Study of the radical chemistry promoted by tributylborane

Article abstract of DOI:10.1007/s11164-012-0512-2

The structures of radicals generated in the oxidation process of trialkylborane were detected based on ultra-high performance liquid chromatographyquadrupole-time-of-flight mass spectrometry (UPLC/Q-ToF MS) combined with the spin trapping method. Structur

Full text of DOI:10.1007/s11164-012-0512-2

Res Chem Intermed (2012) 38:1893–1907
DOI 10.1007/s11164-012-0512-2
Study of the radical chemistry promoted
by tributylborane
•
•
•
Shujuan Liu Zhen Zheng Minrui Li
Xinling Wang
Received: 17 September 2011 / Accepted: 20 February 2012 / Published online: 16 March 2012
Ó Springer Science+Business Media B.V. 2012
Abstract The structures of radicals generated in the oxidation process of trial-
kylborane were detected based on ultra-high performance liquid chromatography-
quadrupole-time-of-flight mass spectrometry (UPLC/Q-ToF MS) combined with the
spin trapping method. Structural identification of the spin adducts produced by 5,5-
dimethyl-1-pyrroline-1-oxide with radicals could be carried out unambiguously
by combining the data obtained by UPLC/Q-ToF MS analyses. Then the oxidation
mechanism was described. To specify the inter-relationships between the oxidation
process of trialkylborane and the concomitant radical chemistry, four kinds of
alkanes providing different H-abstraction reactivity to alkoxy radical were chosen as
radical capturers. The final oxidation products of trialkylborane were characterized
by GC–MS and 11B-NMR. The results indicted that the radical content was not only
affected by the oxidation degree of trialkylborane, but also done by the activity of
alkane. Especially the hydrogen atom abstraction by n-butoxy radical played an
important role in the oxidation process of tributylborane, which would promote the
oxidizability of tributylborane and deepen the oxidation degree of tributylborane.
Keywords Oxygen ꢀ Oxidation ꢀ Radical ꢀ Trialkylborane ꢀ Mechanism
S. Liu ꢀ Z. Zheng ꢀ M. Li ꢀ X. Wang (&)
School of Chemistry & Chemical Engineering, Shanghai Jiao Tong University,
800 Dongchuan Road, Shanghai 200240, People’s Republic of China
e-mail: xlwang@sjtu.edu.cn
Z. Zheng
e-mail: zzheng@sjtu.edu.cn
S. Liu ꢀ Z. Zheng ꢀ M. Li ꢀ X. Wang
State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University,
Shanghai 200240, People’s Republic of China
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Introduction
The ability of organoborane to participate in free radical processes has been identified
since the earliest investigation of its chemistry [1–3]. Trialkylborane has been used as a
radicalinitiator in the field of adhesives [4–6], polymerization [7–12], grafting [13, 14],
and others [15, 16]. The oxidation mechanism of trialkylborane has been studied for
several decades. It is assured that the radical chemistry promoted by trialkylborane is
triggered by the initial oxidation of trialkylborane to peroxyborane R2B(OOR), but the
generation process of radicals is a controversial issue. Two speculations have been
proposed so far: the first is that an alkoxy radical and a borinate radical are produced
through the homolytic cleavage of peroxyborane (Eq. 1) [3, 17]. The second is that an
alkyl radical and a peroxyborane radical are produced through the heterolytic cleavage
of peroxyborane (Eq. 2) [18, 19].
R2BðOORÞ ! R2BOꢁ þ ROꢁ
ð1Þ
ð2Þ
R2BðOORÞ ! R2BOOꢁ þ Rꢁ
There is no confirmation of the radical chemistry since the oxidation process of
trialkylborane is very complex and not controllable, and the structures of generated
radicals are hard to determine by the often-used ESR method due to the ESR spectra
of different radicals with similar hyperfine splitting constants.
In this study, we utilized the UPLC/Q-ToF MS/MS technique to identify the
radicals formed from the oxidation of trialkylborane, and obtained more information
for the oxidation mechanism of trialkylborane. On the other hand, the interrela-
tionships between the oxidation degree of trialkylborane and the concomitant
radical chemistry were reported. Four kinds of alkanes with different C–H reactivity
were used to study in detail the interactions between the oxidation degree of
trialkylborane and the concomitant radical chemistry. The comprehensive conclu-
sions from both experiments and theories would be a reliable theoretical basis for
the applications of trialkylborane, such as promoting grafting and adhesion.
Materials and methods
1,1-Diphenyl-2-picrylhydrazine (DPPH) and 5,5-dimethyl-1-pyrroline-1-oxide
(DMPO) were obtained from Sigma-Aldrich. Both were used without further
treatment. Tributylborane (TBB), 1 mol/L in ethyl ether, could be purchased from
Aldrich and distilled under vacuum at 50 °C to remove ethyl ether before being
used. Alkane compounds: 3-methylpentane, n-decane, and squalane were all from
Alfa Aesar and 2,4-dimethylpentane was obtained from Tokyo Chemical Industry
Co., Ltd. They were used directly.
The oxidation process of tributylborane
Alkane and tributylborane were charged into an argon-filled flask with a magnetic
stirrer. The molar ratio of alkane to tributylborane was kept at 10:1. Half an hour
123

