Homolytic Rearrangements of Ketenimines to Nitriles: Employments of Hammett Dual Parameters

Article abstract of DOI:10.1021/jo971844c

The thermal isomerizations of ketenimines to nitriles involve a "homolytic" transition state (TS) as the major contributor to the structure of the TS. The rates have been fitted to Hammett dual correlations. The magnitudes of |ρ/ρ| represent the relative weights in the TS assuming homolytic character.

Full text of DOI:10.1021/jo971844c

J . Org. Chem. 1998, 63, 1571-1573
1571
Hom olytic Rea r r a n gem en ts of Keten im in es to Nitr iles:
Em p loym en ts of Ha m m ett Du a l P a r a m eter s
Sung Soo Kim,* Bo Liu,1 Chul Hyun Park, and Kwang Ho Lee
Department of Chemistry and Center for Molecular Dynamics, Inha University, Inchon 402-751, South
Korea
Received October 6, 1997
The thermal isomerizations of ketenimines to nitriles involve a “homolytic” transition state (TS)
as the major contributor to the structure of the TS. The rates have been fitted to Hammett dual
correlations. The magnitudes of |F•/F| represent the relative weights in the TS assuming homolytic
character.
In the absence of steric effects, substituents2 control
the rates of radical reactions through polar effects3 and
Ta ble 1. Absolu te a n d Rela tive Ra tes of th e
Isom er iza tion s a t 60 °C in CDCl3
spin delocalization. Polar effects have been thoroughly
investigated by Kim et al.4 A polar TS is thus defined
as an “imbalanced TS”,5 which features entropy control
of rates.4 The concept of a radical substituent constant
(σ•) was originally proposed by Streitwieser and Perrin.6
rates (Y)
p-Br
p-Cl
p-OCH3
m-F
m-Cl
The value of σ• may represent the capacity for dispersion
of the spin density and the magnitude of F•σ• is equivalent
to the increment of free energy of activation derived
therefrom. Recently, numerous σ• scales7-13 have been
defined and used in Hammett dual parameters. The
pioneering work by Singer14 indicates that the rearrange-
ment of ketenimines takes place via cage recombination
of the radical pairs. Later studies15,16 corroborate the
homolytic nature of the reaction. The previous studies,14-16
however, neglected investigations of the TS involved. We
would like to herein report on the structure of the TS for
kY × 105 24.3 ( 0.3 21.6 ( 1.0 20.1 ( 0.2 12.5 ( 0.4 12.2 ( 0.7
a
(s-1
)
kY/kH
3.04
2.70
2.51
1.56
1.53
rates (Y)
p-CH3
p-F
H
m-OCH3
m-CH3
kY × 105 11.9 ( 0.1 9.17 ( 0.3 8.00 ( 0.2 7.49 ( 0.5 7.17 ( 0.5
a
(s-1
)
kY/kH
1.49
1.15
1.00
0.936
0.896
a
The rates were measured more than three times.
Ta ble 2. Ha m m ett Sin gle a n d Du a l Cor r ela tion s for th e
Isom er iza tion s a t 60 °C in CDCl3
log kY/kH ) Fσ; log kY/kH ) F•σ•; log kY/kH ) Fσ + F•σ•
(1) A postdoctoral fellow (1995-1997) by a grant from Inha Uni-
versity.
(2) Substituent Effects in Radical Chemistry; Viehe, H. G.; J anousek,
Z.; Mereny, R., Ed.; NATO ASI Series C; Reidel: Dordrecht, The
Netherland, 1986; Vol. 189.
(3) Russell, G. A. In Free Radicals in Solution; Kochi, J . K., Ed.;
Wiley: New York, 1973; Vol. 1, Chapter 7.
(4) (a) Kim, S. S.; Choi, S. Y.; Kang, C. H. J . Am. Chem. Soc. 1985,
107, 4234. (b) Kim, S. S.; Kim, H. R.; Kim, H. B;. Youn, S. J .; Kim, C.
J . J . Am. Chem. Soc. 1994, 116, 2754. (c) Kim, S. S. Pure Appl. Chem.
1995, 67, 791. (d) Kim, S. S.; Kim, H.; Yang, K. W. Tetrahedron Lett.
1997, 38, 5303.
(5) (a) J encks, D. A.; J encks, W. P. J . Am. Chem. Soc. 1977, 99, 7948.
(b) Bernasconi, C. F. Acc. Chem. Res. 1992, 25, 9.
(6) Streitwieser, A., J r.; Perrin, C. J . Am. Chem. Soc. 1964, 86, 4938.
(7) Agirbas, H.; J ackson, R. A. J . Chem. Soc., Perkin Trans. 2 1983,
739.
(8) Fisher, T. H.; Dershem, S. M.; Prewitt, S. M. J . Org. Chem. 1990,
55, 1040.
approach
σa
σ•
F
F•
|F•/F|
-
-
3.04 0.953 10
3.21 0.976
1.52 0.960
-
20.8
23.4
rb
nc
Exner17
σ
-
0.13
-
0.145 10
0.698 10
Creary9
-
σ
σc•
σc•
σc•
-
1.34
•
0.72 2.19
0.61 1.96
0.75 1.14
6.56
0.61 12.7
0.59 13.8
0.53 -1.28
1.36
0.36 1.56
0.23 1.70
0.58 -0.40
σp
6
5
8
8
5
4
σm σc •
Arnold10
-
σ
σR•
σR•
σR
-
0.634
0.883
0.970
σp
σm σR••
2.42 0.998
J iang and J i11
-
σ
σjj
σjj
σjj
-
-
0.774 10
4.33 0.863 10
•
•
•
σp
σm σjj
7.39 0.976
0.69 0.977
6
5
a
b
Taken from ref 17. Correlation coefficients. c Number of the
(9) Creary, X.; Mehrsheikh-Mohammadi, M. E.; McDonald, S. J . Org.
Chem. 1987, 52, 3254.
points.
(10) Dust, J . M.; Arnold, D. R. J . Am. Chem. Soc. 1983, 105, 1221.
(11) (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.
(12) Bordwell, F. G.; Bausch, M. J . J . Am. Chem. Soc. 1986, 108,
1979.
the thermal isomerizations of diphenyl N-substituted
benzyl ketenimines to the corresponding nitriles (Scheme
1).
(13) Adam, W.; Harrer, H. M.; Kita, F.; Korth, H.-G.; Nau, W. M. J .
Org. Chem. 1997, 62, 1419.
(14) (a) Singer, L. A.; Lee, K.-W. J . Chem. Soc., Chem. Commun.
1974, 962. (b) Singer, L. A.; Lee, K.-W. J . Org. Chem. 1974, 39, 3780.
(c) Lee, K.-W.; Horowitz, N.; Ware, J .; Singer, L. A. J . Am. Chem. Soc.
1977, 99, 2622.
(15) Clarke, L. F.; Hegarty, A. F.; O′Neill, P. J . Org. Chem. 1992,
57, 362.
(16) Neuman, R. C., J r.; Sylwester, A. P. J . Org. Chem. 1983, 48,
2285.
Resu lts a n d Discu ssion
Various ketenimines were synthesized by known meth-
ods14 with some alterations of reaction conditions (Scheme
2). All the NMR spectra excellently match the structures.
Some of them are also consistent with previously reported
spectra.14 Corresponding nitriles show spectral patterns
S0022-3263(97)01844-6 CCC: $15.00 © 1998 American Chemical Society
Published on Web 02/07/1998

