Secondary α-Deuterium Kinetic Isotope Effects Signifying a Polar Transition State in the Thermolysis of Ring-Substituted tert-Butyl Phenylperacetates

Article abstract of DOI:10.1021/jo9814492

Several ring-substituted tert-butyl phenylperacetates (YC₆H₄CH₂CO₃Bu^t) and their deuterated versions (YC₆H₄CD₂CO₃Bu^t) were prepared (Y: p-OCH₃, p-CH₃, p-H, and p-NO₂). Thermolyses at 80°C in CDCl₃ showed excellent first-order kinetics. The rates have been measured as k_YH × 10⁴ and k_YD × 10⁴ s^-1: 11.9 and 9.20 (p-OCH₃), 2.64 and 2.22 (p-CH₃), 1.06 and 0.93 (p-H), 0.164 and 0.156 (p-NO₂). Hammett correlations were derived to yield ρ_YH⁺ = -1.17 and ρ_YD⁺ = -1.12. However, better Hammett plots were obtained with three points (p-OCH₃, p-CH₃, and p-H) showing ρ_YH⁺ = -1.35 and ρ_YD⁺ = -1.28. SDKIE was calculated as 1.293 (p-OCH₃), 1.189 (p-CH₃), 1.140 (p-H), and 1.051 (p-NO₂), showing substantial substituent effects. Values of k_YH/k_YD for p-NO₂ showed little temperature dependence. Hammett correlations and SDKIE were derived from the same kinetic entity that is the bond cleavage.

Full text of DOI:10.1021/jo9814492

J . Org. Chem. 1999, 64, 3821-3824
3821
Secon d a r y r-Deu ter iu m Kin etic Isotop e Effects Sign ifyin g a P ola r
Tr a n sition Sta te in th e Th er m olysis of Rin g-Su bstitu ted ter t-Bu tyl
P h en ylp er a ceta tes
Sung Soo Kim* and Alexey Tuchkin†
Department of Chemistry and Center for Chemical Dynamics, Inha University,
Inchon 402-751, South Korea
Received J uly 23, 1998
Several ring-substituted tert-butyl phenylperacetates (YC6H4CH2CO3But) and their deuterated
versions (YC6H4CD2CO3But) were prepared (Y: p-OCH3, p-CH3, p-H, and p-NO2). Thermolyses at
80 °C in CDCl3 showed excellent first-order kinetics. The rates have been measured as kYH × 104
and kYD × 104 s-1: 11.9 and 9.20 (p-OCH3), 2.64 and 2.22 (p-CH3), 1.06 and 0.93 (p-H), 0.164 and
0.156 (p-NO2). Hammett correlations were derived to yield FYH+ ) -1.17 and FYD+ ) -1.12. However,
+
better Hammett plots were obtained with three points (p-OCH3, p-CH3, and p-H) showing FYH
)
-1.35 and FYD+ ) -1.28. SDKIE was calculated as 1.293 (p-OCH3), 1.189 (p-CH3), 1.140 (p-H), and
1.051 (p-NO2), showing substantial substituent effects. Values of kYH/kYD for p-NO2 showed little
temperature dependence. Hammett correlations and SDKIE were derived from the same kinetic
entity that is the bond cleavage.
In tr od u ction
∆Vq) when the reactions change from one-bond to two-
bond homolysis.
tert-Butyl peresters1-3 undergo dual thermal decom-
position depending upon the stabilities of radical R•
extruded (RCO2OBut f R• + CO2 + t-BuO•). Bartlett et
al.2 showed that two-bond scission exhibits smaller
magnitude activation parameters (∆Hq and ∆Sq)4 than
one-bond cleavage. However, this can be conceived by
comparing structures of transition state (TS) 1 and 2.
