Sulfoximines: A reusable directing group for chemo-and regioselective ortho C-H oxidation of arenes

Article abstract of DOI:10.1002/chem.201200092

Sulfoximines direct: A new protocol for the chemo-and regioselective ortho C-H acetoxylation of arenes in N-benzoylated sulfoximines is reported. The sulfoximine directing group is easily detached from the C-H oxidation product through acid-promoted hydrolysis, isolated, and reused (see scheme). The meta-substituted phenols are synthesized following this strategy and the stereointegrity of the sulfoximine is preserved in this transformation. C(sp³)-H acetoxylation of the methyl group is also demonstrated.

Full text of DOI:10.1002/chem.201200092

COMMUNICATION
DOI: 10.1002/chem.201200092
Sulfoximines: A Reusable Directing Group for Chemo- and Regioselective
À
ortho C H Oxidation of Arenes
M. Ramu Yadav, Raja K. Rit, and Akhila K. Sahoo*[a]
Dedicated to Professor Dr. Hans-Joachim Gais on the occasion of his 70th birthday
The directing-group-assisted, transition-metal-catalyzed
À
C H oxidation of arenes is recognized as an elegant and
versatile approach for the synthesis of useful phenol deriva-
tives.[1,2] The ability of the directing group (DG) to coordi-
nate the transition metal triggers the activation of the
À
remote C H bond and facilitates the formation of the prod-
uct with better regioselectivity.[3] The Sanford and Yu
groups have explored the regioselectivity and the mecha-
À
nism involved in the DG-assisted C H oxidation of
arenes.[4,5] Subsequently, a variety of DGs for ortho C H ox-
À
idations have been extensively investigated.[6] In spite of
these significant advances, the use of nonremovable and
nonmodifiable DGs and the lack of generality limit the
À
Scheme 1. Reusable directing groups in ortho C H functionalizations. [o-
broad synthetic application of this transformation. In addi-
tion, highly chemoselective functionalization in the presence
À
CH-A & F]=ortho C H activation and functionalization.
[7]
À
of a variety of C H bonds remains elusive. The incorpora-
À
tion of easily removable and robust DGs can overcome
some of these limitations.[8] So far, the attachable and de-
tachable directing groups 8-aq (8-amino quinoline),[9a,b] pza
(2-pyrazole-5-ylaniline),[9c] and aam (anthranilamide)[9d]
ortho C H acetoxylation of N-benzoylated methylphenyl-
sulfoximines; this directing group is easily detached from
the C H oxidation product and can be reused.
To test this hypothesis, compound 2a was subjected to the
A
À
À
have been used in the C H functionalization of arenes
known reaction conditions [Pd
ACHTUNGTRENNUNG
ACHTUNGRTEN(NUGN OAc)2 (10 mol%) and PhI-
(Scheme 1). Recently, the easily modifiable DGs sulfur[10a]
and Si-tethered pyridyl derivatives[10b,c,d] have been em-
(OAc)2 in AcOH] at 1008C for 48 h (Table 1, entry 1).[1b,4a]
Interestingly, formation of the ortho-acetoxylation products
À
ployed for the efficient C H oxidation of arenes.
