Towards the synthesis of azoacetylenes

Article abstract of DOI:10.1002/hlca.200390252

The synthesis of azoacetylenes (=dialkynyldiazenes) 1 and 2 has been investigated. They represent a still elusive class of chromophores with potentially very interesting applications as novel bistable photochemical molecular switches or as antitumor agents (Fig. 1). Our synthetic efforts have led us alongside three different approaches (Scheme 1). In a first route, it was envisioned to generate the azo (=diazene) bond by photolysis of N,N′-dialkynylated 1,3,4-thiadiazolidine-2,5-diones that are themselves challenging targets (Scheme 2). Attempts are described to obtain the latter by alkynylation of the parent heterocycle with substituted alkynyliodonium salts. In a conceptually similar approach, the no-less-challenging dialkynylated 9,10-dihydro-9,10-diazanoanthracene (29) was to be generated by alkynylation of the unsubstituted hydrazine 28 (Scheme 6). In a second route, the generation of the N=N bond from Br-substituted divinylidenehydrazines (ketene-azines) 35 was attempted in a synthetic scheme involving an aza-Wittig reaction between azinobis(phosphorane) 36 and (triisopropylsilyl)ketene 37 (Scheme 7). Finally, a third approach, based on the formation of the central azo bond as the key step, was explored. This route involved the extrapolation of a newly discovered condensation reaction of N,N-disilylated anilines with nitroso compounds (Scheme 11, and Tables 1 and 2) to the transformation of N,N-disilylated ynamine 55 and nitroso-alkyne 54 (Scheme 13).

Full text of DOI:10.1002/hlca.200390252

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Helvetica Chimica Acta Vol. 86 (2003)
Towards the Synthesis of Azoacetylenes
by Frÿdÿric Denonne, Paul Seiler, and FranÁois Diederich*
Laboratorium f¸r Organische Chemie der Eidgenˆssischen Technischen Hochschule, ETH-Hˆnggerberg, HCI,
CH-8093 Z¸rich
(Phone: ‡4116322992, fax: ‡4116321109, e-mail: diederich@org.chem.ethz.ch)
Dedicated to Prof. Helmut Schwarz on the occasion of his 60th birthday
The synthesis of azoacetylenes (ˆdialkynyldiazenes) 1 and 2 has been investigated. They represent a still
elusive class of chromophores with potentially very interesting applications as novel bistable photochemical
molecular switches or as antitumor agents (Fig. 1). Our synthetic efforts have led us alongside three different
approaches (Scheme 1). In a first route, it was envisioned to generate the azo (ˆdiazene) bond by photolysis of
N,N'-dialkynylated 1,3,4-thiadiazolidine-2,5-diones that are themselves challenging targets (Scheme 2).
Attempts are described to obtain the latter by alkynylation of the parent heterocycle with substituted
alkynyliodonium salts. In a conceptually similar approach, the no-less-challenging dialkynylated 9,10-dihydro-
9,10-diazanoanthracene (29) was to be generated by alkynylation of the unsubstituted hydrazine 28 (Scheme 6).
In a second route, the generation of the NˆN bond from Br-substituted divinylidenehydrazines (ketene-azines)
35 was attempted in a synthetic scheme involving an aza-Wittig reaction between azinobis(phosphorane) 36 and
(triisopropylsilyl)ketene 37 (Scheme 7). Finally, a third approach, based on the formation of the central azo
bond as the key step, was explored. This route involved the extrapolation of a newly discovered condensation
reaction of N,N-disilylated anilines with nitroso compounds (Scheme 11, and Tables 1 and 2) to the
transformation of N,N-disilylated ynamine 55 and nitroso-alkyne 54 (Scheme 13).
1. Introduction.
(Z)- and (E)-azoacetylenes (ˆdialkynyldiazenes) 1 and 2,
respectively (Fig. 1), represent an elusive class of chromophores with the potential for a
variety of fascinating applications. Similar to azobenzenes (ˆdiphenyldiazenes; 3/4),
they could be introduced as photoswitches [1] into a variety of molecular devices, such
as photoresponsive receptors and sensors [2], conjugated oligomers [3], or dendrimers
[4]. It is well-known that azobenzenes are not thermally bistable, since unfavorable
steric interactions in the (Z)-isomer induce thermal (Z) ! (E) isomerization (Fig. 1)
[5]. Introduction of the ethynediyl moieties between the aryl rings and the central bond
in the (Z)- and (E)-diarylated azoacetylenes, 5 and 6, respectively, should eliminate
these unfavorable steric interactions, thereby leading to novel, thermally bistable
photoswitches. In analogy, we had shown in previous work that a similar chromophoric
extension by ethynediyl insertions into (Z)- and (E)-stilbenes 7/8 [6] provides two-way
photoswitchable, thermally bistable (Z)- and (E)-diarylated 1,2-diethynylethenes
(ˆ hex-3-ene-1,5-diynes, DEEs; 9/10) [7] and tetraethynylethenes (ˆ 3,4-diethynylhex-
3-ene-1,5-diyne, TEEs; 11/12) [8]. Both diarylated DEEs and TEEs feature fully
planar, conjugated p-chromophores [9], including the aryl rings, and thermal isomer-
ization is not observed.
Other interesting applications of (Z)-azoacetylene (ˆdiethynyldiazene, 1; R,R' ˆ H)
include its use as precursor in the formation of C12N6 (13; Fig. 2), the hexaaza analog of

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Fig. 1. Azoacetylenes 1, 2, 5, 6, and their geometric relationship to related classes of photoswitches
cyclo-C18 [10]. Furthermore, (Z)-azoacetylenes could find application as novel
−warheads× in anticancer agents, designed after the enediyne antitumor antibiotics
[11]. Bergman cyclization [12] would provide 3,6-didehydropyrazine biradicals
damaging DNA in cancer cells, in analogy to the work by Chen and co-workers [13]
who showed that (Z)-azaenediyne 14 cyclizes to the intermediate 2,5-didehydropyr-
idine biradical (Fig. 2) [14].

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N
N
H
H
R
R'
N
N
N
N
N
N
13 C12N6
R
N
N
N
N
R'
H
H
H
Ph
Ph
N
Me
H+
Ph
(i-Pr)2O
Ph
Ph
N
N
Me
Me
N
Ph
Ph
Ph
14
Me
Fig. 2. Other potential applications of (Z)-azoacetylenes and their analogy to 1-methyl-N-(2-phenylethynyl)-
propanimine (14)
Despite these fascinating potential applications, no successful or attempted
preparation of azoacetylenes 1/2 has been described to the best of our knowledge. In
fact, their synthesis represents a formidable challenge since the usual routes towards
azo derivatives cannot be applied. The coupling of diazonium salts with nucleophiles
although successful in the preparation of (aryl)(arylethynyl)diazenes [15] would
require the intermediate preparation of alkynyldiazonium salts for our purposes. Such
intermediates have very low stability and have been shown to undergo nucleophilic
attack at the alkynyl C-atom in b-position to the diazonium ion [16]. The condensation
of ethynylated amines (ynamines) with nitroso compounds [17], as well as the
deprotection/oxidation of N,N'-dialkynylated hydrazines [18] require protic conditions
that are incompatible with the ynamine character of the starting materials. Primary and
secondary ynamines tautomerize indeed to the more-stable nitriles and ketenimines,
respectively.
Here, we describe a variety of synthetic pathways (Scheme 1) explored to prepare
various azoacetylenes that, at this stage, remain elusive. Our investigations were
focused on three main routes: i) the use of protecting groups for 1,2-dialkynylhy-
drazines that can be removed under nonhydrolytic conditions, followed by oxidation,
ii) the reductive elimination of dihalogen from bis(2-halogenovinylidene)hydrazines,
and iii) the formation of the NˆN bond through couplings of ynamines and other
precursors under aprotic conditions.