Two-bond homolysis is inevitably accompanied by two
partial π-bond formations in 2. The conjugation of incipi-
ent benzyl carbocation introduces a double bond between
the benzylic carbon and the phenyl ring. Formation of a
CO2 molecule also yields a CdO bond. Therefore, bond-
breakings (R-CO2 and O-O) could be effectively com-
pensated by the formation of double bonds to reduce the
enthalpy of activation (∆Hq). The double bonds then
prohibit the rotational motions which act to reduce the
entropy of activation (∆Sq). Surprisingly, the two-bond
homolysis5 also showed less volume of activation (ca. ∆Vq
) 5 cm3/ml) than the one-bond cleavage (ca. ∆Vq ) 10
cm3/mL). The volume of activation6 is a function of bond
rotation and length. Double bonds developed in 2 could
restrict the free rotation and contract bond length so as
to decrease volume of activation. Therefore, the simul-
taneous formation of two double bonds can bring about
diminutions7 of activation parameters (∆Hq, ∆Sq, and
Secondary R-deuterium kinetic isotope effects (SDKIE)8
were reported for 2 with substituent Y being p-OCH3
(kH/kD ) 1.07 at 60.46 °C), p-H (kH/kD ) 1.13 at 84.98
°C), and p-NO2 (kH/kD ) 1.10 at 85.1 °C). A kH/kD value
of 1.13 for p-H could be consistent with two-bond ho-
molysis. However, the value for p-OCH3 (kH/kD ) 1.06)
appears to hardly reflect electron-donating ability, con-
sidering kH/kD ) 1.10 for p-NO2. The studies by substitu-
ent/temperature,2 pressure,5 and viscosity9 strongly nomi-
nate p-OCH3 as an efficient promoter of two-bond scission.
Resu lts a n d Discu ssion
Peresters 3 and their deuterated counterparts 4 were
prepared according to the known methods.2,10 Reactions
at 80 °C for 5 half-lives in degassed and sealed Pyrex
† A visiting scholar from St. Petersburg State University by a grant
from the Science & Technology Policy Institute under the Korea-
Russia manpower exchange program.
(1) Blomquist, A. T.; Berstein, I. A. J . Am. Chem. Soc. 1951, 73,
5546.
(2) (a) Bartlett, P. D.; Hiatt, R. R. J . Am. Chem. Soc. 1958, 80, 1398.
(b) Bartlett, P. D.; Simons, D. M. J . Am. Chem. Soc. 1960, 82, 1753.
(c) Bartlett, P. D.; Ru¨chardt, C. J . Am. Chem. Soc. 1960, 82, 1756.
(3) Wolf, R. A.; Trocino, R. J .; Rozich, W. R.; Sabeta, I. C.; Ordway,
R. J ., J r. J . Org. Chem. 1998, 63, 3814.
(4) Pryor, W. A.; Smith, K. Int. J . Chem. Kinet. 1971, 3, 387-394.
(5) (a) Neuman, R. C., J r.; Behar, J . V. J . Am. Chem. Soc. 1967, 89,
4549. (b) Neuman, R. C., J r.; Behar, J . V. J . Org. Chem. 1971, 36, 654.
(c) Neuman, R. C., J r.; Behar, J . V. J . Org. Chem. 1971, 36, 657.
(6) Eldik, R. V.; Asano, T.; Le Noble, W. J . Chem. Rev. 1989, 89,
549.
(7) An alternative explanation for variations of activation param-
eters might employ solvent interactions with the polar TS. Solvent
molecules may induce electrostrictions around the polar TS so as to
decrease ∆Sq and ∆Vq. However, reduction of ∆Hq may be hardly
rationalized by solvent effects. Therefore, “double bond formations”
could be a more versatile explanation than solvent effects.
(8) (a) Koenig, T.; Wolf, R. J . Am. Chem. Soc. 1969, 91, 2574. (b)
Koenig, T.; Huntington, J .; Cruthoff, R. J . Am. Chem. Soc. 1970, 92,
5413.
(9) Pryor, W. A.; Smith, K. J . Am. Chem. Soc. 1970, 92, 5403.
(10) Staab, H. A. Chem. Ber. 1956, 89, 1927.
