Bromotrimethylsilane as a selective reagent for the synthesis of bromohydrins

Article abstract of DOI:10.1039/d1ra01980e

Bromotrimethylsilane (TMSBr) is a very efficient reagent in the solvent-free conversion of glycerol into bromohydrins, useful intermediates in the production of fine chemicals. As glycerol is a relevant by-product in biodiesel production, TMSBr has been also tested as a mediator in transesterification in acidic conditions, providing FAME from castor oil in good yields, along with bromohydrins from glycerol. Subsequently the glycerol conversion was optimized and depending on the reaction conditions, glycerol can be selectively converted into α-monobromohydrin (1-MBH) or α,γ-dibromohydrin (1,3-DBH) in very good yields. This journal is

Full text of DOI:10.1039/d1ra01980e

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Bromotrimethylsilane as a selective reagent for the
synthesis of bromohydrins†
Cite this: RSC Adv., 2021, 11, 14453
ab
ab
a
*
*
Donatella Giomi,
and Alberto Brandi
Antonella Salvini,
Jacopo Ceccarelli
ab
Bromotrimethylsilane (TMSBr) is a very efficient reagent in the solvent-free conversion of glycerol into
bromohydrins, useful intermediates in the production of fine chemicals. As glycerol is a relevant by-product
in biodiesel production, TMSBr has been also tested as a mediator in transesterification in acidic conditions,
providing FAME from castor oil in good yields, along with bromohydrins from glycerol. Subsequently the
glycerol conversion was optimized and depending on the reaction conditions, glycerol can be selectively
converted into a-monobromohydrin (1-MBH) or a,g-dibromohydrin (1,3-DBH) in very good yields.
Received 12th March 2021
Accepted 1st April 2021
DOI: 10.1039/d1ra01980e
rsc.li/rsc-advances
extensively applied in pharmaceutical, food, cosmetic,
chemical industries. In particular, a great interest has been
Introduction
devoted to its transformation in more valuable chemicals like
ethers, esters, organic acids, acrolein, chlorohydrins, dihalo-
and dihydroxy-acetones.1,7 The success of the use of glycerol
as a raw material for these productions depends on the purity
achieved in its production and, therefore, on the biodiesel
production process. The low purity of glycerol generally ob-
tained in rst-generation biodiesel production makes further
conversion more difficult. Moreover, the rst-generation
process has received in the past much criticism because it
utilizes as feed stock materials that belong to the foodstuff
supply. Therefore, of particular interest, becomes the
production of FAME from exhaust vegetable oils. The use of
these oils, however, raises another problem connected with
the most common industrial method for the production of
FAME, that is the alkaline transesterication of triglycerides
in the presence of MeOH.8
In fact, a total free fatty acid content in vegetable oils above
0.5% by weight, because of the formation of soaps (Na or K salts
of fatty acids), can provoke their emulsions with water, making
difficult the separation of pure FAME. A simple solution to this
problem is the switch to acidic transesterication catalysts.
Recently our group has addressed both the tasks, glycerol
valorization and exhaust vegetable oils conversion, by showing
that trimethylchlorosilane (TMSCl) is an excellent reagent to
promote very efficiently the transesterication of vegetable oils
and fats to FAME and other fatty acid methyl esters,9,10 and, at
the same time, to promote the selective transformation of
glycerol to mono- and dichlorohydrins.10,11
The bromination of glycerol to produce a-monobromohydrin (1-
MBH, 4) and a,g-dibromohydrin (1,3-DBH, 8) is an important
process for the importance of these bromoderivatives widely
applied as ne chemicals in organic syntheses.1–3 In particular,
1,3-DBH could be oxidized to 1,3-dibromoacetone which is
a versatile building block for several compounds important as
drugs, antibacterials or inhibitors.1
However, few methods to synthesize bromohydrins from
glycerol have been reported exploiting bromine, phosphorous
and bromine, phosphorous tribromide,4 hydrobromic acid,5 as
well as triuoroacetic anhydride and lithium bromide as
brominating agents.6 Multi-step processes have also been used
with dichlorohydrins as intermediates.1
Glycerol is an interesting raw material for obtaining
products of high added value because it is obtained as by-
product of the production of Fatty Acid Methyl Esters
(FAME) from vegetable oils for their use as biodiesel. This is
a well known process providing a fuel that can be used in
combustion plants, or in diesel motorcars without any
particular variation of the combustion system. During the
transesterication of triglycerides, ca. 10% by weight of the
initial mass was transformed in glycerol. In 2020, the global
production of crude glycerol has approached 4 billion liters
and adequate valorization becomes crucial for the chemical
industry. This abundant and important raw material is very
a
`
Dipartimento di Chimica ‘Ugo Schiff’, Universita degli Studi di Firenze, Via della
Lastruccia 3-13, I-50019 Sesto Fiorentino (Fi), Italy. E-mail: donatella.giomi@uni.
