Photobrominations of substituted cumenes by N-bromosuccinimide: Charge delocalizations, inductive effects, and spin dispersions triggered by substituents

Full text of DOI:10.1021/jo9905468

J . Org. Chem. 1999, 64, 9261-9264
9261
Sch em e 1
P h otobr om in a tion s of Su bstitu ted
Cu m en es by N-Br om osu ccin im id e: Ch a r ge
Deloca liza tion s, In d u ctive Effects, a n d Sp in
Disp er sion s Tr igger ed by Su bstitu en ts
Sung Soo Kim* and Chun Soo Kim
Department of Chemistry and Center for Chemical
Dynamics, Inha University, Inchon 402-751, South Korea
Resu lts a n d Discu ssion
Received March 29, 1999
Competitive photobrominations of pairs of substituted
cumene/cumene by NBS were carried out at 10, 40, 60,
and 80 °C in CCl4 (Scheme 1). Analysis of the reaction
mixtures employed the GLC method. Relative rates (kY/
kH) were obtained from eq 5 at each temperature. Y and
H indicate concentrations of substituted cumene and
cumene, respectively with subscripts f meaning final and
i initial concentrations. Hammett plots employed σ+,15
σ,16 and σ• 11 to yield F+, F, and F•, respectively, that were
obtained from Figures 1-3 and the like. Figure 3 exhibits
a typical shotgun type of scatter of the points and
indicates a breakdown the correlations (r ) 0.198). Spin
delocalizations might occur with 3. However, the forego-
ing breakdown suggests that 3 could hardly take place
and not influence the rates. Accordingly, the rates should
be relatively insensitive to change of bond dissociation
energy (BDE) of the bond being broken. Plots of log kY/
kH vs 1000/T provided differential activation parameters
utilizing Eyring equation17 (Figure 4). The rate data were
altogether tabulated in Table 1. Comparisons of Ham-
mett correlation coefficients (Table 1) and Figures 1-3
advocate the validity of F+/σ+ correlations that are
superior to other correlations (F/σ and F•/σ•). Accordingly,
1 should be a major TS determining the rates.
In tr od u tion
Substituents1 control the rates of radical reactions
through charge delocalizations (σ+), inductive effects (σ),
or spin dispersions (σ•). The polar effects of radical
reactions2-7 are due mainly to the charge delocalizations
and involve entropy control of rates.6c,7 Furthermore,
dominance of entropy could maintain the relative rates
(kY/ kH) either constant or higher with increasing tem-
perature (a violation of reactivity/selectivity principle).
The inductive effects7c,8,9 have been operated only with
TS bearing a carbanionic moiety where localization of
negative charge occurs on the benzylic carbon atom.
Substituents could also provoke spin delocalizations10-13
to influence the rates of radical reactions.
Photobrominations of substituted cumenes by N-
bromosuccinimide (NBS)14 previously showed a Hammett
reaction constant (F ) - 0.38 at 70 °C). The F/σ correla-
tions indicated that the rates were controlled by inductive
effects. However, the same brominations of substituted
toluenes6 measured F+ ) -1.40 at 80 °C and suggested
the charge delocalizations taking place with TS. In the
present study, we probe functions of substituents that
influence the rates. Temperature effects are to be inves-
tigated to obtain differential activation parameters telling
structures of TS.
(1) Substituent Effects in Radical Chemistry; Viehe, H. G., J anousek,
Z., Mereny, R., Eds.; NATO ASI Series C.; Reidel: Dordrecht, The
Netherlands, 1986; Vol. 189.
(2) Russell, G. A. J . Org. Chem. 1958, 23, 1407.
(3) Walling, C. Free Radicals in Solution; Wiley: New York, 1957;
Chapter 8.
(4) Russell, G. A. In Free Radicals; Kochi, J . K., Ed.; Wiely: New
York, 1973; Vol. 1, Chapter 7.
(5) Pryor, W. A. Free Radicals; McGraw-Hill: New York, 1966;
Chapter 12.
(6) (a) Walling, C.; Rieger, A. L.; Tanner, D. D. J . Am. Chem. Soc.
