Hexabromoacetone as tribromoacetylating agent of alcohols and amines and as mediator in the conversion of carboxylic acids into amides in the presence of triphenylphosphine

Article abstract of DOI:10.1016/j.tetlet.2009.03.093

Hexabromoacetone has been used as an alternative tribromoacetylating agent of primary alcohols and amines and as mediator in the conversion of carboxylic acids into amides in the presence of triphenylphosphine. The reactions have been performed under mild conditions with moderate to good yields. All the products have been adequately characterized by their physical and spectroscopic properties.

Full text of DOI:10.1016/j.tetlet.2009.03.093

Tetrahedron Letters 50 (2009) 2559–2561
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Tetrahedron Letters
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Hexabromoacetone as tribromoacetylating agent of alcohols and amines
and as mediator in the conversion of carboxylic acids into amides
in the presence of triphenylphosphine
*
*
Fabrício G. Menezes, Rosane Kolling, Adaílton J. Bortoluzzi, Hugo Gallardo , César Zucco
Chemistry Department, Federal University of Santa Catarina, Florianópolis, SC 88040-900, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Hexabromoacetone has been used as an alternative tribromoacetylating agent of primary alcohols and
amines and as mediator in the conversion of carboxylic acids into amides in the presence of triphenyl-
phosphine. The reactions have been performed under mild conditions with moderate to good yields.
All the products have been adequately characterized by their physical and spectroscopic properties.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 16 January 2009
Revised 9 March 2009
Accepted 11 March 2009
Available online 18 March 2009
Keywords:
Hexabromoacetone
Tribromoacetylation
Alcohols
Amines
Carboxylic acids
Triphenylphosphine
Amides
The
a
-trihalogenated carbonyl compounds have been reported
able, but it is easily prepared via the Gilbert’s method.8 Although
known since 1969, it is very seldom cited as a reagent in organic
synthesis, except for its ability to act as a mediator in the conver-
sion of alcohols to alkyl halides.9
as very useful versatile reagents in organic synthesis. Two trichlo-
rinated methyl groups attached to a carbonyl carbon form hexa-
chloroacetone (HCA),
a reagent which has been intensively
studied with interesting results. Noteworthy examples are the tri-
chloroacetylation of alcohols1 and amines,2 the mediator role of
HCA in the conversion of alcohols into alkyl halides3 and of carbox-
ylic acids into amides.3a,4 HCA reactions with aliphatic and aro-
matic diamines lead to the obtainment of the respective
diamides and cyclic ureas, together with heterocyclic compounds
without the carbonyl group, such as bisimidazolidine, tetra-
hydrobisbenzimidazolediene, and bisbenzimidazole derivatives,
particularly in reactions conducted under ultrasonic energy.5
When HCA was reacted with vicinal glycols, in the presence of
different catalysts, alkylene carbonates and chloroform were ob-
tained.6 Also, HCA was found to be a precursor for formation of
The alkyl tribromoacetates prepared can be converted into dif-
ferent Z-a
-bromoacrylates (known for their stereoselectivity),10
and act in the conversion of carboxylic acid into amides with high
yield under mild conditions.11 The tribromoacetamides have prac-
tically not been explored in organic synthesis, in contrast to the tri-
chloroacetamides, which can, for example, be converted into
carbamates,12 and mediate conversion of alcohols into alkyl chlo-
ride3b and obtention of Z-
tion of
a
-chloroacrylamides.10c Another applica-
a
-trihalogenated carbonyl compounds is the use of alkyl
trichloroacetate to get polysubstituted chlorocyclopropanes,13
2-substituted-1,3-dithiane,14 and trihalo-containing acetylenes
and azoles.15
a,a
,
a0
,
a0-tetrachloro-substituted[3.2.1]bicyclic compounds.7
This Letter reports on our initial studies involving the use of an
After the preparation of the HBA by the Gilbert’s method8 it was
characterized through the melting point, elemental CHN analysis,
IR, 13C NMR, and X-ray crystallographic analysis (Fig. 1).16,17
The reactions of HBA with 1.1 equiv of alcohol (methanol, etha-
nol, n-propanol, n-butanol, n-pentanol, and i-amyl alcohol) in the
presence of 1.0 equiv of dimethylformamide (acting as a proton
sponge), at 60 °C, provided, after 10 h of reaction, the respective al-
kyl tribromoacetates (1–6) with yields between 55% and 65%,
depending on the alcohol structure.18 The purification of alkyl
tribromoacetates was carried out by column chromatography on
HCA analog, hexabromoacetone, HBA, as an alternative tribromoa-
cylating agent for both primary alcohols and amines, and as a
mediator in the conversion of carboxylic acids into amides in the
presence of triphenylphosphine. HBA is not commercially avail-
* Corresponding authors. Tel.: +55 48 37219544; fax: +55 48 37216850.
