Solution structure of some λ3 iodanes: An 17O NMR and DFT study

Article abstract of DOI:10.1021/jo070111h

(Figure Presented) The structure of a series of I-O bonded bis(acyloxy)iodoarenes and benzoiodoxolones in chloroform solution has been investigated by ¹⁷O NMR spectroscopy and by density functional theory (DFT) calculations, employing the PBE0 functional together with the LANL2DZ basis set extended with polarization (d) and diffuse (p) functions. This combined approach allowed us to ascertain that, although these classes of λ³ iodanes maintain in chloroform solution their solid state "T-shaped" structure, a degenerate [1,3] sigmatropic shift of iodine between the two oxygens of the acyloxy groups occurs in solution. The energy barrier involved in this process differs in the two classes, thus causing significant differences in the ¹⁷O NMR spectra, at room temperature, of the two classes of compounds.

Full text of DOI:10.1021/jo070111h

17
Solution Structure of Some λ3 Iodanes: An O NMR and DFT
Study
Francesca Mocci,* Gianluca Uccheddu, Angelo Frongia, and Giovanni Cerioni*
Dipartimento di Scienze Chimiche, UniVersita` di Cagliari, Complesso UniVersitario, S.S. 554,
BiVio per Sestu, I-09042 Monserrato (CA), Italy
cerioni@unica.it; francy@dsc.unica.it
ReceiVed January 18, 2007
The structure of a series of I-O bonded bis(acyloxy)iodoarenes and benzoiodoxolones in chloroform
solution has been investigated by 17O NMR spectroscopy and by density functional theory (DFT)
calculations, employing the PBE0 functional together with the LANL2DZ basis set extended with
polarization (d) and diffuse (p) functions. This combined approach allowed us to ascertain that, although
these classes of λ3 iodanes maintain in chloroform solution their solid state “T-shaped” structure, a
degenerate [1,3] sigmatropic shift of iodine between the two oxygens of the acyloxy groups occurs in
solution. The energy barrier involved in this process differs in the two classes, thus causing significant
differences in the 17O NMR spectra, at room temperature, of the two classes of compounds.
Introduction
bound, although with a highly ionic character,1 is usually
assumed also in solution. In fact, dipole moment studies3 by
Exner and Plesnicˇar on the conformation of acyloxy groups of
some bis(acyloxy)iodoarenes in benzene solution favor this
assumption, but definitive evidence is, however, lacking. On
the contrary, in recent years, Koser’s group4 has shown that
[hydroxy(tosyloxy)iodo]benzene is fully ionized in water solu-
tion, originating a monomeric positively charged hydroxy-
(phenyl)iodonium ion. Ochiai’s group5 has been able to stabilize
this cation by interaction with 18-crown-6 ether.
In a recent paper,6 to justify the observation of only one signal
in the 17O NMR for a number of para-substituted bis(acetoxy)-
iodobenzenes in chloroform, we have suggested the possibility
that these compounds had an “ion pair” structure, even if other
structures (e.g., “chelate”) could not be ruled out. Equivalence
Hypervalent iodine chemistry has experienced in the last few
years a fast and important increase of interest, as shown by the
accelerated appearance of reviews1 motivated not only by the
pure chemical interest but also by the growing application of
many compounds of this class in organic synthesis, especially
for their oxidizing properties. Notwithstanding this large body
of literature, a full spectroscopic characterization of many I(III)
organic derivatives, such as λ3 iodanes, is lacking in the solution
state. The solid-state structure,2 “T-shaped” and I-O covalently
(1) (a) Moriarty, R. M. J. Org. Chem. 2005, 70, 2893-2903. (b)
Zhdankin, V. V. Curr. Org. Synth. 2005, 2, 121-145. (c) Stang, P. J. J.
Org. Chem. 2003, 68, 2997-3008. (d) Morales-Rojas, H.; Moss, R. A.
Chem. ReV. 2002, 102, 2497-2521. (e) Zhdankin, V. V.; Stang, P. J. Chem.
ReV. 2002, 102, 2523-2584. (f) Zhdankin, V. V. In ReViews on Heteroatom
Chemistry; Oae, S., Ed.; MYU: Tokyo, 1997; Vol. 17, pp 133-151. (g)
Stang, P. J.; Zhdankin, V. V. Chem. ReV. 1996, 96, 1123-1178. (h)
Varvoglis, A. The Organic Chemistry of Polycoordinate Iodine; VCH: New
York, 1992.
