Mechanistic considerations for
N-X bond fragmentation ofpyridinium salts
2024.01.18
Literature Seminar
B4 Hiroki Kanamori

Contents
1. Examples of N-X bond fragmentation
2. Theoretical background
3. Model calculation

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Fragmentation of pyridinium ion via single
electron reduction
Pyridinium salts: Redox-Active Functional Group Transfer Reagents

SETfragmentation = homolysis or heterolysis

S. L. Rössler, et al. Angew. Chem. Int. Ed. 2020, 59, 9264–9280

3

N-C bond fragmentation via homolytic cleavage
Ni catalysis

photoredox catalysis

F. J. R. Klauck, et al. Angew. Chem. Int.
Ed. 2017, 56, 12336–12339
C. H. Basch, et al. J. Am. Chem. Soc. 2017, 139, 15, 5313–5316

EDA complex

J. Wu, et al. Angew. Chem. Int. Ed. 2019, 58, 5697–5701

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N-O bond fragmentation via homolytic cleavage
Radical translocation

Decarboxylation

J. W. Beatty, et al. Nat. Commun. 2015, 6, 7919
I. Kim, et al. Angew. Chem. Int. Ed. 2018, 57, 15517-15522

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Photocatalytic generation of pyridyl radical cation
via heterolytic cleavage
Heterolytic cleavage as a minor competing pathway
Nmajor pathwayF3C

OCF3NRuII

RuIIIO

OCF3RuII

CN

N

SETCN

OCF3RuIII

RuIII RuII

NCN
OCF3minor pathway

NCN

Select reagents undergo heterolytic cleavage

B. J. Jelier, et al. Angew. Chem.
Int. Ed. 2018, 57, 13784-13789

CN

High-energy trifloxy radical is not generated
S. L. Rössler, et al. Angew. Chem. Int. Ed. 2019, 58, 526–531 6

Postulated mechanism of pyridinium
fragmentation

Keywords• orbital mixing• three electron N∴X bond
S. L. Rössler, et al. Angew. Chem. Int. Ed. 2020, 59, 9264–9280

7Contents
1. Examples of N-X bond fragmentation
2. Theoretical background
3. Model calculation

8

Single electron reduction & fragmentation
eNN

OMe

OMe

pyridinium ionpyridyl radical

NOMepyridine & methoxy radical

Step 1
Single electron reduction
π* orbital of pyridinium ion receives an electron from donor.
Step 2
N-O bond fragmentation→ today’s topic

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How can we obtain pyridine ?
NN

OMe

OMe

out-of-plane electron

in-plane electron

NOMe

in-plane electron

An electron must be transferred from out-of-plane to in-plane before fragmentation.
→ Orbital mixingσ*
π* & σ* are orthogonal→ N-O bond bending

π*10

N-O bond bending

planar radical
no orbital mixing

N-O bond bending
orbital mixing
climbE
π* & σ* mixing lowers
energy11Representative configurations involved infragmentation
π*nπ*

σ*

12

Configuration changes in fragmentation

orbital mixing

π*

σ*σσ*

π*nnσ

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Short summary

π* orbital of pyridinium ion receives an electron from donor.
N-O bond bending is important for π* & σ* orbital mixing.
Orbital mixing is impossible when planar radical.

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Contents
1. Examples of N-X bond fragmentation
2. Theoretical background
3. Model calculation

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Model+N-O out of plane bending angle α
N-O bond length r

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Fragmentation is adiabatic
Non-adiabatic
⾮断熱adiabatic
断熱

smaller T.S. energy
barrier

https://whereareelectrons.blogspot.com/2017/07/blog-post.html 17

Energy correlation diagramnπ*
π*σσ* + σσ*2π*

Assumed a single mixed σ* state that changes character between π*σσ* & σσ*2 .
Fragmentation occurs from σσ*2 .

n218

E. D. Lorance, and I. R. Gould. J. Phys. Chem. A 2005, 109, 12, 2912–2919

Radical states formed by orbital mixing
extensive mixing

rN-O = rc

rN-O < rcOrbital mixing becomes more extensive withincreasing bond length & decreasing angle
small effect of orbital mixing
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E. D. Lorance, et al. J. Am. Chem. Soc. 2002, 124, 51, 15225–15238

Energy potential surface
A: diabatic surfaces

B: adiabatic surfaces

NEnergy ↑conical intersection→

NNbondangleOMebond length →bond length →20

E. D. Lorance and I. R. Gould. J. Phys. Chem. A 2005, 109, 12, 2912–2919

radical-NO2 : large α & small r →T.S.

Contour plots

bond angle ↑-Ph : small α & large r →← -OMe : barrierlessbond length →21

E. D. Lorance and I. R. Gould. J. Phys. Chem. A 2005, 109, 12, 2912–2919

Bent conformations of minimized radicalstructures
EWG: hardly bent
slow fragmentation

reaction rate:-NO2 < -Ph < -OMe

delocalizing group : bent

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E. D. Lorance, et al. J. Am. Chem. Soc. 2002, 124, 51, 15225–15238

The validation of model calculation
Calculation ↑DFT →Activation free energies
Model vs DFT : good agreement (left)
Model vs Experiment : good agreement (right)

Experiment →23

E. D. Lorance and I. R. Gould. J. Phys. Chem. A 2005, 109, 12, 2912–2919

Relationships of exothermicity with bond angle
and bond length

bond angle ↑bond length ↑exothermicity →(α, r) at T.S. are similar among different structures, while those at radical minimum are not.
Slow reaction has a radical minimum that is far apart from T.S. & it is an endothermic reaction.

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E. D. Lorance and I. R. Gould. J. Phys. Chem. A 2005, 109, 12, 2912–2919

Substitutional effect: general discussion
without substitution

with EDG

EDG raises the energies of configurations in which π* is occupied. (B)25

E. D. Lorance, W. H. Kramer, and I. R. Gould. J. Am. Chem. Soc. 2004, 126, 43, 14071–14078

Effect of electron-withdrawing groups
442
1
Z = EWG

decreased mixing

21

more stabilized

less stabilized
EWG makes fragmentation slower.
26

E. D. Lorance, et al. J. Am. Chem. Soc. 2002, 124, 51, 15225–15238

Effect of electron-donating groups
442
1
Z = EDG

increased mixing

2
1

more destabilized

less destabilized
EDG makes fragmentation faster.
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E. D. Lorance, et al. J. Am. Chem. Soc. 2002, 124, 51, 15225–15238

Effect of delocalizing groups
44
2
1
Z = Ph, allyl

2

unchanged mixing
1

more stabilized

less stabilized
Delocalizing group makes fragmentation slower.
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E. D. Lorance, et al. J. Am. Chem. Soc. 2002, 124, 51, 15225–15238

Summary+N-O bond bending is important for π* & σ* orbital mixing.
Fragmentation occurs from three electron N∴X bond.
Conformations at T.S. are similar among different pyridyl radicals.
Fast fragmentation reaction• occurs when pyridyl radical has EDG(s)• is highly exothermic• has radical minimum whose conformation is close to T.S.
4-OMe pyridyl radical undergoes barrierless fragmentation.

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