Allylsilanes isomerization

Conventional synthetic schemes to produce 1,6-disubstituted products, e.g. reaction of a - with d -synthons, are largely unsuccessful. An exception is the following reaction, which provides a useful alternative when Michael type additions fail, e. g., at angular or other tertiary carbon atoms. In such cases the addition of allylsilanes catalyzed by titanium tetrachloride, the Sakurai reaction, is most appropriate (A. Hosomi, 1977). Isomerization of the double bond with bis(benzonitrile-N)dichloropalladium gives the y-double bond in excellent yield. Subsequent ozonolysis provides a pathway to 1,4-dicarbonyl compounds. Thus 1,6-, 1,5- and 1,4-difunctional compounds are accessible by this reaction. 

The reactions of titanium-alkylidenes prepared from thioacetals with unsymmetrical olefins generally produce complex mixtures of olefins. This complexity arises, at least in part, from the concomitant formation of the two isomeric titanacyclobutane intermediates. However, the regiochemistry of the titanacyclobutane formation is controlled when an olefin bearing a specific substituent is employed. Reactions of titanocene-alkylidenes generated from thioacetals with trialkylallylsilanes 30 afford y-substituted allylsilanes 31, along with small amounts of homoallylsilanes 32 (Scheme 14.16) [28]. 

This selectivity is independent of the cisitrans geometry and the a- or y-isomerism of the starting allylsilanes. 

Allylic amide isomerization, 117 Allylic amine isomerization ab initio calculations, 110 catalytic cycle, 104 cobalt-catalyzed, 98 double-bond migration, 104 isotope-labeling experiments, 103 kinetics, 103 mechanism, 103 model system, 110 NMR study, 104 rhodium-catalyzed, 9, 98 Allylnickel halides, 170 Allylpalladium intermediates, 193 Allylsilane protodesilylation, 305 Aluminum, chiral catalysts, 216, 234, 310 Amide dimers, NMR spectra, 282, 284 Amines ... 

As reactions 68 and 69 above exemplify, the substitution of the allylsilane usually takes place with an allylic shift (Se2 ). This can be synthetically useful, for example in the isomerization of allyl sulphones 123 to vinyl sulphones 124 (equation 74)145. The reaction is also highly stereospecific (>90% E) (vide infra). 

Untypical conversion of allyltrisubstituted silanes has been proved to occur via preliminary isomerization of 1-propenyl-trisubstituted silanes, followed by heterocoupling with parent allylsilanes to finally yield the (E+Z) isomers of bis(silyl)propene and propene [50,58]. 

The cross-coupling of allylsilanes with alkenes [50] and styrene [58] also occurs via their preliminary isomerization followed by the reaction of 1-propenylsilane with exemplary olefin-l-decene resulting in l-(triethoxysilyl)-1-decene as a product (Eq. 31). If the cross-coupling takes place, an expected product of this reaction is l-silyl-2-undecene, which is not detected. 

Intramolecular allylsilylation of alkynes is a convenient route to cyclic vinylsilanes.211 Propargylsilanes are available for Lewis acid-promoted carbosilylation of alkynes.212 Unlike allylsilanes, both cr-substituted and unsubstituted propargylsilanes react at the a-position, which can be rationalized by in situ isomerization of these reagents to allenylsilanes (Equation (55)). 

Isomerization of methyl-substituted allylsilanes. Substrates of this type when heated with fluoride ion in THF rearrange to allyl silanes in which the silicon atom is bonded to the less-substituted carbon atom of the allylic group. 

The complex has enjoyed relatively little use in organic synthesis. For iridium-catalyzed homogeneous hydrogenation of alkenes, Crabtree s iridium complex ((1,5-Cycloocta-diene)(tricyclohexylphosphine)(pyridine)iridium(I) Hexafluoro-phosphate) is generally preferred, although this readily prepared Ir complex is active. It is more reactive than its rhodium counterpart in the catalytic isomerization of butenyl- to allylsilanes. ... 

Silver(I) compounds are often used as promoters for substitution reactions of aliphatic halides with carbon nucleophiles. A cyclic (8-bromo ether 29 can be reacted with allyltrimethylsilane (30) imder the influence of AgBp4, yielding a mixture of ally-lated products 31 and 32 (Sch. 7) [15]. Product 31 is formed by direct substitution of the bromine atom in ether 29 by an allyl group and isomeric ether 32 arises from the carboxonium ion which is generated by debromination and subsequent [l,2]-hydrogen shift. A synthesis of optically active 4-allylazetidinone 33 (Ft = phthalimido) has also been achieved by employing the silver-promoted substitution reaction of 4-chloro-azetidinone 34 with allylsilane 30 [16]... 

The Lewis acid-promoted reactions of acrylates and propiolates with allylsilanes usually afford [2-1-2] adducts as described in the next section [470-473]. The corresponding [3-1-2] adducts are obtained as minor products although there are a few exceptions. The ratio of the two kinds of cycloadduct depends on the reaction temperature [470] - the proportion of [3-1-2] adducts increases with increasing temperature. The product ratio from cycloaddition to alkylidenemalonates and their derivatives is markedly temperature-dependent (Scheme 10.170) [474, 475]. Cyclobutanes are major products at low temperature, and [3-1-2] cycloaddition proceeds predominantly at higher temperature. In addition, the [2-1-2] cycloadducts are smoothly isomerized to the [3-1-2] adducts in the presence of a Lewis acid. This behavior clearly shows that [3-1-2] cycloaddition is thermodynamically favored. 

Even when isomeric mixtures of the allylsilane are used, complete stereochemical control is observed for all the stereocenters established in the conjugate addition. The only breakdown in selectivity in the process occurs with the stereocenters adjacent to the carbonyl carbon, which result from protonation on work-up. The less stable cis-fused decalone is readily converted to the trans-fused isomer by treatment with base. Hence, with two steps, complete control over the relative configuration of four contiguous stereogenic centers is achieved. 

The reaction proceeds smoothly with styrene, -substituted styrenes, and /3-vinyl-naphthalene to give the corresponding ( )-vinylsilanes in 83-100% yield. However, isomerization of the double bond gives an allylsilane when a substrate having allylic proton(s) is employed ... 

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