Polar Effects in Radical Reactions - Department of Chemistry ...

Polar Effects in Radical Reactions
Partha Nandi
H
H
H
Department of Chemistry
Michigan State University

Objectives and Motivations
• Origin of polar effects in organic radical reactions
• Improve the ability to design experiments
• Find new ways to expand the scope of known reaction
mechanisms to address reactivity-selectivity problems

Outline
• The polar-effect in traditional organic reaction mechanisms.
Is a polar-effect anticipated for radical reactions?
• Factors responsible for polar-effect in radicals
(1) Geometry and Orbital interactions
(2) Non-perfect synchronization of TS
(3) Solvent polarity and viscosity (“cage effect”)
• Specific examples and illustrations
• Conclusions and remarks

Outline
• Polar-effects in traditional organic reaction mechanisms
Is a polar-effect anticipated for radical reactions?
• Factors responsible for polar-effects in radicals
(1) Geometry and Orbital interaction effects
(2) Non-perfect synchronization of TS
(3) Solvent polarity and viscosity (“cage effect”)
• Specific examples and illustrations
• Conclusions and remarks

Polar Effects in Stereo-type Organic
Reaction Mechanisms?
R-X + Y(s) n
TS1
I. S N 1 Case II. S N 2 case
R
R (s) n
TS2
Less polar solvent
More polar solvent
R-Y + X(s) n
R-X + Y(s) n
X RY
X
Y
More polar solvent
Less polar solvent
R-Y + X(s) n
Same notion works for E1 and E2 mechanism
“Theoretical and Physical Principles of Organic Reactivity”, Pross, A. John Wiley & Sons, Inc. 1996.

Summary of the Solvent Polarity Effect
Reaction Type
R–X-->R +δ …X –δ
Y+ R–X-->Y + δ …R…X –δ
R–X +δ −−> R +δ …X –δ
Y – + R–X--> Y – δ …R…X –δ
Y – + R–X--> Y – δ …R…X +δ
Nature of
Charge Type
Separation
Separation
Dispersion
Dispersion
Annihilation
Effect of polarity
increase on
reaction rate
Large Increase
Large Increase
Small Decrease
Small Decrease
Large Decrease
“Theoretical and Physical Principles of Organic Reactivity”, Pross, A. John Wiley & Sons, Inc. 1996.

Outline
• Polar-effects in traditional organic reaction mechanisms a
brief overview. Is polar-effect anticipated for radical reactions
• Factors responsible for polar-effects in radicals
(1) Geometry and Orbital interaction effects
(2) Non-perfect synchronization of TS
(3) Solvent polarity and viscosity (“cage effect”)
• Specific examples and illustrations
• Conclusions and remarks

Geometry and Orbital Polarization:
E
H 3 M.O
(Not to scale)
Methyl Radical
π
π∗ π∗
σ*
π
nb
σ
MOs for Me
2p z 2px 2p y
C A.Os
“Theoretical and Physical Principles of Organic Reactivity”, Pross, A. John Wiley & Sons, Inc. 1996.

Energy Changes on Pyramidalizations
E
CHF 2
CH 3
CF 3
CH 2F
0 4 8 12
ω, degrees
C
α
ω=α−90
• ESR coupling
• Computations
• MO picture
Zheng, X.; Phillips, D. L. J. Phys. Chem. A. 2000, 104, 1030.
Cramer, C. J. J. Org. Chem., 1991, 56, 5229.
Rozum, I.; Tennyson, J. J. Phys. B. 2004, 37, 957.
• Polarization proportional
to dipole moment

MO for Pyramidalization
C X
X X
SOMO - filled lone pairs repulsion
pyramidalization helps to stabilize the SOMO
by the interaction of p-σ∗

Vibrational Polarization & Pyramidalization
• Geometrical flexibility of radicals can be rationalized
from MO.
• Vibrational polarizability
E
Vibrational polarization turning on
Pyramidal inversion
“Solvation of the Methyl radical and Its implication” Stratt, R. M.; Desjardins, S. G. J. Am. Chem.
Soc. 1984, 106, 256.

