p.c. hiberty, laboratoire de chimie physique universit é de paris-sud, 91405 orsay, france
DESCRIPTION
Correlation Between the Diradical Character of 1,3-Dipoles and their Reactivity Toward Ethylene and Acetylene. P.C. Hiberty, Laboratoire de Chimie Physique Universit é de Paris-Sud, 91405 Orsay, France. An application of ab initio valence bond theory. Some families of 1,3-dipoles. - PowerPoint PPT PresentationTRANSCRIPT
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Correlation Between the Diradical Character of 1,3-Dipoles and their Reactivity Toward Ethylene and Acetylene
P.C. Hiberty, Laboratoire de Chimie PhysiqueUniversité de Paris-Sud, 91405 Orsay, France
An application of ab initio valence bond theory
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Some families of 1,3-dipoles
H2C
HN
Z Z
HN
H2C H2C
HN
Z Z = O, NH, CH2
HC N Z HC N Z HC N Z Z = O, NH, CH2
N N Z N N Z N N Z Z = O, NH, CH2
Azomethine betaines :
Nitrilium betaines :
Diazonium betaines :
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Dipolar cycloadditions
H2C
HN
Z Z
HN
H2C H2C
HN
Z Z = O, NH, CH2
Example: Azomethine betaines :
H2C CH2
+
HC CH
+
HC CH
H2C
HN
Z
H2C CH2
H2C
HN
Z
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Cycloaddition on ethylene (azomethine oxide) :
H2C
HN
O
H2C CH2+
H2C CH2
H2C
HN
O
0
10
20
30
-10
-20
-30
-40
-50
-60
E(kcal/mole)
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Cycloaddition on acetylene (azomethine oxide) :
We expect :
H2C
HN
O
H2C CH2+
H2C CH2
H2C
HN
O
HC CH+
HC CH
H2C
HN
O
0
10
20
30
-10
-20
-30
-40
-50
-60
E(kcal/mole)
15.1
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We observe :(accurate ab initio)
Cycloaddition on acetylene (azomethine oxide) :
H2C
HN
O
H2C CH2+
H2C CH2
H2C
HN
O
HC CH+
HC CH
H2C
HN
O
0
10
20
30
-10
-20
-30
-40
-50
-60
E(kcal/mole)
15.1
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Nitrilium ylide + ethylene or acetylene :
N N CH2
H2C CH2+
H2C CH2
NN
CH2
HC CH+
HC CH
NN
CH2
0
10
20
30
-10
-20
-30
-40
-50
-60
E(kcal/mole)
17.3
Still two identical barriers…
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N N NH
H2C CH2+
H2C CH2
NN
NH
HC CH+
HC CH
NN
NH
0
10
20
30
-10
-20
-30
-40
-50
-60
E(kcal/mole)
41.8
Nitrilium imine + ethylene or acetylene (aromatic product !)
Still two identical barriers, idem for the 9 1,3-dipoles
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Frontier Orbital Theory (FMO)
1,3-dipole Ethylene
HOMO
LUMO
HOMO
LUMO
Small HOMO-LUMO gap:=> Low reaction barrier
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Frontier Orbital Theory (FMO)
1,3-dipole Ethylene
HOMO
LUMO
HOMO
LUMO
Let’s focus on thedipole’s HOMO
The higher this HOMO, the lower the reaction barrier
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Frontier Orbital Theory (FMO)
E(HOMO)
∆H≠
Z = O Z = NH Z = CH2
diazonium betaines N-N-Z
From: DH Hess, KN Houk, JACS 2008, 130, 10187
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Frontier Orbital Theory (FMO)
E(HOMO)
∆H≠
Z = O Z = NH Z = CH2
diazonium betaines N-N-Z
From: DH Hess, KN Houk, JACS 2008, 130, 10187
azomethine Betaines H2C
NHZ
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Frontier Orbital Theory (FMO)
E(HOMO)
∆H≠
Z = O Z = NH Z = CH2
diazonium betaines N-N-Z
nitrilium betaines HN-N-Z
azomethine Betaines
From: DH Hess, KN Houk, JACS 2008, 130, 10187
H2CNH
Z
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Dipole-1,3
BV
HO
BV
HO
acetylene
FMO predicts higher barriers for reaction with acetylenethan with ethylene (at variance with experiment)
Now, taking the FMOs of the dipolarophile into account…
ethylene
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Geometries of transition states
Falsifies Hammond’s principle
H2C CH2
+HC CH
+
~ samegeometries
(the more exothermic the reaction,the earlier the transition state)
C
HN
Z
H2C CH2
C
HN
Z
HC CH
It looks like the kinetics depend on only one ofthe two reactants: the 1,3-dipole.
True for the9 reactions
(more exothermic)
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Ess and Houk’s Distortion/Interaction model
∆E≠ = ∆E≠ + ∆E≠
C
HN
Z
H2C CH2
d i
Distortion energies of the isolated fragments
Interaction energy of the fragments in the TS
∆E≠i
d
∆E≠ is found to be proportional to ∆E≠
Pending questions:
- why is the dipole’s distortion the same with C2H4 and C2H2? - why does’nt matter at all ???- How to relate the barrier to properties of reactants ?
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Try to see things from a different perspective=> Valence Bond theory
H2C
HN
O O
HN
H2C H2C
HN
O
What is the difference between 1,3-dipoles and other reactants ?
