Poly(N,N′-dichloro-N-ethylbenzene-1,3-disulfonamide) and N,N,N′,N′-tetrachlorobenzene-1,3-disulfonamide as novel reagents for the synthesis of N-chloroamines, nitriles and aldehydes

Article abstract of DOI:10.1055/s-0028-1087967

The applications of poly(N,N′-dichloro-N-ethylbenzene-1,3- disulfonamide) (PCBS) and N,N,N′,N′-tetrachlorobenzene-1,3- disulfonamide (TCBDA) as novel reagents for the preparation of N,N-dichloroamines, nitriles, and aldehydes from primary amines under various conditions are described. Also, a simple and effective procedure for the direct oxidative conversion of primary alcohols into nitriles was successfully carried out with TCBDA and PCBS in aqueous ammonia. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0028-1087967

PAPER
945
Poly(N,N¢-dichloro-N-ethylbenzene-1,3-disulfonamide) and N,N,N¢,N¢-Tetra-
chlorobenzene-1,3-disulfonamide as Novel Reagents for the Synthesis of
N-Chloroamines, Nitriles and Aldehydes
P
CB
S
andTCBD
a
A
a
s
N
ove
m
l
Reagents in Ghorbani-Vaghei,* Hojat Veisi
Department of Organic Chemistry, Faculty of Chemistry, Bu-Ali Sina University, 65174 Hamadan, Iran
Fax +98(811)8257407; E-mail: rgvaghei@yahoo.com
Received 17 October 2008; revised 5 November 2008
(TCCA)19 and NaOCl3 were reported for the synthesis of
dichloroamines under various conditions.
Abstract: The applications of poly(N,N¢-dichloro-N-ethylbenzene-
1,3-disulfonamide) (PCBS) and N,N,N¢,N¢-tetrachlorobenzene-1,3-
disulfonamide (TCBDA) as novel reagents for the preparation of
N,N-dichloroamines, nitriles, and aldehydes from primary amines
under various conditions are described. Also, a simple and effective
procedure for the direct oxidative conversion of primary alcohols
into nitriles was successfully carried out with TCBDA and PCBS in
aqueous ammonia.
In continuation of our interest in the synthesis of N-halo-
sulfonamide and application of these reagents in organic
synthesis,27–32 we report here the synthesis of poly(N,N¢-
dichloro-N-ethylbenzene-1,3-disulfonamide) (PCBS) and
N,N,N¢,N¢-tetrachlorobenzene-1,3-disulfonamide (TCBDA)
as novel reagents in organic reactions (Figure 1).
Key words: TCBDA, PCBS, dichloroamines, nitriles, alcohols
Cl
Cl
Cl
N
The oxidation of primary and secondary amines into the
corresponding chloroamines and oxidation of primary
amines into nitriles constitutes a very useful functional
group transformation in organic synthesis.1–3 Consequent-
ly, there is a great deal of interest in finding out methods
for their synthesis under mild conditions. A typical meth-
od is the reaction of alkyl halides with very toxic metal cy-
anides via a nucleophilic pathway; the length of the
carbon chain is increased by one during this process. Due
to the toxicity of metal cyanides, nitriles are generally pre-
pared by the dehydration of amides with SOCl2, TsCl/py-
ridine, P2O5, COCl2, (EtO)3P/I2, or Ph3P/CCl4.4 It has been
reported that nitriles can be prepared from carboxylic acid
or its derivatives,5 amides,6,7 aldehydes,8,9 oximes,10 and
nitro compounds.11 Nitrile synthesis from amines has
been one of the classical routes. Numerous metal-based
oxidants, such as nickel peroxide,12 silver reagents,13 cop-
per reagents,14 OsO4,15 NaOCl,16 PhIO,17 TCCA with
N
Cl
S
S
O
O
O
O
TCBDA
Cl
N
Cl
N
H2C
CH2
S
S
n
O
O
O
O
PCBS
Figure 1 Structures of TCBDA and PCBS
The advantages of TCBDA and PCBS are as follows: The
preparation of TCBDA and PCBS are easy and conve-
nient. TCBDA and PCBS are stable for two to three
months under atmospheric conditions (25 °C, air). After
completion of the reaction, the sulfonamides are recov-
ered and can be reused many times without decreasing the
yield.