Study of the radical chemistry
1895
later, oxygen was injected slowly at the rate of 10 mL/h. The reaction was allowed
to proceed overnight at 30 °C.
UPLC/Q-ToF MS/MS analysis
The reaction mixture, containing 7.5 lmol DMOP, 1.5 lmol tributylborane and
50 mL acetonitrile, was stirred in a beaker at room temperature for 3 min. The
reaction mixture was filtered and injected into the UPLC column for UPLC/Q-ToF
MS/MS measurements.
Evaluation of the concentration of free radicals
A 0.5-mL ethanol solution of tributylborane (3.0 9 10-2 mol/L) and 5 mL ethanol
solution of DPPH (2.0 9 10-4 mol/L) was added to an argon-filled flask, and then a
different amount of oxygen was introduced (0, 2, 4, 5, 10, 15, 30, and 50 mL).
The mixture was shaken vigorously and allowed to stand for 20 min in the dark.
The decrease in absorbance was measured at 515 nm against a blank (ethanol)
with a spectrophotometer.
Characterization
Gas chromatography and mass spectrometry (GC–MS) measurements were
performed on a QP2010 GC–MS. Conditions were as follows: column: DB-5
0.25 mm 9 0.25 lm 9 30 m, column pressure: 10.1 psi, flow-rate: 1.22 mL/min,
injector temperature: 280 °C. Temperature of the column: kept 3.5 min at 50 °C and
then increased to 250 °C at 20 °C/min, holding for 17 min scan: 33–750 amu.
HPLC/Q-ToF MS/MS was performed on UPLC Acquity/QToF MS Premier
(Waters, America). The conditions were: flow rate: 0.2 mL/min, max pressure:
15,000 psi, column temperature: 25 °C, mass range: m/z: 50–500, ion mode:
positive ion mode.
Nuclear magnetic resonance spectroscopy (NMR) was run on Avance-400
(Bruker, Switzerland) at 25 °C. Boric acid was used as external standard, and
CDCl3 was used as a solvent.
Results and discussion
DMPO is a widely used nitrone spin trap, which can trap not only carbon-centered
radicals but also oxygen-centered radicals [20]. The addition of a reactive free
radical across the double bond of DMPO formed a much more stable radical. These
spin-trapped radical species are hard to distinguish, since various radical-derived
nitroxides show similar ESR spectra. HPLC separation with MS detection has
proven reliable in distinguishing the different DMPO/radical adducts. We firstly
conducted experiments for the spin trapping of radicals generated from the
oxidation of tributylborane in which the identification of the DMPO adducts was
123