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Scheme 1. Synthetic Approaches (i-iii) towards Azoacetylenes Explored in this Investigation. Some possible
precursors are shown.
2. Results and Discussion.
2.1. Attempted Preparationof Azoacetyleens
via
Protected 1,2-Dialkynylhydrazines. 2.1.1. The 1,3,4-Thiadiazolidine-2,5-dione Route.
1,3,4-Thiadiazolidine-2,5-dione has been shown by Squillacote et al. to be a versatile
photochemical synthon for the azo group [19]. Starting from N,N-disubstituted
derivatives, a variety of azo compounds were obtained upon irradiation with UV light
(254 nm). Accordingly, the 3,4-dialkynylated 1,3,4-thiadiazolidine-2,5-dione 15a was
selected as our first target (Scheme 2). 1,2-Dialkynylated 1,2-diacylhydrazine deriva-
tives, such as 15a, had not been reported before, making them interesting targets by
themselves. To obtain 15a, it was planned to react the nucleophilic dianion of 1,3,4-
thiadiazolidine-2,5-dione (16; for the synthesis of 16 and related heterocycles, see [20])
with electrophilic ethynyl species. Alkynyliodonium salts [21] were chosen for this
purpose. When their alkyne moiety bears good migrating groups (H, R3Si, Ph), these
salts have been shown to react with a variety of soft nucleophiles, including
deprotonated amides, to give the alkynylated derivatives [22].
Attempted reaction of the dianion of 16 with iodonium salts 17a/17b [21] led only to
the decomposition of the latter. The use of 5-methoxy-3H-1,3,4-thiadiazol-2-one (18)
as a possible precursor for the stepwise alkynylation allowed isolation of 19a/19b in
good yields upon reaction with the alkynyliodonium salts 17b and 17c, respectively.
Unfortunately, demethylation of 19a/19b under generation of the starting material for
the second alkynylation, 20a/20b, failed, with decomposition of the starting material
occurring with either Me3SiI or HI at elevated temperatures. On the other hand,
monoalkynylated thiadiazolidinone 20b could be obtained by alkynylation of 16 with
(i-Pr)3Si-protected alkynyliodonium salt 17c. It was not very stable, though, and
decomposed in all attempts to perform the second alkynylation under formation of the
target 15a. In contrast, it was possible to prepare the more-stable N-methylated 15b by
quenching deprotonated 20b with MeI.
The alkynylation of other 1,2-diacylated hydrazines was subsequently investigated.
The anion obtained with LHMDS from commercially available dimethyl hydrazodi-

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Scheme 2. Alkynylation of 1,3,4-Thiadiazolidine-2,5-dione (16)
a) LHMDS, THF, À 708 ! 208, 12 h; 0 % (20a); 79% (20b). b) 17c, LHMDS, THF, À 708 ! 208, 12 h; 0 %; or
KH, DMF, 08 ! 208, 12 h; 0 %.c) KH, DMF, then MeI (10equiv.), 20 8, 7 h; 48%. d) LHMDS, PhMe, 08 ! 208,
12 h; 59% (19a); 08 ! 1108, 1 h; 58% (19b). e) Me3SiI, MeCN; 808, 12 h; or aq. HI, 208 ! 1008, 1 h; 0%.
LHMDS ˆ lithium hexamethyldisilazane, TfO ˆ (trifluoromethyl)sulfonyloxy.
carboxylate (21) reacted with iodonium salt 17c to provide mono-alkynylated 22 in
39% yield (Scheme 3). However, all attempts to introduce the second alkynyl residue
under formation of 23 failed again. In a control reaction, alkylation of 22 with 4-
nitrobenzyl bromide gave only 19% of 24, probably due to the low stability of the
starting material.
Scheme 3. Alkynylation of Dimethyl Hydrazodicarboxylate (21)
a) LHMDS, 17c, PhMe, 08 ! 208, 12 h; 39% (22). b) KH, DMF, 4-NO2BnBr, 208, 12 h; 19%. Bn ˆ benzyl.
The second alkynylation of the heterocycle 20b (Scheme 2) and dimethyl
hydrazodicarboxylate 22 (Scheme 3) could be thwarted by a side reaction of the
intermediate carbenes (Scheme 4). It is indeed possible that the intramolecular
reaction of the electrophilic carbenes with the ynamide moiety is faster than the desired
silyl migration, resulting in the degradation of the starting material. Attempts to

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Scheme 4. A Possible Side Reaction Preventing the Formation of 23 from 22. A similar mechanism could
prevent the dialkynylation of 20b (Scheme 2).
quench the anions with another acetylenic electrophile, namely tert-butyl 3-bromo-
prop-2-ynoate were not successful.
The introduction of the alkynyl fragment was then attempted in a different fashion
(Scheme 5). Accordingly, 21 was reacted with an excess of ClCꢀCCl in the presence of
DMAP. This time, it was possible to isolate 1,2-bis(dichlorovinyl)hydrazine derivative
25 in 21% yield, together with rather unstable monovinylated 26 (12%). Attempts to
transform 25 into the corresponding dialkynylated hydrazine derivative 27 by reduction
with Rieke-Zn in THF were unsuccessful. No reaction occurred at room temperature,
while heating to reflux led to slow decomposition of the starting material.
Scheme 5. Dichloroethenylation of Dimethyl Hydrazodicarboxylate (22)
a) ClCꢀCCl, DMAP, MeCN/Et2O, À 408 ! 208, 10h; 12% ( 26); 21% (25). b) Rieke Zn, THF, 08 ! 708, 12 h;
0%. DMAP ˆ 4-(dimethylamino)pyridine.
In contrast to the ynamides given in Schemes 2 and 3, 25 proved to be quite stable,
and it was possible to grow crystals suitable for X-ray diffraction (Fig. 3,a). The
dihedral angle C(2)ÀN(1)ÀN(6)ÀC(7) about the hydrazo bond (NÀN bond length:
1.39 ä) was found to be 638. The crystal structure revealed a close intermolecular
CÀH ¥¥¥ O contact (d(C ¥¥¥ O) ˆ 3.20ä, angle C ÀH ¥¥¥ O ꢁ 1578) between the vinylic
C(17)ÀH moiety and the ester carbonyl O-atom O(3) (Fig. 3,b). This H-bond should
be strengthened by the CÀH-acidifying effect of the two Cl-atoms in the vinylic moiety.
2.1.2. The retro-DielsÀAlder Route. In this route, we hoped to dialkynylate the
hydrazo moiety in 9,10-dihydro-9,10-diazanoanthracene (28) under formation of 29
and to subsequently eliminate anthracene by retro-DielsÀAlder reaction under
formation of the desired azoacetylene (Scheme 6). This approach has been used to
generate diimide or (Z)-azomethane [23] and could potentially be applied to the
synthesis of more-elaborate diazenes. The hydrazo compound 28 was obtained by
deprotection of the bis-Fmoc derivative 30 (Fmoc ˆ {[(9H-fluoren-9-yl)methyl]oxy}-
carbonyl) as described in [23c]. It is rather unstable and undergoes retro-DielsÀAlder
reaction in solution within 5 h at room temperature, thereby generating anthracene and
diimide [23a]. However, all attempts to react 28 with ClCꢀCCl under various
conditions failed to give the bis-enamine-type divinyl derivative 31, a direct precursor
of the dialkynylated hydrazine 29. This result was quite surprising since ClCꢀCCl has

