Rate constants for the reaction of cumylperoxyl radicals with Bu3SnH and (TMS)3SiH

Article abstract of DOI:10.1039/a808894b

The rate constants for the reaction of a cumylperoxyl radical with Bu₃SnH and (TMS)₃SiH were determined at 72.5°C to be 1600 and 66 M^-1 s^-1, respectively, by using inhibited hydrocarbon oxidation methodologies.

Full text of DOI:10.1039/a808894b

Rate constants for the reaction of cumylperoxyl radicals with Bu3SnH and
(TMS)3SiH
Chryssostomos Chatgilialoglu,*a Vitaliy I. Timokhin,*b Andriy B. Zaborovskiy,b Daria S. Lutsykb and
Ruslan E. Prystanskyb
a I.Co.C.E.A., Consiglio Nazionale delle Ricerche, Via Gobetti 101, I-40129 Bologna, Italy.
E-mail: chrys@area.bo.cnr.it
b Department of Physico-Chemistry, Institute of Physical Chemistry, National Academy of Sciences of Ukraine, 3A
Naukova Street, 290053, Lviv, Ukraine
Received (in Liverpool, UK) 13th December 1998, Accepted 18th January 1999
The rate constants for the reaction of a cumylperoxyl radical
with Bu3SnH and (TMS)3SiH were determined at 72.5 °C to
be 1600 and 66 m21 s21, respectively, by using inhibited
hydrocarbon oxidation methodologies.
two reducing agents, depending on the presence or absence of
an induction period when they are used as antioxidants. Scheme
2 shows that the initiation and propagation steps are identical for
the two methods, whereas the termination steps are different.
Providing that conditions can be found in which termination
1 is applied, the rate constant for H-atom abstraction from the
silane by cumylperoxyl radicals can be obtained from eqn. (1),9
In the last two decades, radical chain reactions have proven to
be valuable in synthetic organic chemistry.1a The majority of
such chain processes have been carried out by using Bu3SnH or
(TMS)3SiH, which are the most widely used reagents under
reduction conditions.1 For a synthetically useful radical chain
reaction the intermediates must be disciplined. The concept of
discipline2 in free radical reactions is strictly connected with the
kinetic information of each individual step, although upon first
consideration, the importance of the kinetic knowledge might
be less apparent in planning a synthetic strategy.
(-d[O2]/ dt)0 (-d[O2]/ dt)
(-d[O2]/ dt) (-d[O2]/ dt)0
fkSiH[(TMS)3SiH]0
-
=
(1)
(2ktR )1/ 2
i
where (2d[O2]/dt) and (2d[O2]/dt)0 represent the initial rate of
oxidation of cumene with or without (TMS)3SiH.‡ The
stoichiometric factor f, which is the number of cumylperoxyl
radicals trapped by each molecule of silane, is assumed to be
equal to 2.§ The inhibition of thermally initiated (0.021 m of
AIBN at 345.5 K) oxidation of pure cumene by (TMS)3SiH
(concentration range of 1.84 3 1024 to 6.13 3 1024 m) is found
to be the case. Fig. 1 shows that the oxidation rate of cumene
decreases upon increasing the concentration of silane without an
induction period. On the other hand, Fig. 1 also shows the linear
regression analysis of eqn. (1), whose slope provides 2kSiH/(2kt
Ri)1/2 to be 7.1 3 102 m21. Since the rate of radical production11
The reaction of peroxyl radicals with organic substrates is
one of the most important classes of reactions in chemistry,3
being the key step of several processes, e.g. autoxidation of
synthetic polymers, lipid peroxidation and DNA damage. The
use of alkylperoxyl radicals in synthesis is limited to a few
procedures.4,5 In one of them, the conversion of halides to
alcohols has recently been approached by tin hydride reduction
under aerobic conditions.5 The reaction sequence that has been
suggested is shown in Scheme 1. Examples in which the alkyl
radical rearranges itself prior to the reaction with molecular
oxygen are reported,5 although in some cases the reaction is
sluggish and gives undesired products.6 Nakamura and co-
workers were able to run these experiments under conditions
where hydroperoxides were isolated in good yields.7 In one
case, the (TMS)3SiH mediated halide to alcohol transformation
was reported to give a moderate yield.7 In all these transforma-
tions, the reaction of a peroxyl radical with a tin or silicon
hydride is crucial. Kinetic information regarding this step is not
available. In their recent mechanistic studies on model DNA
damage, Greenberg and co-workers8 arbitrarily assumed the
rate constants for the ROO• radical with Bu3SnH to be 5 3 103
m21 s21 at 55 °C in THF. Following these considerations, the
knowledge of the rate constants for the reaction of peroxyl
radicals with Bu3SnH and (TMS)3SiH turns out to be necessary
for better synthetic planning as well as a reference reaction for
mechanistic studies.
Initiation
Production of R at a rate = Ri
Propagation
fast
+
O2
R
ROO
ROOH
kp
+
+
ROO
RH
R
Termination 1
2kt
2 ROO
ROO
non-radical products
kSiH
+
+
(TMS)3Si
(TMS)3SiH
ROOH
(TMS)3Si
or
(TMS)3SiOO
fast
non-radical
products
Herein we report the rate constants of a cumylperoxyl radical
with (TMS)3SiH and Bu3SnH which have been determined by
using inhibited hydrocarbon oxidation methodologies.9,10 In
particular, two different kinetic approaches were used for the
+
(f–1) ROO
Termination 2
kSnH
+
Bu3SnH
+
Bu3Sn
ROOH
ROO
Bu3Sn
O2
RX
R
ROO
Bu3Sn
or
Bu3SnOO
fast
non-radical
products
+
Bu3SnH
(f–1) ROO
R = PhCMe2
ROH
Scheme 1
ROOH
Scheme 2
Chem. Commun., 1999, 405–406
405

