Novel generation of phenylnitrenium ions from S,S-dialkylanilinosulfonium salts in trifluoroacetic acid and formation of anilines by a new intramolecular hydride-shift to the phenylnitrenium ions

Article abstract of DOI:10.1039/a803039a

Reactions of S,S-dialkylanilinosulfonium salts in trifluoroacetic acid give anilines, trifluoroacetanilides and dialkyl sulfoxides via novel intramolecular hydride-shift from the α-C-H of sulfide to phenylnitrenium ions (not completely free from the sulfide) interacting with both the unshared electron pair of the sulfide and the counter ion.

Full text of DOI:10.1039/a803039a

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Novel generation of phenylnitrenium ions from S,S-dialkylanilino-
sulfonium salts in trifluoroacetic acid and formation of anilines by
a new intramolecular hydride-shift to the phenylnitrenium ions
Hiroshi Takeuchi,* Tomohito Taniguchi, Mamoru Masuzawa and Kazuya Isoda
Department of Chemical Science and Engineering, Faculty of Engineering, Kobe University,
Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan
Reactions of S,S-dialkylanilinosulfonium salts in trifluoroacetic acid give anilines, trifluoroacetanilides
and dialkyl sulfoxides via novel intramolecular hydride-shift from the á-C᎐H of sulfide to phenyl-
nitrenium ions (not completely free from the sulfide) interacting with both the unshared electron pair
of the sulfide and the counter ion.
Table 1 Effect of counter ion on the reaction of S,S-dimethyl-
We have inferred that reactions of dialkyl sulfide with phenyl-
nitrenium ions I generated by acid decomposition of phenyl
azides 11,2 or 1-anilinopyridinium salts 23 in trifluoroacetic acid
(TFA) give alkyl 2- and 4-aminophenyl sulfides 4 and 5 via S,S-
dimethylanilinosulfonium salts 3 or intermediates II in which
the nitrenium ions interact with the unshared electron pair of
sulfide and the counter ion (Scheme 1). However, under basic or
anilinosulfonium salt 3a (0.5 mmol) in TFA (30 cm3) containing acid
(1.2 molar equivalent to 3a) at 20 ЊC for 5 h
Yielda (%)
Acid
HY
Counter ion
ϪY
Ratio
(6a ϩ 7a):8a
6a
7a
8a
2.5
HCl
ϪCl
35
21
27
27
22
18
Trace
Trace
Trace
Trace
Trace
Trace
14
None
AcOH
TFSA
H2SO4
HClO4
ϪOCOCF3
ϪOCOCF3
ϪOSO2CF3
ϪOSO3H
ϪClO4
10
14
21
18
33
2.1
1.9
1.3
1.2
0.55
δ+
+
NH Y–
δ+
NH
N3
NH SR2 Y–
:SR2
Y–
R2S
HY, –N2
in TFA
X
δ+
X
or
X
δ+
X
a The yields are based on the amount of 3a used.
1
I
3
II
Table 2 Effect of solvent-nucleophilicity on reaction of S,S-dimethyl-
anilinosulfonium salt 3a (0.50 mmol) in solvent (25 cm3) containing
TFA (1.2 molar equivalent to 3a) at 20 ЊC for 20 h
–R+
R2S
–R+
+
N
Y–
∆
NH2
NH2
Yielda (%)
NH
in TFA
SR
Ratio
Solvent
6a
7a
8a
9.0
36
46
60
(6a ϩ 7a):8a
+
X
X
X
TFA
45
31
11
11
Trace
Trace
Trace
Trace
5.0
2
SR
CF3CH2OH
CH2Cl2
MeOH
0.86
0.24
0.18
5
4
Scheme 1
a The yields are based on the amount of 3a used.
neutral conditions, the anilinosulfonium salts 3 were converted
into 2-aminoarylmethyl methyl sulfides 8 by Sommelet Hauser-
type rearrangement2 without giving 4 and 5 (Scheme 2). In this
paper, the reaction of the tilted anilinosulfonium salts 3 is per-
formed in order to investigate the possibility that 4 and 5 are
formed via 3 in an acidic solvent such as TFA. We here rule
out this possibility, and find instead the formation of anilines
by a novel hydride-shift to phenylnitrenium ions as seen in
Scheme 2.