E-mail addresses: hugo@qmc.ufsc.br, hugo.gallardo@pq.cnpq.br (H. Gallardo),
czucco@qmc.ufsc.br, czucco@fapesc.sc.gov.br (C. Zucco).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.03.093

2560
F. G. Menezes et al. / Tetrahedron Letters 50 (2009) 2559–2561
as the eluent. Table 2 summarizes the results of the reaction of
HBA with the amines. Under the same experimental conditions,
the reactions of HBA with the bulky primary (t-butylamine) and
secondary (piperidine) amines did not provide the respective
amides, and the consumption of a great part of the HBA was not
verified by TLC. The products of the reactions of HBA with the
amines were obtained in the form of white solids, after recrystalli-
zation from 5:1 (v/v) hexane:chloroform, and were characterized
through the melting point, elemental CHN analysis, and IR and
1H, 13C and DEPT NMR spectroscopies.
The spectral data of the alkyl tribromoacetates and the tri-, and
di-bromo-N-alkylacetamides are very similar, following the same
line, thus facilitating the characterization, particularly by IR and
NMR. All products have had their purity verified by TLC and GC
analysis.
The role of HBA as a mediator in the conversion of carboxylic
acids into the respective amides was tested via formation, in situ,
of bromide acid, as previously reported for other carbonyl a-tribro-
Figure 1. Molecular structure of HBA.
minated compounds.11 Reactions between benzoic and aliphatic
carboxylic acids with the different aromatic and aliphatic amines
were carried out in the presence of HBA/triphenylphosphine (0.3/
1.5 equiv in proportion to the carboxylic acids and amines) as a
brominating agent.20 This method is a mixture of those already
known, involving the use of ethyl tribromoacetate in the formation
of amides,11 and of HBA in the formation of alkyl halides from alco-
hols.9 The amides (13–27) were obtained in yields of 53% to 91%. It
is observed that, Table 3, aromatic acids are giving better yields
than aliphatic acids. However, controlled experiments carried out
on acetic acid (0.5 g, 8.3 equiv, and 1.0 g, 16.6 equiv) that yields
the amide fall within the range observed for aromatic acids, 75%
and 85%, respectively. So it is reasonable to conclude that the initial
amount of aliphatic acid is affecting the product yield, since losses
of aliphatic amides, during the purification process, are greater
than those of aromatic ones.
silica gel, using hexane–chloroform mixtures as the eluent. Table 1
summarizes the results obtained for the HBA alcoholysis. The tri-
bromoacetates were obtained as colorless oils, and characterized
through IR, 1H, 13C and DEPT NMR, and MS. From reactions of
HBA with benzyl alcohol and with the secondary alcohols i-propa-
nol and cyclohexanol, under the same experimental conditions, the
respective tribromoacetates were not isolated, despite verifying
the consumption of HBA through TLC analysis.
The formation of alkyl tribromoacetates and N-alkyl tribro-
moacetamides proceeds by a typical addition-elimination mecha-
nism which begins with a nucleophilic addition of alcohols and
amines, respectively, to the carbonyl group of HBA and finishes
with the departure of Br3Cꢀ, as the leaving group, thus forming
bromoform. In the reaction involving tribromoacetates formation
DMF functions as a proton sponge species.
When HBA is reacted with 2 equiv of amine, in chloroform, the
2,2,2-tribromo-N-alkylacetamides (7, 8, 10, 12) are obtained in
yields varying from 65% to 74% for the slightly sterically hindered
amines (ethylamine, n-propylamine, n-butylamine, and n-hexyl-
amine), after 1 h of reaction, at room temperature.19 Under the
same experimental conditions, the reactions with the primary
amines i-propylamine and s-butylamine, which are more sterically
hindered, the 2,2,2-tribromo-N-alkylacetamides (9a, 11a) were ob-
tained with yields between 48% and 50%. However, there were also
isolated 2,2-dibromo-N-alkylacetamides (9b, 11b), with 15% and
20% yields, respectively. The sterically hindered amines would
rather prefer, to some extent, a bromonium abstraction than a car-
bonyl group addition. Such a reaction should form the pentabro-
moacetone, through the respective enol, which goes then to 9b
and 11b acetamides by the respective amine reactions.
The products, most of them already known, were characterized
through melting point, IR, and 1H NMR, and the data are in agree-
ment with those reported in the literature.
The aims of this study were to find better experimental condi-
tions for the tribromoacetylation of alcohols and amines, to explore
other acids, amines, and nucleophiles (such as thioalcohols) to be
converted into amides and thiocarbamates, and to extend the stud-
ies previously carried out with HCA and diamines,5 to HBA.