1572 J . Org. Chem., Vol. 63, No. 5, 1998
Kim et al.
Sch em e 1
Sch em e 2
consistent with proposed structures. The thermolysis at
60 °C of ketenimines to nitriles proceeds smoothly
without side reactions. The rates of the isomerizations
follow first-order kinetics and have been measured by
monitoring the disappearance and appearance of the
benzylic protons of the ketenimine and its nitrile coun-
terpart, respectively. The absolute rate constants (kY)
were obtained from the equation ln(Cat + Cbt)/Cat ) kYt,
Cat: magnitude of the benzylic protons of the ketenimine
at time t, Cbt: the similar entity for the nitrile. The
absolute rate constants and relative rates are shown in
Table 1. The logarithm of the relative rates were then
F| for the meta substituents (|F•/F| ) 1.52,9 2.42,10 and
0.6911). The rates of homolyses of various azoalkanes18-23
were also treated with similar Hammett dual parameter
equations. Here18 again |F•/F| is larger than unity; that
is 1.04 e |F•/F| e 3.60, which indicates the importance of
a TS analogous to 1. Furthermore the order of substitu-
ent effects on rates is very similar in the rearrangement
reactions of ketenimines (Table 1) and the pyrolyses of
azocompounds.18 The rates thus increase in the order H
< p-CH3 < p-OCH3 < p-Cl < p-Br. The parallelism could
be again due to the similar TS structure for the two
reactions.
In conclusion the major substitutent effects on the
rates of ketenimine rearrangement are due to spin
delocalization. Single correlations with σ• alone, however,
yield relatively poor correlations. Therefore polar effects
in the TS are not to be ignored completely. The role of
solvent in forming the polar TS could be unimportant
because the polarity3 has been attributed to factors other
than solvent (e.g. electronegativity and heats of reaction).
The relative rates (kY/kH in Table 1) can be utilized as a
new set of σ• constants in reactions which have both polar
and radical character in the TS. Further work is in
progress in terms of solvent effects and temperature
studies.
plotted against σ,17 σc•,9 σR
and σjj
for the single
• 10
• 11
correlations. For the dual parameter correlations, rates
were best fitted to the dual parameter equation log kY/
kH ) Fσ + F•σ• where σ• ) σc•,9 σR
or σjj
(Table 2).
• 10
• 11
A plot of log kY/kH versus σ17 exhibits a typical shotgun-
type scatter of the points and indicates lack of a correla-
tion (r ) 0.145). Singer et al.14c also observed the
breakdown of a similar Hammett correlation. The single
correlation with the three σ• scales shows some improve-
ments but yet plots are not linear (r ) 0.698,9 0.634,10
and 0.77411). The introduction of dual parameter equa-
tions, however, gives quite satisfactory plots (r ) 0.953,9
0.883,10 and 0.86311) especially with the Creary ap-
proach.9 The values of |F•/F| are much larger than unity
(|F•/F| ) 3.04, 20.8, and 4.33) implicating importance of
TS 1 in Scheme 1. The separate treatment of para and
meta substituents yields even better correlations (r g
0.960 for the six cases). |F•/F| ) 3.21,9 23.4,10 and 7.3911
indicate that para substituents promote spin delocaliza-
tion and stabilize 1 regardless of their electronic char-
acter. Most of the meta substituents9,10 have σ• < 0 and
prevent spin dispersion. Accordingly, the role of 1 is
decreased and contributions of 2 begin to emerge. This
is clearly shown in the reduction of the magnitude of |F•/
Exp er im en ta l Section
Ma ter ia ls. The reagents are commercially available from
major suppliers. Liquids were distilled with center-cut col-
(18) Nau, W. M.; Harrer, H. M.; Adam, W. J . Am. Chem. Soc. 1994,
116, 10972.
(19) Bandlish, B. K.; Garner, A. W.; Hodges, M. L.; Timberlake, J .
W. J . Am. Chem. Soc. 1975, 97, 5856.
(20) Adam, W.; Harrer, H. M.; Nau, W. M. J . Org. Chem. 1994, 59,
3786.
(21) (a) Shelton, J . R.; Liang, C. K.; Kovacic, P. J . Am. Chem. Soc.
1968, 90, 354. (b) Kovacic, P.; Flynn, R. R.; Gormish, J . F.; Kappelman,
A. H.; Shelton, J . R. J . Org. Chem. 1969, 34, 3312.
(22) Shelton, J . R.; Liang, C. K. J . Org. Chem. 1973, 38, 2301.
(17) Exner, O. In Correlation Analysis in Chemistry; Chapman, N.
B.; Shoter, J .; Plenum Press: New York, 1978; Chapter 10.