E
Role of Substituents on SOMO
"C" centered
SOMO
n or filled
σ orbital
X type
NR 2 , OR
Cl, Me, I
Z type
COR, CN,
SOR,
NO, NO 2
C type
C=CH2
Ph, etc
“Orbital Interaction Theory in Organic Chemistry’” 2001, 2nd Ed, Rauk, A. Wiley & sons. Inc.,
π∗
π∗
π

Outline
•Polar-effects in traditional organic reaction mechanisms a
brief overview. Is polar-effect anticipated for radical reactions?
• Factors responsible for polar-effects in radicals
(1) Geometry and Orbital interaction effects
(2) Non-perfect synchronization of TS
(3) Solvent polarity and viscosity (“cage effect”)
•Specific examples and illustrations
•Conclusions and remarks

Hammond’s Postulate and Limitations
E
• Hammond’s postulate - what does it tell us?
R
RC
P
R'
P'
• Instant idea on nature of TS
• Fails to give an accurate location of TS
• Does not consider multiple degrees of freedoms
along the reaction coordinate
• Modern version and extension of Hammond’s postulate - “The
Principle of Nonperfect Synchronization”
Bernasconi, C. F. Acc. Chem. Res. 1992, 25, 9.

Nonperfect Synchronization
• ------- hypothetical resonance
developments synchronous with
charge transfer
• ____ actual situation where
resonance development lags behind
charge transfer
• Smaller degree of resonance
stabilization of TS leads to a higher
barrier
“The Principle of Nonperfect Synchronization: More than a qualitative concept?”
Bernasconi, C. F. Acc. Chem. Res. 1992, 25, 9.

“Nonperfect Synchronization” &
“Imperfect TS”
• Reaction potential energy surface is multi-dimensional
• Bond breaking and formation
• Solvation and desolvation
• Delocalization and localization of charge
• Unequal progress at the TS, termed as “Imperfect TS”
“The Principle of Nonperfect Synchronization: More than a qualitative concept ?” Bernasconi, C. F.
Acc. Chem. Res. 1992, 25, 9.

Outline
• Polar-effects in traditional organic reaction mechanisms a
brief overview. Is polar-effect anticipated for radical reactions?
• Factors responsible for polar-effects in radicals
(1) Geometry and Orbital interaction effects
(2) Non-perfect Synchronization of TS
(3) Viscosity (“cage effect”) and solvent polarity
• Specific examples and illustrations
• Conclusions and remarks

6.5
k disp/ kdim
5.5
5
0
R + R
Viscosity Effects
Fate of diffusive cage pair
k diff
k -diff
for t- Bu
radical
for 2-Propyl radical
1 η (cP)
2 3
k dim
( R/R ) cage
k disp
R-R
R -H + R-H
•Variation of k disp/k dim with viscosity
•Shape matters: t- Bu radical an ellipsoid,
isopropyl “V” shaped
•Similar trend observed for polar radical
reactions
Shuch, H. H.; Fischer, H. Helv. Chim. Acta 1978, 61, 2463.
Minisci, F.; Vismara, E.; Fontana, F.; Morini, G.; Serravalle, M.; Giordano, C. J. Org. Chem. 1987, 52, 730.

Polar Solvent Decelerating the Rate
N
O
TEMPO
Entry
1.
2.
3.
4.
5.
6.
+
CH 2
Solvent
Solvent
n-pentane
Cyclohexane
Benzene
Chlorobenzene
Tetrahydrofuran
Acetonitrile
N
O
k T X 10 -7 , M -1 s -1
50 + 1.5
41 ± 2
18 + 1
17 + 2
23 + 3
9.5 + 0.7
Beckwith, A. L. J.; Bowry, V. W.; Ingold, K. U J. Am. Chem. Soc. 1992, 114, 4983.