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S. Liu et al.
carried out by UPLC/Q-TOF MS and the resulted TIC spectrum was showed in
Fig. 1.
The mass profile of the species with a mass-to-charge ratio (m/z) of 186 could be
assigned to the protonated DMPO/ꢀOC4H9 adduct (Fig. 2). All the major fragment
ions in the MS spectrum were in agreement with the fragmentation pathways of the
DMPO/ꢀOC4H9 adduct (Scheme 1).
Meanwhile, the mass profile of the specie with a mass-to-charge ratio (m/z) of
170 was also found and illustrated in Fig. 3, and the major fragment ions in its MS
spectra were speculated as Scheme 2, which were assigned to the DMPO/ꢀC4H9
adduct.
The UPLC/Q-TOF MS analysis revealed that both n-butyl radial and n-butoxy
radical were produced in the oxidation process of tributylborane, and indicated
that the peroxyborane should be homolytic cleavage after the initial oxidation of
tributylborane because the electron-withdrawing from the neighboring borane
moiety weakened the O–O bond (Eq. 1). The n-butyl radical was formed through
LSJ_S1
100
1: TOF MS ES+
TIC
1.62
114.0914
5.37e4
6.94
228.2325
9.18
803.5411
8.61
284.2934
10.49
284.2936
3.15
170.1531
4.03
244.2270
6.71
228.2313
0
Time
2.00
4.00
6.00
8.00
10.00
12.00
Fig. 1 UPLC/Q-TOF MS chromatogram of the product of DMPO with TBB
LSJ_S1 249 (2.808) Cm (246:255-220:235)
130.0856
1: TOF MS ES+
1.43e4
100
LSJ_S1
100
2.81
130.0855
0
186.1486
2.00
4.00
131.0902
132.0912
187.1520
188.1557
448.9679
69.0702
112.0758
308.2339 322.2512
0
m/z
50 75 100 125 150 175 200 225 250 275 300 325 350 375 400 425 450 475 500
Fig. 2 UPLC/Q-TOF MS analysis of the product of DMPO/ꢀOC4H9 adduct
123

Study of the radical chemistry
1897
•OC4H9
OC4H9
N
O
NH
O•
m/z=186
- C4H9
O
NH
O
O
NH
OH
m/z=130
- H2O
- CH(CH3)2
N
N
O
O
m/z=112
m/z=69
Scheme 1 Proposed fragmentation pathways of the protonated ions DMPO/ꢀOC4H9 adduct
the SN1 electronic donating substitution of n-butoxy radical (Eq. 3) or the
trimerization of borinate radicals (Eq. 4). The latter was the main source of n-butyl
radical due to the high reactive of tributylborane.
BuOꢁ + Bu3B ! Bu2BðOBuÞ þ Buꢁ
ð3Þ
ð4Þ
3Bu2BOꢁ ! Bu3B3O3 þ 3Buꢁ
For further proving the above mechanism, four kinds of alkanes with different C–H
activities (shown in Scheme 3) were used as the capturers of n-butoxy radical due to
the strong hydrogen atom abstraction ability of n-butoxy radical. The oxidation
process of trialkylborane was carried out in serious alkanes at room temperature.
Then the formations of coupling and hydrogen atom abstraction of generated
radicals were separated and distinguished by GC–MS chromatogram. Two of the
TIC spectra are shown as Figs. 4 and 5.
N-butanol, n-octane, and 2,4,6-tributylboroxin were all detected from the reaction
products of tributylborane and alkanes. The results suggested the generation of
n-butoxy, n-butyl, and borinate radicals in the oxidation process of tributylborane.
In the alkane compound, n-butoxy radical was proposed to abstract hydrogen atom
from alkane, thereby yielding a carbon-centered radical on the alkane. N-butyl
123

