Chlorosilanes racemization

Important requirements for formation of hexacoordinate silicon complexes is small bulk and a strongly electron-withdrawing character of the substituents at the Si center (171). These conditions are not met in the chlorosilane racemization and solvolysis processes studied (251,285). Also the faster rate of halogen exchange relative to inversion on silicon observed by Cartledge et al. (286) remains in conflict with the hexacoordinate silicon intermediate (A) pathway [Eq. (64)]. 

According to this mechanism (Scheme 7.19), the process is amenable to asymmetric modifications. Among a variety of silicon reagents that were examined, only tetrachlorosilane proved to be suitable for the asymmetric process, while the application of other chlorosilanes resulted in the formation of racemates. A... 

Summary Our research on a-metalated organosilanes currently focuses on (aminomethyl)silanes containii a defined stereogenic center next to the silicon center. A synthetic route based on the preparation of 2-silyl-substituted pyrrolidines was developed. The racemic product could be synthesized by metalation of JV-Boc-pyrrolidine in the presence of TMEDA and conversion with the corresponding chlorosilane, whereas the enantioenriched form was achieved by metalation in the presence of the chiral amine (-)-sparteine. Subsequent metalation and transformation reactions yielded the formation of the corresponding (aminomethyl)(lithiomethyl)silane. 

The chlorosilane 50 undergoes rapid racemization, which is faster than alcoholysis. The menthoxysilane diastereomers 39 are obtained quantitatively in a ratio 1 2 close to that of the rate constants ky-.k. Fractional crystallization of the mixture of diastereomeric menthoxysilanes 39 allowed isolation of the less soluble diastereomer (-)-(-)-39 ([o]D -66.2°, m.p. 72°C) in almost 70% recovery. 

A detailed examination of the features concerning nucleophilically activated racemizations and hydrolysis (or alcoholysis) of chlorosilanes led to the following observations (268) (i) Kinetic data for racemization (eq. [96]) and for hydrolysis (eq. [97]) indicate very similar processes ... 

The intervention of one mole of nucleophile in the inversion process contrasts with the results obtained for racemization of chlorosilane where two moles of nucleophiles are involved (13). To account for their results, Martin and Stevenson proposed that inversion at tetracoordinate silicon arises from an isomerization of a pentacoordinated intermediate via Berry pseudorotation steps. Several pathways were proposed, the most interesting being a nucleophilic attack at a carbon-oxygen edge of the tetrahedral silicon compound. In the resulting intermediate, a single Berry pseudorotation with the nucleophile as pivot and equatorial departure of the nucleophile causes an inversion at silicon (eq. [110]). A... 

TABLE 12. Rate constants for the racemization of various chlorosilanes in HMPA... 

Summary The highly enantiomerically enriched silyllithium compound lithiomethylphenyl(l-piperidinylmethyl)silane (2) reacts stereospedfically with chlorosilanes, but over a period of several hours slow racemization in solution at room temperature occurs, which can be supressed by a metathesis reaction with [Mg(thf)4]Br2. Quantum chemical calculations of solvated model systems allow an assessment of possible intermediates during the racemization process. 

We were able to prove that it is possible to synthesize the highly enantiomerically enriched silyllithium compound 2 (ee > 98 %) in large amounts and to perform stereospecific reactions with chlorosilanes. Due to the observed racemization of 2 in solution, we believe that a thorough reassessment of previous studies concerning optically active silyllithium species is in order. 

Some of the enantiopure phospholane oxides 116 were used for further transformations. For example, they could be isomerised by treatment with base for several hours in a protic solvent to afford 117 or subjected to a second deprotonation to produce 118 as a single isomer. Attempts to prepare Z)w(phospholanes) by double nucleophilic attack of the anions derived from 116 were unsuccessful. Some of the phospholane oxides were reduced with tri-chlorosilane/pyridine to afford the free phospholane with retention of configuration. The same strategy was applied to 1-phenylphospholane oxide but low yields of racemic products were obtained. This suggests that substrate 115 is probably a special case due to the formation of a more stable benzylic carbanion. ... 

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