Effects of counter ion and solvent nucleophilicity. The ratio
(6a ϩ 7a):8a decreased with the increased softness of the
counter ion (Table 1). The counter ion corresponds to the anion
formed from acid added to the reaction system; the counter
ion is ϪOCOCF3 using TFA only as the acid. The ratio also
decreased with an increase in the solvent-nucleophilicity4
(TFA < CF3CH2OH < CH2Cl2 < MeOH) (Table 2); although
the solvent nucleophilicity of CH2Cl2 has not been calculated,
its order seems to be between the orders of MeOH and
CF3CH2OH.
These results can be explained by considering competing
paths a and b (see Scheme 2). The hard counter ion of 3a
increases the resonance contribution of the sulfonium ion
structure with an S᎐N single bond to give a high ratio of
products from the favoured path a involving an S᎐N scission.
However, a soft counter ion for 3a increases the delocalisation
of the positive charge over the N-atom as well as the S-atom to
enhance the double-bond nature of the bond between the
S- and N-atoms, giving an unfavourable path a. On the con-
trary, path b as a rate-determining step (a cyclisation after the
path b might be very fast) for the formation of 8 would be
insensitive to the counter ion. A more nucleophilic solvent
Results and discussion
Reactions of S,S-dimethylanilinosulfonium salt 3a in TFA
S,S-Dimethylanilinosulfonium picrate was used as a precursor
of anilinosulfonium trifluoroacetate 3a. The dissolution of this
picrate in TFA at 20 ЊC resulted in complete conversion to
3a, as was seen from the immediate colour change of the solu-
tion from the yellow of the picrate to colourless on dissolution.
Reaction of the picrate in TFA (i.e. reaction of 3a in TFA) did
not give 4a and 5a, but instead gave aniline 6a, trifluoro-
acetanilide 7a and 2-aminophenylmethyl methyl sulfide 8a
(Table 1). This result rules out the possibility of the inter-
mediacy of 3 in the formation of 4 and 5 in Scheme 1.
J. Chem. Soc., Perkin Trans. 2, 1998
1743

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NH2
+
R
K
a
Y–
k
HC
H
CHR
CHR
S+
+
+
~H–
(RCH2)2S NH
Y–
X
(RCH2)2S
NH
X
Y–
+
S: NH
X
S
Y–
CH2R
CH2R
CH2R
3
IV
III
X
c
b
6
TFA or
(CF3CO)2O
–H+
TFA
NH2
R
HNCOCF3
R
CHSCH2R
RCH
+
CH2R
+
HCδ+ Hδ+
Sδ+ NHδ+
CH2R
–RCH2
RCH2S NH
X
+S OCOCF3
Y–
X
CH2R
X
–
if Y– = CF3CO2
X
NH2
NH2
8
SCH2R
7
6
+
IV
+
(RCH2)2S
O + (CF3CO)2O
R
X
9
a
b
c
d
H
H
H
H
NO2
Me
X
SCH2R
4
5a
Prn NO2
Scheme 2
Table 3 Reactions of S,S-dialkylanilinosulfonium salts 3b–d (0.50
Table 4 Rate constants and activation parameters for decomposition
of S,S-dialkylanilinosulfonium salts 3b–d in TFAa
mmol) in TFA (5.0 cm3)
Yielda (%)
∆S‡/
kobs
/
∆H‡/
cal molϪ1
KϪ1
∆G‡/
Salt
T/ЊC
t/h
6
7
8
9
Salt
T/ЊC
10Ϫ5 sϪ1
Kcal molϪ1
Kcal molϪ1
3b
3c
3d
50
50
72
5
5
72
93
95
2.0
6.0
3.0
98
0
0
0
—
67
79
3b
20
30
50
20
30
50
20
50
72
1.2
3.7
26
5.2
11
19
12
8.0
Ϫ17
24
24
25
3c
3d
a The yields are based on the amount of 3 used.
Ϫ39
Ϫ57
34
0.27
0.81
2.6
may disfavour path a by solvation with the phenylnitrenium ion
and/or by reaction of the nitrenium ion with solvent. Thus,
the path b suppresses path a, leading to a lower ratio of
(6a ϩ 7a):8.
a ∆H‡, ∆S‡ and ∆G‡ are the values at 25 ЊC.
