Allylsilanes conformation

In most cases, open-chain allylsilanes react with electrophiles with anti stereoselectivity156-162. The simple explanation for this observation follows from the probable conformation of the allylsilane. The preferred conformation 129 will have the small substituent H eclipsing the double bond. 

The Z product is enantiomerically pure, but the E isomer is formed in a 90 10 ratio of enantiomers. One reason for this was revealed by considering the relevant conformations of the allylsilane, shown in equation 96. 

The increasing diastereoselectivity, as the steric requirements of R increase, can be rationalized by considering the conformation of the allylsilane. Again the smallest group (H) is positioned in the plane of the double bond (143), and the steric influence of R determines the ratio of attack on the two faces of the allylsilane. 

This outcome is rational since the (/ )-allylsilanes 74 and 75 are expected to exist in conformation 78 (Scheme 11), in which the C-Si bond overlaps with the Jt bond. This conformation maximizes the ct-tc interaction between the C-Si bond and the 7t system84 (see also later section on allylsilanes). Attack of the electrophile from the opposite face to the trimethylsilyl group in 78 gives the carbenium ion intermediate 79, which is stabilized by a interaction with the C-Si bond substituent12,37,38 loss of silicon from this intermediate gives the E olefin products. Anti selectivity has also been observed in the trifluoroacetolysis of 2-cyclohexenyl-silanes, germanes, and stannanes.85... 

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... 

Epoxidation of the allylsilane (14) is diastereospecific (equation 6). The favored confonnation of (14) is (14a) the peroxy acid approaches the double bond horn the face anti to the bulky silyl group. The epoxides (16) and (17) obtained from the acetonide (15 equation 7) can be readily separated in gram quantities using standard chromatographic techniques. The presence of the conformationally rigid acetonide moie in the epoxides (16) and (17) facilitates their separation the corresponding epoxy diols cannot be separated by chromatography. The racemic epoxide (19), an intermediate for the synthesis of maytansine has been synthesized from (18 equation 8)."... 

There thus exists a preference for anti (or antara) hydroxylation in these cyclohexenylstannanes, where electrophilic substitutions are known to proceed faitiifully with allylic rearrangement. A more likely padiway is shown in Scheme 4, which is supported by results with optically active allylsilanes, whidi require anti attack by MCPBA on the silane conformation maximizing C—Si tr nr interaction. 

E)- and (Z)-Allylsilanes with bulky alkyl groups are osmylated with the same sense of induced diastereoselectivity as observed for allylic alcohols and derivatives. In contrast to allylic alcohols, the preferred ground state conformation of these substrates with the allylic hydrogen atom in the plane of the double bond is also expected to be the most reactive one. Assuming electrophile approach from the opposite side to the silicon atom, the outside position is usually preferred by the alkyl groups3 32,38,39. 

The stereochemical outcome of the similar C-allylation of pentofuranose does not depend on the substituent at the C-4 position of the carbohydrate, but rather depends on the type of substituent at the C-3 position as shown in O Scheme 20. Woerpel explained the stereochemical outcome by stereoelectronically controlled attack of allylsilane to the lower energy envelope conformer as shown below [35]. The reaction of 7 gave a-C-products through the transition state shown for intermediate 9, while the reaction of 8 afforded /3-allyl product 12 through a different transition state 11, in which the methyl substituent occupied the equatorial position. 

The Lewis acid-mediated addition of electrophiles to allylsilanes has been extensively studied [10 c], In most cases the addition of an electrophile to an allylsilane proceeds via an anti Se process. In the ground state structure, simple allylsilanes are known to prefer the conformation wherein the allylic hydrogen eclipses the double bond. The electrophile can then approach the double bond from the same side as the allylmetal (syn Se) or from the side opposite the allyl-... 

