Atropodiastereoselective C-H olefination of biphenyl p-tolyl sulfoxides with acrylates

Article abstract of DOI:10.1002/adsc.201300446

A stereoselective method for the synthesis of axially chiral biaryl scaffolds by C-H bond functionalization was accomplished using chiral sulfoxide both as the directing group enabling the regioselective activation of a C-H bond and as the chiral auxiliary generating an asymmetric environment in the coordination sphere of the metal complex. We have demonstrated the directing ability of the p-tolylsulfinyl group in promoting the Pd(II)-catalyzed C-H olefination of biphenyls.

Full text of DOI:10.1002/adsc.201300446

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DOI: 10.1002/adsc.201300446
À
Atropodiastereoselective C H Olefination of Biphenyl p-Tolyl
Sulfoxides with Acrylates
Thomas Wesch,a Frꢀdꢀric R. Leroux,a and FranÅoise Coloberta,*
a
Laboratoire de Chimie Molꢀculaire (UMR CNRS 7509), Universitꢀ de Strasbourg, ECPM, 25 Rue Becquerel, 67087
Strasbourg, France
Fax : (+33)-3-6885-2742; phone: (+33)-3-6885-2744; e-mail: francoise.colobert@unistra.fr
Received: May 22, 2013; Published online: August 9, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300446.
Abstract: A stereoselective method for the synthe-
sis of axially chiral biaryl scaffolds by C H bond
27% yield and 72% ee as best enantioselectivity
[{Scheme 1, Eq. (1b)]. Both reports showed the diffi-
À
À
functionalization was accomplished using chiral
sulfoxide both as the directing group enabling the
culty to control axial chirality in high yield in this C
H activation process. Moreover in both examples pyr-
idine and thiophene moieties, part of the biaryl scaf-
fold, play the role of non-modulable directing groups.
On the other hand, various different approaches
have been developed to construct axially chiral biaryl
scaffolds.[5] Among them our group has recently re-
ported a highly diastereoselective biaryl Suzuki–
Miyaura cross-coupling reaction mediated by sulfox-
ides between ortho,ortho’-disubstituted aryl iodides
bearing in ortho position a tert-butyl or p-tolylsulfinyl
group and ortho-substituted phenylboronic acids or
esters.[6] We could show that the diastereoselectivity is
facilitated by the formation of a five-membered palla-
dacycle in which the oxygen atom of the sulfoxide co-
À
regioselective activation of a C H bond and as the
chiral auxiliary generating an asymmetric environ-
ment in the coordination sphere of the metal com-
plex. We have demonstrated the directing ability of
the p-tolylsulfinyl group in promoting the Pd(II)-
À
catalyzed C H olefination of biphenyls.
À
Keywords: axial chirality; biaryls; C H functionali-
zation; olefination; sulfoxides
À
Over the past decade, the use of C H bonds as func- ordinates to palladium [Scheme 1, Eq. (2)]. This
tional groups similar to carbon-halogen bonds has methodology shows the efficiency of sulfinyl groups
represented a powerful, valuable and straightforward as chiral controllers in asymmetric biaryl coupling re-
strategy for C C bond formation. Thus, activation of actions.[7]
C H bonds to form C C bonds in a single step has
À
À
À
Inspired by this previous work, we hypothesized
been efficiently developed and the direct functionali- that in view of the coordinating ability of the sulfox-
À
zation of these bonds constitutes a fast and atom-eco- ide moiety, it could be astutely applied in C H activa-
nomical synthetic approach.[1] However asymmetric tion to act as both directing group and chiral auxili-
À
C H bond transformations involving an enantiomeri- ary.