Inspired by the previous results and the concerns pertain-
ing to this transformation, we envisioned the use of sulfox-
Table 1. Optimization of the reaction conditions.[a]
ACHTUNGTRENNUNG
imines[11] as an easily attachable and detachable new DG for
À
C H functionalizations. A direct and elegant approach for
the ortho hydroxylation of benzoic acid in the presence of
2
O2 has been demonstrated by Yu.[5b] The C
(sp ) H acetoxy-
À
Entry
Catalyst
(10 mol%)
Oxidant
PhI(OAc)2
Cosolvent
Yield [%][b]
(3a/4a/[c]
lation of anilides, deriving from the corresponding anilines,
has been performed with palladium catalysis.[1d] With the aid
of bidentate systems, ortho acetoxylations of amides have
been achieved under harsh reaction conditions.[1f] Herein,
we report the initial results on the chemo- and regioselective
G
E
)
1
2
3
4
5
6
7
8
9
Pd
Pd
Pd
Pd
Pd
N
G
–
–
–
–
57 (10:1:3)
19 (3a)
70 (1:1:8)
0
90 (13:1:1)
95 (14:1:1)[d]
57 (18:1:2)
78 (20:1:4)
90 (4:1:2)
91 (2:1:0.6)
Oxone
K2S2O8
TBHP
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
DCE
CHCl3
Ac2O
toluene
CHCl3
CHCl3
PdACHTUNTGRENNUNG
Pd
Pd
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
[a] M. R. Yadav, R. K. Rit, Prof. Dr. A. K. Sahoo
School of Chemistry, University of Hyderabad
Hyderabad-500046 (India)
PdCl2
Pd(TFA)2
10
ACHTUNGTRENNUNG
Fax : (+91)40-23134822
E-mail: akssc@uohyd.ernet.in
[a] Reaction conditions: 2a (50 mg, 0.18 mmol), oxidant (0.36 mmol), and
AcOH/cosolvent (1:1, 1.0 mL). [b] Conversion based on crude 1H NMR
spectroscopy. [c] Mono-deacylated product. [d] AcOH/CHCl3 (2:3,
1.5 mL) was used. TBHP=tert-butyl hydroperoxide.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200092.
Chem. Eur. J. 2012, 18, 5541 – 5545
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5541

Table 2. ortho Acetoxylation of N-benzoylated sulfoximines.[a,b]
3a and 4a, along with a small amount of the mono-deacylat-
ed compound, was observed in 57% yield by NMR spectros-
À
copy. It is highly intriguing that the ortho C H bonds of the
N-benzoylated aryl ring have been exclusively functional-
ized among the two different arenes of compound 2a. Even
though the ortho hydrogen atoms of both aromatic rings are
present at the same proximity to the metal-coordinating ni-
trogen atom, the factors responsible for this chemoselective
2
3
À
CACHTUNGTRENNUNG(sp ) H oxidation are unclear. The acidic sp S-methyl-hy-
drogen atom did not participate in the oxidation reaction.
No traces of the monoacetoxylation product from selective
À
replacement of the more-hindered ortho C H bond were
detected in the crude NMR spectrum. These results clearly
À
demonstrate the chemo- and regioselective C H oxidation
of arenes. From various oxidants screened (entries 2–4),
K2S2O8 turned out to be superior (entry 3), forming the ana-
logues phenol as major product, whereas benzoquinone, Cu-
ACHTUNGTRENNUNG
(OAc)2, and H2O2 were completely ineffective.[12] To reduce
the formation of deacylated product, various combinations
of solvents with AcOH were tested. Chlorinated solvents
appeared effective (entries 5 and 6) and chloroform was
found to be the best cosolvent (entry 6). Among the
amounts of solvent screened, a 2:3 mixture of AcOH:CHCl3
(1.5 mL for 0.2 mmol of substrate) resulted optimum
(entry 6). The exploration of other solvents, such as Ac2O
and toluene, produced moderate amounts of 3a (entries 7
and 8), whereas DMF and DMSO gave 3a in poor yield.[12]
Other Pd catalysts were found to be similarly efficient (en-
tries 9 and 10). As expected, no product was detected in the
absence of either the Pd catalyst or the oxidant. Gratifying-
ly, the presence of air/moisture did not affect the reaction
efficiency.
[a] Reaction conditions:
2 (1.0 mmol), PdACTHUNGTNRUEGN(OAc)2 (10 mol%), K2S2O8
(2.0 mmol), and AcOH/CHCl3 (3:5, 8.0 mL). [b] Isolated yields. [c] For-
mation of mono-deacetylated product. [d] Starting material recovered.
[e] 72 h. [f] Regioisomeric mixture. [g] 2e (0.57 mmol) at 1208C for 96 h.
