Chap 12 Conjugated unsaturated systems

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Chapter 15 Pericyclic Reactions (????) (ref
??? ?17?) (Wade Chapter 15-11, 12 )
2
???????????

• ???D????????
• ???D??????????????????????????????,??????????,?
  ???????????????????????D,??????,???????D????????
• ???????D,???????????D2?,?????????????D2,???????
  ????????7-?????????D3?,????????????D3,???????
  

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Reactions of Organic Compounds

• a polar reaction ?????

• a radical reaction?????

• a concerted reaction ????

A pericyclic reaction (????), a reaction that
occurs as a result of reorganizing the electrons
in the reactant(s)
4
Three Pericyclic Reactions (??????)

• Electrocyclic reactions ?????
• Cycloaddition reactions ?????
• Sigmatropic rearrangements s???(??)??

5
Electrocyclic reactions ?????

• A electrocyclic reaction is an
  intramolecular(???) reaction in which a new s-
  bond is formed between the ends of a conjugated p
  system????????????,??p????????????????s-??

• Electrocyclic reactions are reversible
  ??????????

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Cycloaddition reactions ?????
Two different p bond-containing molecules react
to form a cyclic compound ???????p????????????????
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Cycloadditions are classified according to the
number of p electrons that interact in the
reaction

• 42 cycloaddition ?????(Diels-Alder reaction)

• 22 cycloaddition ?????

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Sigmatropic rearrangements s?????
A s bond is broken in the reactant, a new s bond
is formed in the product, and the p bonds
rearrange. ???s???,???s???, p ????
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Note

• The electrocyclic reactions and sigmatropic
• rearrangements are intramolecular reactions
• ??????s???????????

• The cycloaddition reactions are usually
  intermolecular reactions
• ?????????????

12

• Common features
• among the three pericyclic reactions

• are concerted reactions (????, ?????)
• are not affected by catalysts and
  solvents(??????????)
• are thermal or photochemical reactions
  (????????)
• are reversible (?????)
• are highly stereoselective (???????)

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The configuration of the product formed depends
on

• the configuration of the reactant (??????)
• the number of conjugated double bonds or pairs
  of electrons in the reacting system (?????)
• whether the reaction is a thermal or a
  photochemical reaction (?????????)

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??
??
4e ?? ????
??
??
15
??
6e?? ????
??
??
??
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Conservation of Orbital Symmetry Theory(????????)

• ???????????????????????????,?????????????????,????
  ????????????????????????

Woodward R. B. Hoffmann R
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mirror plane ??
Asymmetric ????
P orbital
Symmetric ???
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A p Molecular Orbital Description of Ethene
?(?)? ??????0)
antibonding p
Asymmetric ???
bonding p
Symmetric ??
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Frontier Molecular Orbital Theory(FMO,??????)

• ???????????HOMO(Highest Occupied Molecular
  Orbital)??????LUMO(Lowest Unoccupied Molecular
  Orbital)?????????
• ????,???????????????????
• ???(thermal reaction)?,???????(ground state
  )???(photochemical reaction)?,????????(excited
  state)?

????
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1,3-butadiene(1,3-???)
(LUMO)
(LUMO)
(HOMO)
(HOMO)
??
???
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1,3,5-hexatriene
22
the electrocyclic reaction ?????
To form the new s bond in the electrocyclic
reaction, the p orbitals at the end of the
conjugated system must overlap head-to-head
??
??
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Only the symmetry of the HOMO is important in
determining the course of the reaction
??????????????????,???????
HOMO??, ????, ????
HOMO???, ????, ????
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4e ?? ????
???,??,HOMO ?2, ???, ??
???,???, HOMO ?3, ??, ??
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6e ?? ????
???,??,HOMO ?3, ??, ??
???,???, HOMO ?4, ???, ??
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6e??
6e?? ???,??,HOMO ?3,??,?? ???,???,HOMO?4,???,??
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??????????
p??? ?? ?? ?? ??
p??? HOMO ??? ?? HOMO ??? ??
4n ??? ?? ?? ??
4n2 ?? ?? ??? ??
4n ?? ????
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??
??
??
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Cycloadditions ?????
??????(FMO,Frontier Molecular
Orbitals)??,?????????,????????????HOMO???????????L
UMO??????????,????????????,???????,??????
?????(FMO)?????,????????HOMO??????,???????????????
?,?????????,??????HOMO??????????,????????LUMO????
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????
????
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Frontier Orbital Analysis of a 4 2
Cycloaddition Reaction
???
42???,???????
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???
?????? ??????
????? ???????
42???,??????, ????, ????
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• ?????????,???????????,???????

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minor
major
??
??

• Endo rule (????)
• ???????????,????????
• Secondary overlap ??(??)??

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1,3-product
1,4-product major
1,2-product major
1,3-product

• ?????????,??????????,????1,2-?1,4-??.

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1,4-product major
1,2-product
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Frontier MO Analysis of the 2 2
Cycloaddition Reaction
p2
22 ????? ??? ??????
p2
p1
p2
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Woodward-Hoffmann rules for cycloaddition
reactions ???????
p????? ???? ????
4n2 ?? ??
4n2 ?? ??
4n ?? ??
4n ?? ??
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Sigmatropic Rearrangements s?????
??? i,j 1,3 , 1,5, 1,7 ??? 1,3 ,
1,5, 3,3, 5,5
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a 1,3 sigmatropic rearrangement
1,3 C ??
a 3,3 sigmatropic rearrangement
3,3 C ??
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Migration of hydrogen (???)
??????????????????????,????C-H????,?????
?????????????????HOMO?????
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HOMO?? ????
HOMO??? ????
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1,3-hydrogen shift 1,3???
??, ???, ????,???
???, ???, ????,??
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1,5-hydrogen shift 1,5???
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Transition state of 1,5-hydrogen shift
???, ??, ????,??
???, ???, ????,???

• s????????
• ?????6????????????????
• 1,3,1,7??????, 1,5??????.

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????????
????????p?????(1j) ?? ??
4n2 ( 1,5 ) ????, ?? ????, ??
4n ( 1,3, 1,7 ) ????, ?? ????, ??
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Migration of carbon (???)
????
???????
???????
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???????
????
???????
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1,3 C migrating
??? 1,3???, HOMO??? C???? C????
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3,3??
??(Cope)?? ??-?s???? 3,3 ??
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3,3????s???,??????????????,?????????????,??
????HOMO?,3,3'??????P??????????,?????????1?1'?????
????,3,3'???????,?????????
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???(Claisen)???????????????-?????s? 3,3 ?????
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??????,??????.
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i,j s???????
1,3 ??, ????
1,3 ??, ????
3,3 ?? (Cope Claisen ??)
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Pericyclic Reactions in Biological Systems
conrotatory ??
7-?????
1,7 s-??
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Examples
????????
3.
12.
-78?, b?a??????1022?, ????
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Summary

• ??
• ??????????????s???????(?????????????????????)?????
  
• ????????????,????????????????
• ????????????????,???????

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??????????
p??? HOMO??? ?? ??
4n2 ???? ?? ??
4n ????? (?C2??) ?? ??
??????????
p????? ?? ??
4n2 ?? ??
4n ?? ??
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s????????
????????p?????(1j) ?? ??
4n2 ( 1,5 ) ????, ?? ????, ??
4n ( 1,3, 1,7 ) ????, ?? ????, ??
i,j s???????
1,3 ??, ????
1,3 ??, ????
3,3 ?? (Cope Claisen ??)
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??

• 1, 2, 4, 5, 7, 9, 11
• 15-30