Silanes absolute configuration

More recently, the Lewis acid promoted asymmetric 1,4-addition of trimethyl(2-propenyl)silane to chiral a,/ -unsaturated /V-acylamides has been published33. Lewis acid mediated reactions of trimethyl(2-propenyl)silanes with a,/I-unsatu rated AT-acyloxazolidinones or iV-enoylsultams show high chemical yield with good diastereomeric excess. The absolute configuration of the new asymmetric center is controlled by the nature of the Lewis acid used. 

The absolute configuration of the silicon complex has been established by an X-ray study and shown to be (S) starting from the (R) silane, involving retention of configuration. ... 

E. C. 3.1.1.3) as the biocatalyst, the prochiral silanes 86 and 88 were transformed into the levorotatory isobutyrates (—)-87 and (—)-89, respectively (Scheme 20)73. The latter conversions were carried out in diisopropyl ether (with 86) and tetrahydrofuran (with 88) as solvent. The yields and enantiomeric purities obtained with the transesterifications of 86 and 88 with CCL and CVL are summarized in Table 4. The absolute configurations of the biotransformation products are unknown. 

Enantioselective enzymatic transesterification has also been used for a kinetic racemate resolution75. Starting from the racemic (hydroxymethyl)silane rac-92 (analytical scale), transesterification with vinyl acetate in water-saturated 2,2,4-trimethylpentane, catalyzed by a commercial crude papain preparation (E.C. 3.4.22.2), yielded the corresponding optically active (acetoxymethyl)silane 93 (sign of optical rotation and absolute configuration not reported) (Scheme 21). The enantiomeric purity of the remaining dextrorotatory (hydroxymethyl)silane (+)-92 was moderate (49% ee). 

Ohaya, Y., and T. Ashida The Structures of a-Naphthylphenylmethyl-silanes, Optically Active Silicon Compounds, and their Absolute Configurations. Acta cristallogr. [Copenhagen] 20, 461—471 (1966). 

Tertiary phosphines, in the absence of special effects 2 ), have relatively high barriers 8) ca. 30-35 kcal/mol) to pyramidal inversion, and may therefore be prepared in otically stable form. Methods for synthesis of optically active phosphines include cathodic reduction or base-catalyzed hydrolysis 3° 31) of optically active phosphonium salts, reduction of optically active phosphine oxides with silane hydrides 32), and kinetic 3 0 or direct 33) resolution. The ready availability of optically pure phosphine oxides of known absolute configuration by the Grignard method (see Sect. 2.1) led to a study 3 ) of a convenient, general, and stereospecific method for their reduction, thus providing a combined methodology for preparation of phosphines of known chirality and of high enantiomeric purity. 

If the silicon atom of allylsilane is stereogenic, disappointing selectivities are obtained [1156], On the other hand, if the silicon substituents are chiral, a chirality transfer may be observed if the allylic group itself bears the asymmetry [54, 736] or asymmetric induction may be observed if another substituent of silicon is chiral [737, 1014], Chirality transfer in into-- or intramolecular reactions of allyl-silanes 2.78 with aldehydes is very efficient under TiCl4 catalysis however, from allylsilanes 2.79, a poor selectivity is observed [737, 1014] (Figure 6.53). More useful results were obtained by Nishitani and Yamakawa [1222] in the cyclization of 6.60, whose chiral ester is derived from (l/ ,25,5i )-phenmenthol 1.4 (R = Ph). The chiral auxiliaty is recovered after lactonization, but the absolute configuration of the cis lactones thus formed has not been determined (Figure 6.53). 

Similar reaction sequences were used to resolve 1 -naphthylphenylvinyl- and -ethyl-silane 32 and 33 (46, 48). The Fredga method was used to establish the relative configuration of these compounds and to correlate them with the known absolute configuration of Sommer s compounds (49). Figure 1 shows the configuration of the compounds obtained by reduction of the less soluble (-)-menthoxy diastereomer. 

Figure 1. Absolute configuration of (+)-l-naphthy phenylvinyl- and -ethyl-silane.

In organosilicon chemistry the molecular structure and absolute configuration of only a few chiral silanes have been determined by the Bijvoet method. These... 

For the determination of the absolute configuration of the major diastereomer of silane (5,S)-3, the hydrogen iodide adduct (5,5)-3 HI was formed by reaction of the compound with anhydrous hydrogen iodide (solution in diethyl ether) at 0 °C. Colorless needles of (5,5)-3-HI were obtained as single crystals from CH2Cl2/n-pentane. 

Recently, Delvos and Oestreich presented the related enantioconvergent silylation of a racemic cyclic allylic phosphate to arrive at an enantioeiuiched cyclic allylic silane (Scheme 31) [70]. A comprehensive experimental analysis revealed that preformed chiral 6-membered NHC-copper(l) complex L7 CuCl adopts two different courses depending on the absolute configuration of the starting material. The (/ )-configured phosphate (/ )-120 reacts through a iyn-SN2 pathway [(R)-120 (R)-II (S )-121], whereas the (5)-configured enantiomer (S )-120 follows an anti-Sj 2 mechanism [(S)-120 —> (5)-II —> (5)-121]. 

The highly electrophilic cationic bis(8-quinolinolato)aluminum complex 407 enabled Yamamoto and coworkers to perform Mukaiyama-Michael additions of silyl enol ethers to crotonylphosphonates 406. The procedure was not only applicable to enol silanes derived from aryl methyl and alkyl methyl ketones (a-unsubstituted silicon enolates) but also to several cycfic a-disubstituted silyl enol ethers, as illustrated for the derivatives of a-methyl tetralone and indanone 405 in Scheme 5.105. Despite the steric demand of that substitution pattern, the reaction occurred in relatively high chemical yield with varying diastereoselectivity and excellent enantiomeric excess of the major diastereomer. The phosphonate residue was replaced in the course of the workup procedure to give the methyl esters 408. The protocol was extended inter alia to the silyl enol ether of 2,6,6-tetramethylcyclohexanone. The relative and absolute configuration of the products 408 was not elucidated [200]. 

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