Asymmetrie hydroxylative phenol dearomatization through in situ generation of iodanes from chiral iodoarenes and m-CPBA

Full text of DOI:10.1002/anie.200901039

Angewandte
Chemie
DOI: 10.1002/anie.200901039
Asymmetric Synthesis
Asymmetric Hydroxylative Phenol Dearomatization through In Situ
Generation of Iodanes from Chiral Iodoarenes and m-CPBA**
Stꢀphane Quideau,* Gildas Lyvinec, Mꢀlanie Marguerit, Katell Bathany, Aurꢀlie Ozanne-
Beaudenon, Thierry Buffeteau, Dominique Cavagnat, and Alain Chꢀnedꢀ
The quest for chiral hypervalent iodine (iodane) reagents
capable of promoting asymmetric induction in oxygenating
reactions, such as the oxidation of sulfides into sulfoxides, the
a-oxygenation of ketones, the dioxygenation of olefins, and
the oxygenative dearomatization of phenols into cyclohexa-
dienone derivatives, continues to challenge the organic
chemistry community.[1] The best results reported so far are:
a) the oxidation of methyl tert-butylsulfide (56% ee) using a
(+)-menthylated variant of the l3-iodane (i.e., iodine(III))
Koser reagent (A, Figure 1),[1a,c] and b) that of methyl p-
tolylsulfide (29% ee) using a chiral l5-iodane (i.e., iodine(V))
developed by Wirth and co-workers (C, Figure 1);[1f] and
e) an intramolecular dearomatizing spirolactonization of a
series of naphthol derivatives (78–86% ee) using a chiral
bis(l3-iodane) spirobiindane-based reagent recently devel-
oped by Kita and co-workers (D, Figure 1).[1h,15]
Our own efforts toward the development of asymmetric
phenol dearomatization reactions[2] and the efficacy of the l5-
iodane 2-iodoxybenzoic acid (IBX) or its stabilized non-
explosive version (SIBX)[3] in mediating hydroxylative phenol
dearomatization (HPD) in a strictly ortho-selective manner
led us to envisage the use of a chiral oxygenating iodane in
this reaction. The HPD reaction is a powerful means for
preparing, in one step, chiral 6-alkyl-6-hydroxycyclohexa-2,4-
dienones from simple (achiral) 2-alkylphenols or related
arenol variants.[4] These systems, trivially referred to as ortho-
quinols, either constitute the structural core of some natural
products or can serve as advanced intermediates in the
synthesis of several others.[4,5] Thus, having access to a chiral
iodane reagent capable of enantioselectively installing a
hydroxy group at an alkylated ortho position of a phenol
would enable the reagent controlled preparation of ortho-
quinols in a nonracemic form.
Our first approach toward this objective was to generate
IBX analogues having either a center or an axis of chirality as
close as possible to the iodine center. The iodoarenes 1a–e
and 2a–c were thus considered as chiral precursors of either
six-membered-ring homologues of IBX or biaryl variants
(Figure 2). Extensive efforts were spent on converting these
iodoarenes (using either racemates or enantiomerically pure
forms) into their corresponding l5-iodane iodoxy derivatives
using various reagent systems known to achieve this type of
Figure 1. Select examples of chiral l3- and l5-iodane reagents. Ts=p-
toluenesulfonyl.
N-(2-iodylphenyl)acylamide (NIPA) reagent developed by
Zhdankin and co-workers (B, Figure 1);[1d] c) the a-oxytosy-
lation of propiophenone (40% ee), and d) the dioxytosylation
of styrene (65% ee) using a chiral Koser-type reagent
[*] Prof. S. Quideau, Dr. G. Lyvinec, M. Marguerit, K. Bathany,
Dr. A. Ozanne-Beaudenon, Dr. T. Buffeteau, Dr. D. Cavagnat
Universitꢀ de Bordeaux
Institut des Sciences Molꢀculaires (CNRS UMR 5255)
Institut Europꢀen de Chimie et Biologie
2 rue Robert Escarpit, 33607 Pessac Cedex (France)
Fax: (+33)5-4000-2215
E-mail: s.quideau@iecb.u-bordeaux.fr
Dr. A. Chꢀnedꢀ
Simafex, 17230 Marans (France)
[**] We thank Simafex, the Association Nationale de la Recherche
Technique (CIFRE Grants No. 457/2005 & 301/2002), and the
Institut Universitaire de France for their financial support.
m-CPBA=meta-chloroperoxybenzoic acid.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200901039.