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Fig. 3. a) ORTEP Plot of 25. Arbitrary numbering. Atomic displacement parameters obtained at 183 K are
drawn at the 50% probability level. Selected bond lengths and bond angles: C(7)ÀN(6) 1.40, N(6)ÀC(11) 1.41
C(11)ÀC(13) 1.32, N(1)ÀN(6) 1.39, N(1)ÀC(2) 1.39, N(1)ÀC(15) 1.41, C(15)ÀC(17) 1.32 ä;
C(7)ÀN(6)ÀC(11) 124.6, C(7)ÀN(6)ÀN(1) 118.3, C(11)ÀN(6)ÀN(1) 116.5, C(2)ÀN(1)ÀN(6) 118.2,
C(2)ÀN(1)ÀC(15) 124.2, C(15)ÀN(1)ÀN(6) 116.78. b) Crystal structure of 25 showing a short intermolecular
CÀH ¥¥¥ O H-bond.
been reported to react with amines under the conditions applied [24]. We surmised that
either i) the hydrazo group in 28 is too nucleophilic and promotes the polymerization of
ClCꢀCCl, or ii) there is no reaction occurring below the decomposition temperature of
one or both of the reactants. A third explanation (iii) would be that bis(1,2-
dichlorovinyl)diazene is generated in a spontaneous retro-DielsÀAlder reaction from
31 and decomposes as it is formed. When 28 was reacted with BnBr, which is not more

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electrophilic than ClCꢀCCl (its alkylation reaction with imidazole requires heating for
2 d in MeCN, while ClCꢀCCl undergoes this conversion in less than 12 h at or below
room temperature; see [25]), the corresponding bis-adduct 32 was obtained in good
yield. This tends to refute hypothesis ii. Attempts to react 1,2-dimethylhydrazine with
ClCꢀCCl only afforded an intractable orange wax, which seems in favor of hypothesis
i, by which hydrazines promote the polymerization of ClCꢀCCl. On the other hand,
hypothesis iii cannot be ruled out, since large amounts of anthracene were recovered as
well, pointing to a retro-DielsÀAlder reaction. It is, however, difficult to say whether
this reaction occurred starting from 28 or from 31.
Scheme 6. Explorationof the retro-DielsÀAlder Route to Azoacetylenes
Fmoc
N
Fmoc
N
30
a)
Cl
Cl
R
R
R
H
N
N
N
N
N
Cl
H
Cl
N
b)
X
N
N
31
29
28
c)
d)
R
Bn
N
Bn
N
+ HN=NH
32
a) Et2NH, MeCN/CH2Cl2, 08, 30min; 35%. b) ClCꢀCCl, DMAP, À 208, 24 h; 0 %.c) CDCl3, 208, 5 h; 100%. d)
MeCN, BnBr (4 equiv.), 08 ! 208, 12 h; 72% (20% from 30).
2.2. The 1,2-Bis(2-halogenovinylidene)hydrazine Route. In 1975, Malament and
Levi reported the debromination of azine 33 [26]. In the presence of cyclohexene,
diazene 34 was obtained together with 1,2-dibromocyclohexane and 3-bromocyclohex-
ene (Scheme 7). We intended to apply this concept to 1,2-divinylidenehydrazine 35,
leading to our target azoacetylene. This divinylidenehydrazine could possibly be
obtained in a Wittig-type reaction between azinobis(phosphorane) 36 [27] and
(triisopropylsilyl)ketene 37, followed by a brominationÀdehydrobromination se-
quence.
(Triisopropylsilyl)ketene 37 was synthesized by the well-known retro-ene reaction
of (tert-butoxy)alkyne 38 [28], which itself was prepared by known procedures [29].
The synthesis of a bromoketenimine from 37 was then attempted (Scheme 8).
Bromoketenimines are indeed not very widespread; only five have been reported [30],
none of which bears a silyl group. In our approach, reaction of ketene 37 with
Ph3PˆNPh provided the corresponding ketenimine 39 in good yield (NMR). This
ketenimine, however, is somewhat sensitive to heat, and distillation reduced the yield of

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Scheme 7. Debromination of Azine 33 and Proposed Synthesis of an Azoacetylene via Divinylidenehydrazine 35
a) Cyclohexene, MeOH, AcOEt, D, 10min; 70%. D ˆ heating to reflux.
isolated material to 45%. Sequential bromination (Br2)Àdehydrobromination (NEt3)
of 39 afforded the desired bromoketenimine 40 with good conversion. This compound
was stable only in solution and decomposed rapidly to a black oil when stored in pure
form at room temperature. It was, therefore, converted, by acidic hydrolysis, to a-
bromoacetamide 41 that was fully characterized.
Scheme 8. Synthesis of Bromoketenimine 40
a) 808, 1 h; 84%. b) Ph3PˆNPh, CH2Cl2, 208, 4 h; 45%. c) Br2, CH2Cl2, 208, then Et3N (3 equiv.), 208, 1 h; 65%.
d) THF, 1n HCl; 208, 3 h; 37%.
The synthesis of the desired 1,2-divinylidenehydrazine 35 was then explored. For
this purpose, the known hydrazobis(triphenylphosphonium) dichloride 42 [27] was
reacted with ketene 37 in the presence of a base (Scheme 9). Immediate conversion of
the starting ketene was observed, but no 1,2-divinylidenehydrazine could be detected in
the formed mixture. There was indeed no sign of the characteristic CˆN stretching
band around 2000 cmÀ1 in the IR spectrum. Unfortunately, it was not possible to assign
structures to the compounds in the complex mixture formed.
It is well possible that 1,2-divinylidenehydrazine 43 is unstable. To the best of our
knowledge, there are only two reported 1,2-divinylidenehydrazines (44 and 45; Fig. 4),
both of them bearing two electron-withdrawing groups [31]. While 44 seems to be
stable, 45 exists in an equilibrium together with two phenoxide radicals resulting from
NÀN bond cleavage. There are not many other 1,2-vinylidenehydrazines (ketene-
hydrazones) reported in the literature. Schweng and Zbiral reported the easy cleavage
of the NÀN bond of some 1,2-vinylidenehydrazines, preventing their full character-

Helvetica Chimica Acta Vol. 86 (2003)
Scheme 9. Attempted Synthesis of 43
3105
a) CCl4, THF, 408, 4 h; 74%. b) Et3N (5 equiv.), CH2Cl2, 208, 15 min, then c) 37 (2 equiv.), CH2Cl2, 08 ! 208,
2 h.
ization [32]. Only a perfluorinated 1,2-vinylidenehydrazine reported by Coe et al.
seems stable [33], pointing again towards a favorable stabilizing effect of electron-
withdrawing substituents on the NÀN bond. In consequence, future efforts on this
route towards azoacetylenes should focus on the coupling of azinobis(phosphorane) 36
with ketenes bearing electron-withdrawing substituents.
O
t-Bu
N
O
N
O
t-Bu
2
2
44
45
Fig. 4. Known divinylidenehydrazines 44 and 45 [31]
2.3. The NˆN Connective Approach. 2.3.1. The Photocoupling Route. In 1978, Yim
et al. reported the irradiation of (arylimino)(triphenyl)phosphoranes (Ph3PˆNAr) to
form the corresponding diarylated azo derivatives in excellent yield [34]. Accordingly,
we sought to perform the same reaction with (alkynylimino)(triphenyl)phosphorane 46
(Scheme 10).
Scheme 10. Attempted Photocoupling of 46
a) MeCN, 108, 30min, then NH 3, 208, 12 h; 89%. b) BuLi, THF, À 788, 15 min. c) 17c, À 788 ! 208, 1 h; 22%
(crude). d) PhMe, hn, 12 h; 0 %.
Compound 46 was obtained by alkynylation of the aminophosphonium salt 47 with
alkynyliodonium salt 17c. This reaction was not very efficient but the formation of the
desired (alkynylimino)(triphenyl)phosphorane could be observed in ca. 22% yield in
the crude mixture. 1H-, 13C-, and 31P-NMR spectra pointed indeed toward the
formation of 46 (see Exper. Part). This compound is, however, extremely sensitive to

