A closer look at the hydroiodination of propiolic acid

Article abstract of DOI:10.1080/00397910600942826

The crystal structures of cis-3-iodoacrylic acid (1), trans-3-iodoacrylic acid (2), trans-3-iodoacrylic acid methyl ester (3), 3,3-diiodopropanoic acid (4), and trans-2,3-diiodoacrylic acid (5) are reported. Compounds 1 and 2 are the kinetic and thermodynamic products, respectively, of the hydroiodination of propiolic acid. Compound 4 results from addition of a second equivalent of hydroiodic acid to 1 or 2, whereas 5 results from the addition of trace elemental iodine to propiolic acid. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910600942826

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A Closer Look at the Hydroiodination of
Propiolic Acid
James P. Donahue a
a Department of Chemistry , Tulane University , New Orleans, Louisiana,
USA
Published online: 24 Nov 2006.
To cite this article: James P. Donahue (2006) A Closer Look at the Hydroiodination of Propiolic Acid, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 36:23,
3461-3471, DOI: 10.1080/00397910600942826
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Synthetic Communicationsw, 36: 3461–3471, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600942826
A Closer Look at the Hydroiodination of
Propiolic Acid
James P. Donahue
Department of Chemistry, Tulane University, New Orleans,
Louisiana, USA
Abstract: The crystal structures of cis-3-iodoacrylic acid (1), trans-3-iodoacrylic acid
(2), trans-3-iodoacrylic acid methyl ester (3), 3,3-diiodopropanoic acid (4), and trans-
2,3-diiodoacrylic acid (5) are reported. Compounds 1 and 2 are the kinetic and thermo-
dynamic products, respectively, of the hydroiodination of propiolic acid. Compound 4
results from addition of a second equivalent of hydroiodic acid to 1 or 2, whereas 5
results from the addition of trace elemental iodine to propiolic acid.
Keywords: cis-3-Iodoacrylic acid, hydroiodination, propiolic acid, trans-3-iodoacrylic
acid
INTRODUCTION
Although the syntheses of cis- and trans-3-iodoacrylic acid via the hydroiodi-
nation of propiolic acid were first described more than 120 years ago,[1] it has
not been until the more recent work by Moss et al.,[2] Abarbri et al.,[3] and
Zoller and Uguen[4] that procedures optimized for the isolation of one or
the other stereoisomer have been available. The cis isomer, which is the
kinetic product of hydroiodination, is preferentially formed when relatively
mild reaction conditions are employed (8–108C, CuI catalysis,[2] or 4 h at
658C[3]), whereas formation of the thermodynamically favored trans isomer
is promoted by more vigorous conditions (16 h at 1308C[3] or 57% HI
solution starting with isolated cis isomer, 85–908C, 24 h[4]). Since
Received in the USA April 19, 2006
Address correspondence to James P. Donahue, Department of Chemistry, Tulane
University, 6400 Freret Street, Room 2015, New Orleans, LA 70118-5698, USA.
E-mail: donahue@tulane.edu
3461