Outline
• Polar-effects in traditional organic reaction mechanisms a
brief overview. How do we think about radicals?
• Factors responsible for polar-effects in radicals
(1) Solvent polarity effects
(2) Internal pressure and viscosity (“cage effect”)
(3) Non-perfect synchronization of TS
(4) Geometry and Orbital polarization
• Specific examples and illustrations
• Conclusions and remarks

Early Examples of Polar Radical Reactions
+ Br 2
hv
CH2 H Br
δ− δ+
Kim, S. S.; Choi, S. Y.; Kang, C. H. J. Am. Chem. Soc. 1985, 107, 4234.
O
R C
heat
O
O O R C O
OC(Me) 3
R + CO2 + tBuO
Barlett, P. D.; Hiatt, R. R. J. Am. Chem. Soc. 1958, 80, 1398.
Br

Specific Examples and Illustrations
(1) NO catalyzed oxidations
(2) MGM, ICM and MCM catalyzed isomerizations
(3) Silyl enol ether mediated 5-exo and 6-endo cyclizations,
and general approach to 4-allyl oxyl radical cyclizations

Aerobic Oxidation of Benzyl Alcohols
O
N
O
PhCHOH
O 2
by NHPI or PINO
O + H-CHOHPh
N
PhCH(OOH)OH
PhCHO
O
O
– δ + δ
O H CHOHPh
– OH
PhCH(O)OH
H atom abstraction
-H 2O
PhCH(OH) 2
-PhCHOH
Minisci, F.; Punta, C.; Recupero, F.; Fontana, F.; Pedulli, G. F. J. Org. Chem. 2002, 67, 2671.
Annunziatini, C.; Gerini, M. F.; Lanzalunga, O.; Lucarini, M. J. Org. Chem. 2004, 69, 3431.
O
N
O
O H

R 2
Structural Modifications in PINO
R 1
R 3
PINO
O
O
N
O
Entry
1
2
3
4
5
HO
+
R 1
H
F
H
MeO
MeO
R 2
MeO
R 2
MeOCO
H
H
H
R 1
R 3
NHPI
R 3
H
H
H
H
H
O
O
N
OH
ρ
+
-0.70
-0.69
-0.68
-0.60
-0.54
Annunziatini, C.; Gerini, M. F.; Lanzalunga, O.; Lucarini, M. J. Org. Chem. 2004, 69, 3431.
HO

Electronic Perturbation & Implications
log(k x/k H)
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
p-OMe
p-Me
m-Me
p-Cl
m-OMe
m-CN
m-Cl
p-CN
p-NO 2
m-NO 2
-0.4
-0.9 -0.5
σ
-0.1 0.3 0.7
+
X
OH
PINO
O
Hammet plot where
ρ= – 0.69
Minisci, F.; Recupero, F.; Cecchetto, A.; Gambarotti, C.; Punta, C.; Faletti, R.; Paganelli, R.;
Padulli, G. Eur. J. Org. Chem. 2004, 109.
X
H

Barrier Height for H Atom Abstraction
O
N
O
O + H-CHOHPh
N
>NO radical
H 2 NO
(O=CH)NHO
(O=CH)N(CH 3 ) O
(O=CH) 2 NO
O
O
B3LYP/
6-31G (kJ mol -1 )
103
78
83
49
– δ + δ
O H CHOHPh
B3LYP/
6-311++G
109
79
86
49
-PhCHOH
115
92
92
66
O
N
O
CCSD (T)ccpVDZ/B3LY
P/6-311++G
Hermans, I.; Vereecken, P. A.; Jacobs, A.; Peters, J. Chem. Commun., 2004, 1140.
O H

Potential Energy Surface Topology
E
30
20
10
0
-10
-20
-30
-40
ZPE corrected Energy
( kJmol -1 )
>NOH + CH 3OO
Pre-reaction complex
TS/Saddle point
Reaction Co-ordinate
Post reaction complex
(O=CH) 2 NO
>NO + CH 3OOH
Hermans, I.; Vereecken, P. A.; Jacobs, A.; Peters, J. Chem. Commun., 2004, 1140.