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H2O and could not be isolated in sufficiently pure form for proper characterization.
Attempts to perform the photocoupling reaction (medium-pressure Hg lamp, 125 W)
on the crude product mixture were not successful; no evidence for the formation of any
azo compound or for any change in the composition of the mixture was obtained. On
the other hand, the findings of Yim et al. were readily reproduced. Therefore, the
observed lack of reactivity of crude 46 could be a consequence of light absorption
mainly by the impurities in the crude product mixture. Hence, this approach should be
re-examined when a better synthesis of 46 has been devised.
2.3.2. Development of a New Condensation Method towards Azo Compounds. Since
no acidic functionalities can be present in the NˆN bond-forming reaction needed for
the synthesis of azoacetylenes, we sought to devise a new route towards azo
compounds, starting from nonprotic amines. Two research teams had previously
reported the synthesis of diazonium salts by the condensation of nitrosyl chloride with
either imino(triphenyl)phosphoranes (Ph3PˆNR) [35a] or N,N-bis(trimethylsilyl)ani-
line [35b] (for an application to the synthesis of phenylethynyl diazonium salts, see
[35c]). It was thus decided to investigate the coupling of N,N-disilylated anilines with
nitrosobenzene (Scheme 11 and Table 1).
Scheme 11. Coupling Reaction of N,N-Bis(trimethylsilyl)aniline 48a with Nitrosobenzene 49a
O
SiMe3
SiMe3
a)
+
Br
N
N
48a
49a
O
N
N
Br
N
Br
N
+
+
N
N
O
51a
52a
50a
a) See Table 1.
Table 1. Coupling Attempts between N,N-Bis(trimethysilyl)aniline 48a and Nitrosobenzene (49a)
Entry
Reaction conditions
Results
1
2
3
4
5
6
CH2Cl2, 208, 24 h
No reaction
Starting material destroyed
50a: 28%
No reaction
51a: 6%; 52a: 21%; 50a: 10 %
51a: 20%; 50a: 58%
CH2Cl2, Me3SiOSO2CF3 (1 equiv.), 08 ! 208, 2 h
CH2Cl2, Me3SiN(SO2CF3)2 (1 equiv.), À 708 ! 208, 3 h
DMF, CsF, 208, 12 h
THF, Bu4NF on SiO2 (0.05 equiv.), À 708 ! 208, 12 h
THF, MeOK (1 equiv.), 08 ! 208, 12 h
While attempts to couple (triphenyl)(phenylimino)phosphorane (Ph3PˆNPh) with
nitrosobenzene in the presence of various Lewis acids (TiCl4, BF3 ¥ OEt2, SnCl4) led
only to the decomposition of starting material, the conversion of the disilylated anilines
was more successful. At first, the transformation of 48a with nitrosobenzene (49a) in
the presence of Me3SiN(CF3SO2)2 (Table 1, Entry 3) yielded only the azoxybenzene
derivative 50a, resulting probably from the deoxygenation of nitrosobenzene dimer

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(the deoxygenation of nitrosobenzene dimer has already been reported, albeit in the
presence of reducing agents; see [36]). Instead of Lewis acid catalysts, Lewis bases
were evaluated as reaction promoters (Table 1, Entries 5 and 6). Indeed, in the
presence of FÀ or MeOÀ anions, the desired azobenzene derivative 51a could be
obtained together with the corresponding azoxy derivative 52a and azoxybenzene 50a
resulting from the homo-coupling of two nitrosobenzene molecules. The MeOÀ-
mediated formation of azobenzene 51a probably starts by monodesilylation of the
disilylated aniline, which then adds to nitrosobenzene, followed by elimination of
trimethylsilyloxide (step i in Scheme 12). The latter can promote the desilylation of
another disilylated aniline, generating bis(trimethylsilyl)ether in a catalytic process.
The scope of this new reaction was then briefly investigated (Table 2) and shown to
be limited to aromatic silylamines. Aliphatic silylamine 48c promoted only the self-
coupling of the nitroso derivatives 49a and 49c, leading to symmetric azoxybenzenes
50a/50c, respectively. It must be noted that no such homo-coupling of the nitroso
components occurs in the absence of silylamine! The reaction between 48a and 49c also
afforded a small amount (6%) of nitrosoaniline 53 resulting from nucleophilic aromatic
substitution.
Table 2. Reactionof N,N-Disilyl Amines 48a 48c and Nitroso Compounds 49a 49c inthe Presence of MeOK.
Reaction conditions: a) MeOK, THF, 08 ! 208, 12 h.
R
R
R'
O
O
SiMe3
SiMe3
O
a)
+
+
+
N
N
N
N
N
N
R
N
R'
N
R'
R'
R'
48
49
51
52
50
R
R'
Yields
(48a)
Ph (49a)
51a: 20 %
0
50a: 58%
Br
48a
t-Bu (49b)
51b: 61%a)
0
0
NO
H
N
48a
(49c)
(53): 6% 52c: 16%
50c: 33%
Et2N
Br
NEt2
(48b)
49a
51d: 12%a)
52d: 16%a) 50a: 16%a)
Cl
MeO
(48c) 49a
49c
0
0
0
0
50a: 70 %
50c: 61%
48c
a) Obtained with Bu4NF on SiO2 (0.5 equiv.).

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Helvetica Chimica Acta Vol. 86 (2003)
Our results are tentatively explained by competition between the addition of mono-
desilylated aniline to nitrosobenzene (49a) (step i) and the silyl-amine-promoted
reduction of nitrosobenzene or its dimer [37] by the MeOÀ ion (step ii, Scheme 12).
Although it has never been reported that nitrosobenzene is reduced by alkoxides, it is
the case for nitrobenzene [38]. The disilyl amine somehow accelerates this reduction
(steps iii and iv), since no reaction occurs without it; it possibly assists as an electron
shuttle in one-electron transfer reduction steps. In the case of alkylamine 48c (Table 2),
the desilylation is probably slower compared to the aromatic amine due to the higher
N-basicity. Hence, the heterocoupling reaction (step i) may become too slow to be
competitive with the reduction of nitrosobenzene or its dimer, and is no longer
observed. Another reduction mechanism must be at play in the case of the FÀ-catalyzed
reaction.
Scheme 12. Possible Mechanism for the Formation of Hetero- and Homo-Coupled Products in the MeOÀ-
Mediated Conversion of Disilylated Anilines with Nitrosobenzene
2.3.3. Applicationof the New N ˆN Bond-Forming Reaction to Acetylenic
Derivatives. Although the yields of the new coupling reaction (Sect. 2.3.2) are rather
modest, the condensation between nitrosoalkyne 54 and N,N-disilylated ynamine 55
under formation of azoacetylene 56 was explored (Scheme 13). tert-Bu-Substituted
alkynes were chosen in the hope that the steric bulk would impart some stability to
starting materials and product. As a result, homo- and heterocoupling would lead to the
identical product 56. The use of silyl protecting groups on the alkynes was not advised,

