Intramolecular addition of benzylic radicals onto ketenimines. Synthesis of 2-alkylindoles

Article abstract of DOI:10.1039/b310593h

The inter- and intramolecular addition of free radicals onto ketenimines is studied. All the attempts to add intermolecularly several silicon, oxygen or carbon centered radicals to N-(4-methylphenyl)-C,C-diphenyl ketenimine were unsuccessful. In contrast, the intramolecular addition of benzylic radicals, generated from xanthates, onto the central carbon of a ketenimine function with its N atom linked to the ortho position of the aromatic ring occurred under a variety of reaction conditions. These intramolecular cyclizations provide a novel radical-mediated synthesis of 2-alkylindoles.

Full text of DOI:10.1039/b310593h

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Intramolecular addition of benzylic radicals onto ketenimines.
Synthesis of 2-alkylindoles
Mateo Alajarín, Angel Vidal* and María-Mar Ortín
Departamento de Química Orgánica, Facultad de Química, Universidad de Murcia,
Campus de Espinardo, 30100 Murcia, Spain. E-mail: vidal@um.es; Fax: 34 968 364149;
Tel: 34 968 367418
Received 3rd September 2003, Accepted 1st October 2003
First published as an Advance Article on the web 23rd October 2003
The inter- and intramolecular addition of free radicals onto ketenimines is studied. All the attempts to add
intermolecularly several silicon, oxygen or carbon centered radicals to N-(4-methylphenyl)-C,C-diphenyl ketenimine
were unsuccessful. In contrast, the intramolecular addition of benzylic radicals, generated from xanthates, onto the
central carbon of a ketenimine function with its N atom linked to the ortho position of the aromatic ring occurred
under a variety of reaction conditions. These intramolecular cyclizations provide a novel radical-mediated synthesis
of 2-alkylindoles.
On the basis of the results of Tidwell and co-workers on the
radical reactions of ketenes, it is conceivable that radicals R
could add initially to carbon atoms Cα or Cβ of the ketenimine 1
to give either a tert-alkyl radical 2 or an imidoyl radical 3
Introduction
6
ؒ
In the field of modern organic synthesis, free radical chemistry1
is gaining considerable importance and getting its place along-
side the more familiar ionic chemistry, due to its growing
versatility, predictability and functional group tolerance. In
(Scheme 1). Afterwards, radicals 2 and 3, depending on the reac-
tion conditions, could undergo reduction, dimerization or be
recent years, radical chemistry has become a powerful and
captured by other species present in the reaction medium.
standard strategy in the total synthesis of natural products2
and in the preparation of heterocyclic compounds.3
The known chemistry of ketenimines is mainly based on the
addition of nucleophiles to their electrophilic sp-hybridized
carbon atom, and on their participation in pericyclic processes.4
However, the radical chemistry of ketenimines remains
unexplored. To the best of our knowledge no inter or intra-
molecular free radical additions to ketenimines have been
reported. Moreover, the free radical chemistry of hetero-
cumulenes is practically unknown,5 with the exception of
those of ketenes and isothiocyanates which are currently
under investigation by the groups of Tidwell6 and Nanni,7
respectively.
In the last few years, we have been involved in the study of
the chemical behaviour of ketenimines. In our hands, these
heterocumulenes have been successfully employed in the syn-
thesis of nitrogen-containing heterocycles, via their partici-
pation in electrocyclic ring closures,8 intramolecular [2 ϩ 2]9
and [4 ϩ 2]10 cycloadditions, and imino-ene type reactions.11
Now we have focused our attention on the development of
inter- and intramolecular radical processes involving keten-
imines as substrates.
We present here our results in the study of the intermolecular
addition reactions of different types of free radicals onto keten-
imines and those concerning the intramolecular addition of
benzylic radicals onto a ketenimine function linked to an ortho
position on the aromatic ring by means of its nitrogen atom.12
These intramolecular cyclizations represent a novel method-
ology for the synthesis of indoles.13
Scheme 1 Presumed main paths for the intermolecular addition of
free radicals to ketenimines.
The ionic chemistry of ketenimines involves the attack of
nucleophiles at the Cα carbon atom.4 As it occurs for ketenes, it
may be expected that the free radical chemistry of ketenimines
parallels their ionic chemistry and thereby the tendency for
nucleophilic radicals to attack at Cα. The regiochemistry of
radical additions to ketenimines must also be influenced by the
stability of the radical resulting from the initial addition. Thus,
the addition of radicals to ketenimines should preferentially
give type 2 alkyl radicals (versus the less stable sp2 carbon
centered type 3 radicals), especially if the stability of radical 2 is
adequately enhanced by placing stabilizing substituents on the
sp3 carbon atom, as is the case shown in Scheme 1.
Unfortunately, all the attempts to add several silicon, oxygen
or carbon-centered radicals to ketenimine 1 under reaction
Results and discussion
conditions very similar to those in which these classes of radi-
᎐
cals react with compounds containing C᎐C or C᎐C bonds were
᎐
᎐
Intermolecular addition of free radicals to ketenimines
unsucessful. In these reactions ketenimine 1 was recovered
unaltered or gave rise to complex reaction mixtures that we
could not resolve.
Our first try at the radical chemistry of ketenimines was the
ؒ
intermolecular addition of different types of free radicals (R )
ؒ
to the easily available, and stable, N-(4-methylphenyl)-C,C-di-
phenyl ketenimine 1,14 with the aim of establishing the feasibil-
ity of these intermolecular processes and their regioselectivity.
Experiments with the silyl radical [(CH3)3Si]3Si , generated
by reacting tris(trimethylsilyl)silane and AIBN in benzene or
toluene at reflux temperature,15 did not provide evidence for
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 4 2 8 2 – 4 2 9 2
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
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intermolecular attack on ketenimine 1. No reaction was
observed even after we added a stoichiometric excess of tri-
s(trimethylsilyl)silane/AIBN. Similarly, from the reaction of
ketenimine 1 with four equivalents of the commercially avail-
able oxygen centered radical TEMPO16 (2,2,6,6-tetramethyl-1-
piperidinyloxy free radical), in dichloromethane at room
temperature, compound 1 was recovered. In sharp contrast,
when we carried out the reaction of 1 with TEMPO in benzene
or toluene at reflux temperature we only obtained complex
reaction mixtures.
As already mentioned, the reaction of ketenimine 1 with
carbon radicals was also explored. For this purpose, amongst
all the reported methods for producing carbon radicals we
selected those involving (a) the reaction of xanthates with
organic peroxides as radical initiators,17 and (b) the reaction of
alkyl phenyl selenides with the system tris(trimethylsilyl)silane/
AIBN.18 Using the xanthate-based methodology we did not
observe a reaction when we added only a catalytic amount of
the peroxide initiator to the reaction mixture. In fact, the radi-
cal precursor was totally consumed only when a stoichiometric
amount of peroxide was added. In these cases complex reaction
mixtures were formed in which none of the expected radical
addition products could be detected. Finally, the addition of
carbon radicals to ketenimine 1 also failed when this hetero-
cumulenic compound was submitted to reaction with several
secondary alkyl or benzyl phenyl selenides in the presence of an
excess of tris(trimethylsilyl)silane/AIBN.
Although we did not obtain any successful results in our
attempts to add free radicals to ketenimines intermolecularly,
this study gave us a background of radical reaction conditions
under which a ketenimine function could survive, which was
actually very useful in the study of the intramolecular
processes.
Scheme 2 Reagents and conditions: (a) KSC(S)OEt, acetone, rt, 1 h;
(b) PPh3, diethyl ether, rt, 6 h; (c) PhR4C᎐C᎐O, dichloromethane, rt,
᎐
᎐
30 min; (d) lauroyl peroxide (1.5 eq.), cyclohexane, reflux, 30 h;
(e) lauroyl peroxide (1.2 eq.), methanol/1,2-dichloroethane, reflux, 24 h.
Intramolecular addition of benzylic radicals onto ketenimines
We reasoned that the entropic assistance inherent to intra-
molecular reactions would hopefully allow us to achieve the
intramolecular addition of radicals to ketenimines. To that
end, ketenimines 7 (Scheme 2) were specifically designed for
generating benzylic radicals which could undergo a favoured
5-exo-dig cyclization onto the central carbon of the ketenimine
function. In compounds 7 two substituents are placed on the
terminal carbon atom of the ketenimine function in order to
obtain stabilized tertiary radicals in the cyclization step, thus
making that cyclization a more favourable process. Compounds
7 bear a xanthate group from which the benzylic radical
could be generated under mild reaction conditions. Moreover,
the xanthate group is compatible with our well-established
methodology for the preparation of ketenimines, that is, the
aza-Wittig reaction of phosphazenes and ketenes.8–11
Scheme 3 Reagents and conditions: (a) t-butyl peroxide (1.2 eq.),
chlorobenzene, reflux, 24 h.
initiator to a 0.01 M solution of ketenimine 7c (R1 = Cl, R2 =
R3 = H, R4 = Ph) in boiling cyclohexane produced indole 8,
which incorporates the lauroyloxy fragment, in 38% yield,
along with a small amount (4%) of the reduced indole 9c. The
reaction initiated by lauroyl peroxide and conducted in a
mixture of methanol/1,2-dichloroethane (1 : 3; v/v) led to
2-(α-methoxy-α,α-diphenyl)methylindole 10c in 43% yield.20
When a stoichiometric amount of t-butyl peroxide was
added portionwise to a 0.01 M solution of ketenimine 7c in
boiling chlorobenzene, 5-chloro-2-diphenylmethylindole 9c
was obtained in 60% yield, as the only product which could be
isolated from the reaction mixture.
Using methanol/1,2-dichloroethane as reaction medium the
cyclization of ketenimine 7d (R1 = CH3, R2 = R3 = H, R4 = Ph)
was also achieved to give 10d (34%) (Scheme 2).20 Cyclization
of ketenimines 7a,b,d–h under the t-butyl peroxide/chloro-
benzene conditions furnished further examples of 2-substituted
indoles 9 in moderate yields (Scheme 3) (Table 1).
