Diaryl sulfoxides from aryl benzyl sulfoxides: A single palladium-catalyzed triple relay process

Article abstract of DOI:10.1002/anie.201307172

A novel approach to produce diaryl sulfoxides from aryl benzyl sulfoxides is reported. Optimization of the reaction conditions was performed using high-throughput experimentation techniques. The [Pd(dba)₂]/NiXantPhos catalyst system successfully promotes a triple relay process involving sulfoxide α-arylation, C-S bond cleavage, and C-S bond formation. The byproduct benzophenone is formed by an additional palladium-catalyzed process. It is noteworthy that palladium-catalyzed benzylative C-S bond cleavage of sulfoxides is unprecedented. A wide range of aryl benzyl sulfoxides, as well as alkyl benzyl sulfoxides with various (hetero)aryl bromides were employed in the triple relay process in good to excellent yields (85-99 %). Moreover, aryl methyl sulfoxides, dibenzyl sulfoxides, and dimethylsulfoxide could be utilized to generate diaryl sulfoxides involving multiple catalytic cycles by a single catalyst. Three for one: The [Pd(dba)₂]/NiXantPhos (dba=dibenzylideneacetone) catalyst system successfully promotes a triple relay process involving sulfoxide α-arylation, C-S bond cleavage, and C-S bond formation to give diaryl sulfoxides (see picture). Aryl benzyl sulfoxides, as well as alkyl benzyl sulfoxides reacted with various (hetero)aryl bromides. Copyright

Full text of DOI:10.1002/anie.201307172

.
Angewandte
Communications
DOI: 10.1002/anie.201307172
Homogeneous Catalysis
Diaryl Sulfoxides from Aryl Benzyl Sulfoxides: A Single Palladium-
Catalyzed Triple Relay Process**
Tiezheng Jia, Ana Bellomo, Sonia Montel, Mengnan Zhang, Kawtar EL Baina, Bing Zheng, and
Patrick J. Walsh*
Abstract: A novel approach to produce diaryl sulfoxides from
aryl benzyl sulfoxides is reported. Optimization of the reaction
conditions was performed using high-throughput experimen-
tation techniques. The [Pd(dba)2]/NiXantPhos catalyst system
successfully promotes a triple relay process involving sulfoxide
À
À
a-arylation, C S bond cleavage, and C S bond formation. The
byproduct benzophenone is formed by an additional palla-
dium-catalyzed process. It is noteworthy that palladium-
À
catalyzed benzylative C S bond cleavage of sulfoxides is
unprecedented. A wide range of aryl benzyl sulfoxides, as well
as alkyl benzyl sulfoxides with various (hetero)aryl bromides
were employed in the triple relay process in good to excellent
yields (85–99%). Moreover, aryl methyl sulfoxides, dibenzyl
sulfoxides, and dimethylsulfoxide could be utilized to generate
diaryl sulfoxides involving multiple catalytic cycles by a single
catalyst.
Scheme 1. Synthetic approaches to diaryl sulfoxides. dba=dibenzylide-
neacetone.
S
ulfoxides are widely occurring in natural products,[1]
bond cleavage of allylic sulfenate esters by a Mislow–Braver-
man–Evans rearrangement of the allylic sulfoxides was
described (Scheme 1b).[8c] Subsequently, the sulfenate
anions were arylated with aryl iodides or bromides to yield
diaryl sulfoxides. The substrate scope of these reactions is
narrow.
synthetic bioactive compounds,[2] and materials.[3] They are
also very important structural motifs in marketed therapeu-
tics, such as Nexium for heartburn and esophagitis,[4a] and
Provigil for narcolepsy.[4b] Furthermore, in recent years,
sulfoxides have attracted attention as ligands in catalysis.[5]
Despite the importance of sulfoxides, rapid syntheses of
diversified diaryl sulfoxides remains challenging.
