Rhodium(iii)-catalyzed oxidative olefination of N-allyl sulfonamides

Article abstract of DOI:10.1039/c3ob40272j

Rhodium(iii)-catalyzed oxidative couplings between N-sulfonyl allylamines and activated olefins have been achieved. Only olefination occurred for acrylates, and the butadiene product can be further cyclized under palladium-catalyzed aerobic conditions. The coupling with N,N-dimethylacrylamide followed a cyclization pathway.

Full text of DOI:10.1039/c3ob40272j

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Rhodium(III)-catalyzed oxidative olefination of N-allyl
sulfonamides†
Shui Hu,a,b Dongqi Wang,b Jiexiang Liua and Xingwei Li*b
Cite this: Org. Biomol. Chem., 2013, 11,
2761
Received 6th February 2013,
Accepted 9th March 2013
DOI: 10.1039/c3ob40272j
www.rsc.org/obc
Rhodium(III)-catalyzed oxidative couplings between N-sulfonyl enamides,20 and acrylamides21 (Scheme 1). Besides the limited
allylamines and activated olefins have been achieved. Only olefi- scope, the Z/E selectivity of these couplings can be low in
nation occurred for acrylates, and the butadiene product can be some cases. When an activated olefin was applied, the initial
further cyclized under palladium-catalyzed aerobic conditions. olefination product may be further complicated by in situ
The coupling with N,N-dimethylacrylamide followed a cyclization Michael cyclization (Scheme 1). Thus it is necessary to expand
pathway.
the scope of the olefins. We have recently reported rhodium(III)-
catalyzed diversified couplings of N-allyl sulfonamides with
internal alkynes under oxidative conditions, leading to the syn-
thesis of pyridines, dihydropyridines, and cyclopentenones.22
These reactions occurred via a C–H activation pathway,
suggesting that sulfonamide is a viable directing group in the
activation of olefinic C–H bonds.23 We now report rhodium(III)-
catalyzed oxidative coupling of N-allyl sulfonamides with acti-
vated olefins.
We initiated our studies with the screening of the con-
ditions for the coupling of N-tosyl allylamine (1a) and methyl
acrylate (Table 1). On the basis of our outcomes in the coup-
ling of 1a with alkynes, acetone was designated as the solvent
(100 °C). It was found that while oxidants such as Cu(OAc)2,
Ag2O, and Ag2CO3 all effected this coupling reaction when
[RhCp*Cl2]2 was used in 5 mol% (Table 1, entries 1–3), AgOAc
proved to be a superior oxidant (entry 4), and product 3a was
isolated in 76% yield. The structure of 3a was elucidated on
Ever since the report of the pioneering work by Fujiwara and
Moritani in 1967,1 oxidative olefination of C–H bonds has
attracted increasing attention. This synthetically important
reaction represents
a step-economic and waste-reducing
process because unreactive C–H bonds are ubiquitous and no
prior functionalization is necessary.2 This strategy has been
applied to the synthesis of many useful complex structures.
Amongst the transition metal catalysts used, palladium com-
plexes have been predominant. While early examples have
shown that ruthenium(II)3 and rhodium(III)4 complexes can be
active catalysts in this transformation, studies using these
metals lag behind. It is only in recent years that rhodium(III)5
and ruthenium(II)6 species came to be realized as highly active
catalysts for this purpose. In particular, rhodium(III) catalysts
readily effect the olefination of a large scope of arenes with
high selectivity and functional group compatibility. Thus
simple arenes7 and arenes with heteroatom directing groups
such as hydroxyl,8 amide,9 pyridyl,10 carbamate,11 imine12 and
carboxyl13 have been olefinated under operationally simple
conditions.14
While oxidative olefination of arenes has been extensively
explored, the oxidative C–H/C–H couplings of two olefins have
gained less attention.15 Pioneered by Gusevskaya,16 Ishii,17
and Loh,18 palladium catalysis still dominates in olefin–olefin
oxidative couplings. Rh(III)-catalyzed such couplings have been
reported, but the substrates are limited to acrylic acids,19
aSchool of Chemical Engineering, Hebei University of Technology, Tianjin 300130,
PR China
bDalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023,
PR China. E-mail: xwli@dicp.ac.cn
†Electronic supplementary information (ESI) available: Synthetic procedures
and characterization data. See DOI: 10.1039/c3ob40272j
Scheme 1 Rh(III)-catalyzed olefin–olefin coupling.