Ketenimines 7 were prepared in high overall yields from the
2-azidobenzyl bromides or chlorides 4 in three steps (Scheme
2). The substitution of the bromine or chlorine atom in com-
pounds 4 by the xanthate group to provide xanthates 5 was
carried out by reaction with the commercially available salt
KSC(S)OEt, in acetone at room temperature. Staudinger
treatment19 of the azidoxanthates 5 with triphenylphosphane,
in diethyl ether solution at room temperature, yielded the tri-
phenylphosphazenes 6. Reaction of the triphenylphosphazenes
6 with 1 equivalent of methyl phenyl ketene or diphenyl ketene,
in dichloromethane at room temperature for a short period
of time, gave ketenimines 7. Compounds 7 were purified by
column chromatography on silica gel and were fully character-
ized (see experimental section). Their IR spectra showed strong
absorptions around 2000 cmϪ1 attributable to the N᎐C᎐C
The structural characterization of indoles 8, 9 and 10 relies
on their analytical and spectroscopic data.21 In this respect, the
IR spectrum of indole 8 shows a strong absorption at 1706 cmϪ1
᎐ ᎐
grouping.
Radical cyclization of ketenimines 7 was explored under
different conditions (Schemes 2 and 3). Gradual addition of a
stoichiometric excess (1.5 eq.) of lauroyl peroxide as radical
corresponding to the C᎐O vibration. In the 1H NMR spectrum
᎐
of this compound the indolic NH appears as a broad singlet at
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 4 2 8 2 – 4 2 9 2
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Table 1 2-substituted indoles 9 from ketenimines 7
Compound
R1
R2
R3
R4
Yield (%)
9a
9b
9c
9d
9e
9f
H
H
H
H
H
NO2
H
H
H
H
H
H
CH3
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
CH3
50
24
60
47
64
60
52
44
Br
Cl
CH3
H
H
9g
9h
C6H4
H
H
H
δ = 8.49, and the C(3)H proton is observed at δ = 6.04. The 13
C
NMR spectrum shows the signal due to the C᎐O group at
᎐
δ = 179.3. The incorporation of the methoxy group on the
indoles 10 was demonstrated by the presence in their 1H NMR
spectra of a singlet near δ = 3.1, integrating three protons, and
the appearance in their 13C NMR spectra of a signal at δ = 52.4.
Concerning the structural determination of the indoles 9, the
methine proton of the substituent at C2 resonates as a singlet in
their 1H NMR spectra at δ = 6.00–6.42 for compounds 9a–g, or
as a quartet at δ = 4.29 for 9h. The C(3)H proton appears at
δ = 6.00–6.42. Their 13C NMR spectra show the signals due to
the methine carbon of the mentioned substituent at δ = 39.2–
51.4, and that of C3 at δ = 99.6–103.7. The MS and analytical
data of indoles 8, 9 and 10 are in perfect agreement with the
proposed structures.
A reasonable mechanism for explaining the conversions
7
8, 7 9 and 7 10 is shown in Scheme 4. The thermal
ؒ
decomposition of the peroxide initiator produces radical (R0) ,
which exchanges the xanthate group with ketenimines 7 giving
rise to the expected benzylic radicals 11, which then undergo a
5-exo-dig addition of the radical moiety onto the electrophilic
central carbon atom of the ketenimine function, followed by a
prototropic imine–enamine equilibrium favouring the aromatic
indole form 13. The stabilized tertiary triarylmethyl-type radi-
cals 13 did not react with a new molecule of 7 to sustain the
radical chain sequence.22 Instead they underwent reduction to
give indoles 9, or electron transfer to the lauroyl peroxide to
furnish carbocations 14, which under the reaction conditions
were quenched by the carboxylate anion generated in the redox
process,23 to furnish indole 8, or if methanol was used as
solvent, to afford indoles 10.23b
The conversion of ketenimines 7 into the indoles 9 is a reduc-
tive process and the hydrogen atom donor toward the inter-
mediate radicals 13 is not obvious.24 In this respect, we were not
able to isolate any product from the crude reaction mixtures
that provides us with evidence about which species present in
the reaction medium acts as the source of hydrogen atoms. We
reasoned that the yield of these reactions might be improved
by addition to the reaction mixture of hydrogen donors.
With this aim we chose 1,4-cyclohexadiene which is compatible
with the ketenimine function. The addition of 5 equivalents of
1,4-cyclohexadiene to the initial reaction mixture of the treat-
ment of ketenimine 7d with t-butyl peroxide led to a cleaner
reaction, but the yield of the corresponding indole 9d did not
improve.
Scheme 4 Proposed mechanism for the conversions 7 8, 7 9 and
10.
7
15 should be formed by the coupling of radical 13d with that
arising from the ortho-xylene (Scheme 5).
Then, we directed our efforts towards obtaining mechanistic
insights into the radical conversions here described. Attempts
of trapping intermolecularly the benzylic radicals 11 by carry-
ing out the radical cyclization of ketenimines 7 in the presence
of an excess of allyl acetate or benzyl acrylate failed. These
results prove that the cyclization of radicals 11 is a more
favourable path due to the great stability of the tertiary radicals
12 and 13. We also conducted an experiment aimed at capturing
radicals 13. It is known that t-butyl peroxide abstracts hydrogen
atoms from substituted toluenes to yield benzylic radicals.25
Thus we performed the radical cyclization of ketenimine 7d
using t-butyl peroxide as radical initiator in boiling ortho-
xylene26 pleasantly obtaining the indole 15 in 52% yield. Indole
An attractive alternative that we considered for generating
the benzylic radicals 11 was the substitution of the O-ethyl
xanthate group present in the ketenimines 7 by other radical-
precursor functional groups, such as N-phenylthioxocarb-
amate, S-methyl xanthate and phenylthionocarbonate. The
reaction of these new derivatives with the good hydrogen donor
tris(trimethylsilyl)silane18,27 in the presence of AIBN as radical
initiator could also provide the desired benzylic radicals 11, and
accordingly the indoles 9.
The treatment of the azidobenzyl alcohols 16 with phenyliso-
thiocyanate, carbon disulfide/methyl iodide or phenyl chloro-
thionoformate/DMAP, under standard conditions, yielded the
N-phenylthioxocarbamate 17, the S-methyl xanthate 18 and the
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 4 2 8 2 – 4 2 9 2
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Table 2 Selected NMR data for compounds 17–19 and 20–22
Compound
δH CH2
δC CH2
δC C᎐S
δC C᎐O
᎐
᎐
17
18
19
20
21
22
5.58
5.54
5.47
4.52
4.52
4.46
69.5a
70.9
71.3
33.0
33.6
34.7
188.1
215.6
194.9
167.6
191.4
172.1
a Very broad signal.
The O- to S- transpositions observed in the formation of
triphenylphosphazenes 20–22 belong to a known class of
reactions. Probably, these compounds are formed via a
Newman–Kwart28 or Schönberg29 rearrangement, but this was
unexpected for us given the mild reaction conditions involved in
the Staudinger treatment of azides 17–19.
Attempts at intramolecular addition of benzylic radicals onto
carbodiimides and isothiocyanates
The success of the radical cyclization of ketenimines 7 led us
to study similar reactions of structurally related compounds
in which the ketenimine moiety has been substituted by
other heterocumulenic functions such as carbodiimide or
isothiocyanate.
Scheme 5 Reagents and conditions: (a) t-butyl peroxide (1.2 eq.), ortho-
xylene, reflux, 24 h.
To test the feasibility of the intramolecular addition of
benzylic radicals onto carbodiimides we prepared compound
23 by reaction of triphenylphosphazene 6d and 4-methyl-
phenylisocyanate (Scheme 7). However, all the attempts at radi-
cal cyclization of the xanthate-carbodiimide 23 under similar
reaction conditions to those utilized in the cyclization of keten-
imines 7 failed and just yielded complex reaction mixtures.
phenylthionocarbonate 19, respectively. The Staudinger reac-
tion of azides 17–19 with triphenylphosphane, in diethyl ether
solution at room temperature, afforded the triphenyl-
phosphazenes 20–22, in which the benzyl group has experi-
enced an oxygen to sulfur migration (Scheme 6).
Scheme 6 Reagents and conditions: (a) Ph–NCS, NaH, tetrahydro-
furan, rt, 16 h to give 17; (a) CS2, NaH, tetrahydrofuran, rt 2 h and
50 ЊC 1 h, then CH3I, 50 ЊC, 6 h to give 18; (a) Ph–O–C(S)Cl, DMAP,
acetonitrile, rt, 1 h to give 19; (b) PPh3, diethyl ether, rt, 6 h.
Scheme 7 Reagents and conditions: (a) 4-CH3–C6H4–NCO, dichloro-
methane, rt, 30 min.
The isothiocyanate 26 was easily obtained by reaction of tri-
phenylphosphazene 6d with carbon disulfide. The addition, in
small portions, of a stoichiometric amount of lauroyl peroxide
to a solution of isothiocyanate 26 (0.015 M) in boiling cyclo-
hexane did not give any product resulting from the intra-
molecular cyclization of the intermediate benzylic radical 27,
instead the dimeric bis(isothiocyanate) 28 was obtained in
quantitative yield (Scheme 8).
The determination of the structure of compounds 20–22 was
mainly based on their 1H and 13C NMR spectroscopic data. Of
particular relevance were the observed chemical shifts for the
protons of the benzylic methylene group in their 1H NMR
spectra, and the chemical shifts of the carbon atom of that
group in their 13C NMR spectra. In all cases, a notable upfield
shift was observed for the nuclei mentioned above when
compared with the chemical shifts of the same nuclei in the
corresponding azides 17–19 (Table 2). These shift differences
support that in compounds 20–22 the benzyl group is on the
sulfur atom. Also in the 13C NMR spectra of the triphenyl-
phosphazenes 20–22 the signals that appear at higher δ values
have undergone an upfield shift with respect the same signals
in the spectra of the azides 17–19, accounting for the presence
of a carbonyl instead of a thiocarbonyl group in compounds
20–22.
