Trimethylsilyl group stereochemistry

Rothwell and colleagues352 studied the titanium mediated [2 + 2 + 2] cycloaddition of alkenes with monoynes and diynes. Among the reactions studied, the reaction between styrene (29) and diyne 609 in the presence of titanium catalyst 610 proved cleanest (equation 175). The reaction yielded 614 via a [2 + 2 + 2] cycloaddition followed by a titanium mediated suprafacial [1,5] H-shift involving 611-613. The cis relationship between the trimethylsilyl group and the phenyl group indicated that the initially formed titananorbornene 611 had an endo stereochemistry. 

Silyl-directed Beckmann fragmentation. A trimethylsilyl group in the p- or p -position of cyclic ketoxime acetates controls the regio- and stereochemistry of the double bond formed on Beckmann fragmentation catalyzed by trimethylsilyl triflate. 

Only small variations in peak intensities for stereoisomers were observed, and these differences could not be used for identification purposes. The per(trimethylsilyl)ated inositols are fragmented by single or consecutive eliminations of CH3 , McsSiO , and MesSiOH from the molecular ion. Most of the other ions in the spectra are commonly found also in the mass spectra of per(trimethylsilyl)ated hexoses. The variation in peak intensities in the spectra of stereoisomers was considerable these differences were attributed to the bulky trimethylsilyl groups, which restrict the thermal interconversion between different conformations. Furthermore, certain elimination reactions for trimethylsilyl groups proceed by way of cyclic intermediates, and thus may be influenced by the stereochemistry. 

Entry 5 is an example of use of an a-trimethylsilylallyl group to prepare a vinylsilane. The stereochemistry is consistent with a cyclic TS having the trimethylsilyl substituent in a quasi-axial position to avoid interaction with the bridgehead hydrogen of the bicyclic ring. 

Rhodium-catalyzed silylformylation proceeds smoothly in branched 1-alkynes at 25 °C as shown in Table 3. The stereochemistry at the chiral carbon involved in alkynes is retained intact under the silylformylation conditions. (A)-28, (rhodium particles co-condensed with mesitylene. 3-Trimethylsilyl-l-propyne 40 reacts similarly to give 41 (Equation (7)). " / //-Butylacetylene does not work as a substrate for the silylformylation because of the bulky tert-huty group on the i/>-carbon. In contrast to /< r/-butylacetylene, trimethylsilylacetylene 42 gives 43 in a fair yield (Equation (8)). ... 

The absolute configuration of the -glucose and (3-galactouronic acid groups was confirmed as D by GC analysis of the trimethylsilyl ethers of l(L-a-methylbenzyl-amino)-l-deoxyalditols derived from the water-soluble fraction from acid hydrolysis of (27). The stereochemistry at C-5 was secured by the usual method (hydrolysis and Lemieux degradation of... 

The Mitsunobu reaction is one of the staple reactions for clean nucleophilic substitution with inversion of configuration. It came as a surprise, therefore, to find that a Mitsunobu reaction on atlyEic alcohol 143,1 [Scheme 8 143) using terf-butyl 2-(trimethylsilyl)ethylsulfonylcarbamate (143.2) as the nucleophile occurred with retention of configuration.320 The unusual stereochemistry was explained by a double inversion process in which neighbouring group participation first leads to the intermediate 1433. A subsequent second nucleophilic substitution by 1433 then gave the product 143.4 in 86% yield ... 

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]. 

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