Allylsilane products

The cyclohexyloxy(dimethyl)silyl unit in 8 serves as a hydroxy surrogate and is converted into an alcohol via the Tamao oxidation after the allylboration reaction. The allylsilane products of asymmetric allylboration reactions of the dimethylphenylsilyl reagent 7 are readily converted into optically active 2-butene-l, 4-diols via epoxidation with dimethyl dioxirane followed by acid-catalyzed Peterson elimination of the intermediate epoxysilane. Although several chiral (Z)-y-alkoxyallylboron reagents were described in Section 1.3.3.3.3.1.4., relatively few applications in double asymmetric reactions with chiral aldehydes have been reported. One notable example involves the matched double asymmetric reaction of the diisopinocampheyl [(Z)-methoxy-2-propenyl]boron reagent with a chiral x/ -dialkoxyaldehyde87. 

Disilane trapping reagents behave in a conceptually similar fashion. Tsuji and coworkers found that a variety of simple dienes (143, e.g., butadiene, isoprene, 2-phenyl-l,3-butadiene, 2-trimethylsilyloxy-l,3-butadiene) undergo efficient Pd(dba)2-catalyzed linear dimerization disilane trapping with a variety of simple disilanes 144 to afford bis(allylsi-lane) products 145 (41-92% yield) (Scheme 47). Cyclic disilanes afford macrocyclic bis(allylsilane) products. ... 

The ( -y-(silyl) allyl boronates 47 and 48 were subsequently introduced in order to access more highly oxygenated aldehyde addition products. For example, the allylic silane addition products can undergo subsequent oxidation to provide 1,2- and 1,4-diol products. The allylsilane products are also allylmetal reagents Roush and co-workers have demonstrated their ability to undergo addition reactions with Lewis acid activated aldehydes to form tetrahydrofuran products. ... 

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 reaction between a-alkoxyaldehydes and allylsilanes is highly stereoselective in favor of chelation-controlled products if tin(IV) chloride is used as the Lewis acid, whereas boron trifluoride gives modest stereoselectivity in favor of the nonchelation-controlled product58. 

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. 

Almost 15 years ago Sakurai and Hosomi, in pioneering work, showed that intermolecular addition of an allylsilane to a,j6-unsaturated ketones in the presence of titanium(IV) chloride as the Lewis acid gave the desired 1,4-addition products1 4. In the case of 4,4a,5,6,7,8-hexahy-dro-2(3//)-naphthalenone, reaction was shown to proceed by 1,4-addition with exclusive production of the ris-fused product in high chemical yield. 

In the Sakurai reaction, the product-determining step should be the nucleophilic addition of the allylsilane to the Lewis acid coordinated enone28. 

In contrast, the fluoride ion induced cyclization proceeds via an anticlinal approach which minimizes steric congestion between the allylsilane and the substituent in the 3-position of the cyclohexenone50. Thus, the product having a cis relationship between the vinyl group and R2 at the ring junction is formed preferentially 48 " 51. The controlling element is the substituent R2. In its absence (R2 — H) the diastereoselectivity disappears (see Table 1). 

Allylsilanes, being homologues of vinylsilanes, undergo a similar regio-controlled attack (I) by electrophiles, this time at the /-position, with resulting loss of the silyl group providing products of substitution with a net shift of the double bond ... 

In general, simple deprotonated allylsilanes react in a y-regioselective manner with electrophiles, vinylsilanes being obtained as products. 

In the case of alkylation using allylsilancs in the presence of aluminum chloride as a catalyst, allylsilanes containing one or more chlorine substituents on the silicon react with aromatic compounds at room temperature or below 0 C to give alkylated products. 2-aryl-1 -silylpropanes.- while allyltrimethylsilane did not give the alkylated product but instead dimerized to give the allylsilylation product.. S-itrimethylsilyli-d-itrimethylsilylrnethyl)- 1-pentene (Eq. (1 )). In the alkylation reaction, the reactivity of allylsilanes increased as the number of chlorine... 

