Thermal isomerizations of ketenimines to nitriles: Evaluations of sigma- dot (σ·) constants for spin-delocalizations

Article abstract of DOI:10.1021/jo991186r

Rate constants (k(Y)) of the isomerizations of 11 diphenyl N- (substituted benzyl) ketenimines were measured at 40, 50, 60, and 70°C. Activation parameters ΔH((+))(Y) and ΔS((+))(Y) were obtained using the Eyring equation. The relative rates (k(Y)/k(H)) were fitted into Hammett single correlations (log k(Y)/k(H) = ρσ and log k(Y)/k(H) = ρ·σ·). The single correlations have been compared with Hammett dual correlations (log k(Y)/k(H) = ρσ + ρ·σ·). Separate treatments of para and meta substituents yielded even better correlations. Para substituents control the rates through spin-delocalizations and inductive effects. The former outweighs the latter when the latter exerts a modest but distinct influence on the rates. On the other hand, inductive effects are the 'major' or the sole interactions triggered by meta substituents.

Full text of DOI:10.1021/jo991186r

J . Org. Chem. 2000, 65, 2919-2923
2919
Th er m a l Isom er iza tion s of Keten im in es to Nitr iles: Eva lu a tion s of
Sigm a -Dot (σ•) Con sta n ts for Sp in -Deloca liza tion s
Sung Soo Kim,* Yu Zhu,† and Kwang Ho Lee
Department of Chemistry and Center for Chemical Dynamics, Inha University,
Inchon 402-751, South Korea
Received J uly 26, 1999
Rate constants (kY) of the isomerizations of 11 diphenyl N-(substituted benzyl) ketenimines were
measured at 40, 50, 60, and 70 °C. Activation parameters ∆HqY and ∆SqY were obtained using the
Eyring equation. The relative rates (kY/kH) were fitted into Hammett single correlations (log kY/kH
) Fσ and log kY/kH ) F•σ•). The single correlations have been compared with Hammett dual
correlations (log kY/kH ) Fσ + F•σ• ). Separate treatments of para and meta substituents yielded
even better correlations. Para substituents control the rates through spin-delocalizations and
inductive effects. The former outweighs the latter when the latter exerts a modest but distinct
influence on the rates. On the other hand, inductive effects are the “major” or the sole interactions
triggered by meta substituents.
In tr od u ction
devised to address rates of the radical reactions. The σ•
scales formulated by Arnold,14 Creary,15 and J iang and
J i16 could be the least contaminated with polar effects
and thereby used as a measure of capacity of spin-
delocalizations. Arnold14 measured R-hydrogen hyperfine
coupling constants of substituted benzyl radicals for
definition of σ•R. Creary15 set up the σ•c scale by measuring
rates of rearrangements of methylenearylcyclopropanes.
J iang and J i16 derived σ•jj from rates of thermal cyclo-
addition of R,â,â-trifluorostyrenes.
The concept of Hammett substituent constants for
radical reactions (σ•) was introduced for the first time by
Streitwieser and Perrin1 as an index of spin-delocaliza-
tions. However, the polar effects2-5 caused by substitu-
ents have been found to outweigh the spin-delocalization
effects in determining the rates of numerous radical
reactions. Furthermore, separation of the latter from the
former engenders extreme difficulty. Recently, substitu-
ent effects on rates of radical reactions6-11 have attracted
considerable interest. Numerous σ• scales12-16 have been
Thermal isomerizations of ketenimines to nitriles17-19
are known to take place via cage recombinations of
radical pairs. Subsequently,20 the rates of isomerizations
at 60 °C of various diphenyl N-(substituted benzyl)-
ketenimines were measured that follow excellent first-
order kinetics. The rates are better fitted with Hammett
dual correlations indicating concurrent contributions of
1 and 2 to the transition state (TS). Para substituents
are capable of spin-delocalizations thereby favoring 1 as
a major contributing structure. However the contribu-
tions of 2 become important via inductive effects with
meta substituents. These are shown in Scheme 1. We
wish to now report the temperature dependence of the
substituent effects of the isomerizations.
† A visiting scholar from Shanghi Institute of Organic Chemistry
under the program of the Brain Pool (1996-1998) supported by the
Korean Federation of Science and Technology Societies.