Figure 2. Starting chiral iodoarenes and isolated iodanes.
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oxidation (e.g., KBrO3/H2SO4, NaOCl/H2O-TBABr, NaIO4/
H2O, MeCO3H/H2O, H2O2/H2O-Ac2O, Oxone/H2O, TBA-
OX/MeSO3H-CH2Cl2, DMDO/acetone or CH2Cl2; TBA =
amount of the epoxidized ortho-quinol rac-7a (8% yield),
and the observation of another minor product (vide infra).
Although the outcome of this reaction confirmed the
catalytic action of 1 f, which is oxidized in situ by m-CPBA
into some oxygenating iodane species (vide infra), m-CPBA
was also capable of trapping the initially formed ortho-quinol
intermediate through epoxidation of its D-4,5 bond. The use
of equimolar amounts of 1 f and m-CPBA did not improve the
reaction outcome, but a twofold excess of 1 f increased the
yield of 6a to the detriment of the epoxidation event (Table 1,
entries 2 and 3). On the contrary, only the epoxide 7a was
isolated in 47% yield when a large excess of m-CPBA
(10 equiv) and a catalytic amount of 1 f (0.1 equiv) were
added to a less concentrated solution of 5a in acetone, a
better solvent than CH2Cl2 for dissolving m-CPBA; the
higher dilution was applied with the aim of limiting the
dimerization process (Table 1, entry 4). The exclusive regio-
and diastereoselectivity observed for 7a (and other related
epoxides described below) in favor of a cis epoxidation of the
D-4,5 bond, relative to the orientation of the OH group at C6
of the orthoquinol intermediate, is a directivity gift offered by
this allylic hydroxy group.[9]
To additionally evaluate the scope and limitations of the
in situ generation of HPD-mediating iodanes, two additional
phenols (i.e., 5b and 5c) and 2-methylnaphthol (5d) were
utilized as starting materials (Table 2). Under catalytic
conditions, 2,4,6-trimethylphenol (5b) led to a complex
mixture from which the racemic dimer 6b[4b] and epoxide
7b could each be isolated in low yields. Traces of the ortho-
quinol m-chlorobenzoate 7b’ were also detected (Table 2,
entry 1). The latter compound, resulting from a surprising
(only observed when using 5b) participation of m-chloroben-
zoic acid in the dearomatization process, could be isolated in
11% yield from a reaction in which a twofold excess of 1 f was
used (Table 2, entry 2). Again, the use of such an excess of the
iodoarene prevented the epoxidation event, but the expected
formation of 6b in high yield did not occur.
tetra-n-butylammonium,
TBA-OX = tetra-n-butylammo-
nium oxone, DMDO = dimethyldioxirane),[6] but in most
cases satisfactory conversions were not observed.
The only bits of encouragement were the oxone-mediated
oxidation of 1a into the l5-iodane 4a in 62% yield and the
DMDO-mediated oxidation of 1d into the l3-iodane 3d in
yields ranging from approximately 50 to 90%, together with
the isolation of a white solid whose structure could be
attributed to the l5-iodane 4d (little structural evidence was
obtained to confirm this attribution). To test the oxygenating
and asymmetry-induction capabilities of these materials, (À)-
4a was engaged in a HPD reaction using 2,6-dimethylphenol
(5a) in THF at À788C. The expected ortho-quinol dimer 6a
(vide infra)[4b,d] was obtained in quantitative yield, but in only
20% ee. Under the same reaction conditions, the l3-iodane 3d
was totally inefficient, but each of its optical antipodes could
oxidize methyl p-tolylsulfide into the corresponding sulfox-
ides in good yields (ca. 50 to 70%), albeit in poor enantio-
meric excesses (7–8% ee).[7] Finally, each optical antipode of
the presumed l5-iodane 4d successfully mediated the HPD of
5a into 6a in yields of 70–75% with opposite but less than
10% ee (see the Supporting Information).