Formation of 6 and 7 via an intramolecular reaction. The reac-
tion of 3a in benzene (70% v/v)–TFA (30% v/v) for 2 h at 25 ЊC
or under reflux did not produce diphenylamine and 2- and
4-aminobiphenyls which are formed by the intermolecular reac-
tion5 of the phenylnitrenium ion with benzene, but instead gave
6a and 7a (see Experimental section). This result indicates that
an intermediate III formed by path a is subject to an intra-
molecular reaction which is much faster than the inter-
molecular reaction with benzene, leading to the formation of
6 and 7. Thus, the intramolecular reaction would be an intra-
molecular hydride-shift to the nitrenium ion interacting with
sulfide as seen in Scheme 2. The hydride-shift from α-C᎐H of
the sulfide should give a carbocation IV as well as 6. In fact, the
reaction of 3c or 3d in TFA produced sulfoxide 9c (i.e. 9a) or 9d,
respectively (Table 3); the formation of 9 can be interpreted by a
retro-Pummerer reaction of IV (Scheme 2).
picrate) to colourless, showing the conversion of the precursors
to anilinosulfonium trifluoroacetates 3b–d in TFA. The solution
was allowed to stand under the conditions described in Table 3,
affording 6 and 7 without giving 8 (Table 3).
This preferential formation of 6 and 7, not 8, results from
path a being favoured over path b by a high positive charge on
the S-atom of 3 rather than the N-atom in the cases of 3b
and 3d which have an electron-withdrawing NO2, and by the
stability of the 4-tolylnitrenium ion compared to the phenyl-
nitrenium ion in the case of 3c (Scheme 2). The observation that
the total yield of 6 and 7 is nearly 100% (see Table 3) means
that the rate of decomposition of 3 should be equal to the rate
of total production of 6 and 7.
Rate constants and activation parameters for the decomposition
of S,S-dialkylanilinosulfonium salts 3b–d in TFA
We obtained the first-order rate constants kobs for the decom-
position of 3b–d (i.e. the rate constants for the total production
of the corresponding 6 and 7) at three different temperatures,
determining the activation parameters (Table 4).
The formation of 6 and IV by an intramolecular concerted
mechanism (path c in Scheme 2) can be clearly ruled out by the
fact that the rate of decomposition of 3d is lower than that of
3b as seen in Table 4. If the concerted mechanism were correct,
its rate, giving a secondary carbocation IVd, should be higher
than path a, forming a primary carbocation IVb (i.e. IVa).
Thus, 3 equilibrates with III (i.e. K in Scheme 2), followed by an
intramolecular hydride-shift (i.e. k in Scheme 2) giving 6 and
The nitrenium ion in III interacts with both the unshared
electron pair on sulfur and the counter ion (Scheme 2). We have
proposed a similar interaction of the parent nitrenium ion
(NH2ϩ)6 or the arylsulfenium ion (ArSϩ).7 The following find-
ings also support the formation of the phenylnitrenium ions
interacting in such a way.
Reactions of S,S-dialkylanilinosulfonium salts 3b–d in TFA
We dissolved S,S-(dimethyl- or di-n-butyl)-N-(4-nitro-
phenyl)imino-λ4-sulfanes and S,S-dimethyl-4-methylanilino-
sulfonium picrate as precursors of 3b or 3d and 3c, respectively,
in TFA at 20 ЊC. The colour of the solution immediately con-
verted from orange (of the λ4-sulfanes) or yellow (of the
1744
J. Chem. Soc., Perkin Trans. 2, 1998

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IV. In this case, 7 was obtained from 6 under the reaction con-
ditions. The intermediate IIId is more unstable than IIIb
because of the substitution of two butyl groups on the S-atom
rather than two methyl groups, so that the formation of IIIb
would be favoured over formation of IIId. Thus, the rate of
decomposition of 3d may be lower than that of 3b.
1465, 1320, 1290, 1260 (C᎐N), 1190, 1120, 1097, 1082, 1000,
900, 850 (C᎐NO2), 800 (para-substituted phenyl), 762, 707, 655,
626, 540, 502 and 480; δH(CDCl3) 0.98 (6H, t, J 6.0, Me), 1.0–
2.0 (8H, m, MeCH2CH2), 2.9 (4H, t, J 5.0, SCH2) and 7.38 (4H,
ABq, J 9.6, phenyl H); m/z 226 (Mϩ Ϫ Bu ϩ 1), 170 (Mϩ Ϫ
2Bu ϩ 2), 138 (Mϩ Ϫ 2BuS ϩ 2), 57, 45 and 41 (Found: C,
59.85; H, 7.85; N, 9.65. C14H22N2O2S requires C, 59.55; H, 7.8;
N, 9.95%).