(2) (a) Zanos Gougoutas, J.; Clardy, J. C. J. Solid State Chem. 1972, 4,
226-229. (b) Etter, M. C. J. Am. Chem. Soc. 1976, 98, 5326-5331. (c)
Alcock, N. W.; Countryman, R. M.; Esperas, S.; Sawyer, J. F. J. Chem.
Soc., Dalton Trans. 1979, 854-860.
(3) Exner, O.; Plesnicˇar, B. J. Org. Chem. 1974, 39, 2812-2814.
(4) Wilkinson Richter, H.; Cherry, B. R.; Zook, T. D.; Koser, G. F. J.
Am. Chem. Soc. 1997, 119, 9614-9623.
(5) Ochiai, M.; Miyamoto, K.; Shiro, M.; Ozawa, T.; Yamaguchi, K. J.
Am. Chem. Soc. 2003, 125, 13006-13007.
(6) Cerioni, G.; Uccheddu, G. Tetrahedron Lett. 2004, 45, 505-507.
10.1021/jo070111h CCC: $37.00 © 2007 American Chemical Society
Published on Web 04/28/2007
J. Org. Chem. 2007, 72, 4163-4168
4163

Mocci et al.
CHART 1. Structures of Compounds 1, 2, and 3
TABLE 1. 17O NMR Data,a,b at 25 °C and in CHCl3 Solution, of
the Carboxylic Groups of Derivatives 1a-j and of Their
Corresponding Free Carboxylic Acids
We note that, although with the possible exception in 1g, iodine
bonding influences 17O chemical shifts of the carboxylic groups
roughly to a similar extent (∆δ ) 43 ( 8 ppm, excluding 1g),
seemingly irrespective of electronic and/or steric factors. In a
qualitative way and for limited and quite homogeneous series,
it is, however, possible to distinguish an influence of steric
hindrance or of acid strength as it can be deduced from the
comparison of the lower ∆δ values for 1c versus 1a,b or 1i,j
versus 1h. The influence of the acid strength is indicated by
the lowering of the ∆δ values on going from 1d to 1g. The
quite low ∆δ value (∆δ ) 24 ppm) found for 1g seems thus
due to an unexpected high value of the shift of the trifluoroacetic
acid. The relatively high deshielding overall observed (∆δ >
35 ppm) on going from the free acids to derivatives 1 indicates
the presence of a significant interaction between iodine and the
two oxygens of the carboxylic group. This interaction does not
seem to be sensitive to changes in the electronic distribution
on the phenyl ring since para-substitution showed a remarkable
constancy of the chemical shifts of the acyloxy group, with a
measured δ ) 299 ( 3 ppm.6
δ
ν1/2
(RCOOI)
δ
ν1/2
(COOH)
compound
(RCOOI)
(COOH)
∆δ
1a
1b
1c
1d
1e
1f
1g
1h
1i
301
298
295
292
281
275
270
285
307
303
1050
1350
1270
1100
2150
∼2200
1030
1340
640
254.2
248.9
242.4
250.2
244.1
238.7
246.5
234.1
266.3
263.1
161
218
206
245
226
240
166
420
535
465
47
49
43
42
37
36
24
51
41
40
1j
1365
a δ(17O), ppm, from δH O ) 0 ppm; ν1/2(17O) half-height line width, Hz.
2
Because of the lines broadness, δ, for derivatives 1, and ∆δ values are
approximated to integer values since the chemical shifts are estimated with
a confidence of (2 ppm. A better estimate ((1 ppm or less) is obtained
from the much sharper signal of the free carboxylic acids. b 17O NMR spectra
had already been reported for 1a6,7a and 1g;7a our measurement is not in
complete agreement, in the case of 1g, with the literature data both for the
chemical shift (263 ppm)7a and, particularly, with their finding that a “sharp”
line is observed when compared to that of 1a.
