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Issue 7, 2008
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Reactions of silyl-stabilised sulfur ylides with organoboranes: enantioselectivity, mechanism, and understanding

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Abstract

The reaction of trimethylsilyl-substituted sulfonium ylides with organoboranes (Ph3B, Et3B) has been studied and although homologated products were obtained in good yield (after oxidation to the corresponding alcohols), the enantiomeric excesses were low with our camphor-based chiral sulfide (up to 40% ee, cf. corresponding phenyl-substituted sulfonium ylides gave >95% ee). Cross-over experiments were conducted to ascertain the nature of this difference in selectivity. Thus, aryl- and silyl-substituted sulfonium ylides (1 equiv.) were (separately) reacted with Et3B (1.5 equiv.) followed by Ph3B (1.5 equiv.) The experiments were repeated changing the order of addition of the two boranes. It was found that the aryl-substituted sulfonium ylide only trapped the first borane that was added indicating that ate complex formation was non-reversible and so was the selectivity determining step. In contrast the silyl-substituted sulfonium ylide only trapped Ph3B (it is more reactive than Et3B) indicating that ate complex formation was reversible and so 1,2-migration was now the selectivity determining step. The reactions have been studied computationally and the experimental observations have been reproduced. They have further revealed that the cause of reversibility in the case of the silyl-substituted sulfonium ylides results from ate complex formation being less exothermic and a higher barrier to 1,2-migration.

Graphical abstract: Reactions of silyl-stabilised sulfur ylides with organoboranes: enantioselectivity, mechanism, and understanding

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Article information


Submitted
29 Nov 2007
Accepted
28 Jan 2008
First published
28 Feb 2008

Org. Biomol. Chem., 2008,6, 1185-1189
Article type
Paper

Reactions of silyl-stabilised sulfur ylides with organoboranes: enantioselectivity, mechanism, and understanding

D. Howells, R. Robiette, G. Y. Fang, L. S. Knowles, M. D. Woodrow, J. N. Harvey and V. K. Aggarwal, Org. Biomol. Chem., 2008, 6, 1185
DOI: 10.1039/B718496D

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