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Table 1 Optimization studies
Entry
Catalyst
Loading (mol%)
Oxidant
Yielda (%)
1
2
3
4
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
5
5
5
5
4
3
4
4
Cu(OAc)2
Ag2CO3
Ag2O
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
42
53
nd
76
75
60
72
78
5
6
7b
8c
Conditions: N-tosyl allylamine (0.3 mmol), methyl acrylate
(0.45 mmol), [RhCp*Cl2]2 (3–5 mol%), oxidant (2.1 equiv.), acetone
(3 mL), 100 °C, 24 h, sealed tube under argon. a Isolated yield. b 4.5
equiv. of AgOAc was used. c 36 h.
the basis of 1H and 13C NMR analyses. The isolated yield of 3a
stayed essentially the same when the catalyst loading was
lowered to 4 mol% (entry 5). However, further decrease of the
loading to 3 mol% caused an appreciable decrease of the cata-
lytic efficiency (entry 6). Using a prolonged reaction time or an
excess of AgOAc all failed to significantly improve the yield of
3a (entries 7–8). Thus the conditions given in entry 5 were
deemed optimal.
Scheme 2 Oxidative olefination of N-sulfonyl allylamines.a,b
The scope and limitations of this reaction were next
explored by following the optimized conditions (Scheme 2). A
series of acrylates coupled with sulfonamide 1a in moderate to
good yield and the product was obtained as essentially a single
stereoisomer (>95 : 5) on the basis of 1H NMR analysis (3aa–
3ad). Introduction of a methyl to the 2-position of the allyl
group is well-tolerated, and coupling products 3ba–3bd were
isolated in comparable yield. In contrast to the high stereo-
selectivity for acrylates, the coupling between 1a and acrylonitrile
afforded a mixture of two isomeric (Z,Z) and (Z,E) dienes in
70% isolated yield and in 9 : 1 ratio, where the acrylonitrile
moiety prefers to adopt a (Z) geometry. This selectivity
dropped to 3.9 : 1 when an N-methallyl sulfonamide substrate
was used, although the total yield of the product stayed essen-
tially the same. The sulfonamide directing group is not limited
to NHTs; both electron-donating (3gb) and -withdrawing
groups (3db, 3eb and 3fb) in the phenyl ring of the sulfona-
mide can be tolerated, although the isolated yields of these
products are generally lower than those obtained from 1a. In
contrast, when an Ms directing group was applied, no desired
coupling reaction occurred, indicative of the limitation of this
reaction. In addition, the olefin coupling partner is limited to
an activated olefin because no reaction occurred for styrene
even under more harsh conditions.
was obtained, indicating that C–H activation (cyclometalation)
is involved in the turnover-limiting step. This KIE value is also
consistent with that obtained in the Rh(III)-catalyzed oxidative
coupling of 1a and diphenylacetylene.24
ð1Þ
ð2Þ
To better define the substrate scope, N,N-dimethylacryla-
mide was allowed to couple with 1a under the standard con-
ditions. To our surprise, no analogous butadiene could be
isolated, and the major product obtained (28%) was a 2,3-dihy-
dropyrrole with an exo-cyclized CvC bond (4a), as a result of
twofold oxidation. Under these catalytic conditions, no isomeri-
zation of the exo-cyclic CvC was detected. Optimization by
simply using an excess of AgOAc (4.2 equiv.) increased the
yield of 4a to 51%. Thus other direct analogues such as 4c and
To probe the mechanism of this reaction, the kinetic
isotope effect was studied for the cleavage of the C–H bond of
1a in the competitive coupling of ethyl acrylate with an equi-
molar mixture of 1a and 1a-d2 (eqn (1)). A value of kH/kD = 4.2
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attempted but failed to prepare 5a via palladium-catalyzed one
pot aerobic oxidation starting from 1a and ethyl acrylate, indi-
cating that palladium catalysis failed at the oxidative olefina-
tion stage. It is noteworthy that although 5a,b and 4a,c,e are at
the same oxidation level, they differ in connectivity. Achieving
molecular diversity and complementary selectivities via sub-
strate control and condition control constitutes an important
task in synthesis.