[h] 96 h.
To probe the effect of the (S)-methyl-(S)-phenylsulfox-
ACHTUNGTRENNUNGimine (MPS) directing group on the chemo- and regioselec-
À
tive ortho C H acetoxylation of arenes, the optimized reac-
tion conditions (Table 1, entry 6) were surveyed to various
N-benzoylated MPSs (Table 2). At first, the regioselective
ortho acetoxylation of meta-substituted N-benozylated sulf-
tion of b-naphthyl derivative 2 f under the catalytic condi-
tions gave 3 f and 4 f in 56 and 15% yield, respectively.
These results suggest that the selectivity is governed by the
steric, as well as the electronic effect of the meta substi-
tuent.[4c] The electronically neutral compound 2g and the
para-substituted N-benzoylated MPSs 2h–k gave the desired
mono- and diacetoxylated products in moderate to good
overall yields. However, the reaction of N-[para-chloroben-
zoyl]-MPS (2j) gave the mono-deacylated compound as
a major product. The halogen substituents in 3e and 3j’ can
be amenable to further synthetic transformations. Acetoxy-
lation products 3l–m were isolated in excellent yields from
electron-rich ortho-substituted N-benzoylated MPSs 2l–
ACHTUNGTRENNUNGoximines was investigated. The reaction of N-(meta-methyl-
benzoyl)-MPS (2a) gave 3a as a single regioisomer in 65%
yield; the ortho-diacetoxylated product 4a and the mono-de-
acylated product were also isolated in small amounts. We
believe that the compound 4a is obtained from 3a. To verify
this observation, 3a was subjected to the optimized condi-
tions and 4a was isolated, albeit in poor yield. Interestingly,
the TBS (tert-butyldimethylsilyl) group was tolerated under
these reaction conditions and the product 3b was obtained
in 47% yield. A meta-methoxy group did not exhibit the
secondary directing effect and produced 3c exclusively;[4c]
the bulky methoxy group inhibits the formation of diace-
toxylated product 4c. However, a poor level of regioselec-
tivity was observed in the case of N-(meta-fluorobenzoyl)-
MPS (3d).[4c] Unfortunately, the bromo-substituted 3e was
obtained in poor yield, even though the reaction was run for
96 h; the precursor 2e was recovered in 42% yield. The cor-
responding debromination product was observed in negligi-
ble amounts (<5%) by crude NMR spectroscopy. The reac-
m.[1f,5b] Similarly, the less-hindered ortho C H bond was re-
À
placed with an acetoxy group in 3n. Gratifyingly, a moderate
yield of the diacetoxylated product 4g was isolated from the
reaction of 3g. This reaction cleanly delivers two new mole-
cules of mono- and diacetoxylated products. In case of mod-
erate yields of products, the mass-balance can be justified
with the recovery of the precursors.
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Chem. Eur. J. 2012, 18, 5541 – 5545

Sulfoximines as Reusable Directing Group
COMMUNICATION
We next investigated the steric and electronic effects of
sulfoximine derivatives on the ortho acetoxylation of arenes
(Table 3). The electron-rich (S)-methyl-(S)-(4-methylphe-
Table 3. Steric and electronic effects of sulfoximine derivatives on the
ortho acetoxylation of arenes.[a,b]
Scheme 2. Acetoxylation of chiral N-benzoylated sulfoximines.
Finally, the robustness of this reaction was demonstrated
through gram-scale ortho acetoxylations of arenes. Thus, the
reactions of 2l and (S)-2l were independently performed
under the optimized catalytic conditions and the products
3l[14] and (S)-3l were obtained in 83 and 87% yield, respec-
tively [Eq. (1)].