3462
J. P. Donahue
these reports describing procedures for the synthesis of pure cis- or
trans-3-iodoacrylic acid, these compounds have been increasingly employed
as starting materials and building blocks in the synthesis of complex
organic molecules with important pharmacological or medicinal properties.
The value of these molecules as starting materials arises from the facility
with which they undergo carbon–carbon bond forming reactions via the
Stille coupling protocol.[5] Examples of important natural products whose
synthesis has been accomplished using cis- or trans-3-iodoacrylic acid, or
their methyl esters, include strychnine,[6,7] cylindramide,[8] freelingyne
(a sesquiterpene),[9] xerulin (a cholesterol biosynthesis inhibitor),[10,11] the
macrolides elaiophylin[12] and elaiolide[13] (antibiotics and anthelmintics),
(2)-macrolactin A (an antiviral agent),[14,15] (þ)-brefeldin C[16] and (þ)-
brefeldin A[17,18] (antiviral and antitumour agents), eunicenone A,[19] (þ)-
aloperine (an anti-inflammatory agent),[20] and formamicin (an antifungal,
antibacterial, and antitumor agent).[21,22]
In the course of earlier work with polyunsaturated dicarboxylates as con-
nectors between dimolybdenum units,[23] trans-3-iodoacrylic acid was
prepared by the hydroiodination of propiolic acid. While following a literature
procedure,[4] it was noted that this reaction is attended by the formation of
other products in greater or lesser amounts depending upon the specific con-
ditions. Because of the importance of cis and trans iodoacrylic acids as
organic synthons, and because they had not previously been characterized
by crystallography, X-ray crystal structure determinations of these two
simple organic acids were undertaken. Two other organic acids that were
unexpected by-products of this reaction were identified and serve to
emphasize the importance of precise stoichiometry and reaction temperature
upon the product manifold. Reported here are the crystal structures of both
cis- and trans-3-iodoacrylic acid, the methyl ester of the latter, and the struc-
tures of the unexpected by-products 3,3-diiodopropanoic acid and trans-2,3-
diiodoacrylic acid. The crystal structure of trans-3-iodoacrylic acid has
been briefly described in an earlier publication.[23]
RESULTS AND DISCUSSION
Figure 1 presents the structures of cis- and trans-3-iodoacrylic acid [(1) and
(2), respectively], with crystal data and data collection and refinement
details presented in Table 1. Bond lengths and angles are collected in
˚
Tables 2 and 3. The carbon-iodine bond lengths [2.070(4) and 2.077(6) A]
˚
and the carbon–carbon double bond lengths [1.321(6) and 1.317(8) A] are
unexceptional. As is typically the case, these simple acids crystallize in a
“head-to-head” fashion though hydrogen bonding in the carboxyl groups.
The methyl ester of trans-3-iodoacrylic acid (3), prepared by refluxing the
acid in MeOH with H2SO4 as catalyst (see Fig. 2), is a low melting solid
that crystallizes upon slow cooling of the melt. Its crystallization was

Hydroiodination of Propiolic Acid
3463
Figure 1. Thermal ellipsoid plots of cis-3-iodoacrylic acid (1), trans-3-iodoacrylic
acid (2), trans-3-iodoacrylic acid methyl ester (3), 3,3-diiodopropanoic acid (4) and
trans-2,3-diiodoacrylic acid (5). Ellipsoids are drawn at the 50% probability level,
except for 5, which has ellipsoids drawn at the 30% level.
an unexpected result because existing information in the literature on this
compound appears to suggest that it is an oil at ambient temperature and
pressure.[24] Figure 1 illustrates a thermal ellipsoid plot of trans-3-iodoacrylic
acid methyl ester (3), and Table 4 presents selected bond distances and angles.
If the hydroiodination of propiolic acid is attempted at temperatures
greater than 1008C with excess HI, the trans-3-iodocrylic acid that is
isolated is mixed with substantial quantities of another product from which
it is readily distinguished by crystal morphology. The desired trans-3-
iodoacrylic acid crystallizes as large, flat needle crystals, whereas the
1
second product crystallizes as large, white, block-shaped crystals. The H
NMR spectrum of this second species identifies it as 3,3-diiodopropanoic
acid (4), a product resulting from the addition of a second equivalent of HI

Table 1. Crystal and refinement data for compounds 1–5
cis-3-Iodoacrylic
acid, 1
trans-3-Iodoacrylic
acid, 2
trans-3-Iodoacrylic
acid methyl ester, 3
3,3-Diiodo-
propanoic acid, 4
trans-2,3-Diio-
doacrylic acid, 5
Parameter
Formula
FW
C3H3IO2
197.95
White
C3H3IO2
197.95
White
Needle
Orthorhombic
Cmca
C4H5IO2
211.98
White
Prism
C3H4I2O2
325.86
White
C3H2I2O2
323.85
White
Crystal color
Crystal morphology
Crystal system
Space group
Plate
Block
Monoclinic
P21/c
12.7395(8)
6.6028(4)
8.8693(6)
90
Needle
Triclinic
P¯ı
Monoclinic
P21/n
7.275(2)
8.003(2)
9.359(3)
90
Orthorhombic
Cmc21
6.649(1)
5.9507(9)
15.626(3)
90
˚
a, A
6.485(1)
5.637(1)
27.519(5)
90
4.9604(7)
11.506(2)
11.797(2)
91.482(2)
95.041(2)
90.558(3)
670.4(2)
4
˚
b, A
˚
c, A
a, deg.
b, deg.
g, deg.
106.239(4)
90
90
90
90
90
102.643(1)
90
3
˚
V, A
523.2(2)
4
260
1005.8(3)
8
260
618.2(2)
4
260
727.96(8)
4
260
Z
Temperature
260
0.71073
l
0.71073
0.71073
0.71073
0.71073