kp[RH]0Ri/fkSnH was found to be 11.6 3 1029 s21.§ Taking kp
= 1.3 m21 s21 12 [RH]0 = 7.188 m, Ri = 0.89 3 1026 m s21 11
, ,
and an average of f = 0.46, we calculated kSnH = 1559 m21 s21
at 345.5 K.
In summary, the rate constants of 66 and 1600 m21 s21 at
345.5 K were obtained for the reaction of cumylperoxyl radicals
with (TMS)3SiH and Bu3SnH, respectively. Therefore, the
alkylperoxyl radicals abstract a hydrogen atom from the tin
hydride 24 times faster than from the silane. (TMS)3SiH is a
notably less reactive hydrogen donor than Bu3SnH, mainly due
to the ca. 5 kcal mol21 difference in bond dissociation
energies.14 For comparison, primary alkyl radicals and acyl
radicals abstract hydrogen from the silane five and 16 times
slower, respectively, than from the tin hydride at the same
temperature and in much faster processes.14
Fig. 1 Plots of (2) 2d[O2]/dt and (5) (2d[O2]/dt)0/(2d[O2]/dt) 2
(2d[O2]/dt)/(2d[O2]/dt)0 vs. [(TMS)3SiH]; [AIBN] = 0.021 m at 345.5 K.
Notes and references
† This relation can be used when the initial oxidation rate and the initiation
rate remain essentially constant under the experimental conditions.
‡ This is possible if the reactions in termination 1 dominate the systems and
if the products of these reactions are not active towards initiation.
§ Since the chain length of oxidation [i.e. n = (2d[O2]/dt)exp/Ri] is less than
12, the initial rate of oxidation is corrected for oxygen absorption by the
and the termination for cumylperoxyl radicals12 are Ri = 1.09 3
1026 m s21 and 2kt = 3.2 3 104 m21 s21, respectively, we
calculated kSiH = 66.3 m21 s21 at 345.5 K.
The inhibition of thermally initiated (0.0172 m of AIBN at
345.5 K) oxidations of pure cumene by Bu3SnH (concentration
range of 1.07 3 1023 to 3.20 3 1023 m) shows an induction
period (t). Termination 2 in Scheme 2 describes the inhibition
mechanism and the initial rate of oxidation (before exiting from
the induction period), given by eqn. (2), of peroxyl radicals
initiator, and nitrogen evolution from the initiator, i.e. 2d[O2]/dt
=
(2d[O2]/dt)exp 2 Ri, where the (2d[O2]/dt)exp is the experimental initial
rate of cumene oxidation.
1 (a) For example, see: D. P. Curran, N. Porter and B. Giese,
Stereochemistry of Radical Reactions, VCH, Weinheim, 1995; W. B.
Motherwell and D. Crich, Free Radical Chain Reactions in Organic
Synthesis, Academic Press, London, 1992; (b) For reviews on
(TMS)3SiH, see: C. Chatgilialoglu, Acc. Chem. Res., 1992, 25, 188; C.
Chatgilialoglu, C. Ferreri and T. Gimisis, in The Chemistry of Organic
Silicon Compounds, ed. S. Rappoport and Y. Apeloig, Wiley, London,
1998, vol. 2, pp. 1539–1579.
2 A terminology introduced by Barton. For example, see: D. H. R. Barton,
Aldrichim. Acta, 1990, 23, 3; D. H. R. Barton and S. I. Parekh, Half a
Century of Free Radical Chemistry, CUP, Cambridge, UK, 1993.
3 For some representative reviews, see: K. U. Ingold, Acc. Chem. Res.,
1969, 2, 1; N. A. Porter, in Organic Peroxides, ed. W. Ando, Wiley,