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S. Liu et al.
LSJ_S1 292 (3.292) Cm (289:297-266:272)
170.1538
1: TOF MS ES+
3.40e3
LSJ_S1
100
100
3.42
170.1527
2.48
170.1542
0
2.00
4.00
209.1650
322.2502
323.2482
302.2234
114.0908
210.1668
81.0686
186.9550
137.9863
404.2879
420.2823
268.2282
433.3120
0
50 75 100 125 150 175 200 225 250 275 300 325 350 375 400 425 450 475 500
LSJ_S1 304 (3.424) Cm (303:312-332:347)
170.1529
1: TOF MS ES+
1.97e4
100
171.1576
332.2643
115.0758
453.3429
m/z
204.1607
306.2165
268.2262
433.2845
420.2836
152.1443
69.0704
0
50 75 100 125 150 175 200 225 250 275 300 325 350 375 400 425 450 475 500
Fig. 3 UPLC/Q-TOF MS analysis of the product of DMPO/ꢀC4H9 adduct
radical was not energetic enough to react with alkane, instead, it tended to couple
with another radical or to be oxidized. The only one form organoborane found in
this system was accounted for the peak ascribed to tributylboroxin, which was the
trimerization product of borinate radicals. This cyclic structure was full proof of the
homolytic cleavage of the peroxyborane after tributylborane undergoing a single
oxidation by oxygen. The changes of n-butoxy radical and n-butyl radical with
oxidation degree of tributylborane could be speculated from the various trends of
n-butanol and n-octane, respectively (Table 1). At the low oxidation degree of
tributylborane, more un-oxidized tributylborane could react with n-butoxyl radical
through SN1 reaction to produce n-butyl radical. At the high oxidation degree of
tributylborane, more tributylborane was oxidized to peroxylborane, which cleaved
to n-butoxy radical and borinate radical. So more n-butoxy radical was generated,
however, the amount of n-butyl radical declined due to its diffusion control reaction
with oxygen.
123

Study of the radical chemistry
1899
Pathway 1:
C4H9•
C4H9
C4H9
N
NH
N
O
O
O
m/z=170
-NH2OH
m/z=170
- C4H9
NH
O
m/z=114
m/z=81
Pathway 2:
C4H9•
- C4H9
C4H9
N
N
O
NH
O
O
m/z=170
m/z=115
Scheme 2 Proposed fragmentation pathways of the protonated ions of DMPO/ꢀC4H9 adduct
3-methylpentane
n-decane
2,4-dimethylpentane
squalane
Scheme 3 The structures of alkanes chosen as capturers of n-butoxyl radical
The four kinds of alkanes show different C–H bond reactivity to the hydrogen
atom abstraction by n-butoxy radical. The interplay of a number of potential
contributions influences the selectivity towards radical attack at different sites
123

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S. Liu et al.
3-Methylpentane
2,4,6-Tributylboroxin
n-Butanol
n-Octane
0
2
4
6
8
10
12
Retention time (min)
Fig. 4 GC–MS spectrum of the products of 3-methylpentane, TBB, and oxygen
n-Decane
2,4,6-Tributylboroxin
n-Octane
n-Butanol
2
4
6
8
10
12
Retention time (min)
Fig. 5 GC–MS spectrum of the products of n-decane, TBB, and oxygen
[21–24]. These include thermochemical effects (radical stability and bond
dissociation energy), as well as steric, stereo-electronic, and polar effects. The
selectivity of attack on C–H bonds generally follows the expected order
tertiary [ secondary [ primary, correlating with the order of decreasing bond
strength. Camara et al. [25] concluded the relative reactivity per hydrogen at 298 K
was 3.1 (tertiary):1 (secondary):ꢂ 1 (primary). However, steric retardation has been
123

Study of the radical chemistry
1901
Table 1 Effect of oxygen on the radicals generated from TBB in different models
Model compound
3-Methylpentane
Ma
n-Butanol (%)
n-Octane (%)
0.5
1
39.9
56.8
67.7
71.5
17.2
25.5
33.1
1.4
5.2
7.8
3.3
3.0
0.7
1.1
0.99
0.3
0.4
0.4
0.03
0.04
0.07
–
2
4
2,4-Dimethylpentane
n-Decane
0.5
1
4
1
4
4.5
b
?
0.5
2
5.9
Squalane
0.5
0.6
4
0.9
b
?
2.2
a
b
M was the mole ratio of oxygen to TBB, oxygen was continuously injected
proposed for tertiary and secondary C–H bonds in 2,4-dimethylpentane. Dokolas
[26] demonstrated that 2,4-dimethylpentane was three times less reactive than
3-methylpentane towards radical abstraction. For the linear n-decane, methylene
groups were difficultly attacked by n-butoxy radical since they were much less
reactive than methine groups at branch alkane. The reactivity of squalane decreased
further because of the more steric effect. Therefore, the C–H activity on different
alkane decreased in the order 3-methylpentane, 2,4-dimethylpentane, n-decane, and
squalane.
In the oxidation process of tributylborane, the hydrogen atom abstraction of
n-butoxy radical from higher active alkanes, such as 3-methylpentane and 2,4-
dimethylpentane, would promote the reversible homolytic cleavage reaction of
peroxyborane, and produce more radicals (n-butoxy and borinate radicals), and then,
result in more n-butanol. Besides, more n-butyl radicals originated from this
homolytic cleavage reaction, which coupled with each other in the form of n-octane.
In the alkanes with inert C–H bond, such as n-decane and squalane, the reversible
homolytic cleavage reaction of peroxyborane was more favorable to form
peroxyborane instead of radicals. As a result, n-butoxy radical and n-butyl radical
were rarely detected.
11B-NMR spectrum was also used to characterize the oxidation products of
tributylborane in different alkanes. Various oxidation products could be discrim-
inate without ambiguity [19, 27–29].
Table 2 listed the chemical shifts of detected organoborane compounds. The
oxidation products of tributylborane could be categorized into three groups: one
form of mono-oxidized tributylborane Bu2B(OBu); three forms of di-oxidized
species, including BuB(OBu)(OOBu), B3O3Bu3 and BuB(OBu)2; one form of
123