Density, g/cm23
m, mm21
u range, deg
2.513
5.988
2.614
6.229
2.277
5.075
2.973
8.551
3.209
9.284
3.15–27.51
29 to 9
210 to 10
210 to 12
3407
2.96–27.48
28 to 8
27 to 6
234 to 35
3315
2.61–27.51
28 to 8
27 to 7
220 to 12
1882
3.28–27.51
215 to 16
26 to 8
211 to 10
4880
1.73–27.49
26 to 6
213 to 14
215 to 14
4657
Limiting indices, h
Limiting indices, k
Limiting indices, l
Reflections collected
Independent reflections
Parameters refined
GoF
1183
67
634
47
610
43
1666
69
2919
109
1.073
1.197
1.076
1.194
1.049
R1,a wR2b
0.025, 0.065
0.001
0.70
0.024, 0.061
20.001
0.65
0.022, 0.057
0.001
0.75
0.029, 0.083
0.001
1.78
0.043, 0.105
0.001
2.38
Largest shift/esd
Largest peak, final
cycle
aR1 ¼S||Fo| 2 |Fc||/S|Fo|.
w ¼ 1/[s2(Fo2) þ (aP)2 þ (bP)], where P ¼ [max(Fo2 or 0) þ 2(Fc2)]/3.
1
2 2
2
b
2
2 2
wR2 ¼ [S[w(Fo 2 Fc) ]/S[w(Fo) ]] .

3466
J. P. Donahue
˚
Table 2. Bond parameters (A, deg.) for 1
I(1)–C(3)
O(1)–C(1)
2.070(4)
1.310(4)
1.228(4)
1.477(5)
1.321(6)
O(2)–C(1)
C(1)–C(2)
C(2)–C(3)
I(1)–C(3)–C(2)
C(1)–C(2)–C(3)
O(1)–C(1)–O(2)
128.0(3)
126.9(3)
123.5(3)
across the carbon–carbon bond (Fig. 2). This assignment was subsequently
confirmed by X-ray crystallography, and the thermal ellipsoid plot is
presented in Fig. 1 (4). Bond distances and selected bond angles are
gathered in Table 5. Although it is an unintended product and previously
unknown molecule, it is probable that 3,3-diiodopropanoic acid could be
deliberately prepared in reasonable yield by a slight modification of this
method.
When the hydroiodination of propiolic acid is attempted at ambient
temperature without reflux of the reaction mixture or addition of catalytic
CuI, subsequent workup of the reaction by extraction yields small amounts
of organic material, which, upon recrystallization, is clearly different from
any of the other products. This material was identified by crystallography as
trans-2,3-diiodoacrylic acid (5 in Figs. 1 and 2), a product that undoubtedly
results from reaction of propiolic acid with traces of I2, which occur as a
minor impurity in commercial HI solutions. The synthesis of this molecule
by reaction of propiolic acid with I2 has been previously reported,[25–28] but
none of these previous reports was accompanied by a crystallographic
study. The bond distances and angles listed in Table 6 for 5 are for one of
two independent (and disordered) molecules in the asymmetric unit.
As shown in Fig. 3, trans-2,3-diiodoacrylic acid is disordered in the crys-
talline state. The disorder is such that the positions of the two iodine atoms are
fixed, and the remainder of the molecule is distributed between two highly
˚
Table 3. Bond parameters (A, deg.) for 2
I(1)–C(3)
O(1)–C(1)
2.077(6)
1.218(7)
1.304(7)
1.489(7)
1.317(8)
O(2)–C(1)
C(1)–C(2)
C(2)–C(3)
I(1)–C(3)–C(2)
C(1)–C(2)–C(3)
O(1)–C(1)–O(2)
123.8(4)
122.5(5)
125.1(5)