21
TEMPO18 or Et3N,19 IBX/I2,20 I2/NH3, 1,3-diiodo-5,5-
dimethylhydantoin in aqueous NH3,22,23 and ruthenium
reagents24 have been used for carrying out this transfor-
mation. However, there are only a few reports on the di-
rect oxidative conversion of primary alcohols into nitriles
in a one-pot procedure, that is, using NH4HCO3,
In order to apply these novel reagents, we were interested
in studying the possibility of chlorination of amines under
various conditions (methods A and B, Scheme 1). Thus,
benzylamine, as a model substrate, was dissolved in
dichloromethane and treated with an equimolar amount of
TCBDA at room temperature. After 10 minutes, TLC
analysis showed the complete absence of the amine and
the workup of the reaction mixture afforded N,N-dichlo-
robenzylamine in quantitative yield.
(Bu4N)2S2O8,
and
a
catalytic
amount
of
Cu(HCO2)2·Ni(HCO2)2 in aqueous KOH and i-PrOH,25
and MnO2, NH3, and MgSO4 in THF and i-PrOH for ben-
zylic and cinnamic alcohols. Recently, I2 and DIH22 in
26
aqueous NH3 were reported by Togo for this conversion.
N,N-Dichloroamines are very useful intermediates in syn-
thetic organic chemistry. Trichloroisocyanuric acid
Cl
method A: TCBDA or PCBS, CH2Cl2, 25 °C
RNH2
R
N
method B: TCBDA or PCBS, solid-state, grinding, 25 °C
Cl
R = alkyl, aryl
SYNTHESIS 2009, No. 6, pp 0945–0950
x
x
.
x
x
.2
0
0
9
Advanced online publication: 24.02.2009
DOI: 10.1055/s-0028-1087967; Art ID: Z23008ST
Scheme 1
© Georg Thieme Verlag Stuttgart · New York

946
R. Ghorbani-Vaghei, H. Veisi
PAPER
Table 1 Formation of Chloramines from Amines by Reaction with TCBDA and PCBS Under Various Conditions
Entry
Amine
Producta
TCBDA
(Method A)
PCBS
(Method A)
TCBDA
(Method B)
PCBS
(Method B)
Time
(min)
Yield
Time
(min)
Yield
(%)
Time
(min)
Yield
(%)
Time
(min)
Yield
(%)
(%)b
98
NCl2
NH2
1
2
3
5
10
8
10
15
10
96
1
1
1
98
1
96
NCl2
NH2
92
96
90
90
96
99
1.5
2
96
98
NH2
NCl2
MeO
HO
MeO
HO
NH2
NCl2
4
5
6
15
20
15
90
90
89
15
25
15
85
87
80
1
98
95
90
1.5
2
96
90
85
NH2
NCl2
1.5
1.5
Cl
Cl
NCl2
NH2
2.5
Cl2N
H2N
NH2
NCl2
7
8
10
15
90
92
12
18
90
87
1
96
92
2
92
90
N
H
N
1.5
2.5
Cl
H
N
Cl
N
9
5
5
96
95
5
7
95
90
1
1
99
98
1
2
98
96
N
H
N
Cl
O
O
10
N
N
H
Cl
NH2
NCl2
11
12
13
10
300
300
90
0
10
300
300
85
0
2
10
10
90
0
2
10
10
90
0
NH2
NCl2
H
N
Cl
N
0
0
0
0
a The products were characterized by comparison of their spectroscopic and physical data with those of samples synthesized by reported pro-
cedures.
b Yields refer to pure isolated products.
The reaction reported in Scheme 1 was then applied to aniline and diphenylamine did not react under these con-
other primary and secondary amines. As shown in ditions.
Table 1, various types of amines (aromatic and aliphatic)
Since, in recent years, there has been an increasing inter-
were cleanly and rapidly converted to the corresponding
est in reactions that proceed in the absence of solvents due
N,N-dichloroamines in CH2Cl2 using PCBS and TCBDA
to their reduced pollution, low cost, simplicity in process,
and handling, we decided to test the N-chlorination of
at 25 °C. The benzylic amines and aliphatic amines were
more effective in this reaction. Secondary amines reacted
benzylamine under solvent-free conditions (grinding)
also to give the chloramines (Table 1, entries 8–10). But,
Synthesis 2009, No. 6, 945–950 © Thieme Stuttgart · New York

PAPER
PCBS and TCBDA as Novel Reagents
947
with these reagents. We found that the reaction was rapid
(1 min) and proceeds with an excellent yield (98%,
Table 1, entry 1) using TCBDA, and in excellent yield
(96%, 1 min, Table 1, entry 1) using PCBS. Thus, to test
the generality and versatility of this procedure for the N-
chlorination of amines, we have examined a number of ar-
omatic and aliphatic amines using the optimized condi-
tions (Table 1). Also, the results of Table 1 show that this
method is the better than other methods using TCCA19 for
the preparation of chloramines, as summarized Table 2.