This constancy and the small changes among the ∆δ values,
mostly reflecting the influence of the R group in RCOOH, seem
to indicate a very small influence of changes in other parts of
the molecule on the interactions between iodine and carboxylic
groups.
in the 17O NMR of the two “estereal” oxygens had already been
observed.7 In the case of compounds 1a and 1g (see Chart 1)
in CH2Cl2 solution, a degenerate [1,3] sigmatropic shift of iodine
between the two oxygens was suggested7a as not unreasonable,
while for some acyloxysilanes7b and -germanes7b and some Sn-
(IV) stannane dicarboxylates,7c an asymmetric chelate bonding
was preferentially suggested. Other possible ways to reach
equivalence of the two oxygens are full ionization and inter-
molecular exchange between the acyloxy groups. Thus it seemed
to us of interest to extend our previous observations6 to seek a
better understanding of the I-O bonds and of the structure(s)
in solution of acyloxyiodoarenes and related λ3 iodanes, and,
possibly, to ascertain the way leading to the observed6,7a
equivalence of the oxygens of their acyloxy groups.
The measured chemical shifts and the signals’ half-height line
widths of derivatives 1 are clearly different from those of the
free acids, thus ruling out both a full ionization or an “ac-
cidental” hydrolysis, as a possible cause of observing only a
single signal, in the 17O NMR, for the two oxygens of the
estereal group. The lack of ionization is also in agreement with
results based on conductivity and freezing point measurements.8
A systematic study of carboxylic acids by 17O NMR seems
not to be present in the literature. A contribution to the observed
deshielding (∆δ > 35 ppm) of derivatives 1 compared to that
of the free acids could be partly given by the rupture of the
intermolecular hydrogen bonds present in the free acids.
Intermolecular exchange between acyloxy groups can also
be discharged; in fact, when studying, in CDCl3 solution, a
degenerate isomerization of acetoxy ligands about iodine in
some chiral acetoxyiodo(III)binaphthyls,9 Ochiai and co-workers
clearly demonstrated the absence of bimolecular ligand ex-
change. We further checked this point on our derivatives,
measuring a 13C NMR spectrum of an equimolar mixture of 1a
and 1g, and not observing any changes from the spectra of these
derivatives measured as pure compounds.
Results and Discussion
The structures of studied derivatives are shown in Chart 1.
The 17O NMR data of derivatives 1, collected in Table 1,
confirm our previous observation6 on a series of p-substituted
bis(acetoxy)iodobenzenes; only one broad signal is observed,
in the 17O NMR, for the two oxygens of the carboxylic groups
in this class of λ3 iodanes, not only in the case of the acetoxy
derivatives6,7a but also for a generality of acyloxy substituents.
(7) (a) Ho, Z.-C.; Livant, P.; Lott, W. B.; Webb, T. R. J. Org. Chem.
1999, 64, 8226-8235. (b) Lycˇka, A.; Holecˇek, J.; Handlirˇ, K.; Pola, J.;
Chvalovskyˆ, V. Collect. Czech. Chem. Commun. 1986, 51, 2582-2589.
(c) Lycˇka, A.; Holecˇek, J. J. Organomet. Chem. 1985, 294, 179-182.
(8) Johnson, W. D.; Sherwood, J. E. Aust. J. Chem. 1971, 24, 2281-
2286.
(9) Ochiai, M.; Takaoka, Y.; Masaki, Y.; Nagao, Y.; Shiro, M. J. Am.
Chem. Soc. 1990, 112, 5677-5678.
4164 J. Org. Chem., Vol. 72, No. 11, 2007

Solution Structure of λ3 Iodanes
TABLE 2. 17O NMR Data,a at 25 °C, of Derivatives 1a, 2, and 3
compound
δ(O1)
ν
1/2(O1)
δ(O2)
ν
1/2(O2)
δ(O3)
ν
1/2(O3)
δ(O4)
ν
1/2(O4)
δ(O5)
ν
1/2(O5)
1a
2a
2b
2cb
3a
3b
3c
301
360
370
340
1050
800
1500
301
219
190
198
1050
520
1700
323
324
325
308
310
309
425
630
203
203
198
213
216
219
440
700
470
440
580
280
285
325
22
53
80
600
650
585
a δ(17O), ppm, from δH O ) 0 ppm; ν1/2(17O) half-height line width, Hz. Because of the lines broadness observed in most cases, all δ values are approximated
2
to integer values with a confidence of (2 ppm. b The value of ν1/2 could not be estimated due to partial signals overlapping.