Conclusion
In summary, in this report we have described the difficulties
encountered in the intermolecular addition of free radicals to
ketenimines and the feasibility of this process in its intra-
molecular version. We have shown how ketenimines undergo
intramolecular addition of free carbon-centered radicals, gen-
erated from xanthates, providing a new radical tin-free route to
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 4 2 8 2 – 4 2 9 2
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Preparation of 3-azido-2-bromomethylnaphthalene 4g
A solution of 3-amino-2-naphthoic acid (0.94 g, 5 mmol) in
anhydrous tetrahydrofuran (20 ml) was added dropwise over
30 min to a stirred solution of lithium aluminium hydride
(0.47 g, 12.5 mmol) in tetrahydrofuran (20 ml) at 0 ЊC. The
mixture was stirred at room temperature under an atmosphere
of nitrogen for 16 h, after which it was cooled and quenched by
careful addition of water (20 ml). The mixture was basified with
10% sodium hydroxide (20 ml) and then extracted with diethyl
ether (4 × 50 ml). The combined extracts were dried over
anhydrous magnesium sulfate. The solvent was removed under
reduced pressure and the resulting material was triturated with
cold diethyl ether to give 3-amino-2-hydroxymethylnaphthalene
(0.34 g, 39%).
A solution of sodium nitrite (0.43 g, 6.25 mmol) in water
(5 ml) was added dropwise over 5 min to a solution of 3-amino-
2-hydroxymethylnaphthalene (0.87 g, 5 mmol) in water (10 ml)
and concentrated sulfuric acid (2 ml) at 0 ЊC. The mixture was
stirred at that temperature for 30 min. Then a solution of
sodium azide (0.57 g, 8.75 mmol) in water (5 ml) was added
dropwise over 10 min. After stirring for 5 h the precipitated
3-azido-2-hydroxymethylnaphthalene was isolated by filtration,
washed with water and air dried (0.85 g, 85%).
Scheme 8 Reagents and conditions: (a) CS2, benzene, reflux, 12 h;
(b) lauroyl peroxide (1.0 eq.), cyclohexane, reflux, 24 h.
indoles. The success of these cyclizations may be due to the
exceptional stability of the radicals resulting from the cycliz-
ation step.
A solution of bromine (0.64 g, 4 mmol) in anhydrous ben-
zene (5 ml) was added dropwise over 30 min to a stirred solu-
tion of triphenylphosphane (1.05 g, 4 mmol) in anhydrous
benzene (15 ml) at 0 ЊC. Then 3-azido-2-hydroxymethylnaph-
thalene (0.80 g, 4 mmol) and triethylamine (0.41 g, 4 mmol)
were added and the stirring was continued for 3 h. After fil-
tration the solvent was removed under reduced pressure and the
residue was purified by column chromatography [silica gel,
using hexanes/diethyl ether (9 : 1) as eluent] to give 3-azido-2-
bromomethylnaphthalene 4g (0.71 g, 68%) (Found: C, 50.1; H,
2.9; N, 16.3. C11H8BrN3 requires C, 50.4; H, 3.1; N, 16.0%);
νmax(film)/cmϪ1 2134, 2113, 1622, 1594, 1501, 1288, 1212, 868,
843 and 756; δH (200 MHz; CDCl3; Me4Si) 4.59 (2 H, s), 7.37–
7.53 (3 H, m), 7.67–7.77 (2 H, m), 7.81 (1 H, s); δC (50 MHz;
CDCl3; Me4Si) 29.1, 116.0, 126.0, 126.5, 127.6, 127.9, 128.4 (s),
130.8 (s), 130.9, 133.9 (s), 136.7 (s).
Experimental
All melting points were determined on a Kofler hot-plate melt-
ing point apparatus and are uncorrected. IR spectra were
obtained on a Nicolet Impact 400 spectrophotometer. NMR
spectra were recorded on a Bruker AC200 or on a Varian Unity
300 and are reported in ppm on the δ scale. J values are given in
Hz. Mass spectra were recorded on a Hewlett-Packard 5993C
spectrometer or on a VG-Autospec spectrometer. Micro-
analyses were performed on a Carlo Erba EA-1108 instrument.
Materials
2-Azidobenzyl chloride 4a,30 2-azido-5-chlorobenzyl chloride
4c,31 2-azido-5-methylbenzyl chloride 4d,32 2-azido-4-nitro-
benzyl bromide 4e,32 2-azido-3-methylbenzyl chloride 4f,31
2-azidobenzyl alcohol 16a,33 2-azido-5-methylbenzyl alcohol
16b,34 methyl phenyl ketene35 and diphenyl ketene36 were
prepared by literature procedures.
Preparation of dithiocarbonic acid O-ethyl ester S-(2-azido-
benzyl) esters 5
A solution of potassium O-ethyl xanthate (0.80 g, 5 mmol) in
anhydrous acetone (20 ml) was added dropwise to a solution of
the corresponding 2-azidobenzyl bromide or 2-azidobenzyl
chloride 4 (5 mmol) in the same solvent (15 ml), while stirring at
room temperature. After 1 h, the KBr was separated by fil-
tration, the solvent was removed under reduced pressure and
the residue was purified by column chromatography [silica gel,
using hexanes/diethyl ether (9 : 1) as eluent].
Preparation of 2-azido-5-bromobenzyl bromide 4b
To a solution of 2-azidobenzyl alcohol 16a (2.98 g, 20 mmol)
in anhydrous tetrahydrofuran (15 ml) N-bromosuccinimide
(3.56 g, 20 mmol) was added. The reaction mixture was stirred
at room temperature for 24 h. After separation of the precipi-
tated succinimide by filtration the solvent was removed under
reduced pressure to leave a yellow solid, which was purified by
column chromatography [silica gel, using hexanes/ethyl acetate
(4 : 1) as eluent] to give 2-azido-5-bromobenzyl alcohol (3.42 g,
75%).
A solution of bromine (0.64 g, 4 mmol) in anhydrous
benzene (5 ml) was added dropwise over 30 min to a stirred
solution of triphenylphosphane (1.05 g, 4 mmol) in anhydrous
benzene (15 ml) at 0 ЊC. Then 2-azido-5-bromobenzyl alcohol
(0.91 g, 4 mmol) and triethylamine (0.41 g, 4 mmol) were added
and the stirring was continued for 3 h. After filtration the sol-
vent was removed under reduced pressure and the residue was
purified by column chromatography [silica gel, using hexanes/
diethyl ether (9 : 1) as eluent] to give 2-azido-5-bromobenzyl
bromide 4b (0.95 g, 82%) (Found: C, 28.6; H, 1.5; N, 4.5.
C7H5Br2N3 requires C, 28.9; H, 1.7; N, 4.8%); νmax(film)/cmϪ1
2125, 2083, 1482, 1436, 1302, 1217, 1196, 1102, 893, 874 and
811; δH (200 MHz; CDCl3; Me4Si) 4.38 (2 H, s), 7.01 (1 H, d,
J = 8.5), 7.44 (1 H, dd, J = 8.5 and 2.3), 7.49 (1 H, d, J = 2.3);
δC (50 MHz; CDCl3; Me4Si) 27.1, 117.6 (s), 120.1, 130.8 (s),
132.9, 134.1, 137.8 (s).
Dithiocarbonic acid O-ethyl ester S-(2-azidobenzyl) ester (5a).
(0.92 g, 73%); yellow oil (Found: C, 47.15; H, 4.2; N, 16.8.
C10H11N3OS2 requires C, 47.4; H, 4.4; N, 16.6%); νmax(film)/
cmϪ1 2121, 1582, 1491, 1451, 1292, 1218, 1150, 1112, 1048 and
749; δH (300 MHz; CDCl3; Me4Si) 1.41 (3 H, t, J = 7.1), 4.31
(2 H, s), 4.64 (2 H, q, J = 7.1), 7.03–7.10 (2 H, m), 7.23–7.40
(2 H, m); δC (75 MHz;, CDCl3; Me4Si) 13.8, 35.7, 70.0, 118.2,
124.7, 127.0 (s), 129.1, 131.1, 138.6 (s), 214.0 (s).
Dithiocarbonic acid O-ethyl ester S-(2-azido-5-bromobenzyl)
ester (5b). (1.58, 95%); yellow oil (Found: C, 36.3; H, 3.0; N,
12.5. C10H10BrN3OS2 requires C, 36.15; H, 3.1; N, 12.65%);
νmax(film)/cmϪ1 2127, 2079, 1299, 1263, 1225, 1119, 1105, 1051,
1004, 893, 872, 804 and 738; δH (300 MHz; CDCl3; Me4Si) 1.43
(3 H, t, J = 7.0), 4.25 (2 H, s), 4.65 (2 H, q, J = 7.0), 7.00 (1 H, d,
J = 8.4), 7.41 (1 H, dd, J = 8.4 and 2.4), 7.54 (1 H, d, J = 2.4);
δC (75 MHz; CDCl3; Me4Si) 13.9, 35.2, 70.3, 117.5 (s), 119.8,
129.5 (s), 132.0, 133.9, 137.9 (s), 213.4 (s).
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Dithiocarbonic acid O-ethyl ester S-(2-azido-5-chlorobenzyl)
ester (5c). (1.16 g, 81%); yellow oil (Found: C, 41.5; H, 3.3; N,
14.4. C10H10ClN3OS2 requires C, 41.7; H, 3.5; N, 14.6%);
νmax(film)/cmϪ1 2128, 2084, 1590, 1490, 1303, 1219, 1150, 1113,
1049, 897 and 815; δH (300 MHz; CDCl3; Me4Si) 1.43 (3 H, t,
J = 7.1), 4.25 (2 H, s), 4.65 (2 H, q, J = 7.1), 7.07 (1 H, d,
J = 8.6), 7.28 (1 H, dd, J = 8.6 and 2.4), 7.40 (1 H, d, J = 2.4);
δC (75 MHz; CDCl3; Me4Si) 13.8, 35.2, 70.4, 119.4, 129.0, 129.1
(s), 129.8 (s), 131.0, 137.3 (s), 213.4 (s).
7.52 (9 H, m), 7.70–7.80 (6 H, m); δC (75 MHz; CDCl3; Me4Si)
13.9, 39.2, 69.4, 116.8, 120.7 (d, J = 10.2), 128.1, 128.6 (d,
J = 11.9), 130.0, 130.1, 130.2 (d, J = 2.5), 130.7 (d, J = 121.3),
131.7 (d, J = 2.8), 132.6 (d, J = 9.6), 150.1 (d, J = 0.7), 217.0 (s);
δP (121.4 MHz; CDCl3; H3PO4) 3.6; m/z (EI) 487 (Mϩ, 5%), 366
(100).
Dithiocarbonic acid O-ethyl ester S-(5-bromo-2-triphenyl-
phosphoranylideneamino)benzyl ester (6b). (2.41 g, 85%); mp
123–125 ЊC; colourless prisms (Found: C, 59.5; H, 4.3; N, 2.4.