cally pure [C M]* intermediate are still very rare and
À
Herein we describe the first examples where an
limited to fairly specific substrates or transforma- easily available enantiopure p-tolyl sulfoxide[8] acts as
tions.[2] Moreover, the synthesis of axially chiral scaf- directing group for C H activation and we evidence
À
À
folds by C H bond activation was the subject of only its ability as chiral controller in the atroposelective
two works. In 2000 S. Murai[3] reported the Rh-cata- construction of highly functionalized axially chiral
À
lyzed atroposelective C H activation/alkenylation of biaryls [Scheme 1, Eq. (3)]. Sulfoxide moieties are
2-(1-naphthyl)-3-methylpyridine. The best enantio- known to be easily transformed into myriads of syn-
meric excess (49%) was obtained by the use of thetically valuable functional groups by sulfoxide/
a chiral ferrocenyl phosphine ligand giving the cou- metal interconversion followed by trapping with
pling product in 37% yield [Scheme 1, Eq. (1a)]. a range of electrophiles and consequently represent
More recently, J. Yamaguchi and K. Itami[4] described a new site for further functionalization.[9]
À
the Pd-catalyzed atroposelective C H arylation of thi-
Quite surprisingly, the potential of sulfoxides in the
À
ophenes with arylboronic acids in 2012. Chiral bisoxa- context of C H bond activation had remained under-
zolines allowed the synthesis of the arylthiophenes in estimated until 2011, when the first reports concern-
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Thomas Wesch et al.
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Scheme 1. Control of axial chirality by C H functionalization using a chiral sulfinyl group.
À
ing the application of 2-pyridyl sulfoxide as directing ic ring, followed by the second, intramolecular C H
group were independently published by several bond activation and Pummerer rearrangement, giving
groups. These early reports concern palladium-cata- access to dibenzothiophene scaffolds. Simultaneously
À
lyzed directed C H functionalization of arenes such and independently our group developed a sulfoxide-
as acetoxylations,[10] alkenylations and arylations[11] mediated intermolecular C H bond activation and
À
and olefination and coupling reactions with boron re- cyclization towards dibenzothiophene sulfoxides.[14]
agents.[12] However, the key point in all these reac-
However, the use of sulfoxides as both directing
À
tions is the use of pyridyl-substituted sulfoxides. group and chiral auxiliary in C H functionalization
Indeed, it is rather the pyridyl moiety, known and ex- processes has no precedent in the literature.
tensively used in the field of C H bond activation,
that plays the role of the directing group. The sulfox- lyzed alkenylation of biaryl moieties A with acrylates
ide is therefore simply used as an “easily removable via C H activation employing an enantiopure sulfox-
linker” prone to temporarily install the chelating ide as traceless directing group. In this way, configura-
À
Our preliminary effort focused on a palladium-cata-
À
group.
tionally stable biaryls B, powerful intermediates to-
Very recently, the first example concerning the use wards a family of atropoenantiopure biaryl moieties
of the chelating properties of sulfoxides to direct the C, become accessible (Scheme 2).
À
C H bond activation event was reported by Anton-
Biaryl sulfoxides A were synthesized by means of
chick et al.[13] The key step of this transformation is a palladium-catalyzed Suzuki–Miyaura coupling reac-
the sulfoxide-directed cyclometallation of the aromat- tion between aryl iodides bearing in the ortho posi-
À
Scheme 2. C H activation/alkenylation reaction of biphenyl p-tolyl sulfoxide with acrylates.
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Atropodiastereoselective C H Olefination of Biphenyl p-Tolyl Sulfoxides with Acrylates
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Scheme 3. Direct C H activation/alkenylation reaction of biphenyl p-tolyl sulfoxide 1a-S with methyl and tert-butyl acrylate.
