Upload
doanhuong
View
267
Download
2
Embed Size (px)
Citation preview
1,3-Dipolar Cycloadditions
Lawrence M. Wolf
Group Meeting
06-15
Cycloadditions: FMO and Beyond
Lawrence M. Wolf
Group Meeting
15-10
Various Dipoles and MO for allylFleming, I.
Hiberty, P. C. et al. J. Am. Chem. Soc. 1983, 105, 719
Isoelectronic w/ allyl anion
Fleming, I. Molecular Orbitals and Organic Reactions, Wiley, 2010
1,3-Dipolar Cycloaddition
Huisgen proposed a concerted process on the
basis of the following observations:
(1) lack of trappable intermediates
(2) stereospecificity
(3) substituent effects favoring two-bond processes
Firestone proposes a diradical based mechanism
that is consistent with all three of the above:
Both (1) and (2) can be rationalized on the basis of
the reactivitiy of the diradical a smaller barrier of the reactivitiy of the diradical a smaller barrier of
bond rotation to reaction
Huisgen
Huisgen
Firestone
Huisgen, R. Angew. Chem. Int. Ed. 1963, 2, 633.
Huisgen, R. Angew. Chem. Int. Ed. 1963, 2, 565.
Firestone, R. A. J. Org. Chem. 1968, 33, 2285.
Huisgen, R. J. Org. Chem. 1968, 33, 2291
Scrambling Experiment
Firestone, R. A.; Houk, K. N. et. al. J. Am. Chem. Soc. 1985, 107
7227.
Barrier of rotation for n-propyl
radical is ~0.1-0.4 kcal/mol.
Even if barrier for cyclization (kc)
is 0.1 kcal/mol, and barrier to
rotation (kr) is 0.4, ~27% of cis
product would form from trans
and vice versa.
R = D
107,
FMO Theory: Salem-Klopmann EquationFleming, I.
Klopman, G. J. Am. Chem. Soc. 1968, 90, 223
Salem, L. J. Am. Chem. Soc. 1968, 90 543
EquationFleming, I. Molecular Orbitals and Organic Reactions, Wiley, 2010
Sustmann, R. Tetrahedron Lett. 1971, 29, 2717
FMOs in 1,3-Dipolar Cycloadditions
A
HO controlled:
-Rate Acceleration
- Dipole
- R,X,C
- Dipolarophile
- C,Z
LU controlled:
-Rate Acceleration:
- Dipole:
- C,Z
- Dipolarophile:
- R,X,C
Houk, K. N.; Sims. J. et. al. J. Am. Chem. Soc. 1973, 95, 7287.
Houk K. N.; Sims, J. et. al. J. Am. Chem. Soc. 1973, 95, 7301
B
Cycloadditions
B D
+
C,Z,X
DB
A
Z,C
DB
A
DB
A
X
C,Z,X
HO
HO
LU
, 7287.
, 7301
Relative dipole FMO energies
Houk K. N.; Sims, J. et. al. J. Am. Chem. Soc. 1973, 95, 7301
Rates of Cycloadditions vs. HOMO
Sustmann, R.; Trill, H. Angew. Chem. Int. Ed. 1972, 11, 838
vs. HOMO-LUMO energies
Applying FMO to aryl azide [3+2]Fleming, I.
Houk
Fleming, I. Molecular Orbitals and Organic Reactions, Wiley, 2010
Houk K. N.; Sims, J. et. al. J. Am. Chem. Soc. 1973, 95, 7301
Other methods for Predicting Regiochemistry
Allopolarization:
- Differences in charge at potential reactive
sites
DFT based reactivity descriptors:
- ρ(r) (electron denisty)
- µ (chemical potential)
- η (chemical hardness)
HSAB Principle
Fukui, K. et. al. J. Chem. Phys. 1972, 20, 722
Ess. D. H.; Jones, G. O.; Houk, K. N. Adv. Synth. Catal. 2006, 348
Geerlings P. et. al. J. Phys. Org. Chem. 2003, 16, 615.