Now, the synthesis of bromohydrins from glycerol was
studied using trimethylbromosilane (TMSBr) as a reagent,
preliminarily also analyzing its effectiveness in the trans-
esterication of triglycerides.
it; antonella.salvini@uni.it
bLaboratorio congiunto VALORE, “Valorizzazione di masse algali e sottoprodotti agro-
industriali e riduzione di gas serra in atmosfera”, Via Morosini snc, I-50019 Sesto
Fiorentino (Fi), Italy
† Electronic supplementary information (ESI) available: 1H and 13C NMR spectra
of reaction mixtures and puried products. See DOI: 10.1039/d1ra01980e
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Table 1 Transesterification reactions of 1 and 2 with MeOH promoted
by TMSBr
Entry Oil TMSBr/oila MeOH/oila T (ꢀC) t (h)
FAMEb (%)
1
2
3
4
5
6
1
1
1
1
2
2
2.2
2.2–6.6
2.2–4.4
2.2
3
4.5
4.5–9
4.5
4.5
4.5–35
4.5–35
60
60
100
100
60
8
30
60
72
12
28
42
60
90
8 + 6
12 + 6 93
3
60
a
Molar ratio. b Determined via 1H NMR analyses.
Fig. 1 Sunflower oil and castor oil (ricinolein).
in the range 4.35–4.10 ppm), according to a conversion of 60%.
No trace of halogenated derivatives of glycerol, like 4, has been
observed through 13C NMR analysis of the reaction mixture
(DMSO-d6), in contrast to what was observed using TMSCl for
the transesterication.9
Concerning castor oil, previous data related to the use of
TMSCl10 evidenced the need to operate with an excess of MeOH
to allow, in addition to transesterication of 2 to give 6, the
methanolysis of estolides 7, i.e. dimers obtained by esterica-
tion of the free ricinoleic acid with the hydroxy group on C-12
carbon of another molecule of ricinoleic methyl ester (Scheme
1).
Therefore, a mixture of TMSBr, MeOH, and castor oil 2
(molar ratios TMSBr/2 3, MeOH/2 4.5) was heated at 60 ꢀC for 8
hours then added of a further portion of MeOH (nal MeOH/2
ratio 35) and heated for additional 6 hours. Delightfully, 1H
NMR monitoring of the reaction mixture12 showed the forma-
tion of ricinoleic methyl ester with a conversion of 90% of the
oil (Table 1, entry 5). A slightly better conversion (93%) was
observed by increasing the time of the rst step of the reaction
from 8 to 12 hours. The 1H NMR analysis of the crude mixture,
Results and discussion
Based on the results obtained with TMSCl,9–11 and taking into
account that glycerol is obtained as a co-product of the biodiesel
production, it was decided to preliminarily evaluate the effec-
tiveness of TMSBr in transesterication of triglycerides. In fact,
despite the higher cost of the new reagent, this system can
become interesting if one wishes to produce bromohydrins as
a co-product. Furthermore, it seemed interesting to evaluate the
different reactivity of the two systems in the presence of Br
instead of Cl. Two model triglycerides were chosen for the
transesterication studies, i.e. sunower oil and castor oil
(Fig. 1). By analogy with previous studies with TMSCl,9,10
mixtures of oil, MeOH, and TMSBr were heated. The nature of
the two oils makes already a difference at this point. Whereas
the mixture with castor oil is a homogeneous solution,
according to the presence of ricinolein as the major triglyceride
(ca. 90%), the mixture with sunower oil is an emulsion that
becomes clear only by increasing the temperature.