Table 3 Reaction of N,N-Dichloroamines with Triethylamine
Entry Amine
1
Producta
Time (h) Yield (%)
CN
NCl2
1.5
1.2
96
93
CN
NCl2
2
CN
NCl2
3
2
96
95
90
82
90
Table 2 Comparison of the Present Method with the Published
Method in Terms of Reaction Times and Yields
MeO
MeO
Cl
CN
NCl2
4
1
Substrate
Conditions Time
Yield (%) Ref.
Cl
19
4-chlorobenzylamine
4-chlorobenzylamine
TCCA
1 h
77
–
–
CN
CN
NCl2
5
2
TCBDA
20 min 90
HO
HO
NC
(Method A)
NCl2
6
3.5
2
4-chlorobenzylamine
TCBDA
1.5 min 95
–
Cl2N
(Method B)
19
NCl2
dibenzylamine
dibenzylamine
TCCA
0.5 h
75
–
–
CN
7
TCBDA
15 min 92
(Method A)
a The products were characterized by comparison of their spectroscop-
ic and physical data with those of samples synthesized by reported
procedures.
dibenzylamine
TCBDA
(Method B)
1.5 min 92
–
19
benzylamine
benzylamine
TCCA
1 h
98
98
–
–
Table 4 Oxidative Conversion of Primary Amines into Nitriles with
TCBDA and PCBS
TCBDA
1 min
(Method A)
Entry Amine
Producta
TCBDA
PCBS
benzylamine
TCBDA
(Method B)
1 min
98
–
Time Yield TimeYield
(h) (%) (h) (%)
CN
NH2
1
2
1
1
96
95
1.2 92
1.5 90
Since N,N-dichloroamines may be synthetic intermediates
for the preparation of nitriles and carbonyl compounds,33
we have treated the prepared N,N-dichloroamines with 3
equivalents of triethylamine in CH2Cl2 (Scheme 2). The
results shown in Table 3 indicate that the desired nitriles
are obtained in good yields.
CN
NH2
CN
NH2
3
1.5 90
1.5 90
2
2
2
3
90
88
90
80
Et3N, CH2Cl2
MeO
MeO
HO
Cl
NCl2
R
R
CN
r.t.
CN
NH2
4
Scheme 2
HO
CN
CN
Furthermore, we have checked the possibility to carry out
the reaction in a one-pot procedure for preparing nitriles
from amines. Best results were obtained in DMF and by
introducing triethylamine directly in the reaction mixture.
In this case, the reaction can be carried out in a single step:
under these conditions, benzylamine afforded benzoni-
trile within one hour in 96% yield (Scheme 3). The results
are shown in Table 4.
NH2
5
1
2
2
91
90
95
Cl
NH2
6
H2N
NC
NH2
CN
7
2.5 87
TCBDA or PCBS
NH2
R = aryl
a The products were characterized by comparison of their spectro-
scopic and physical data with those of samples synthesized by report-
ed procedures.
R
R CN
Et3N, DMF, 25 °C
Scheme 3
Synthesis 2009, No. 6, 945–950 © Thieme Stuttgart · New York

948
R. Ghorbani-Vaghei, H. Veisi
PAPER
Et3N
Et3N
••
••
H
H
elimination
– NH4Cl
elimination
– NH4Cl
TCBDA or PCBS
DMF, 25 °C
Cl
NH2
R
R
CN
N
N
R
R
Cl
Cl
Scheme 4
A plausible mechanism for the conversion of a primary were obtained in good to high yields (Table 5, entries 1–
amine into nitrile is shown in Scheme 4. According to this 8).
mechanism, the initial N-chlorination of the amine with
TCBDA or PCBS occurs to form the N,N-dichloroamines.
