Rhodium-catalyzed oxidative annulation of sulfoximines and alkynes as an approach to 1,2-benzothiazines

Article abstract of DOI:10.1002/anie.201304456

Revitalization by oxygen: A rhodium(III)-catalyzed oxidative C-H/N-H activation/annulation sequence provided access to a variety of substituted 1,2-benzothiazine derivatives from readily available NH-sulfoximines and alkynes (see scheme; Cp=C₅Me₅). The oxidation system consisted of molecular oxygen in combination with a catalytic amount of Fe(OAc)₂. Copyright

Full text of DOI:10.1002/anie.201304456

Angewandte
Chemie
DOI: 10.1002/anie.201304456
CÀH/NÀH Activation
Rhodium-Catalyzed Oxidative Annulation of Sulfoximines and
Alkynes as an Approach to 1,2-Benzothiazines**
Wanrong Dong, Long Wang, Kanniyappan Parthasarathy, Fangfang Pan, and Carsten Bolm*
Within the last few years, oxidative activation reactions of CÀ strategy for the preparation of 1,2-benzothiazines from
H/NÀH or C-H/OÀH bond combinations with subsequent
sulfoximines and alkynes [Eq. (2)]. We report herein our
success with this approach.
alkyne or alkene annulations have become the synthetically
most useful CÀC and CÀX (X = N, O, etc.) bond-forming
[1]
processes. Various metals proved applicable, and complexes
II [2]
III [3]
II[4]
based on Pd , Rh , or Ru
have been utilized for the
catalytic construction of a wide range of heterocycles. To the
best of our knowledge, 1,2-benzothiazines have not been
among the heterocyclic products, despite their usefulness as
[
5]
scaffolds in drug development and as synthetic intermedi-
[
6]
ates. Harmata and co-workers have introduced the most
advanced approaches towards 1,2-benzothiazines, including
annulations of sulfonimidoyl chlorides with alkynes or
To find suitable reaction conditions, we used the sulfox-
imine 1a and diphenylacetylene (2a) as model substrates and
examined the effect of the rhodium complex, oxidant, and
solvent. The screening revealed that [Cp*Rh(MeCN) ][BF ]
[
7]
alkenes, palladium catalysis in domino reaction sequences
[6,8]
starting with a Buchwald–Hartwig N-arylation,
and Sono-
3
4 2
gashira-type cross-coupling reactions of S-2-bromophenyl-S-
methylsulfoximines with terminal alkynes [Eq. (1); DMF =
was most effective, with the formation of 3a in 89% yield
(Table 1, entry 1). The rhodium complexes [{RhCp*Cl } ],
2
2
[
9]
N,N-dimethylformamide]. None of these 1,2-benzothiazine
syntheses involve the oxidative CÀH/NÀH activation of
[{Rh(OAc) } ], [{RhCl(cod)} ], and [Rh(PPh ) Cl] were less
2 2 2 3 3
simple alkyl aryl sulfoximines.
Table 1: Catalyst screening for the CÀH/NÀH activation of sulfoximine
[
a]
1
a with alkyne 2a.
