with alkenyl iodides in the presence of a palladium ) catalyst and TASF to form dienes of high stereospecificity (equation 160)274. Since the alkenylsilane preparation and coupling reaction are conducted under neutral conditions, without the involvement of strong reducing agents, this coupling reaction has wide applicability. 

Alkenylsilanes can be prepared from aldehydes and ketones using lithio(chloromethyl)trimethylsilane. The adducts are subjected to a reductive elimination by lithium naphthalenide. This procedure is stereoselective for the E-isomer with both alkyl and aryl aldehydes.82... 

E)-Alkenylsilanes and -sulfides.2 Dibromomethyltrimethylsilane (1), prepared from ClSi(CH3)3 and CH2Br2, is reduced by commercial CrCl2 in THF to a reagent that converts aldehydes into (E)-vinylsilanes. A related reaction provides (E)-vinyl sulfides selectively from C6H5SCHC12. 

Third, chromium-based reagents prepared by reduction of 1,1-dihalo compounds with CrCl2 are suitable for the transformation of aldehydes to (i4)-iodoalkenes, (it)-alkenylsilanes, " and ( )-alkenylboronic esters with one-carbon homologation (Equation (8)). The amount of CrCl2 can be reduced to a catalytic amount by using manganese (or zinc) and MesSiCl. " ... 

Vinylsilanes and allylsilanes are prepared by dehydrogenative silylation of alkenes catalysed by Rh and other complexes [221]. A particularly effective catalyst for alkenylsilanes is Ru3(CO)12 [222], Using excess 1-hexene, 1-silyl-1-hexene 575 was... 

Dimethyltitanocene (213), called the Petasis reagent, can be used for alkenation of carbonyls (aldehydes, ketones, esters, thioesters and lactones). This reagent is prepared more easily than the Tebbe reagent by the reaction of titanocene dichloride with MeLi. However, this reagent may not be a carbene complex and its reaction may be explained as a nucleophilic attack of the methyl group at the carbonyl [67], Alkenylsilanes are prepared from carbonyl compounds. Tri(trimethylsilyl)titanacyclobutene (216), as a... 

Alkenylsilanes.1 A reagent (2), prepared from 1, TiCU, zinc, and TMEDA, is efficient for conversion of esters or thiolesters into alkenylsilanes with (Z)-selectiv-ity. 

The best procedure for the preparation of these compounds involves alkenylsilanes and iodosylbenzene activated by either triethyloxonium tetrafluoroborate or boron trifluoride etherate. The reaction proceeds stereospecifically with retention of configuration. 

Alkenylsilanes, mainly vinyl silanes and allyl silanes or related compounds, being widely used intermediates for organic synthesis can be efficiently prepared by several reactions catalyzed by transition-metal complexes, such as dehy-drogenative silylation of alkenes, hydrosilylation of alkynes, alkene metathesis, silylative coupling of alkenes with vinylsilanes, and coupling of alkynes with vinylsilanes [1-7]. Ruthenium complexes have been used for chemoselective, regioselective and stereoselective syntheses of unsaturated products. 

Hydrosilylation of 1 -alkynes catalyzed by transition-metal complexes is still the most versatile method for preparing 1-alkenylsilanes but the reaction often gives a mixture of three possible isomers (Eq. 12)... 

The mixed 1,2-bimetallic Zn/Si reagent 108 is a versatile species, which reacts with aldehydes in high diastereo-selectivity (Scheme 40).314 It is prepared by a bromine/lithium exchange reaction, followed by a transmetallation with ZnCl2- The reaction with acetaldehyde leads initially to the alkenylzinc species 109, which reacts with MesSiCl providing the alkenylsilane 110 in 41% yield and with a diastereoselectivity >9 1. 

Preparation of 1,4-dienes can be affected by the palladium-catalyzed crosscoupling of alkenylsilanes with allyl carbonates or 1,3-butadiene monoxides (Scheme 6) [36,37] or alternatively by the reaction of allylsilanes with alkenyl triflates (Eq. 36) [32]. 

An anionic trimethylsilyl moiety [22] generated by treatment of hexamethyldisilane with TASF undergoes palladium-catalyzed silylation of iodoolefins [Eq.(26)]. The reaction is stereospecific and thus allows us to prepare stereodefined alkenylsilanes [23]. 

Among the many available procedures for preparing alkenylsilanes are hydrosilylation of alkynes and partial reduction of alkynylsilanes. Hydrosilylation of 1-alkynes with triethylsilane in the presence of catalytic chloroplatinic acid results in regioselective... 