C28H25BrNOPS2 requires C, 59.3; H, 4.45; N, 2.5%); νmax(nujol)/
cmϪ1 1576, 1437, 1244, 1211, 1110, 1047, 815, 720 and 693;
δH (300 MHz; CDCl3; Me4Si) 1.46 (3 H, t, J = 7.2), 4.61 (2 H, s),
4.68 (2 H, q, J = 7.2), 6.25 (1 H, d, J = 8.6), 6.88 (1 H,
dd, J = 8.6 and 2.6), 7.41–7.55 (10 H, m), 7.70–7.77 (6 H, m);
δC (75 MHz; CDCl3; Me4Si) 13.9, 38.6, 69.7, 108.4, 121.9 (d,
J = 10.4), 128.8 (d, J = 12.2), 130.6 (d, J = 99.7), 130.7, 131.9,
132.6 (d, J = 9.9), 149.3 (s), 216.3 (s); δP (121.4 MHz; CDCl3;
H3PO4) 4.4; m/z (EI) 567 (Mϩ ϩ 2, 5%), 565 (Mϩ, 4), 183 (100).
Dithiocarbonic acid O-ethyl ester S-(2-azido-5-methylbenzyl)
ester (5d). (1.18 g, 88%); yellow oil (Found: C, 49.4; H, 4.8; N,
15.45. C11H13N3OS2 requires C, 49.3; H, 4.9; N, 15.7%);
νmax(film)/cmϪ1 2125, 2083, 1498, 1297, 1214, 1146, 1112, 1049
and 808; δH (300 MHz; CDCl3; Me4Si) 1.42 (3 H, t, J = 7.2), 2.29
(3 H, s), 4.28 (2 H, s), 4.65 (2 H, q, J = 7.2), 7.01 (1 H, d, J = 8.0),
7.11 (1 H, dd, J = 8.0 and 2.0), 7.18 (1 H, d, J = 2.0); δC (75
MHz; CDCl3; Me4Si) 13.8, 20.8, 35.8, 70.0, 118.2, 126.7 (s),
129.8, 131.8, 134.6 (s), 135.9 (s), 214.1 (s).
Dithiocarbonic acid O-ethyl ester S-(5-chloro-2-triphenyl-
phosphoranylideneamino)benzyl ester (6c). (2.35 g, 90%); mp
139–140 ЊC; colourless prisms (Found: C, 64.2; H, 4.7; N, 2.6.
C28H25ClNOPS2 requires C, 64.4; H, 4.8; N, 2.7%); νmax(nujol)/
cmϪ1 1584, 1437, 1348, 1245, 1211, 1183, 1114, 1050, 876, 816,
721 and 693; δH (300 MHz; CDCl3; Me4Si) 1.43 (3 H, t, J = 7.1),
4.60 (2 H, s), 4.66 (2 H, q, J = 7.1), 6.28 (1 H, dd, J = 8.5 and
1.0), 6.73 (1 H, dd, J = 8.5 and 2.7), 7.26–7.29 (1 H, m), 7.37–
7.55 (9 H, m), 7.67–7.77 (6 H, m); δC (75 MHz; CDCl3; Me4Si)
13.9, 38.6, 69.7, 121.1 (s), 121.2 (d, J = 13.4), 127.7, 128.7 (d,
J = 12.0), 129.7 (d, J = 2.0), 130.5 (d, J = 99.2), 131.4 (s), 131.9
(d, J = 2.6), 132.5 (d, J = 9.7), 148.7 (s), 216.3 (s); δP (121.4
MHz; CDCl3; H3PO4) 4.9; m/z (EI) 523 (Mϩ ϩ 2, 3%), 521 (Mϩ,
8), 183 (100).
Dithiocarbonic acid O-ethyl ester S-(2-azido-4-nitrobenzyl)
ester (5e). (1.06, 71%); yellow oil (Found: C, 40.0; H, 3.5;
N, 18.6. C10H10N4O3S2 requires C, 40.3; H, 3.4; N, 18.8%);
νmax(film)/cmϪ1 2223, 2122, 1525, 1347, 1290, 1221, 1149, 1112,
1044, 1002, 877, 815, 736 and 654; δH (300 MHz; CDCl3; Me4Si)
1.43 (3 H, t, J = 7.1), 4.37 (2 H, s), 4.65 (2 H, q, J = 7.1), 7.64
(1 H, d, J = 8.4), 7.93 (1 H, dd, J = 8.4 and 2.1), 8.00 (1 H, d,
J = 2.1); δC (75 MHz; CDCl3; Me4Si) 13.8, 35.1, 70.6, 113.2,
119.4, 131.7, 134.7 (s), 140.0 (s), 147.3 (s), 213.0 (s).
Dithiocarbonic acid O-ethyl ester S-(2-azido-3-methylbenzyl)
ester (5f). (0.89 g, 67%); yellow oil (Found: C, 49.2; H, 4.8;
N, 15.7. C11H13N3OS2 requires C, 49.4; H, 4.9; N, 15.7%);
νmax(film)/cmϪ1 2099, 1463, 1434, 1292, 1214, 1149, 1112, 1049,
943, 787 and 759; δH (300 MHz; CDCl3; Me4Si) 1.42 (3 H, t,
J = 7.1), 2.42 (3 H, s), 4.39 (2 H, s), 4.65 (2 H, q, J = 7.1), 7.00–
7.12 (2 H, m), 7.25 (1 H, dd, J = 7.0 and 2.0); δC (75 MHz;
CDCl3; Me4Si) 13.8, 18.0, 37.0, 70.1, 125.9, 128.8, 129.5 (s),
131.2, 133.0 (s), 137.0 (s), 213.9 (s).
Dithiocarbonic acid O-ethyl ester S-(5-methyl-2-triphenyl-
phosphoranylideneamino)benzyl ester (6d). (1.88 g, 75%); mp
104–105 ЊC; colourless prisms (Found: C, 69.2; H, 5.6; N, 2.7.
C29H28NOPS2 requires C, 69.4; H, 5.6; N, 2.8%); νmax(nujol)/
cmϪ1 1609, 1438, 1247, 1197, 1108, 1048, 1026, 999, 857, 814,
717 and 696; δH (300 MHz; CDCl3; Me4Si) 1.42 (3 H, t, J = 7.2),
2.16 (3 H, s), 4.66 (2 H, q, J = 7.2), 4.67 (2 H, s), 6.31 (1 H, dd,
J = 8.0 and 1.0), 6.61 (1 H, dd, J = 8.0 and 2.0), 7.11 (1 H, t,
J = 2.0), 7.37–7.51 (9 H, m), 7.71–7.78 (6 H, m); δC (75 MHz;
CDCl3; Me4Si) 14.0, 20.5, 39.2, 69.4, 120.5 (d, J = 9.9), 125.9 (s),
128.6 (d, J = 12.2), 129.5 (d, J = 23.2), 130.6, 131.3 (d, J = 99.7),
131.6, 132.6 (d, J = 9.9), 147.4 (s), 217.0 (s); δP (121.4 MHz;
CDCl3; H3PO4) 3.2; m/z (EI) 501 (Mϩ, 7%), 380 (100).
Dithiocarbonic acid O-ethyl ester S-[(3-azido-2-naphthyl)-
methyl] ester (5g). (1.38 g, 91%); yellow oil (Found: C, 55.2; H,
4.1; N, 13.7. C14H13N3OS2 requires C, 55.4; H, 4.3; N, 13.85%);
νmax(film)/cmϪ1 2107, 1598, 1464, 1377, 1287, 1219, 1113, 1052,
871, 760 and 749; δH (300 MHz; CDCl3; Me4Si) 1.41 (3 H, t,
J = 7.1), 4.44 (2 H, s), 4.64 (2 H, q, J = 7.1), 7.36–7.50 (3 H, m),
7.66–7.76 (2 H, m), 7.85 (1 H, s); δC (75 MHz; CDCl3; Me4Si)
13.9, 36.2, 70.1, 115.7, 125.8, 126.4, 126.6 (s), 127.1, 127.7,
130.4, 130.7 (s), 133.4 (s), 136.9 (s), 213.9 (s).
Dithiocarbonic acid O-ethyl ester S-(4-nitro-2-triphenyl-
phosphoranylideneamino)benzyl ester (6e). (2.10 g, 79%); mp
150–152 ЊC; colourless prisms (Found: C, 63.0; H, 4.6; N, 5.1.
C28H25N2O3PS2 requires C, 63.2; H, 4.7; N, 5.3%); νmax(nujol)/
cmϪ1 1607, 1560, 1510, 1341, 1294, 1217, 1109, 1044, 862, 813,
740, 718 and 694; δH (300 MHz; CDCl3; Me4Si) 1.42 (3 H, t,
J = 7.1), 4.66 (2 H, q, J = 7.1), 4.70 (2 H, s), 7.14 (1 H, s), 7.39–
7.55 (11 H, m), 7.72–7.83 (6 H, m); δC (75 MHz; CDCl3; Me4Si)
13.9, 38.3, 69.9, 111.4, 113.7 (d, J = 11.0), 129.0 (d, J = 12.2),
129.5 (d, J = 99.7), 129.7 (d, J = 2.0), 132.2 (d, J = 2.7), 132.5 (d,
J = 9.9), 137.8 (d, J = 3.1), 148.0 (s), 151.1 (s), 215.9 (s); δP (121.4
MHz; CDCl3; H3PO4) 7.5; m/z (EI) 532 (Mϩ, 6%), 411 (100).
Preparation of dithiocarbonic acid O-ethyl ester S-(2-triphenyl-
phosphoranylideneamino)benzyl esters 6
To a solution of the corresponding azide 5 (5 mmol) in
anhydrous diethyl ether (15 ml) triphenylphosphane (1.31 g,
5 mmol) was added. The reaction mixture was stirred at room
temperature in an atmosphere of nitrogen for 6 h. Then, the
precipitated compounds 6 were isolated by filtration.
These compounds were used in the following step without
further purification. For analytical samples compounds 6 were
recrystallized from diethyl ether.