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J . J . Am. Chem. Soc. 1994, 116, 2754. (c) Kim, S. S. Pure Appl. Chem.
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Chem. 1998, 63, 1185. (f) Kim, S. S.; Tuchkin, A. J . Org. Chem. 1999,
64, 3821. (g) Kim, S. S.; Kim, C. S. J . Org. Chem. 1999, 64, 9261.
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Z., Mere´ny, R., Ed.; NATO ASI Series C; Reidel: Dordrecht, The
Netherlands, 1986; Vol. 189.
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1979. (b) Bordwell, F. G.; Zhang, X. J . Org. Chem. 1990, 55, 6078. (c)
Bordwell, F. G.; Harrelson, J . A., J r. J . Org. Chem. 1991, 56, 4448. (d)
Bordwell, F. G.; Cheng, J .-P.; Satish, A. V.; Twyman, C. L. J . Org.
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D.; Fried, H. E. J . Org. Chem. 1993, 58, 3060.
Resu lts a n d Discu ssion
High ligh ts of Kin etic Da ta . Preparations of numer-
ous substituted ketenimines followed the previous
method.20 All the NMR spectra of ketenimines and
nitriles match their structures. Ketenimines were dis-
solved in CDCl3 and placed into capped NMR tubes that
(8) (a) Pasto, D. J .; Krasnansky, R.; Zercher, C. J . Org. Chem. 1987,
52, 3062. (b) Pasto, D. J . J . Am. Chem. Soc. 1988, 110, 8164.
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K. J . Org. Chem. 1996, 61, 746. (b) Wu, Y.-D.; Wong, C.-L. J . Org.
Chem. 1995, 60, 821.
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Org. Chem. 1997, 62, 1419.
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Org. Chem. 1987, 52, 3254
(16) (a) J iang, X.-K.; J i, G.-Z. J . Org. Chem. 1992, 57, 6051. (b) J iang,
X.-K. Acc. Chem. Res. 1997, 30, 283.
(17) Lee, K.-W.; Horowitz, N.; Ware, J .; Singer, L. A. J . Am. Chem.
Soc. 1977, 99, 2622.
(11) Zhang, X.-M. J . Org. Chem. 1998, 63, 3590.
(12) Agirbas, H.; J ackson, R. A. J . Chem. Soc., Perkin Trans. 2 1983,
739.
(13) Fisher, T. H.; Dershem, S. M.; Prewitt, S. M. J . Org. Chem.
1990, 55, 1040.
(14) Dust, J . M.; Arnold, D. R. J . Am. Chem. Soc. 1983, 105, 1221.
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2285.
(19) Clarke, L. F.; Hegarty, A. F.; O’Neill, P. J . Org. Chem. 1992,
57, 362.
(20) Kim, S. S.; Liu, B.; Park, C. H.; Lee, K. H. J . Org. Chem. 1998,
63, 1571.
10.1021/jo991186r CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/26/2000

2920 J . Org. Chem., Vol. 65, No. 10, 2000
Kim et al.
Sch em e 1
were later immersed in a constant-temperature bath for
thermal isomerizations. The rates of isomerizations were
measured by monitoring variations of the peaks of
benzylic protons of the ketenimines and their nitrile
counterparts. The former gradually diminished and the
latter increased as the reactions progressed. The rates
followed excellent first-order kinetics with ln(Cat + Cbt)/
Cat ) kYt where Cat and Cbt are the peak area of benzylic
protons of the ketenimine and the nitrile, respectively,
at time t. Absolute rate constants of isomerizations (kY)
were obtained at 40, 50, 60, and 70 °C. Activation
parameters (∆Hq and ∆SqY) have been derived from
Y
Eyring plot21 (Table 1). The relative rates (kY/kH) were
fitted with a single parameter using either σ22 (entries
1-3) or σ• 14-16 (entries 4-6, 10-12, 16-18). Entries 7-9,
13-15, and 19-21 employed both σ22 and σ• 14-16 for dual
correlations. Furthermore, the correlations have been
separately treated as with meta/para substituents (en-
tries 1, 4, 7, 10, 13, 16, 19), para substituents (entries 2,
5, 8, 11, 14, 17, 20), and meta substituents (entries 3, 6,
9, 12, 15, 18, 21). The results at 40 °C are presented as
entries 1-21 in Table 2. Similar data at 50, 60, and 70
°C corresponding to Table 2 are given in the Supporting
Information.