10.1021/jo9814492 CCC: $18.00 © 1999 American Chemical Society
Published on Web 05/12/1999

3822 J . Org. Chem., Vol. 64, No. 11, 1999
Kim and Tuchkin
p-Methoxyphenol (k ) 1.6 × 109 M-1 s-1 ) reacted faster
than phenol (k ) 3.3 × 108 M-1 s-1 ). However, the former
reaction (Ea ) 4 kcal mol-1) exhibited higher activation
energy than the latter (Ea ) 2.8 kcal mol-1). p-OCH3
stabilizes the cationic site, which stimulates an increase
in the extent of cleavage of the phenolic O-H in 5. An
Sch em e 1
_
enthalpy increase14 due to such “heterolytic” bond scission
could outweigh its reduction caused by the stabilization
of the positive charge via conjugations making a double
bond. A value of 4 - 2.8 ) 1.2 kcal mol-1 corresponds to
tubes containing tert-butyl phenylperacetate (0.05 M), I2
(0.2-0.4 M), and acetonitrile (0.02 M, internal standard)
in CDCl3 gave rise to formations of benzyl iodide (90.8),
tert-butyl alcohol (54.3), tert-butyl benzyl ether (4.5),
methyl iodide (43.3), acetone (43.3) (refer to Scheme 1).
The products were identified by comparing the chemical
shifts and quantified by integrations of the peaks of
appropriate protons of products. The figures in the
parenthesis correspond to mol % of products based on
the amount of the perester decomposed. The material
balance indicates 95.3% conservation of benzyl and
102.1% of tert-butoxy.
the differential enthalpy of activation (∆∆Hq
) ∆Hq
Y-H
Y
- ∆HqH, where ∆Hq is the enthalpy of activation with
Y
substituent Y, and ∆HqH is the enthalpy of activation with
the substituent being hydrogen), which carries a positive
sign because p-OCH3 provokes more bond rupture than
p-H. This enthalpic disadvantage can then be overcome
by a much larger differential entropy term (∆∆Sq
)
Y-H
6.62 cal K-1 mol-1) to achieve rate enhancement. The
differential entropy could be expressed as ∆∆Sq
)
Y-H
∆SqY - ∆SqH, where entropy terms are defined similarly
as with their enthalpic mates. Entropy of activation was
calculated from previous data13 as ∆Sq ) -5.10 eu for
Y
p-OCH3 and ∆Sq ) -11.72 eu for p-H. To our surprise,
H
the signs of entropy are negative, which indicates that
the decrease15 in entropy exceeds its increase during the
activation process. The positive sign of the differential
entropy term (∆∆Sq
) 6.62 cal K-1 mol-1) tells again
Y-H
that p-OCH3 engenders more bond cleavage to create
translational entropy16 larger than that of p-H. Our
previous radical reactions17,18 also involved a similar polar
TS showing F+ < 0 and indicated entropy control of rates.
The rates (kYH and kYD) were measured by observing
periodically the disappearance of 3 and 4 during the
decompositions. The magnitude of benzyl protons (δ )
3.6-3.7 ppm) of 3 surviving thermolysis were monitored
by FT NMR and used as concentration terms (Co and Ct)
with ln(Co/Ct) ) kYHt. Protons of the tert-butyl of 4 (δ )
1.2-1.4 ppm) were excellently resolved by a paramag-
Plots12 of ∆∆Sq
vs σ+ gave good linear lines with
Y-H
negative slopes. The negative entity repeatedly suggests
that electron-donating substituents (e.g., p-OCH3) promote
bond-breakings to loosen TS structures.
Structures 2 and 5 share similar features so that
comparable substituent effects may act on the bond-
breakings. SDKIE19 could be derived from the promotion
of out-of-bending vibrations of methylene when a carbon
atom involving bond-cleavage undergoes a hybridization
change from sp3 to sp2. As previously mentioned, electron-
11
netic shift reagent Eu(fod)3 from protons of other tert-
butyl groups, i.e., tert-butyl alcohol and benzyl tert-butyl
ether. Peak area of the tert-butyl of 4 was integrated to
be utilized as concentrations of 4. The value of kYD has
been derived from ln(Co/Ct) ) kYDt. All of the plots (ln-
(Co/Ct) vs t)12 showed excellent linearities, with r g 0.997
in most of the cases, making it possible to assign reliable
values of kYH and kYD. The rates and Hammett correla-
tions are tabulated in Table 1. A value for FYH+ of -1.17
(or -1.35) is consistent with a polar structure of 2 and
appears comparable to F+ ) -1.0 at 80 °C of previous
studies.2 Although 2 involves two-bond homolysis, cleav-
age of CO2-OC(CH3)3 may not be seriously influenced
by substituent effects. Therefore, substituents could
primarily control the degree of bond-breaking of CH2-
CO2 only.