Herein, we report a general, high-yielding method for the
synthesis of diaryl sulfoxides. Remarkably, mechanistic stud-
ies reveal that a single palladium catalyst promotes at least
three distinct reactions in this transformation (Scheme1).[9]
We recently reported the palladium-catalyzed functional-
Two classic methods to generate diaryl sulfoxides are the
oxidation of sulfides[6] and the nucleophilic substitution of
electrophilic sulfoxide derivatives.[7] Madec, Poli, and co-
workers reported the preparation of aryl sulfoxides by the
palladium-catalyzed arylation of sulfenate anions with aryl
iodides and bromides (Scheme 1a,b).[8] In the presence of
20 equivalents of KOH, sulfenate anions could be generated
from the retro-Michael reaction of b-sulfinyl esters (Sche-
ization of weakly acidic sp3-hybridized C H bonds
À
(pKa values 28–35 in DMSO) by a deprotonative cross-
coupling process (DCCP). Using this approach, we developed
coupling protocols for diarylmethanes,[10] sulfoxides,[11] sul-
fones,[12] amides,[13] and chromium-activated benzylic amines
(to produce enantioenriched diarylmethylamines).[14] During
our study on the a-arylation of sulfoxides using the ligand
L1,[15] we noticed that a diaryl sulfoxide by-product (3a) was
generated in up to 20% yield from benzyl phenyl sulfoxide
(1a) with 4-tert-butyl bromobenzene (2a) [10 mol% Pd-
(OAc)2, 15 mol% L1, 3 equiv LiOtBu, in cyclopentyl methyl
ether (CPME) at 1108C for 12 h; Table 1, entry 1]. We
initiated the optimization by performing a ligand screen using
the reaction conditions described in Table 1. Of the 112
sterically and electronically diverse mono- and bidentate
phosphines examined, two classic ligands [SPhos (L2) and
DavePhos (L3)] along with NiXantPhos (L4)[16] outper-
formed L1 (Table 1). On a laboratory scale, the yields of 3a
with SPhos, DavePhos, and NiXantPhos were 25%, 33%, and
22%, respectively (Table 1, entry 1 versus entries 2–4).
Because the role of the base is crucial to DCCP reactions,
me 1a).[8a,b] In a second report, a palladium-catalyzed C O
À
[*] T. Jia, Dr. A. Bellomo, Dr. S. Montel, M. Zhang, K. EL Baina,
B. Zheng, Prof. P. J. Walsh
Roy and Diana Vagelos Laboratories
Penn/Merck Laboratory for High-Throughput Experimentation
Department of Chemistry, University of Pennsylvania
231 S. 34th St. Philadelphia, PA 19104-6323 (USA)
E-mail: pwalsh@sas.upenn.edu
Homepage: http://titanium.chem.upenn.edu/walsh/
[**] We thank the National Science Foundation [CHE-0848460 (GOALI)
and 1152488] for financial support. K.E.B thanks the Universitꢀ
Pierre et Marie Curie and CROUS de Paris for financial support. B.Z.
thanks the China Scholarship Council [2011635137] for financial
support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201307172.
260
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Table 1: Optimization of diaryl sulfoxide formation by cross-coupling of
benzyl phenyl sulfoxide (1a) with 4-tert-butylbromobenzene (2a).
Table 2: Substrate scope of aryl bromides in palladium-catalyzed
formation of diaryl sulfoxides with 1a.
