Silyl ether geometry

It seems likely that the reaction proceeds through a prototropic ene reaction pathway, a pathway that has not been previously recognized as a possible mechanism in the Mukaiyama aldol condensation. Usually an acyclic antiperiplanar transition-state model has been used to explain the formation of the syn diastereomer from either ( )- or (Z)-silyl enol ethers [91]. The cyclic ene mechanism, however, now provides another rationale for the syn diastereoselectivity irrespective of enol silyl ether geometry (Sch. 32). 

A high degree of syn selectivity can be obtained from the addition of enamines to nitroalkenes. In this case, the syn selectivity is largely independent of the geometry of the acceptor, as well as the donor, double bond. Next in terms of selectivity, are the addition of enolates. However, whether one obtains syn or anti selectivity is dependent on both the geometry of the acceptor and the enolate double bond, whereas anti selectivity of a modest and unreliable level is obtained by reaction of enol silyl ethers with nitroalkenes under Lewis acid catalysis. 

Further evidence for the above-mentioned mechanism of HOMO elevation by group 14 elements is provided by studies of thioethers. The decrease in oxidation potential of silyl ethers as compared to ethers is not realized in the case of a-silylthioethers whereas a-stannyl substituents in thioethers cause a considerable cathodic shift in oxidation potential. Moreover, the effect is geometry-dependent. Values for substituted cyclic dithianes 15 are summarized in Table 21. The difference between Si and Sn in this case is illustrative. The lone nonbonding pair in the 3p orbital of sulfur is much too low in energy compared to... 

The transition state is thought to be an open structure. Assuming that a particular silyl enol ether geometry is used, the substituents will tend to occupy opposite faces of the transition state and thus give a particular diastereomer (syn-anti) preferentially. Because of the open transition state geometry, the diastereoselec-tivity is not high. 

Asymmetric aldol reactions.4 The borane complex 3 can also serve as the Lewis acid catalyst for the aldol reaction of enol silyl ethers with aldehydes (Mukaiyama reactions).5 Asymmetric induction is modest (80-85% ee) in reactions of enol ethers of methyl ketones, but can be as high as 96% ee in reactions of enol ethers of ethyl ketones. Moreover, the reaction is syn-selective, regardless of the geometry of the enol. However, the asymmetric induction is solvent-dependent, being higher in nitroethane than in dichloromethane. 

The kinetics of the desilylation reactions of a range of sulfonylated and meth-oxylated norbomyl silyl ethers establish a correlation between the geometry of the 6-bond s-relay and the rate of desilylation. These desilylation rates generally decrease in the order W > sickle-like > U as shown in the table accompanying Scheme 1.55."... 

Diastereoselecttve aldol reactions. The diastereoselectivity in the Lewis acid-catalyzed aldol reaction of chiral oi-hydroxy aldehydes is independent of the geometry of the enol silyl ether. Also, the reaction does not involve prior Si-Ti or Si-Sn exchange. 

Asymmetric Intramolecular Hydrosilation. Intramolecular hydrosilation of allylic alcohols followed by oxidation is a convenient method for the stereoselective preparation of 1,3-diols. An enantioselective version is achieved by use of diene-free BINAP-Rh+ (eq 6). Both silyl ethers derived from cinnamyl alcohol and its cis isomer give (iJ)-l-phenylpropane-l,3-diol in high ee regardless of alkene geometry. 

Additional insight into the competition between the various cyclic transition states is provided by a recent study of the reactions of crotylboronates (22) and (23) with the two isomers of oxime silyl ether (24 Scheme 8). The stereoselectivities of these reactions were found to be independent of the geometry of (24), both isomers of which were shown to be conflgurationally stable under the reaction conditions. Since the oxime stereochemistry defines the site of coordination to the boron atom, it seems likely that the (Z)-oxime isomer reacts preferentially through the chair-like transition state (28), while the ( )-oxime reactions proceed preferentially via boat-like transition state (27). Evidently, the chair-like arrangement... 

Additions of enol silanes to p-alkoxy aldehyde (85 equation 25) are reported in Table 17. High selectivity (chelation control) was obtained with TiCU via complex (78 entries 1, 2). The same preference for isomers (86) and (87) was obtained with BF3 via complex (80), which simulates chelation. The influence of chelation on simple stereoselection is also evident in the reactions of achiral aldehydes (90) and (92) with silyl enol ethers (Z)-(91) and ( )-(93), which are usually moderately anti selective in their reactions with aldehydes incapable of chelation high syn selectivity was obtained irrespective of the enol ether geometry (equations 26 and 27). - ... 

When the alkenyl component is an O-terf-butyldimethylsilyl (TBDMS) enol ether, another anomaly occurs independent of enol ether geometry, the anti product is favored (Scheme 6.8) [62]. With trimethylsilylpropargyl ethers, the anti selectivity is 95-98%, making this reaction an excellent route for the preparation of anti 1,2-diols. In these cases, transition structures similar to Figure 6.6c and d are operative, the dominant influence being mutual repulsion between the carbanion substituent, R, and the 0-silyl group. 

Problem 173 The ZSiCX valence angle in methyl(silyl)ether is 120°. Could this angle be determined by across-angle repulsion Would you expect the coordination geometry of the N atom in methyl(disilyl)amine to be planar What about dimethyl(silyl)amtne (The former is found to be planar, the second nearly so.)... 

The main advantage of using silyl ethers in cross<oupling reactions is the ability to incorporate them into molecules by a number of methods. Cyclic silyl ethers, as a class, nicely illustrate this attribute. The well-known hydrosilylation of alkynes to form vinylsilanes can easily be rendered intramolecular by attachment of the silane as, for example, a homopropargyl silyl ether to form an oxasilacyclopentane 108 (Scheme 7.28) [53]. In this stmcture, the double-bond geometry is defined by the stereochemical course of hydrosilylation and the ether tether defines the location of the silicon atom with respect to the alkene. Thus, the siHcon-oxygen bond in this molecule serves to direct the hydrosilylation, as well as to activate the siHcon for cross-coupling. 

It is quite interesting that such a silicon effect depends strongly on the geometry of the molecule. Yoshida and coworkers found a linear correlation on plotting the oxidation potentials of a-silylated ethers, where the rotation around the C-O bond is restricted, against the HOMO energy-torsion angle (Si-C-O-C) curve obtained by MO... 

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