1963, 85, 3129. (b) Pearson, R. E.; Martin, J . C. J . Am. Chem. Soc.
1963, 85, 354, 3142. (c) Kim, S. S.; Choi, S. Y.; Kang, C. H. J . Am.
Chem. Soc. 1985, 107, 4234.
(7) (a) Kim, S. S.; Kim, H. R.; Kim, H. B.; Youn, S. J .; Kim, C. J . J .
Am. Chem. Soc. 1994, 116, 2754. (b) Kim, S. S. Pure Appl. Chem. 1995,
67, 791. (c) Kim, S. S.; Kim, H.; Yang, K. W. Tetrahedron Lett. 1997,
38, 5303. (d) Kim, S. S.; Choi, W. J .; Zhu, Y.; Kim, J . H. J . Org. Chem.
1998, 63, 1185. (e) Kim, S. S.; Tuchkin, A. J . Org. Chem. 1999, 64,
3821.
(8) (a) Pryor, W. A.; Tang, F. A.; Tang, R. H.; Church, D. F. J . Am.
Chem. Soc. 1982, 104, 2885.
(9) (a) Henderson, R. W.; Ward, R. D. J . Am. Chem. Soc. 1974, 96,
7556. (b) Henderson, R. W. J . Am. Chem. Soc. 1975, 97, 213.
(10) Creary, X.; Mehrsheikh-Mohammadi, M, E.; McDonald, S. J .
Org. Chem. 1987, 52, 3254.
Hydrogen abstractions18 from substituted phenols by
tert-butoxy radical revealed F+ ) -0.90 at 22 °C. p-
Methoxyphenol (k ) 1.6 × 109 M-1 s-1) reacted faster
than unsubstituted phenol (k ) 3.3 × 108 M-1 s-1).
However, the former reaction exhibited higher activation
energy (Ea ) 4 kcal mol-1) than the latter (Ea ) 2.8 kcal
mol-1). p-OCH3 stabilizes the cationic site, which may
stimulate an increase in the extent of cleavage of phenolic
O-H in 4. An enthalpic increment due to such increase
of “heterolytic” bond scission (factor a ) could outweigh
(11) Dust, J . M.; Arnold, D. R. J . Am. Chem. Soc. 1983, 105, 1221.
(12) (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.
(13) Kim, S. S.; Liu, B.; Park, C. H.; Lee, K. H. J . Org. Chem. 1998,
63, 1571.
(15) Brown, H. C.; Okamoto, Y. J . Am. Chem. Soc. 1958, 80, 4979.
(16) The values of σ were taken from the work of March. March, J .
In Advanced Organic Chemistry; Wiley: New York, 1992.
(17) Eyring, H. J . Chem. Phys. 1935, 3, 107.
(18) Das, P. K.; Encinas, M. V.; Steenken, S.; Scaiano, J . C. J . Am.
Chem. Soc. 1981, 103, 4162.
(14) Gleicher, G. J . J . Org. Chem. 1968, 33, 332.
10.1021/jo9905468 CCC: $18.00 © 1999 American Chemical Society
Published on Web 11/18/1999

9262 J . Org. Chem., Vol. 64, No. 25, 1999
Notes
F igu r e 1. Hammett correlations at 80 °C with σ+.
F igu r e 4. Eyring plot for activation parameters.
be independent of substituent effects. Accordingly, the
extent of bond cleavage (e.g., O-H in 4) was to remain
invariable regardless of substituents. However, a positive
value of 1.2 kcal mol-1 can be justified only when p-OCH3
promotes the bond cleavage so that a may outweigh b. a
tends to increase ∆H*Y while b acts to reduce it, thereby
8
counteracting with each other to determine ∆H* (e.g.,
Y
p-OCH3: ∆H*Y ) 3.42 kcal mol-1, p-H: ∆H* ) 2.22 kcal
H
mol-1).20 Photobrominations of substituted toluenes6c also
bear similar polar TS resembling 4. Therefore, the
electron-donating substituents again yielded ∆∆H*
>
Y-H
0 (e.g., 0.73 kcal mol-1 for p-methoxytoluene: refer to
Table II of ref 6c for ∆∆H*
> 0 with other electron-
Y-H
donating substituents).