R
Product Yield [%][a]
R
Product Yield [%][a]
4-tBu
4-NMe2 3b
4-F
4-Cl
4-CF3
4-NO2
3a
91
2-Me
1-naphthyl 3h
3-OMe
2-pyridyl
3-pyridyl
4-pyridyl
3g
90[b]
93
Entry Catalyst
Ligand Catalyst/ligand T [oC] Base
Yield [%][a]
91[b]
95
(mol%)
3c
3d
3e
3 f
3i
3j
3k
3l
93
1
Pd(OAc)2 L1
10:15
10:15
10:15
10:15
10:15
10:15
10:15
10:15
5:7.5
110 LiOtBu
110 LiOtBu
110 LiOtBu
110 LiOtBu
110 NaOtBu
80 NaOtBu
80 NaOtBu
80 NaOtBu
80 NaOtBu
80 NaOtBu
55 NaOtBu
20
25
33
22
83
85
55
91
91
68
28
93
89[d]
2
3
4
5
6
7
8
9
Pd(OAc)2 L2
Pd(OAc)2 L3
Pd(OAc)2 L4
Pd(OAc)2 L4
Pd(OAc)2 L4
[Pd2(dba)3] L4
[Pd2(dba)3] L4
[Pd2(dba)3] L4
[Pd2(dba)3] L4
[Pd2(dba)3] L4
85[c]
94
94[b,c] (86[e])
85[b,c]
[a] Yield of isolated product. [b] 1108C. [c] 24 h. [d] 6 h. [e] 1a (5 mmol),
2k (10 mmol), CPME (50 mL), 36 h.
The reaction conditions identified for the formation of 3a
were evaluated for a series of aryl bromides with 1a (Table 2).
Electron-donating groups on the aryl bromides, such as 4-tert-
butyl and 4-N,N-dimethylamino, were well tolerated, thus
providing 3a and 3b, respectively, in 91% yield. The cross-
coupling reactions proceeded smoothly with 1a and various
aryl bromides bearing electron-withdrawing groups, including
4-fluoro (2c), 4-chloro (2d), 4-trifluoromethyl (2e), and 4-
nitro (2 f), thus providing 3c–f in 85–95% yield. Excellent
chemoselectivity in coupling at bromide over the chloride was
achieved with 4-chloro bromobenzene (2d). The sterically
more demanding 2-bromotoluene (2g) and 1-bromonaphtha-
lene (2h) furnished the products 3g (90%) and 3h (93%),
respectively. 3-Bromoanisole generated the coupling product
3i in 93% yield.
10
11
2.5:3.8
5:7.5
[a] Yield of isolated product. CPME=cyclopentyl methyl ether.
we next examined 12 bases and four solvents using the ligands
SPhos (L2), DavePhos (L3), and NiXantPhos (L4; see the
Supporting Information). The highest yield was with
15 mol% NiXantPhos (L4), 10 mol% Pd(OAc)2, and
NaOtBu in CPME (Table 1, entry 5). Forging ahead with
NiXantPhos, a similar yield was obtained when the temper-
ature was reduced to 808C (entry 6). Examination of four
Bioactive sulfoxides often contain heterocycles.[18] Our
protocol is effective for the synthesis of heterocyclic sulf-
oxides, as demonstrated with couplings to yield 2-, 3-, and 4-
pyridyl phenyl sulfoxides (3j–l) in 89, 94, and 85% yield,
respectively. To achieve these yields, higher reaction temper-
atures (1108C) and longer reaction times (24 h) were needed.
It is noteworthy that 3j is the key motif for an anti-
inflammatory agent[18a] and 3l is derived from a hair papilla
cell proliferation agent.[18b] To show the scalability of our
method, we performed the reaction using 5 mmol benzyl
phenyl sulfoxide (1a) and 10 mmol 3-bromopyridine (2k).
The product, 3k was isolated in 86% yield.
palladium
sources
[Pd(OAc)2,
[Pd(cod)Cl2],
[Pd-
(NCCH3)2Cl2], and [Pd2(dba)3]] in six solvents was under-
taken using NiXantPhos (L4) and NaOtBu at 808C for
12 hours. Among the variables examined, [Pd2(dba)3] in
CPME rendered the highest assay yield on microscale.
Unfortunately, we were unable to translate these conditions
to lab scale, and only 55% yield of 3a was obtained. We
hypothesized that contamination of commercial [Pd2(dba)3]
with nanoparticles was
a
potential problem (Table 1,
We next turned our attention to the substrate scope of aryl
benzyl sulfoxides in coupling reactions with bromobenzene
(2m) to generate diaryl sulfoxides. Electron-donating 4-
methyl and 4-N,N-dimethylamino groups were well tolerated,
thus giving 3m and 3b in 99 and 92% yield, respectively.