The overall difficulty in oxidizing our starting iodoarenes
into iodanes led us to look for another approach. We turned
our attention toward methodologies based on the in situ
generation of iodanes from iodoarenes in the presence of a
co-oxidant, a process having the possibility of proceeding with
only catalytic amounts of iodoarenes.[8] We therefore verified
the potential of this process in an HPD reaction, again using
2,6-dimethylphenol (5a) as a test substrate in various
combinations with 2-iodophenyl acetic acid (1 f) and m-
CPBA, which was recently introduced by Kita and co-
workers[8e] and Ochiai and co-workers[8d] as a terminal co-
oxidant of choice in related iodoarene-catalyzed reactions. By
using 0.1 equivalent of 1 f and 1.0 equivalent of m-CPBA in
CH2Cl2 at room temperature for 2 hours (Table 1, entry 1), 5a
was partially converted into the expected racemic dimer 6a,
which was isolated in a 55% yield. Some starting 5a was also
recovered (16% yield), together with the isolation of a small
Under catalytic conditions, carvacrol (5c) was converted
into biscarvacrol (6c)[2a,4b] and epoxide 7c, together with
significant amounts of the para-quinone 7c’ (Table 2, entries 3
and 4). We presume that 7c’ resulted from a 1,2-hydride shift
(also known as an NIH shift)[10] of the epoxide 7c in the acidic
medium used (see arrows in Table 2). In fact, the same
transformation also occurred with the disubstituted epoxide
7a, but only traces (up to less than 10% as estimated by NMR
analysis) of the corresponding quinone were observed (vide
supra). This epoxide chemistry was again of no concern when
using a twofold excess of 1 f, which led to the isolation of
exclusively 6c in 41% yield (Table 2, entry 5).
Table 1: Preliminary evaluation of reaction conditions for in situ
generation of HPD-mediating iodanes from iodoarenes.[a]
The reaction outcome was much simpler when using 2-
methylnaphthol (5d), the hydroxylative dearomatization of
which led to a nondimerizing ortho-quinol rac-6d and/or to its
epoxide derivative rac-7d (Table 2, entries 6–9). Most
remarkably, the ortho-quinol 6d was obtained in 82% yield
when using a twofold excess of the iodoarene 1 f with an
equimolar amount of m-CPBA (Table 2, entry 8), whereas
only the epoxide 7d was generated in high yield when using a
larger excess of m-CPBA (here limited to 2.5 equiv) and a
catalytic amount of 1 f (Table 2, entry 9).
Entry
1 f [equiv]
m-CPBA
[equiv]
Recovered
5a [%]
6a [%]
7a [%][b]
1
2
0.1
1
2
1
1
1
10
16
18
13
n.d.
55
58
64
n.d.
8
3
n.d.
47
3
4[c]
0.1
[a] The reactions were conducted in CH2Cl2 for 2 h, [5a]=0.33m, unless
otherwise noted. [b] Relative stereochemistry determined by NOESY
experiments. [c] [5a]=0.05m in acetone. n.d.=not detected.
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Table 2: Iodoarene-mediated oxygenating dearomatization of select arenols in
the presence of m-CPBA.[a]
chemistry of their stereoisomers by using vibrational
circular dichroism (VCD).[11] The VCD spectra of each
atropisomer of 2b and of each enantiomer of 6d were
recorded, using CDCl3 as the solvent, and then com-
pared with the predicted VCD spectra of (S)-2b and
(R)-6d; the predicted VCD spectra were calculated by
using density functional theory (DFT) calculations
using the B3LYP functional and 6-311 + G* basis set,
except for the iodine atom for which the LANL2DZ
basis set was used (see the Supporting Information).
These comparisons unambiguously established that the
axial chirality of (À)-2b is S and the configuration at C6
of (À)-6d is R. Thus, the sense of asymmetric induction
is that (S)-(À)-2b gives rise to (R)-(À)-6d as the major
product, and (R)-(+)-2b leads to (S)-(+)-6d (Table 3).
