Radical Cations: Generation, Reactivity, and Stability

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