Induced thermolysis of tert-butyl phenylperacetates by thiophenol: Simultaneous occurrence of homolysis and single electron transfer

Article abstract of DOI:10.1021/jo0106290

Thermolysis of tert-butyl phenylperacetates in the presence of thiophenol takes place via dual mechanism. The two-bond homolysis indicates ρ_H⁺ = -1.16, testifying to polar transition states. The single electron transfer yields a radical anion intermediate which undergoes fragmentation with ρ_ET = 1.01.

Full text of DOI:10.1021/jo0106290

7738
J . Org. Chem. 2001, 66, 7738-7740
In d u ced Th er m olysis of ter t-Bu tyl P h en ylp er a ceta tes by
Th iop h en ol: Sim u lta n eou s Occu r r en ce of Hom olysis a n d Sin gle
Electr on Tr a n sfer †
Sung Soo Kim,* Alexey Tuchkin,‡ and Chun Soo Kim
Department of Chemistry and Center for Chemical Dynamics, Inha University,
Inchon 402-751, South Korea
sungsoo@inha.ac.kr
Received J une 19, 2001
Thermolysis of tert-butyl phenylperacetates in the presence of thiophenol takes place via dual
mechanism. The two-bond homolysis indicates F+H ) -1.16, testifying to polar transition states.
The single electron transfer yields a radical anion intermediate which undergoes fragmentation
with FET ) 1.01.
Sch em e 1
In tr od u ction
Benzoyl peroxide1-3 and tert-butyl perbenzoates4 ex-
hibit fast decompositions in the presence of nucleophiles
including sulfides. 1,2-Dioxetanes5 were reduced to cor-
responding vicinal diols by various thiols. The thermoly-
sis1-5 followed second-order kinetics, first-order in per-
oxidic substrate and in electron donor, respectively. We
wish to herein report the reactions of ring-substituted
tert-butyl phenylperacetates (TBPA) with thiophenol that
involve concurrent homolysis and a single electron trans-
fer (SET) mechanism.
Sch em e 2
Resu lts a n d Discu ssion
Reactions at 80 °C of degassed and sealed Pyrex tubes
containing TBPA (0.05 M), thiophenol (0.5-3.5 M),
bibenzyl (0.01 M, internal standard), and potassium
fluoride (acid scavenger: 50 mg) in CDCl3 gave rise to
formations of phenylacetic acid, tert-butyl alcohol, tolu-
ene, and phenyl disulfide. Scheme 1 shows the two-bond
homolysis of TBPA which has been thoroughly investi-
gated by Bartlett et al.6 and us.7 The initial transfer of
an electron from thiophenol to TBPA results in forma-
tions of the radical anion 1 and the radical cation 2, which
eventually yield phenyl acetic acid and tert-butyl alcohol
(Scheme 2). Alternatively, they might decompose to give
second-order rate constant for SET reactions. Equation
1 could be transformed into a simpler form of eq 2 when
a large excess (relative to concentration of TBPA) of
thiophenol is utilized. Integration of eq 2 produces ln Co/C
) kobsdt where Co and C denote the initial and final mole
concentrations of TBPA, respectively. Ring-substituted
TBPA (YPhCH2C(O)O2But, Y: p-CH3, H, p-Cl, m-Cl, and
p-NO2) were heated at 80 °C with several different
concentrations of thiophenol. The values of Co and C were
determined by NMR technique for ln Co/C ) kobsdt.
Accordingly kobsd has been obtained from the plot of ln
Co/C vs t. All the plots showed good to excellent linearities
with correlation coefficients r ) 0.990 in most cases,
where the slopes correspond to magnitude of kobsd. Then,
•
PhCH2 , CO2, and t-BuO- via two-bond homolysis,6,7and
•
PhCH2CO2 may not intervene the reactions at all.
The rates of decompositions of TBPA in the presence
of thiophenol can be written as with eq 1. kH corresponds
to rate constant for unimolecular homolysis while kET is
* To whom correspondence should be addressed. Fax +82-32-867-
5604.
kH and kET were determined from plot of kobsd ) kH
+
† The late Professor Glen A. Russell has been a Distinguished
Foreign Adviser for this investigation administered by KRF. We are
heavily indebted to his mentorship.
kET[PhSH]. These plots are presented in Supporting
Information. The rate data are shown with Table 1.
‡ A visiting scholar from St. Petersburg State University by a grant
from the Science & Technology Policy Institute under Korea-Russia
manpower exchange program.
-d[TBPA]/dt ) kH[TBPA] + kET[TBPA][PhSH] (1)
-d[TBPA]/dt ) (kH + kET[PhSH])[TBPA]
(1) Pryor, W. A.; Bickley, H. T. J . Org. Chem. 1972, 37, 2885.
(2) Walling, C.; Zhao, C. Tetrahedron 1982, 38, 1105.
(3) Pryor, W. A.; Hendrickson, W. H., J r. J . Am. Chem. Soc. 1983,
105, 7114.
(4) Pryor, W. A.; Hendrickson, W. H., J r. J . Am. Chem. Soc. 1975,
97, 1580.
) kobsd [TBPA]
(2)
(5) Adam, W.; Epe, B.; Schiffman, D.; Vargas, F.; Wild, D. Angew.
Chem., Int. Ed. Engl. 1988, 27, 429.
(6) Bartlett, P. D.; Ru¨chardt, C. J . Am. Chem. Soc. 1960, 82, 1756.
(7) Kim, S. S.; Baek, I. S.; Tuchkin, A.; Go, K. M. J . Org. Chem.
2001, 66, 4006.
10.1021/jo0106290 CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/13/2001