(14) Formation of an H-OC(CH3)3 bond in 5 could be relatively
immune to substituent effects and may hardly influence the differential
term (∆∆HqY-H). Substituents primarily control the bond cleavage and
subsequent double bond formation, both of which are responsible for
formation of enthalpy of activation (∆HqY).
(15) An entropy decrease could be caused by formations of YC6H5d
O double bond and H-OC(CH3)3 σ bond in 5. Formation of the σ bond
could significantly progress via hydrogen-bonding and remarkably
reduce translational entropy. The double bond prohibits rotations to
diminish rotational entropy.
(16) The bond scission creates translational entropy, which could
be however partly canceled out by a decrease of rotational entropy due
to double bond formation. Translations involve three dimensions,
whereas rotations occur with two dimensions. Therefore, an increase
of translational entropy may outweigh a decrease of rotational entropy.
(17) Kim, S. S.; Choi, S. Y.; Kang, C. H. J . Am. Chem. Soc. 1985,
107, 4234.
Hydrogen abstractions13 from substituted phenols by
the tert-butoxy radical revealed F+ ) -0.90 at 22 °C.
(11) Rondeau, R. E.; Sievers, R. E. J . Am. Chem. Soc. 1971, 93, 1522.
(12) Refer to Supporting Information.
(13) Das, P. K.; Encinas, M. V.; Steeken, S.; Scaiano, J . C. J . Am.
Chem. Soc. 1981, 103, 4162.
(18) Kim, S. S.; Kim, H. R.; Kim, H. B.; Youn, S. J . J . Am. Chem.
Soc. 1994, 116, 2754.
(19) Collins, C. J .; Bowman, N. S. Isotope Effects in Chemical
Reactions; Van Nostrand Reinhold: New York, 1970.

Thermolysis of Ring-Substituted t-Butyl Phenylperacetates
J . Org. Chem., Vol. 64, No. 11, 1999 3823
Ta ble 1. Ra te Da ta for Th er m olysis of P er ester s
kYH(YD)
YC6H4CH2(D2)CO3But
8 YC6H4CH2(D2)• + CO2 + t-BuO•
T, °C
substituent (Y)
kYH × 104 s-1
no. of runsa
kYD × 104 s-1
no. of runsa
kYH/kYD
b
80
80
80
p-OCH3
11.9 ( 0.2
6c
3
9.20 ( 0.29
2.22 ( 0.02
0.93 ( 0.01
0.156 ( 0.002
1.34 ( 0.03
3.62 ( 0.15
10.23 ( 0.23
7
4
3
5
3
3
3
1.293
1.189
1.140
1.051
1.074
1.063
1.066
p-CH3
p-H
p-NO2
p-NO2
p-NO2
p-NO2
2.64 ( 0.02
1.06 ( 0.01
0.164 ( 0.001
1.44 ( 0.04
3.85 ( 0.02
10.91 ( 0.37
5d
5e
3
80
100
110
120
3
3
log(kYH/kHH) ) FYH+ σ+
FYH+ ) -1.17 (r ) 0.9970)f
FYH+ ) -1.35 (r ) 0.9997)g
log(kYD/kHD) ) FYD+ σ+
FYD+ ) -1.12 (r ) 0.9970)f
FYD+ ) -1.28 (r ) 0.9998)g
a
The figures indicate number of kinetic runs. kYH was measured by monitoring the disappearance of methylene of 3 unless otherwise
b
indicated in footnotes c, d, and e. Rate of diminution of tert-butyl of 4 has been used to obtain kYD
.