- η (chemical hardness)
- S (Softness)
- s(r) (Local softness)
- f(r) (Fukui function, reactivity
index)
Regiochemistry
Differences in charge at potential reactive
348, 2337.
A New Reactivity Model
- Slopes for reactions with ethylene and acetylene are
equivalent despite significantly different FMO energies
-
Houk, K. N.;
Houkl, K. N.;
(acetylene)
Slopes for reactions with ethylene and acetylene are
equivalent despite significantly different FMO energies
does not fit trend
, K. N.; Ess. D. H. J. Am. Chem. Soc. 2007, 129, 10646.
, K. N.; Ess. D. H. J. Am. Chem. Soc. 2008, 130, 10187.
Transition State Geometries
As χZ decreases, the earlier is the TS.
Houkl, K. N.;
decreases, the earlier is the TS
, K. N.; Ess. D. H. J. Am. Chem. Soc. 2008, 130, 10187.
Activation Energy → Distortion + Interaction Activation Energy → Distortion + Interaction
Distortion energy in [3+2]
Activation, Distortion, and Interaction energies
= +
Stepwise mechanism (diradical)
.
Houkl, K. N.;
H-L gap (stability):
oxide > imine > ylide
, K. N.; Ess. D. H. J. Am. Chem. Soc. 2008, 130, 10187.
Effect of Distortion on Energy Levels
.
Houkl, K. N.;
Effect of Distortion on Energy Levels
, K. N.; Ess. D. H. J. Am. Chem. Soc. 2008, 130, 10187.
Substituent Effects
(1) ∆Edǂ is nearly constant , and reactivity is
dictated by ∆Eiǂ
(2) ∆Eiǂ is similar across the series of alkenes,
and dipole, alkene, or both distortion
energies control reactivity
(3) Substituents influence both ∆Edǂ and ∆Ei
ǂ
.
Houkl, K. N.; Ess. D. H. J. Am. Chem. Soc. 2008, 130, 10187., 10187.
Strained multiple bonds
Houk, K. N.; Ess. D. H.; Jones, G. O.; Schoenebeck, F. J. Am. Chem. Soc. J. Am. Chem. Soc. 2009, 131, 8121
Cycloalkynes
Better fit with distortion model over strain-
release modelHouk, K. N.; Ess. D. H.; Jones, G. O.; Schoenebeck, F. J. Am. Chem. Soc. J. Am. Chem. Soc. 2009, 131, 8121
Problem
Houk, K. N.; Arndsten, B. A. J. Am. Chem. Soc. 2008, 130, 10052.
Houk, K. N.; Arndsten, B. A. et. al. J. Org. Chem. 2010, ASAP
, 10052.
, ASAP
Solution
Houk, K. N.; Arndsten, B. A. J. Am. Chem. Soc. 2008, 130, 10052.
Houk, K. N.; Arndsten, B. A. et. al. J. Org. Chem. 2010, ASAP
, 10052.
, ASAP
Nature of reactive state: Valence Bond Theory
3 Remaining Questions:
(1) Why does reactivity follow that of the H-L gap
dipoles but not dipolarophiles?
(2) Why does ∆Edǂ correlate with ∆Eǂ while this is not
general for many other reactions?
(3) Why are the geometries of the distorted dipoles so
strikingly similar whether added to ethylene or
acetylene?
Hiberty, P. C.;
Nature of reactive state: Valence Bond Theory
while this is not
Why are the geometries of the distorted dipoles so
Ψ: valence bond wavefunction of 1,3 dipole
WK: weight of HLSP functions (closed and
open shell representations Ф)
Example: Diazonium Betaines
=
=
=
, P. C.; Braida, B. et. al. J. Am. Chem. Soc. 2010, 132, 7631.