These results raise several questions about the
mechanistic aspects of the reaction. Why was it so
difficult to independently oxidize carboxylated iodoar-
enes into iodanes, whereas this oxidation easily occurs
in situ in the presence of the phenolic substrate? What
kind of iodane (i.e., l3 or l5) is actually generated in
situ? What is the modus operandi of the (enantioselec-
tive) oxygen atom transfer at the ortho-position of the
phenolic substrate? At this stage, we can only offer
working hypotheses for future investigations
(Scheme 1). The presence of the phenolic substrate in
situ must be a determining factor of the advancement of
the oxidation of the iodoarene I. Thus, an initially
formed iodosyl species, shown here as its cyclic tauto-
Entry Arenol
1 f/m-
Product(s) (% yield)[b,c]
CPBA
1
2
5b
5b
0.1:1.0 6b (16)
2.0:1.0 6b (27)
7b (7)
n.d.
7b’ (trace)[d]
7b’ (11)
3
4
5
5c
5c
5c
0.1:1.0 6c (19)
0.1:1.5 6c (32)
2.0:1.0 6c (41)
7c (6)
7c (10)
n.d.
7c’ (23)
7c’ (37)
n.d.
6
7
8
9
5d
5d
5d
5d
0.1:1.0 6d (13)
1.0:1.0 6d (67)
2.0:1.0 6d (82)
0.1:2.5 n.d.
7d (37)
n.d.
n.d.
7d (91)
[a] The reactions were conducted in CH2Cl2 at room temperature for 2–3 h,
[5]=0.33m. [b] 15–35% of 5 were recovered, except for entries 4, 8, and 9.
[c] Relative stereochemistry of epoxides determined by NOESY experiments.
[d] In 7b’, R=m-chlorobenzoyl. n.d.=not detected.
The naphthol 5d was thus selected as a most convenient
susbtrate to evaluate the capacity of the chiral iodoarenes
1a–e and 2a–c (Figure 2) to induce asymmetry during the
reaction. The most significant results are given in Table 3.
Iodoarenes having a chiral benzylic center, such as 1d and 1e,
afforded maximum ee values of 21–23% for the ortho-quinol
6d and 14-17% for its epoxide 7d (Table 3, entries 1–4).
Iodoarenes featuring axial chirality were much better induc-
ers of asymmetry (Table 3, entries 5–10).
The best performances were achieved with the binaphthyl
iodoarene 2b. For example, using only a catalytic amount of
(À)-2b and a 2.5-fold excess of m-CPBA, the expected
epoxide 7d was obtained in high yield and 29% ee (Table 3,
entry 5). Most significantly, the use of equimolar amounts of
either (À)-2b or (+)-2b and m-CPBA enabled the exclusive
formation of the ortho-quinol 6d in good yields and with
ee values of 45–47%, with the major isomer being (À)-6d or
(+)-6d, respectively (Table 3, entries 6 and 7). As previously
observed (Table 2, entries 7 and 8), the use of a twofold excess
of the iodoarene (+)-2b led to a better yield of 6d, in this case
up to 83%, and resulted in a slight increase of the ee value up
to 50% in favor of (+)-6d (Table 3, entry 8).
Table 3: Enantioselective iodoarene-mediated hydroxylative dearomati-
zation of 2-methylnaphthol (5d) using m-CPBA.[a]
Entry Ar*I
Ar*I
[equiv] [equiv]
m-CPBA 6d [%] 7d [%] ee [%][b]
1
2
3
4
5
6
7
8
9
10
(+)-1d
(+)-1d
(À)-1e
(À)-1e
0.1
1.0
0.1
1.0
2.5
1.0
2.5
1.0
2.5
1.0
1.0
1.0
2.5
1.0
–
70
–
72
–
67
71
83
–
89
–
91
–
90
–
–
–
90
–
14 (6S)[c]
21 (6S)
17 (6R)
23 (6R)
29 (6R)
45 (6R)
47 (6S)
50 (6S)
23 (6R)
37 (6R)
(S)-(À)-2b 0.1
(S)-(À)-2b 1.0
(R)-(+)-2b 1.0
(R)-(+)-2b 2.0
(+)-2c
(+)-2c
0.1
1.0
71
[a] The reactions were conducted for 2–3 h, [5d]=0.33m. [b] Enantio-
meric excesses were determined by HPLC analysis on a chiral stationary
phase (Chiralcel AS-H column). [c] Absolute configuration at C6 of the
major enantiomer (see below).