Thermolysis of tert-Butyl Phenylperacetates
J . Org. Chem., Vol. 66, No. 23, 2001 7739
Ta ble 1. Ra te Da ta for Th er m olysis of Rin g-Su bstitu ted ter t-Bu tyl P h en ylp er a ceta tes w ith Th iop h en ol a t
80 °C in CDCl3
a
b
substituent (Y)
kobsd × 104, s-1
kH × 104, s-1
kET × 104, s-1 M-1
0.15 ( 0.02
p-CH3
2.65(0.932), 2.68(0.976)
2.76(1.516), 2.80(1.899)
1.38(1.10), 1.51(1.661)
1.66(2.377)
1.35(1.514), 1.41(1.744)
1.72(2.517), 1.96(3.459)
0.76(0.631), 1.06(0.989)
1.40(1.785), 1.57(2.068)
0.938(0.519), 1.18(0.723)
1.63(1.143), 2.03(1.414)
3.09(2.074)
2.53 ( 0.03
(2.63)
p-H
1.14 ( 0.01
(1.06)
0.87 ( 0.06
0.219 ( 0.006
0.32 ( 0.02
0.53 ( 0.05
1.38 ( 0.08
p-Cl
(0.84)
m-Cl
p-NO2
0.47 ( 0.07
(0.42)
0.16 ( 0.01
(0.16)
F+H ) -1.16(0.997)
FH ) -1.53(0.947)
FET ) 1.01(0.996)
F+ET ) 0.89(0.992)
a
b
The figures in parentheses indicate mole concentrations (M) of thiophenol employed. The values in parentheses are the rate constant
(kH) reported by us with iodine trapping of the intermediate radicals of ref 7. The two figures show an excellent agreement.
Ta ble 2. Ra tio of kET/kH Ca lcu la ted fr om th e Yield of
Tolu en e a n d P h en yla cetic Acid fr om Th em olysis of
ter t-Bu tyl P h en ylp er a ceta te w ith Th iop h en ol a t 80 °C
in CDCl3
Ta ble 3. Differ en ce of kET Occu r r in g w ith Differ en t
Solven t System s
kaobsd × 104,
kH × 104, kET × 104,
solvent
s-1
s-1
s-1 M-1
6.08
[PhSH], M
[PhCH3]
[PhCH2CO2H]
kET/kH
5% CH3OH/CDCl3 5.51(0.74), 7.50(1.027)
9.39(1.375)
1.09
1.20
1.41
1.99
0.0771
0.1048
0.0371
0.0181
0.0303
0.0151
0.196
0.205
0.204
CDCl3
1.38(1.10), 1.51(1.661)
1.66(2.377)
1.14
0.219
a
F+H ) -1.16 can be excellently compared with our7 FH+
) -1.18 at 80 °C, considering the different reaction
conditions. The negative value of F+H indicates the polar
transition state involved with the two-bond homolysis.
Formation of radical anion 1 and cation 2 could be a rate-
determining electron transfer. Fragmentation of 1 gave
rise to phenylacetate anion and tert-butoxy radical.
Phenylacetic acid is formed from the acetate anion and
thiophenol radical cation. The magnitude of kET increases
with electron-withdrawing groups to give FET ) 1.01. The
SET mechanism shows better correlation with σ rather
than σ+, indicating the negative charge is localized. The
size of FET ) 1.01 could be also consistent with SET. SN2
reactions8 are rather immune to electronic effect of
substituent because requirements of bond formation and
cleavage are opposite. Therefore, much smaller Hammett
F value occurred. The value of kET/kH for decomposition
of C6H5CH2C(O)O2But is measured to be 0.219/1.14 )
0.192 (Table 1). However, kET/kH can be also obtained
from the ratio of kET [C6H5SH]/kH ) [C6H5CH2CO2H]/
[C6H5CH3] when the perester is completely decomposed
for 5 half-life. The molar concentration of toluene and
phenylacetic acid were measured at 1.20, 1.41, and 1.99
M of thiophenol. The figures of kET/kH (0.196, 0.205, and
0.204) (Table 2) is very close to kET/kH ) 0.192 that is
obtained by measuring the disappearance of the perester
(Table 1).
The figures in paretheses correspond to mole concentration
of thiophenol (M) where the perester was decomposed.
Con clu sion
tert-Butyl phenylperacetates interact with thiophenol
via SET to give radical ion pair. The radical anion of the
perester fragments into phenylacetate anion and tert-
butoxy radical. Phenylacetate anion and thiophenol radi-
cal cation give phenylacetic acid. tert-Butoxy radical
abstracts hydrogen from thiophenol to yield tert-butyl
alcohol. It is interesting to note that kH decreases but
kET becomes larger when the substituent becomes electron-
withdrawing. The former (kH) is better correlated with
σ+ while the latter (kET) is with σ.
Exp er im en ta l Section
Ma ter ia ls a n d Meth od s. Substituted phenylacetic 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 procedures9 if necessary. A
Varian Gemini 2000 NMR spectrometer was used for the
analysis of the reaction mixtures. tert-Butyl phenylperacetates
(YC6H4CH2CO3But, Y ) p-CH3, p-H, p-Cl, m-Cl, p-NO2) were
prepared according to the method described in Experimental
Section of reference 7.
Th er m a l Rea ction s of ter t-Bu tyl P h en ylp er a ceta tes.
Weighed samples of a perester (0.05 M), thiophenol (0.5-2.5
M), bibenzyl (0.01 M, internal standard), and potassium
fluoride (50 mg) were dissolved in CDCl3 (5 mL). The solutions
were divided into several Pyrex ampules that were degassed
and sealed by freeze-pump-thaw method. The ampules (i.d.
4 mm, length 3 cm, 2/3 full) were immersed in a constant-
temperature bath at appropriate temperature. Less than 20 s
were required for complete thermal equilibration, which was
measured by a copper-constantan thermocouple. At various
intervals, the tubes were removed from the oil bath, quenched
in ice-water, and opened. The reaction mixture was filtered
and ready for NMR analysis of the remaining perester.
Disappearance of the benzylic peak (δ ) 3.6-3.7 ppm) was
monitored. Rate constants were then calculated by the method
Formation of phenylacetic acid might be also realized
via SN2 reaction. tert-Butyl phenylperacetate were heated
at 80 °C with variable concentrations of thiophenol in
5% CH3OH/CDCl3. The comparison is made for decom-
position of the perester in pure CDCl3 and 5% CH3OH/
CDCl3 (Table 3). kET is accelerated and kH is nearly
identical when 5% CH3OH is added. The acceleration of
6.08/0.219 ) 27.7 can be due to the increase of polarity
of the solvent that may stabilize the radical anion and
cation. However, the rate should be slower by addition
of CH3OH if the reaction were to follow SN2.
(8) Isaacs, N. Physical Organic Chemistry, 2nd ed.; Longman
Scientific & Technical: England, 1995; Chapter 10.
(9) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. F. Purification of
Laboratory Chemicals, 2nd ed; Pergamon press: Oxford, U.K., 1980.

