Oxidative breakdown of iodoalkanes to catalytically active iodine species: A case study in the α-tosyloxylation of ketones

Article abstract of DOI:10.1002/cctc.201300774

Catalysis of the oxidative processes by iodoarenes has become a promising direction in synthesis. The mechanism, involving the well-known isolable hypervalent iodine species, is generally limited to aromatic iodides, since the corresponding aliphatic spec

Full text of DOI:10.1002/cctc.201300774

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DOI: 10.1002/cctc.201300774
Oxidative Breakdown of Iodoalkanes to Catalytically
Active Iodine Species: A Case Study in the
a-Tosyloxylation of Ketones
Wusheng Guo,[a] Oriol Vallcorba,[b] Adelina Vallribera,[a] Alexandr Shafir,*[a] Roser Pleixats,*[a]
and Jordi Rius[b]
Catalysis of the oxidative processes by iodoarenes has become
a promising direction in synthesis. The mechanism, involving
the well-known isolable hypervalent iodine species, is generally
limited to aromatic iodides, since the corresponding aliphatic
species are normally highly unstable. Nevertheless, in this work
catalytic amount of several primary, secondary or tertiary io-
doalkanes were found to promotes the a-tosyloxylation of
a range of ketones with RSO3H, including aliphatic sulfonates.
Scheme 1. Hypervalent iodine in the a-tosyloxylation of ketones.
Ox=oxidant.
The process, was found to proceed through the oxidative
breakdown of the iodoalkane to an inorganic catalytic species
ꢀ
(likely IOꢀ or IO2 ), thus falling within the previously described
catalysis using molecular iodine or inorganic iodides. The cata-
lyst eventually becomes deactivated through the precipitation
of an iodine overoxidation product, the structure of which was
solved ab initio from the powder diffraction data as a hitherto
unreported phase of the iodic acid (HIO3).
that, in principle, ArI can be recovered and reused, variants of
such processes have been developed (including asymmetric
versions) following the early reports[4] by the groups of Ochiai
and Kita, whereby ArI was used as a catalyst in the presence of
a cheaper terminal oxidant.[5] In these cases, the catalytic cycle
involves the in situ regeneration of the organoiodine(III) re-
agent (Scheme 1b). The burgeoning field of hypervalent iodine
catalysis was recently the subject of two Minireviews.[5d,e]
In light of the close relationship between hypervalent iodine
catalysis and classical stoichiometric applications, it is not sur-
prising that the field has been limited to iodoarenes. After all,
with the exception of some very bulky or heavily fluorinated
derivatives, hypervalent iodoalkanes are notoriously unsta-
ble,[1b] which makes their use in stoichiometric applications un-
feasible. Indeed, upon generation, the ꢀIX2 moiety in such spe-
cies has a leaving group ability that is orders of magnitude
higher than that of an iodide.[6] During the course of our own
investigations into immobilized organoarenes,[7] we observed
that the products of the oxidative breakdown of certain iodoal-
kanes were still catalytically active. We wondered whether
such activity paralleled the catalysis by simple molecular
iodine and inorganic iodides that was reported recently.[8] We
also became interested in the catalyst deactivation pathways
frequently suffered by such systems.
Iodoarenes are known to undergo oxidation at iodine to give
the corresponding hypervalent iodine reagents. In particular,
partial oxidation of ArI gives rise to highly reactive T-shaped
ArIXX’ species, exemplified by PhI(OAc)2 and PhI(OH)(OTs)
(Koser reagent, Ts=para-toluenesulfonyl).[1] In synthesis, such
a reagent may act as an oxidant capable of delivering one of
the Xꢀ fragments to the product, such as in the oxidative
a-tosyloxylation of aliphatic ketones, which usually proceeds
via an iodonium enolate intermediate (Scheme 1a for a mecha-
nism proceeding through a C-enolate; alternative paths involv-
ing iodonium O-enolates have also been invoked).[2] This kind
of oxidative CꢀH functionalization provides rapid access to val-
uable building blocks for the construction of thiazoles, ox-
azoles, selenazoles, and imidazoles, among others.[3] Given
[a] W. Guo, A. Vallribera, A. Shafir, R. Pleixats
Wusheng Guo, Prof. Adelina Vallribera,
Dr. Alexandr Shafir, Prof. Roser Pleixats
Department of Chemistry
To confirm the catalytic activity of the species obtained
upon the in situ oxidative decomposition of iodoalkanes, the
performance of 2-iodobutane was compared to that of iodo-
benzene in the tosyloxylation of propiophenone by following
a process reported by Togo et al. that involved the employ-
ment of meta-chloroperbenzoic acid (mCPBA) as the terminal
oxidant.[5b] Indeed, the use of a catalytic amount of PhI
(10 mol%) gave, as expected, a 75% yield of the a-tosyloxylat-
ed product after 10 h at 658C. Under the same conditions,
a 10 mol% loading of sBuI also proved active, and it afforded
a respectable 65% yield of the product (Scheme 2), albeit with
Universitat Autꢀnoma de Barcelona
08193-Cerdanyola del Vallꢁs, Barcelona (Spain)
Fax: (+34)93-5812477
E-mail: alexandr.shafir@uab.cat
roser.pleixats@uab.cat
[b] O. Vallcorba, J. Rius
Dr. Oriol Vallcorba, Prof. Jordi Rius
Institut de Ciꢁncia de Materials de Barcelona, CSIC
Campus de la UAB, 08193-Bellaterra, Barcelona (Spain)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300774.