Chichester, 1992; C. von Sonntag and H.-P. Schuchmann, Angew.
Chem., Int. Ed. Engl., 1991, 30, 1229; C. Walling, in Active Oxygen in
Chemistry, ed. C. S. Foote, J. C. Valentine, A. Greenberg and J. F.
Liebman, Blackie, London, 1995, pp. 24–65.
kp[RH]0 Ri
(-d[O2 ]/ dt) =
(2)
fkSnH[Bu3SnH]0
trapped by each molecule of Bu3SnH. The use of Boozer et al.’s
induction period method,13 of which an example is illustrated in
Fig. 2 (i.e. suppression of the oxygen uptake by the presence of
Bu3SnH) allows the determination of the t values. Then the
stoichiometric factor f can be obtained from f
= Rit/
[Bu3SnH]0.9,10 All the data are summarized in Table 1. From the
slope of the linear plot between 2d[O2]/dt and 1/[Bu3SnH]0, the
4 For example, see: G. L. Hill and G. M. Whitesides, J. Am. Chem. Soc.,
1974, 96, 870; D. H. R. Barton, D. Crich and W. B. Motherwell,
J. Chem. Soc., Chem. Commun., 1985, 1066; H. C. Brown, M. M.
Midland and G. W. Kabalka, Tetrahedron, 1986, 42, 5523.
5 E. Nakamura, T. Inubushi, S. Aoki and D. Machii, J. Am. Chem. Soc.,
1991, 113, 8980; S. Moutel and J. Prandi, Tetrahedron Lett., 1994, 35,
8163; S. Mayer and J. Prandi, Tetrahedron Lett., 1996, 37, 3117; M.
Sawamura, Y. Kawaguchi, K. Sato and E. Nakamura, Chem. Lett., 1997,
705; M. Sawamura, Y. Kawaguchi and E. Nakamura, Synlett, 1997,
801.
6 For example, see: D. L. Boger and J. A. McKie, J. Org. Chem., 1995, 60,
1271.
7 E. Nakamura, K. Sato and Y. Imanishi, Synlett, 1995, 525.
8 K. A. Tallman, C. Tronche, D. J. Yoo and M. M. Greenberg, J. Am.
Chem. Soc., 1998, 120, 4903.
9 E. T. Denisov and I. V. Khudyakov, Chem. Rev., 1987, 87, 1313; E. T.
Denisov, Liquid-Phase Reaction Rate Constants, Plenum, New York,
1974; N. M. Emanuel, E. T. Denisov and Z. K. Maizus, Liquid Phase
Oxidation of Hydrocarbons, Plenum, New York, 1967.
Fig. 2 Oxygen consumption during the AIBN-initiated (0.0172 m) oxidation
of cumene in the presence of 2.64 3 1023 m of Bu3SnH at 345.5 K.
10 J. A. Howard, Y. Ohkatsu, J. H. B. Chenier and K. U. Ingold, Can. J.
Chem., 1973, 51, 1543; J. H. B. Chenier, S. B. Tong and J. A. Howard,
Can. J. Chem., 1978, 56, 3047; S. Korcek, J. H. B. Chenier, J. A.
Howard and K. U. Ingold, Can. J. Chem., 1972, 50, 2285.
11 J. P. Van Hook and A. V. Tobolsky, J. Am. Chem. Soc., 1958, 80, 779;
E. Niki, Y. Kamija and N. Ohta, Bull. Chem. Soc. Jpn., 1969, 42,
3220.
12 D. G. Hendry, J. Am. Chem. Soc., 1967, 89, 5433.
13 C. E. Boozer, G. S. Hammond, C. E. Hamilton and J. N. Sen, J. Am.
Chem. Soc., 1955, 77, 3233.
Table 1 Kinetic data for the oxidation of cumene in the presence of
Bu3SnHa
(d[O2]/dt)b/m s21
[Bu3SnH]0/m
n
t/min
f
10.94 3 1026
7.01 3 1026
3.15 3 1026
4.67 3 1026
a
1.07 3 1023
1.64 3 1023
2.64 3 1023
3.20 3 1023
11.8
7.5
3.4
5.0
10
15
21
20
0.52
0.51
0.44
0.35
[AIBN] = 0.0172 m, Ri = 0.89 3 1026 m s21 at 345.5 K. d[O2]/dt =
(d[O2]/dt)exp 2 Ri.
b
14 C. Chatgilialoglu, Chem. Rev., 1995, 95, 1229.
Communication 8/08894B
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Chem. Commun., 1999, 405–406