The variations of rates in Table 1 stay in the range of
less than 3-fold. Such modest substituent effects engen-
der differential free energies of activation ∆∆G*Y-H of
less than 1 kcal mol-1. On the other hand, a larger
magnitude of the same entity (∆∆G*Y-H > 1 kcal mol-1
)
was observed for the radical reactions involving polar
TS.5 The modest alterations of the rates suggest that
substituent effects may be mainly derived from other
than polar interactions via delocalizations of positive
charge.5 Para substituents accelerate the rates (kY)
regardless of their signs of σ (σ > 0 or σ < 0) indicating
minor role of inductive effects. Meta substituents reveal
even weaker substituent effects on the rates.
Ha m m ett Sin gle Cor r ela tion s. Entries 1-3 cor-
respond to Figures 1-3, respectively. The Hammett
correlations are derived from inductive effects. Figures
1 and 2 exhibit shotgun-type scatter of the points. A
satisfactory straight line is recognized only with Figure
3 that reveals Hammett F ) 0.602 (r ) 0.958). Hydrogen
abstractions from substituted toluenes by tert-butyl radi-
cal23 also gave a positive value of F ) 0.50 (r ) 0.968).
Tri-n-butyltin radical5d abstracts cyanide from substi-
tuted benzyl isocyanides with F ) 0.70 (r ) 0.999). Both
(21) Eyring, H. J . Chem. Phys. 1935, 3, 107.
(22) Exner, O. In Correlation Analysis in Chemistry; Chapman, N.
B., Shorter, J ., Ed.; Plenum Press: New York, 1978; Chapter 10.
(23) Pryor, W. A.; Tang, F. Y.; Tang, R. H.; Church, D. F. J . Am.
Chem. Soc. 1982, 104, 2885.

Thermal Isomerizations of Ketenimines to Nitriles
J . Org. Chem., Vol. 65, No. 10, 2000 2921
Ta ble 2. Ha m m ett Sin gle a n d Du a l Cor r ela tion s of th e Isom er iza tion s a t 40 °C
entry
approacha
Hammett22
σb
σ•c
F
F•
|F•/F|
rd
ne
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
σp/σm
σp
σm
0.137
0.172
0.602
0.181
0.209
0.958
0.652
0.748
-0.805
0.928
0.950
0.978
0.599
0.761
-0.535
0.888
0.942
0.996
0.724
0.936
-0.039
0.808
0.972
0.974
11
6
6
10
6
5
10
6
5
8
5
4
8
5
4
11
6
6
11
6
6
Creary15
σ•p/σ•m
σ•p
1.118
1.205
-3.102
1.886
1.650
2.693
5.668
7.908
-7.892
11.20
12.35
0.178
1.134
1.446
-0.074
1.266
1.468
-0.330
σ•m
Hammett22/Creary15
Arnold14
σp/σm
σp
σm
σ•p/σ•m
σ•p
0.648
0.534
1.012
2.91
3.09
2.66
σ•m
σ•p/σ•m
σ•p
σ•m
Hammett22/Arnold14
J iang and J i16
σp/σm
σp
σm
σ•p/σ•m
σ•p
0.592
0.595
0.574
18.92
20.76
0.31
σ•m
σ•p/σ•m
σ•p
σ•m
Hammett22/J iang and J i16
σp/σm
σp
σm
σ•p/σ•m
σ•p
0.277
0.215
0.618
4.57
6.83
0.53
σ•m
a
b
The authors have originally defined the substituent constants. σ is the Hammett substituent constant for the inductive effects taken
from ref 22. c σ• has been defined by three different scales14-16 that measure the capacity of spin-delocalizations triggered by substituents.
Correlation coefficients. e Number of the points (e.g., substituents) forming the correlations.
d
F igu r e 3. log kY/kH ) Fσm.
F igu r e 1. log kY/kH ) Fσm/p
.
F igu r e 4. log kY/kH ) F•σ•m/p
.