7740 J . Org. Chem., Vol. 66, No. 23, 2001
Kim et al.
of least squares, utilizing ln Co/Ct ) kobsdt. Accordingly kobsd
has been obtained from the plot of ln Co/C vs t. kH and kET can
be calculated with kobsd ) kH + kET [PhSH]. These are indicated
in the Supporting Information.
Ack n ow led gm en t. We warmly thank Korea Re-
search Foundation (KRF) for a Nondirected Research
Fund (1996-1999). Technical assistance was provided
by Mr. Chong Hyuck Yoon. We are also indebted to
Brain Korea 21 provided from Ministry of Education
through KRF.
Com p lete Th er m a l Rea ction s of ter t-Bu tyl P h en yl-
p er a ceta tes. Samples of tert-butyl phenylperacetate (0.05 M),
thiophenol (1.20, 1.41, 1.99 M), and bibenzyl (0.005 M) were
placed into a 2 mL volumetric flask, and CDCl3 was added to
the line. This solution and KF (30 mg) were divided into the
ampules that subsequently underwent degassing three times
and sealing. After reaction, few drops of trifluoroacetic acid
were added to destroy the complex of C6H5CH2COOH‚KF. The
filterate was then analyzed with NMR for phenylacetic acid
(benzylic: δ ) 3.6-3.7), bibenzyl (methylene: δ ) 2.9), tert-
butyl alcohol (tert-butyl: δ ) 1.26), toluene (methyl: δ ) 2.31).
Su p p or tin g In for m a tion Ava ila ble: Graphical plots of
ln Co/Ct vs t and kobsd ) kH + kET [PhSH] are provided to show
the accurate measurements of the rate constants. This mate-
rial is available free of charge via the Internet at http://
pubs.acs.org.
J O0106290