Helvetica Chimica Acta Vol. 86 (2003)
3109
as they might be cleaved under the desilylating conditions of the reaction. Both alkynes
54 and 55 were obtained according to the literature procedures [39]. It has to be
mentioned that nitroso-alkyne 54 cannot be obtained by reacting the lithium salt of 3,3-
dimethylbut-1-yne with nitrosyl tetrafluoroborate. The use of the toxic Hg salt 57 is,
thus, mandatory. When the coupling reaction was attempted at À 408, no conversion
was observed, whereas raising the temperature above À 208 led to the destruction of
the starting nitrosoalkyne. This was not surprising, since 54 is known to decompose at
temperatures above À 408 [39c].
Scheme 13. Attempted Coupling between Alkynyl Derivatives 54 and 55
SiMe3
t-Bu
t-Bu
N
SiMe3
55
t-Bu
N
b)
X
N
t-Bu
a)
56
t-Bu
Hg
2
N
O
54
57
c)
CO
CO
CO
OC
OC
CO
Co
Co
Co
t-Bu
N
t-Bu
OC
N
t-Bu
N
X
O
N
Ar
N Ar
Co
CO
CO
OC
CO
CO
58
a) NOCl, CH2Cl2, À 708, 15 min then b) or c). b) MeOK or Bu4NF, THF, À 408 ! 208, 3 h; 0%. c) [Co2(CO)8],
CH2Cl2, À 708 ! 208, 2 h; 49%.
Accordingly, the nitroso compound 54 was stabilized through its Co complex 58.
The reaction with octacarbonyldicobalt provided a brown solid showing IR absorptions
at 2000 and 1450 cmÀ1, which could be assigned to CO and NO stretching bands,
1
respectively. The H-NMR spectrum indicated the presence of the t-Bu group. We
could, however, not obtain a satisfying mass spectrum nor an elemental analysis of this
molecule, as its instability in solution precluded any thorough purification. This
compound was nonetheless submitted to the coupling reaction with the N,N-disilylated
aniline 48a, but no conversion of 48a to the desired coupled product could be observed.
Only desilylated 4-bromoaniline could be recovered. The complex 58 eventually had
completely decomposed after an extended reaction time.
3. Conclusions and Outlook. After this work, the targeted azoacetylenes 1 and 2
remain elusive. Although the alkynylation of amides with alkynyliodonium salts
provides a suitable entry into simple N-alkynyl amides, this method does not seem
appropriate for the construction of dialkynylated hydrazides (Schemes 2 and 3), the
direct precursors of azoacetylenes. Even though the anions of the monoalkynylated
hydrazides 20b and 22 did not react with tert-butyl 3-bromoprop-2-ynoate, the second
alkynylation could possibly be achieved in future work by applying the recently

3110
Helvetica Chimica Acta Vol. 86 (2003)
published protocol for Cu-catalyzed hetero-cross-coupling of bromoacetylenes to
amides [40]. More indirect approaches to dialkynylated 1,3,4-thiadiazolidine-2,5-
diones, such as 15a direct precusors to azoacetylenes remain to be explored. By N,N-
dialkylation or diacylation of heterocycle 16, versatile synthons for CꢀC moieties, such
as aldehydes or diethyl vinyl phosphates, could be introduced and subsequently
transformed into the alkynyl residues. Similar indirect routes could also be pursued for
the dialkynylation of 9,10-dihydro-9,10-diazanoanthracene (28).
Our attempts to synthesize bis(2-bromovinylidene)hydrazine 35 as a precursor to
azoacetylenes were also not successful. Although the envisioned aza-Wittig reaction
could be applied to the synthesis of bromo(triisopropylsilyl)ketenimine 40, the reaction
of azinobis(phosphorane) 36 with (triisopropylsilyl)ketene 37 did not afford bis(viny-
lidene)hydrazine 43. In view of the low stability of the ketenimine 40, the synthesis of
35 could prove very difficult.
Although aprotic coupling conditions for the reaction between N,N-disilylated
anilines and nitroso compounds could be successfully developed (Tables 1 and 2), their
application to the construction of azoacetylenes requires conditions that allow the
coupling partners to react at or below À 408, owing to the low stability of the
nitrosoacetylene 54 above that temperature. Obtaining azoacetylenes through an NˆN
bond-forming reaction could possibly be more successful by the light-promoted
coupling of pure (alkynylimino)phosphorane 46. However, a better access to these
compounds needs first to be developed.
Another approach to the synthesis of azoacetylenes not investigated in this work
could be based on the addition of alkynyl cuprates to properly substituted hyponitrites
(ROÀNˆNÀOR ! R'ÀCꢀCÀNˆNÀCꢀCÀR'). This approach is related to the
synthesis of the parent cis-3-azahex-3-ene-1,5-diyne from tosylimines [14a]. Sulfonyl-
ated hyponitrites are not reported but the acyl derivatives show very low stability at
room temperature, undergoing radical NÀO bond cleavage [41]. On the other hand,
most dialkyl hyponitrites are stable at room temperature [42]; therefore, the success of
this approach would require to find a leaving group showing enough stability and, at the
same time, sufficient nucleofuge character to react with alkynyl cuprates. It is clear that
many routes towards azoacetylenes remain to be explored. But, given the fascinating
research objectives that can be targeted once these compounds are in hand (see the
Introduction), all these efforts seem to be most worthwhile.
Experimental Part
General. Reagents and solvents (reagent-grade) were used without further purification unless otherwise
stated. THF and Et2O were freshly distilled from sodium benzophenone ketyl, and CH2Cl2 was freshly distilled
from CaH2. Anh. DMF, stored over molecular sieves, was purchased from Fluka. (i-Pr)2NH and Et3N were
distilled over CaH2, and used immediately after distillation. All reactions were performed in oven-dried or
flame-dried glassware under an inert atmosphere (N2 or Ar) unless otherwise stated. Evaporation and
concentration invacuo were performed at 358. Further drying of new compounds was carried out at ca. 0.1 Torr.
Column chromatography (CC) refers to flash chromatography on SiO2 60 (0.02 Æ 0.063 mm) from Fluka; head
pressure of ca. 0.3 bar. TLC: Polygram SIL G/UV254 SiO2-coated plates from Macherey-Nagel; visualization by
UV light (254 nm) or by coloring with a permanganate or phosphomolybdate soln. M.p.: B¸chi B-540 melting-
point apparatus; uncorrected. IR Spectra [cmÀ1]: Perkin-Elmer 1600 FT-IR spectrometer. NMR Spectra (1H,
13C; d [ppm], J [Hz]: VarianGemini 200 , VarianMercury 300 , or VarianGemini 300 spectrometer at 298 K with
residual solvent peaks as internal reference. EI-MS: VG-Tribrid instrument operating at 70eV. ESI-MS:

