Substituent-controlled reactivity in the Nazarov cyclisation of allenyl vinyl ketones |
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Authors: | Marx Vanessa M Stoddard Rhonda L Heverly-Coulson Gavin S Burnell D Jean |
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Affiliation: | Department of Chemistry, Dalhousie University, Nova Scotia, B3H 4R2, Canada. |
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Abstract: | Alkyl substitution α to the ketone of an allenyl vinyl ketone enhances Nazarov reactivity by inhibiting alternative pathways involving the allene moiety and through electron donation and/or steric hindrance. This substitution pattern also accelerates Nazarov cyclisation by increasing the population of the reactive conformer and by stabilising the oxyallyl cation intermediate. Furthermore, α substitution by an alkyl group does not alter the regioselectivity of interrupted Nazarov reactions when the oxyallyl cation intermediate is intercepted by addition of an oxygen nucleophile, or by [4+3] cyclisation with acyclic dienes. The regioselectivity of the Nazarov process for allenyl vinyl ketones was determined to be a result of an electronic bias in the oxyallyl cation intermediate. Computational data are consistent with this observation. |
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Keywords: | allenes carbocations cyclic compounds cycloaddition Nazarov reaction |
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