Substrates bearing the electron-withdrawing 4-F (1c), 4-Cl
(1d), and 4-CF3 (1e) groups furnished the products in 86–
92% yield. The congested 2-tolyl and 1-naphthyl benzyl
sulfoxides 1g and 1h afforded 3g (93%) and 3h (91%),
respectively. Aryl heteroaryl sulfoxides were provided from
heteroaryl benzylsulfoxides in excellent yields under slightly
modified reaction conditions (see Table 3 for details). 3-
Pyridyl (3k), 2-furanyl (3n), and 2-thienyl (3m) phenyl
sulfoxides were thus prepared. The compound 3n could be
utilized as a key motif for an antituberculotic reagent.[18c]
Alkyl aryl sulfoxides could also be prepared, as demonstrated
entry 7).[17] Changing to [Pd(dba)2] overcame this issue, thus
generating 3a in 91% yield (Table 1, entry 8). We were able
to decrease the Pd/ligand loading from 10:15 mol% to
5:7.5 mol% while maintaining the high yield (Table 1,
entry 9). Reduction to 2.5:3.8 mol%, however, resulted in
lower yield (entry 10). Therefore, 5 mol% [Pd(dba)2] and
7.5 mol% NiXantPhos were employed. Decreasing the
temperature to 558C led to only 28% yield of 3a (entry 11).
Further optimization of the loading of NaOtBu and the ratios
of 1a and 2a revealed that 3 equivalents of NaOtBu and a 1:2
ratio of 1a/2a was optimal (see the Supporting Information).
Thus, our best reaction conditions were 5 mol% [Pd(dba)2],
7.5 mol% NiXantPhos (L4), 1a (as the limiting reagent),
2 equivalents of 2a, and 3 equivalents NaOtBu in CPME at
808C for 12 hours.
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Table 3: Substrate scope of aryl benzyl sulfoxides in palladium-catalyzed
formation of diaryl sulfoxides with bromobenzene (2m).
R
Product Yield [%][a]
R
Product Yield [%][a]
4-Me
4-NMe2 3b
4-F
4-Cl
4-CF3
2-Me
3m
99
92
1-naphthyl 3h
91
3-pyridyl
2-furanyl
2-thienyl
3k
3n
3o
89[c]
90
3c
3d
3e
3g
92
91[b]
86
85[c]
90[c]
cyclohexyl 3p
93[c]
[a] Yield of isolated product. [b] 24 h. [c] 1108C.
by generation of cyclohexyl phenyl sulfoxide (3p) in 90%
yield.
Scheme 2. Generation of 4g and a-arylation and benzylation of 4a.
To generate diaryl sulfoxides, the Pd/NiXantPhos catalyst
must promote multiple distinct reactions, including cleavage
À
and formation of C S bonds. Based on experiments discussed
NaOtBu base, thus generating a diarylmethyl tert-butyl ether
(J), which we have isolated from the reaction mixture. The
yield of this byproduct is always significantly lower than
expected based on the reaction stoichiometry. Further studies
demonstrated that the Ar2CH-O-tBu undergoes a base-
induced E2 elimination to generate Ar2CH-ONa, which is
oxidized by palladium in a fourth catalytic cycle. When
3 equivalents of 4-nitro bromobenzene are employed, nitro-
benzene is isolated in 76% yield. If 2 equivalents of the aryl
bromide is used, we propose the eliminated isobutylene is
reduced to isobutane (see the Supporting Information for
details).
