Transition States and Origins of 1,4‐Asymmetric Induction in Alkylations of 2,2,6‐Trialkylpiperidine Enamines
Identifieur interne : 000D87 ( Main/Curation ); précédent : 000D86; suivant : 000D88Transition States and Origins of 1,4‐Asymmetric Induction in Alkylations of 2,2,6‐Trialkylpiperidine Enamines
Auteurs : Joann Um ; Naeem Kaka ; David Hodgson [Royaume-Uni] ; K. Houk [États-Unis]Source :
- Chemistry – A European Journal [ 0947-6539 ] ; 2010-06-01.
English descriptors
- KwdEn :
- Aldehyde, Alkene, Alkylation, Alkylations, Allyl, Allyl bromide, Angew, Asymmetric induction, Axial, Axial methyl, Axial methyl group, Bromide, Chem, Conformation, Conformer, Enamine, Enantiomeric ratios, Equatorial, Equatorial position, Ethyl iodide, Gmbh, Good agreement, Hodgson, Intermolecular nucleophilic substitution, Isopropyl, Isopropyl enamine, Isopropyl group, Kgaa, Lone interaction, Lone pair, Methyl, Methyl enamine, Model enamines, Olefinic protons, Phys, Piperidine, Piperidine ring, Preferential attack, Relative enthalpies, Transition state, Transition states, Transition structures, Unfavorable interaction, Verlag, Verlag gmbh, Weinheim, Weinheim chem.
- Teeft :
- Aldehyde, Alkene, Alkylation, Alkylations, Allyl, Allyl bromide, Angew, Asymmetric induction, Axial, Axial methyl, Axial methyl group, Bromide, Chem, Conformation, Conformer, Enamine, Enantiomeric ratios, Equatorial, Equatorial position, Ethyl iodide, Gmbh, Good agreement, Hodgson, Intermolecular nucleophilic substitution, Isopropyl, Isopropyl enamine, Isopropyl group, Kgaa, Lone interaction, Lone pair, Methyl, Methyl enamine, Model enamines, Olefinic protons, Phys, Piperidine, Piperidine ring, Preferential attack, Relative enthalpies, Transition state, Transition states, Transition structures, Unfavorable interaction, Verlag, Verlag gmbh, Weinheim, Weinheim chem.
Abstract
The asymmetric C‐alkylation of chiral enamines derived from terminal epoxides and lithium 2,2,6‐trialkylpiperidides has previously been shown to provide α‐alkylated aldehydes by intermolecular nucleophilic substitution in good levels of asymmetric induction. We now report a computational study of the origins of asymmetric induction in these reactions. Computational modeling with density functional theory (B3LYP/6‐31G(d)) agrees closely with the experimental observations. This stereoselectivity is attributed to a preferential conformation of the enamine and the piperidine ring that places the C‐6 alkyl substituent in an axial position due to A1, 3 strain. Preferential attack occurs away from the axial group, for steric reasons. The effects of changing the C‐6 substituent from methyl to isopropyl were studied, and twist transition states were found to contribute significantly in the latter alkylations.
Url:
- https://api.istex.fr/document/035F03995891F79B507BD6081B3B87643B691835/fulltext/pdf
- http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3049728
DOI: 10.1002/chem.201000046
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Joann Um<affiliation><wicri:noCountry code="subField">(+1) 310‐206‐1843</wicri:noCountry>
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<profileDesc><textClass><keywords scheme="KwdEn" xml:lang="en"><term>Aldehyde</term>
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<term>Allyl bromide</term>
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<term>Intermolecular nucleophilic substitution</term>
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<term>Lone pair</term>
<term>Methyl</term>
<term>Methyl enamine</term>
<term>Model enamines</term>
<term>Olefinic protons</term>
<term>Phys</term>
<term>Piperidine</term>
<term>Piperidine ring</term>
<term>Preferential attack</term>
<term>Relative enthalpies</term>
<term>Transition state</term>
<term>Transition states</term>
<term>Transition structures</term>
<term>Unfavorable interaction</term>
<term>Verlag</term>
<term>Verlag gmbh</term>
<term>Weinheim</term>
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<term>Conformer</term>
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<term>Good agreement</term>
<term>Hodgson</term>
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<term>Isopropyl</term>
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<term>Model enamines</term>
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<front><div type="abstract" xml:lang="en">The asymmetric C‐alkylation of chiral enamines derived from terminal epoxides and lithium 2,2,6‐trialkylpiperidides has previously been shown to provide α‐alkylated aldehydes by intermolecular nucleophilic substitution in good levels of asymmetric induction. We now report a computational study of the origins of asymmetric induction in these reactions. Computational modeling with density functional theory (B3LYP/6‐31G(d)) agrees closely with the experimental observations. This stereoselectivity is attributed to a preferential conformation of the enamine and the piperidine ring that places the C‐6 alkyl substituent in an axial position due to A1, 3 strain. Preferential attack occurs away from the axial group, for steric reasons. The effects of changing the C‐6 substituent from methyl to isopropyl were studied, and twist transition states were found to contribute significantly in the latter alkylations.</div>
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