tion the widely used p-tolylsulfinyl group and various 2b-SaS=4.6:1)[16] than for 2a (Table 1, entry 1). Biaryl
commercially available arylboronic acids.[15]
1c-S with a chlorine atom and 1d-S with a trifluoro-
Our study commenced by selection of 2-p-tolylsulfi- methyl group in position 2’ gave the alkenylation
nylbiphenyl 1a-S as test substrate to explore the role products 2c and 2d in 43% and 30% yield and lower
À
of sulfoxide as directing group in the C H alkenyla- atroposelectivity (2c-SaS/2c-SaR=2.5:1 and 2d-SaS/
À
tion with methyl acrylate. A direct C H activation/ 2d-SaR=2.8:1) (Table 1, entries 2 and 3). With a me-
olefination reaction was readily accomplished with thoxy group in position 2’, an excellent yield for the
À
AgOAc as oxidant in the presence of palladium(II) C H alkenylation was observed (87%) but lower con-
acetate as catalyst precursor, and selective product trol of the chiral axis (2e-SaS/2e-SaR=1.8:1)
formation was observed. Dichloroethane was identi- (Table 1, entry 4). Starting from biaryl 1f-S with
fied as the best solvent affording 2a in 76% yield a naphthyl moiety, 2f was obtained in excellent 86%
after 6 h at 808C (Scheme 3). We also noticed the for- yield and 3.1:1 atropodiastereomeric ratio (Table 1,
mation of di-ortho-alkenylated products (through a se- entry 5).
À
quential double C H alkenylation) with 14% yield.
No product was obtained in the absence of palladium be further improved with a substituent at position 6
To our delight, the atropodiastereoselectivity could
catalyst or AgOAc.
of biaryl substrates 1g and 1h. For example, with a me-
The general structure of the new product 2a was thoxy group at position 6 (1g-S), the C-H alkenylation
1
unambiguously determined by H and 13C NMR stud- coupling product 2g was obtained in 56% yield show-
ies showing the presence of two diastereoisomers in ing both atropodiastereomers in a 5:1 ratio (Table 1,
a 1:2.1 ratio. The coalescence temperature of 2a has entry 6). Moreover, with a trifluoromethyl group at
1
been determined by variable temperature H NMR position 6 (1h-S), an excellent atropodiastereoselec-
ACHTUNGTRENNUNG
studies [DG#(363 K)=17.0 kcalmolÀ1]. This result in- tivity was observed giving the alkenylation product 2h
dicates that biaryl 2a is configurationally stable at the in 36% yield and 10:1 diastereomeric ratio (Table 1,
reaction temperature and that the enantiopure sulfi- entry 7). Finally 2’,6-dimethoxy-2-p-tolylsulfinylbi-
À
nyl moiety is not only acting as a directing group for phenyl 1i-S gave in the C H alkenylation process the
C H activation in position 2’ but also induces atropo- corresponding tetra-ortho-substituted biaryl as
À
diastereoselectivity during the alkenylation process. a single atropodiastereomer 2i-SaS in 40% yield. The
The coupling reaction with tert-butyl acrylate gave the absolute configuration of 2i-SaS was determined by
corresponding biaryl in a lower yield (50%) and the single crystal X-ray diffraction analysis[17] (Figure 1)
same atropodiastereoselectivity.
and we deduced the absolute configuration of all
1
With these optimized reaction conditions in hand, major atropoisomers 2a–h by comparison of H NMR
À
the C H alkenylation of 2-p-tolylsulfinylbiphenyls spectra.
bearing substituents either in position 6 or 2’ as R1
We noticed that 2-p-tolylsulfinylbiphenyls bearing
and/or R2 was investigated (Table 1). A wide array of electron-withdrawing groups at the aryl ring are less
2-p-tolylsulfinylbiphenyls 1b-S–1i-S was subjected to active under the reaction conditions (Table 1, en-
À
the C H alkenylation with methyl acrylate and the tries 2, 3 and 7). Consequently, an electrophilic metal-
corresponding coupling products 2b–i were isolated in lation mechanism is probably favored. Moreover,
good to excellent yields showing moderate to excel- a total regioselectivity was observed during the olefi-
lent atropodiastereoselectivity.