The substituent effect of vinylsilanes is similar to that of allylsilanes. The reactivity of vinylsilanes increased as the number of chlorine atoms on the silicon increased, but decreased as the number of methyl groups increased. However, vinyltrimethylsilane does not react with benzene to give alkylated products. " In the aluminum chloride-catalyzed alkylation of arenes with allylsilanes or vinylsilanes, one or more chlorine substituents on the silicon atom of silanes are required. 

Nametkin and co-workers hrst reported the alkylation of benzene derivatives with allylchlorosilanes in the presence of aluminum chloride as catalyst. " 2-(Aryl)propylsilanes were obtained from the alkylation of substituted benzenes (Ph—X X = H, CL Br) with allylsilanes such as allyldichlorosilane and allyltrichlo-rosilane.The yields ranged from 34 to 66% depending upon the substituents on the benzene ring, but information concerning reaction rates and product isomer distribution was not reported. 

A three-component domino process consisting of an ene reaction followed by the addition of an allylsilane to afford polysubstituted tetrahydropyrans in generally good yield was described by Marko and coworkers [115]. However, the nature of the products formed in this process depends heavily on the Lewis acid employed as a catalyst. Thus, reaction of the allylsilane 4-333 with an aldehyde in the presence of BF3-Et20 led to the domino products 4-334, whereas in the presence ofTiCL the diol 4-332, and in the presence of Et2AlCl the alcohol 4-335, were obtained (Scheme 4.74). The latter compound can then be transformed in stepwise manner to 4-334, using the same aldehyde as previously. However, it is also possible to use another aldehyde to prepare tetrahydropyrans of type 4-336. 

No unusual products were formed with allylsilanes. Under similar conditions, allylsilanes made only 1,3-disubstituted propanes, and pro-penylsilanes were recovered if excess allylsilane was used. 

The mechanism of this new reaction is shown in Scheme 14. Coordination of the diene to palladium(II) makes the diene double bond electrophilic enough to be attacked by the allylsilane. The attack by the allylsilane takes place on the face of the diene opposite to that of the palladium (anti). This is the first example of an anti attack by an allylsilane on a 7T-(olefin)metal complex. Benzoquinone (BQ)-induced anti attack by chloride ion produces the product 58. 

As expected, there was no formation of stilbenes or a dinitrile product and, more surprisingly, in all of the reactions reported only 5-7% of the allyltrimeth-ylsilane self-metathesis product was observed. It was proposed that this lack of allylsilane self-metathesis was due to the steric bulk of the TMS group reducing the reactivity of the Me3SiCH2 substituted alkylidene. In a more recent report by Blechert and co-workers it was noted that allyltrimethylsilane and its hydrocarbon equivalent (4,4-dimethylpent-l-ene) had comparable reactivities in the cross-metathesis reaction [28], further suggesting that the selectivity arises from steric rather than electronic effects. 

As with the allylsilane cross-metathesis reactions, significant quantities of allyl stannane self-metathesis were not detected in any of the reactions and the trans isomer predominated in the cross-metathesis products. Identical reactions were carried out using allyltributyl stannane, in place of allyltriphenyl stannane, but the yields of the cross-metathesis products were consistently lower and in many cases dropped below 25%. 

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],... 

Schafer reported that the electrochemical oxidation of silyl enol ethers results in the homo-coupling products. 1,4-diketones (Scheme 25) [59], A mechanism involving the dimerization of initially formed cation radical species seems to be reasonable. Another possible mechanism involves the decomposition of the cation radical by Si-O bond cleavage to give the radical species which dimerizes to form the 1,4-diketone. In the case of the anodic oxidation of allylsilanes and benzylsilanes, the radical intermediate is immediately oxidized to give the cationic species, because oxidation potentials of allyl radicals and benzyl radicals are relatively low. But in the case of a-oxoalkyl radicals, the oxidation to the cationic species seems to be retarded. Presumably, the oxidation potential of such radicals becomes more positive because of the electron-withdrawing effect of the carbonyl group. Therefore, the dimerization seems to take place preferentially. 

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