Dithiocarbonic acid O-ethyl ester S-(2-triphenylphosphor-
anylideneamino)benzyl ester (6a). (2.05 g, 84%); mp 121–122 ЊC;
colourless prisms (Found: C, 68.8; H, 5.25; N, 2.7. C28H26-
NOPS2 requires C, 69.0; H, 5.4; N, 2.9%); νmax(nujol)/cmϪ1
1589, 1436, 1340, 1310, 1209, 1107, 1041, 998, 847, 745, 718 and
694; δH (300 MHz; CDCl3; Me4Si) 1.41 (3 H, t, J = 7.1), 4.65
(2 H, q, J = 7.1), 4.69 (2 H, s), 6.40 (1 H, d, J = 7.9), 6.57 (1 H, t,
J = 7.2), 6.80 (1 H, td, J = 7.6 and 1.5), 7.27–7.32 (1 H, m), 7.38–
Dithiocarbonic acid O-ethyl ester S-(3-methyl-2-triphenyl-
phosphoranylideneamino)benzyl ester (6f). (2.51 g, 79%); mp
136–138 ЊC; colourless prisms (Found: C, 69.3; H, 5.5; N, 2.7.
C29H28NOPS2 requires C, 69.4; H, 5.6; N, 2.8%); νmax(nujol)/
cmϪ1 1593, 1435, 1232, 1207, 1113, 1052, 748, 716 and 700;
δH (300 MHz; CDCl3; Me4Si) 1.33 (3 H, t, J = 7.2), 1.84 (3 H, s),
4.23 (2 H, s), 4.54 (2 H, q, J = 7.2), 6.63 (1 H, td, J = 7.4 and
1.8), 6.91 (1 H, d, J = 7.4), 7.08 (1 H, d, J = 7.4), 7.35–7.51 (9 H,
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m), 7.55–7.62 (6 H, m); δC (75 MHz; CDCl3; Me4Si) 13.8, 21.3,
39.4, 69.2, 118.9, 127.9 (d, J = 1.5), 128.4 (d, J = 12.2), 129.3 (d,
J = 9.3), 130.0 (d, J = 2.4), 131.4 (d, J = 2.2), 132.3 (d, J = 9.6),
132.5 (d, J = 97.4), 133.2 (d, J = 4.8), 147.6 (s), 215.8 (s);
δP (121.4 MHz; CDCl3; H3PO4) Ϫ2.8; m/z (EI) 501 (Mϩ, 4%),
294 (100).
Ketenimine (7e). (0.76 g, 85%); yellow oil (Found: C, 64.0; H,
4.3; N, 6.1. C24H20N2O3S2 requires C, 64.3; H, 4.5; N, 6.2%);
νmax(film)/cmϪ1 1997, 1527, 1483, 1438, 1348, 1222, 1195, 1119,
1047, 816, 722 and 694; δH (300 MHz; CDCl3; Me4Si) 1.40 (3 H,
t, J = 7.2), 4.62 (2 H, s), 4.63 (2 H, q, J = 7.2), 7.21–7.30 (2 H,
m), 7.34–7.41 (8 H, m), 7.70 (1 H, d, J = 8.4), 8.06 (1 H, dd,
J = 8.4 and 2.2), 8.15 (1 H, d, J = 2.2). δC (75 MHz; CDCl3;
Me4Si) 13.8, 35.9, 70.6, 77.6 (s), 117.5, 122.1, 127.3, 128.2,
129.2, 131.6, 132.8 (s), 139.2 (s), 141.0 (s), 148.1 (s), 194.2 (s),
213.0 (s).
Dithiocarbonic acid O-ethyl ester S-(3-triphenylphosphoranyl-
ideneamino-2-naphthyl)methyl ester (6g). (2.46 g, 91%); mp
67–69 ЊC; colourless prisms (Found: C, 71.2; H, 5.1; N, 2.5.
C32H28NOPS2 requires C, 71.5; H, 5.25; N, 2.7%); νmax(nujol)/
cmϪ1 1595, 1438, 1339, 1281, 1213, 1197, 1114 and 1051; δH (300
MHz; CDCl3; Me4Si) 1.43 (3 H, t, J = 7.0), 4.66 (2 H, q, J = 7.0),
4.84 (2 H, s), 6.63 (1 H, s), 7.06–7.21 (3 H, m), 7.39–7.52 (9 H,
m), 7.59 (2 H, d, J = 7.8), 7.76–7.83 (6 H, m); δC (75 MHz;
CDCl3; Me4Si) 14.0, 39.3, 69.6, 114.5 (d, J = 10.3), 121.7, 125.3,
127.2 (s), 127.3, 128.5, 128.6, 128.7 (d, J = 12.2), 130.7 (d,
J = 99.7), 131.8 (d, J = 2.3), 132.7 (d, J = 9.8), 133.0 (s), 134.8 (s),
148.5 (s), 216.6 (s); δP (121.4 MHz; CDCl3; H3PO4) 5.3; m/z (EI)
537 (Mϩ, 20%), 183 (100).
Ketenimine (7f). (0.70 g, 84%); yellow oil (Found: C, 71.8; H,
5.4; N, 3.2. C25H23NOS2 requires C, 71.9; H, 5.55; N, 3.35%);
νmax(film)/cmϪ1 2012, 1592, 1492, 1463, 1217, 1176, 1115, 1051,
762, 699 and 648; δH (300 MHz; CDCl3; Me4Si) 1.36 (3 H, t,
J = 7.1), 2.24 (3 H, s), 4.42 (2 H, s), 4.57 (2 H, q, J = 7.1),
7.02–7.10 (2 H, m), 7.17–7.24 (2 H, m), 7.28–7.35 (9 H, m);
δC (75 MHz; CDCl3; Me4Si) 13.8, 19.0, 37.1, 69.9, 73.6 (s),
126.2, 126.5, 128.0, 128.6, 128.9, 129.3 (s), 130.8, 131.9 (s),
134.3 (s), 138.3 (s), 187.1 (s), 213.2 (s).
Ketenimine (7g). (0.72 g, 79%); yellow oil (Found: C, 73.9; H,
5.0; N, 3.0. C28H23NOS2 requires C, 74.1; H, 5.1; N, 3.1%);
νmax(film)/cmϪ1 1997, 1595, 1495, 1461, 1436, 1196, 1119, 1047,
883, 748, 724 and 694; δH (300 MHz; CDCl3; Me4Si) 1.45 (3 H,
t, J = 7.2), 4.68 (2 H, q, J = 7.2 Hz), 4.75 (2 H, s), 7.26–7.32
(2 H, m), 7.38–7.50 (10 H, m), 7.77–7.83 (3 H, m), 7.99 (1 H, s);
δC (75 MHz; CDCl3; Me4Si) 13.8, 37.0, 70.0, 78.0 (s), 120.8,
126.6, 126.8, 127.5, 127.6, 127.9, 128.9, 129.8 (s), 130.0, 132.4
(s), 133.3 (s), 133.8 (s), 137.4 (s), 190.9 (s), 213.8 (s).
Preparation of ketenimines 7
To a solution of the corresponding triphenylphosphazene 6
(2 mmol) in anhydrous dichloromethane (25 ml) a solution of
methyl phenyl ketene or diphenyl ketene (2 mmol) in the same
solvent (2 ml) was added. After stirring at room temperature for
30 min the solvent was removed under reduced pressure and the
resulting material was chromatographed on a silica gel column,
using hexanes/diethyl ether (9 : 1) as eluent.
Ketenimine (7h). (0.46 g, 68%); yellow oil (Found: C, 66.7; H,
5.5; N, 4.0. C19H19NOS2 requires C, 66.8; H, 5.6; N, 4.1%);
νmax(nujol)/cmϪ1 2003, 1598, 1582, 1490, 1253, 1219, 1112, 1048,
757 and 692; δH (300 MHz; CDCl3; Me4Si) 1.43 (3 H, t, J = 7.2),
2.13 (3 H, 3), 4.60 (2 H, s), 4.66 (2 H, q, J = 7.2), 7.08–7.15 (1 H,
m), 7.18–7.27 (5 H, m), 7.29–7.36 (2 H, m), 7.47 (1 H, d,
J = 7.2); δC (75 MHz; CDCl3; Me4Si) 12.3, 13.9, 36.5, 67.5 (s),
70.1, 122.8, 124.6, 125.3, 127.4, 128.8, 129.0, 130.9, 131.2 (s),
135.3 (s), 140.4 (s), 194.6 (s).
Ketenimine (7a). (0.73 g, 90%); yellow oil (Found: C, 71.1; H,
5.1; N, 3.25. C24H21NOS2 requires C, 71.4; H, 5.4; N, 3.5%);
νmax(film)/cmϪ1 1998, 1596, 1579, 1491, 1454, 1218, 1111, 1048,
760 and 697; δH (300 MHz; CDCl3; Me4Si) 1.40 (3 H, t, J = 7.1),
4.58 (2 H, s), 4.63 (2 H, q, J = 7.1), 7.19–7.27 (4 H, m), 7.34–
7.36 (9 H, m), 7.48 (1 H, dd, J = 6.9 and 2.1); δC (75 MHz;
CDCl3; Me4Si) 13.9, 36.4, 70.1, 78.1 (s), 123.0, 126.6, 127.9,
128.0, 128.9, 129.0, 130.7, 131.6 (s), 133.8 (s), 139.4 (s), 190.8
(s), 214.0 (s).
Preparation of indole 8
Ketenimine (7b). (0.89 g, 92%); yellow oil (Found: C, 59.5; H,
4.0; N, 2.8. C24H20BrNOS2 requires C, 59.75; H, 4.2; N, 2.9%);
νmax(film)/cmϪ1 1991, 1494, 1475, 1464, 1220, 1172, 1112, 1047,
873, 818, 763 and 694; δH (300 MHz; CDCl3; Me4Si) 1.45 (3 H,
t, J = 7.2), 4.55 (2 H, s), 4.68 (2 H, q, J = 7.2), 7.23–7.30 (2 H,
m), 7.35–7.44 (10 H, m), 7.67 (1 H, d, J = 1.7); δC (75 MHz;
CDCl3; Me4Si) 13.9, 35.8, 70.3, 78.6 (s), 121.1 (s), 124.4, 126.8,
128.0, 129.0, 132.0, 133.4 (s), 133.7, 134.0 (s), 138.5 (s), 192.0
(s), 213.4 (s).