F igu r e 2. log kY/kH ) Fσp.
with m-Cl(m-Br), m-F, and H showing the weight of
inductive effects on the rates illustrated by 2.
abstraction reactions by nucleophilic radicals5d,23 main-
tain polar TS structure with the partial negative charge
being localized24 on the benzylic carbon atom. The cor-
relation coefficient r ) 0.958 could be considered fair
when comparing with r ) 0.96823 and r ) 0.999.5d
However an excellent linearity (r ) 0.999) is attained
Entries 4-6 utilize σ• defined by Creary15 while entries
10-12 and 16-18 indicate the correlations with σ• scales
invented by Arnold14 and by J iang and J i,16 respectively.
Entries 4-6, 10-12 and 16-18 disclose surprisingly
parallel trend of variations of F• and r. Entries 4-6
correspond to Figures 4-6, respectively. Figure 4 shows
a poor linear relation (r ) 0.652) that could be, however,
better than r ) 0.181 of Figure 1. Such poor improvement
nonetheless suggests that spin-delocalization could be
somewhat more important than inductive effects. The
Hammett plot was further reformed in Figure 5 (r )
(24) Deprotonations of nitroalkanes traverse “carbanionic” TS struc-
ture where the negative charge is also localized on the carbon atom
(Bordwell, F. G.; Boyle, W. J ., J r.; Yee, K. C. J . Am. Chem. Soc. 1970,
92, 5926.). The conjugation may not occur when the reorganization
(from sp3 to sp2) energy can be hardly compensated by delocalization
of negative charge on carbanion (Impastato, F. J .; Walborsky, H. M.
J . Am. Chem. Soc. 1962, 84, 4838).

2922 J . Org. Chem., Vol. 65, No. 10, 2000
Kim et al.
F igu r e 5. log kY/kH ) F•σ•p.
F igu r e 7. log kY/kH ) 0.648σm/p + 1.886σ•m/p
.
F igu r e 6. log kY/kH ) F•σ•m
.
0.748) when only para substituents were engaged sug-
gesting promotion of spin-delocalizations via 1. However,
meta substituents seem to be incapable of spin-delocal-
izations to break down the correlations (Figure 6). The
values of σ• of electron-withdrawing meta substituents
devised by Fisher,13 Arnold,14 and Creary15 carry negative
F igu r e 8. log kY/kH ) 0.534σp + 1.650σ•p.
•
signs (σm < 0). Free radicals are in principle electron-
deficient groups and can be destabilized7,8,11,13 by the
inductive effects of electron-attracting groups.
Ha m m ett Du a l Cor r ela tion s. Entries 7-9, 13-15,
and 19-21 use dual substituent constants incorporating
inductive effects (σ) and spin-delocalizations (σ•) into log
kY/kH ) Fσ + F•σ•.25 Once again, a similar tendency of
variations of F, F•, and r can be deciphered in the three
groups of entries, respectively. Entries 7-9 are il-
lustrated in Figures 7- 9, respectively. Figure 7 shows
improved Hammett correlations (r ) 0.928) compared to
Figures 1 (r ) 0.181) and 4 (r ) 0.652). The linear
correlations are further improved when para and meta
substituents were separately treated. Accordingly, Fig-
ures 8 and 9 exhibit good Hammett correlations with r
) 0.950 and r ) 0.978, respectively.
p-Cl and p-Br exhibit faster rates than p-OMe, al-
though the former carry much smaller spin-delocalization
constants (σc• ) 0.12 for p-Cl and 0.13 for p-Br)15 than
the latter (σc• ) 0.24).15 This “anomaly” can be explained
by consideration of participation of inductive effects of
the substituents (σ ) 0.24 for p-Cl, 0.26 for p-Br, and
-0.27 for p-OMe).22 Figure 5 shows a negative deviation
for p-Me (σc• ) 0.11; σ ) -0.14)15,22 and a positive one
F igu r e 9. log kY/kH ) 1.012σm + 2.693σ•m
.
for p-F (σc• ) -0.08; σ ) 0.15).15,22 The directions of
deviation match sign of σ of the substituents indicating
again the secondary role of inductive effects. Therefore,
rates triggered by para substituents are determined
mainly by contribution of 1, which is supplemented partly
with role of 2.
m-Cl, m-Br, and m-F share very comparable rates that
are ca. 1.7 times larger than that for H. The three
substituents retain a negative spin-delocalization con-
stant (e.g., σ• ) -0.04 for m-Cl and σ• ) -0.05 for m-F).26
The rate enhancements could be explained only when
inductive effects (σ ) 0.37 for m-Cl and m-Br, and σ )
0.34 for m-F) eminently outweigh spin-delocalizations.
