Copper(I) halide mediated tandem 1,4-aryl migration-oxidative amidyl radical cyclisation of bromosulfonamides

Article abstract of DOI:10.1055/S-0029-1219151

Reaction of N-butyl-N-(2-bromo-2-methylpropionyl)-arylsulfonamides with CuBr/tripyridylamine leads to amidyl radicals via 1,4-aryl migration with concomitant loss of SO₂, which can further undergo cyclisation to oxindoles or reduction to amides

Full text of DOI:10.1055/S-0029-1219151

610
LETTER
Copper(I) Halide Mediated Tandem 1,4-Aryl Migration–Oxidative Amidyl
Radical Cyclisation of Bromosulfonamides
1
, 4-Aryl Migration
n
–Oxidative
5-
exo-Amidy
r
l
R
adica
e
l
Cyclisatio
w
n
J. Clark,* David R. Fullaway, Nicholas P. Murphy, Hemal Parekh
Department of Chemistry, University of Warwick, Coventry, West Midlands, CV4 7AL, UK
Fax +44(2476)524112; E-mail: a.j.clark@warwick.ac.uk
Received 8 October 2009
Dedicated to Prof. G. Pattenden on his 70th birthday.
onamide 1b at various temperatures and in various sol-
Abstract: Reaction of N-butyl-N-(2-bromo-2-methylpropionyl)-
arylsulfonamides with CuBr/tripyridylamine leads to amidyl radi-
cals via 1,4-aryl migration with concomitant loss of SO2, which can
further undergo cyclisation to oxindoles or reduction to amides with
the ratio dependent upon the temperature and solvent utilised.
vents (Table 1). We initially reacted compound 1b with
1.1 equivalents of CuBr and 1.1 equivalents of ligand 7 at
room temperature. The reaction was extremely slow due
to the insolubility of the CuBr–7 complex in both toluene
(entry 3) and CH2Cl2 (entry 1), but when heated to solubi-
lise the complex, two products were isolated, namely the
1,4-aryl rearrangement product 3b and the oxindole 8b
(entries 2 and 4). The oxindole 8b was unexpected, but its
structure was unambiguously confirmed via the synthesis
of an authentic sample and the lack of any sulfur content
as determined by elemental analysis.9
Key words: copper, radical reactions, rearrangement, tandem reac-
tions, sulfonamides
We recently reported that trichloroacetamide1 1a and 2-
bromo-2-methyl-propionamide2 1b undergo reaction with
copper(I) halides and pentamethyldiethylenetriamine 4 to
furnish 2-aryl amides 3a,b via 1,4-aryl migration. At re-
flux, minor amounts of competing halide reduction to give
2a,b were also observed but levels only became signficant
at room temperature (2a/3a = 61:39, 2b/3b = 53:47,
Scheme 1).1,2 Aryl transfer from sulfonamides with loss
of SO2 during radical reactions is well established and a
range of migration types including 1,4- and 1,5-aryl mi-
grations have been described.3 The majority of these pub-
lished procedures involve reactions mediated by toxic
organostannane reagents under high-dilution conditions.
