Stereochemistry allylsilanes

Allylsilanes are available by treatment of allyl acetates and allyl carbonates with silyl cuprates17-18, with antarafacial stereochemistry being observed for displacement of tertiary allyl acetates19. This reaction provides a useful asymmetric synthesis of allylsilanes using esters and carbamates derived from optically active secondary alcohols antarafacial stereochemistry is observed for the esters, and suprafacial stereochemistry for the carbamates20,21. 

The intramolecular coupling of enolethers with enolethers, styrenes, alkyl-substituted olefins, allylsilanes, and vinylsilanes was systematically studied by Moeller [69]. Many of these coupling reactions turned out to be compatible with the smooth formation of quaternary carbon atoms (Eq. 11) [70], which were formed diastere-oselectively and led to fused bicyclic ring skeletons having a ds-stereochemistry [71]. The cyclization is compatible with acid-sensitive functional groups as the allylic alkoxy group. Moeller has demonstrated in some cases that these reactions can be run without loss of selectivity and yield in a simple beaker with either a carbon rod or reticulated carbon as anode without potential control and a 6-V lantern battery as power supply [71]. 

The use of silylallyl anion in organic synthesis has been extensive24,29. The regio-and stereochemistry of these reactions can be controlled. For example, alkylation of the anion generated from the corresponding allylsilane with an electrophile E+ takes place selectively at the /-position due to steric hindrance (equation 197)352. 

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... 

The effects of the cr—JT interaction on the ground-state properties of allyltrimethylmetal compounds are paralleled by the effect on reactivity towards electrophilic reagents. Mayr demonstrated that allyltrialkylsilanes, allyltrialkyl-germanes, and trialkylstannanes react with diphenylcarbenium ions at rates 105,5.6 x 105, and 109, respectively, relative to propene.158 The reaction rates were also found to be sensitive to the inductive effects of the other substituents attached to the metal. A theoretical evaluation of the factors determining the regiochemistry and stereochemistry of electrophilic addition to allylsilanes and other allyl systems is reported by Hehre et al.159 They predict a preference for electrophilic attack anti with respect to the silane substituent, a prediction that is supported by many experimental studies.82,160... 

This type of thermal 1,3-silyl migration is mechanistically very interesting, because two typical intramolecular 1,3-silyl migrations in allylsilanes 190 (equation 118) and /3-ketosilanes 191 (equation 119) were shown to occur concertedly but with different stereochemistry at the silicon center1. 

The BF3-catalyzed reaction of a-aminoaldehydes with 10 is valuable for highly stereoselective synthesis of 2,3,5-trisubstituted pyrrolidines with all -cis configurations (Equation (50)).197 The stereochemical outcome like chelation-controlled stereochemistry might result from the inherent conformational arrangement of the aldehyde-BF3 complex. />-Quinoneimines, o-quinones, and a-alkoxyhydroperoxides undergo similar types of [3 + 2]-cycloadditions with allylsilanes to afford dihydro indoles, dihydrobenzofurans, and 1,2-dioxolanes, respectively.164,175,198... 

The kind of a fluoride ion activator and the leaving group in electrophiles affects the stereochemistry in the cross-coupling reaction of allylsilanes as exemplified with 2-cyclohexenyl(difluoro)phenylsilane (Eq. 31) 135]. 

A chiral carbon present in the alcohol portion of acetals might control the stereochemistry of the allylation. A few examples conducted in the presence of a titanium Lewis acid are shown in Eqs (82) [225], (83) [226], and (84) [227]. In contrast, an allylsilane with a chiral auxiliary derived from arabinose on the silicon atom has been used for asymmetric synthesis, although diastereoselectivity was low [228],... 

Analogous to the allylation with allylsilanes and -stannanes, the transformations, vinylallylation, propargylation, allenylation, alkenylation, alkynylation, and arylation, are viable by the use of an appropriate reagent in the presence of a titanium Lewis acid these are surveyed in the review articles cited both in the Introduction and in this section. The stereochemistry of the reaction of a (vinylallyl)silane in the presence of TiCU has been reported [234]. Equation (113) shows that the major reaction of this silane and isobutyraldehyde occurred mainly in the anti sense with a ratio of anti to syn attack of 90 10 at the terminus remote from the silyl group. Essentially the same stereochemical outcome was observed for the same reaction with the corresponding trimethylsilyl derivative. The intramolecular reaction with an acetal, however, proceeded less selectively the anti syn ratio was 60 40 (Eq. 114) [234]. 

Allylsilanes are easily protodesilylated, completing the reduction process. In the presence of Lewis acids, allylsilanes also react with electrophiles like aldehydes, ketones and acid halides through an Se2 mechanism involving anti stereochemistry. These reactions are extensively discussed in Volumes 2-4. 

Stereochemistry of addition reactions involving allylsilanes 23.4.12 Intermolecular additions to aldehydes, ketones and acetals... 

Stereochemistry of internal allylsilane additions to aideh es and acetals... 

A variety of aldehydes can function as heteroallenophiles in this [3 + 2] annelation (Figure 13 and Table 9). Reactions of the C(3)-substituted allenylsilanes (66) gave predominantly the ds-substituted dihydrofurans (71). The cis stereochemistry was anticipated, based on the well-documented stereochemical course of Lewis acid catalyzed additions of 3-substituted allylsilanes to aldehydes. ... 

Table 11 summarizes the important results of asymmetric induction in addition reactions of optically active allylsilanes (90) and (92). The origin of the ir-facial selectivity in these reactions can be traced to the anti selectivity that is commonly observed for 5e2 reactions of lylsilanes (Figure 14). The syn stereochemistry with respect to the double bond configuration (E or Z) is also very high even with aldehydes bearing sterically bulky substituent gro q)s, as illustrated by entry (/ )-(Z)-(92b). 

Analogous systems related to the models for allylsilane-acetal cyclizations have been studied. In these cases, however, cyclizadon of the acetals (105 Scheme SO) fonts the corresponding syn and anti bicyclic ethers (106). The audiors concluded diat the stereochemistry of cyclizadon of acetal models (105) is dependent upon the mechanism of activation. In the presence of TMS-OTf the acetals (105 -c) reacted through an SN2-type process, while the isopropyl acetal (105d) reacted through prior ionization to oxonium ion (107). 

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