Under heating a mixture of dried sunower oil 1, TMSBr and
anhydrous MeOH (molar ratios TMSBr/1 2.2, MeOH/1 4.5) at
ꢀ
60 C a poor conversion of the oil in FAME has been observed
1
aer 8 hours through H NMR monitoring (Table 1, entry 1).12
Increasing the amount of TMSBr and MeOH (by adding further
portions of the starting reagents) the conversion was slightly
increased (28%, Table 1, entry 2) aer 30 hours at the same
temperature. By raising the reaction temperature to 100 ꢀC,
aer 60 hours the conversion climbed up to 42% (Table 1, entry
3).
The best experimental conditions led to a 60% conversion of
1 into FAME aer 72 hours at 100 ꢀC (Table 1, entry 4). 1H NMR
analysis (CDCl3) of the reaction mixture aer removing under
vacuum all the volatile compounds showed the presence of
FAME, mono-, di-, and triglycerides besides glycerol 3 (signals
Scheme 1 Transesterification of castor oil (2) with TMSBr.
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Table 2 Reactions of glycerol with TMSBr
T
(ꢀC)
t
(h)
Entry
TMSBr/3a
AcOH/3a
3b
4b
8b
1
2
3
4
5
6
2.5
2.5
2.5
1.5
1.5
1.5
—
60
60
60
20
20
20
12
6
9
24
24
48
29
3
—
82
46
20
59
29
—
18
51
60
12
68
ca. 100
—
3
0.03
0.03
—
0.03
0.03
Scheme
triglycerides.
2 Role of TMSBr as mediator for transesterification of
20
a
b
Molar ratio. Molar ratios (%) estimated via 13C NMR (DMSO-d6)
spectra for semi-quantitative analyses (see ESI).
aer removing volatiles under vacuum, showed the presence of
ricinoleic methyl ester (6) as the major compound, a small
amount of estolide 7 (identied by the characteristic quintet at
4.9 ppm of the H-120 proton), and glycerol derivatives (signals in
the range 4.1–3.5 ppm). The 13C NMR analysis of the reaction
crude (in DMSO-d6 to dissolve even glycerol) allowed the iden-
tication of monobromohydrin 4 alongside minor amounts of
dibromohydrin 8 and traces of glycerol.13
The reaction mixture, aer removing under vacuum all the
volatile compounds, was subjected to Kugelrhor distillation at
reduced pressure giving, in order of decreasing volatility,
dibromohydrin 8 (9%), monobromohydrin 4 (25%), and rici-
noleic methyl ester (6) (80%), while the estolide 7 was recovered
in 12% yield as distillation residue (Scheme 1).
The results of Table 1 show a quite signicant difference in
the reactivity of the two triglycerides 1 and 2 with TMSBr.
Sunower oil appears much less reactive, likely because of
a lower solubility in the reaction medium, hampering the
reaching of full conversion, even at higher temperature and
longer reaction times, as well as the formation of bromo
derivatives of glycerol.
intermediate 10 that collapses to FAME by loss of glycerol and
TMSBr. The use of TMSBr in higher than catalytic amount can
be explained by its decomposition by side processes, like
hydrolysis and methanolysis (Scheme 3), as well as bromination
of glycerol (see below).
A comparison of the reactivity of TMSBr with TMSCl clearly
shows a higher efficiency for TMSCl that requires lower
temperatures and reaction times, providing higher conversions
of the triglycerides into FAME.9 This different behavior can be
justied on the basis of the HSAB theory14 suggesting for the
soer TMSBr a poor interaction with the hard oxygen atom in
the transition state A (Scheme 2) rendering the formation of 9
less feasible. However, the use of TMSBr in the synthesis of
The more polar triglyceride 2, giving rise to a clear solution
even at room temperature, can be almost quantitatively con-
verted into FAME at lower temperature, along with bromohy-
drins 4 and 8.
As previously reported for TMSCl,9 the role of TMSBr as
mediator for transesterication of triglycerides can be ratio-
nalized as shown in Scheme 2.
TMSBr activates the ester group by coordination to the
carbonyl moiety leading to the electrophilic intermediate 9.
Then, nucleophilic attack of methanol affords the tetrahedral
Fig. 2 (a) Experiments performed at 60 ꢀC (Table 2, entries 2 and 3). (b)
Scheme 3 Side processes involving TMSBr decomposition.
