Radical Cation

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Radical Cations•+: Generation, Reactivity, Stability

R

A

R

A

MacMillan Group Meeting
4-27-11
by
Anthony Casarez

Three Main Modes to Generate Radical Cations

  Chemical oxidation
D A D A

  Photoinduced electron transfer (PET)
h!

1) D

A

D

A*

D

A

2) D

A

h!

D*

A

D

A

  Electrochemical oxidation (anodic oxidation)

D

Anode

D

Chemical Oxidation

  Stoichiometric oxidant: SET

O N Bn H N

Me

O N

Me

t-Bu

Ce(NH4)2(NO3)6 DME

Bn H

N

t-Bu

hexyl

hexyl

Me3SiO O

FeCl3, DMF

Me3SiO

MacMillan et al. Science 2007, 316, 582.
Booker-Milburn, K. I. Synlett 1992, 809.

Photoinduced Electron Transfer

  PET: Organic arene

CN NC OMe CN OMe

h!, MeCN, MeOH
CN

  PET: Metal mediated
h!, MgSO4, MeNO2, Ru(bpy)32+
R MeN NMe

MeO

MeO

R

Arnold, D. R.; Maroulis, A. J. J. Am. Chem. Soc. 1976, 98, 5931. Ischay, M. A.; Lu, Z.; Yoon, T. P. J. Am. Chem. Soc. 2010, 132, 8572.

Electrochemical Oxidation

  Anodic oxidation

Me O

Me O

anode, MeOH DCM, NaClO4 2,6-lutidine
MeO MeO

N N H

anode, 2,6-lutidine
Et

N N H

MeCN, Et4NClO4

CO2Me

CO2Me

Et

Ponsold, K.; Kasch, H. Tetrahedron Lett. 1979, 4463.
M. J. Gašić et al. J. Chem. Soc. Chem. Comm. 1993, 1496.

Primary Fate of Radical Cations

Key Points

  A radical cation will be generated from the electrophore on the molecule with the lowest oxidation potential (usually π or n, where n = nonbonding electrons).   The chemistry of the resultant radical cation is determined from the functionality around its periphery.   Deprotonation of the radical cation is a major pathway, resulting in a radical which adheres to typical radical reactivity patterns.

  Secondary reactions play a major role in our generation and use of radical cations.

Schmittle, M.; Burghart, A. Angew. Chem. Int. Ed. Engl. 1997, 36, 2550.

Primary Fate of Radical Cations

Symbol
CH
AH
AB
CC
CX
Nu
CA
R
ET
Rad
RA
H
Dim

Classification
C–H deprotonation
A–H deprotonation
A–B bond cleavage
C–C bond cleavage
C–X bond cleavage
Nu attack
cycloaddition
rearrangement
electron transfer
radical attack
radical anion attack
hydrogen transfer
dimerization

Primary Product
π-C• + H+ or n-C• + H+
π-A• + H+
π-A• + B+ or π-A+ + B•
π-C• + C+
π-C• + X+
Nu-π-A-B•+
cycloaddition
rearrangement
π-A-B or π-A-B2+
R-π-A-B+
RA-π-A-B
H-π-A-B+
(π-A-B)22+

A
C
O, N, S, X
A
C
C
A
A
A
A
A
A
A
A

B
H
H
B
C
Si, Sn
B
B
B
B
B
B
B
B

Schmittle, M.; Burghart, A. Angew. Chem. Int. Ed. Engl. 1997, 36, 2550.

Primary Fate of Radical Cations

Symbol
CH
AH
AB
CC
CX
Nu
CA
R
ET
Rad
RA
H
Dim

Classification
C–H deprotonation
A–H deprotonation
A–B bond cleavage
C–C bond cleavage
C–X bond cleavage
Nu attack
cycloaddition
rearrangement
electron transfer
radical attack
radical anion attack
hydrogen transfer
dimerization

Primary Product
π-C• + H+ or n-C• + H+
π-A• + H+
π-A• + B+ or π-A+ + B•
π-C• + C+
π-C• + X+
Nu-π-A-B•+
cycloaddition
rearrangement
π-A-B or π-A-B2+
R-π-A-B+
RA-π-A-B
H-π-A-B+
(π-A-B)22+

A
C
O, N, S, X
A
C
C
A
A
A
A
A
A
A
A

B
H
H
B
C
Si, Sn
B
B
B
B
B
B
B
B

Schmittle, M.; Burghart, A. Angew. Chem. Int. Ed. Engl. 1997, 36, 2550.

Thermochemical Cycle: Calculating pKaʼs

R-H

!G[pKa(RH•+)]

R• + H+

Eox(RH) !G[pKa(RH)]

Eox(R–)

R-H

R– + H+

  ΔG = –nF[Eox(RH) – Eox(R–)]

  pKa(RH•+) = ΔG/2.303RT

Nicholas, A. M. de P.; Arnold, D. R. Can. J. Chem. 1982, 60, 2165.

Deprotonation
  Acidity of a radical cation is severely increased compared to the neutral counterpart. H H H H H

H–B+

B

R-H
PhCH2CN
PHCH2SO2Ph
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