To show the practical utility of this strategy, we examined
the reusability of the directing group. Thus, hydrolysis of the
N-benzoyl moiety of 3 was investigated (Table 4). Unfortu-
nately, the N-benzoyl moiety of 3a was not cleaved under
base-induced hydrolysis.[15,16a] However, hydrolysis of 3a
with HCl (12 n) at 808C was found successful and the de-
sired 2-hydroxy-5-methyl benzoic acid (5a) was extracted in
79% yield (entry 1). Neutralization of the acidic mother
liquor, followed by standard work-up, delivered the (S)-
methyl-(S)-phenylsulfoximine (1a) in good yield. The reac-
tion of crude 1a afforded 2a efficiently. Similarly, the corre-
[a] Reaction conditions:
2 (1.0 mmol), PdACTHUNGTNRUEGN(OAc)2 (10 mol%), K2S2O8
(2.0 mmol), and AcOH/CHCl3 (3:5, 8.0 mL). [b] Isolated yields. [c] Start-
ing material recovered. [d] Formation of mono-deacetylated product.
[e] 2v (100 mg, 0.47 mmol) was used.
nyl)sulfoximine imparts slightly lower efficiency, resulting in
3o in 83% yield compared to 3l (95% yield, Table 2). Poor
reactivity was noticed in the case of (S,S)-diphenyl-substitut-
ed sulfoximine 2p. The bulkiness on the sulfur atom retards
the reaction efficiency: the (S)-ethyl-substituted sulfoximine
2q shows identical reactivity in comparison to 3a and deliv-
ered 3q in 65% yield, whereas the (S)-isopropyl-substituted
sulfoximine gave 3r in poor yield. Moreover, a similar trend
of selectivity and reactivity was observed for the ortho-ace-
toxylation products of N-benzoylated (S,S)-dimethylsulfox-
Table 4. Reusability of the sulfoximine directing group.[a]
ACHTUNGTRENNUNGimines 3s–u compared to 3g, 3a, and 3l, respectively. The
cyclic (S,S)-tetramethylenesulfoximine showed analogous re-
activity with respect to the (S)-methyl-(S)-phenylsulfoximine
and gave 3v in 85% yield. Based on these experimental re-
sults, (S)-methyl-(S)-phenylsulfoximine is conceived as a po-
tential directing group for the ortho acetoxylation of arenes.
Sulfoximines serve as potential chiral ligands for asym-
metric synthesis.[11d,13] Therefore, we examined the effect of
catalytic conditions to the acetoxylation of chiral N-benzoy-
lated sulfoximines. Gratifyingly, the acetoxylation of (S)-2k
and (S)-2l gave (S)-3k[14] and (S)-3l with >99% ee in 43
and 89% yield, respectively (Scheme 2). This observation
reveals that the stereointegrity of the sulfoximine moiety is
preserved in this transformation. We believe that this strat-
egy would create a wide array of new optically active sulfox-
imines bearing complex molecules in an efficient manner.[13]
Entry
3
Yield of 5 [%][b] Yield of 1a [%][c] Yield of 2 [%][d]
1
2
3
4
5
3a
3g[e]
3k
3m
(S)-3l
79
87
71
74
61
73
85
87
84
73
68
92
75
89[f]
82[f]
[a] Reaction conditions: 3 (100 mg), HCl (12 n, 5.0 mL), 808C, 48 h.
[b] Extracted from the crude reaction mixture. [c] Isolated yields of 1a.
[d] Isolated yields of the corresponding product
(1.0 mmol) was used. [f] Yield of the corresponding decarboxylated prod-
uct 6.
2 from 1a. [e] 3g
Chem. Eur. J. 2012, 18, 5541 – 5545
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www.chemeurj.org
5543