Proposal
=
=
=
ΨreactantHypothesis:
∆Eǂ = C1E(ζc) + C2
Hiberty, P. C.; Braida, B. et. al. J. Am. Chem. Soc. 2010,
(ααααa+ζa) •
(ααααb+ζb) •
(ααααc+ζc) •
ΨTS
′
′
′
ααααa•
ααααb•
ααααc•
, 132, 7631.
Weights of Limiting Valence Bond Representations
Hiberty, P. C.; Braida, B. et. al. J. Am. Chem. Soc. 2010,
Weights of Limiting Valence Bond Representations
, 132, 7631.
Correlations with bi-radical character
∆Eǂ
(kcal/mol)
weights of equilibrium geometries
∆E
(kcal/mol)
weights of equilibrium geometries
∆Eǂ
(kcal/mol)
E(ζc)
Hiberty, P. C.; Braida, B. et. al. J. Am. Chem. Soc. 2010,
radical character
E
(kcal/mol)
(i) A critical weight of the diradical
resonance form
Linear dipoles: 0.34
Bent dipoles: 0.426
(ii) A critical energy gap between
ground state and diradical
Linear dipoles: 91 kcal/mol
Bent dipoles: 76 kcal/mol
, 132, 7631.
Dynamics: Early and Late Barriers
Xu, L.; Doubledar, C. E.; Houk, K. N. J. Am. Chem. Soc. 2010, 132, 3029, 3029
Transition Vectors and Bending Mode Contributions
Xu, L.; Doubledar, C. E.; Houk, K. N. J. Am. Chem. Soc. 2010, 132, 3029
Transition Vectors and Bending Mode Contributions
Start from TS with 0.6
kcal in the reactant
direction
, 3029
Energy Partitioning
Acetylene
Ethylene
Xu, L.; Doubledar, C. E.; Houk, K. N. J. Am. Chem. Soc. 2010, 132, 3029
Erc = 0.6 kcal/mol
, 3029
Trajectory Analysis
Xu, L.; Doubledar
Decrease in amplitude is in
accord with %V to Eavl
Doubledar, C. E.; Houk, K. N. J. Am. Chem. Soc. 2010, 132, 3029
Following a TrajectoryXu, L.; , L.; Doubledar, C. E.; Houk, K. N. J. Am. Chem. Soc. 2010, 132, 3029
Snapshots continued
Xu, L.;
Importance of dipole vibration is Importance of dipole vibration is
apparent for dipoles 1-3
Importance of rotation is apparent
for dipoles 4-6
, L.; Doubledar, C. E.; Houk, K. N. J. Am. Chem. Soc. 2010, 132, 3029
Time Gaps between bond formations (concerted Xu, L.;
Gaps between bond formations (concerted vs stepwise), L.; Doubledar, C. E.; Houk, K. N. J. Am. Chem. Soc. 2010, 132, 3029
C-C, C-N, C-O
Vibrational
period=30 fs
Inconsistent w/ a
cyclodiradical
intermediate
Conclusions
• The energy required for distortion was discovered to correlate well with activation energy while not being general among other reactions
• The correlation held constant when FMO theory was insufficient (vs alkyne; nitrilium ylide)
• Alternatively, the reactivity can be explained in terms of the difference in • Alternatively, the reactivity can be explained in terms of the difference in energy between the ground state and pure
• The diradicaloid model can be interpreted as being a special case of the distortion model
• Dynamics simulations revealed the important contribution of energy to the total distortion energy and the relative roles of all three forms of energy (vib., rot., trans.)
The energy required for distortion was discovered to correlate well with activation energy while not being general among other reactions
The correlation held constant when FMO theory was insufficient (alkene
Alternatively, the reactivity can be explained in terms of the difference in Alternatively, the reactivity can be explained in terms of the difference in energy between the ground state and pure diradical reactive state (TS)
model can be interpreted as being a special case of the
Dynamics simulations revealed the important contribution of vibrationalenergy to the total distortion energy and the relative roles of all three