F igu r e 2. Hammett correlations at 80 °C with σ.
1 and 4 share structural resemblance that should
promise similar substituent effects on the bond scission.
However, magnitude of the activation parameter (∆H*
)
Y
could be varied by dissimilar molecular structures be-
tween cumene and phenol. The approach of Br• to cumylic
hydrogen could be retarded by steric crowding of two
methyl groups in 1. The steric hindrance may cause
equivalently less extent of C-H cleavage of 1. Further-
more, cumylic C-H (BDE ) 84.4 kcal mol-1 21 is weaker
)
than phenolic O-H (BDE ) 88.3 kcal mol-1).22 The lesser
extent of bond breakage and weaker BDE may cooperate
to further alleviate substituent effects on disturbance of
∆H*Y. Accordingly, values of ∆∆H*
became trivial
Y-H
showing almost invariable fluctuations of ∆∆H*Y-H against
σ+ (Figure 5). Similar values of ∆∆H*
close to zero
Y-H
were also observed with fragmentation reactions of
benzyl methyl substituted benzyl carbinyloxy radicals.7a
The enthalpic disadvantage triggered by p-OCH3 with
F igu r e 3. Hammett correlations at 80 °C with σ•.
its reduction caused by the stabilization of the positive
charge via conjugations making a double bond (factor b).
A value of 4-2.8 ) 1.2 kcal mol-1 is equivalent to the
4 (∆∆H*
) 1.2 kcal mol-1) has been overcome by a
Y-H
much larger differential entropy term (∆∆S*
) 6.62
Y-H
differential enthalpy of activation (∆∆H*
) ∆H*
-
cal K-1 mol-1) to achieve the rate enhancement (k ) 1.6
× 109 M-1 s-1 for p-OCH3; k ) 3.3 × 108 M-1 s-1 for p-H).
Y-H
Y
∆H*H, ∆H* and ∆H* are enthalpy of activation when
Y
H
The differential entropy is expressed as ∆∆S*
) ∆S*
substituents are Y and hydrogen, respectively). The
Y-H
Y
positive sign of ∆∆H* ) 1.2 kcal mol-1 conflicts with
Y-H
(20) Enthalpies of activation can be obtained from ∆H* ) Ea - RT.
(21) McMillen, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982,
33, 493
(22) Lucarini, M.; Pedrielli, P.; Pedulli, G. F. J . Org. Chem. 1996,
61, 9259.
the classical concept of a linear free energy relationship
(LFER).19 LFER19 assumed that the TS structure should
(19) Hammett, L. P Chem. Rev. 1935, 17, 22.

Notes
J . Org. Chem., Vol. 64, No. 25, 1999 9263
Ta ble 1. Ra te Da ta of P h otobr om in a tion s of Su bstitu ted Cu m en es by NBS in CCl4
(kY/kH)a
p-Cl
T (°C)
p-OMe
p-Me
H
p-CN
0.60
p-NO2
F+ b (r)
Fb (r)
F• c (r)
10
40
60
80
1.71d
1.72
1.31
1.31
1.30
1.30
1
1
1
1
0.92
0.51
0.51
0.53
0.52
-0.33 (0.998)
-0.33 (0.996)
-0.33 (0.998)
-0.33 (0.997)
-0.43 (0.981)
-0.43 (0.982)
-0.42 (0.980)
-0.43 (0.980)
-2.17(0.215)
-2.01(0.201)
-1.97(0.196)
-1.99(0.198)
0.92
0.61
1.73
(0.92)f
0.93g
(0.61)f
0.61g
1.73e
p-OMe
p-Me
H
p-Cl
p-CN
p-NO2
h
∆∆H*
0.04 ( 0.01
1.19 ( 0.02
-0.03 ( 0.01
0.45 ( 0.03
0
0
0.02 ( 0.02
-0.084 ( 0.06
0.05 ( 0.02
-0.84 ( 0.06
0.08 ( 0.06
-1.05 ( 0.20
∆∆S*YY--HHi
a
Error limits were far less than 2% being average deviations of three runs unless otherwise stated. Each sample has been always
subject to GLC analysis three times. b Hammett substituent constants (σ+ and σ) were taken from March, J . In Advanced Organic Chemistry;
Wiley: New York, 1992. c σ• was taken from ref 11. An average value of five runs. e An average value of nine runs. f The values were
d
g
taken from those at 40 °C. The relative rate (kp-Cl/kp-CN) at 80 °C has been obtained as kp-Cl/kp-CN ) 1.55 from competition reactions of
p-chlorocumene and p-cyanocumene with NBS. A similar value of (kp-Cl/kH)/(kp-CN/kH) ) 1.52 was observed to indicate validity of the
competition method. Unit: kcal mol-1
. .