Helvetica Chimica Acta Vol. 86 (2003)
3111
Finnigan Mat TSQ 7000 instrument. MALDI-MS: IonSpec Ultra instrument, with 2,5-dihydroxybenzoic acid
(DHB) or 2,4,5-trihydroxyacetophenone/diammonium citrate 2 :1 (THA) as a matrix. Elemental analyses were
performed by the Mikrolabor at the Laboratorium f¸r Organische Chemie at ETH Z¸rich.
X-Ray Crystal Structure of 25. Crystals were grown by slow evaporation of a soln. of 25 in PhMe. Crystal
data at 183 K for C8H8N2O4Cl4 (Mr 337.96): monoclinic space group P2(1)/n, Dc ˆ 1.673 g/cm3, Z ˆ 4, a ˆ
7.7520(2), b ˆ 13.1318(3), c ˆ 13.6012(3) ä, a ˆ 90.00, b ˆ 104.241(1), g ˆ 90.008, Vˆ 1342.02(5) ä3. Bruker-
Nonius Kappa-CCD, MoKa radiation, l ˆ 0.7107 ä. The structure was solved by direct methods [43] and refined
by full-matrix least-squares analysis [44] including an isotropic extinction correction. All heavy atoms were
refined anisotropically (H-atoms isotropic, whereby H-positions are based on stereochemical considerations).
Final R(F) ˆ 0.030, wR(F2) ˆ 0.0911 for 172 parameters and 2798 reflections with I > 2s(I) and q < 27.488.
Crystallographic data (excluding structure factors) for the structure reported in this paper have been deposited
with the Cambridge Crystallographic Data Centre, deposition No. CCDC 208636. Copy of the data can be
obtained free of charge, on application to the CCDC, 12 Union Road, Cambridge CB2 1EZ UK (fax:
‡44(1223)336033; e-mail: deposit@ccdc.cam.ac.uk).
3-Methyl-4-[(triisopropylsilyl)ethynyl]-1,3,4-thiadiazolidine-2,5-dione (15b). A soln. of 20b (100 mg,
335 mmol) in DMF (2 ml) was added to a suspension of KH (74 mg, 368 mmol) in DMF (2 ml). After stirring
for 15 min at 208, MeI (208 ml, 475 mg, 3.35 mmol) was added to the mixture. After stirring for 7 h at 208, the
mixture was diluted with CH2Cl2 (30ml) and extracted with H 2O (2 Â 20ml). The org. phase was dried
(MgSO4) and concentrated invacuo . The residue (130mg of a yellow oil) was purified (CC; CH 2Cl2) to yield 15b
(50mg, 48%). A total of 40mg (40%) of starting material was also recovered. Colorless oil. IR (neat): 1128 m,
1292m, 1691s, 1732s, 2171s, 2865 s, 2943s. 1H-NMR (CDCl3, 300 MHz): 1.11 (m, 21 H); 3.47 (s, 3 H). 13C-NMR
‡
(CDCl3, 75 MHz): 11.2; 18.6; 31.9; 81.7; 85.7; 161.1; 170.6. EI-MS: 312 (M ). HR-EI-MS: 269.0770 ([M À (i-
‡
‡
Pr)] , C11H17N2O2SSi ; calc. 269.0780).
5-Methoxy-3H-1,3,4-thiadiazol-2-one (18). A suspension of hydrazinecarbothioic acid O-methyl ester
(1.0g, 9.42 mmol) in CH 2Cl2 (20ml) was cooled to 0 8, and a soln. of triphosgene (1.0g, 3.30mmol) in CH 2Cl2
(20ml) was slowly added so that the temp. was kept below 10 8. The mixture was stirred for 30min at 20 8, then
sat. aq. NaHCO3 soln. (20ml) was added, and stirring was continued for 15 min. The mixture was filtered, and
the solid was washed with CH2Cl2. The phases were separated, and the aq. phase was washed with CH2Cl2 (3 Â
20ml). The combined org. phases were dried (MgSO 4) and concentrated invacuo . The residual solid was
recrystallized from MeOH/H2O 1:1 to give 18 (1.1 g, 88%). White solid. M.p. (dec.) 136 1378 ([20a]: 1358). IR
(CH2Cl2): 1269s, 1421s, 1591s, 1706s, 2305s, 2986s, 3413m. 1H-NMR (CDCl3, 300 MHz): 3.98 (s, 3 H); 9.00
(br. s, 1 H). 13C-NMR (CDCl3, 75 MHz): 57.3; 159.9; 170.1.
5-Methoxy-3-[(trimethylsilyl)ethynyl]-3H-1,3,4-thiadiazol-2-one (19a). To HMDS (0.89 ml, 4.23 mmol) in
PhMe (8 ml) at 08, BuLi (2.63 ml of a 1.6m soln. in hexane, 4.21 mmol) was slowly added, and the mixture was
stirred at 08 for 15 min. A soln. of 18 (490mg, 3.71 mmol) in PhMe (32 ml) was added, and the mixture was
stirred for 20min at 0 8. Finally, 17b (2.0g, 4.44 mmol) was added in portions, and the mixture was stirred
overnight at 208. After filtration through a plug of SiO2, the precipitate was washed with PhMe (500 ml). The
combined filtrates were concentrated invacuo to afford 1 g of an orange solid that was purified (CC; PhMe) to
give 19a (500 mg, 59%). Pale-yellow solid. M.p. 74 758. IR (CH2Cl2): 837s, 1265s, 1589s, 1699s, 2247m, 2262m,
2951s. 1H-NMR (CDCl3, 300 MHz): 0.24 (s, 9 H); 4.04 (s, 3 H). 13C-NMR (CDCl3, 75 MHz): À 0.1; 57.8; 78.8;
‡
‡
‡
88.6; 157.4; 167.2. EI-MS: 228 (M ). HR-EI-MS: 213.0149 ([M À Me] , C7H9N2O2SSi ; calc. 213.0152).
5-Methoxy-3-[(triisopropylsilyl)ethynyl]-3H-1,3,4-thiadiazol-2-one (19b). To HMDS (0.75 ml, 3.56 mmol)
in PhMe (7 ml) at 08, BuLi (2.22 ml of a 1.6m soln. in hexane, 3.55 mmol) was slowly added, and the mixture was
stirred at 08 for 15 min. A soln. of 18 (413 mg, 3.12 mmol) in PhMe (30ml) was added dropwise, and the
resulting mixture was stirred for 20min at 0 8. Finally, 17c (2.0g, 3.74 mmol) was added in portions, and the
mixture was stirred for 1 h at 1108. After filtration through a plug of SiO2, the precipitate was washed with PhMe
(500 ml), and the combined filtrates were concentrated invacuo to afford 1.34 g of an orange solid, which was
purified (CC; PhMe/pentane 1:1) to give 19b (570mg, 58%). Pale-yellow solid. M.p. 55 57 8. IR (CH2Cl2):
1265s, 1589s, 1695s, 2248m, 2255m, 2965s. 1H-NMR (CDCl3, 300 MHz): 1.11 (m, 21 H); 4.04 (s, 3 H). 13C-NMR
‡
(CDCl3, 75 MHz): 11.2; 18.6; 57.8; 75.5; 90.4; 157.0; 167.4. EI-MS: 312 (M ). HR-EI-MS: 269.0772 ([M À (i-
‡
‡
Pr)] , C11H17N2O2SSi ; calc. 269.0780).
3-[(Triisopropylsilyl)ethynyl]-1,3,4-thiadiazolidin-2,5-dione (20b). Method A. BuLi (1.16 ml of a 1.6m soln.
in hexane, 1.86 mmol) was added dropwise at 08 to a stirred soln. of HMDS (393 ml, 301 mg, 1.86 mmol) in THF
(9 ml). After stirring for 15 min, 16 (100 mg, 847 mmol) in THF (18 ml) was added dropwise. After 30min, the
cloudy mixture was cooled to À 708, and 17c (905 mg, 1.69 mmol) in THF (9 ml) was slowly added. After
warming to 208, stirring was continued for 12 h. The mixture was poured into sat. aq. NH4Cl soln. (20ml) and