Table 5 Oxidative Conversion of Primary Alcohols into Nitriles
with TCBDA and PCBS
This is followed by a b-elimination of HCl by triethyl-
amine, leading to the nitrile.19,20
Entry Alcohol
Producta
TCBDA
PCBS
We found that TCBDA was suitable for the conversion of
benzylamines to the corresponding carbonyl compounds
in good to high yields under reflux conditions in the pres-
ence of HCl (10%) adsorbed on SiO2 (Scheme 5).19
Time Yield Time Yield
(h) (%) (h) (%)
CN
OH
1
2
1
95 1.5 90
HCl (10%)
adsorbed
CN
H
on SiO2
OH
TCBDA
1.1 86 1.5 80
Cl
RCHO
NCl
R
NH2
R
N
reflux
hydrolysis
R
– HCl
CH2Cl2
reflux
Cl
CN
OH
3
1.5 90
2
80
MeO
40 min., 90%
40 min., 80%
MeO
HO
Br
R =
CN
OH
4
2
85 2.5 75
HO
MeO
CN
CN
OH
5
2.5 88 2.5 81
Br
40 min., 75%
HO
OH
6
1
95 1.5 90
Scheme 5
Cl
Cl
OH
Our reagents TCBDA and PCBS were also tested for the
oxidation of alcohols and found that these reagents were
able to oxidize alcohols under mild conditions. Our next
study was a practical and facile method for the oxidative
conversion of alcohols to the corresponding nitriles di-
rectly, using PCBS and TCBDA as novel reagents in
aqueous ammonia (Scheme 6).
CN
7
8
3.5 90
85
4
5
90
82
5
CN
OH
a The products were characterized by comparison of their spectro-
scopic and physical data with those of samples synthesized by report-
ed procedures.
TCBDA or PCBS
R
CN
OH
A plausible reaction pathway for the conversion of prima-
ry alcohols into nitriles is shown in Scheme 7. According
to this pathway, the initial O-chlorination of the alcohol
with TCBDA or PCBS occurs to form the O-chloro com-
pounds a, followed by b-elimination of HCl by ammonia
to form the aldehyde b. This aldehyde reacts with ammo-
nia to form an aldimine c. Then, the aldimine c reacts with
TCBDA or PCBS in the presence of ammonia to form an
N-chloroaldimine d, followed by b-elimination of HCl by
ammonia to generate the corresponding nitrile.26
R
aq NH3, 60 °C
Scheme 6
Thus, the present reaction was carried out by treating ben-
zyl alcohol (1 mmol) with TCBDA (1.1 mmol) in aqueous
ammonia (30%, 3 mL) at 60 °C to provide the correspond-
ing nitriles in high yields (Table 5). When the same reac-
tion was carried out in 0.5 mmol of the reagent, instead of
1.1 mmol, the yield of nitrile decreased. A similar result
was obtained with PCBS under these conditions. Various
primary and benzylic alcohols were treated with TCBDA
and PCBS in aqueous ammonia under the same condi-
tions, as shown in Table 5. The corresponding nitriles
We found that under these conditions, dibenzylamine re-
acted with TCBDA or PCBS in aqueous NH3 to provide
benzonitrile in good yield (Scheme 8).
Synthesis 2009, No. 6, 945–950 © Thieme Stuttgart · New York

PAPER
PCBS and TCBDA as Novel Reagents
949
In conclusion, PCBS and TCBDA are novel and efficient
reagents for the formation of N,N-dichloroamines, ni-
triles, and aldehydes from primary amines, and good re-
agents for oxidative conversion of primary alcohols into
the corresponding nitriles in good to high yields in aque-
ous ammonia. Prominent advantages of these new meth-
ods are their mild reaction conditions, operational
simplicity, easy reaction workup, and generality of the re-
actions.
TCBDA or PCBS
NH3, 60 °C
R
CN
OH
R
reagent
– HCl
H
Cl
NH3
reagent
R-CH=NH
R-C=N
d
H
RCHO
b
O
R
– HCl
– H2O
c
a
Cl
reagent: TCBDA or PCBS
All commercially available chemicals were obtained from Merck
and Fluka companies, and used without further purifications unless
otherwise stated. Nuclear magnetic resonance (NMR) spectra were
recorded on Bruker Avance 300 MHz and Jeol 90 MHz FT NMR
spectrometers. Infrared (IR) spectroscopy was conducted on a
Perkin Elmer GX FT-IR spectrometer. All yields refer to isolated
products.