A solution of the ketenimine 7c (0.26 g, 0.6 mmol) in anhydrous
cyclohexane (60 ml) was heated at reflux temperature under an
atmosphere of nitrogen and lauroyl peroxide was added (0.36 g,
0.9 mmol) portionwise (0.1 mmol every 3 h). After 3 h since the
last addition the solvent was removed under reduced pressure
and the crude residue was purified by silica gel column chrom-
atography, using hexanes/diethyl ether (7 : 3) as eluent.
After removing the solvent from the column chromatography
under reduced pressure the resulting viscous oil was dried at
room temperature under high vacuum for 12 h, and used as
such for characterization.
Ketenimine (7c). (0.81 g, 92%); yellow oil (Found: C, 65.65;
H, 4.4; N, 3.0. C24H20ClNOS2 requires C, 65.8; H, 4.6; N, 3.2%);
νmax(film)/cmϪ1 1995, 1599, 1493, 1478, 1219, 1174, 1113, 1047,
899, 822, 761, 695 and 646; δH (300 MHz; CDCl3; Me4Si) 1.43 (3
H, t, J = 7.1 Hz), 4.53 (2 H, s), 4.65 (2 H, q, J = 7.1), 7.22–7.41
(12 H, m), 7.49 (1 H, d, J = 1.8); δC (75 MHz; CDCl3; Me4Si)
13.9, 35.9, 70.3, 78.6 (s), 124.12, 126.8, 127.9, 129.0, 129.1,
130.7, 133.2 (s), 133.4 (s), 133.8 (s), 137.9 (s), 191.9 (s), 213.4 (s).
Indole (8). (0.12 g, 38%) (Found: C, 76.9; H, 7.2; N, 2.6.
C33H38ClNO2 requires C, 76.8; H, 7.4; N, 2.7%); νmax(film)/cmϪ1
3438, 1706, 1467, 1445, 1312, 1142, 1061, 919, 872, 800, 761 and
702; δH (300 MHz; CDCl3; Me4Si) 0.88 (3 H, t, J = 7.0), 1.21–
1.26 (16 H, m), 1.52–1.57 (2 H, m), 2.31 (2 H, t, J = 7.4), 6.04
(1 H, d, J = 1.4), 7.09 (1 H, dd, J = 8.6 and 2.0), 7.15 (1 H, d,
J = 8.6), 7.28–7.33 (10 H, m), 7.46 (1 H, d, J = 1.4), 8.49 (1 H,
br); δC (75 MHz; CDCl3; Me4Si) 14.2, 22.8, 24.8, 29.1, 29.3,
29.4, 29.5, 29.7, 32.0, 34.0, 43.5, 79.0 (s), 102.8, 112.1, 120.2,
122.6, 125.6 (s), 127.2, 128.0, 128.3, 129.0 (s), 134.4 (s), 144.3
(s), 145.0 (s), 179.3 (s); m/z (EI) 518 (Mϩ ϩ 2, 9%), 516 (Mϩ,
12), 314 (100).
Ketenimine (7d). (0.79 g, 94%); yellow oil (Found: C, 71.7; H,
5.4; N, 3.2. C25H23NOS2 requires C, 72.0; H, 5.55; N, 3.35%);
νmax(film)/cmϪ1 1998, 1599, 1492, 1454, 1213, 1150, 1110, 1052,
818, 792, 695 and 646; δH (300 MHz; CDCl3; Me4Si) 1.40 (3 H,
t, J = 7.2), 2.32 (3 H, s), 4.55 (2 H, s), 4.64 (2 H, q, J = 7.2),
7.07 (1 H, d, J = 8.1), 7.20–7.38 (12 H, m); δC (75 MHz;
CDCl3; Me4Si) 13.8, 21.1, 36.4, 70.0, 78.0 (s), 123.0, 126.5,
127.9, 128.9, 129.8, 131.5, 134.0 (s), 136.7 (s), 138.1 (s), 190.2
(s), 214.1 (s).
Preparation of indoles 9
A solution of the corresponding ketenimine 7 (0.6 mmol) in
anhydrous chlorobenzene (60 ml) was heated at reflux temper-
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 4 2 8 2 – 4 2 9 2
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ature under an atmosphere of nitrogen and t-butyl peroxide was
added (0.10 g, 0.72 mmol) portionwise (0.12 mmol every 4 h).
After 4 h since the last addition the solvent was removed under
reduced pressure and the crude residue was purified by silica gel
column chromatography, using hexanes/diethyl ether (9 : 1) as
eluent.
After removing the solvent from the column chromatography
under reduced pressure the resulting solid material was tritur-
ated, dried at room temperature under high vacuum for 12 h,
and used as such for characterization. Compounds 9 were
stored in an atmosphere of nitrogen to avoid air oxidation.
2-Diphenylmethylbenzo[f]indole (9g). (0.11 g, 52%) (Found:
C, 90.2; H, 5.6; N, 4.2. C25H19N requires C, 90.1; H, 5.7; N,
4.2%); νmax(nujol)/cmϪ1 3411, 1600, 1584, 1279, 1268, 861, 744
and 698; δH (300 MHz; CDCl3; Me4Si) 5.59 (1 H, s), 6.19 (1 H,
d, J = 0.9), 7.21–7.36 (12 H, m), 7.57 (1 H, s), 7.69 (1 H,
br s), 7.80 (1 H, dd, J = 7.0 and 2.2), 7.88 (1 H, dd, J = 7.0 and
2.2), 7.96 (1 H, s); δC (75 MHz; CDCl3; Me4Si) 51.4, 102.3,
105.8, 117.5, 122.6, 123.6, 127.2, 127.3, 128.1, 128.7, 129.1,
130.3 (s), 131.0 (s), 137.3 (s), 141.8 (s), 145.2 (s); m/z (EI) 333
(Mϩ, 100%).
2-(1-Phenylethyl)indole (9h). (0.058 g, 44%) (Found: C, 86.7;
H, 6.9; N, 6.4. C16H15N requires C, 86.8; H, 6.8; N, 6.3%);
νmax(nujol)/cmϪ1 3409, 1618, 1601, 1299, 1153, 1031, 792, 750
and 704; δH (300 MHz; CDCl3; Me4Si) 1.71 (3 H, d, J = 7.2),
4.29 (1 H, q, J = 7.2), 6.42 (1 H, s), 7.03–7.13 (2 H, m), 7.19–
7.41 (6 H, m), 7.57 (1 H, dd, J = 6.6 and 2.4), 7.67 (1 H, br s);
δC (75 MHz; CDCl3; Me4Si) 21.3, 39.2, 99.6, 110.5, 119.7, 120.2,
121.4, 126.9, 127.7, 128.5 (s), 128.8, 136.2 (s), 143.1 (s), 144.6
(s); m/z (EI) 221 (Mϩ, 69%), 220 (100).
2-Diphenylmethylindole (9a). (0.085 g, 50%) (Found: C, 89.3;
H, 5.9; N, 4.7. C21H17N requires C, 89.0; H, 6.0; N, 4.9%);
νmax(nujol)/cmϪ1 3400, 1601, 1585, 1290, 1078, 1031, 749 and
700; δH (300 MHz; CDCl3; Me4Si) 5.54 (1 H, s), 6.06 (1 H, d,
J = 1.0), 7.00–7.34 (13 H, m), 7.48 (1 H, d, J = 6.8), 7.71 (1 H,
br s); δC (75 MHz; CDCl3; Me4Si) 51.1, 102.9, 110.7, 119.8,
120.3, 121.6, 127.0, 128.4 (s), 128.7, 129.1, 136.3 (s), 140.9 (s),
142.1 (s); m/z (EI) 283 (Mϩ, 100%).
5-Bromo-2-diphenylmethylindole (9b). (0.052 g, 24%) (Found:
C, 69.5; H, 4.2; N, 3.7. C21H16BrN requires C, 69.6; H, 4.45; N,
3.9%); νmax(nujol)/cmϪ1 3300, 1615, 1600, 1279, 1050, 742 and
703; δH (300 MHz; CDCl3; Me4Si) 5.55 (1 H, s), 6.03 (1 H, d,
J = 2.0), 7.06 (2 H, d, J = 8.6), 7.16–7.36 (10 H, m), 7.60 (1 H, d,
J = 2.0), 7.82 (1 H, br s); δC (75 MHz; CDCl3; Me4Si) 51.1,
102.4, 112.1, 113.0 (s), 122.8, 124.4, 127.2, 128.8, 129.0, 130.2
(s), 134.9 (s), 141.8 (s), 142.3 (s); m/z (EI) 363 (Mϩ ϩ 2, 99%),
361 (Mϩ, 87), 203 (100).
Preparation of indoles 10
A solution of the corresponding ketenimine xanthate 7
(0.6 mmol) in a mixture of methanol and 1,2-dichloroethane
(1 : 3; v/v) (60 ml) was heated at reflux temperature under an
atmosphere of nitrogen and lauroyl peroxide was added (0.10 g,
0.72 mmol) portionwise (0.12 mmol every 4 h). After 4 h since
the last addition the solvent was removed under reduced pres-
sure and the crude residue was purified by chromatography
[silica gel, using hexanes/diethyl ether (9 : 1)].
5-Chloro-2-diphenylmethylindole (9c). (0.12 g, 60%) (Found:
C, 79.1; H, 5.0; N, 4.2. C21H16ClN requires C, 79.4; H, 5.1; N,
4.4%); νmax(nujol)/cmϪ1 3417, 1600, 1575, 1309, 1218, 1137,
1061, 919, 867, 796, 748 and 701; δH (300 MHz; CDCl3; Me4Si)
5.50 (1 H, s), 6.00 (1 H, d, J = 1.2), 7.03 (2 H, d, J = 1.2), 7.14–
7.34 (10 H, m), 7.42 (1 H, s), 7.74 (1 H, br s); δC (75 MHz;
CDCl3; Me4Si) 51.0, 102.5, 111.6, 119.6, 121.8, 125.3 (s), 127.1,
128.7, 129.0, 129.4 (s), 134.5 (s), 141.8 (s), 142.4 (s); m/z (EI) 319
(Mϩ ϩ 2, 20%), 317 (Mϩ, 82), 240 (100).
5-Chloro-2-(ꢀ-methoxy-ꢀ,ꢀ-diphenyl)methylindole
(10c).