The points for m-Cl(m-Br), m-F, and H show better
(25) The dual formalism appears similar to Yukawa-Tsuno equation,
log(kY/kH) ) Fσ + Fr(σ+ - σ). The equation expresses LFER derived
from normal and extra resonance effects. Fr/F is a measure of relative
contributions of polar and resonance effects, that could be equivalent
to F•/F in our reactions. (a) Tsuno, Y.; Ibata, T.; Yukawa, Y. Bull. Chem.
Soc. J pn. 1959, 32, 960, 965, and 971. (b) Young, P. R.; J encks, W. P.
J . Am. Chem. Soc. 1977, 99, 8238; 1979, 101, 3288 and 4678.
(26) Spin-delocalization constant (σ•) of m-Br has not been reported.
However σ• of m-Br could be comparable in magnitude with that of
m-Cl.

Thermal Isomerizations of Ketenimines to Nitriles
J . Org. Chem., Vol. 65, No. 10, 2000 2923
Ta ble 3. Activa tion P a r a m eter s of Hom olysis of
In itia tor s
Con clu sion
σ•R, σ•c, and σ•jj are derived from different scales whose
values are not identical with each other (e.g., σ•R ) 0.017,
σ•c ) 0.12, and σ•jj ) 0.22 for p-Cl). Nonetheless, the
correlations utilizing three different parameters give
remarkably parallel tendency with Hammett single and
dual equations. The parallelism supports the σ• as bona
fide spin-delocalization constants. Spin-delocalizations
and inductive effects cooperate to determine the rates.
The former is favored by para substituents and the latter
by meta substituents. m-Cl, m-Br, and m-F exert solely
inductive effects to control the rates. Perturbations of the
activation parameters (∆H*Y and ∆S*Y ) by substituents
are too small to elucidate TS structure. However, the
negative values of ∆S*Y may be due to restriction of bond
rotation in the TS.
initiators
∆Hq (kcal mol-1
)
∆Sq (eu)
a
c
C6H5C(O)OOC(CH3)3
(CH3)3COOC(CH3)3
CH3C(O)OO(O)CCH3
37.5
37.8
31.3
34.3
25.4
16.1
13.8
11.6
5.6
b
d
C6H5CH2NdNCH2C6H5
(C6H5)2CdCdNCH2C6H5
-1.1
a
b
Taken from ref 30. Taken from ref 31. c Taken from ref 32.
d
Taken from ref 28.
linearity (r ) 0.999) than the case including all the
substituents (r ) 0.958) in Figure 3. This may suggest
contribution of 2 as a sole element determining the rates.
On the contrary, m-Me (σ• ) 0.03 and σ ) -0.06) and
m-OMe (σ• ) -0.02 and σ ) 0.12) mark slightly smaller
rates than rate for H. If inductive effects prevailing in 2
were implicated, m-OMe should show somewhat faster
rates than m-Me. However the isomerization rates of
m-OMe are always slightly slower than those of m-Me.
Accordingly, the role of substituent effects is reversed
when spin-delocalizations occurring in 1 emerge to influ-
ence the rates. The relative importance of 1 and 2 can
Exp er im en ta l Section
Ma ter ia ls. The reagents are commercially available and
were purified, if necessary, according to standard procedures.34
The preparations of diphenyl N-(substituted benzyl)keten-
imines were described in a previous paper.20
1H NMR Da t a for Ket en im in es (CDCl3 w it h 0.03%
TMS). Diphenyl N-(m-bromobenzyl) ketenimine gave 1H NMR:
7.0-7.8 (m, 14H), 4.8 (s, 2H). The other ketenimines were
described previously.20
27
be also verified from values of |F•/F| in Table 2. Para
substituents (entries 8, 14, 20) show much larger values
of |F•/F| than corresponding meta substituents (entries 9,
15, 21).