Scheme 7
CN
TCBDA or PCBS
N
H
2
aq NH3
60 °C, 4 h, 90%
Scheme 8
Poly(N,N¢-dichloro-N-ethylbenzene-1,3-disulfonamide)(PCBS)
and N,N,N¢,N¢-Tetrachlorobenzene-1,3-disulfonamide
Based on these results, a plausible reaction pathway for (TCBDA)
A sample of white, finely powdered poly(N-ethylbenzene-1,3-di-
the conversion of dibenzylamine into nitrile is shown in
Scheme 9. Thus, initial N-chlorination of the amine oc-
curs to give the N-chloro secondary amine e, followed by
a b-elimination of HCl to form the imino compound f.
Once imino compound f is formed, it reacts with aqueous
NH3 to form the 1,1-diamino compound g, which smooth-
ly decomposes to the primary amine h and imine j. The
primary amine h react with TCBDA or PCBS to form the
N-chloroamine i, followed by a b-elimination of HCl to
generate imine j. Imine j reacts with TCBDA or PCBS to
form the N-chloroimine k, and finally elimination of HCl
by NH3 occurs to provide the corresponding nitrile.21
sulfonamide) (1 g) or benzene-1,3-disulfonamide (1 g, 4.9 mmol)
was dissolved in a solution of NaOCl (50 mL, 14%) at 25 °C for 30
min. The color of the solution did not change. After this time, AcOH
(20 mL, 50%) was added to the solution. The insoluble chlorinated
reagent was removed by filtration and washed with H2O (5 mL) to
give PCBS or TCBDA, respectively.
N,N,N¢,N¢-Tetrachlorobenzene-1,3-disulfonamide
Yield: 1.28 g (80%); white solid; mp 145–147 °C.
IR (KBr): 3050, 2950, 2900, 1570, 1462, 1417, 1377, 1304, 1167,
1082, 807, 776, 675 cm–1.
1H NMR (FT-250 MHz, CDCl3): d = 7.95–8.09 (m, CHarom, 1 H),
8.11–8.58 (m, CHarom, 2 H), 8.79 (s, CHarom, 1 H).
MS: m/z = 374 ([M + H]+), 339, 337, 321, 319, 305, 303, 272, 269,
267, 156, 154, 139, 125, 120, 104, 91, 77, 63.
CN
N
H
2
Anal. Calcd for C6H4Cl4N2O4S2: C, 19.25; H, 1.06; N, 7.48; S,
17.11. Found: C, 19.05; H, 1.16; N, 6.94; S, 16.28.
reagent
– HCl
Cl
Poly(N,N¢-dichloro-N-ethylbenzene-1,3-disulfonamide)
Yield: 0.9 g (78%); white solid; mp 175–178 °C.
H
IR (KBr): 3050, 2950, 2900, 1578, 1462, 1418, 1377, 1303, 1168,
1081, 809, 779, 674, 603, 570 cm–1.
1H NMR (FT-250 MHz, CDCl3): d = 7.56–8.48 (br, CHarom).
R
N
R
R-C=N
Cl
e
H
H
k
Oxidation of Benzylamines to N,N-Dichlorobenzylamine in Sol-
vent; Typical Procedure; Method A
– HCl
R-CH-NH
Cl
i
Benzylamine (214 mg, 2 mmol) was dissolved in CH2Cl2 (3 mL)
and treated with TCBDA (0.6 mmol) or PCBS (0.5 g) at 25 °C. Af-
ter the addition, the mixture was stirred for the required time until
completion. After 5–10 min, TLC analysis showed the complete ab-
sence of the amine, the mixture was filtered on Celite and the sol-
vent was evaporated to yield N,N-dichlorobenzylamine, which was
isolated without further purification (oil, 98%).
R
N
R
reagent
– HCl
f
NH3
reagent
••
NH2
R-CH2-NH2
+
R-CH=NH
R
N
R
H
N,N-Dichlorobenzylamine under Solvent-Free Conditions
(Grinding); Typical Procedure; Method B
h
g
j
Benzylamine (214 mg, 2 mmol) and the reagent PCBS (0. 5 g) or
TCBDA (0.6 mmol) were added to a mortar and the mixture was
pulverized with a pestle. A spontaneous reaction took place [1–2.5
min, Table 1, monitored by TLC (4:1, hexane–acetone)]. After
completion of the reaction, CH2Cl2 (5 mL) was added, and the in-
reagent: TCBDA or PCBS
Scheme 9
Synthesis 2009, No. 6, 945–950 © Thieme Stuttgart · New York

950
R. Ghorbani-Vaghei, H. Veisi
PAPER
soluble reagent was removed by filtration. The filtrate was evapo-
rated under reduced pressure to give the product in good yield.