Me
Me
N
Me
N
N
N
N
45
Me
Me
N
N
Me
N
N
78
N
N
Me
Me
Me
56
67
Figure 1 Common ligands used in copper-mediated ATRC reac-
tions
X
H
R
R
R
R
R
R
CuX, 4
+
CH2Cl2
reflux
SO2
SO2
Contrary to the reaction of 1a with CuBr–72 no reduction
to 2b was detected in the NMR spectrum of the crude ma-
terial (entry 1). Mechanistically 3b and 8b (Scheme 2) are
likely to be formed via a 5-exo-ipso substitution of radical
9 (X = H) to give spirocyclohexadienyl radical 10 which
upon re-aromatisation and loss of SO2 furnishes the
amidyl radical 11 (X = H).10,3h Radical 11 is partioned be-
tween two competing reaction pathways (a bimolecular
reduction to give 3b or an intramolecular cyclisation to
give 12). Reduction of 11 furnishes the observed 1,4-aryl
transfer product 3b, while the oxindole 8b may originate
via radical cyclisation of the amidyl radical 11 (X = H) to
give the cyclohexadienyl radical 12. Oxidation of this rad-
ical 12, mediated by the CuBr2 formed in situ from the ini-
tial step 1b → 9, followed by elimination of a proton
provides the observed oxindole product 8b.11,12 No six-
membered cyclic sulfonamide 15 was isolated. This is in
contrast to Bu3SnH-mediated reactions of related sulfon-
amides where cyclic sulfonamide intermediates were de-
tected.3 Interestingly, the ratio of products seems to be
O
NHBu
3a,b
O
N
O
N
Bu
Bu
2a,b
1a (R = Cl, X = Cl)
1b (R = Me, X = Br)
40 °C = 49%, 2a/3a = 7:93
40 °C = 49%, 2b/3b = 5:95
Scheme 1 Reaction of amides 1a,b with CuX–4
Conventional copper(I)-mediated atom-transfer radical
cyclisations of haloacetamides have been thoroughly in-
vestigated and a range of ligands (e.g., 4–7) have been re-
ported to facilitate these types of transformations
(Figure 1).4–8 We previously screened ligands 4 and 7 in
the 1,4-aryl transfer of trichloroacetamide 1a and discov-
ered that ligand 4 was optimal for production of 3a.1 In
this paper we report the affect of ligand 7 in the 1,4-aryl
transfer reaction of the related 2-bromo-2-methyl-propi-
SYNLETT 2010, No. 4, pp 0610–0614
Advanced online publication: 22.12.2009
DOI: 10.1055/s-0029-1219151; Art ID: D28709ST
0
1.
0
3.
2
0
1
0
© Georg Thieme Verlag Stuttgart · New York

LETTER
1,4-Aryl Migration–Oxidative 5-exo-Amidyl Radical Cyclisation
611
Ar
3b (Ar = Ph)
3c (Ar = 4-tol)
2b
O
NHBu
•
•
H
H
X
X
X
X
Br
•
5-exo
ipso
•
?
– SO2
CuBr2
CuBr
•
O
O
SO2
•
SO2
SO2
7
7
N
N
O
N
X
X
N
O
N
NBu
O
Bu
Bu
O
Bu
Bu
10
Bu
13
12
11
1b X = H
9
1c X = Me
X
X
X
•
O
– SO2
[O]
O
N
N
SO2
SO2
X
Bu
Bu
O
N
O
N
Bu
Bu
16b (X = H)
16c (X = Me)
8b (X = H)
8c (X = Me)
15
14
Scheme 2 Possible mechanism for the formation of oxindole 8b
was poorer than for the other solvents, while both H2O and
MeCN led to byproducts and consequently more complex
reaction mixtures. Increasing the reaction time from 18
hours to 48 hours (compare entries 5 and 6) had little in-
fluence on the product ratio. Although we postulated that
oxindole 8b was likely formed via cyclisation of 11 (11 →
12 → 13 → 8b) we could not rule out its formation from
extrusion of SO2 from the undetected cyclic sulfonamide
15 (i.e., 15 → 16b where 16b = 8b).13 In order to probe
this further we prepared the 4-tolyl derivative 1c2 and in-
vestigated its reaction with CuBr–7 under two sets of con-
ditions (CH2Cl2, 40 °C, 18 h and toluene, 110 °C, 24 h).
By analogy to 1b the expected products would be the 2-
aryl amide 3c and the 6-methyl oxindole 8c (assuming
pathway 11 → 12 → 13 → 8c) or 5-methyl oxindole 16c
(assuming pathway 9 → 14 → 15 → 16c). We also pre-
pared authentic samples of both of these oxindoles 8c and
16c via an intramolecular Friedel–Crafts acylation of the
Table 1 Effect of Solvent and Temperature on the Reaction of 1a
Bu
N
H
CuBr, 7
N
+
1b
O
Ph
18 h
O
3b
8b
Entry Solvent
Temp
(°C)
Conv
(%)
Mass balance Ratio
(%)
65
78
100
83
72
66
54
51
74
3b/8ba
1.0:0.0b
1.4:1.0
1
2
3
4
5
6
7
8
9
CH2Cl2
CH2Cl2
toluene
toluene
toluene
toluene
THF
r.t.