Pyridine has been added to stabilize
p-CH3OC6H4CH2I (YC6H4CH2I + C5H5N f YC6H4CH2NC5H5+I-). The remaining pyridine then interfered with the shift reagent Eu(fod)3
which was therefore treated with methyl iodide (C5H5N + CH3I f C5H5NCH3 I-). c Three runs measured the disappearance of tert-
+
butyl of 3. Two runs measured disappearance of tert-butyl of 3. e One run measured disappearance of tert-butyl of 3. f p-NO2 was included
d
g
for calculation of correlation coefficients. p-NO2 was excluded for calculation of correlation coefficients.
discussed with entropic control of reactions (vide ante and
refer to Supporting Information).13,17,18 p-NO2 shows little
variation of kYH/kYD with temperatures, which could be
a sign of the entropic dominance. p-NO2 causes the most
sluggish rate, which could discourage two-bond homoly-
sis. When p-NO2 is excluded, Hammett plots become
improved (Table 1) and Figure 1 maintains a better
linearity. These anomalies might mean a changeover of
reaction mode. Viscosity studies9 proposed a dual ho-
molysis mechanism because p-NO2 strongly destabilizes
the “cationic moiety” of 2. Further studies are in progress
to probe such delicate changes of bond-breakings.
Con clu sion s
Although the peresters undergo two-bond homolysis,
cleavage of only one bond can be critically influenced by
substituent effects. Electron-donating substituents ac-
F igu r e 1.
celerate the rate of homolysis and heighten the extent of
bond-breaking. The cleavages bestow looseness upon the
TS, which has been figured as values of kYH/kYD. Substit-
donating substituents, i.e., p-OCH3, could boost cleavages
of CH2-CO2 in 2 and activate bending vibrations of the
methylene to give larger values of kYH/kYD (refer to Table
uents control SDKIE, i.e., kYH/kYD, which can be an
1). A theoretical maximum of SDKIE20 was assigned as
analogue of the differential entropy of activation (∆∆SqY-H).
kH/kD ) 1.41. Therefore, changes of kYH/kYD (1.051-1.293)
appear substantial to demonstrate strong substituent
Exp er im en ta l Section
interactions occurring with the bond rupture. The bond-
breaking imparts looseness to the TS structure. An
excellent linear relation is observed with three points (Y
) p-CH3O, p-CH3, and p-H) in Figure 1. The linearity
indicates that the looseness of 2 is systematically con-
trolled by substituents. Looseness of the TS of SN2
reactions21-23 was also correlated with SDKIE. Substit-
uents22 engendered quite comparable variations of SD-
KIE (e.g., kH/kD ) 1.039 for p-NO2 and 1.126 for p-OCH3)
when the reactions traversed a symmetrical TS.
Indicated in Table 1 is the fact that electron-donating
substituents accelerate the rates of homolysis (kYH and
kYD) and intensify the looseness of 2. Entropy could also
be a measure of such looseness. Quite similar behaviors
of electron-donating substituents have already been
Ma ter ia ls a n d Meth od s. Substituted phenyl acetic acid,
N,N′-carbonyldiimidazole, tert-butyl hydroperoxide, and other
reagents were purchased from the major suppliers. Liquids
were distilled with center-cut collection, and solids were
purified according to standard procedures.24 A Varian Gemini
2000 NMR spectrometer was used for the analysis of the
reaction mixtures.
Rin g-su bstitu ted ter t-bu tyl p h en ylp er a ceta tes (YC6H4-
CH2CO3Bu t, Y) p-CH3O, p-CH3, a n d p-H)10 were prepared
by the reactions of the corresponding carboxylic acid, N,N′-
carbonyldiimidazole, and tert-butyl hydroperoxide. ter t-Bu tyl
p-n itr oph en ylper acetate (p-NO2C6H4CH2CO3Bu t)2 has been
derived from the reactions of p-NO2C6H4CH2COCl and tert-
butyl hydroperoxide. All the peresters showed IR stretching
at 1760-1770 cm-1 1H NMR data for the peresters (CDCl3
.
with 0.03% TMS): p-OCH3 7.2(d,2H), 6.8(d,2H), 3.79(s,3H),
3.58(s,2H), 1.26(s,9H); p-CH3 7.1-7.2(m,4H), 3.60(s,2H), 2.33-
(s,3H), 1.26(s,9H); p-H 7.4-7.2(m,5H), 3.65(s,2H), 1.26(s,9H);
p-NO2 8.2(d,2H), 7.4(d,2H),3.76(s,2H), 1.28(s,9H). p-NO2 and
(20) Streitwieser, A.; J agow, R. H.; Fahey, R. C.; Suzuki, S. J . Am.