FIGURE 1. 17O NMR spectra of 1a at 25 °C (bottom) and 45 °C (top).
To discriminate among the remaining hypotheses, that is,
chelate or ion pair structures, or a degenerate [1,3] sigmatropic
shift, and to gain a better comprehension of the phenomenon,
at least in the case of these classes of λ3 iodanes, we extended
our 17O NMR investigation of bis(acyloxy)iodobenzenes 1 to
the related benzoiodoxolones 2 and 3 (Table 2) and modeled
compounds 1a, 2a, and 3a using DFT calculations.
beyond detection, while those of O3 and O4 and that of
derivatives 1 sharpened, as shown for 1a in Figure 1 and for
2a in Figure 2.
The behavior (and the uniqueness) of oxygens O1 and O2 of
derivatives 1 is typical of a dynamical phenomenon after
coalescence, while that of oxygens O1 and O2 of derivatives 2
indicates a situation before coalescence. The fact that these
signals become undetectable at 45 °C seems to indicate that
they reach coalescence around this temperature. The sharpening
of the signals of O3 and O4 is simply what is expected by
increasing the temperature in the case of quadrupolar nuclei as
17O.10 A proper dynamic NMR analysis is prevented by the
severe broadening (quantitatively not predictable) of the oxygen
signals with temperature lowering; in fact, obtaining decoales-
cence of the signal of derivatives 1 would be, for these reasons,
useless or even impossible. However, a rough estimate of the
energy barriers for the oxygens exchange in compounds 1a and
2a has been obtained assuming, for both compounds, as
chemical shift difference between O1 and O2 in the absence of
exchange, the chemical shift difference between O3 and O4 of
2a, and a ν1/2 value of 500 Hz; under these assumptions, the
exchange rate constant was varied until the best fitting of
An important difference is found in the 17O NMR spectra of
derivatives 2 and 3 versus that of 1. The carboxylic group now
part of the iodoxolone ring of derivatives 2 and 3 originates
two different signals. This feature is not completely surprising
since stabilization of the I-O bond by ring closure could be
expected. Much more unexpectedly, also the other carboxylic
group in derivatives 2 showed two different signals. A com-
parison of the 17O signals in compounds 2 and 3 with the only
signal of derivatives 1 allows a safe attribution of the resonances.
The average values of the chemical shifts assigned to O1 and
O2 of each of derivatives 2 are similar to the corresponding
values measured for compounds 1, while those assigned to O3
and O4 better match the corresponding unequivocal signals of
compounds 3. We also note that the signals of 2 and 3 are, in
most cases, sharper than those of 1 and that in derivatives 2 the
pair of signals assigned to O3 and O4 is sharper than that
assigned to O1 and O2. Moreover, in the 17O NMR spectra
obtained at 45 °C, O1 and O2 signals of derivatives 2 broadened
(10) Butler, L. G. In 17O NMR Spectroscopy in Organic Chemistry;
Boykin, D. W., Ed.; CRC Press: Boca Raton, FL, 1991; Chapter 1, p 2.
J. Org. Chem, Vol. 72, No. 11, 2007 4165

Mocci et al.
FIGURE 2. 17O NMR spectra of 2a at 25 °C (bottom) and 45 °C (top), showing the vanishing of the O1 and O2 signals at 45 °C.
computed and experimental spectra was obtained. The ∆Gq
values estimated by this crude approximation are 1a = 44 kJ/
mol and 2a = 54 kJ/mol.
CHART 2. Schematic Representation of the [1,3]
Sigmatropic Shift of Iodine between the Two Oxygens of an
Acyloxy Group in 1a (I-Cac-O1 Angle Bolded)
To obtain further evidence that the equivalence of the 17O
NMR observed signals is due to a dynamic phenomenon and
to explain the difference between the classes of compounds 1
and 2, DFT calculations were performed on derivatives 1a-
3a, studying both the equilibrium structures and the preferential
path to mutual exchange of the estereal oxygens in 1a and 2a.
DFT calculations were performed at the hybrid PBE0 level11
using the LANL2DZ12 basis set extended with polarization (d)
and diffuse (p) functions.12c,d Because of the low dielectric
constant of CHCl3 (4.8 at 293 K), it seemed reasonable to carry
on the calculations in vacuo; however, possible solvent effects
were checked by employing the polarizable continuum model
(PCM)13 in single-point energy calculations on the in vacuo
optimized geometries.