The low ∆H‡ of 3d compared to that of 3b (Table 4) may
arise from the greater stability of IVd compared to IVb. The
lower ∆H‡ value of 3c compared to that of 3b (Table 4) can be
explained by the facile formation of a more stable 4-tolyl-
nitrenium ion rather than the 4-nitrophenylnitrenium ion.
From the above results, showing that the rate of decom-
position depends upon the ease of formation of both III and
IV, kobs might be represented as kK. The variation in which the
softer sulfonium ion 3 changes to the harder nitrenium ion
of III would mostly affect ∆S‡, leading to the large negative
∆S‡ due to an increase in the ion pair-association with the
counter-anion. The magnitudes of ∆H‡ and ∆S‡ and those of
the free energy are given in Table 4.
2,2,2-Trifluoroacetanilide derivatives 7a, 7b and 7c were syn-
thesized by reactions of trifluoroacetic anhydride with 6a, 6b
and 6c, respectively, and identified by the following data.
2,2,2-Trifluoroacetanilide 7a. The white crystalline com-
pound had mp 87–88 ЊC (lit.,11 87.6 ЊC); δH(CDCl3) 6.9–7.7
(5H, m, phenyl H) and 7.9–8.6 (1H, br, NH); m/z 189 (Mϩ), 120
(Mϩ Ϫ CF ), 92 (Mϩ Ϫ CF C᎐O), 77 (Phϩ), 65 and 51 (Found:
᎐
3
3
C, 50.55; H, 3.2; N, 7.5. Calc. for C8H6F3NO: C, 50.8; H, 3.2;
N, 7.4%).
2,2,2-Trifluoro-4Ј-nitroacetanilide 7b. The white crystalline
compound had mp 139–140 ЊC; νmax(nujol)/cmϪ1 3285 (NH),
1740 (C᎐O), 1695, 1618, 1565 (N᎐O), 1500, 1338 (N᎐O), 1288,
The thermal reaction of 11,2 or 23 with sulfides in TFA has
been proposed to produce 4 and 5 without giving 6 and 7 via
nitrenium species such as I and II, not by a bimolecular aro-
matic substitution of 1 or 2 (Scheme 1). However, the reaction
of 3 in TFA yields 6 and 7 without giving 4 and 5 via III
(Scheme 2). These facts imply that the nitrenium ion in III must
be different from the nitrenium ion in II. Thus, the nitrenium
ion in III may be in a lower energy state than II, and is interact-
ing strongly with sulfide as compared with the interaction of II
with sulfide. Because of such a strong interaction, III could give
6 and IV by the intramolecular hydride-shift from the α-C᎐H of
the sulfide. The details of the interaction between the nitrenium
ion and sulfide as shown in II will be reported in the future.
᎐
᎐
᎐
1253, 1203, 1148 (C᎐F), 1118, 903, 862 (C᎐NO2), 828 (para-
substituted phenyl), 755, 739, 693, 670, 655 and 502; δH(CDCl3–
[2H6]dimethyl sulfoxide) 5.6 (4H, ABq, J 9.0, phenyl H) and
8.3–8.8 (1H, br, NH); m/z 234 (Mϩ), 218, 204, 188, 168, 165
(Mϩ Ϫ CF3), 140, 134, 119, 109, 91, 69, 64 and 40 (Found: C,
41.3; H, 2.25; N, 11.95. Calc. for C8H5F3N2O3: C, 41.05; H,
2.15; N, 11.95%).
2,2,2-Trifluoro-4Ј-methylacetanilide 7c. The white crystalline
product had mp 102–102.5 ЊC; νmax(nujol)/cmϪ1 3280 (NH),
1700 (C᎐O), 1613, 1553, 1512, 1415, 1357, 1308, 1273, 1243,
᎐
1202, 1160 (C᎐F), 1132, 945, 820 (para-substituted phenyl),
728, 710 and 505; δH(CDCl3) 2.3 (3H, s, Me), 7.26 (4H, ABq,
J 9.0, phenyl H) and 7.6–8.4 (1H, br, NH); m/z 203 (Mϩ), 184
(Mϩ Ϫ CF ), 156 (Mϩ Ϫ CF C᎐O), 134, 106, 91, 65 and 40
᎐
3
3
Experimental
(Found: C, 53.4; H, 3.8; N, 6.85. Calc. for C9H8F3NO: C, 53.2;
H, 3.95; N, 6.9%).
