Investigation of asymmetric induction at prochiral silicon and phosphorus
The goal of this study was to induce asymmetry into a prochiral silicon or phosphorus atom from a directly bonded, carbon-based chiral auxiliary. The bicyclo (2.2.1) heptyl group was initially selected as a model chiral auxiliary. Several stereoselective synthetic methods were utilized for preparing silicon and phosphorus substituted bicyclo (2.2.1) heptanes. Although the exo-substituted compounds were prepared from literature methods, the endo-phosphorus substituted materials required original synthetic developments. These key starting materials were converted to a series of silyl and phosphorus substrates which contained a variety of spectator and leaving groups at the heteroatom center. Two identical leaving groups were present at both the prochiral phosphorus and silicon atoms. Both the exo- and endo-silyl isomers were studied to test the influence of a change of the auxiliary on the efficiency of the induction process. Several phosphonic acid derivatives and one phosphonium salt derivative were also prepared in the exocyclic series; upon treatment with nucleophiles these derivatives gave approximate 1:1 mixtures of diastereomers. Both the exo-2-methyldiphenoxysilyl and exo-2-methyldi-$\alpha$-naphthoxysilyl substrates gave a 3:2 ratio of diastereomers upon treatment with t-butyllithium. The endo-2-methyldiphenoxysilyl isomer also gave an approximate 3:2 ratio of diastereomers for this reaction. The above studies were extended to include the 1,7,7-trimethylbicyclo (2.2.1) heptyl substituent (bornyl group), which is available from the naturally occurring and optically active molecule, camphor. The exo-3- and endo-3-methyldiphenoxysilyl-1,7,7-trimethylbicyclo (2.2.1) heptanes (tentative stereo assignments) were prepared as an equal molar mixture. Treatment of this mixture of substrates with t-butyllithium gave a 55:45 ratio of one set of diastereomers, and a 73:27 ratio of another set of diastereomers. Nuclear magnetic resonance spectroscopy (1D and 2D) was used to determine the diastereomeric ratios, characterize the reaction products and establish the stereochemical outcome of the above reactions. These collective experimental results demonstrate that asymmetric induction at a prochiral silicon center from a directly bonded, carbon-based chiral auxiliary is a feasible process. Additional modifications will be required to achieve asymmetric induction at prochiral phosphorus.
Andrew Carlton Peterson,
"Investigation of asymmetric induction at prochiral silicon and phosphorus"
(January 1, 1992).
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