Geometry optimization of compounds 1a-3a reveals that,
as in the solid state,2 also in vacuo the stable conformers of the
modeled derivatives have a T-shaped structure. We found four
stable conformers for 1a, two for 2a, and only one for 3a; the
conformations differ in the orientation of the acetoxy groups
plane, which can be either coplanar or perpendicular to the I-C
bond. In all cases, the most stable conformer is analogous to
that found in the solid state:2 in 1a and 2a, the acetoxy group
plane is coplanar with the I-C bond, while in 3a, the plane
containing the methoxy group carbon and oxygen and the iodine
is perpendicular to the I-C bond. Molecular models of all the
optimized structures are shown in Figure 1S of the Supporting
Information; total energies and optimized coordinates are also
provided as Supporting Information.
Considering that the [1,3] sigmatropic shift of iodine between
the acetoxy oxygens necessarily involves the variation of the
I-Cac-O1 angle (see Chart 2), we calculated the potential
energy profile for this process by varying this angle in step of
5°, from the computed equilibrium value of ca. 77°, found in
the most stable conformers of 1a and 2a, up to 45°. For each
constrained value of the I-Cac-O1 angle, the structures were
optimized leaving all of the other geometrical parameters free
to vary. To better estimate the energy barriers, further calcula-
tions were performed to locate the transition states (TS) close
to the maximum energy point.
The total energy (electronic + nuclear repulsion energy)
differences of the in vacuo or solvated structures from that of
the corresponding most stable rotamer obtained with uncon-
strained minimization are reported in Table 3, together with the
values of the dihedral angles that are affected by the I-Cac-
O1 angle variation.
The energy profiles do not vary substantially including the
solvent in the energy calculation, making us confident that the
equilibrium geometry and the exchange path should be similar
in vacuo and in chloroform; the estimated energy barriers,
including the solvent contribution, are 48 and 59 kJ/mol for 1a
and 2a, respectively. Should the entropic contribution to the
free energy barriers be negligible in comparison to the total
energy term, then the calculated barriers must be quite close to
the free energy barriers. In fact, the ∆E values found for the
transition structures are close to the ∆Gq values roughly
estimated from the 17O NMR spectra (1a = 44 kJ/mol, 2a =
54 kJ/mol). The difference of the energy barriers for the
(11) Adamo, C.; Barone, V. J. Chem. Phys. 1999, 110, 6158-6179.
(12) (a) Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical Chemistry;
Schaefer, H. F., III, Ed.; Plenum: New York, 1976; Vol. 3, pp 1-28. (b)
Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284-298. (c) Check, C.
E.; Faust, T. O; Bailey, J. M.; Wright, B. J.; Gilbert, T. M; Sunderlin, L.
S. J. Phys. Chem. A 2001, 105, 8111-8116. (d) Basis sets were obtained
from the Extensible Computational Chemistry Environment Basis Set
Database, Version 02/25/04, as developed and distributed by the Molecular
Science Computing Facility, Environmental and Molecular Sciences
Laboratory, which is part of the Pacific Northwest Laboratory, P.O. Box
999, Richland, Washington 99352, U.S.A., and funded by the U.S.
Department of Energy.
(13) (a) Cossi, M.; Scalmani, G.; Rega, N.; Barone, V. J. Chem. Phys.
2002, 117, 43-54. (b) Cance`s, M. T.; Mennucci, B.; Tomasi, J. J. Chem.
Phys. 1997, 107, 3032-3041. (c) Cossi, M.; Barone, V.; Mennucci, B.;
Tomasi, J. Chem. Phys. Lett. 1998, 286, 253-260. (d) Mennucci, B.;
Tomasi, J. J. Chem. Phys. 1997, 106, 5151-5158.
