Silyl alkenes

Deuterium-labeling studies pointed to the operation of a nonstandard Chalk-Harrod mechanism for these reactions involving a silyl-alkene insertion step.133... 

An important observation in the area of Zr-catalyzed carboaluminations of alkenes is that made by Wipf and Ribe that addition of water leads to substantial acceleration of the C—C bond-forming process [35]. Thus, as illustrated in Scheme 6.14, whereas catalytic alkylation of the silylated alkene 41 does not afford any of the desired product, upon addition of one equivalent of water, 42 is formed in 85 % yield with 80 % ee. As is also depicted in Scheme 6.14, carboaluminations of unsaturated alcohols are less efficient (—> 43, but better than reactions without water), while those involving alkenes that bear an a-branched substituent are less selective (—> 44). Another impressive example of rate... 

Independent discovery of the silylformylation of alkynes was reported by the Matsuda and Ojima groups. The general reaction involves addition of both CO and tertiary hydrosilane to an alkyne to yield silyl alkenals, catalyzed by rhodium or rhodium-cobalt mixed metal clusters [Eq. (46)]. 

In an unconventional cyclopentenone synthesis, Negishi cyclised vinyllithiums derived from the 1-iodo- 1-silyl alkenes 103 onto preformed lithium carboxylate salts.57 He later found58 that amides 104 function in this reaction rather better than the carboxylate salts, and that the silyl substituent is not necessary for cyclisation. Nitriles, on the other hand, fail to cyclise. 

For cyclisation, simple vinyl iodides need to be Z, or polymerisation occurs. However, a silyl substituent geminal to the lithium atom promotes causes geometrical instability in the vinyllithium, and even E 1-iodo-l-silyl alkenes 159 and 160 cyclise successfully 76... 

Silylated alkenes, 191, are useful reagents to be used in tandem reactions of lithiated chain intermediates. Thus, it was reported that the reactions of the lithiated alkene 193,... 

Addition reactions of the Si-Si bonds across carbon-carbon triple bonds have been most extensively studied since the 1970s by means of palladium catalysts. In the early reports, palladium complexes bearing tertiary phosphine ligands, mostly PPh3, were exclusively employed as effective catalysts, enabling the alkyne bis-silylation with activated disilanes, i.e., disilanes with electronegative elements on the silicon atoms such as hydro [36], fluoro [37], chloro [38], and alkoxy-disilanes [39,40] and those with cyclic structure (Scheme 4) [41-44]. The bis-silylation reactions could be successfully applied to terminal alkynes and acetylenedicarboxylates to give (Z)-l,2-bis(silyl)alkenes, which are otherwise difficult to synthesize. 

Under extremely high pressure, the isonitrile-palladium catalyst promotes intramolecular bis-silylation of bis(silyl)acetylenes 51 to give tetrakis(silyl) alkenes 52, which are otherwise difficult to synthesize the reaction under atmospheric pressure hardly proceeds even at 200°C (Eq. 22) [48,49]. 

The results of irradiation ( i > 290 nm) of a series of aldehydes and ketones (91) in the presence of the silyl acetals (92) have been reported. The reactions are both solvent and silyl group dependent and the best results are obtained when the solvents used are /z-hexane, THF, diethyl ether or benzene and with the silyl group TBDMS. The products are the oxetanes (93) and the silyl-migrated product (94) in a ratio greater than 95 5 respectively. There is no evidence for the formation of the isomeric oxetane. Other studies from this research group" have examined the photochemical addition of a series of aryl aldehydes (95) to the cyclic silyl alkenes (96) brought about by irradiation at X,>290 nm in methylene chloride solution. The additions encountered take place with regio and exo selectivity as shown by the yields and ratios of the products (97). 

In fact, allyl alcohol 13 was not used at the start of this synthesis because low ees result if there is no substituent on the alkene trans to the alcohol (R2 in the mnemonic should be something other than H). It is much better here to add a removable group and so the silyl alkene 14 was used instead of 13. The silyl group allowed the isolation of the epoxy alcohol 15 in a much higher yield than was possible using allyl alcohol itself and the ee of 15 was >95%. The alcohol was converted to a mesylate 16. 