References
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Nitriles from N,N-Dichloroamines
The N,N-dichloroamine (1 mmol) was dissolved in CH2Cl2 (3 mL)
and treated with Et3N (3 mL) at 25 °C. After completion of the re-
action [1–3.5 h, Table 3, monitored by TLC (3:1, hexane–acetone)],
the mixture was washed with H2O (10 mL) and HCl (10%, 5 mL).
The organic layer was dried (MgSO4), filtered, and concentrated in
vacuo to obtained the desired product in high yield.
Nitriles from Amines; General One-Pot Procedure
Amine (1 mmol) was dissolved in DMF (3 mL) and treated with
Et3N (2.56 mL, 3 mmol) and with PCBS (0. 5 g) or TCBDA (0.6
mmol) at 25 °C. After completion of the reaction [1–2 h, Table 4,
monitored by TLC (3:1, hexane–acetone)], the mixture was
quenched with H2O (10 mL), and extracted with CH2Cl2 (2 × 15
mL). The combined organic layers were washed with H2O (5 mL),
HCl (10%, 5 mL), dried (MgSO4), and the solvent evaporated to
give the crude product. The crude product was purified by TLC us-
ing n-hexane–acetone (80:20) as eluent to afford the pure products.
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(15) Gao, S.; Herzig, D.; Wang, B. Synthesis 2001, 544.
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(17) Moriarty, R. M.; Vaid, R. K.; Duncan, M. P.; Ochiai, M.;
Inenaga, M.; Nagao, Y. Tetrahedron Lett. 1988, 29, 6913.
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Aldehydes from Amines; General One-Pot Procedure (Scheme
5)
Amine (1 mmol) was dissolved in CH2Cl2 (5 mL) and treated with
TCBDA (0.5 mmol) at 25 °C. After the addition, the mixture was
stirred for 10 min under reflux. Then, silica gel (1 g) treated with aq
10% HCl was added and the mixture was refluxed for 30 min. After
completion of the reaction, CH2Cl2 (5 mL) was added, and the in-
soluble SiO2 was removed by filtration. The filtrate was quenched
with H2O (10 mL) and extracted with CH2Cl2 (2 × 15 mL). The or-
ganic layers were dried (MgSO4), and the solvent was evaporated.
The product benzaldehyde was purified by column chromatography
on silica gel (hexane–acetone, 9:1).
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Oxidative Conversion of Primary Alcohols into Nitriles; Gener-
al Procedure
To a mixture of benzyl alcohol (108 mg, 1 mmol) and aq ammonia
(3 mL) was added PCBS (0.8 g) or TCBDA (1.1 mmol) at 25 °C.
The mixture obtained was stirred at 60 °C. After completion of the
reaction [1–2.5 h, Table 5, monitored by TLC (6:1, hexane–
acetone)], the mixture was quenched with aq HCl (10%, 10 mL),
and extracted with CH2Cl2 (2 × 15 mL). The organic layer was dried
(MgSO4) and evaporated to provide the corresponding nitrile in
good yield. If necessary, the product was purified by column chro-
matography on silica gel (hexane–EtOAc, 4:1).
(25) Chen, F.; Li, Y.; Xu, M.; Jia, H. Synthesis 2002, 1804.
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H. Tetrahedron Lett. 2006, 47, 4505.
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2005, 16, 647. (b) Ghorbani-Vaghei, R.; Akbari-
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Chehardoli, G.; Ghorbani Choghamani, A.; Yazdi Hosain,
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Acknowledgment
We are thankful to Bu-Ali Sina University, Center of Excellence of
Chemical Methods (CEDCM) for financial support, and the Univer-
sity of Sheffield for NMR, Mass spectra and CHN. Also the senior
author acknowledges Prof. I. Coldham from the University of Shef-
field for his useful comments and his kindness for hosting him as a
research visitor.
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Synthesis 2009, No. 6, 945–950 © Thieme Stuttgart · New York