3112
Helvetica Chimica Acta Vol. 86 (2003)
extracted with Et2O (3 Â 20ml). The org. phases were dried (MgSO 4) and concentrated invacuo to yield 450mg
of a yellow oil, which was purified (CC; MeOH/CH2Cl2 0:1 ! 1:9) to provide 20b (190mg, 79%). Unstable
1
yellow oil. H-NMR (CD3OD, 300 MHz): 1.10 (m, 21 H). 13C-NMR (CD3OD, 75 MHz): 12.5; 19.0; 73.2; 93.7;
162.5; 172.5.
Method B. To a stirred suspension of KH (374 mg, 1.86 mmol) in THF (9 ml) at 08, a soln. of 16 (100 mg,
847 mmol) in THF (18 ml) was added dropwise. After 30min, the mixture was cooled to À 788, and 17c (905 mg,
1.69 mmol) in THF (9 ml) was slowly added. After warming to 208, stirring was continued for 24 h. The orange
mixture was poured into sat. aq. NH4Cl soln. (20ml) and extracted with Et 2O (3 Â 20ml). The combined org.
phases were dried (MgSO4) and concentrated invacuo to yield 450mg of a yellow oil, which was purified (CC,
MeOH/CH2Cl2 0:10 ! 1 :9) to give 20b (130mg, 51%).
Dimethyl 1-[(Triisopropylsilyl)ethynyl]hydrazine-1,2-dicarboxylate (22). To HMDS (434 ml, 332 mg,
2.06 mmol) in PhMe (2 ml) at 08, BuLi (1.29 ml of a 1.6m soln. in hexane, 2.06 mmol) was slowly added, and
the mixture was stirred at this temp. for 15 min. A soln. of 21 (138 mg, 936 mmol) in PhMe (8 ml) and THF
(5 ml) was added dropwise, and the mixture was stirred for 20min at 0 8. Finally, 17c (1.0g, 1.87 mmol) was
added portionwise, and stirring was continued for 12 h at 208. The yellow mixture was poured into sat. aq. NH4Cl
soln. (30ml) and extracted with Et 2O (3 Â 20ml). The combined org. phases were dried (MgSO 4) and
concentrated invacuo to yield 500 mg of a yellow oil, which was purified (CC, AcOEt/PhMe 0 :1 ! 1:9) to
provide 22 (120mg, 39%). Unstable colorless oil. IR (neat): 1068 s, 1213s, 1457s, 1517s, 1734s, 2182s, 2929s, 3308s.
1H-NMR (CDCl3, 300 MHz): 1.06 (s, 21 H); 3.78 (s, 3 H); 3.85 (s, 3 H); 7.10(br. s, 1 H). 13C-NMR (CDCl3,
75 MHz): 11.3; 18.5; 53.4; 54.8; 71.1; 94.6; 154.4; 155.0. EI-MS: 110 (100), 124 (60), 180 (99), 285 (37), 328 (0.5,
‡
M ).
Dimethyl 1-(4-Nitrobenzyl)-2-[(triisopropylsilyl)ethynyl]hydrazine-1,2-dicarboxylate (24). A soln. of 22
(110mg, 335 mmol) in DMF (2 ml) was added to a suspension of KH (80mg, 402 mmol) in DMF (2 ml), and the
mixture was stirred at 208 for 30min. 4-Nitrobenzyl bromide (145 mg, 670 mmol) was then added, and the
mixture was stirred for 12 h. The mixture was poured into H2O (20ml) and extracted with CH 2Cl2 (3 Â 20ml).
The combined org. phases were dried (MgSO4), concentrated invacuo , and purified (CC, PhMe/AcOEt 1 :0 !
98 :2) to yield 24 (30mg, 19%). Pale-yellow oil. IR (neat): 1348 s, 1441s, 1524s, 1608w, 1755s, 2179m, 2865m,
2945m, 3031w. 1H-NMR (CDCl3, 300 MHz): 1.06 (s, 21 H); 3.82 (s, 6 H); 4.61 (d, J ˆ 15.0, 1 H); 5.04 (br. d, J ˆ
15.0, 1 H); 7.57 (d, J ˆ 8.7, 2 H); 8.15 8.18 (d, J ˆ 8.7, 2 H). 13C-NMR (CDCl3, 75 MHz): 11.0; 18.4; 52.6; 54.2;
‡
54.8; 77.2; 93.5; 123.6; 129.4; 142.8; 147.6; 153.7; 155.0
C22H33N3NaOSi ; calc. 486.2036).
.
HR-MALDI-MS: 486.20 36 M([‡ Na] ,
‡
Dimethyl 1,2-Bis[(E)-1,2-dichloroethenyl]hydrazine-1,2-dicarboxylate (25) and Dimethyl 1-[(E)-1,2-Di-
chloroethenyl)hydrazine-1,2-dicarboxylate (26). A soln. of 21 (200 mg, 1.35 mmol) and DMAP (181 mg,
1.49 mmol) in MeCN (5 ml) was cooled to À 408, and ClCꢀCCl (1.35 ml, 4.05 mmol) was slowly added. The
mixture was stirred at 208 for 12 h, and then the solvent was evaporated invacuo . The brown oily residue was
dissolved in CH2Cl2 and washed with H2O. The org. phase was dried (MgSO4) and concentrated invacuo to yield
0.5 g of a brown oil, which was purified (CC, CH2Cl2) to afford 25 (100 mg, 21%). Further elution (CH2Cl2/
AcOEt 9 :1) provided 26 (40mg, 12%).
Data of 25: White solid. IR (CH2Cl2): 1276s, 1333s, 1441s, 1614w, 1754s, 2853w, 3018w, 3092w. 1H-NMR
(CDCl3, 300 MHz): 3.89 (s, 6 H); 6.39 (s, 2 H). 13C-NMR (CDCl3, 75 MHz): 54.6; 119.4; 128.6; 151.5. EI-MS: 59
‡
(100), 163 (33), 165 (20), 257 (20), 259 (20), 301 (12), 303 (12), 338 (1.5, M ). Anal. calc. for C8H8N2O4Cl4
(337.97): C 28.53, H 2.39, N 8.29; found: C 29.02, H 2.45, N 8.05. X-Ray: see Fig. 3.
Data of 26: Unstable, colorless oil. IR (CH2Cl2): 1243s, 1302s, 1442s, 1489m, 1617m, 1746s, 2959m, 3033m,
3400m. 1H-NMR (CDCl3, 300 MHz): 3.79 (s, 3 H); 3.86 (s, 3 H); 6.26 (s, 1 H); 6.8 (br. s, 1 H). 13C-NMR
(CDCl3, 75 MHz): 53.4; 54.6; 116.8; 130.3; 155.7.
11,12-Dibenzyl-9,10-dihydro-9,10-diazanoanthracene (32). To a soln. of 30 (991 mg, 1.52 mol) in CH2Cl2
(70ml), MeCN (7 ml) was added. The soln. was cooled to 0 8, and Et2NH (7 ml) was added over a period of
15 min. After stirring at 08 for 25 min, the solvents were evaporated invacuo at 08. The resulting yellow solid was
suspended in Et2O (8 ml) and re-evaporated to remove any residual Et2NH. The residual solid was re-dissolved
in pre-cooled MeCN (7 ml) at 08, and BnBr (1.56 g, 9.11 mmol) was added. The mixture was stirred for 3 h at 08
and then for 12 h at 208. Evaporation invacuo afforded a residue, which was dissolved in CH2Cl2 and washed
with sat. aq. NH4Cl soln. The org. phase was dried (MgSO4) and concentrated invacuo to yield 1 g of yellow oil
solid, which was purified (CC; PhMe/hexane 0:1 ! 1:0) to give32 (150mg, 20%). White solid. M.p. 160 162 8.
IR (CH2Cl2): 730s, 1255s, 1275s, 1459m, 1468m, 2847m, 3029m, 3053m. 1H-NMR (CDCl3, 300 MHz): 3.25
(br. s, 2 H); 3.50(br. s, 2 H); 4.84 (s, 2 H); 7.26 (m, 18 H). 13C-NMR (CDCl3, 75 MHz): 61.6; 61.8; 124.9; 126.5;