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owing to its poor miscibility with CH3CN. All three io-
dobutane isomers as well as 2-iodopropane proved
active (Table 2, entries 3–6); the use of simple iodo-
methane (Table 2, entry 7) also gave a product that
was isolated in 75% yield. The formation of the cata-
lyst was unaffected by the presence of an aromatic
group (Table 2, entry 8). The similarity of the catalytic
behavior of such a wide range of iodoalkanes is also
strong evidence that, unlike aryl iodides, the catalytic cycle
does not involve a hypervalent organoiodane, but rather pro-
ceeds through a common inorganic iodine species.
Scheme 2. Tosyloxylation of propiophenone by PhI and sBuI.
no improvement at longer reaction times. As discussed below,
the loss of catalytic activity after approximately 10 h was ac-
companied by the precipitation of a white solid that was
1
found to be H NMR spectroscopy silent.
Additional control experiments were consistent with this hy-
pothesis. Thus, an enantioenriched menthol-derived iodocyclo-
hexane (Table 2, entry 9), although catalytically active, afforded
a racemic product (see the Supporting Information).
To gain further insight into the catalyst stability, the perfor-
mance of sBuI was examined under a range of reaction condi-
tions (Table 1). Thus, an mCPBA loading of 1.5 equivalent was
Table 1. Optimization of the reaction conditions.[a]
Table 2. Screening of iodoalkanes as potential tosyloxylation
precatalysts.[a]
Entry[a]
RI
Yield[b] [%]
Entry
Solvent
mCPBA
[equiv.]
2-BuI
[mol%]
T
[8C]
t[b]
[h]
Yield[c]
[%]
1
2
3
4
5
6
7
n-iodododecane
n-iodohexane
n-iodobutane
sec-iodobutane
tert-iodobutane
2-iodopropane
MeI
69
78
78
75
78
79
75
1
2
3
4
5
6
7
8
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
toluene
EtOAc
CHCl3
1.0
1.1
1.3
1.5
2.0
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
10
10
10
10
10
10
10
10
10
10
10
5
65
65
65
65
65
50
40
75
50
50
50
50
50
10
10
10
10
10
22
22
10
22
22
22
22
22
50
58
65
74
69
77
73
45
34
21
59
65
69
8
79
9
10
11
12
13
9
69
CH3CN
CH3CN
10
11
12
13
14
15
C4F9I
CF3CH2I
I(CF2)6I
I2
NaI
KI
<5
<5
<5
69/61[c]
77
20
[a] [Substrate]=0.2m. [b] Reaction was stopped when no further evolu-
tion was detected by GC. [c] Corrected GC yield (PhCl used as an internal
standard).
73
[a] [Substrate]=0.2m. [b] Yield of isolated product. [c] 5 and 2.5 mol% I2.
found to be optimal (Table 1, entries 1–5),[9] and the best yields
were achieved at 508C, albeit at the cost of a somewhat
slower reaction (Table 1, compare entries 4 and 6–8). However,
no further improvement could be achieved either by changing
the solvent (Table 1, entries 6 and 9–11) or by varying the cata-
lyst loading (Table 1, entries 12 and 13). In all cases, the forma-
tion of a white solid was observed towards the end of the re-
action. These results indicated that although the oxidative
breakdown of sBuI gave a catalytically active iodine species,
further evolution of such species into a white insoluble materi-
al resulted in catalyst deactivation.