Homolytic Rearrangements of Ketenimines to Nitriles
J . Org. Chem., Vol. 63, No. 5, 1998 1573
lections, and solids were recrystallized according to standard
procedures.24
An a lytica l P r oced u r es. The ketenimine solutions (0.3 M
in CDCl3) were placed in capped NMR tubes. The tubes were
then placed in a thermostated bath for the thermal reactions.
At several time intervals the tubes were quenched in liquid
nitrogen, thawed, and transferred to the cavity of a Varian
Gemini 2000 NMR spectrometer for 1H NMR measurement.
1:1 correlation was observed in integrated areas for the
benzylic protons of the reacted ketenimines and corresponding
nitriles produced. The absolute rate constants (kY) were
calculated using the equation ln(Cat + Cbt)/Cat ) kYt.
Diphenyl N-substituted benzyl ketenimines were prepared
according to a known method14 with some modifications.
Substituted (benzylamino)triphenylphosphonium bromides
(0.05 mol) in 300 mL of liquid NH3 were treated with NaNH2
(0.055 mol). However, previous preparations14 utilized KOH
in anhydrous ether. The method employing NaNH2 in liquid
NH3 improved the yield (90-95%) and shortened the reaction
time from 20-40 h to 2-4 h for the preparations of triph-
enylphosphine substituted benzylimines (Scheme 2).
1H NMR d a ta for th e k eten im in es (CDCl3 with 0.03%
TMS): p-Br: 7.0-7.8 (m, 14H) 4.8 (s, 2H); p-Cl: 7.0-7.8 (m,
14H) 4.8 (s, 2H); p-OCH3: 6.8-7.8 (m, 14H) 4.7 (s, 2H) 3.8 (s,
3H); m-F: 7.0-7.4 (m, 14H) 4.8 (s, 2H); m-Cl: 7.1-7.4 (m,
14H) 4.7 (s, 2H); p-CH3: 7.2-7.8 (m, 14H) 4.8 (s, 2H) 2.4 (s,
3H); p-F: 7.1-7.8 (m, 14H) 4.8 (s, 2H); H: 7.3-7.4 (m, 15H)
4.8 (s, 2H); m-OCH3: 6.8-7.4 (m, 14H) 4.8 (s, 2H) 3.7 (s, 3H);
m-CH3: 7.0-7.4 (m, 14H) 4.7 (s, 2H) 3.6 (s, 3H).
Ack n ow led gm en t. We warmly thank the Korea
Research Foundation (KRF) for financial support in
1995/1996. We are heavily indebted to Professor Law-
erence A. Singer who has served a foreign advisor for
the international cooperative program by KRF. We also
thank the reviewers for the very kind efforts to improve
the manuscript.
(23) Peyman, A.; Hickl, E.; Beckman, H. D. Chem. Ber. 1987, 120,
713.
(24) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. F. Purification of
Laboratory Chemicals, 2nd ed.; Pergamon Press: Oxford, 1980.
J O971844C