(0.090 g, 43%); mp 120–122 ЊC (from n-hexane); colourless
prisms (Found: C, 76.3; H, 5.4; N, 4.1. C22H18ClNO requires C,
76.0; H, 5.2; N, 4.0%); νmax(nujol)/cmϪ1 3430, 1450, 1307, 1255,
1140, 1065, 1050, 804 and 711; δH (300 MHz; CDCl3; Me4Si)
3.13 (3 H, s), 6.48 (1 H, s), 7.07–7.36 (8 H, m), 7.51–7.54 (5 H,
m), 8.17 (1 H, br s); δC (75 MHz; CDCl3; Me4Si) 52.4, 83.8 (s),
104.5, 112.0, 120.1, 122.6, 125.5 (s), 127.6, 127.8, 128.2, 128.9
(s), 134.2 (s), 141.2 (s), 142.7 (s); m/z (EI) 349 (Mϩ ϩ 2, 12%),
347 (Mϩ, 35), 315 (100).
2-Diphenylmethyl-5-methylindole (9d). (0.084 g, 47%)
(Found: C, 88.6; H, 6.5; N, 4.8. C22H19N requires C, 88.85; H,
6.4; N, 4.7%); νmax(nujol)/cmϪ1 3408, 1600, 1589, 1311, 1291,
1219, 1157, 1030, 802, 747 and 701; δH (300 MHz; CDCl3;
Me4Si) 2.40 (3 H, s), 5.55 (1 H, s), 6.07 (1 H, s), 6.93 (1 H, d,
J = 8.2), 7.09 (1 H, d, J = 8.2), 7.18–7.32 (11 H, m), 7.65 (1 H, br
s); δC (75 MHz; CDCl3; Me4Si) 21.5, 51.2, 102.4, 110.3, 120.0,
123.2, 127.0, 128.7, 129.1, 134.6 (s), 140.9 (s), 142.3 (s); m/z (EI)
297 (Mϩ, 100%).
2-(ꢀ-Methoxy-ꢀ,ꢀ-diphenyl)methyl-5-methylindole
(10d).
(0.067 g, 34%); mp 89–91 ЊC (from n-hexane); colourless
prisms (Found: C, 84.7; H, 6.2; N, 4.0. C23H21NO requires C,
84.4; H, 6.5; N, 4.3%); νmax(nujol)/cmϪ1 3454, 1663, 1603, 1450,
1313, 1295, 1224, 1153, 1075, 909, 795 and 737; δH (300 MHz;
CDCl3; Me4Si) 2.43 (3 H, s), 3.14 (3 H, s), 6.48 (1 H, d, J = 1.5),
6.99 (1 H, dd, J = 8.3 and 1.3), 7.15–7.36 (8 H, m), 7.52–7.57
(4 H, m), 7.98 (1 H, br s); δC (75 MHz; CDCl3; Me4Si) 21.5,
52.3, 83.9 (s), 104.9, 110.7, 120.4, 124.0, 127.4, 127.7, 128.0 (s),
128.1, 129.1 (s), 134.3 (s), 139.4 (s), 143.3 (s); m/z (EI) 327 (Mϩ,
38%), 298 (100).
2-Diphenylmethyl-6-nitroindole (9e). (0.13 g, 64%) (Found: C,
76.7; H, 4.9; N, 8.8. C21H16N2O2 requires C, 76.8; H, 4.9; N,
8.5%); νmax(nujol)/cmϪ1 3398, 1592, 1537, 1505, 1337, 1072, 880,
827, 781, 749 and 701; δH (300 MHz; CDCl3; Me4Si) 5.62 (1 H,
s), 6.20 (1 H, d, J = 0.9), 7.17–7.35 (10 H, m), 7.47 (1 H, d,
J = 8.8), 7.93 (1 H, dd, J = 8.8 and 1.8), 8.15 (1 H, d, J = 1.8),
8.46 (1 H, br s); δC (75 MHz; CDCl3; Me4Si) 51.2, 103.7, 107.7,
115.5, 119.9, 127.4, 128.9, 129.0, 133.4 (s), 134.7 (s), 141.2 (s),
142.9 (s), 147.6 (s); m/z (EI) 328 (Mϩ, 100%).
Preparation of indole 15
Indole 15 was obtained by reaction of ketenimine 7d with
t-butyl peroxide under similar reaction conditions to that of the
preparation of indoles 9 but using ortho-xylene as solvent.
2-Diphenylmethyl-7-methylindole (9f). (0.11 g, 60%) (Found:
C, 88.6; H, 6.5; N, 4.9. C22H19N requires C, 88.85; H, 6.4; N,
4.7%); νmax(nujol)/cmϪ1 3438, 1600, 1554, 1298, 1254, 1081,
1032, 796, 748 and 700; δH (300 MHz; CDCl3; Me4Si) 2.34 (3 H,
s), 5.57 (1 H, s), 6.06 (1 H, d, J = 1.2), 6.92–7.01 (2 H, m), 7.21–
7.35 (11 H, m), 7.75 (1 H, br s); δC (75 MHz; CDCl3; Me4Si)
16.7, 51.1, 103.6, 118.0, 119.8 (s), 120.0, 122.3, 127.0, 127.9 (s),
128.6, 129.1, 135.9 (s), 140.5 (s), 142.2 (s);. m/z (EI) 297 (Mϩ,
100%).
Indole 15. (0.13 g, 52%) (Found: C, 89.6; H, 6.5; N, 3.4.
C30H27N requires C, 89.7; H, 6.8; N, 3.5%); νmax(film)/cmϪ1
3454, 1600, 1446, 1409, 1313, 1290, 1265, 1037, 872, 797, 741
and 704; δH (300 MHz; CDCl3; Me4Si) 1.56 (3 H, s), 2.41 (3 H,
s), 3.94 (2 H, s), 6.24 (1 H, d, J = 2.2), 6.82–7.32 (18 H, m);
δC (75 MHz; CDCl3; Me4Si) 19.3, 21.5, 41.6, 55.1 (s), 110.3,
120.1, 123.2, 125.5, 126.6, 126.7, 127.9, 128.8 (s), 129.5, 130.0,
130.2, 134.4 (s), 136.5 (s), 139.1 (s), 143.8 (s), 144.8 (s); m/z (EI)
401 (Mϩ, 12%), 296 (100).
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Preparation of N-phenylthioxocarbamate 17
temperature under an atmosphere of nitrogen for 6 h. Then, the
precipitated compounds 20–22 were isolated by filtration.
To a solution of 2-azidobenzyl alcohol 16a (0.74 g, 5 mmol)
and phenyl isothiocyanate (0.67 g, 5 mmol) in anhydrous tetra-
hydrofuran (20 ml) was added sodium hydride (60% in oil; 0.21
g, 5.25 mmol). The reaction mixture was stirred at room tem-
perature in an atmosphere of nitrogen for 16 h, after which the
tetrahydrofuran was removed under reduced pressure. The
resulting material was partitioned between dichloromethane
(25 ml) and water (25 ml). The organic layer was separated and
dried over anhydrous magnesium sulfate. After evaporation of
the solvent the residue was chromatographed [silica gel, using
hexanes/diethyl ether (7 : 3) as eluent] to give 17 (0.88 g, 62%);
mp 94–96 ЊC (from diethyl ether); colourless prisms (Found: C,
59.4; H, 4.3; N, 20.0. C14H12N4OS requires C, 59.1; H, 4.25; N,
19.7%); νmax(nujol)/cmϪ1 3231, 2131, 2086, 1556, 1526, 1294,
1223, 1206, 1180, 1059, 1036, 912, 750 and 695; δH (300 MHz;
CDCl3; Me4Si) 5.58 (2 H, s), 7.09–7.43 (9 H, m), 8.54 (1 H, br s);
δC (75 MHz; CDCl3; Me4Si) 69.5 (br), 118.3, 121.8 (br), 124.8
(s), 125.6 (br), 129.0, 130.0, 130.6, 136.9 (s, br), 138.7 (s), 188.1
(s); m/z (EI) 284 (Mϩ, 7%), 256 (38), 136 (100).
Triphenylphosphazene 20. (2.25 g, 87%); mp 81–82 ЊC (from
diethyl ether); colourless prisms (Found: C, 74.4; H, 5.0; N, 5.3.
C32H27N2OPS requires C, 74.1; H, 5.25; N, 5.4%); νmax(nujol)/
cmϪ1 3057, 1589, 1482, 1434, 1210, 1181, 1110, 1042, 1019, 999,
847, 823, 746, 718 and 696; δH (300 MHz; CDCl3; Me4Si) 4.52
(2 H, s), 6.43 (1 H, d, J = 7.4), 6.61 (1 H, td, J = 7.4 and 0.8),
6.79 (1 H, td, J = 7.4 and 1.8), 7.02 (1 H, td, J = 7.4 and 1.2),
7.18–7.23 (2 H, m), 7.29–7.52 (13 H, m), 7.52–7.77 (6 H, m);
δC (75 MHz; CDCl3; Me4Si) 33.0, 117.5, 120.1, 120.9 (d,
J = 9.9), 124.0, 127.8, 128.7 (d, J = 11.6), 128.9, 130.0, 130.5 (s),
131.7 (d, J = 2.3), 132.6 (d, J = 9.8), 138.2 (s), 149.6 (s), 167.6 (s);
δP (121.4 MHz, CDCl3, H3PO4) 4.03; m/z (EI) 400 (3%), 278
(19), 277 (100), 240 (25).
Triphenylphosphazene 21. (2.22 g, 91%); mp 168–170 ЊC
(from diethyl ether); colourless prisms (Found: C, 68.7; H,
5.1; N, 2.7. C28H26NOPS2 requires C, 69.0; H, 5.4; N, 2.9%);
νmax(nujol)/cmϪ1 1639, 1609, 1435, 1128, 1112, 1038, 998, 892,
878, 813, 719 and 695; δH (300 MHz; CDCl3; Me4Si) 2.15 (3 H,
s), 2.39 (3 H, s), 4.52 (2 H, s), 6.30 (1 H, d, J = 8.0), 6.60 (1 H,
dd, J = 8.0 and 2.2), 7.09 (1 H, s), 7.39–7.51 (9 H, m), 7.71–7.77
(6 H, m); δC (75 MHz; CDCl3; Me4Si) 13.1, 20.5, 33.6, 120.4 (d,
J = 9.9 Hz), 126.1 (s), 128.6 (d, J = 11.6), 128.7, 130.4 (s), 130.7,
131.3 (d, J = 103.2), 131.6 (d, J = 2.9), 132.6 (d, J = 9.9), 147.3
(s), 191.4 (s); δP (121.4 MHz; CDCl3; H3PO4) 2.41; m/z (EI) 487
(Mϩ, 13%), 183 (100).