Interestingly, fluorinated iodoalkanes (Table 2, entries 10–12),
which presumably could give a reasonably stable hypervalent
species and, thus, operate through an organoiodane-based
catalytic cycle (such as that depicted in Scheme 1b), proved in-
active under the catalytic conditions tested. Additional experi-
ments established that a reaction between C4F9I and mCPBA
did not lead to oxidative breakdown.[10a] As suggested by a ref-
eree, a stable Koser-type iodane C4F9I(OH)OTs was prepared fol-
lowing a method developed by Zhdankin et al.;[10b] a reaction
between propiophenone and a stoichiometric amount of this
reagent, however, proved sluggish, and only a trace amount of
the corresponding tosyloxylated product was delivered after
24 h at 508C. It would appear, therefore, that this resistance to
oxidative breakdown of the fluorinated iodoalkanes, coupled
The formation of an active tosyloxylation catalyst was found
to be quite general for a range of iodoalkanes tested (Table 2).
Long-, medium-, and short-chain primary iodoalkanes (Table 2,
entries 1–3) all performed quite well, but a somewhat dimin-
ished yield was obtained for C12H25I (Table 2, entry 1) likely
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to the peculiar reactivity of the corresponding hydroxytosylate
reagents (and, perhaps, their low solubility in CH3CN) would all
contribute to the lack of catalytic activity of such reagents
under our conditions.
tries 1–5). However, with the exception of KI, only a 30–36%
yield could be achieved for 3-pentanone (Table 3, entry 6). In-
terestingly, the catalytic activity of I2 in this reaction was previ-
ously found to proceed through the in situ decomposition of
TsOH to p-iodotoluene, which would act as the true catalyst
[Eq. (1)].[8a] It was argued, therefore, that an aromatic sulfonic
acid was required if I2 were to be used as a precatalyst. A relat-
ed proposal by a different group suggested that a catalytic io-
dobenzene was formed through the iodination of the benzene
solvent employed:
In contrast, molecular iodine and inorganic iodides, as previ-
ously reported,[8] also proved catalytically active (Table 2, en-
tries 13–15). In addition, the systems obtained from nBuI, sBuI,
and tBuI showed similar kinetic profiles (Figure 1), although
with some differences likely resulting from different rates of io-
doalkane breakdown. Interestingly, the reaction rates were
found to be only slightly lower than those obtained with
a more-studied PhI catalyst (Figure 1).
p-MeC6H4SðOÞ2OH þ I2 ! p-MeC6H4I
ð1Þ
However, in our hands, even the simplest aliphatic sulfonic
acid, MeSO2OH, was coupled in good yields, by using either io-
dobutane or molecular iodine as the precatalyst (10 mol% I;
Table 3, entry 7); no aromatic solvent was used. The result sug-
gests that a species other than an aromatic iodide may act as
the catalyst in this process. In fact, even 10-camphorsulfonic
acid gave a modest yield (22%) of the product; the results
were somewhat improved by using sBuI (40%; Table 3,
entry 8). Control tests in the absence of any iodine source
(Table S1, Supporting Information)[11] failed to produce the de-
sired product.
Figure 1. Kinetic profiles of nBuI, sBuI, and tBuI versus that of PhI. The con-
version of propiophenone according to GC is shown and was corrected by
using PhCl as an internal standard with conditions as in Table 2.
During the course of the model a-tosyloxylation reactions,
the GC peak for the iodoalkane disappeared immediately and,
as mentioned above, a white solid appeared gradually.[9b]
Given that the nature of the white solid might shed some light
on the mode of catalyst deactivation, the solid obtained in
entry 4 of Table 1 (denoted M1) was separated and examined.
The material was found to be insoluble in organic solvents and
In addition to its mechanistic implications, the oxidative
a-tosyloxylation of ketones with the use of iodoalkanes as cat-
alyst precursors might offer, under certain conditions, an at-
tractive synthetic procedure. Thus, before delving into the
nature of the catalytic species, we briefly tested the perfor-
mance, as catalyst precursors, of the iodobutanes along with
KI and I2, in the a-tosyloxylation of additional substrates. As
seen in Table 3, all five catalysts performed quite well with pro-
piophenone and the para-substituted propiophenones, includ-
ing those with a halide or a nitro substituent (Table 3, en-
1
soluble in water, and its H NMR spectrum in D2O was void of
signals. To our surprise, the powder X-ray diffraction (PXRD)
pattern of M1 (Figure S1) produced a pattern identical to that
of a substance identified in an earlier publication[12a] as
a mixed iodine oxide I4O9, formulated as I(IO3)3, and described
as a yellow, air-sensitive solid obtained through the ozonation
of I2 or MeI.[12,13] Our surprise
stemmed from the fact that, de-
spite the coincidence of the dif-
Table 3. a-Tosyloxylation of ketones with iodine sources as catalysts[a]
fraction patterns, product M1
was white and showed no sign
of decomposition in air. Inciden-
tally, a material with the same
diffraction pattern, denoted as
M2, could be prepared in the
absence of the two substrates
(ketone and pTsOH) simply by
oxidizing RI (or I2) with mCPBA
(Table S2).[12b] The structure of
M1 was solved from powder
data[14] by using the cluster-
based direct methods imple-
mented in XLENS_pd6[14a] and re-
fined with RIBOLS[14b] by apply-
ing an unrestrained Rietveld re-
finement. The crystal structure of
Product
Yield[b] [%]
Entry
R
R’
R’’
KI
I2
1
2
3
4
5
6
7
8
p-CH3Ph
Ph
p-BrPh
p-ClPh
p-NO2Ph
Et
H
H
H
H
H
Me
H
64
70
82
73
66
36
64
29
65
76
86
72
58
32
76
40
68
70
81
75
65
31
80
33
59
80
81
70
63
66
72
27
55
64
73
66
56
30
61
22
p-CH3Ph
Ph
Ph
Me
10-camphor
H
[a] [Substrate]=0.2m. [b] Yield of isolated product.