Hydroiodination of Propiolic Acid
3467
Figure 2. Products resulting from the hydroiodination of propiolic acid.
overlapping and nearly coplanar orientations that are related to one another via
a noncrystallographic twofold rotation axis. The position of this twofold axis
may be easily visualized in Fig. 3 as the perpendicular bisector of the line
segments joining atoms C(1A) with C(1B), C(2A) with C(2B), and C(3A)
with C(3B). Because this twofold axis is noncrystallographic, that is, not
required to exist by the symmetry of the crystal system, the site occupancy
of these two orientations is not required to be 1:1. Refinement of the site
occupancy as an optimal fit yielded a 7:3 occupancy ratio between the A
and B orientations, respectively, where the A and B positions are those with
atom labels bearing A or B in Fig. 3. A second molecule of trans-2,3-diio-
doacrylic acid occurs in the asymmetric unit (not shown), is disordered in
the same fashion, and was refined in the same manner as just described.
Because of this disorder problem, anisotropic refinement was only
attempted for the iodine atoms.
CONCLUSIONS
The foregoing results provide further, more complete characterization of the
hydroiodination of propiolic acid and emphasize that the system is susceptible
to a second hydroiodination in the presence of excess HI. To suppress this
undesired further reaction, the synthesis of trans-3-iodoacrylic acid is best accom-
plished with a stoichiometric quantity of HI and elevated reaction temperatures
˚
Table 4. Bond parameters (A, deg.) for 3
I(1)–C(3)
O(1)–C(1)
2.074(7)
1.324(9)
1.202(8)
1.474(9)
1.322(9)
1.47(1)
O(2)–C(1)
C(1)–C(2)
C(2)–C(3)
C(4)–O(1)
I(1)–C(3)–C(2)
C(1)–C(2)–C(3)
123.8(5)
119.4(5)

3468
J. P. Donahue
˚
Table 5. Bond parameters (A, deg.) for 4
I(1)–C(3)
I(2)–C(3)
2.150(5)
2.147(4)
1.498(7)
1.520(7)
1.304(7)
1.224(7)
C(1)–C(2)
C(2)–C(3)
O(1)–C(1)
O(2)–C(1)
I(1)–C(3)–I(2)
C(1)–C(2)–C(3)
110.5(2)
112.0(4)
(125–1308C). Because the reaction products (cis- and trans-iodoacrylic acid and
3,3-diiodopropanoic acid) crystallize in morphologically distinct forms (thick
plates, needles, and blocks, respectively), these structural studies offer a con-
venient means for qualitative identification of the species isolated.
EXPERIMENTAL
Propiolic acid and HI (57% wt solution) were obtained from commercial sources
and used as received. 1H NMR spectra were obtained on a Varian XL-200AA
spectrometer with chemical shifts referenced to the protonated solvent
residual. The elemental analysis was performed by Canadian Microanalytical
Service, Delta, British Columbia, Canada, upon a highly crystalline sample.
All crystals were coated with Paratone oil and mounted on a quartz fiber or
nylon Cryoloop affixed to a goniometer head. All diffraction data were collected
using a Bruker Smart 1000 CCD area-detector system using v scans of 0.3 deg/
frame with 30, 60, or 90 second frames such that 1271 frames were collected for
a full hemisphere of data. The first 50 frames were recollected at the end of the
data collection to monitor for crystal decay, but no significant decomposition
was observed. Cell parameters were determined using the program
SMART.[29] Data reduction and integration were performed with the software
package SAINT,[30] which corrects for Lorentz and polarization effects,
˚
Table 6. Bond parameters (A, deg.) for 5
I(2)–C(3A)
I(1)–C(2A)
2.01(2)
2.11(1)
1.38(2)
1.48(2)
1.30(2)
1.24(2)
131(1)
119(1)
C(3A)–C(2A)
C(2A)–C(1A)
O(1A)–C(1A)
O(2A)–C(1A)
I(2)–C(3A)–C(2A)
I(1)–C(2A)–C(3A)