Experiments performed at 20 ꢀC (Table 2, entries 5 and 6).
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unreacted TMSBr and (Me3Si)2O (see Scheme 3) was separated
and the pale yellow lower one was subjected to distillation
under reduced pressure to give 1,3-DBH (8), isolated in 80%
yield as a colorless liquid (Scheme 4).
With the aim to produce 1-MBH, the reaction of glycerol (3)
was performed at room temperature with a minor amount of
TMSBr (1.5 molar ratio). Without acid catalysis the reaction was
very slow but in the presence of 3% of AcOH a roughly 1 : 1
mixture of glycerol and 1-MBH was formed aer 24 hours, along
with traces of 1,3-DBH (Table 2, entries 4 and 5). Increasing
reaction times to 48 hours, glycerol was additionally converted
with a rise in the amount of 1-MBH (3/4 ratio 1 : 3), but the
selectivity of the reaction was reduced as 20% of 1,3-DBH is also
formed (Table 2, entry 6). Therefore, to produce 1-MBH the
conditions of entry 5 (Table 2 and Fig. 2b) are the most conve-
nient as, aer separation of 1-MBH, unreacted glycerol can be
recovered and recycled. Distillation at reduced pressure of the
reaction crude allowed the isolation of 1-MBH in 50% yield,
while unreacted 3 was recovered (36%) from the distillation
residue (Scheme 4).
The achieved selectivity in formation of 1-MBH and 1,3-DBH
is signicant as both compounds can be produced practically as
a single product applying different reaction protocols.
An analysis of the literature data shows that chlorination or
bromination by TMSCl or TMSBr occurs under the presence of
catalysts or co-reagents.15 Only in one case the transformation is
reported without using a catalyst, but it concerns exclusively
tertiary or benzylic alcohols.16 On the other hand, to our
knowledge, glycerol has not been utilized as a substrate in
halogenation studies with halogenosilanes. Moreover, it is
noteworthy the catalytic role of AcOH in glycerol chlorination
with TMSCl, evidenced by our group, and now conrmed with
TMSBr.
Scheme 4 Bromination of glycerol with TMSBr.
bromohydrins from glycerol appears interesting. Therefore, the
reaction of TMSBr and glycerol was studied following the track
opened with TMSCl.11
By heating a yellow/orange emulsion of glycerol and TMSBr
(2.5 molar ratio) at 60 ꢀC, aer 12 hours, under monitoring via
13C NMR analysis (in DMSO-d6 to dissolve even glycerol), the
formation of both 1-MBH (4) and 1,3-DBH (8) was observed in
roughly 5 : 1 ratio, besides ca. 29% unreacted glycerol (Table 2,
entry 1). However, according to previous observations of
a catalytic effect of acetic acid in the reaction of TMSCl with
glycerol,11 the experiment was repeated mixing glycerol (3),
TMSBr (2.5 molar ratio), and 3% of AcOH (related to 3).
Aer heating 6 hours at 60 ꢀC, a roughly 2 : 1 mixture of 1,3-
DBH and 1-MBH has been obtained, with only traces of
unreacted glycerol, and by increasing the reaction times at 9
hours, 1,3-DBH was the almost exclusive product in the reaction
crude (Table 2, entries 2 and 3, and Fig. 2a).
The reaction mixture obtained aer 9 hours heating
appeared as a biphasic system: the upper phase containing
The probable reaction pathway producing bromohydrins
from glycerol and TMSBr is described in Scheme 5. The role of
catalytic AcOH in accelerating the reaction can be rationalized
through activation of TMSBr. The reaction with glycerol likely
forms a silyl derivative 11 that undergoes an activated epoxide
formation that eventually experiences a selective opening by
nucleophilic bromide substitution.
Concerning the reactivity of TMSBr and TMSCl, the experi-
mental results demonstrate a higher reactivity for TMSBr in
formation of bromohydrins, performed at lower temperatures
and reaction times, with respect to TMSCl. These results can be
explained by the better nucleophilic character of bromide
anion, compared to chloride, in a polar alcoholic medium as
glycerol.