40
33
100
0
c,d
r.t.
50
–
12
1.0:1.0
0.3:1.0
0.2:1.0e
2.0:1.0
2.0:1.0f
1.0:0.0g
110
110
50
100
100
88
1
corresponding amides 17 and 18 (Scheme 3) and key H
NMR and 13C NMR data are presented.14 As observed for
compound 1b the nature of the solvent and the tempera-
ture at which the reaction was carried out affected the ratio
of products.15 Thus, the amide 3c was the major product
when 1c was heated at 40 °C in CH2Cl2 over 18 hours (1c/
8c = 1.2:1.0, 55%) while the oxindole 8c was the major
product at 110 °C in toluene for 24 hours, (1c/
8c = 0.6:1.0, 94%), this parallels the reactivity of 1b (see
entries 2 and 5, Table 1). Comparison of the spectral de-
tails of the oxindole produced with authentic samples of
8c and 16c unambiguously indicated that the product was
6-methyl oxindole 8c providing further evidence for the
proposed mechanistic pathway 10 → 8c.
H2O
50
100
100
MeCN
50
a Ratio determined by 1H NMR (300 MHz) of the crude mixture.
Reaction time 18 h.
b Reaction time = 168 h.
c Not measured due to low conversion.
d Reaction time = 96 h.
e Reaction time = 48 h.
f 5% of mass balance contained reduced product 2b.
g Only 10% of 3b was isolated. The rest of the mass balance (64%)
was uncharacterised material.
solvent and temperature dependent (changing the solvent
from CH2Cl2 to toluene and increasing the temperature
leads to relatively more oxindole 8b, compare entries 1
and 2 and entries 4 and 5). The use of THF marginally
favoured the 1,4-aryl-transfer product 3b but conversion
The generation of amidyl radicals by the loss of SO2 from
sulfonamides has recently been reported by Zard16 and
amidyl radical cyclisations are well known in the
literature17 with the majority of procedures involve 4-
Synlett 2010, No. 4, 610–614 © Thieme Stuttgart · New York

612
A. J. Clark et al.
LETTER
between between CuBr–4 and CuBr–7 in atom-transfer
radical cyclisation reactions. In particular, radical reac-
tions that use amine ligands (e.g. CuBr–4 or CuBr–6),
generally lead to greater reduction of intermediate radi-
cals than those derived from pyridine ligands (e.g. CuBr–
5 or CuBr–7).3b The reason for this observation is not
clear, but the source of the ‘H’ mediating the reduction
was proposed to originate from the ligand (e.g.. 4 or 6) it-
self.3b Thus, the rate of reduction of 11 is likely to be faster
with CuBr–4 than with CuBr–7 explaining why no oxin-
dole is formed with the former ligand. In addition, the re-
dox potential of CuBr–4 and CuBr–7 complexes are
sufficiently different26 that the rate of the oxidation step
12 → 13 is also likely to be ligand dependent.
In conclusion, we have shown that reaction of N-butyl-
N-(2-bromo-2-methylpropionyl)arylsulfonamides 1b–h
with CuBr–7 leads to amidyl radicals via 1,4-aryl migra-
tion with concomitant loss of SO2 which can further un-
dergo cyclisation to oxindoles 8b–h or reduction to
amides 3b–h with the ratio dependent upon the solvent
and temperature employed.