Chem. Soc. 1958, 80, 2326.
(21) (a) Lee, I. Adv. Phys. Org. Chem. 1992, 27, 57 (b) Lee, I. Chem.
Soc. Rev. 1995, 223.
(22) Westaway, K. C.; Pham, T. V.; Fang, Y.-r. J . Am. Chem. Soc.
1997, 119, 3670 and references therein.
(23) Glad, S. S.; J ensen, F. J . Am. Chem. Soc. 1997, 119, 227.
(24) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. F. Purification of
Laboratory Chemicals, 2nd ed.; Pergamon Press: Oxford, U.K., 1980.

3824 J . Org. Chem., Vol. 64, No. 11, 1999
Kim and Tuchkin
p-OCH3 exhibited melting points of 53-54 and 38-39 °C,
respectively.
intervals, the tubes were removed from the bath, quenched in
ice-water, and opened for NMR analysis.
Rin g-su bstitu ted ter t-bu tyl p h en ylp er a ceta tes-r,r-d 2
(YC6H4CD2CO3Bu t, Y) p-CH3O, p-CH3, p-H, a n d p-NO2)2
were prepared by the reactions of the corresponding acid
chloride (YC6H4CD2COCl) and tert-butyl hydroperoxide. IR
stretching: 1764-1771 cm-1. 1H NMR data (CDCl3 with 0.03%
TMS): p-H 7.4-7.2(m,5H), 1.26(s,9H); p-CH3O 7.2(d,2H), 6.8-
(d,2H), 3.79(s,3H), 1.26(s,9H); p-NO2 8.2(d,2H), 7.5(d,2H), 1.28-
(s,9H); p-CH3 7.2-7.1(m,4H), 2.33(s,3H), 1.26(s,9H). p-CH3O
and p-NO2 showed melting points of 38-39 and 52-53 °C,
respectively.
Rin g-Su bstitu ted P h en ylp er a cetic Acid -r,r-d 2 (YC6H4-
CD2CO2H, Y) p -CH 3O, p -CH 3, a n d p -H). The mixture of
phenylacetonitrile (2.48 g, 0.021 mol) and Bu4NBr (0.01 g) was
stirred with 1% NaOD in D2O (15 mL) for 2 h at room
temperature. More than 99% deutration has been achieved by
the repetition of the foregoing procedure. Phenylacetonitrile-
R,R-d2 (2.40 g, 0.02 mol), NaOD (1.0 g, 0.025 mol), Bu4NBr
(0.01 g), and D2O (15 mL) were refluxed for 5 h. After the
mixture cooled to 0 °C, concentrated hydrochloric acid (3 mL,
0.03 mol) was added. The precipitate of phenylacetic acid-R,R-
d2 was filtered off and dissolved in ether (50 mL). The ether
solution was separated from the aqueous solution and dried
with MgSO4. After evaporation of the ether, the solid was
recrystallized from hexane to give 2.04 g (74% yield) of the
product. 1H NMR measurements indicated 99% deutera-
tion. 1H NMR data (CDCl3 with 0.03% TMS): p -OCH 3
7.3-7.1(d,2H), 6.9-6.8(d,2H), 3.8(s,3H); p -CH 3 7.2-7.1-
(dd,4H), 2.3(s,3H); p-H 7.26-7.34(m,5H).
p-Nitr op h en yla cetic a cid -r,r-d 2 (p-NO2C6H4CD2CO2H)
was prepared by direct nitration of phenylacetic acid-R,R-d2.
In a 100 mL two-necked round-bottom flask were placed
concentrated sulfuric acid (20 mL) and 56% nitric acid (20 mL).
The mixture was cooled to 5 °C in an ice bath, and phenylacetic
acid-R,R-d2 was added slowly at such a rate that the temper-
ature remained at 10 C°. After the addition was completed
(ca. 30 min) with removal of the ice bath, the mixture was
stirred for 1 h and poured onto crushed ice (100 g). A slightly
yellow solid was separated by filtration, washed with ice water,
and dissolved in ether (50 mL). The ether solution was
separated from the aqueous portion and dried with MgSO4.