Entry
Catalyst
[Cp*Rh(MeCN) ][BF ]
Metal salt/oxidant
Yield [%]
1
2
3
4
5
6
7
8
9
Fe(OAc)2
Fe(OAc)2
89
35
trace
–
3
4 2
[{RhCp*Cl } ]
Stimulated by our recent findings on the copper-catalyzed
2 2
[10a]
[{Rh(OAc)
2
} ]
2
Fe(OAc)
2
direct arylation of NÀH sulfoximines with heteroarenes
and the oxidative cross-coupling of NÀH sulfoximines with
[{RhCl(cod)}2]
[Rh(PPh ) Cl]
Fe(OAc)2
Fe(OAc)2
Fe(acac)2
Fe(OTf)2
Cu(OAc)
2
PhI(OAc)2
–
3
3
[
10b]
terminal alkynes
and feeling challenged by the excellent
[Cp*Rh(MeCN) ][BF ]
trace
trace
38
–
3
4 2
III
work of Cramer and co-workers on the Rh -catalyzed
[Cp*Rh(MeCN) ][BF ]
3
3
4 2
oxidative CÀH/NÀH activation of N-acyl sulfonamides to
[Cp*Rh(MeCN)
][BF ]
4
2
[
11]
[Cp*Rh(MeCN) ][BF ]
4 2
afford benzosultams,
we wondered about an analogous
3
1
1
0
1
[Cp*Rh(MeCN) ][BF ]
–
K S O
8
–
–
–
3
4 2
2
2
[
*] W. Dong, L. Wang, Dr. K. Parthasarathy, Prof. Dr. C. Bolm
Institute of Organic Chemistry, RWTH Aachen University
Landoltweg 1, 52056 Aachen (Germany)
[a] Reaction conditions: sulfoximine 1a (0.30 mmol), alkyne 2a
(0.36 mmol), catalyst (5.0 mol%), metal salt/oxidant (20 mol%), tolu-
ene (2.5 mL), 1008C, 48 h. cod=1,5-cyclooctadiene, Cp*=pentame-
thylcyclopentadiene.
E-mail: carsten.bolm@oc.rwth-aachen.de
F. Pan
Institute of Inorganic Chemistry, RWTH Aachen University
Landoltweg 1, 52056 Aachen (Germany)
active (Table 1, entries 2–5). With respect to the oxidation
system, a remarkable discovery was made: Under an atmos-
phere of molecular oxygen (1 atm), simple Fe(OAc)2
[
**] W.D. and F.P. thank the China Scholarship Council for predoctoral
stipends. K.P. acknowledges support by the Alexander von Hum-
boldt Foundation. We also thank Prof. Dr. J. Cheng (Changzhou
University) for helpful discussions and Prof. Dr. U. Englert as well as
Dr. R. Wang (both RWTH Aachen University) for the collection of X-
ray crystallographic data.
(20 mol%) served as an effective mediator to give the desired
product in 89% yield (Table 1, entry 1). To the best of our
knowledge, this oxidant/mediator combination is unprece-
dented. Other iron and copper salts [Fe(acac) , Fe(OTf) , and
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201304456.
2
2
Cu(OAc) ] were less effective (Table 1, entries 6–8), and the
2
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
11573

.
Angewandte
Communications
commonly applied oxidants PhI(OAc) and K S O proved
Table 2: Scope of the rhodium-catalyzed CÀH/NÀH activation of
[a]
2
2
2
8
sulfoximines with alkynes.
inactive (Table 1, entries 9 and 10). When the reaction was
carried out in the absence of the rhodium catalyst and the iron
salt, no conversion occurred (Table 1, entry 11). Details of the
solvent screening are reported in the Supporting Information.
To evaluate the scope of the reaction, we treated
substituted sulfoximines 1b–g with 2a under the optimized
reaction conditions (Table 2). In general, all transformations
proceeded well to afford the corresponding 1,2-benzothia-
zines in 72–89% yield (Table 2, entries 1–7). The reaction of
the meta-substituted sulfoximine 1b revealed high regiose-
lectivity in favor of activation of the less hindered CÀH bond
to provide the two expected regioisomers 3b and 3b’ in
a 90:10 ratio in 88% combined yield (Table 2, entry 2).
Comparison of the results for the conversion of the para-
substituted sulfoximines 1c–f shows that the influence of
electronic effects induced by a methoxy, nitro, or halo group
was minor and that chloro and bromo substituents were well
tolerated (Table 2, entries 3–6). If the S-methyl group (as in
Entry
1
2
Product 3
Yield [%]
89
1
1
2
3
4
5
6
a
b
c
d
e
f
a
a
a
a
a
a
3a: R =H
1
[b]
3b: R =3-Me
88
1
3c: R =4-MeO
86
72
79
81
1
3d: R =4-NO
2
1
3e: R =4-Cl
1
3 f: R =4-Br
1
a–f) was changed to an S-phenyl substituent (as in 1g), the
yield of the resulting 1,2-benzothiazine (in this case 3g) was
essentially unaffected (85%; Table 2, entry 7).