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this solid,[14c] combined with thermogravimetric analysis, X-ray
photoelectron spectroscopy, and elemental analysis data,[14d,e]
showed it to be a hitherto unreported crystal form of iodic
acid, HIO3.[15] The iodine atoms form hexagonal layers parallel
to the crystalline bc planes, and each iodine is surrounded by
and Iꢀ species. Further oxidation might then lead to the inac-
tive I+5 products (i.e., M1 or KIO3); this explains the drop in
ꢀ
the catalytic activity over time. The I+5 (specifically IO3 ) spe-
cies might also form as a result of the known propensity of hy-
poiodite IOꢀ to disproportionate into iodide and iodate. The
same active species may be obtained from the oxidation of
iodide or molecular iodine (the initial brown color observed
with RI also indicates the formation of at least some I2).
In summary, we have shown that, in addition to ArI, simple
iodoalkanes can also serve as precursors to species that are
active in the catalytic oxidative a-tosyloxylation of aliphatic ke-
tones with sulfonic acids. However, unlike ArI, the catalytic ac-
tivity of which is based on the intermediacy of PhIX2 species,
the iodoalkanes appear to quickly degrade into catalytically
active inorganic iodites (and/or hypoiodites). Overoxidation
and/or disproportionation of such species leads to catalyst de-
activation through the formation of a white solid that was
identified as a previously unreported polymorph of HIO3 (ob-
served as a white precipitate, i.e., M1), or, in some cases, as
KIO3.
oxygen atoms in
a
distorted octahedral coordination
(Figure 2); each HIO3 molecule is connected to neighbor mole-
cules through three weak I···O interactions and a (probable)
O2···O3’ hydrogen bond [2.62(3) ꢁ]. In spite of the different
crystal packing, bond lengths are similar to those reported in
other HIO3 structures.[15b–d]
Acknowledgements
Figure 2. Crystal structure of M1. Left: Molecular scheme of the IꢀO weak in-
teractions (a, 2.5 to 2.8 ꢁ); right: packing of the distorted iodine octahe-
dra. Iodine centers form hexagonal layers normal to a.
Financial support from the Ministerio de Ciencia e Innovaciꢂn
(MICINN) of Spain [Projects CTQ2009-07881 and CTQ2011-22649
and Consolider Ingenio 2010 (CSD2007-00006)]. A Ramꢂn y Cajal
scholarship to A.S. [Ministerio de Educaciꢂn y Ciencia (MEC),
Spain, RYC-2006-001425], Departament d’Universitats, Recerca
i Societat de la Informaciꢂ (DURSI)-Generalitat de Catalunya
(SGR2009-1441), and China Scholarship Council (CSC scholarship
to W.G.) are gratefully acknowledged.
Consistent with the notion that the precipitation of the
white solid is symptomatic of catalyst deactivation, neither iso-
lated M1 nor the commercially available sample of HIO3 or its
anhydride, I2O5, showed any activity in the a-tosyloxylation of
propiophenone (Table S3). It was also found that although an
analogous overoxidation process did take place in the case of
KI, the diffraction pattern obtained for the corresponding
white solid corresponded to that of KIO3 rather than the acid.
On the basis of these results and previous research[16] on re-
lated processes, we propose that an inorganic iodine (+1)
and/or (+3) intermediate might be the true catalyst for cases
in which alkyl iodides are used (Scheme 3). As previously pro-
Keywords: hypervalent compounds · iodine · iodoalkanes ·
powder diffraction · tosyloxylation
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8.45229(16), b=6.96194(13), c=4.49821(8) ꢁ, V=264.694(8) ꢁ3, Z=4,
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Received: September 14, 2013
Published online on December 13, 2013
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