After evaporation of the ether, the solid was recrystallized from
anhydrous ether two times to give 33% yield (3.0 g). p-
Nitrophenylacetic acid-R,R-d2 retained 99% deuteration as
measured by 1H NMR. 1H NMR data (CDCl3 with 0.03%
TMS): 8.2 (d,2H), 7.5(d,2H). The mp (154-155 °C) agrees with
the literature value (154.4-154.8 °C).2
Th er m a l Rea ction s of Su bstitu ted ter t-Bu tyl P h en yl-
p er a ceta tes. Weighed samples of perester (50 mg), acetoni-
trile (7 mg, internal standard), and iodine (100-150 mg) were
dissolved in CDCl3 (5 mL). The solutions were divided into
several Pyrex ampules, which were degassed and sealed by a
freeze-pump-thaw method. The ampules (i.d. 4 mm, length
3 cm, 2/3 full) were immersed in a constant-temperature bath
for at least 200 s for thermal equilibration. Less than 20 s were
required for complete thermal equilibration, which was mea-
sured by a Copper-Constantan thermocouple. At various
The integrations of the benzylic peak of a perester at δ )
3.6-3.7 ppm and the peak of acetonitrile (δ ) 2.0 ppm) have
been made. When integrated values of the perester and
acetonitrile are designated as IP and IS, respectively, the
2
following equation can be obtained: IP/IS ) /3C/CS. C and CS
represent the concentrations of the perester and acetonitrile,
respectively. Rate constants were then produced by the method
of least squares utilizing ln(Co/Ct) ) kYHt. Co and Ct are
concentrations of the perester at time 0 and t, respectively. Co
has been fixed as a concentration of a perester that already
underwent homolysis for more than thermal equilibration
time. When substituent Y is p-CH3, p-H, and p-NO2, plots of
ln(Co/Ct) vs t showed excellent linear relations (r g 0.997).
However, the reactions of p-methoxy perester did not show
simple first-order kinetics. The product analysis suggested that
p-methoxybenzyl iodide is unstable under the reaction condi-
tions, which could interfere with homolysis of the perester.
Pyridine has been added to trap p-methoxybenzyl iodide so
as to observe first-order kinetics (refer to Supporting Informa-
tion for plot of ln(Co/Ct) vs t).
Th er m a l Rea ction s of Su bstitu ted ter t-Bu tyl P h en yl-
p er a ceta tes-r,r-d 2. Weighed samples of perester (50 mg),
CH2Cl2 (25 mg, internal standard), and iodine (300-350 mg)
were dissolved in CDCl3 (5 mL). Aliquots (0.7 mL) of the
solutions were placed into degassed, sealed Pyrex ampules
using a freeze-pump-thaw method. The ampules were im-
mersed in the constant-temperature bath for thermal equili-
bration. The tubes were then removed periodically from the
bath, quenched in ice-water, and opened for NMR analysis.
Shift reagent (Eu(fod)3, 0.1-0.2 mg) was added into the
reaction mixture for resolution of the proton peaks of various
tert-butyl groups. The tert-butyl of the perester retains a
chemical shift at δ ) 1.26 ppm. The reaction mixture showed
peaks at δ ) 1.26 and 5.1 ppm of CH2Cl2, which were
integrated to give values of IP and IS, respectively. When
concentrations of the pereseters and CH2Cl2 are defined as C
and CS, respectively, IP/IS ) 9/2C/CS could be obtained. The rate
constants (kYD) were likewise obtained from plots of ln(Co/Ct)
) kYDt (refer to Supporting Information).
Ack n ow led gm en t. We warmly thank the Korea
Science & Engineering Foundation for generous support
(KOSEF 95-0501-06-01-3). We are also very grateful to
the reviewers for their efforts to improve the manu-
script. Professor Bong Rae Cho has kindly provided
valuable comments. Technical assistance was extended
by Mr. J i Won Yang.
Su p p or tin g In for m a tion Ava ila ble: Plots of ln(Co/Ct) )
k
YHt and ln(Co/Ct) ) kYDt are provided for the thermal
decomposition of YC6H4CH2CO3But and YC6H4CD2CO3But at
80 °C, respectively. Plots of ∆∆Sq vs σ+ are also given. This
Y-H
material is available free of charge via the Internet at
http://pubs.acs.org.
J O9814492