MO Picture for H Atom Abstraction
B B H-A
Energy (Not to scale)
H-A
B H A
B-H
B-H A
“The stability of alkyl radicals”, Tsang, W. J. Am. Chem. Soc., 1985, 107, 2872.
“Kinetics and Thermochemistry of CH 3 , C 2 H 5 , i- C 3 H 7 , Study of equilibrium of R + HBr”
Russel, J. J.; Seetula, J. A.; Gutman, D. J. Am. Chem. Soc., 1990, 112, 1347.
A

N
OH
69.6
Enthalpy Effect
N
OH
O
79.2 88.1
O-H Bond Dissociation
Energy (kcal/mol)
Minisci, F.; Recupero, F.; Cecchetto, A.; Gambarotti, C.; Punta, C.; Faletti, R.;
Paganelli, R.; Padulli, G. Eur. J. Org. Chem. 2004, 109-119.
O
N
O
OH

Fundamental Steps of TEMPO Catalyzed
2
R
R
N
O
R H + O X R + HOX
N
OH
N
O
N
O
O 2
R
R
N
Mn(II), Co(II)
H
Oxidations
OH
O R O O R O O
N
O
N
O
N
O
+
O + H +
Minisci, F.; Recupero, F.; Cecchetto, A.; Gambarotti, C.; Punta, C.; Faletti, R.; Paganelli, R.; Padulli, G.
Eur. J. Org. Chem. 2004, 109-119.
R
N
OH
N
OH
N
OR

Polar Non-radical Mechanism?
N
O
N
O
+ H
OH
+ H
OH
B
-H
N
HO O
N
O O
H
-BH
H B
N
OH
N
OH
+ O
+ O
Minisci, F.; Recupero, F.; Cecchetto, A.; Gambarotti, C.; Punta, C.; Faletti, R.; Paganelli, R.; Padulli, G.
Eur. J. Org. Chem. 2004, 109-119.

Specific Examples and Illustrations
(1) NO catalyzed oxidations
(2) MGM, MCM and ICM catalyzed isomerization
(3) Silyl enol ether mediated 5-exo and 6-endo cyclizations,
and general approach to 4-allyl-oxyl radical cyclizations.

Polar Radical Pathway of MGM
HOOC
COOH
2-methyleneglutaric acid
XOOC
COOX
X = H / R /
MGM or
Methyleneglutarate-mutase
XO 2C
CO 2X
HOOC
COOH
3-methylitaconic acid
XOOC
Newcomb, M; Miranda, N. J. Am. Chem. Soc. 2003, 125, 4080.
COOX

Potential Energy Profile Analysis
Energy (kcal/mol)
HOOC
Reactant
12.1
COOH
HO 2C
12.1
CO 2H
Reaction Co-ordinate
Tri-radical intermediate
8.6
12.1
Newcomb, M; Miranda, N. J. Am. Chem. Soc. 2003, 125, 4080.
HOOC
COOH

Apparent Paradox in MGM Catalyzed
Isomerization
• Rate Constant for cyclization estimated to be 2000 s -1
• Estimation is coupled with partioning of intermediate cyclopropyl
carbinyl radical, overall rate constant is estimated to be 10E-3 s -1 ,
• Unusual mechanism is possibly involved with polar effects operative
HO 2C
O 2C
Scheme A:
Scheme B:
CO 2H
CO 2
HO 2C
O 2C
CO 2H
CO 2
HO 2C
O 2C
CO 2H
CO 2
Newcomb, M; Miranda, N. J. Am. Chem. Soc. 2003, 125, 4080.

Catalytic Mechanism Devoid of 3-exo
O 2C
O 2C
H
H
CO 2
H
CO 2
Cyclization
Methylene-glutarate-Mutase
O 2C
H
CO 2
H
H
O 2C
O 2C
H
H
H
• Fragmentation results formation of a radical that is stabilized by a
through space polar-captodative orbital interactions.
H
H
CO 2
CO 2