4166 J. Org. Chem., Vol. 72, No. 11, 2007

Solution Structure of λ3 Iodanes
TABLE 3. Selected Dihedrals and PBE0/LANL2DZDP Total Energy Differences, from That of the Most Stable Rotamer, at Variable
I-Cac-O1 Angle. Energies were Calculated Either in vacuo or Using the PCM Model for CHCl3
1a
2a
∆E (kJ/mol)
Dihedral angles (deg)
∆E (kJ/mol)
Dihedral angles (deg)
I-Cac-O1
a
a,b
a
(deg)
in vacuo
solvated
0c
Ci-I-O1-Cac
-1.0
Co-Ci-I-O2
in vacuo
solvated
Ci-I-O1-Cac
Co-Ci-I-O2
76.2
77.0
70
65
60
55
50
45
55.8, TS
53.6, TS
0
53.5
0
0
5.0
0.0
0.0
0.0
0.0
3.3
11.8
25.7
34.7
17.5
5.8
5.7
12.2
26.6
35.1
19.7
8.2
-1.3
0
0.1
-79.2
-80.5
-80.4
-42.7
56.4
89.8
89.4
84.9
85.6
83.3
74.1
4.4
14.0
29.6
48.2
30.0
16.5
14.9
31.3
50.8
27.7
14.9
0.0
0.0
-0.1
-0.2
-87.2
-87.3
0.0
0.0
-0.5
-0.5
45.2
48.3
57.0
59.0
-42.4
2.7
a For a direct comparison among the dihedrals of the considered atomic configurations of 1a, the conformational enantiomers with the smallest Co-Ci-
I-O2 angle between the unconstrained acetoxy unit and the clockwise Co were considered. b O2 belongs to the unconstrained moiety, while Co is the closest
one in the clockwise direction. c As discussed in the Supporting Information when taking into account the solvent effect on the energy, the conformer with
Co-Ci-I-O ) 90° is predicted to be the more stable; accordingly, the values reported in this column are relative to that structure.
exchange of the oxygens in the acetoxy groups of the
two compounds is thus estimated to be ca. 10 kJ/mol experi-
mentally and 11 kJ/mol theoretically and explains the different
number of oxygen signals observed experimentally. It is
worth noting that, while in the unconstrained global minimum
energy structure of both 1a and 2a the two carboxylic groups
(i.e., the two acetoxy moieties in 1a, and the acetoxy moiety
and the iodoxolone in 2a) lay in the same plane, in the transition
state of both compounds the acetoxy group involved in the
exchange moves out from the plane, as can be seen in Table 3
considering the variation of the Ci-I-O1-Cac dihedrals.
For values of I-Cac-O1 larger than that of the TS, the
considered dihedral is nearly 0° in both compounds, while for
smaller values, it is ca. 80° in 1a and 87° in 2a. We also point
out that for I-Cac-O1 = 40° the conformation is stable and
the rotation around the I-O bond from this value to that of the
most stable conformer has a very low barrier (see Supporting
Information). It is thus apparent that a degenerate [1,3] sigma-
tropic shift of iodine between O1 and O2 is the actual pathway
leading to the equivalence observed by the 17O NMR for
compounds 1, and this observation rules out chelate or ion pair
structures.
45° leads, in vacuo, to an exchange barrier of 50.7 kJ/mol,
intermediate between the situations described above.
The sigmatropic shift is impossible for the exchange between
O3 and O4 for evident geometrical reasons; thus the breaking
of the I-O3 bond could not be compensated by the simultaneous
formation of the I-O4 bond, and in fact, the barrier for the
exchange is calculated to be ca. 173 kJ/mol.14 The height of
this barrier is consistent with the observation of two 17O NMR
signals for the oxygens of the iodoxolone ring in either
compounds 2 and 3.
A dynamic phenomenon with an energy barrier similar to
those of the sigmatropic shift had been observed by Ochiai,
who gave evidence9 of the stereochemical nonrigidity in λ3
iodanes through pseudorotation on iodine. However, while
pseudorotation leads to equivalence of a pair of ligands in a
chiral situation, the [1,3] sigmatropic shift leads to equivalence
of the two oxygens in the same ligand, and the two phenomena
are independent each other.
As far as the 13C NMR spectra of these derivatives are
concerned, it has already been observed and discussed15 in a
systematic way that, in λ3 and λ5 iodanes, the usual “heavy atom
effect” of iodine on the 13C chemical shift of the ipso carbon
Ci is compensated or overwhelmed. This point has been
empirically related to the oxidation state of iodine. We can add
that the range of 13C chemical shift of the ipso carbon is very
narrow, with the δ values being equal to 122.6 ( 1.1 ppm for
derivatives 1 and 118.6 ( 0.4 ppm in the case of 2 and 3.
