Aldehydes allylsilanes, stereochemistry

For intramolecular aldehyde/allylsilane reactions (1- 2, M = SiR3) see Yamamoto, Y. Sasaki, N. The Stereochemistry of the Sakurai Reaction in Chemical Bonds - Better Ways to Make Them I. Bernal, Ed. Elsevier Science Publ., 1989 pp 363-442. 

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... 

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... 

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). 

J.4.I.4 Stereochemistry of internal allylsilane additions to aldehydes and acetals... 

In general, the Lewis acid-promoted allylation and propargylation of aldehydes wifh y-substituted allylsilanes [357] and aUenylsilanes [358] are syn-selective. Information about the preferred orientation of fhe reactive double bonds in fhe transition structure is essential for rationahzing fhe stereochemistry. Denmark et al. examined the Lewis acid-promoted cychzation of fhe formyl-substituted allylsilane 119 to clarify fhe transition structure (Scheme 10.124) [359]. It was found that fhe allylation occurs from the synclinal and antiperiplanar arrangements, and the former is somewhat important. Thus, synchnal and antiperiplanar transition state models have been frequently used to explain fhe origin of the simple diastereoselection [14, 334]. 

Stereoselective reactions of crotylsilicates (97)-(100) and aldehydes have been described (Scheme 17). These reactions readily proceed at room temperature in the absence of a Lewis acid catalyst, and type I diastereoselection is clearly evident. Evidence supporting a cyclic transition state has been provided through studies of optically active crotylsilicates such as (101 Scheme 18). The absolute stereochemistry of the products requires that the reaction is suprafacial with respect to the allylsilane moiety, in contrast to the anti stereochemical outcome of 5e reactions of allylsilanes. Cyclic transition states are also implicated in the reactions of the crotyltrifluorosilanes and CsF, but the crotyltrifluorosilane/BuiNF reaction apparently proceeds via an uncomplexed allyl anion species. ... 

Note that the aldehyde approaches the alkene from the direction anti to the silicon atom. Therefore, when a chiral allylsilane or allylstannane with a substituent in the a-position is used, chirality transfer takes place, to generate the homoallylic alcohol with essentially no loss in enantiomeric purity. For example, reaction of the aldehyde 157 with the chiral allylsilane 158, using boron trifluoride etherate as the catalyst, gave predominantly the syn product 159 (1.151). The absolute stereochemistry can be determined by using a model in which the hydrogen atom on the a-carbon of the allylsilane eclipses the alkene (the so-called inside hydrogen effect ) in order to minimize steric interactions (1.152). 

The acid-promoted Prins reaction between a homoaUyhc alcohol and an aldehyde is a weU-estabhshed synthesis of tetrahydropyrans (Scheme 4) [ 14,15]. While substituted tetrahydropyrans are often assembled by cyclizations forming a C - O bond, the Prins reaction undergoes cyclization by C - C bond formation. The Prins reaction of the silyl-modified substrates [16], which can be regarded as the intramolecular Hosomi-Sakurai reaction, is effectively activated by the allylsilane unit. The stereochemistry of the 2,6-cfs-form produced in the case of the exo-allylsilane unit is elucidated by the 6-membered transition state model, hi the cyclization of the ewdo-aUylsilane substrates, since the silyl group would be fixed on the axial position of the 6-membered transition states, the tetrahydropyrans with both 2,6-cis and fraws-forms can be synthesized (Panek Sect. 3.3.9). This type of cychzation was also... 

Panek s synthesis is highUghted by the efficient construction of a cis-and trans-2,6-disubstituted tetrahydropyran ring. As described in Sect. 2.4, Panek s annulation between aldehydes and chiral allylsilanes can be regarded as a Hosomi-Sakurai-Prins reaction, in which the stereochemistry of cis-... 

Removable Silicon Tethers. Cox and coworkers have explored the use of removable silicon tethers as a method of controlling the relative stereochemistry of the addition of allylsilane to aldehydes.This strategy relies on the construction of un-symmetrical bis(silyl)propenes with a pendant aldehyde (eq 13). When the dimethylsilyl ether was used as the tether only the corresponding diene was observed, but by increasing the steric bulk of the silane to the diethylsilyl ether the construction of the oxasilacycle was possible. Use of the diisopropyl tether completely suppressed formation of the diene by-product however, the 1,3-stereoinduction was eroded as four diastereomers were observed instead of two. Tamao-Fleming oxidation of the two diastereomers (2.7 1-9.7 1 dr) derived from the diethylsilyl ether tether yielded the stereodefined 1,2,4-triol in the case of the major... 

This book chapter is limited to Lewis acid-mediated reactions, and does not discuss the important field of Lewis base-mediated allylations, nor does it describe the reactions of allylsilanes with other electrophiles such as epoxides, imines, and allyl-X (X = -Cl, -OR, -OAc). The SaJcurai reaction has been covered under different forms in reviews focusing on The Stereochemistry of the Sakurai reaction , Intramolecular Addition Reactions of Allylic and Propargylic Silanes ," Selective Reactions Using Allylic Metals , Catalytic Enantioselective Addition of Allylic Organometallic Reagents to Aldehydes and Ketones , and Modem Carbonyl Chemistry . ... 

The most popular promoters are BF3 OEt2, TiCU, and SnCU. They are followed, arguably, by AICI3, AlC Et, and TMSOTf. Protic acids are used more rarely since they tend to cause an unwanted protodesilylation. BF3, TiCU, and SnCL have somewhat different properties. BF3 does not react with allylsilanes directly, and has only one coordination site. TiCU is also inert towards allylsilanes, even at room temperature, and has two coordination sites. TiCU forms 1 1 or 1 2 adducts with aldehydes, depending on the stoichiometry of the reagents. It can also form chelates with a- and P-alkoxyaldehydes, which influences the stereochemistry of the reaction (chelation control). TiCU is best used at low temperatures (-78 °C) to prevent the formation of unwanted chlorination products that appear at room temperature. The major characteristic of SnCU is that, in the presence of an allylsilane, it undergoes a transmetallation within minutes at -80 °C. Hence,... 

Allylborane reagents that contain a silyl- or silylmethyl substituent at ar-position react with aldehydes highly diastereoselectively to give alkenylsilanes and allylsilanes, respectively, with defined stereochemistry (Schemes 3-58 and 3 59).[ ° 2 ] To explain the stereochemical outcome, staggered acyclic transition states are suggested. 

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