In summary, HBA was shown to be a good tribromoacetylating
agent for primary alcohols and amines, both slightly sterically hin-
dered, under mild conditions. In the reactions with i-propylamine
and s-butylamine, the 2,2-dibromo-N-alkylacetamide subproducts
were obtained along with 2,2,2-tribromo-N-alkylacetamides. Fur-
thermore, HBA acting as a brominating agent, in the presence of
The purifications of the products were carried out by column
chromatography on silica gel, using hexane–chloroform mixtures
Table 2
Di- and tri-bromoacetylation of primary amines with HBA
O
O
O
RNH2 (2 eq.)
Br3C CBr3
Br3C NHR Br HC NHR
+
2
Table 1
CHCl3
r.t.; 1 h
Tribromoacetylation of alcohols with HBA
HBA
O
O
ROH (1.1 eq.)
Entry
Amine
Yield (%)
Br3C CBr3
HBA
Br3C OR
DMF (1.0 eq.)
60 ºC, 10 h
TBACAa
DBACAb
7
8
Ethylamine
74
70
50
69
48
65
—
—
15
—
20
—
Entry
Alcohol
Yield (%)
n-Propylamine
i-Propylamine
n-Butylamine
s-Butylamine
Hexylamine
1
2
3
4
5
6
Methanol
Ethanol
n-Propanol
n-Butanol
n-Pentanol
i-Amyl alcohol
65
65
60
62
58
55
9(a,b)
10
11(a,b)
12
a
Isolated 2,2,2-tribromo-N-alkylacetamides.
Isolated 2,2-dibromo-N-alkylacetamides.
b

F. G. Menezes et al. / Tetrahedron Letters 50 (2009) 2559–2561
2561
Table 3
15. Martins, M. A. P.; Emmerich, D. J.; Pereira, C. M. P.; Cunico, W.; Rossato, M.;
Zanatta, N.; Bonacorso, H. G. Tetrahedron Lett. 2004, 45, 4935.
Conversion of aliphatic and benzoic acids into the respective amides mediated by
hexabromoacetone/triphenylphosphine
16. Hexabromoacetone, HBA: mp 108 °C (Lit.8 109 °C). IR (KBr)
m 1725, 1070, 763,
563. 13C NMR (100 MHz, CDCl3) d (ppm) 173.6 (C@O), 24.8 (CBr3). Anal. Calcd
for C3OBr3: C, 6.78. Found: C, 6.82. Yield: 70%.
1) HBA (0.3 eq.), PPh3 (1.5 eq.),
O
O
CH2Cl2, r.t., 3h
17. X-ray crystallographic data: An irregular block was cut off from a soft crystal
and selected for crystallographic measurements. X-ray analysis was carried out
R
OH
R
NR'R''
2) R'R''NH (1.0 eq.),
Et3N (3.0 eq.), r.t., 15 min
on an Enraf-Nonius CAD-4 diffractometer with monochromatic Mo-K
a
radiation (0.71069 Å), at room temperature. Cell parameters were
determined from 25 centered reflections in the h range 3.93–12.20°. One
thousand nine hundred and eight poorly diffracted intensities were collected
Entry
R
R0
R00
Yield (%)
using the
1020 with [I > 2
x
–2h scan method, of which 1790 (Rint = 0.1170) are unique and
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
Ph
n-But
s-But
t-But
c-Hex
H
H
H
H
H
H
H
H
Et
H
H
H
H
H
H
86
72
70
75
85
89
81
82
70
83
90
91
60
57
53
r
(I)], with h angle ranging from 1.98o to 25.00o. Due to high
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
absorption coefficient of HBA, a numerical absorption correction should be
indicated. However, once the selected crystal had an irregular shape, the
empirical absorption correction (w-scan) was applied with better results with
Ph
respect to numerical absorption correction. All atoms were refined with
anisotropic thermal parameters. Crystal data: formula: C3Br6O, FW = 531.49,
monoclinic, P21/n, a = 6.292(7) Å, b = 20.619(1) Å, c = 8.242(1) Å, b = 107.88(1)o,
4-n-But-Ph
4-NO2-Ph
3,5-DiNO2-Ph
Et
c-Hex
Ph
V = 107.88(1) Å3, Z = 4,
q l
calc = 3.469 Mg/m3, = 23.611 mmꢀ1, F(0 0 0) = 944,
w
-scan transmission factors: 0.015 and 0.101, parameters = 92, GOOF
(F2) = 0.958, R1 [I > 2
r(I)] = 0.0986, wR2 (all data) = 0.2574. Crystallographic
4-MeO-Ph
4-MeO-Ph
4-MeO-Ph
Me
Et
n-Prop
data (excluding structure factors) for the structure in this Letter have been
deposited with the Cambridge Crystallographic Data Centre as supplementary
publication number CCDC 713085. Copies of the data can be obtained, free of
charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, (fax:
+44-(0)1223-336033 or e-mail: deposit@ccdc.cam.ac.Uk).