Preparation of S-methyl xanthate 18
To a solution of 2-azido-5-methylbenzyl alcohol 16b (0.82 g,
5 mmol) and carbon disulfide (38 g, 0.5 mol) in anhydrous tetra-
hydrofuran (60 ml) was added sodium hydride (60% in oil,
3 g, 75 mmol). The reaction mixture was stirred in an atmos-
phere of nitrogen at room temperature for 2 h and then at 50 ЊC
for 1 h, after which methyl iodide (10.64 g, 75 mmol) was
added. The reaction mixture was stirred at 50 ЊC for 6 h. After
cooling the tetrahydrofuran was removed under reduced pres-
sure, and the resulting material was partitioned between di-
chloromethane (25 ml) and water (25 ml). The organic layer was
separated and dried over anhydrous magnesium sulfate. After
evaporation of the solvent the residue was chromatographed
[silica gel, using hexanes/ethyl acetate (9 : 1) as eluent] to give 18
(1.2 g, 95%); mp 48–49 ЊC (from n-hexane); colourless prisms
(Found: C, 47.2; H, 4.3; N, 16.4. C10H11N3OS2 requires C, 47.4;
H, 4.4; N, 16.6%); νmax(nujol)/cmϪ1 2128, 2103, 1587, 1318,
1245, 1205, 1067, 972, 814, 772 and 726; δH (300 MHz; CDCl3;
Me4Si) 2.33 (3 H, s), 2.57 (3 H, s), 5.54 (2 H, s), 7.05–7.09 (1 H,
s), 7.19 (2 H, br s); δC (75 MHz; CDCl3; Me4Si) 19.2, 20.8, 70.9,
118.2, 125.6 (s), 130.8, 131.4, 134.7 (s), 136.1 (s), 215.6 (s).
Triphenylphosphazene 22. (2.37 g, 89%); mp 163–164 ЊC
(from diethyl ether); colourless prisms (Found C, 74.0; H,
5.5; N, 2.3. C33H28NO2PS requires C, 74.3; H, 5.3; N, 2.6%);
νmax(nujol)/cmϪ1 1717, 1609, 1438, 1331, 1188, 1114, 1087, 1022,
999, 746, 720 and 691; δH (300 MHz; CDCl3; Me4Si) 2.16 (3 H,
s), 4.46 (2 H, s), 6.33 (1 H, d, J = 8.0), 6.63 (1 H, dd, J = 8.0 and
2.0), 7.10–7.22 (4 H, m), 7.31–7.36 (2 H, m), 7.40–7.53 (9 H, m),
7.73–7.80 (6 H, m); δC (75 MHz; CDCl3; Me4Si) 20.5, 34.7,
120.4 (d, J = 9.9), 121.6, 125.8, 126.1 (s), 128.6 (d, J = 12.2),
129.4, 130.6, 130.8 (s), 131.3 (d, J = 102.6), 131.7 (d, J = 2.3),
132.6 (d, J = 9.9), 147.2 (s), 151.6 (s), 172.1 (s); δP (121.4 MHz;
CDCl3; H3PO4) 2.84; m/z (EI) 533 (Mϩ, 5%), 194 (100).
Preparation of isothiocyanate 26
Preparation of thionocarbonate 19
To a solution of triphenylphosphazene 6d (1.25 g, 2.5 mmol) in
anhydrous benzene (15 ml) carbon disulfide was added (10 ml).
The mixture was stirred at reflux temperature for 12 h. The
solvent was removed under reduced pressure and the residue
was chromatographed [silica gel, using hexanes/dichloro-
methane (4 : 1) as eluent] to give 26 (0.69 g, 98%); mp 29–30 ЊC
(from n-hexane); colourless prisms (Found: C, 50.7; H, 4.3; N,
4.8. C12H13NOS3 requires C, 50.85; H, 4.6; N, 4.9%); νmax(nujol)/
cmϪ1 2102, 1251, 1218, 1113, 1050, 1011, 947, 914, 875, 824
and 744; δH (300 MHz; CDCl3; Me4Si) 1.44 (3 H, t, J = 7.1),
2.35 (3 H, s), 4.37 (2 H, s), 4.67 (2 H, q, J = 7.1), 7.06 (1 H, dd,
J = 8.1 and 1.5), 7.15 (1 H, d, J = 8.1), 7.23 (1 H, d, J = 1.5);
δC (75 MHz; CDCl3; Me4Si) 13.9, 21.3, 36.6, 70.4, 126.9, 128.0
(s), 129.7, 131.3, 131.7 (s), 136.6 (s), 137.7 (s), 213.1 (s); m/z (EI)
283 (Mϩ, 51%), 162 (100).
A solution of phenyl chlorothionoformate (0.86 g, 5 mmol) in
acetonitrile (5 ml) was added dropwise to a stirred solution of
2-azido-5-methylbenzyl alcohol 16b (0.82 g, 5 mmol) and 4-di-
methylaminopyridine (0.92 g, 7.5 mmol) in acetonitrile (30 ml).
The reaction mixture was stirred in an atmosphere of nitrogen
for 1 h, after which the acetonitrile was removed under reduced
pressure. The resulting material was partitioned between di-
chloromethane (25 ml) and water (25 ml). The organic layer was
separated and dried over anhydrous magnesium sulfate. After
evaporation of the solvent the residue was chromatographed
[silica gel, using hexanes/ethyl acetate (9 : 1) as eluent] to give 19
(0.96 g, 64%); mp 82–84 ЊC (from diethyl ether/n-hexane);
colourless prisms (Found: C, 60.0; H, 4.3; N, 14.3. C15H13N3O2S
requires C, 60.2; H, 4.4; N, 14.0%); νmax(nujol)/cmϪ1 2127, 2095,
1297, 1284, 1217, 1203, 1012, 849, 772, 713 and 686; δH (300
MHz; CDCl3; Me4Si) 2.33 (3 H, s), 5.47 (2 H, s), 7.11–7.45 (8 H,
m); δC (75 MHz; CDCl3; Me4Si) 20.8, 71.3, 118.2, 121.9, 124.9
(s), 126.6, 129.5, 131.0, 131.4, 134.7 (s), 136.1 (s), 153.5 (s),
194.9 (s).
Reaction of isothiocyanate 26 with lauroyl peroxide
A solution of isothiocyanate 26 (0.17 g, 0.6 mmol) in anhydrous
cyclohexane (60 ml) was heated in an atmosphere of nitrogen to
reflux and lauroyl peroxide added (0.24 g, 0.6 mmol) portion-
wise (0.1 mmol every 3 h). After 3 h since the last addition the
solvent was removed under reduced pressure and the crude
residue was purified by silica gel column chromatography [using
hexanes/dichloromethane (4 : 1) as eluent] to give 28 (0.095 g,
98%); mp 134–135 ЊC (from n-hexane); colourless prisms
Preparation of triphenylphosphazenes 20–22
To a solution of the corresponding azide 17–19 (5 mmol) in
anhydrous diethyl ether (15 ml) triphenylphosphane (1.31 g,
5 mmol) was added. The reaction mixture was stirred at room
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 4 2 8 2 – 4 2 9 2
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9 (a) M. Alajarín, P. Molina and A. Vidal, Tetrahedron Lett., 1996, 37,
(Found: C, 66.4; H, 4.9; N, 8.3. C18H16N2S2 requires C, 66.6; H,
5.0; N, 8.6%); νmax(nujol)/cmϪ1 2144, 948, 902 and 816; δH (300
MHz; CDCl3; Me4Si) 2.33 (6 H, s), 2.92 (4 H, s), 7.00 (2 H, dd,
J = 8.1 and 1.5), 7.07 (2 H, d, J = 1.5), 7.09 (2 H, d, J = 8.1);
δC (75 MHz; CDCl3; Me4Si) 21.3, 33.3, 126.3, 127.2 (s), 128.3,
131.2, 134.5 (s), 137.1 (s), 137.9 (s); m/z (EI) 324 (Mϩ, 71%), 162
(100).
8945–8948; (b) M. Alajarín, P. Molina, A. Vidal and F. Tovar,
Tetrahedron, 1997, 53, 13449–13472; (c) M. Alajarín, A. Vidal,
F. Tovar, A. Arrieta, B. Lecea and F. P. Cossío, Chem. Eur. J., 1999,
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F. Tovar, J. Org. Chem., 2000, 65, 3633–3643; (e) M. Alajarín,
A. Vidal, F. Tovar, M. C. Ramírez de Arrellano, F. P. Cossío,
A. Arrieta and B. Lecea, J. Org. Chem., 2000, 65, 7512–7515;
( f ) M. Alajarín, A. Vidal and R.-A. Orenes, Eur. J. Org. Chem.,
2002, 4222–4227.
10 (a) M. Alajarín, A. Vidal, F. Tovar and C. Conesa, Tetrahedron Lett.,
1999, 40, 6127–6130; (b) M. Alajarín, A. Vidal and F. Tovar,
Tetrahedron Lett., 2000, 41, 7029–7032; (c) M. Alajarín, A. Vidal
and M.-M. Ortín, Synthesis, 2002, 2393–2398.
11 M. Alajarín, A. Vidal, F. Tovar and P. Sánchez-Andrada,
Tetrahedron Lett., 2002, 43, 6259–6261.
12 A short communication of this work has appeared: M. Alajarín,
A. Vidal and M.-M. Ortín, Tetrahedron Lett., 2003, 44, 3027–
3030.
Acknowledgements
This work was supported by the MCYT and FEDER (Project
BQU2001–0010), and Fundación Séneca-CARM (Project PI-1/
00749/FS/01). One of us (M.-M. O.) thanks the Fundación
Séneca-CARM for a fellowship.
13 For reviews on radical cyclizations leading to indoles, see: (a) G. W.
Gribble, Contemp. Org. Synth., 1994, 1, 145–172; (b) G. W. Gribble,
J. Chem. Soc., Perkin Trans. 1, 2000, 1045–1075.
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