7
g
a
3g
85
Pleasingly, other alkynes also reacted well (as determined
by reactions with 1a as the coupling partner). In the series of
symmetrically substituted diphenylacetylenes, the 4,4’-dime-
thoxy-, 4,4’-difluoro-, and 4,4’-dichloro derivatives all gave the
corresponding products 3h–j in high yields (up to 93%;
Table 2, entries 8–10). When di(2-thienyl)acetylene (2e) was
used, the 1,2-benzothiazine 3k was obtained in 87% yield
3
4
8
9
a
a
a
a
a
a
b
c
d
e
f
3h: R , R =4-MeOC H
93
82
86
87
67
72
6
4
3
4
3i: R , R =4-FC H
6
4
3
4
1
1
0
1
3j: R , R =4-ClC H
6 4
3
4
3k: R , R =2-thienyl
3
4
12
13
3l: R , R =Et
3
4
(
Table 2, entry 11). With aliphatic alkyne substrates, the
g
3m: R , R =nPr
yields were slightly lower, as indicated by conversions of
hex-3-yne (2 f) and oct-4-yne (2g) to provide 1,2-benzothia-
zines 3l and 3m in 67 and 72% yield, respectively (Table 2,
entries 12 and 13).
Unsymmetrically substituted alkynes were expected to
lead to regioisomeric products. Depending on the substitution
pattern, the regioselectivity was either high (up to 95:5) or nil
8
5
1
4
a
a
h
i
(
85:15)
82
(
Table 2, entries 14–18). Thus, 1-phenyl-1-propyne (2h) and
15
(
95:5)
phenyl propiolate (2i) underwent the oxidative annulation
effectively to give the two regioisomeric products (3n/3n’ and
3
o/3o’) in high yields and isomer ratios of 85:15 and 95:5,
respectively (Table 2, entries 14 and 15). The chemical
identity of 3n was established unequivocally by X-ray
crystal-structure analysis (see the Supporting Information
for details). In contrast, no regioselectivity was observed
when the unsymmetrical diaryl acetylenes 2j, 2k, and 2l were
used. Instead, the corresponding regioisomer pairs 3p/3p’,
3
3
[
12]
16
a
a
a
j
3p: R =Ph,
3p’: R =4-MeOC H ,
90
(50:50)
6
4
4
4
R =4-MeOC H
R =Ph
6
4
3
3
17
k
l
3q: R =Ph,
3q’: R =4-NO C H ,
80
(50:50)
2
6
4
3
q/3q’, and 3r/3r’ were obtained in equal amounts in 90, 80,
4
4
R =4-NO C H
R =Ph
2
6
4
and 78% yield, respectively (Table 2, entries 16–18).
[
1,13]
3
3
On the basis of earlier studies,
we deduced a reason-
18
3r: R =Ph,
3r’: R =2-naphthyl,
78
(50:50)
4
4
R =2-naphthyl
R =Ph
able mechanism for the transformation (Scheme 1). The
catalytic cycle is most likely initiated by the transfer of an
[a] Reaction conditions: sulfoximine 1 (0.50 mmol), alkyne 2
(0.60 mmol), [Cp*Rh(MeCN) ][BF (5.0 mol%), Fe(OAc) (20 mol%),
toluene (3.0 mL), 1008C, 48 h. [b] Ratio of regioisomers 3b and 3b’:
0:10.
[14]
acetate group from Fe(OAc) to [Cp*Rh(MeCN) ][BF ] .
3
4
]
2
2
2
3
4 2
The reaction of the resulting rhodium species with the
9
sulfoximine and subsequent activation of an ortho CÀH
bond forms a five-membered rhodacycle II. Coordinative
insertion of alkyne 2 into the rhodium–carbon bond of
intermediate II gives a seven-membered rhodacycle IV.