Kin etic P r oced u r es. Measurements of rates of the isomer-
izations are analogous to those previously described.20 Solu-
tions of ketenimines (0.3 M in CDCl3) were placed in tightly
capped NMR tubes. The tubes were then placed in a constant-
temperature bath for the thermal reactions. At several inter-
vals the tubes undergoing thermolysis were quenched in liquid
nitrogen, thawed, and transferred to the cavity of a Varian
Gemini 2000 NMR spectrometer for 1H NMR measurements.
A 1:1 correspondence was observed for the integrated peak
areas for benzylic protons of ketenimines (δ: 4.7-4.8) and
corresponding nitriles (δ: 3.6-3.8) indicating the amount of
a ketenimine reacted was equal to quantity of a nitrile
produced. Peak areas of the benzylic protons were used for
concentrations of a ketenimine and its nitrile isomer. Therefore
the rates can be measured with ln(Cat + Cbt)/Cat ) kYt. Cat and
Cbt are the magnitudes of the benzylic protons of a ketenimine
and corresponding nitrile at reaction time t, respectively. First-
order rate constants (kY) were determined by standard least-
squares method. Correlation coefficients for the linearity are
greater than r ) 0.9990.
Activa tion P a r a m eter s Mir r or in g TS Str u ctu r es.
Enthalpies of activation show very little variation and
stay in the range of ∆Hq ) 25.0 kcal mol-1 (Table 1).
Y
Thermolysis of diphenylazomethanes28 and thermal re-
arrangements of methylenecyclopropanes29 involve “ho-
molytic” TS structures resembling 1. Both reactions28,29
also showed minor substituent effects on the magnitude
of ∆HqY. Homolysis of several initiators30-32 usually shows
positive large entropy of activation (∆Sq ) (Table 3).
Y
However the reactions of the ketenimines reveal ∆Sq
<
Y
0. This may be due to hindered rotations of phenyl ring33
taking place by formations of double bonds through
conjugation occurring in 1 and 2. m-Cl, m-Br, and m-F
show slightly more negative values of ∆Sq than para
Y
substituents. The positive charge in 2 could be more
effectively delocalized into the two phenyl groups com-
pared to spin-delocalizations occurring in 1. The rotations
of phenyl rings could be more vigorously restricted in 2
than in case of 1. Similar effects28 were also observed in
the thermolysis of benzyl diazenes.
Ack n ow led gm en t. We warmly thank the Korea
Research Foundation for the financial support made in
the program year of 1998 (Grant No. 98-015-D00178).
Mr. Hwan Kyu J ung and Min Soo Kim offered technical
assistance. The authors are heavily indebted to the
reviewers for the enlightening comments that remark-
ably improve the scientific quality of the manuscript.
(27) Magnitude of |F•/F| may deliver only qualitative significance
because σ• and σ were formulated with different scales.
(28) Bandlish, B. K.; Garner, A. W.; Hodges, M. L.; Timberlake, J .
W. J . Am. Chem. Soc. 1975, 97, 5856.
(29) Creary, X.; Engel, P. S.; Kavaluskas, N.; Pan, L.; Wolf, A. J .
Org. Chem. 1999, 64, 5634.
(30) Blomquist, A. T.; Berstein, I. A. J . Am. Chem. Soc. 1951, 73,
5546.
(31) Raley, J . H.; Rust, F. F.; Vaughan, W. E. J . Am. Chem. Soc.
1948, 70, 1337.
(32) Smid, J .; Rembaum, A.; Szwarc, M. J . Am. Chem. Soc. 1956,
78, 3315.
Su p p or tin g In for m a tion Ava ila ble: Comparison of the
Hammett single and dual correlations for the isomerizations
at 40, 50, 60, and 70 °C. This material is available free of
charge via the Internet at http://pubs.acs.org.
J O991186R
(33) We thank Professor T. T. Tidwell who has kindly pointed out
the restricted rotations as the cause of entropy reductions.
(34) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. F. Purification of
Laboratory Chemicals, 2nd ed.; Pergamon Press: Oxford, 1980.