Alkenyllithiums generated from trisylhydrazones react with electrophiles such as primary alkyl bromides, aldehydes, ketones, dimethylformamide, CO2, chloro-trimethylsilane, or 1,2-dibromoethane to furnish substituted alkenes, allylic alcohols, a,(3-unsaturated aldehydes, a,P-unsaturated acids, alkenylsilanes, or alkenyl bromides, respectively, as exemplified below for the preparation of an allylic alcohol and an a,P-unsaturated aldehyde. ... 

Takai, K., Kataoka, Y., Okazoe, T., Utimoto, K. Stereoselective synthesis of (E)-alkenylsilanes from aldehydes with a reagent prepared by chromium(ll) reduction of trimethyl(dibromomethyl)silane. Tetrahedron Lett. 1987, 28, 1443-1446. 

Deprotonation. Exposure of 1-alkenes to the strong base and then a chlorosilane gives (Z)-2-alkenylsilanes. Alkenoic acids are similarly converted to 4-silyl-2-alkenoic acids. The preparation of an acetone enolate equivalent is by lithiation of methyl isopropenyl ether at low temperature. ... 

Coupling reactions involving aryl and alkenylsilanes in which the silicon atom bears an electron-withdrawing group are also valuable. Thus, 1,4-dienes arise from ally lie carbonates and alkenylfluorosilanes. Functionalized biaryls are prepared, and promotion of such reactions by NaOH has been noted. ... 

For the preparation of ( )-alkenylsilanes the Me3SiCH(ZnBr)2-TiCl2 reagent is inferior to MejSiCH(ZnBr)2-CrCl2 in terms of stereoselectivity. 

Azasilacyclopentanes are most conveniently prepared by the amidomercuration-demercuration of dimethyl(chloroalkyl)alkenylsilanes of the form (131) <87JOM(326)i59,85ZOB706). Thus, treatment of dimethyl(chloromethyl)vinylsilane (Ola, n = 0) or dimethyl(chloromethyl)allylsilane (131b, n = 1) with aniline in THF in the presence of mercury acetate, followed by reduction with sodium borohydride, afforded the azasilacyclopentanes (Ola), (132b). 

Intramolecular acylations proceed with greater success for alkenylsilanes than the simple alkenyl substrates. In several cases, ( )-silyl substrates have been cyclized to enones, ring size constraints precluding reaction with retention of alkene geometry (equation 8). - The cyclization approach can be used to prepare enones where the double bond lies exocylic to the ring. In the example shown, retention of configuration is also evident, although the product was easily isomerized under acidic conditions (equation 9). 

The oxidation of alkylsilyl alkoxides has been extended to the preparation of a-hydroxy ketones from alkenylsilanes. Thus, 5-decenyltrimethylsilane was treated with meta-chloroperbenzoic acid and the resulting a-silyl epoxide oxidized with hydrogen peroxide in the presence of fluoride ion to give 6-hydroxy-5-decanone in 88% yield145 (equation 149). 

Alkenyldichloroboranes. The species that are useful for Suzuki coupling can be prepared from alkenylsilanes with BClj in dichloromethane. 

Substituted oxygen heterocycles have been prepared by the reaction of —si1oxypropargy1silanes (192) with aliphatic and aromatic aldehydes (Scheme 55).158 A related cyclisation of an alkenylsilane to an oxonium ion is described in the next section. The intramolecular addition of allyl and propargyl silanes to enones using a Lewis acid... 

Useful preparation reactions include the formation of 3-cyanomethylphthalide from methyl o-bromobenzoate and acrylonitrile. An intramolecular Heck reaction involving a nitroalkene gives both saturated and unsaturated products. Alkenylsilanes are arylated. with a concomitant loss of the silyl group. Fully substituted alkylidenecyclopropanes undergo a most unusual Heck reaction, leading to 2-aryl-l,3-dienes. ... 

Although mechanistically different, functionalized alkenylsilanes are prepared stereoselectively by the reaction of 1-alkynes with iodotrimethylsilane (123) and diethylzinc. At hrst oxidative addition of 123 to Pd(0) generates 125. Then insertion of 1-octyne to 125 affords the alkenylpalladium 126. Transmetallation with Et2Zn gives 127 and reductive elimination provides the alkenylsilane 124. The reaction can be regarded as a Heck-type reaction of alkyne with MesSi-I, followed by Negishi coupling [37]. 

It should be noted that the required alkenylsilanes can readily be prepared by either hydrosilylationt of alkynes or silylation of alkenyl halides. ... 

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). 

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