A. K. Sahoo et al.
sponding ortho-hydroxybenzoic acids 5g and 5k were ob-
tained from 3g and 3k, respectively, and 1a was successfully
recovered and reused (entries 2 and 3). Surprisingly, the
ortho-substituted ortho’-acetoxylated compounds 3m and
(S)-3l underwent decarboxylation during hydrolysis and the
corresponding 3,4-dimethylphenol (6m) and 3-methylphenol
(6l) were isolated in 89 and 82% yield, respectively (en-
tries 4 and 5). Following this method, the so far difficult
meta-substituted phenols can be synthesized.[16b] Interesting-
ly, the configuration of the chiral sulfoximine 1a was re-
tained in the acid-mediated hydrolysis of (S)-3l; (S)-1a was
obtained in 85% yield with >99% ee (entry 5). Looking
into the effective cleavability and reusability of the sulfox-
The direct oxidative functionalization of unactivated C-
3
[20]
À
AHCTUNGTREG(NNUN sp ) H bonds remains challenging. Having succeeded in
À
the selective ortho C H acetoxylation of arenes, we ex-
À
plored the present catalytic conditions for the C H acetoxy-
lation of a methyl group. To our surprise, the desired acetox-
ylation product 8[14] was obtained from 7, albeit in moderate
yield [Eq. (2)].
ACHTUNGTRENNUNGimine directing group, we hope that this strategy would be
useful in fabricating complex phenol derivatives of pharma-
ceutical interest.
Based on the previous findings on Pd-catalyzed oxidative
In conclusion, we have shown that the use of sulfoximines
as a directing group exemplifies a novel approach to the
chemo- and regioselective ortho acetoxylation of arenes.
Even though our initial results are not comparable with the
À
C H functionalizations of arenes with K2S2O8 as oxidant,
the proposed catalytic cycle is likely to proceed involving
a PdII/IV species, as shown in Scheme 3. Interestingly, hetero-
approach reported by Yu for the C H oxidation of aromatic
À
A
carboxylic acids, we hope that the sulfoximine directing
group would contribute to the development of new and
foximines have the ability to coordinate the Pd species.
Owing to the oxidation state of the S atom (+VI) and the
polarization of the S=O bond in sulfoximines, the coordina-
tion of S to PdII can be excluded. Similarly, the chelation of
the O atom of the polarized S=O bond is also believed un-
likely, because it involves a seven-membered transition
state. The spectroscopic and theoretical studies revealed
that the conjugate delocalization of the nitrogen lone pair is
À
useful C H functionalizations. Notably, the stereointegrity
of chiral sulfoximines is preserved. The facile attachment
and detachment of the robust sulfoximine moiety makes it
highly reusable. Realization of milder reaction conditions
2
3
À
for sp and sp C H functionalizations, unraveling of mecha-
nistic details, generation of axial chirality through asymmet-
À
ric C H activation, and exploration of novel synthetic appli-
À
prohibited, owing to its orthogonal orientation to the N
cations are being actively pursued in our laboratory.
C(O) bond.[17] Among the nitrogen and carbonyl-oxygen
atom, we presume that the chelation of N to PdII is more
susceptible due to the localized electron density.[18] Howev-
er, the coordination of the carbonyl-oxygen atom cannot be
completely ruled out. Thus, the first step in the catalytic
cycle involves the chelation of N to PdII, followed by palla-
Acknowledgements
We acknowledge the DST (Grant No. SR/S1/OC-34/2009), the University
of Hyderabad, and ACRHEM for support. M.R.Y and R.K.R thank
CSIR, India for fellowships. We thank P. Sanpui for the X-ray crystallo-
graphic analysis. We sincerely thank Prof. T. P. Radhakrishnan for valua-
ble suggestions.
À
dation of the ortho C H bond of the arene to produce the
crucial five-membered cyclopalladated intermediate A.
Next, oxidation of the PdII species of A with K2S2O8 in the
presence of AcOH gives the PdIV species B.[19] Finally, re-
ductive elimination of B generates the desired ortho-ace-
toxylated product and the active PdII species for the next
catalytic cycle.
À
Keywords: C H activation · directing group · oxidation ·
regioselectivity · sulfoximines
À
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Scheme 3. Proposed catalytic cycle.
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Sulfoximines as Reusable Directing Group
COMMUNICATION
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Received: January 9, 2012
Published online: March 29, 2012
Chem. Eur. J. 2012, 18, 5541 – 5545
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5545