The origin of the difference in the energy barrier between
compounds 1a and 2a seems to be related to the structural
constraint imposed by the iodoxolone ring, which in compounds
2 forces the I-O4 bond and, consequently, the I-O3 bond to
lay in the plane of benzene. To verify this point, we calculated
the energy barrier for the exchange in 1a constraining one of
the two I-O2 bonds to be coplanar with benzene (Co-Ci-I-O
) 0°) and studying the acetoxy oxygens exchange in the same
way as discussed above. The total energy difference between
the transition state and the minimum energy constrained
structure is (in vacuo) 57.7 kJ/mol, a value much higher than
that of 46.0 kJ/mol observed in the unconstrained geometry and
very close to that of 57.0 kJ/mol found for 2a at the same theory
level; this result confirms the structural constraint as the reason
for the difference in the exchange energy barrier.
Conclusions
Coupling the 17O NMR spectroscopy to quantum mechanics
calculations allowed us to gain a deeper understanding about
the structural and dynamical aspects of bis(acyloxy)iodoarenes
and benzoiodoxolones in chloroform, giving substantial evidence
that the T structure observed in the solid state is also adopted
(14) The barrier was estimated as the total energy difference between
the structure of 2a with the carboxylate plane parallel and perpendicular to
that of benzene; the latter has been obtained constraining the Ci-Co-Cac-
O3 dihedral angle to 90° while the first is the global minimum energy
structure.
(15) (a) Katritzky, A. R.; Gallos, J. K.; Durst, H. D. Magn. Reson. Chem.
1989, 27, 815-822. (b) Katritzky, A. R.; Duell, B. L.; Gallos, J. K.; Durst,
H. D. Magn. Reson. Chem. 1989, 27, 1007-1011. (c) Katritzky, A. R.;
Savage, G. P.; Gallos, J. K.; Durst, H. D. J. Chem. Soc., Perkin Trans. 2
1990, 1515-1518.
To further investigate the dependence of the barrier on the
Co-Ci-I-O2 angle, calculations were performed on 1a studying
the exchange imposing a constraint of 45°, a value intermediate
between that of 0 and ca. 90° (a value found in most of the
configurations involved in the unconstrained exchange; see
Table 3). As it could be expected, constraining this dihedral to
J. Org. Chem, Vol. 72, No. 11, 2007 4167

Mocci et al.
functional was chosen over the popular B3LYP19 functional after
comparing its capability of reproducing the relevant experimental
structural parameters of the studied compounds. The effective core
potential valence basis set LANL2DZ12 (i.e., D95V12a basis set for
the first row elements and the Los Alamos ECP plus DZ on
iodine12b) extended with polarization (d) and diffuse (p) functions12c,d
was employed for all atoms. The LANL2DZ basis set was chosen
in view of the previous literature on I(III) hypervalent compounds,20
and the addition of polarized and diffuse functions improves the
results, obtaining geometrical parameters closer to those found
experimentally (see Table S4 in Supporting Information). Numerical
integration was performed using a pruned grid having 99 radial
shells and 509 angular points per shell. For unconstrained geometry
(both minimum energy and transition structures), vibrational
analysis was carried out at the same level of theory to check the
character of the stationary point; the normal mode corresponding
to the imaginary frequency in the transition structure is similar in
compounds 1a and 2a and clearly involves the oxygens exchange.
In order to find the global minima, we optimized the geometry
of the studied compounds starting from several conformations,
constructed with the freely available program Molden,21 differing
each by 90° in the value of the dihedral angle around the I-O
bonds.
The three TSs (two for 1a, unconstrained or with the plane of
one of the acetoxy groups constrained to be coplanar with the I-Ci
bond, and one for 2a) were obtained starting from the atomic
configuration where the I-Cac-O1 angle is 55° and the plane of
the acetoxy group involved in the iodine shift is parallel to the
I-Ci bond.
The solvent effect has been studied by employing the polarizable
continuum model (PCM)13 in single-point energy calculations on
theinvacuooptimizedgeometries,usingthecurrentimplementation13a
in Gaussian 03 of PCM, performing a reaction field calculation
using the IEF-PCM model.13b-d All PCM calculations were carried
out at 298.15 K using an average tesserae area of 0.2 Å2.