4-n-But-Ph
Ph
Ph
Ph
18. General procedure for obtainment of alkyl 2,2,2-tribromoacetates: The following
reagents were placed in a round-bottomed flask: 6.70 mmol of the respective
alcohols (methanol, ethanol, n-propanol, n-butanol, n-pentanol, and i-amyl
alcohol), previously dried by traditional methods,21 and 0.43 mL of DMF
(5.60 mmol). After that 3.00 g (5.64 mmol of HBA was added and the solution
was stirred at 60 °C, for 10 h. The products were purified by column
chromatography on silica gel using hexane–chloroform mixtures as the
eluent. The products were obtained as transparent oils, with yields of 55% to
triphenylphosphine, gave good results in the conversion of benzoic
acids into the respective amines.
Acknowledgments
65%. Methyl tribromoacetate, 1: IR (KBr)
m
2955, 1751, 1235, 758, 609. 1H NMR
(400 MHz, CDCl3) d (ppm) 4.01 (s, 3H). 13C NMR (100 MHz, CDCl3) d (ppm)
163.0, 56.3, 28.6 (Lit.22 161.8, 55.8, 28.4). MS: m/z 310 (M+), 250.8, 230.9, 171.9,
92.9, 78.9, 59.0. Yield: 65%.
The authors are grateful to the Brazilian entities UFSC, CAPES,
INCT-cat, and FAPESC. Also, we would like to thank Dr. Faruk Nome
and Dr. Jacks P. Priebe for the GC and GC–MS analysis.
19. General procedure for the reactions of HBA with amines: obtainment of 2,2,2-
tribromo-N-alkylacetamides or mixtures of 2,2,2-tribromo-N-alkylacetamides and
2,2-dibromo-N-alkylacetamides: Firstly, 1 g (1.88 mmol) of HBA was added
References and notes
portionwise to
a round-bottomed flask containing 3.76 mmol of amine
(ethylamine, n-propylamine, i-propylamine, n-butylamine, s-butylamine, and
hexylamine), previously distilled,23 in 5 mL of chloroform. The solution was
then maintained under magnetic stirring at room temperature for 1 h,
monitored by TLC. After the total consumption of HBA, the chloroform was
eliminated in a rotavapor, and the residue was dried and purified by column
chromatography on silica gel using mixtures of hexane–chloroform as the
eluent, providing the respective pure brominated acetamides after evaporation
of the solvent and recrystallization in 5:1 (v/v) hexane–chloroform. 2,2,2-
tribromo-N-isopropylacetamide, 9a: mp 80–81 °C (Lit.24 80 °C). IR (KBr) mmax
(cmꢀ1) 3302, 2970, 1662, 1518, 1238, 1141, 750, 682, 596. 1H NMR (400 MHz,
CDCl3) d (ppm) 6.65 (br, 1H, NH), 4.15–4.04 (m, 1H, CH), 1.29 (d, J = 6.8 Hz, 6H,
CH3). 13C NMR (100 MHz, CDCl3) d (ppm) 161.6 (C@O), 44.6 (CH), 37.5 (CBr3),
22.2 (CH3). DEPT (100 MHz, CDCl3) d (ppm) 44.6 (CH), 22.2 (CH3). Anal. Calcd
for C5H8NOBr3: C, 17.78; H. 2.39; N. 4.15. Found: C, 17.65; H, 2.50; N, 4.25.
Yield: 50%. 2,2-dibromo-N-isopropylacetamide, 9b: mp 149–150 °C. IR (KBr) mmax
(cmꢀ1) 3282, 2971, 1654, 1551, 1458, 1194, 788, 676, 596. 1H NMR (400 MHz,
CDCl3) d (ppm) 6.32 (br, 1H, NH), 5.79 (s, 1H, CHBr2), 4.12–3.96 (m, 1H, CH),
1.22 (d, J = 6.8 Hz, 6H, CH3). 13C NMR (100 MHz, CDCl3) d (ppm) 163.8 (C@O),
43.2 (CH), 37.1 (CHBr2), 22.3 (CH3). DEPT (100 MHz, CDCl3) d (ppm) 43.2 (CH),
37.1 (CHBr2), 22.3 (CH3). Anal. Calcd for C5H9NOBr2: C, 23.19; H, 3.50; N, 5.41.
Found: C, 23.27; H, 3.59; N, 5.47. Yield: 15%.
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