7.03
6.75
Br
AlCl3
O
7.04
N
O
N
Bu
Bu
17
16c
7.08
Br
O
AlCl3
6.86
6.81
O
O
N
N
7.14
N
Bu
Bu
8c
6.68
Bu
19
6.71
18
Scheme 3 Synthesis of regioisomeric oxindoles 8c and 16c and re-
presentative 1H NMR data (ppm)
exo18 or 5-exo19–24 cyclisations onto alkenes. Cyclisation
of amidyl radicals directly into aromatic groups is not so
well established and only reduction was reported for the
structurally related amidyl radical 20.25
X
O
1d X = F
Br
1e X = Br
1f X = I
NMe
Table 2 Effect of Substituent X on the Reactions of 1d–h
1g X = CN
1h X = OMe
SO2
Bu
X
O
N
N
CuBr / 7
H
Bu
20
1d–h
N
+
O
Ar
Figure 2 Structures of amidyl radical 20 and sulfonamides 1d–h
O
3d–h
8d–h
We next briefly investigated the effect of changing the
electronic nature of the para substituent (Figure 2). The
synthesis of these substrates has been described previous-
ly.2 Treating each substrate with 1 equivalent of CuBr–7
in either CH2Cl2 or toluene led to both the amides 3d–h
and the oxindoles 8d–h in varying ratios (Table 2). In all
cases, the rates of reaction were faster than for the parent
compound 1b. The relative order of the rate of reaction (in
CH2Cl2) was found to be 1b = ca. 1c < 1d = ca. 1e = ca. 1f
< 1g = ca. 1h indicating both electron-donating and -with-
drawing groups accelerated the transformation. The inter-
mediate cyclohexadienyl radical 10 will be stabilised by
both electron-donating and electron-withdrawing substit-
uents which should favour cyclisation and is reflected in
this rate. The reactions carried out in toluene took longer
Compd Solvent
Temp
(°C)
Time
(h)a
Yield
(%)b
Ratio of
3d–h/8d–hc
1d
1d
1e
1f
CH2Cl2
toluene
CH2Cl2
CH2Cl2
CH2Cl2
toluene
CH2Cl2
toluene
40
110
40
7
18
7
59
45
66
80
53
66
73
45
1.4:1.0
0.5:1.0
2.4:1.0
2.7:1.0
4.0:1.0
1.3:1.0
4.0:1.0
1.0:1.0f
40
8
1g
1g
1h
1h
40
4
110
40
18
18d
18e
80
to reach completion, and this may be reflected in the rela- a Time taken to reach 100% conversion.
b Combined yield of 3d–h and 8d–h.
tive solubilities of CuBr–7 in both solvents. As before,
c Ratio determined by 1H NMR (300 MHz) of the crude mixture.
d The reaction had proceeded to 100% conversion in 3 h.
e Under indentical conditions the reaction of 1c proceeded to 56%
conversion.
changing the conditions from CH2Cl2 (40 °C) to toluene
(110 °C) increased the relative proportion of the oxindole
8 in the mixture (Table 2). In CH2Cl2, the main product is
always the amide 3b–h with the ratio of the two products
3b–h/8b–h increasing from 1.2:1.0 to 4.0:1.0. The order
of this increase being similar to that reflected in the rela-
tive rates of the reactions, 1c = ca. 1b = ca. 1d < 1e = ca.
1f < 1g = ca. 1h. We previously reported that when 1b–d
are reacted with a different copper complex (CuBr–4) no
oxindole 8b–d is detected and 3b–d is formed.2 This indi-
cates that the relative ratio of amide 3b,c to oxindole 8b,c
is temperature, solvent, and ligand dependent. A differ-
ence in reactivity and selectivity has also been observed
f A minor uncharacterised product (5%) was also isolated.
Acknowledgment
We acknowledge the EPSRC for DTA studentships (N.P.M.) and
for a project studentship EP/F016573 (H.P.).
References and Notes
(1) Clark, A. J.; Coles, S. R.; Collis, A.; Debure, T.; Guy, C.;
Murphy, N. P.; Wilson, P. Tetrahedron Lett. 2009, 50, 5609.
Synlett 2010, No. 4, 610–614 © Thieme Stuttgart · New York

LETTER
1,4-Aryl Migration–Oxidative 5-exo-Amidyl Radical Cyclisation
(11) For a review on aromatic homolytic substitution, see:
613
(2) Clark, A. J.; Coles, S. R.; Collis, A.; Fullaway, D. R.;
Murphy, N. P.; Wilson, P. Tetrahedron Lett. 2009, 50, 6311.