Graphics of molecular models were generated using the freely
available VMD22 software.
in solution; further, the “free” carboxylic groups of derivatives
1 and 2 show a dynamic behavior, observable only in the 17O
NMR. This behavior is ascribed to a [1,3] sigmatropic shift of
the iodine atom between the two oxygen atoms of the carboxylic
groups, and the energy involved in this process varies signifi-
cantly between bis(acyloxy)iodoarenes and benzoiodoxolones.
Experimental Section
The new compounds 2b and 3c were prepared by the following
procedures.
1-Propionoxy-1H-1λ3-benzo[d][1,2]iodoxol-3-one (2b) was ob-
tained by modification of Merkushev’s method16 used to obtain
derivatives 1. To 2a (1.0 g, 3.2 mmol), obtained by Baker’s
method,17 were added a stoichiometric amount of dry propionic
acid (0.237 g, 3.2 mmol) and then 3 mL of dry chloroform,
obtaining a clear solution. This solution was then heated to 45 °C
under a residual pressure of 35 mmHg, until dryness. The white
solid obtained in this way was washed with cold, dry acetonitrile
and filtered under vacuum, to leave, in quantitative yield, pure 2b
as a white powder. As far as the melting point is concerned, we
observed a first transition point at 120-122 °C, characterized by a
sensible volume increase with partial melting, and a second
transition point at 220-223 °C, when melting is complete. This
1
last melting point coincides with that of 2-iodosobenzoic acid. H
NMR (300 MHz, CDCl3): δ 8.23 (dd, J ) 7.6, 1.3 Hz, 1H), 8.00
(d, J ) 8.4 Hz, 1H), 7.95 (td, J ) 8.4, 1.5 Hz, 1H), 7.72 (td, J )
7.6, 1.3 Hz, 1H), 2.56 (q, J ) 7.6 Hz, 2H), 1.25 (t, J ) 7.6 Hz,
3H); 13C NMR (75.5 MHz, CDCl3) δ 179.2, 168.0, 135.9, 132.8,
131.0, 129.1, 128.8, 118.3, 27.0, 9.5. Anal. Calcd for C10H9IO4:
C, 37.50; H, 2.83. Found: C, 37.25; H, 2.91.
1-iso-Propoxy-1H-1λ3-benzo[d][1,2]iodoxol-3-one (3c) was pre-
pared according to the method described17 for obtaining 3a,b. To
compound 2a (1.0 g. 3.2 mmol), obtained by Baker’s17 method,
was added 5 mL of isopropyl alcohol; the obtained dispersion was
boiled until complete solubilization and was then left for 12 h in
the refrigerator. Colorless needles of 3c separated and were filtered
and washed with cold, dry MeCN to obtain 0.97 g (quantitative
yield) of 3c as a white solid, mp 192-195 °C. 1H NMR (300 MHz,
CDCl3): δ 8.27 (dd, J ) 7.7, 1.1 Hz, 1H), 7.88 (m, 2H), 7.69 (td,
J ) 7.3, 1.1 Hz, 1H), 4.33 (h, J ) 6.2 Hz, 1H), 1.35 (d, J ) 6.2
Hz, 6H); 13C NMR (75.5 MHz, CDCl3) δ 167.9, 134.8, 132.7,
130.8, 130.7, 126.0, 119.1, 75.5, 25.2. Anal. Calcd for C10H11IO3:
C, 39.22; H, 3.62. Found: C, 39.05; H, 3.70.
Acknowledgment. We thank Prof. Giuseppe Saba, Cagliari
University, for helpful discussions and reading the manuscript
in advance. Financial support by Cagliari University, ex 60%
fund, is acknowledged.
Supporting Information Available: Extended and detailed
computational results (comparison of the B3LYP and PBE0
functionals performance, rotational barriers, Cartesian coordinates,
and energies of constrained and unconstrained optimized geometry).
Computational Methods. Structure optimizations of compounds
1a, 2a, and 3a were performed at the DFT level employing the
recently developed parameter free PBE0 functional,11 as imple-
mented in the commercially available Gaussian 03 suite of
programs.18 As discussed in the Supporting Information, the PBE0
1
General synthetic procedures for the known compounds, their H
1
and 13C NMR data, copies of the H and 13C NMR spectra of the
new compounds 2b and 3c. This material is available free of charge
via the Internet at http://pubs.acs.org.
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