Chem 634 Fall 2014
Pericyclic Reactions – Part II
Reading:
CS-B Chapter 6
Grossman Chapter 4
Announcements
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DIP-Cl
OH
Ph
O
Me
X
Me
Me
OH
DIP-Cl
R
R
Cl
B
Me
DIP-Cl
Me
> 98% ee
Me
BCl
H
H
Rs
Me
2
R
O
RL
Akin to a chiral DIBAL.
HC Brown (Nobel 1979)
Brown, TL, 1991, 32, 6691
Stereoselectivity
Small Hydroborating Agent:
Me
RL
Me
1) B2H6
RL
2) H2O2 /NaOH
RM
X
OH
RM
H
Me
Vs
RL
Me
H
H
RM
BH2
Favored
RL
Me
H
BH2
H
RM
RL
OH
RM
RL
RM
Me
Stereoselectivity
Large Hydroborating Agent:
Me
RL
RL
2) H2O2 /NaOH
RM
H
RM
RL
Vs
Favored
H
H
B
Me
RL
Me
RM
H
RL
RM
OH
RM
B
H
Me
X
Me
1) 9BBN
H
B
Me
RL
OH
RM
[3+2] Cycloadditions
3-atoms + 2-atoms (but still 4π e- + 2π e-)
Most common/useful are 1,3-dipolar cycloadditions.
Common Dipoles:
Ozone (for ozonolysis)
Nitrone
Diazoalkane
[3+2] Cycloadditions – More Dipoles
“Click Reaction”
-  Often w/ Cu(I)
-  Very fast
-  Used for
bioligation
(Barry Sharpless)
Azide
Carbonyl ylide
Azomethine ylide
Lewis acid and asymmetric catalytic versions known.
[2+2] Cycloadditions
FMO Analysis:
•
•
No net bonding… “forbidden”
This geometry is suprafacial on both
π bonds => [2πs + 2πs]
Suprafacial = same face of π-system
Antarafacial = opposite faces of π-system
Alternative Transition State Geometry
Problem: Steric Hindrance!
H
H
H
H
H
H
H
H
Solution: Remove steric hindrance!
Chelatropic Reactions
•  Special class of cycloaddition/cyclreversion reactions
•  2 bonds broken or formed to a single atom
•  Characterize the same as for cycloadditions (# of atoms)
Sigmatropic Reactions
•  Reorganization of σ and π bonds (migration of a σ-bond)
•  Number of σ and π bonds remains constant
•  Classify by [m,n]-rearrangement or [m,n]-shift (m, n = number of atoms in
fragment)
[1,3]-Sigmatropic Rearrangement
Does this rearrangement proceed under thermal conditions?
Supra- or antara-facial??
For FMO, break into HOMO and LUMO:
[1,3]-Sigmatropic Rearrangements
Alkyl Shift?
FMO:
[3,3]-Sigmatropic Rearrangements
Suprafacial on both components!
Highly predictable TS –> “chair-like” (can predict stereochem)
Claisen Rearrangement
Modified Claisen Rearrangements
Johnson Orthoester Claisen
Modified Claisen Rearrangements
Eschenmoser–Claisen
Modified Claisen Rearrangements
Ireland–Claisen
Ireland–Claisen: 1,3-Chirality Transfer
Ireland–Claisen: Enolate Geometry (1)
(Recall Ireland enolate model to rationalize enolate stereochemistry.)
Ireland–Claisen: Enolate Geometry (2)
Ireland–Claisen: Enolate Geometry (3)
Cope Rearrangement (all carbon)
Cope Rearrangement
Cope Rearrangement: Tricks to Favor Product Formation
Thermoneutral, so how can we favor product?
Brown Chem. Commun. 1973, 319
Oxy-Cope
Aza-Claisen Rearrangement
[2,3]-Sigmatropic Rearrangements
[2,3]-Sigmatropic Rearrangements
[2,3]-Sigmatropic Rearrangements
[2,3]-Sigmatropic Rearrangements
Group Transfer Reactions (Last Type of Pericyclic Reaction)
Group transferred to another reaction partner
Group Transfer Reactions
Theory #3: Dewar–Zimmerman: Aromatic Transition State
Steps:
1.  Choose basis set of 2p AO’s (or 1s for H atoms)
2.  Assign phases (any phases)
3.  Connect orbitals that interact in the starting material
4.  Connect lobes that begin to interact in the reaction
5.  Count the number of phase inversions
6.  Identify topology
1.  Odd # of phase inversions = Möbius
2.  Even # of phase inversions = Hückel
7.  Assign Transition State as aromatic (thermally allowed) or antiaromatic
(photochemically allowed)
System/Topology Aroma1c An1aroma1c Huckel (4n+2) e-­‐ (4n) e-­‐ Mobius (4n) e-­‐ (4n+2) e-­‐ Example of D–Z Theory
Thank you for a great semester!
Good luck!