Solvent Polarity Effect and Limited
G
Ph
Ph
Ph
O
S
Acid Catalysis
CoA G O
S CoA G S CoA
G= CO 2H (MCM catalyzed rearrangement)
G=CH 3 (Isobutyryl CoA Mutase catalyzed rearrangement)
PhSe
Ph
O
X=H/Me/SEt
X
X
hv
O
Bu 3SnH
Ph
Ph
O
Ph X
Ph
Daublain, P.; Horner, J. H.; Kuznetsov, A.; Newcomb, M. J. Am. Chem. Soc. 2004, 126, 5368.
X
O
Ph
Ph
Ph
Ph
O
O
X
X
O

Solvent Polarity and Acid Catalysis
logk
11
9
7
5
CF 3CH 2OH
MeCN
CH 2Cl 2
THF
Cyclohexane
Gasphase
E T (30)
AcOH
30 40 50 60 70 80
k obs X 10 -6
TFA(M)
CH 2Cl 2
Hexane
A: Observed rate constant for reaction of 1st intermediate
B: Rate constant of reactions of 1st intermediate in presence of
TFA in CH 2 Cl 2 and in hexane
Daublain, P.; Horner, J. H.; Kuznetsov, A.; Newcomb, M. J. Am. Chem. Soc. 2004, 126, 5368.

“Partial Protonated” radical
Ph
Ph
O
H O
X
O
CF 3
H 2N CH C
N
CH 2
NH
O
Histidine
• Catalytic role of His244 in the catalytic site of MCM catalyzed reaction
• Mechanistic reconsiderations - Role of surrounding water in nucleophilic
assistance
G
Nu OX
S
X= or H
CoA
G Nu
+
OX
CoA
S
Nu OX
OH
G SCoA
Daublain, P.; Horner, J. H.; Kuznetsov, A.; Newcomb, M. J. Am. Chem. Soc. 2004, 126, 5368.

Specific Examples and Illustrations
(1) NO catalyzed oxidations
(2) MGM catalyzed isomerization
(3) Silyl enol ether mediated 5-exo and 6-endo cyclizations,
and general approach to 4-allyl-oxyl radical cyclizations

Cyclization of Silyl-enol Ether Radical
Cations
OTBDMS OTBDMS
O
O
I
hv
hv, AIBN
6-endo
O O
6-endo
Bunte, J. O.; Heilmann, E. K.; Hein, B.; Mattay, J.; Eur. J. Org. Chem. 2004, 3535.
Curran, D. P.; Chang, C. T.; J. Org. Chem. 1989, 54, 3140.
+
O
5-exo

Selectivity in 5-exo or 6-endo Radical
Cyclization
Process
Rate Constant
5-exo
2.0E-6
+
6-endo
2.6E-6
• No apparent preference in the formation of a tertiary vs a
primary radical
• Can be explained in terms of assuming 5-exo process
reversible, besides suitable thermodynamics
Bunte, J. O.; Heilmann, E. K.; Hein, B.; Mattay, J.; Eur. J. Org. Chem. 2004, 3535.

Steps Involved in photo-cylization of
silyl-enol ether
SiR3 SiR
O O 3 Nu
hv
-e
O
Mesolytic
Si–O cleavage
H atom abstraction
O
O
6-endo
cyclization
Bunte, J. O.; Heilmann, E. K.; Hein, B.; Mattay, J.; Eur. J. Org. Chem. 2004, 3535.

Geometrical Changes Induced by One
Electron Oxidation
O Si
-e
d(C-C):1.42(+0.08) A
O Si
a(O-Si-C):100.8(-8.2)
d(C-O):1.28(-0.08) A
d(Si-O):1.80(+0.11) A
o
o
o
a(C-O-Si):136.1(+8.4)
• Weakened Si-O bond leads to a facile S N2- like substitution
induced by solvent or other nucleophile
Bunte, J. O.; Heilmann, E. K.; Hein, B.; Mattay, J.; Eur. J. Org. Chem. 2004, 3535.

Selectivities in Five vs Six-membered
Photo-cyclizations of Silyl-enol Ethers
Case I
Case II
H
+
H + H
OTMS H
O O
H
H H
H O
OTMS
31%
41% 28%
H + H
H
H
H
O H O
90% 10%
Bunte, J. O.; Heilmann, E. K.; Hein, B.; Mattay, J.; Eur. J. Org. Chem. 2004, 3535.