IR spectra were obtained on a Hitachi EPI-G3 spectrometer.
1H NMR spectra were taken with a Nippondenshi PMX-60SI
instrument (J values are given in Hz). GLC–MS were recorded
with a Shimadzu QP-5000 spectrometer linked to a Shimadzu
GC-17A column (15 m × 0.25 mm) coated with DC-1 (0.25 µm
film thickness), made by J & W Scientific. GLC was performed
with a Shimadzu GC-8A instrument using a glass column (1
m × 3 mm) packed with 10% SE-30 on 60–80 mesh Chromo-
sorb W (AW-DMCS). Two runs agreed to within 3% error for
the yields of the products which are determined by replicated
GLC analyses.
TFA, CF3CH2OH, dimethyl sulfoxide 9a and aniline 6a were
purified by distillation of reagent grade materials (Nacalai
Tesque). The following compounds were reagent grade (Nacalai
Tesque) and were used without further purification; trifluoro-
methanesulfonic acid (TFSA), H2SO4, diphenylamine, 2-amino-
biphenyl, 4-nitroaniline 6b, 4-methylaniline 6c. Benzene,
AcOH, CH2Cl2 and MeOH were purified by standard methods
before use. HClO4 used in TFA was dehydrated by addition of
trifluoroacetic anhydride (TFAA); HClO4 and TFAA were
reagent grade (Nacalai Tesque). 4-Aminobiphenyl8 and di-n-
butyl sulfoxide 9d9 were prepared by the methods described in
the literature.
Reactions of S,S-dimethylanilinosulfonium salt 3a in TFA
The reaction of 3a in TFA was carried out with a variety of
acids as shown in Table 1. After the reaction, aq. Na2CO3 was
added until the solution reached pH > 7. The organic layer
was extracted with CH2Cl2 and the yields were determined by
GLC analysis. The products were isolated by thin layer chrom-
atography using silica gel (Merk 60F254) after column chrom-
atography (Fujisiriaru Chem. BW-127ZH). The structures of
the products 6a, 7a and 8a were characterised by comparison of
their mass spectra and retention times of GLC–MS with those
of authentic samples; we have already described the character-
isation of the structure of 8a.2 The effect of the counter ion is
summarised in Table 1.
The effect of solvent-nucleophilicity was investigated by
variation of solvent as shown in Table 2. The procedures used
for isolation and characterisation of the products and the
determination of their yields are indicated above.
The reaction of 3a (5.0 mmol) was carried out in a mixture
of TFA (7.5 cm3) and benzene (17.5 cm3) for 2 h at 25 ЊC or
under reflux. The reaction mixture was treated as described
above. According to the GLC analysis, diphenylamine and
2- and 4-aminobiphenyl were not detected, but 6a and 7a were
formed in 12% and trace or 25 and 46% yields for the reaction
at 25 ЊC or under reflux, respectively.
S,S-Dimethylanilinosulfonium picrate, S,S-dimethyl-N-(4-
nitrophenyl)imino-λ4-sulfane and S,S-dimethyl-4-methyl-
anilinosulfonium picrate were prepared by the literature
method,10 and had mp 129.6 ЊC (lit.,10 130–130.6 ЊC), 167 ЊC
(lit.,10 166–167 ЊC) and 167–169 ЊC (lit.,10 165–166 ЊC), respect-
ively. The following λ4-sulfane was synthesized by a similar
method to that described in the above literature, and confirmed
as indicated below.
Reactions of S,S-dialkylanilinosulfonium salts 3b–d in TFA
The detailed reaction conditions and the results are indicated in
Table 3. Isolation of the products 6b, 6c, 7b and 7c was carried
out as shown above. The products 9a and 9d were identified
by comparison of GC–MS and IR spectra with authentic
specimens. The reaction mixtures showed no peaks except those
of 6, 7 and 9 for the GLC analysis, and thus we thought that 8b
and 8c were not formed in the reactions.
