Sigmatropic Rearrangments (I + II)

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23 Terms

1
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1,2-Wittig Rearrangement

  • Benzylic or propargylic stabilization required

  • Radical mechanism, not concerted.

<ul><li><p>Benzylic or propargylic stabilization required </p></li><li><p>Radical mechanism, not concerted. </p></li></ul><p></p>
2
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1,2-Stevens Rearrangement

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3
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1,2-Wagner Meerwein Shift

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4
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1,3-alkyl shifts

  • Methyl shifts of allylic methyl systems

  • Stereoinversion required, if a concerted thermal process

  • Usually occurs with strained systems (vinyl cyclopropane or vinylcyclobutane)

<ul><li><p>Methyl shifts of allylic methyl systems</p></li><li><p>Stereoinversion required, if a concerted thermal process</p></li><li><p>Usually occurs with strained systems (vinyl cyclopropane or vinylcyclobutane)</p></li></ul><p></p>
5
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1,5-Hydride and alkyl shifts

  • Hydride and alkyl shifts of conj. dienes

  • Suprafacial, stereoretentive

  • Super fast in strained systems (i.e cyclopentadiene)

<ul><li><p>Hydride and alkyl shifts of conj. dienes </p></li><li><p>Suprafacial, stereoretentive</p></li><li><p>Super fast in strained systems (i.e cyclopentadiene)</p></li></ul><p></p>
6
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1,7-Hydride shifts

  • Needs antarafacial component (migration from opposite face)

  • Allylic shift of conj. trienes (hydride or methyl)

<ul><li><p>Needs antarafacial component (migration from opposite face)</p></li><li><p>Allylic shift of conj. trienes (hydride or methyl)</p></li></ul><p></p>
7
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[2,3]-Wittig Rearrangement

  • Also common with propargylic substrates

  • Proton alpha to heteroatom but not conj. to allylic group is deprotonated

<ul><li><p>Also common with propargylic substrates </p></li><li><p>Proton alpha to heteroatom but not conj. to allylic group is deprotonated</p></li></ul><p></p>
8
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Still-Wittig

  • Same concept as regular [2,3] rearrangements, except Li transmetalates with SnBu3

<ul><li><p>Same concept as regular [2,3] rearrangements, except Li transmetalates with SnBu3 </p></li></ul><p></p>
9
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[2,3]-Wittig Rearrangement Stereochemistry

Always Trans-olefin formed

10
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Sommelet-Hauser Ammonium Ylide Rearrangement

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11
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[2,3]-Mislow Evans Rearrangement

  • Can prepare via Mitsunobu, mcPBA oxidation (1 eq.)

  • Can cleave with nucleophile (Thiophiles: P(OEt)3, NaBH4, PhSH, etc.) to form alcohol

<ul><li><p>Can prepare via Mitsunobu, mcPBA oxidation (1 eq.)</p></li><li><p>Can cleave with nucleophile (Thiophiles: P(OEt)3, NaBH4, PhSH, etc.) to form alcohol </p></li></ul><p></p>
12
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[3,3]-Cope Rearrangement

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13
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Strain-Driven Cope Rearrangements

  • Divinyl cyclopropane, cyclobutane. Remember, ring strain release is about 26 kcal/mol for cyclopropane

<ul><li><p>Divinyl cyclopropane, cyclobutane. Remember, ring strain release is about 26 kcal/mol for cyclopropane</p></li></ul><p></p>
14
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Anionic Oxy-Cope Rearrangement

  • Anion stabilizes biradical TS

  • Lower BDE of broken bond

  • Requires base to form anion (KH, KHMDS, etc.)

<ul><li><p>Anion stabilizes biradical TS</p></li><li><p>Lower BDE of broken bond</p></li><li><p>Requires base to form anion (KH, KHMDS, etc.)</p></li></ul><p></p>
15
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Claisen Rearrangement

  • Aromatic, alkyl versions. Aromatic is more common

<ul><li><p>Aromatic, alkyl versions. Aromatic is more common</p></li></ul><p></p>
16
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Johnson-Claisen Rearrangement

  • Use of triethylorthoacetate to form ester after rearrangement

<ul><li><p>Use of triethylorthoacetate to form ester after rearrangement</p></li></ul><p></p>
17
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Eschenmoser-Claisen Rearrangement

  • Forms amide after rearrangement

<ul><li><p>Forms amide after rearrangement </p></li></ul><p></p>
18
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Ireland-Claisen Rearrangement

  • Chair TS, stereoselective

  • Z, E alkenes = trans

  • E, E alkenes = cis

  • Deprotonation to enolate, then trapping, then rearrangement

<ul><li><p>Chair TS, stereoselective </p></li><li><p>Z, E alkenes = trans </p></li><li><p>E, E alkenes = cis</p></li><li><p>Deprotonation to enolate, then trapping, then rearrangement</p></li></ul><p></p>
19
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Eschenmoser Claisen reagent

<p></p><p></p>
20
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Tandem Aza-Cope Rearrangement

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21
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Overman Rearrangement

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22
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Boekelheide Reaction

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Fischer Indole Synthesis

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