nation reaction, which reasonably suggests that only
À
In order to increase the atropodiastereoselectivity one specific C H bond is activated. This selectivity
À
during the C H alkenylation process, compared to suggests the formation of a 6-membered palladacycle
our first reaction (Scheme 3), 2’-methyl-2-p-tolylsulfi- intermediate D resulting from the coordination of
nylbiphenyl 1b-S was prepared.[15] Starting from biaryl palladium to the lone pair of the sulfur atom and
À
sulfoxide 1b-S, we performed the C H alkenylation a subsequent stereoselective insertion of the metal
with methyl acrylate and the coupling product 2b was into the appropriately positioned C H bond
obtained in an excellent yield of 87% and, as expect- (Scheme 4). Subsequently the insertion of the olefin
ed, with a better atropodiastereoselectivity (2b-SaR/ partner into the Pd C bond leads to the formation of
À
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Table 1. Direct C H activation/alkenylation reaction of biphenyl p-tolyl sulfoxides 1b-S–1i-S with methyl
acrylate.[a]
[a]
Biaryl sulfoxides were synthesized by a palladium-catalyzed Suzuki–Miyaura coupling reaction.[15]
Determined by H NMR.
Isolated yield.
Atropoenriched starting products racemized at the coupling temperature (see the Supporting Infor-
mation).
[b]
[c]
[d]
1
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Atropodiastereoselective C H Olefination of Biphenyl p-Tolyl Sulfoxides with Acrylates
ladium and sulfur favors the cyclometallation step
from the sterically less hindered side, opposite to the
bulky p-tolyl group, affording atropoenriched inter-
mediate D. Subsequent b-hydride elimination leads
then to the atropodiastereomer and liberation of
Pd(0). Pd(II) is regenerated by oxidation of Pd(0)
with silver(I).[18]
In summary, we have demonstrated the directing
ability of an enantiopure p-tolylsulfinyl group in the
À
Pd(II)-catalyzed C H olefination of biphenyls. Elec-
tron-deficient acrylates serve as efficient coupling
partners, giving access to ortho-alkenylated products
in good yields and atropodiastereoselectivities. The
significance of this strategy can be further highlighted
by the fact that, in contrast to the majority of other
À
asymmetric C H bond activations where the asym-
metric discrimination operates posterior to the inser-
À
tion of the metal into the C H bond, here the activa-
tion of the C H bond is the asymmetrically discrimi-
À
nating step. This manuscript represents a proof of
concept for the use of sulfoxides as both directing
group and chiral auxiliary and opens new perspectives
in the field of C H bond activation reaction using
a chiral sulfoxide, a very special motif which presents
a unique opportunity for the development of technol-
À
À
ogies for asymmetric transformation via C H bond
Figure 1. X-ray crystal structure of 2i-SaS.
activation.
Further investigations are ongoing to extend the
scope of the reaction and to apply the process in the
synthesis of more complex biaryls.
Experimental Section
À
General Procedure for the C H Alkenylation with
Methyl Acrylate
2-(p-Tolylsulfinyl)biphenyl 1a–i (0.8 mmol) was dissolved in
1,2-dichloroethane (2.6 mL, 3.25M). After addition of
AgOAc (4.8 mmol, 6 equiv.), PdACTHNUTRGNENUG(OAc)2 (0.08 mmol,
10 mol%) and methyl acrylate (1.6 mmol, 2 equiv.), the reac-
tion mixture was put in a preheated oil bath at 808C and
stirred for 6 h. Then the dark reaction mixture was cooled
down to room temperature and filtered over Celite. After
distillation of the solvent under reduced pressure, the cou-
pling product was purified by silica gel column chromatogra-
phy.
Supporting Information
Experimental protocols and characterization data for new
compounds, copies of H, 13C NMR spectra are available in
the Supporting Information.
1
À
Scheme 4. Plausible mechanism for the direct C H activa-
tion/alkenylation reaction of biphenyl p-tolyl sulfoxides 1a-
S–1i-S with acrylate.
Acknowledgements
E. The atroposelectivity observed during this reaction
results probably from diastereomeric discrimination
We thank the CNRS (Centre National de la Recherche Scien-
during the formation of D. Coordination between pal- tifique) and the “Ministere de l’Education Nationale et de la
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Thomas Wesch et al.
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Recherche”, France for financial support. T.W. is very grate-
ful to the French Embassy in Germany for a postdoctoral
grant.
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