Unit: cal K-1 mol-1
h
i
F igu r e 6. Plot of ∆∆Sq vs σ+.
Y-H
F igu r e 5. Plot of ∆∆Hq
vs σ+.
Y-H
- ∆S*H, where entropy terms with subscripts are defined
similarly as with their enthalpic partners. Entropy of
activation with 4 was calculated by us from previous
has been also detected by secondary R-deuterium kinetic
isotope effects occurring in thermolysis of tert-butyl
phenylperacetates.7e
data18 as ∆S* ) -5.10 cal K-1 mol-1 for p-OCH3 and
Y
The same logics for the substituent effects can be
∆S* ) -11.72 cal K-1 mol-1 for p-H. The positive sign
applied to 1 in order to explain values of ∆∆S*Y-H. The
H
of the differential entropy term (∆∆S*
) -5.10 +
entropic perturbations have been equated as ∆∆S*
Y-H
Y-H
11.72 ) 6.62 cal K-1 mol-1) tells again that p-OCH3
engenders more bond cleavage than does p-H. The bond
) Rσ+ + c (Figure 6). The negative slope (R ) -1.40) in
Figure 6 indicates that TS could become loosened by
electron-donating substituents (e.g., p-OCH3). R may
represent intensity of substituent effects controlling
degree of bond cleavage. Accordingly R and F+ are
relevant entities measuring looseness of TS structure.
Present reactions (eq 1) reveal F+ ) -0.33 at 80 °C and
R ) -1.40. Brominations of substituted toluenes (YC6H4-
CH2-H + Br• f YC6H4CH2• + HBr)6c gave much larger
values (F+ ) -1.40 at 80 °C and R ) -10.8) because of
the less steric hindrance allowing bromine atom to easily
gain access to benzylic hydrogen for greater bond-
scission yields translational entropy.23 ∆∆S*
) 6.62
Y-H
cal K-1 mol-1 is equivalent to neat increment of trans-
lational entropy derived from further bond cleavage
invoked by p-OCH3. Therefore, both differential activa-
tion parameters (∆∆H*
) 1.2 kcal mol-1, ∆∆S*
)
Y-H
Y-H
6.62 cal K-1 mol-1) argue for p-OCH3 to increase degree
of breakage of O-H in 4. Our previous radical reactions6c,7
also involved a similar polar TS showing F+ < 0 and
indicated entropy control of rates. Plots24 of ∆∆S*
vs
Y-H
σ+ gave good linear lines with negative slopes. The
negative entities repeatedly suggest that electron-donat-
ing substituents (e.g., p-OCH3) promote bond breakings
to loosen TS structures. Quite similar loosening of TS
breaking. Values of ∆∆G*
at 80 °C were calculated
Y-H
as -0.38 and 0.45 kcal mol-1 for p-OMe and p-NO2,
respectively. The rates are therefore delicately controlled
through the entropic term derived from bond cleavage
creating translational motions.23 When Q ) ln kY/kH and
(23) Absolute rate theory (ref 17) postulates that a vibration of the
bond being broken is replaced by a translational mode. One of the
reviewers has kindly pointed out that the translation should be better
termed loose vibration/internal rotation. Therefore, the translation is
spelled as italics to deliver such notions. S.S.K. is very grateful to the
reviewer for the very instructive comments.
ln kY/kH ) -(∆G* - ∆G*H)/RT ) -∆∆G*Y-H/RT, Q )
Y
-∆∆G*Y-H/RT can be divided into entropy and enthalpy
terms. Since ∆∆H*Y-H/RT is close to zero, eq 6 becomes
Q ) ∆∆S*Y-H/R, which is devoid of temperature term and
(24) See the Supporting Information.