Overall, this methodology appears signicant because allows
the direct and selective conversion of glycerol into bromohy-
drins in high yields, operating in mild and solvent-free
conditions.
This reactivity appears very attractive because recent results
concerning the synthesis of 1,3-DBH (8) still apply a two-step
process involving rst the transformation of glycerol into 1,3-
dichlorohydrin then converted into 1,3-dibromohydrin.1b
Scheme 5 Reaction pathway for bromohydrins formation.
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were formed again: the upper one, containing unreacted TMSBr
and hexamethyldisiloxane, was removed by the use of a Pasteur
pipette; the lower pale yellow phase, containing bromo deriva-
tives of glycerol, was subjected to distillation under vacuum.
While traces of 1-MBH (4) were detected in the distillation
Experimental
General information
Chemicals were purchased from commercial suppliers and
1
used as received. H and 13C NMR spectra were recorded with
ꢀ
residue, 1,3-DBH (8) (1.74 g, 80%) was collected at 75–82 C/1
Varian Mercury plus 400 operating at 400 MHz and 100 MHz,
respectively. Dry methanol was obtained by PureSolv™ Micro
100 Liter solvent purication system.
mmHg as a colorless liquid; 1H NMR (CDCl3) d (ppm): 4.02
(quintet, J ¼ 5.3 Hz, 1H), 3.63–3.56 (m, 4H), 2.22 (br s, 1H); 13C-
NMR (CDCl3) d (ppm): 69.5 (d), 35.0 (t).
Transesterication of sunower oil (1)
Synthesis of 1-monobromohydrin (1-MBH) (4)
(A) In a screw-cap Pyrex tube Nꢀ 18, dry MeOH (0.164 g, 0.207
mL, 5.13 mmol) and TMSBr (0.384 g, 0.330 mL, 2.51 mmol) were
added to sunower oil (1) [1 g, 1.14 mmol (Mw calc. ¼ 878 g
molꢁ1)] under nitrogen atmosphere. The reaction mixture was
heated for 8 hours at 60 ꢀC, under magnetic stirring. Aer
cooling, the separation of two phases was observed. The upper
phase, containing FAME, was separated and heated at 50 ꢀC for
2 hours under reduced pressure (5–10 mmHg) to distill off
volatiles. 1H NMR (CDCl3) analysis on the residue showed only
a poor transesterication of the starting triglycerides (ca.
12%).12,17
Operating as above, the biphasic mixture composed of glycerol
(0.921 g, 10 mmol), TMSBr (2.30 g, 1.98 mL, 15 mmol) and acetic
acid (0.02 g, 0.19 mL, 0.33 mmol) was heated at 20 ꢀC for 24
hours, under magnetic stirring. The pale orange lower phase
was subjected to evaporation at 50 ꢀC/5–10 mmHg to remove all
ꢁ2
ꢀ
traces of volatile compounds. Distillation at 50–60 C/3 ꢂ 10
mmHg allowed to isolate 1-MBH (4) (0.775 g, 50%) as a colorless
1
liquid; H-NMR (CDCl3) d (ppm): 4.03–3.86 (m, 1H), 3.84–3.64
(m, 2H), 3.57–3.43 (m, 2H), 2.09 (br s, 2H); 13C NMR (CDCl3)
d (ppm): 71.3 (d), 64.2 (t), 35.2 (t).
(B) Operating as above but heating at 100 ꢀC for 72 hours, 1H
NMR (CDCl3) analysis on the upper phase, aer removal of the
volatiles, showed ca. 60% conversion of 1 into biodiesel.
Traces of 1,3-DBH (8) were detected in the head of distilla-
tion (1H NMR), while unreacted glycerol (ca. 0.332 g, 36%) was
recovered from the distillation residue.