Even though the resulting ee values are still moderate,
these are the first examples of asymmetric iodoarene-
mediated phenol dearomatization reactions, using an external
oxygenating species, to be reported. To ascertain the stereo-
chemical filiation between the iodoarene reagent 2b and the
ortho-quinol product 6d, we determined the absolute stereo-
mer form III, could react with the phenolic substrate by ligand
exchange, hence driving the oxidation reaction, to furnish a
trigonal-bipyramidal aryloxy-l3-iodane of type III’.
Then, using a chiral iodoarene such as (R)-2b, the
approach of an arenol such as 5d would occur from the
least hindered face of III to give III’, the configurational
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the iodoarene I. The sense of asymmetric induction using (R)-
2b to furnish (S)-6d indicates a preference for an orientation
of the sterically demanding aryloxy ligand in III’ with its
methyl group syn to the carboxylic acid function of the
binaphthyl ligand, as depicted in Scheme 1b. However, since
m-CPBA could additionally oxidize the iodosyl species III up
to its iodoxy variant V,[14a] the possibility that the arenol could
ligate the iodine(V) atom, by ligand exchange, should not be
overlooked. Steric repulsion is likely to intervene between the
aryloxy ligand and the ortho-methoxynaphthyl unit of the
resulting 2-iodoxynaphthoic acid V’, and could be relieved by
a hypervalent twist,[14b] during which the oxygen atom and the
aryloxy ligands will move in and out of the plane, respectively,
to promote the ortho-oxygenation reaction (Scheme 1c). The
ortho-quinol 6d would then be obtained after hydrolytic
release of the l3-iodane III in much the same way as in the
case of IBX-mediated HPD reactions.[3a,4]
The speculative nature of these descriptions led us to
begin a search for evidence of l3- and/or l5-iodanyl species
during the reaction. We monitored the HPD reaction of 5d to
give 6d, using equimolar amounts of rac-2b and m-CPBA, by
using electrospray ionization mass spectrometric (ESI-MS)
analysis of aliquots diluted in 0.1% (v/v) AcOH/MeOH.
Interestingly, prior to addition of 5d, the most prominent
peak was at m/z 501, and could correspond to the OH/OMe
ligand exchange product of V. After addition of 5d, a peak at
m/z 487, which could be attributed to [V+H]+, was clearly
seen in all spectra. Most importantly, a new peak also
appeared at m/z 627 over the course of the 3 hour reaction.
This peak can be assigned to the [M+H]+ ion of V’ (see the
Supporting Information). Although these observations tend
to strongly support the iodine(V) path depicted in Scheme 1c,
additional investigations are still required before the iodine-
(III) path can be completely disregarded.
In summary, we have developed an iodoarene-mediated
hydroxylative phenol dearomatization reaction using m-
CPBA as a co-oxidant. Using iodoarenes as organocatalysts
with m-CPBA in excess enables subsequent regio- and
diastereoselective epoxidation. Using chiral iodobiarenes
enables enantioselectivities up to 50% ee, unveiling for the
first time the potential of chiral iodoarenes for reagent control
in asymmetric phenol dearomatization reactions using an
external oxygenating source. Future work will focus on the
use of other chiral iodobiarenes to additionally examine the
potential of this reaction and to challenge the mechanistic and
stereochemical control models proposed herein.
Scheme 1. Proposed mechanistic descriptions of iodoarene-mediated
hydroxylative phenol dearomatization using m-CPBA. a) General mech-
anism proposal, b) the iodine(III) pathway, and c) the iodine(V) path-
way.
Received: February 23, 2009
Published online: May 15, 2009
stability of its iodine(III) center being maintained by secon-
dary I···O interactions with the OMe and/or the CO2H
group(s).[12] Therefore, having an iodine(III) center loaded
with an aryloxy and a hydroxy ligand in such a sterically
biased system may lead to a direct and stereoselective ligand-
coupling event.[13] Even though coupling of these ligands, both
of which are in apical positions, might appear spatially
unlikely, topological transformations by, for example, Berry
pseudorotation,[13a] will push them toward each other and
promote coupling with concomitant reductive elimination of
Keywords: asymmetric synthesis · dearomatization ·
hypervalent compounds · iodine · vibrational circular dichroism
.
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