ð3Þ
Scheme 3 Proposed catalytic cycle.
Interestingly, when we applied these aerobic oxidative con-
ditions to a mixture ((Z,Z) : (Z,E) = 3.9 : 1) of 3be, a thermodyna-
4e were obtained in similar yields (eqn (2)). Shifts of reaction mically stable pyrrole 6 was obtained in 58% yield (eqn (4)),
selectivity when moving from acrylate to acrylamide substrates where the (Z,E) isomer seems to give a higher reactivity
have been reported in previous systems,25 and this is undoubt- because only the (Z,Z) isomer remained after the reaction,
edly caused by the electronic effects of the olefin. In addition, which was recovered in 32% yield.
the relatively high donating capacity of arylamides can also
make a difference. Notably, palladium-catalyzed aerobic oxi-
dative coupling of N-sulfonyl allylamine with olefins has been
ð4Þ
reported by Stahl,26 but onefold oxidation products were
obtained.
While no solid mechanistic studies have been performed, a
In summary, we have demonstrated the oxidative olefina-
plausible mechanism to account for the formation of 4a–e has
tion of N-sulfonyl allylamines using activated olefins. Simple
been proposed on the basis of previous reports (Scheme 3). A
olefination occurred for acrylates to give a butadiene deriva-
rhodium(III) amidate was proposed to be an active intermedi-
tive. This butadiene can further undergo palladium catalyzed
ate. In line with Stahl’s proposal,26 this rhodium amidate
aerobic oxidative cyclization, leading to a 2,5-dihydropyrrole
species rather undergoes a Rh–N bond insertion into an
with an exo-cyclic double bond. N,N-Dimethylacrylamide fol-
incoming acrylamide to give a rhodium(III) alkyl intermedi-
lowed a twofold oxidation pathway. These observed selectivities
ate.27 Subsequent insertion of the Rh–C bond into the termi-
complement that reported in palladium-catalyzed aerobic oxi-
nal olefin yields a rhodium(III) primary alkyl intermediate.
dative coupling reactions between these two substrates, and
β-Hydrogen elimination affords an exo-cyclized pyrolidine
the different selectivities in olefination reactions and in sub-
intermediate and further oxidation by Ag(I) furnishes the final
sequent transformations likely render this method useful in
product.
the synthesis of complex structures. Exploration of other novel
To better demonstrate the synthetic utility of these buta-
directing groups for C–H activation is underway in our
diene products, further transformations of 3 have been per-
laboratory.
formed. Clearly, the current conditions failed to allow any
(oxidative) cyclization of these butadienes. In contrast, palla-
dium catalysis has been well studied for aerobic oxidation reac-
tions leading to C–N couplings.28 Therefore palladium-
catalyzed intramolecular oxidative C–N coupling for 3 was per-
formed. Gratifyingly, smooth cyclization of 3ad and 3bd to 5a
and 5b, respectively, was achieved using a Pd(OAc)2 (5 mol%)–
Acknowledgements
We thank the NSFC and the Dalian Institute of Chemical
Physics, Chinese Academy of Sciences, for financial support of
this work.
pyridine (10 mol%) catalyst under O2 (balloon pressure) (eqn
(3)). On the basis of 1H and 13C NMR analyses, 5a and 5b were
identified as a 2,5-dihydropyrrole bearing an exocyclic CvC
bond. In particular, in the 13C NMR spectrum (CDCl3) of 5a
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the intermediacy of palladium alkyl A (eqn (3)).29 We also
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