Calculated Energy Profile for Case I
Energy
kcal/mol
H
+
H + H
OTMS H
O O
H
H H
H O
H O
NOT TO SCALE
-1.15
+21-25
+19.72
H O
-2.49
0.0
O
0.0
+21-25
+17.9
H H O
-8.39
-9.40
Bunte, J. O.; Heilmann, E. K.; Hein, B.; Mattay, J.; Eur. J. Org. Chem. 2004, 3535.
H
H
O

Potential Energy Profile for Case II
Energy
H
O
OTMS
25
2.79 2.79
H
O
Numbers in kcal/mol
H + H
22
H
H
H
O H O
O
-7.46
19
0.0
23.5
-6.16
H
H O
O
H
H
Bunte, J. O.; Heilmann, E. K.; Hein, B.; Mattay, J.; Eur. J. Org. Chem. 2004, 3535.

How General Are These 5-exo vs
R 1 ,
H H
H Me
H C(Me) 3
H
Ph
Me H
R 2
Me Me
R1
Me C(Me) 3
6-endo Selectivities?
O Me
R 2
5-exo
5-exo/6-endo
98/2
69/31
46/54
7/93
98/2
82/18
37/63
R 1
O
R 2
H-Y
Y H-Y Y
R 1
• Substituents that increase in HOMO
coefficient along C-5 lead to more 6endo
product
• Substituents that can not cause the
increase in C-5 HOMO coefficient lead to
more 5-exo product
O
H
6-endo
Hartung, J.; Kneuer, R.; Rummey, C.; Bringmann, G. J. Am. Chem. Soc. 2004, 126, 12121.
R 2

O
Reaction Model for Analysis
R 2
alkoxy radical
R
O
2 O
R2 5-exo-chair
R 2
R
O
2
R 2
5-exo-boat
O
6-endo-chair
O
6-endo-boat
R 2
R 2
R 2
O
O
O
O R 2
Starting geometry TS cyclized radical
Hartung, J.; Kneuer, R.; Rummey, C.; Bringmann, G. J. Am. Chem. Soc. 2004, 126, 12121.
O
R 2

Ph
Cl
Ph
Cl
Ph
Cl
A True Ion or A Polar Radical ?
δ+ δ−
Cl CCl3 Cl CCl 3
CCl 4
Ph
Cl
Cl
Cl
Ph
Ph
+
Cl
Cl
hv Cl
N
N Ph
Dimerizations
Ph
Ph
Cl
Ph
Cl CCl 3
Cl Cl
Ph
CCl 4
Ph
Cl Cl
Ph
+ CCl3 Cl Cl
Cl Cl
+
Ph
Cl
Cl Cl
Ph
Cl Cl
Cl
+
Cl
Cl
Cl
Cl Cl
- Polar atom transfer
-Ylide
- Dissociative electron
transfer
Jones, M. B.; Jackson, J. E.; Soundararajan, N.; Platz, M. S. J. Am. Chem. Soc. 1988, 110, 5597.

Ph
Cl Cl CCl3 Ph
Cl Cl CCl3 Ph
Cl
δ+ δ-
CCl 4
Ph
Cl Cl CCl 3
Substituting Cl by CN:
1
3
2
Answer
Ph
Cl Cl
carbene
PhCCl
PhCCl
PhCCl
PhCCl
(pOMe)-
PhCCl
Cl donor
CCl 4
CCl 3 CN
CCl 2 (CN) 2
CCl 3 CN
CCl 3 CN
• Expected to retard the rate for ylide mechanism (2)
• Accelerate the rate for polar atom transfer (1 & 3)
solvent
MeCN
PhMe
PhMe
MeCN
PhMe
k (M -1 s -1 )
3.8E4
1.4E7
8.4E8
4.0E8
1.1E8
Jones, M. B.; Jackson, J. E.; Soundararajan, N.; Platz, M. S. J. Am. Chem. Soc. 1988, 110, 5597.
Jones, M. B.; Maloney, V. M.; Platz, M. S. J. Am. Chem. Soc. 1992, 114, 2163.