(3) (a) Loven, R.; Speckamp, W. N. Tetrahedron Lett. 1972, 13,
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Tetrahedron Lett. 1977, 18, 635. (d) Köhler, J. J.;
Speckamp, W. N. J. Chem. Soc., Chem. Commun. 1978,
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Commun. 1980, 142. (f) Clive, D. L. J.; Boivin, T. L. B.
J. Org. Chem. 1989, 54, 1997. (g) Motherwell, W. B.;
Pennell, A. M. K. J. Chem. Soc., Chem. Commun. 1991,
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Ujjainwalla, F. Tetrahedron Lett. 1997, 38, 137.
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Synlett 1999, 441.
(13) Godfrey, C. R. A.; Hegarty, P.; Motherwell, W. B.;
Ujjainwalla, F. Tetrahedron Lett. 1998, 39, 723.
(14) 2-Bromo-N-butyl-2-methyl-N-4-tolylpropanamide (1.50 g,
4.8 mmol) was added to anhyd AlCl3 (1.62 g, 12.1 mmol)
under a stream of nitrogen. The mixture was heated at 50 °C
for 10 min and then maintained at 160 °C for 1 h. The
mixture was washed with H2O (5 × 50 mL) and extracted
with Et2O, dried over MgSO4, and evaporated to give 16c
(0.45 g, 41%) after chromatography (PE–EtOAc, 10:1);
Rf = 0.40 (PE–EtOAc, 10:1). IR: nmax = 2962, 2929, 2865,
1705, 1619, 1599, 1493, 1381, 1351, 1192 cm–1. 1H NMR
(400 MHz, CDCl3): d = 7.04 (1 H, d, J = 8.0 Hz), 7.03 (1 H,
s), 6.75 (1 H, d, J = 8.0 Hz), 3.69 (2 H, t, J = 8.0 Hz), 2.34 (3
H, s), 1.65 (2 H, quin, J = 8.0 Hz), 1.35 (2 H, sext, J = 8.0
Hz), 1.35 (3 H, s), 0.94 (3 H, t, J = 8.0 Hz). 13C NMR (75.5
MHz, CDCl3): d = 181.3, 139.7, 136.1, 131.7, 127.7, 123.3,
108.1, 44.1, 39.6, 29.5, 24.5, 21.1, 20.1, 13.8. ESI-HRMS:
m/z calcd for Na+C15H21NO: 254.1515; found [Na+]:
254.1522.
(i) da Mata, M. L. E. N.; Motherwell, W. B.; Ujjainwalla, F.
Tetrahedron Lett. 1997, 38, 141. (j) Bonfand, E.; Forsland,
L.; Motherwell, W. B.; Vázquez, S. Synlett 2000, 475.
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1999, 40, 4885. (b) Clark, A. J.; De Campo, F.; Deeth, R. J.;
Filik, R. P.; Gatard, S.; Hunt, N. A.; Lastécouères, D.;
Thomas, G. H.; Verlac, J.-B.; Wongtap, H. J. Chem. Soc.,
Perkin Trans. 1 2000, 671. (c) Clark, A. J.; Geden, J. V.;
Thom, S.; Wilson, P. J. Org. Chem. 2006, 71, 1471.
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Washiyama, M.; Itoh, K. J. Org. Chem. 1993, 58, 464.
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Felluga, F.; Pagnoni, U. M.; Valentin, E.; Mucci, A.;
Bellesia, F. Synthesis 2008, 3131. (b) Ghelfi, F.; Stevens,
C. V.; Laureyn, I.; Van Meenen, E.; Rogge, T. M.;
De Buyck, L.; Nikitin, K. V.; Grandi, R.; Libertini, E.;
Pagnoni, U. M.; Schenetti, L. Synthesis 2007, 1882.
