E-allylsilanes

As shown in Scheme 78, with or without zinc chloride, benzylic Grignard reagents couple with vinyl bromide to form allylbenzene derivatives in high ee (188). This method has been used to prepare optically active allylsilanes in up to 95% ee. Vinylic bromides with E configurations lead to the E allylsilanes with high ee, and Z bromides lead to Z allylsilanes with lower ee. 

E)-Allylsilanes These silanes can be prepared regio- and stereoselectively by hydroalumination of (chloromethyl)dimethylsilyl-l-alkynes (1) to give the cis-adduct 2. Reaction of CH3Li (3 equiv.) with 2 results in (E)-allyltrimethylsilanes (3). 

The chiral allylsilanes (3b) were used to determine the stereochemistry of the electrophilic substitution reactions (SE1). Typical results are shown in equations (II) it rid (III). The (Z)-allylsilanes give products of (R)-configuration and the (E)-allylsilanes give the (S)-isomers. In each case, the electrophile attacks the double bond anti to the leaving group.6... 

Treatment of the (.V)-( A )-[(.svr-allylo y )dimcsitylsilyl]stannanc with -BuLi provides a (R)-(.E)-allylsilane with high enantioselectivity, in which 1,3-chirality transfer occurs during the [2,3]-sila-Wittig rearrangement (Equation (118)).290... 

Rearrangement of 1-alkenylsilanesf Alkenyl(chloromethyl)dimethylsilanes, readily available from hydrosilylation of alkynes with (chloromethyl)dimethylsilane, rearrange in the presence of aluminum chloride to allyl- and/or cyclopropylsilanes. (Z)-l-Alken-ylsilanes rearrange exclusively to (Z)-allylsilanes (equation I). (E)-l-Alkenylsilanes rearrange to (E)-allylsilanes as the major product (equation II). Cyclopropylsilanes are formed... 

Allylsilanes, monosubstituted at C-3, react with aldehydes to give more of the syn (= erythro) product than the anti (Scheme 11) the (E)-allylsilane (21) is very clean in this respect, especially with the more heavily substituted aldehydes, but the (Z)-allylsilane (22) is not very selective.31 Antiperiplanar transition... 

D.i.e. Allylsilanes, All of the examples discussed so far have relied on the use of an endocyclic alkene substrate to resolve the /3- versus /3 -hydride elimination regiocontrol problem discussed in Sect. B.i. A more general approach to the problem has been described by Tietze and co-workers and involves the nse of allylsilanes as the alkene component (Scheme ll),W5],[46]... 

Diastereoselective Synthesis of (Z)- and (E)-Allylsilanes. DBBS has been utilized in the diastereoselective synthesis of (Z)- and ( )-allylsilanes following a hydroboration-substitution-displacement strategy starting from 1-alkynes (eq 17). ... 

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. 

Chelation control does not operate in the addition of 2-butenyl Grignard reagents to a-oxyalkanals, since with (racemic) 2-benzyloxy-, 2-(benzyloxymethoxy)- and 2-(t< rt-butyldi-methylsilyoxy)propanal similar ratios of isomers are formed28. Several cations were investigated, but the best choices, e.g., allylboronates or allylsilanes, were not included in this study. 

Optically active (Z)-l-substituted-2-alkenylsilanes are also available by asymmetric cross coupling, and similarly react with aldehydes in the presence of titanium(IV) chloride by an SE process in which the electrophile attacks the allylsilane double bond unit with respect to the leaving silyl group to form ( )-s)vr-products. However the enantiomeric excesses of these (Z)-allylsilanes tend to be lower than those of their ( )-isomers, and their reactions with aldehydes tend to be less stereoselective with more of the (E)-anti products being obtained74. 

When the chain bearing the allylsilane is one carbon longer, i.e., by the use of pentenylsilanes, cycloheptane rings can be formed. Both the (Z)- and (A )-isoiner of the (3-pentenyl)silanes can be synthesized selectively, but only the (Z)-(3-pentenyl)silane cyclized stereospecifically (complete 1,4-asymmetric induction). Both cthylaluminum dichloride and boron trifluoride diethyl ether complex promote the seven-membered ring formation35 43-48. 

Intermolecular allylation of aldehydes with 1 -trialkylsilyl-1,3-dienes 22 in the presence of a stoichiometric amount of triethylsilane and a catalytic amount of Ni(cod)2 and PPI13 shows novel regio- and stereoselectivity (Scheme 6) [20-22], When a toluene solution of a 1-silyl-1,3-diene and an aldehyde is refluxed in the presence of trialkylsilane under the catalysis of Ni(cod)2 and PPh3, ( )-allylsilane (E)-23 is obtained exclusively. On the other hand, when the reaction is carried out in THF upon heating at 50 °C as... 

The assembly of the benzazepane framework 2-338 (e. g., found in cephalotaxine [187]) starting from 2-336 has been achieved by the group of Schinzer [188] utilizing a combination of a Beckmann rearrangement to give the iminiumion 2-337 and an allylsilane cydization (Scheme 2.79). 

An interesting application of the cydization of alkenyl thioacetals is the stereoselective preparation of olefmic diols. Thus, oxidative cleavage of the silicon—carbon bond [32] in the ring-closed metathesis products, i.e. cyclic allylsilanes such as 35 and 36, affords (Z)-alk-2-ene-1,5-diols 37 and 38 (Scheme 14.18) [33],... 

Only recently a selective crossed metathesis between terminal alkenes and terminal alkynes has been described using the same catalyst.6 Allyltrimethylsilane proved to be a suitable alkene component for this reaction. Therefore, the concept of immobilizing terminal olefins onto polymer-supported allylsilane was extended to the binding of terminal alkynes. A series of structurally diverse terminal alkynes was reacted with 1 in the presence of catalytic amounts of Ru.7 The resulting polymer-bound dienes 3 are subject to protodesilylation (1.5% TFA) via a conjugate mechanism resulting in the formation of products of type 6 (Table 13.3). Mixtures of E- and Z-isomers (E/Z = 8 1 -1 1) are formed. The identity of the dominating E-isomer was established by NOE analysis. 

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