Transesterication of castor oil (2)
Conclusions
In a 15 mL screw-cap Pyrex® glass tube, dry MeOH (0.156 g,
0.197 mL, 4.87 mmol) and TMSBr (0.496 g, 0.428 mL, 3.24
mmol) were added to castor oil (2) [1 g, 1.08 mmol (Mw calc. ¼
929 g molꢁ1)] under nitrogen atmosphere. The dull yellow
reaction mixture was heated for 12 hours at 60 ꢀC, under
magnetic stirring. Aer cooling to room temperature, a further
amount of dry MeOH (1.057 g, 1.33 mL, 33 mmol) was added
and the mixture was heated for additional 6 hours. Methanol
and the other volatile compounds were removed by heating for
2 hours at 50 ꢀC under reduced pressure (5–10 mmHg) leading
to a yellow-brown liquid. NMR analyses, performed both in
CDCl3 and DMSO-d6 as solvents, allowed to determine the
conversion of castor oil into methyl esters as well as the pres-
ence of bromohydrins in the reaction mixture. Distillation
under vacuum (3 ꢂ 10ꢁ2 mmHg) in a Kugelrhor apparatus
allowed to recover four fractions. The rst fraction collected at
50–55 ꢀC/3 ꢂ 10ꢁ2 mmHg, was 1,3-DBH (8) (0.021 g, 9%) while
1-MBH (4) (0.042 g, 25%) was recovered at 100 ꢀC/3 ꢂ 10ꢁ2
mmHg and methyl ricinoleate (6)10 (0.810 g, 80%) between 150–
TMSBr is a less efficient mediator than TMSCl for triglycerides
transesterication, albeit it is still able to provide trans-
esterication in good yields in acidic conditions with substrates
leading to a homogeneous solution.
On the other hand, TMSBr is a very efficient reagent, more
than TMSCl, in the solvent-free conversion of glycerol into
halohydrins. Depending on the reaction conditions, glycerol
can be selectively converted into 1-MBH or 1,3-DBH in very good
yields. On the whole, we can affirm that while TMSCl is the
reagent of choice for the transesterication of triglycerides to
give biodiesel, TMSBr is a very efficient reagent for glycerol
conversion into bromohydrins.
Author contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the nal version of
the manuscript.
ꢁ2
ꢀ
220 C/3 ꢂ 10 mmHg. The distillation residue (0.115 g; 12%
yield) was a brownish liquid, containing the dimer 7 (ref. 10) as
the main product.
Conflicts of interest
There are no conicts to declare.
Synthesis of 1,3-dibromohydrin (1,3-DBH) (8)
In a 15 mL screw-cap Pyrex® glass tube, TMSBr (3.83 g, 3.3 mL,
25 mmol) was added to a mixture of glycerol (0.921 g, 10 mmol)
Acknowledgements
and AcOH (0.02 g, 0.19 mL, 0.33 mmol), under nitrogen atmo- The authors thank MUR (Rome-Italy) and Ente Cassa di Ris-
ꢀ
sphere. The biphasic mixture was heated at 60 C for 9 hours parmio di Firenze for the nancial support and acknowledge
under magnetic stirring. At the end of the reaction, two phases Irene Ceccolini for her contribution with Bachelor thesis work.
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12 The yield of FAME has been determined via 1H NMR
analyses by comparing the integral of OCH3 protons of
FAME (singlet at d 3.66) produced in the transesterication
process with that of –CH2–COO– protons (multiplet at
3 For recent applications of bromohydrins, see also: (a) S. Lee
and M. H. Seo, J. Nanosci. Nanotechnol., 2017, 17, 7579; (b)
M. Mojzych, Z. Bernat, Z. Karczmarzyk, J. Matysiak and
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`
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13 The reaction crude appeared as a yellow-orange, clear
solution (only one phase) conrming the almost total
conversion of glycerol into bromohydrins soluble in the
reaction medium. The higher solubility of castor oil in
MeOH allowed even an easier conversion of glycerol.
14 R. G. Pearson, J. Am. Chem. Soc., 1963, 85, 3533.
15 For the use of catalysts and co-reagents such as K2CO3, SeO2,
BiCl3, DMSO, and more recently natural sodium
montmorillonite, see: (a) M. Lissel and K. Drechsler,
Synthesis, 1983, 314; (b) J. G. Lee and K. K. Kang, J. Org.
Chem., 1988, 53, 3634; (c) C. Le Roux, H. Gaspard-
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˚
[kept on activated molecular sieves (4 A) under vacuum for
24 hours], no improvements were observed (1H NMR
analysis).
´
G. Vicente, M. Martınez and J. Aracil, Bioresour. Technol.,
14458 | RSC Adv., 2021, 11, 14453–14458
© 2021 The Author(s). Published by the Royal Society of Chemistry