(c) Bellesia, F.; Daniel, C.; De Buyck, L.; Galeazzi, R.;
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A. F.; Roncaglia, F. Tetrahedron 2006, 62, 746. (d) Bryans,
J. S.; Chessum, N. E. A.; Huther, N.; Parsons, A. F.; Ghelfi,
F. Tetrahedron 2003, 59, 6221. (e) Cagnoli, R.; Ghelfi, F.;
Pagnoni, U. M.; Parsons, A. F.; Schenetti, L. Tetrahedron
2003, 59, 9951. (f) Ghelfi, F.; Parsons, A. F. J. Org. Chem.
2000, 65, 6249. (g) Ghelfi, F.; Bellesia, F.; Forti, L.;
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Tetrahedron 1999, 55, 5839. (h) Benedetti, M.; Forti, L.;
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41, 14031.
Data for the Mixture of 8c and 19
IR: nmax = 2963, 2931, 2871, 1707, 1606, 1462, 1383, 1343,
1223 cm–1. ESI-HRMS: m/z calcd for Na+C15H21NO:
254.1515; found [Na+]: 254.1519.
Data for 19
1H NMR (400 MHz, CDCl3): d = 7.14 (1 H, t, J = 8.0 Hz),
6.81 (1 H, d, J = 8.0 Hz), 6.71 (1 H, d, J = 8.0 Hz), 3.70 (2
H, t, J = 8.0 Hz), 2.40 (3 H, s), 1.65 (2 H, quin, J = 8.0 Hz),
1.44 (3 H, s), 1.37 (2 H, sext, J = 8.0 Hz), 0.94 (3 H, t, J = 8.0
Hz). 13C NMR (75.5 MHz, CDCl3): d = 181.3, 142.3, 134.2,
132.8, 127.4, 124.7, 106.1, 44.9, 39.6, 29.5, 22.4, 20.1, 18.2,
13.8.
Data for 8c
1H NMR (400 MHz, CDCl3): d = 7.08 (1 H, d, J = 8.0 Hz),
6.86 (1 H, d, J = 8.0 Hz), 6.68 (1 H, s), 3.69 (2 H, t, J = 8.0
Hz), 2.38 (3 H, s), 1.65 (2 H, quin, J = 8.0 Hz), 1.37 (2 H,
sext, J = 8.0 Hz), 1.34 (3 H, s), 0.95 (3 H, t, J = 8.0 Hz). 13
C
NMR (75.5 MHz, CDCl3): d = 181.3, 142.3, 137.6, 133.2,
122.6, 122.1, 109.2, 43.8, 39.5, 29.6, 24.6, 21.8, 20.1, 13.8.
(15) A typical procedure is illustrated for reaction of 1b with
CuBr (entry 2, Table 1). Substrate 1b (0.3 mmol) was added
to dry CH2Cl2 (2 mL), and CuBr (0.33 mmol) and
tripyridylamine (0.33 mmol) were added. The reaction
mixture was heated at 40 °C for 18 h. Upon cooling, the
crude mixture was passed through a small silica plug (eluting
with EtOAc, 20 mL to remove copper residues). After
evaporation of the solvent and chromatography (PE–EtOAc,
8:1) a mixture of amide 3b and oxindole 8b were isolated in
the ratio of 1.4:1.0, combined yield 78%. Spectroscopic data
for 3b were identical to an authentic sample prepared
previously2,9 and data for 8b were identical to that reported
above.14
(8) (a) Clark, A. J.; Dell, C. P.; McDonagh, J. P. C. R. Acad. Sci..
Ser. IIC 2001, 4, 575. (b) Clark, A. J.; Battle, G. M.; Bridge,
A. Tetrahedron Lett. 2001, 42, 1999. (c) Clark, A. J.; Battle,
G. M.; Bridge, A. Tetrahedron Lett. 2001, 42, 2003.
(d) Clark, A. J.; Battle, G. M.; Bridge, A. Tetrahedron Lett.
2001, 42, 4409. (e) Clark, A. J.; Geden, J. V.; Thom, S.;
Wilson, P. J. Org. Chem. 2006, 71, 1471. (f) Clark, A. J.;
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4848.
(16) Moutrille, C.; Zard, S. Z. Chem. Commun. 2004, 1848.
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S. Z. Synlett 1996, 1148. (b) Esker, J. L.; Newcomb, M.
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