Palladium p-benzyl intermediates were pioneered by
Fiaud and Kuwano,[19] who have developed them into useful
synthetic intermediates. We are not aware, however, of
previous observation of sulfenates as leaving groups in p-
benzylation reactions. The sulfenate generated in cycle B
undergoes palladium-catalyzed cross-coupling with aryl bro-
mides in cycle C and produces the diaryl sulfoxide. As further
support of the intermediacy of aryl sulfenate (H), we added
benzyl chloride in the standard catalysis conditions with
below, a tricatalytic cycle is proposed in Figure 1. The first
cycle (A), is the a-arylation of aryl benzyl sulfoxides. We
recently reported a similar catalytic arylation of methyl
phenyl sulfoxides in up to 95% yield and one example of the
arylation of a benzyl phenyl sulfoxide (80% yield).[11] Support
for cycle A was gained when the temperature was decreased
from 1108C to 808C (Scheme 2a) in the reaction of 1a with 2-
tolyl bromide (2g). We isolated the diarylmethyl sulfoxide 4g
(intermediate E; Figure 1) in 5% yield along with the 35% of
the diaryl sulfoxide 3g (Scheme 2a). These results support the
proposed a-arylation cycle A. Subsequently, diphenylmethyl
phenyl sulfoxide (4a) was synthesized independently and
subjected to the optimized reaction conditions with 4-tert-
butyl bromobenzene (Scheme 2b). The diaryl sulfoxide
product 3a was generated in 88% yield.
In the second catalytic cycle (B) a novel cleavage of the
C S bond occurs (Figure 1). Arene coordination of the aryl
diarylmethyl sulfoxide moiety (F) to palladium is followed by
formation of a p-benzyl intermediate (G) and expulsion of
aryl sulfenate (H). The intermediate G is attacked by the
À
Figure 1. Proposed mechanism of the palladium-catalyzed triple relay process.
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diphenylmethyl phenyl sulfoxide (4a, Scheme 2c). Benzyl
phenyl sulfoxide (1a) was isolated in 88% yield.
Experimental Section
General procedure for catalysis: [Pd(dba)2] (2.88 mg, 0.005 mmol)
and ligand L4 (4.14 mg, 0.0075 mmol) were added to an oven-dried
microwave vial equipped with a stirbar under a nitrogen atmosphere,
and 1.0 mL dry CPME was then added. After the catalyst/ligand
solution was stirred for 2 h at 248C, NaOtBu (28.8 mg, 0.30 mmol,
3 equiv) was added to the reaction vial followed by benzyl phenyl
sulfoxide (21.6 mg, 0.10 mmol, 1.0 equiv). The microwave vial was
sealed and 4-tert-butyl bromobenzene (34.6 mL, 0.20 mmol, 2.0 equiv)
was added by syringe under a nitrogen atmosphere. Note that if the
benzyl sulfoxide or aryl bromide is a solid, it was added to the reaction
vial before NaOtBu. The reaction mixture was heated to 808C by oil
bath and stirred for 12 h. The sealed vial was cooled to room
temperature, opened to air, and the reaction mixture was passed
through a short pad of silica gel. The pad was then rinsed with 10:1
dichloromethane/methanol. The solvent was removed under reduced
pressure, and the residue was purified by flash chromatography on
silica gel (eluted with EtOAc/hexanes 1:4) to give the product
(23.5 mg, 91% yield) as a white solid.
As mentioned, we recently reported the first palladium-
catalyzed a-arylation of methyl sulfoxides with aryl halides.[11]
The products generated from this a-arylation are aryl benzyl
sulfoxides, which are the starting materials in Tables 1–3.
Thus, we were interested in exploring the synthesis of diaryl
sulfoxides directly from aryl methyl sulfoxides.[20] The pKa of
methyl aryl sulfoxides is significantly higher than that of
benzyl aryl sulfoxides, however. Fortunately, the [Pd(dba)2]/
NiXantPhos system proved to be suitable for this trans-
formation (Scheme 3a). Thus, starting with methyl phenyl
Received: August 15, 2013
Revised: September 25, 2013
Published online: November 24, 2013
Keywords: cross-coupling · homogeneous catalysis ·
.
À
palladium · S O compounds · synthetic methods
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