Silyl allene

Several stoichiometric methods for transition metal-promoted transformations of allenes have been studied, involving metals such as iron [77] and titanium [78, 79]. The titanium-mediated reactions developed by Sato and co-workers have probably the greatest synthetic impact, as exemplified by the conversion of silylated allene 150 to 1,4-diene 152 (Scheme 14.38) [78],... 

A chiral bis-oxazoline catalyses asymmetric Nozaki-Hiyama allenylation of aldehydes.179 For example, benzaldehyde is converted to silylated allene (62) using a bromoalkynylsilane, BrCH2-C = C-SiR R2 the product is readily desilylated quantitatively without loss of ee. 

Silyl allenes such as 48 and enol acetates such as 50 were particularly effective substrates for these reactions. The oxidative cascade reaction of trisubstituted alkene substrate 52 demonstrated a powerful application of ETIC chemistry to form bicycle 53 through consecutive carbon-carbon and carbon-oxygen bond construction. Highly diastereoselective reactions were observed when the nucleophile was... 

Thiopropenoylsilanes 42, readily accessible from silylated allenes, self-dimerise in a head-to-head fashion to give silylated 1,2-dithiins sometimes the corresponding 1,3-dithiin is also formed <03TL2831>. 

The 3,6-disilylated-3-vinyl-l,2-dithiins 38, obtained by the self-dimerisation of silylated allenes, undergo a Lewis aeid-promoted rearrangement to the bicyclic endodisulfide <05TL4711>. Tethered bilayer lipid membranes have been obtained using 4-hydroxy-l,2-dithianes as the anchor for coupling reactions with the lipid <05AJC738>. 

Me2SiCl give exclusively Me3SiCH2C=C—NEt2, a 2 1 mixture of the latter compound and H2C=C=C(SiMe3)NEt2 is produced in the silylation of the potassium compound. In the case of the lithiated c//rae//iy/amino allene equal amounts of the silylated allene and acetylene are obtained [9]. 

Allenyl ketones presynthesised, or generated in situ by acylation of silyl-allenes, or by palladium(0)-catalysed isomerisation of conjugated or non-conjugated alkynyl ketones, can be cyclised to furans. The ring closure has been effected with silver or palladium catalysis acylation of silylallenes leads to the furan directly. In the presence of water, palladium(II)-catalysed closure of / ,7-alkynyl ketones is believed to proceed via the enol, not the allene. l,2,3-Trienyl-4-ols have also been shown to cyclise to give furans. ... 

The use of other electrophiles in the Danheiser annulation for the formation of cyclopentenes is relatively unexplored. Ynones have been reported to imdergo cycloaddition, although examples are limited. Thus treatment of butynone 64 with silyl allene 47 under the usual conditions delivered cyclopentadiene 65 in 53% yield. Danheiser has reported that nitroalkenes do not provide annulation products furnishing instead acyclic alkynes. For example, treatment of nitroalkene 66 with silylallene 34 under standard conditions gives alkyne 67 rather than the cyclopentene 68. afi-Unsaturated aldehydes have been reported to be problematic, failing to provide isolable cyclopentene products. Treatment of methacrolein (69) with silylallene 34 under the standard conditions produced only a complex mixture of unidentified products. 

To promote Peterson olefination, the use of KH results in the formation of o-isotoluene derivatives [2d, 7], On the other hand, treatment of allenic alcohols with concentrated sulfuric acid produces the diene-allenes. The preference for Z geometry of the methyl-substituted a C=C in silylated allenic alcohol is attributed to the allylic methyl group at the C-1 position, favoring the axial position in the chair-like transition state, thus avoiding the severe steric interaction between the equatorial allylic methyl and the equatorial rigid bicyclic ligand on the boron. 

Handling, Storage, and Precautions 1-methyl-l-(trimethyl-silyl) allene is stable indefinitely when stored under nitrogen in the refrigerator. 

Trimethylsilyl)aUenes are unsuitable for this [3 + 2] annulation, as the intermediate carbocations undergo chloride-initiated desilylation to produce alkynic byproducts. This unwelcome reaction pathway is suppressed when the bulkier (/-butyldimethyl-silyl)allenes are employed. 

Related Reagents. Allenyllithium 1-Methyl-l-(trimethyl-silyl)allene Allyltrimethylsilane Propargylmagnesium Bromide 3-Tiimethylsilyl- 1-propyne. 

Despite reports to the contrary early in 1980/ Danheiser and Carini have demonstrated that trimethylsilylallenes function as propargylic anion equivalents in reactions with ketones or aldehydes in the presence of titanium tetrachloride, regiospecifically forming homopropargylic alcohols. Unexpectedly, some silyl-allenes give mixtures of homopropargylic alcohols and trimethylsilylvinyl chlorides, but exposure of the crude reaction product to potassium fluoride in DMSO induces elimination from the latter to furnish exclusively the required alcohols ie.g. Scheme 113). 

Acylsilanes are yet another class of valuable synthetic intermediates, and new methods for their synthesis are always welcome. In the first of several examples reported this year, the silyl allene (168) has been used as a common intermediate for the preparation of a diverse range of acylsilanes with a,p-olefinic, a,p-acetylenic, and a-keto substituents. Alkyl-substituted acylsilanes are now available via a high-yielding process in which an acyl-lithium (generated in situ from an alkyl-lithium and CO) is trapped by MeaSiCl. Cyclic acylsilanes have been prepared from cyclic vinylsilanes by way of an epoxidation-reductive ringopening-oxidation sequence. ... 

From the chiral epoxide 56, the TMM cycloaddition precursor 55 for construction of the tetraquinane stmcture was prepared in a straightforward manner. Oxidation of epoxy alcohol 56 by the Swem protocol followed by treatment with Bestmann-Ohira reagent (E) produced alkyne 70. Iron-catalyzed Sisr2 -type reaction of 70 afforded an allene moiety as a diastereo-meric mixture in a 1 1 ratio and this mixture was used in the next step without the separation of isomers. Allenyl alcohol 71 was protected as a TBDPS ether to give 0-silylated allene 72. Deprotection of the acetal of 72 was delicate as the allenyl moiety was not stable under acidic condition. Fortunately, treatment of 72 with p-toluenesulfonic acid monohydrate in presence of formaldehyde afforded aldehyde 73, and subsequent treatment with p-toluene sulfonyUiyrazide furnished the precursor 55 for the tandem cycloaddition reaction (Scheme 21). 

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