Finally, reductive elimination affords the 1,2-benzothiazine
I
and a Rh species. The iron salt mediates the reoxidation of
this Rh species by molecular oxygen to regenerate the active
Rh species, which enters the next catalytic cycle.
I
III
1
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a coordinated molecule of sulfoximine 1a. The molecular
structure of V was indirectly confirmed by X-ray crystal-
[
12]
structure analysis of rhodacycle VI, which was obtained by
the treatment of V with NaI in MeOH at room temperature
for 30 min.
To demonstrate that such rhodacycles could be involved in
the catalytic formation of 1,2-benzothiazines, we treated V
with 2a in the presence of Fe(OAc) in toluene at 1008C and
2
obtained 3a in high yield after 2 h. In the same manner, 1,2-
benzothiazine 3a was obtained from rhodacycle VI and 2a in
7
8% yield after 15 h.
Finally, in the search for new synthetic applications of 1,2-
benzothiazines, we discovered an unprecedented oxidative
CÀC bond-cleavage reaction. Thus, the treatment of 3a with
meta-chloroperoxybenzoic acid (mCPBA) in CH Cl at 08C
2
2
and warming to room temperature for 8 h led to ortho-
benzoyl-N-benzoylsulfoximine 4a in 96% yield. 1,2-Benzo-
thiazines 3d and 3 f reacted in an analogous manner to
provide products 4b and 4c, respectively, in high yields
[
Eq. (4)].
Scheme 1. Plausible catalytic pathway for the formation of 1,2-benzo-
thiazines.
The intramolecular kinetic isotope effect (KIE) of kH/
k = 2.22 found by using [D ]1a as a substrate [Eq. (3)]
D
1
indicated that the CÀH bond cleavage was directly involved in
[
15]
the product-determining cyclometalation step.
In summary, we have developed a rhodium-catalyzed
process for the synthesis of 1,2-benzothiazines from NH-
sulfoximines and alkynes. The combination of simple
Fe(OAc) (20 mol%) and O (1 atm) enables metal reoxida-
2
2
tion to close the catalytic cycle. Mechanistically, the trans-
formation is proposed to involve a one-pot CÀH/NÀH
activation followed by an annulation of the alkyne. This
hypothesis was substantiated by KIE studies and the isolation
of a reactive rhodacycle. The method is high-yielding and
suitable for the synthesis of small libraries of functionalized
1,2-benzothiazines.
The proposed intermediacy of a five-membered rhoda-
cycle was substantiated by the isolation of a complex upon the
heating of a mixture of 1a and [Cp*Rh(MeCN) ][BF ] to
3
4 2
1
13
1
008C in toluene for 16 h. H NMR, C NMR, and IR
spectroscopy in combination with mass spectrometry sug-
gested the formation of rhodacycle V (Figure 1), in which L is
Experimental Section
General procedure: The sulfoximine (0.5 mmol), the alkyne
(0.6 mmol),
[Cp*Rh(MeCN) ][BF ]
(13.5 mg,
0.025 mmol,
3
4 2
5
mol%), and Fe(OAc) (18 mg, 0.1 mmol, 20 mol%) were placed
2
in a Schlenk tube (20 mL) with a stirring bar. Under an oxygen
atmosphere (1 atm), dry toluene (3.0 mL) was added, and the
reaction mixture was stirred at 1008C for 48 h and then cooled to
room temperature. The mixture was filtered through a short pad of
Celite and washed with dichloromethane (3 ꢀ 20 mL). The filtrate was
concentrated, and the product was purified by column chromatog-
raphy on silica gel with a mixture of hexanes and ethyl acetate as the
eluent.
Figure 1. Structure of rhodacycles V and VI as well as an ORTEP
diagram of complex VI (thermal ellipsoids are drawn at 50% proba-
bility).
Received: May 23, 2013
Revised: August 8, 2013
Published online: September 6, 2013
[
12]
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.
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Communications
Keywords: alkynes · annulation · 1,2-benzothiazines ·
CÀH activation · sulfoximines
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