Conclusions & Remarks
1. Polar effect in radical reactions originates from a polar TS that is often
achieved through, electronic perturbation (from Orbital interactions,
medium polarity etc) Geometrical changes and Nonperfect
synchronization (NPS)
2. NO catalyzed oxidations, MGM, MCM and ICM catalyzed isomerizations,
and radical cyclizations were shown as representative examples where
polar effects were found to be operative
3. Distinguishing a polar radical TS and completely ionic TS can often be
challenging
4. Higher level computations can help in understanding polar effects in
radical reactions

Diels-Alder Reaction in Water
• Enhanced hydrophobic interaction in the TS
• Internal Pressure
• Higher polarizability of TS
• Increased Endo selectivity
• Problems of solubility & possible remedy by trying
cosolvents, constrained medium (zeolite, micellar
medium etc).
EDG
EWG
Bresslow, R.; Rideout, D. J. Am. Chem. Soc. 1980, 102, 7816.
Otto, S.; Engberts, J. B. Pure Appl. Chem., 2000, 7, 1365.
+
EDG
EWG

Acknowledgement
1. Parents - Asim K Nandi & Parbati Nandi
2. Dr. Jackson, Dr. Dye, Dr. Wagner, Dr. Wulff
3. Michael (SiGNa)
4. Labmates - Simona, Misha, Andrea, Jennifer,
Tulika, Karrie, and Kaushik
5. Friends - Sampa, Supriyo, Sanjukta, Aparajita,
Sam, Parul, Bani, Brad
6. Roommate - Neil

Typical Dipole-Moments of Radicals
Species
CH 3
CF 3
CH 3CH 2
CH2CHCH2 CH2CHCHOH ClCH 2CHCH 3
ClCHCHCH 2
HCC
N 3
HOO
http://www.colby.edu/chemistry/webmo/mointro.html
Dipole Moments in Debye
0.00141
0.00615
0.43937
0.07868
2.38577
2.87736
1.25684
0.80287
0.00103
2.52010

Pyramidalization Effects on Energy
and Dipole Moment
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Measure of Solvent Polarity
• Energy of charge transfer as a intramolecular (E T ) or
intermolecular (Z) process
Ph
Ph
E T Scale
Ph
N
O
Ph
Ph
I
COOCH 3
N
CH 2CH 3
Z Scale
hv
Reichardt, C. Chem. Rev. 1994, 94, 2319.
COOCH 3
N
CH 2CH 3
I

Catalytic Mechanism of MCM Catalyzed
Isomerization
CoAS
CoAS
O
O
H H
CO 2
H
CO 2
CoAS
CoAS
H
CO 2
O
CO 2
H
O
CoAS
H
CoAS
H
H
H
H
O
O
CO 2
H
CO 2
H

MO of NO
π∗ 2p
π 2p
E σ∗ 2s
NO
σ∗ 2p
σ 2p
σ 2s
σ∗1s
σ 1s
π∗ 2p
π 2p

NC
Captodative Effects
OMe
6
CN
12
OMe
16
Barrier of C-C rotation in kcal/mol

Factors Responsible For the Selectivities
• Rate constants for cyclizations
• Life-time of the radical intermediates
• Length of the new bond formed (Beckwith-Houk model)
• Endothermicity or exothermicity
• Steric or geometrical optimization
Bunte, J. O.; Heilmann, E. K.; Hein, B.; Mattay, J.; Eur. J. Org. Chem. 2004, 3535.

Calculated Charge and Spin Distribution
+0.29
-0.05
+0.18
+0.17
-0.04
O Si
TMS:+0.17
0.03
0.52
0.03
0.18
0.03
0.69
0.03
O Si O
0.15 0.01
0.20
charge spin spin
Bunte, J. O.; Heilmann, E. K.; Hein, B.; Mattay, J.; Eur. J. Org. Chem. 2004, 3535.
0.05