9264 J . Org. Chem., Vol. 64, No. 25, 1999
Notes
2.9 (m, 1H, CH), 7.3 (d, 2H, C6H4), 7.6 (d, 2H, C6H4); p-Cl, δ 1.2
(d, 6H, CH3), 2.9 (m, 1H, CH), 7.1 (d, 2H, C6H4), 7.3 (d, 2H, C6H4).
Kin etic Stu d ies. Substituted cumene (1-4 mmol), cumene
(1-4 mmol), and nitrobenzene (1 mmol) were dissolved in CCl4
to give a total volume of 3 mL. This mixture and NBS (3 mmol)
were placed into sealed, degassed Pyrex ampules by a freeze-
pump-thaw method. The ampules were then immersed in a
constant-temperature bath and irradiated with a 250 W incan-
descent lamp. The reaction conditions (e.g., substrate concentra-
tions, light intensity, and reaction time) were arranged so that
consumption of the substrates may stay within range of 10-
20%. After reactions, the tubes were quenched by cooling in the
dark, opened, and worked up by filtering the liquid from the
residual insolubles. The solutions were then stored in small vials
wrapped with foil until GLC analysis. The photolyzed solutions
were analyzed on 30 m DB-1 capillary column with FID and
temperature programming from 80 to 190 °C.
explains why the relative rates (kY/kH) are independent
of temperature.
Q ) ∆∆S*Y-H/R - ∆∆H*Y-H/RT
(6)
Con clu sion
Substituents control extent of the bond cleavage, which
is followed by formation of partial double bond. Bond
scission creates translational entropy while double bond
formation reduces rotational entropy. Creation of trans-
lational entropy outweighs loss of rotational one to
determine entropy of activation. The substituent effects
perturb free energy of activation in the range of less than
1 kcal mol-1. The entropy term is 1000 times more
sensitive than its enthalpic partner to detect such delicate
changes of bond scission caused by substituents.
Ack n ow led gm en t. We warmly thank the Korea
Science & Engineering Foundation (KOSEF 95-0501-
06-01-3) for the financial support. Mr. Kwang Hyun
Cha, Sang Hak Lim and Dr. Gang Lin offered as-
sistance. Professor Bong Rae Cho has kindly provided
editorial comments. We also thank Inha University. The
authors are very grateful to the reviewers for the
instructive comments to remarkably improve the sci-
entific quality of the manuscript.
Exp er im en ta l Section
Ma ter ia ls a n d Meth od s. Substituted cumenes (Y: p-Me,
p-H, p-NO2) and other organic materials were purchased from
the major suppliers. Liquids were distilled with center-cut
collection. NBS was rapidly recrystallized from boiling water and
dried in a dark vacuum oven for 3 days at ambient temperature
(purity 99.8% by iodometric titration). A Varian 3300 gas
chromatograph and Varian Gemini 2000 NMR spectrometer
were employed.
p-Meth oxy-, p-cya n o-, a n d p-ch lor ocu m en es were pre-
pared by the known methods:25,26 1H NMR (CDCl3, 200 MHz)
p-OMe, δ 1.2 (d, 6H, CH3), 2.9 (m, 1H, CH), 3.83 (s, 3H, OCH3),
6.9 (d, 2H, C6H4), 7.2 (d, 2H, C6H4); p-CN, δ 1.2 (d, 6H, CH3),
Su p p or tin g In for m a tion Ava ila ble: Plot of ∆∆Sq
vs
Y-H
σ+ for various radical reactions involving polar TS. This
material is available free of charge via the Internet at
http://pubs.acs.org.
J O9905468
(26) Furniss, B. S.; Hannaford, A. J .; Smith, P. W. G.; Tatchell, A.
R. Vogel’s Textbook of Practical Organic Chemistry, 5th ed.; Longman
Scientific & Technical: New York, 1989; p 1083.
(25) Marvel, C. S.; McElvain, S. M. In Organic Syntheses; Gilman,
H., Eds.; Wiley: New York, 1932; Collect. Vol. I, p 170.