Hydroiodination of Propiolic Acid
3469
Figure 3. Illustration of the disorder in trans-2,3-diiodoacrylic acid about a noncrys-
tallographic twofold axis. The iodine atoms are the only ones that are not disordered.
whereas absorption corrections were applied by using the program
SADABS.[30] Space groups for all the compounds structurally characterized
by X-ray crystallography were initially determined with the aid of the
program XPREP, which is part of the SHELX software package.[31] In all struc-
tures, the positions of the iodine atom(s) were found via direct methods using
the SHELX software, and the positions of the remaining nonhydrogen atoms
were revealed by a subsequent cycle of least-squares refinement followed by
difference Fourier synthesis. For both cis- and trans-3-iodoacrylic acid, the
positions of the hydrogen atoms were visible in the final electron density
maps and were refined. In the remaining structures, the hydrogen atoms were
added in calculated positions and refined as riding atoms with isotropic dis-
placement parameter values equal to 1.2 times those of the atoms to which
they were attached. All refined structures were checked for higher symmetry
using the crystallographic checking system provided by the International Union
of Crystallography (http:// journals.iucr.org/c/services/authorservices.html).
cis-3-Iodoacrylic Acid
cis-3-Iodoacrylic acid was synthesized via the method described by Abarbri
et al.[3] Diffraction-quality, thick colorless plate crystals were obtained by
slow evaporation of an Et2O solution.
trans-3-Iodoacrylic Acid and 3,3-Diiodopropanoic Acid
trans-3-Iodoacrylic acid was synthesized by the method described Zoller and
Uguen[4] starting from isolated cis-3-iodoacrylic acid. The crude product was

3470
J. P. Donahue
recrystallized from hexanes with a slight amount of CH2Cl2 added to slow
crystallization. The white crystalline mass that formed proved to be ꢀ1:1
mixture of trans-3-iodoacrylic acid (long, flat, white needle crystals) and
3,3-diiodopropanoic acid, 3 (large, white block crystals). The following data
1
correspond to 3: Mp (uncorrected) ¼ 84.5–85 8C. H NMR: d (200 MHz;
CDCl3): 11.0 (1 H, s br, -CO2H), 5.25 (1 H, t, J 7.3, -CHI2), 3.81 (2 H, d,
J 7.3, -CH2). Anal. calcd. for C3H4I2O2: C, 11.06%; H, 1.24%. Found: C,
11.14%; H, 1.17%.
trans-3-Iodoacrylic Acid Methyl Ester
The methyl ester of trans-3-iodoacrylic acid was synthesized by the following
method: trans-3-iodoacrylic acid (12.00 g, 61 mmol) was dissolved in 100 mL
of distilled MeOH, and 0.5 mL of concentrated H2SO4 was added. This solution
was vigorously refluxed for 24 h, cooled, and neutralized with aqueous
NaHCO3. The MeOH solvent was removed under reduced pressure, and the
resulting reside was partitioned between 100 mL of H2O and Et2O. The Et2O
layer was separated, and the aqueous layer was extracted with 3 ꢀ 50-mL
portions of Et2O. The combined Et2O extracts were filtered through
anhydrous Na2SO4 and concentrated under reduced pressure to a volume of
25 mL. The remaining Et2O was removed by distillation at ambient pressure,
and the residual oil was crystallized upon cooling and standing. Pure trans-3-
1
iodoacrylic acid methyl ester (2.56 g, 20% yield), the H NMR spectrum of
which agreed with literature data,[24] was obtained. A diffraction-quality
crystal was obtained directly from the cooled reaction melt.
trans-2,3-Diiodoacrylic Acid
Small quantities of trans-2,3-diiodoacrylic acid were obtained by treating
propiolic acid with 57% aqueous HI at ambient temperature overnight.
Workup of the reaction mixture as described for the preparation of cis-3-
iodoacrylic acid[2,4] yielded a small quantity of crude product that was
mixture of elemental S8 (resulting from extraction with aqueous thiosulfate)
and trans-2,3-diiodoacrylic acid. Diffraction-quality white needle crystals of
the latter were obtained by extracting the crude product with a minimal
volume of Et2O and slowly evaporating the solution to dryness.
ACKNOWLEDGMENTS
The author thanks Professors F. A. Cotton and C. A. Murillo of Texas A&M
University for hospitality and helpful discussions.

Hydroiodination of Propiolic Acid
3471
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