1902
S. Liu et al.
Table 2 Chemical shift of tributylborane and tributylborane oxides tested by 11B-NMR
Chemical shift (ppm) 86.8
54.9
35.7
33.1
31.1
18.0
Compound B(Bu)3 Bu2B(OBu) BuB(OBu)(OOBu) B3O3Bu3 BuB(OBu)2 B(OBu)3
tri-oxidized tributylborane B(OBu)3. The mono-peroxide tributylborane,
Bu2B(OOBu), was too reactive to be detected.
Figures 6, 7, 8, and 9 were the 11B-NMR spectra, which traced the oxidation
process of tributylborane in different alkanes. From the 11B-NMR result, the
oxidation process of tributylborane could be deduced as Scheme 4.
In case of deficient oxygen (O2:TBB\1:1), Bu3B was first oxidized to
Bu2B(OOBu), which was so vulnerable that it could decompose through homolytic
cleavage or react with the residual Bu3B to form Bu2B(OBu). With the further
increase of oxygen (O2:TBB[1:1), Bu2B(OBu) would be subjected to overoxidation
to BuB(OBu)(OOBu). Although BuB(OBu)(OOBu) was not as reactive as
Bu2B(OOBu), it could still oxidize Bu2B(OBu) to BuB(OBu)2, and some of the
BuB(OBu)2 tended to be overoxidized to BuB(OBu)3.
Table 3 illustrates the results derived from the above 11B-NMR spectra. It
suggested that the oxidizability of tributylborane was affected by different alkanes.
In the presence of 3-methylpentane, the hydrogen atom abstraction was beneficial
for the homolytic cleavage of Bu2B(OOBu). Fewer Bu2B(OBu) could be formed
and the oxidizability of tributylborane was enhanced, so that Bu2B(OBu) was
completely oxidized if oxygen was enough (O2:TBB=1). Meanwhile, more
tributylborane was completely oxidized to B(OBu)3. In squalane, the oxidizability
A: O2:Bu3B=1:1
B: O2:Bu3B=2:1
C: O2:Bu3B=4:1
-O-B-O-
ppm
40
30
20
10
Fig. 6 Effect of oxygen on the 11B-NMR spectra of the mixture of 3-methylpentane and TBB
123

Study of the radical chemistry
1903
A: O2:Bu3B=1:1
B: O2:Bu3B=2:1
C: O2:Bu3B=4:1
BuB(OBu)2
B(OBu)3
Bu2B(OBu)
BuB(OBu)(OOBu)
B3O3Bu3
70
60
50
40
30
20
10
0
ppm
Fig. 7 Effect of oxygen on the 11B-NMR spectra of the mixture of 2,4-dimethylpentane and TBB
BuB(OBu)2
A: O2:Bu3B=1:1
B: O2:Bu3B=2:1
C: O2:Bu3B=4:1
B(OBu)3
Bu2B(OBu)
A
B3O3Bu3
BuB(OBu)(OOBu)
B
C
60
50
40
30
20
10
ppm
0
Fig. 8 Effect of oxygen on the 11B-NMR spectra of the mixture of n-decane and TBB
of tributylborane decreased tremendously. After the equimolar reaction of
tributylborane with oxygen, the remaining Bu2B(OBu) content was 8.2%. More-
over, tributylborane was not susceptible to complete oxidation. In different models,
the oxidizability of tributylborane decreased in the order 3-methylpentane [
123