Combination of three resonance structures:
Not reactive Not reactive Reactive
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H2C
HN
O O
HN
H2C H2C
HN
O
Réactant’s geometry : 48.4% 18.0% 33.7%
Valence Bond theory
Ab initio calculation of the weights for each VB structure.Method: « breathing orbital valence bond »:
• Orbitals are pure atomic orbitals• each VB structure has its specific set of orbitals
• the diradical character is not marginal
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H2C
HN
O O
HN
H2C H2C
HN
O
Réactant’s geometry : 48.4% 18.0% 33.7%
Transition state’sgeometry :
41.7% 19.7% 38.6%
Valence Bond theory
• the diradical character is not marginal• It increases from reactant’s geometry to transition state’s one
Ab initio calculation of the weights for each VB structure.Method: « breathing orbital valence bond »:
• Orbitals are pure atomic orbitals• each VB structure has its specific set of orbitals
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Same calculations, for all 1,3-dipoles:
33.738.041.3
Geometry:
Reactants : Transition state:
21.326.526.3
21.625.127.7
H2C
HN
Z Z = O Z = NH Z = CH2
HC N Z Z = O Z= NHZ = CH2
N N Z Z = OZ = NHZ= CH2
38.643.246.6
32.135.735.4
31.634.436.4
What if the distortion would serve mainlyto increase the diradical character ?
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Proposed mechanism :
barrierless
critical diradical
character
1) dipole distortion 2) barrierless reaction
X ZY
X Y Z
X Y Z
X Y Z
a
b
c
XY
Z
E
• The 1,3-dipole distorts until it has reached a critical diradical character (definition to be specified)• It attacks !
This mechanism would explain why dipolarophile doesn’t matter
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If this mechanism is the right one, prediction :
33.738.041.3
21.326.526.3
21.625.127.7
H2C
HN
Z Z = O Z = NH Z = CH2
HC N Z Z = O Z= NHZ = CH2
N N Z Z = OZ = NHZ= CH2
The higher the diradical characterof the reactant, the easier the reaction
Probable correlationdiradical weight vs barrier
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Reaction barrier vs diradical weight of the 1,3-dipole :
Diradical weight
H2C
HN
ZZ = O Z = NH Z = CH2
N N Z
Z = OZ = NHZ= CH2
HC N ZZ = O Z= NHZ = CH2
(acetylene)(ethelène)
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Diradical weight
H2C
HN
ZZ = O Z = NH Z = CH2
N N Z
Z = OZ = NHZ= CH2
HC N ZZ = O Z= NHZ = CH2
(acetylene)(ethylene)
Reaction barrier vs diradical weight of the 1,3-dipole :
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Diradical weight
H2C
HN
ZZ = O Z = NH Z = CH2
N N Z
Z = OZ = NHZ= CH2
HC N ZZ = O Z= NHZ = CH2
(acetylene)(ethylene)
Reaction barrier vs diradical weight of the 1,3-dipole :
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An alternative measure of diradical character : Transition energy ∆E
H2C
HN
Z
H2C
HN
Z
Z
HN
H2C
H2C
HN
Z
Ground state
Pure diradical
∆E
Strong diradical character=> Small ∆E
Correlation between ∆Eand reaction barrier ?
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Reaction barrier vs transition energy ∆E :
Ground state pure diradical (kcal/mol)∆E
kcal/molR2 = 0.99
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Transition energies ∆E (reactants’ geometries)
État fondamental pur diradical (kcal/mole)∆E
0
50
100
150
200
250
Gap
diazonium betaines nitrilum betaines azomethine betaines
78
9
4
5 6
1
23
∆E ( ) rather scattered
∆E(Ground Diradical)
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Transition energies ∆E ( = transition states’ geometries)
État fondamental pur diradical (kcal/mole)∆E
0
50
100
150
200
250
Gap
diazonium betaines nitrilum betaines azomethine betaines
78
9
4
5 6
1
23
∆E ( ) much less scatteredThe dipoles have the same diradical character (∆E) once they have reached their TS geometry
∆E(Ground Diradical)
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Proposed mechanism :
barrierless
critical diradical
character
1) dipole distortion 2) barrierless reaction
X ZY
X Y Z
X Y Z
X Y Z
a
b
c
XY
Z
E
• linear 1,3-dipoles: ∆E = 91 ± 10 kcal/mol• bent 1,3-dipoles: ∆E = 76 ± 10 kcal/mol
∆E (ground state pure diradical)
Critical value for ∆E :
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« Give me insight, not numbers » (Charles Coulson)
• 1,3-dipoles are special reactants (violate ordinary laws)• The diradical character is the correlating quantity• A mechanism is proposed, consistent with accurate ab initio data• Reaction barriers estimated from reactants’ properties
1,3-dipolar cycloadditions
• VB is more insightful in this case • VB vs OM: describe reality with two different languages
Valence bond vs Molecular Orbitals; 2 exact theories
Valence bond, just seing things from a different perspective(as Prof. Keating would say…)
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Try to see things from a different perspective (Prof. Keating, Dead Poet Society)
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Try to see things from a different perspective (Prof. Keating, Dead Poet Society)
B. Braida, Laboratoire de Chimie Théorique,Université de Paris 6, 75252 Paris, France
C. Walter and B. Engels, Institut für Organische Chemie97074 Würzburg, Germany
Thanks to :