Preparation First representatives of alkenyliodonium salts, dichlorovinyl(phenyl)iodonium species, were reported by Thiele and Haakh in the early 1900s [436]. The first general synthetic approach to alkenyl(phenyl)iodonium salts was developed by Ochiai in the mid-1980s [437,438], This method is based on the reaction of silylated alkenes 299 with iodosylbenzene in the presence of Lewis acids, leading to the stereoselective formation of various alkenyliodonium tetrafluoroborates 300 in good yield (Scheme 2.85). 

Addition of the silicon-boron bond aCTOss carbon-carbon triple bonds (i.e., silabora-tion) is most effectively catalyzed by a paUadium(0)-t rt-alkyl isocyanide complex to give (Z)-l-boryl-2-silyl alkenes with high regio- and stereoselectivity, which are useful for synthesis of stereodefined alkenylsilanes. 

In this context, it is notable that while solutions of 1-pentene and Co2(CO)8 proved to be essentially unreactive when irradiated, solutions of Co2(CO)g, 1-pentene and HSiEt3 were active for pentene isomerization with small concentrations of silylalkenes also found as an end product [75]. However, a chain reaction appears unlikely for the hydrosilation pathway the apparent fate of the cobalt radicals is to form 18 e species such as HCo(CO)4 and Et3SiCo(CO)4 which themselves are photo or thermal catalysts for pentene isomerizations. The silyl alkene products appear to result from the formation of Et3SiCo(CO)3( 1-pentene) followed by insertion of pentene into the Si-Co bond, then P-hydride elimination. 

Dienes such as cyclohexadiene or cyclopentadiene react with trichlorosilane in the presence of 0.1 mol % of a palladium catalyst generated in situ by mixing 1 and a chiral phosphane (eq 96). The use of chiral MOP-phen (3-diphenylphosphino-3 -methoxy-4,4 -biphenanthryl) afforded the highest enantiomeric excess, as the silylated alkene was obtained in 99% yield and 80% ee. 

Silyl alkenes are used in the preparation of (Z)-heineicosane, a housefly... 

A proposed mechanism for this transformation includes the dissociation of a pyridine and an imine ligand. Subsequently, the diene is coordinated in an r) -manner. Oxidative addition of the silane gives rise to a hexacoordinated iron complex. The diene is inserted into the Fe-Si bond to form an n -allyl(hydrido)iron complex. Haptotropic migration affords the corresponding Ti -alkyliron complex that subsequently undergoes reductive elimination to release the silylated alkene (Scheme 4—339). ... 

SILYLAMINOSILYLENE, DISILANE, SILANIMINE, SILACYCLOHEXADIENONES, BIS(SILYL)-ALKENES, AND HYDROSILANIMINE... 

Reactions of silylenes with alkynes present an alternative approach for the silylation of alkynes without the aid of transition-metal catalysts (4). The bis-silylation reaction has been accomplished in a stereospecific manner via a 1,4-silyl migration with the easily available NHC-stabilized silylaminosilylene Ar SiMe3)N(Cl)Si- hPr) (Ar = 2,6-iPr2C6H3, hPr = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) under metal-free conditions (5), representing the first successful approach for the selective bis-silylation of alkynes with a donor-supported silylene as the silylation reagent. Furthermore, the alcoholysis of these bis-silylated alkenes gave trimethoxylsilyl-substituted alkenes. 

The bis-silylated alkene ArN=SiCl(hPr)CH=CR(SiMe3) (1.50 g, 2.3 mmol) was dissolved in methanol (20 mL) and the resulting mixture was stirred overnight. The volatiles were removed under vacuum and the residue was extracted with n-hexane (20 mL). It was filtered and the filtrate was concentrated to dryness to leave a red oil. The red oil was distilled imder vacuum (92°C, 0.2 mbar) to remove 2,6-iPr2C6H3NH2, affording the product as a red oil (yield 0.75 g, 92%). 

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