The regiochemical and the stereochemical course of electrophilic additions to allylsilanes has been modeled computationally by Hehre [15]. In this study the conformational profile of 2-silylbut-3-ene was determined and three energy minima were observed (Chart 10-1). In the two most stable conformers the C-Si bond is perpendicular to the C-C double bond. Experimental evidence has been obtained (microwave [16a,b], electron diffraction [16c], infrared and Raman [16d]) which is in agreement with the computational results. 

Allylation of imines with allyltrimethylsilane usually requires a stoichiometric amount of a strong Lewis acid such as SnCl4, TiCLj, or BL, [413-419], Laschat et al. have reported that Snf h promotes the allylation of imines derived from aromatic aldehydes and galactopyranosylamine with low to high diastereoselectivity (Scheme 10.145) [413]. The preferred formation of the S-configured diastereomers can be rationalized by conformational fixation of the imines by SnCl4, which prohibits attack of the allylsilane from the Re side, that is, the front side of the imine. 

The titanium tetrachloride-promoted addition of allyltrimethylsilane to methylcyclohexenones and methylcycloheptenones has been explored (68,69) results are summarized in Eqs. [l]-[6] of Scheme 32. With the exception of 4-substituted cyclohexenones and cycloheptenones, good to excellent facial selection is observed (Eqs. [2], [5], and [6], Scheme 32). The major products obtained are those predicted by axial attack of the nucleophile on the more stable conformer of the acceptor. Although there are some similarities, the results show some significant deviations from the corresponding cuprate additions. Addition to the bicyclic enone 32.1 occurs from the convex face, producing exclusively the cis-fused decalone (Eq. [7], Scheme 32) (5). In this case, the product is formally the result of equatorial attack of the allylsilane on the a,/ -unsaturated ketone. 

The same pattern is seen, with only one lone pair, in tetrahydropyrans, when nucleophiles like allylsilanes and allylstannanes attack axially in Lewis acid catalysed reactions in pyranyl sugars.562 The same pattern is also found in imminium ions563 as Stevens demonstrated in several syntheses. He showed that nucleophiles attack from the direction that most easily sets up an anti and axial lone pair, especially if it can create a chair conformation, as in the reduction of the imminium ion 5.143 to give monomorine-I 5.144.564... 

For acyclic chiral alkenes the diastereofacial discrimination is often low due to their high conformational mobility. However, in certain allylsilane derivatives the diastereoselectivity of the methylenation is high for (Z)-olefins (Table 3), but is considerably lower for the corresponding ( )-olefins38,93. This effect can be explained by the allylic 1,3-strain model39. 

The regio- and stereochemistry of electrophilic additions to allylsilanes were explained in terms of attack of the electrophile at the terminal olefin position and anti to the silyl group, allowing better stabilization by the / -silyl substituent. The model was based on 6-31G //3-21G calculations for two conformations of CH2=CHCH(CH3)SiH3334. 

An alternative argument based on the allylsilane reacting in a conformation in which the hydrogen is the only group small enough to eclipse the double bond, as in 39, gives an... 

This selectivity reversal can be explained by the different orientation of the allylsilane chain in the two intermediates. In the case of the formation of the five-membered ring, the chain can adopt a position leading to an envelope conformation (A) or a twist conformation (D) (Scheme 14). It seems that for this intermediate the envelope cOTiformation is preferred as it leads to... 

We have found that 4-isobutenyl-dienone cyclizations can also produce cycloheptanes under fluoride ion catalysis provided that the C(4) position is substituted only with the allylsilane-containing side chain (Eq. 12). As mentioned earlier, C(3),C(4)-disubstituted cyclohexenones prefer a conformation with the C(4) substitutent orient axially. In this situation the absence of an equatorial C(4) substitutent favors a transoid con-... 

Let us look first at the allylsilane 46 that does not give good stereocontrol the conformation 51, which will lead to the E-isomer 49, must either be protonated syn to the silyl group or endo on the bicyclic system. Since neither of these pathways is likely to be favourable, it is not surprising that this diastereoisomer does not lead... 

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