S,S-Di-n-butyl-N-(4-nitrophenyl)imino-ë4-sulfane
This yellow crystalline compound had mp 68–69 ЊC; νmax
-
(KBr)/cmϪ1 3960, 3930, 2860, 1580 (N᎐S), 1570 (N᎐O), 1485,
᎐
᎐
J. Chem. Soc., Perkin Trans. 2, 1998
1745

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2 H. Takeuchi, K. Itou, H. Murai and K. Koyama, J. Chem. Res. (S),
1991, 347; H. Takeuchi, K. Itou, H. Murai and K. Koyama,
J. Chem. Res. (M), 1991, 3156.
3 H. Takeuchi and K. Koyama, J. Chem. Soc., Perkin Trans. 1, 1988,
2277.
4 N. S. Isaacs, Reactive Intermediates in Organic Chemistry, Wiley,
London, New York, Sydney and Toronto, ch. 2, p. 106.
5 H. Takeuchi, K. Takano and K. Koyama, J. Chem. Soc., Chem.
Commun., 1982, 1254; H. Takeuchi and K. Takano, J. Chem. Soc.,
Perkin Trans. 1, 1986, 611.
6 H. Takeuchi, S. Hayakawa and H. Murai, J. Chem. Soc., Chem.
Commun., 1988, 1287; H. Takeuchi, S. Hayakawa, T. Tanahashi,
A. Kobayashi, T. Adachi and D. Higuchi, J. Chem. Soc., Perkin
Trans. 2, 1991, 847; H. Takeuchi, D. Higuchi and T. Adachi,
J. Chem. Soc., Perkin Trans. 1, 1991, 1525.
7 H. Takeuchi, T. Yanase, K. Itou, H. Oya and T. Adachi, J. Chem.
Soc., Chem. Commun., 1992, 916; H. Takeuchi, H. Oya, T. Yanase,
K. Itou, T. Adachi, H. Sugiura and N. Hayashi, J. Chem. Soc.,
Perkin Trans. 2, 1994, 827.
8 G. T. Morgan and L. P. Walls, J. Soc. Chem. Ind. London Trans.
Commun., 1930, 49, 15.
9 K. Orito, T. Hatakeyama, M. Takeo and H. Suginome, Synthesis,
1995, 1357.
Rate constants and activation parameters for decomposition of
S,S-dialkylanilinosulfonium salts 3b–d in TFA
S,S-Dimethyl-N-(4-nitrophenyl)imino-λ4-sulfane
(precursor
of 3b), or S,S-di-n-butyl-N-(4-nitrophenyl)imino-λ4-sulfane
(precursor of 3d), 0.5 mmol, was dissolved in TFA (5.0 cm3).
The solution was allowed to stand for various times (1, 3 and
5 h) at a given temperature. S,S-Dimethyl-N-(4-methylanilino)-
sulfonium picrate (precursor of 3c), 0.5 mmol, was dissolved in
27, 22 and 6.0 cm3 of TFA at 20, 30 and 50 ЊC, respectively, and
allowed to stand for various times; 1, 2 and 4 h at 20 ЊC, 0.5, 1
and 2 h at 30 ЊC and 0.5, 0.75 and 1 h at 50 ЊC. After the
reaction mixture was treated as shown above, the first-order rate
constants kobs of decomposition of 3b–d at a given temperature
were obtained by determination of the amount of 3b–d as a
function of time, and are shown in Table 4. In this case, the
molar amount of 3 was obtained by deducting the total molar
amount of the products 6 and 7 from the molar quantity of the
precursor of 3. The total molar amount of the products was
measured by GLC analysis.
Activation parameters ∆H‡, ∆S‡ and ∆G‡ were calculated
from the equation derived from transition state theory, and are
shown in Table 4.
10 A. K. Sharama, T. Ku, A. D. Dawson and D. Swern, J. Org. Chem.,
1975, 40, 2758.
11 F. Swarts, Bull. Sci. Acad. R. Belg., 1922, 8, 343.
References
Paper 8/03039A
Received 23rd April 1998
Accepted 19th May 1998
1 H. Takeuchi, S. Hirayama, M. Mitani and K. Koyama, J. Chem.
Soc., Perkin Trans. 1, 1988, 521.
1746
J. Chem. Soc., Perkin Trans. 2, 1998