1904
S. Liu et al.
A: O2:Bu3B=1:1
B: O2:Bu3B=2:1
C: O2:Bu3B=4:1
BuB(OBu)2
Bu2B(OBu)
A
B(OBu)3
B
C
70
60
50
40
30
20
10 ppm 0
Fig. 9 Effect of oxygen on the 11B-NMR spectra of the mixture of squalane and TBB
2,4-dimethylpentane [ n-decane [ squalane. It corresponded with the activities of
these alkanes toward the hydrogen atom abstraction by n-butoxy radicals. The result
indicated that the ability of alkane with active C–H bond to capture the radical was
beneficial for the oxidizability of tributylborane, which promoted the homolytic
cleavage of Bu2B(OOBu) and the complete oxidation of tributylborane to B(OBu)3.
It was the reasons that more radicals were generated when the oxidation of
tributylborane proceeded in active alkanes.
2,2-Diphenyl-1-picrylhydrazyl (DPPH) was used as another radical scavenger to
evaluate the concentration of radicals. DPPH has an absorption band at 515 nm in
its radical form. The absorption intensity decreases after DPPH being scavenged
with another active radical, so the final absorption intensity at 515 nm depends on
the initial concentration of radicals.
Figure 10 shows that the UV absorption intensity of the mixture of DPPH and
tributylborane decreased at 515 nm with the increase of oxygen. This phenomenon
suggested that amount of active radicals increased with the increase of oxidation
degree of tributylborane. The result was consistent with the above conclusion: the
deep oxidation degree of tributylborane was favorable to more generated radicals.
Conclusions
In this paper, the structures of radicals generated in the oxidation process of
tributylborane were characterized, and then the oxidation mechanism of trialkyl-
borane was proposed. In the oxidation of tributylborane, three kinds of radicals were
found, namely n-butoxy, n-butyl, and borinate radicals. N-butoxy radical was
123

Study of the radical chemistry
1905
R3B
O2
R2B(OOR)
R3B
RO
R BO
•
2
•
+
R2B(OR)
R
•
R3B
R
R
O
O2
R2B(OR)
O2
B
B
R B(OR) + R
•
+ R
•
2
O
O
B
R
O2
RB(OR)(OOR)
R2B(OR)
RB(OR)2
RB(OR)(OOR)
R= n-Bu
B(OR)3
Scheme 4 The oxidation mechanism of tributylborane
generated from the homolytic cleavage of mono-peroxide tributylborane. The
formation of n-butyl radical was originated from n-butoxy and borinate radicals.
The hydrogen atom abstraction by n-butoxy played an important role in the
oxidation process of tributylborane, which would promote the oxidizability of
tributylborane and deepen the oxidation degree of tributylborane, and accordingly,
give birth to more radicals.
123

1906
S. Liu et al.
Table 3 Effect of oxygen on the oxidation of TBB in different alkanes measured by 11B-NMR
Model compound
3-Methylpentane
O2:TBB (molar
ratio)
BBu3
(%)
Bu2B(OBu)
(%)
–O–B–O–
(%)
B(OBu)3
(%)
1
2
4
1
2
4
1
2
4
1
2
4
–
–
–
–
–
–
–
–
–
–
–
–
–
62.9
12.2
5.7
37.1
87.8
94.3
4.9
–
–
2,4-
0.9
–
89.8
38.9
9.2
Dimethylpentane
61.1
90.8
2.4
–
n-Decane
2.7
–
94.9
59.5
25.2
91.8
85.4
65.8
40.5
74.8
–
–
Squalane
8.2
–
14.6
34.2
–
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
O2 increase
400
500
600
Wavelength, nm
Fig. 10 UV spectra of the mixture of DPPH and TBB solution
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Study of the radical chemistry
1907
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