Helvetica Chimica Acta Vol. 86 (2003)
3113
‡
127.1; 128.2; 129.7; 138.5; 140.1. ESI-MS: 389 (100, MH ). Anal. calc. for C28H24N2 (388.51): C 86.56, H 6.23, N
7.21; found: C 86.51, H 6.35, N 7.05.
(Triisopropylsilyl)ethenone (37) [45]. Alkyne 38 (13 g, 51.08 mmol) was heated to 808 for 1 h. After the end
of the gas evolution, stirring was continued for 30min, then the product was distilled at 60 8/0.02 Torr to afford
pure 37 (8.5 g, 84%). Colorless liquid. B.p. 90 100 8/1 Torr. IR (neat): 883m, 995m, 1106w, 1273w, 1367w, 1462m,
1552s, 2106s, 2866s, 2944s. 1H-NMR (CDCl3, 300 MHz): 1.06 (m, 21 H); 1.65 (s, 1 H). 13C-NMR (CDCl3,
75 MHz): À 7.3; 12.1; 18.2; 178.9.
1-(tert-Butoxy)-2-(triisopropylsilyl)ethyne (38) [29b]. To a soln. of LDA, prepared from BuLi (69 ml of a
1.6m soln. in hexane, 110.40 mmol) and (i-Pr)2NH (17.5 ml, 13.51 g, 133.5 mmol) in THF (100 ml) at À 608, (Z)-
2-bromo-1-(tert-butoxy)ethene (8.95 g, 49.98 mmol) in THF (20ml) was added. The mixture was stirred for 2 h
while allowing it to warm to 208. A dark-red color slowly developed. The mixture was then cooled to À 208, and
(i-Pr)3SiCl (12.81 ml, 11.54 g, 59.88 mmol) was added. After stirring for 4 h at 208, the mixture was poured into
sat. aq. NaHCO3 soln. (100 ml), the org. layer was separated, and the aq. layer was extracted with petroleum
ether (3 Â 20ml). The combined org. phases were washed with 0.5 n HCl (2 Â 100 ml), H2O (100 ml), and sat.
aq. NaCl soln. (150ml), and dried (MgSO 4). Solvent evaporation at 208 afforded crude 38 as a dark red liquid
(16 g), which was used directly in the next reaction. The crude product was purified by filtration through a short
column (6 g of SiO2 containing 3% (v/v) Et3N) and elution with pentane (250ml) to give 38 (15 g, 100%). Pale-
yellow liquid. IR (CH2Cl2): 883m, 1148m, 1250w, 1370m, 1462m, 1717m, 2166s, 2864s, 2942s. 1H-NMR (CDCl3,
300 MHz): 1.06 (s, 21 H); 1.42 (s, 9 H). 13C-NMR (CDCl3, 75 MHz): 11.7; 17.7; 18.8; 26.9; 86.6; 107.5.
N-[2-(Triisopropylsilyl)ethenylidene]aniline (39). A soln. of 37 (200 mg, 1.01 mmol) in CH2Cl2 (5 ml) was
added to Ph3PˆNPh (356 mg, 1.01 mmol) in CH2Cl2 (5 ml), and the mixture was stirred until disappearance of
the band at 2100 cmÀ1 in the IR spectrum (ca. 4 h). Evaporation invacuo afforded a pale-yellow solid residue,
which was taken up in hexane (ca. 10ml). Filtration afforded a filtrate that was concentrated invacuo to give
200 mg of a yellow oil. Distillation under reduced pressure afforded 39 (110mg, 45%). Colorless liquid. B.p.
100 1108/0.03 Torr. IR (CCl4): 1119s, 1256s, 1437s, 1483s, 1491s, 1591s, 1943s, 2000s, 2865s, 2890m, 3045w, 3078w.
1H-NMR (CDCl3, 300 MHz): 1.06 (m, 21 H); 3.21 (s, 1 H); 7.24 (m, 3 H); 7.33 (m, 2 H). 13C-NMR (CDCl3,
‡
75 MHz): 11.8; 18.5; 31.1; 123.4; 126.3; 129.3; 141.5; 181.2. EI-MS: 274 (0.2, M ), 230 (11), 131 (100), 103 (99),
75 (97), 61 (35), 28 (30). Anal. calc. for C17H27NSi (273.49): C 74.66, H 9.95, N 5.12; found: C 74.67, H 9.87, N
5.06.
N-[2-Bromo-2-(triisopropylsilyl)ethylidene]aniline (40). A soln. of Br2 (34 ml, 105 mg, 658 mmol) in CH2Cl2
(5 ml) was added dropwise to 39 (180mg, 658 mmol) in CH2Cl2 (5 ml) until a red color persisted. The soln.
slowly turned yellow and then orange. IR Monitoring showed the disappearance of the characteristic band at
2000 cmÀ1 band while a weaker band around 1667 cmÀ1 appeared. After cooling with an ice bath, Et3N (300 ml,
219 mg, 2.17 mmol) was added, and the soln. was stirred at 208 for 1 h. A new band at 1993 cmÀ1 appeared. The
mixture was concentrated invacuo and the solid residue was taken up in hexane (15 ml) and filtered. The filtrate
was concentrated to afford 40 (150mg, 65%). This compound could not be sufficiently purified (CC or
distillation) to obtain an elemental analysis. It decomposes rapidly when stored neat under Ar at 208. Yellow
liquid. IR (CCl4): 1465w, 1488w, 1592w, 1993s, 2857m, 2946m. 1H-NMR (CDCl3, 300 MHz): 1.12 (m, 18 H); 1.20
‡
(m, 3 H); 7.35 (m, 5 H). EI-MS: 351 (0.5, M ), 308 (2), 276 (6), 230 (100), 188 (35), 160 (25), 131 (22).
2-Bromo-N-phenyl-2-(triisopropylsilyl)acetamide (41). Crude 40 (150mg) was dissolved in THF (5 ml) and
stirred for 3 h together with 1n HCl (10ml). The org. phase was dried (MgSO 4) and concentrated invacuo . The
resulting brown oil was purified (CC; PhMe/hexane 0:1 ! 1:0) to afford 41 (150mg, 37%). White solid. Mp
75 788. IR (CH2Cl2): 1311m, 1441s, 1524s, 1599m, 1667s, 2808s, 2946s, 3397m. 1H-NMR (CDCl3, 300 MHz): 1.14
(d, J ˆ 7.5, 9 H); 1.19 (d, J ˆ 7.5, 9 H); 1.44 (sept., J ˆ 7.5, 3 H); 4.14 (s, 1 H); 7.13 (m, 1 H); 7.34 (m, 2 H); 7.50
(m, 2 H); 8.35 (br. s, 1 H). 13C-NMR (CDCl3, 75 MHz): 11.9; 18.6; 36.4; 119.9; 124.7; 129.1; 137.4; 167.1. EI-MS:
‡
‡
371 (0.56, M ), 369 (0.41, M ), 328 (4), 326 (4), 248 (15), 85 (100). Anal. calc. for C17H28BrNOSi (370.40): C
55.13, H 7.62, N 3.78, Br 21.57; found: C 55.07, H 7.79, N 3.82, Br 21.65.
{[(Triisopropylsilyl)ethynyl]imino}triphenylphosphorane (46). BuLi (1.16 ml of a 1.6m soln. in hexane,
1.87 mmol) was added dropwise to a suspension of 47 (335 mg, 935 mmol) [46] in THF (12 ml) at À 708 under
Ar. Stirring was continued for 15 min at this temp., then a soln. of 17c (500 mg, 935 mmol) in THF (5 ml) was
added dropwise. The mixture was stirred for 1 h at À 708 and then slowly warmed to 208 (3 h). An NMR analysis
of the crude mixture revealed the formation of the desired product, alongside starting material and other
products. A quick filtration over SiO2 (Et2O) allowed isolation of ca. 10mg (3%) of 46. Yellow oil. 31P-NMR
1
(CDCl3, 300 MHz): 7.50. H-NMR (CDCl3, 300 MHz): 1.10 (m, 21 H); 7.46 (m, 9 H); 7.84 (m, 6 H). 13C-NMR
(CDCl3, 75 MHz): 11.1; 18.5; 101.0 (d, J ˆ 150.0); 113.2 (d, J ˆ 19.0); 128.4 (d, J ˆ 36.0); 130.6 (d, J ˆ 11.0);
131.9; 133.0( d, J ˆ 120.0).

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Helvetica Chimica Acta Vol. 86 (2003)
Reactionbetween 48a and 49a inthe Presence of Me 3SiN(SO2CF3)2. A soln. of HN(SO2CF3)2 (300 mg,
1.07 mmol) and allyl(trimethyl)silane (186 ml, 134 mg, 1.17 mmol) in CH2Cl2 (4 ml) was added dropwise to 49a
(114 mg, 1.07 mmol) in CH2Cl2 (5 ml) at À 708. The blue soln. turned deep-red, then clear-orange. Aniline 48a
(301 ml, 337 mg, 1.07 mmol) was then added. The mixture was warmed to 208 over 3 h and poured into sat. aq.
NaHCO3 soln. The org. phase was dried (MgSO4) and concentrated invacuo to afford 0.3 g of a dark-green
solid, which was purified (CC; PhMe/hexane 0:1 ! 1:0) to give azoxybenzene (50a) (30mg, 28%). Yellow
solid. Mp 34 368 (commercial sample: 32 348). 4-Bromoaniline was recovered as well (50mg, 27%). Brown
solid. M.p. 60 62 8 (commercial sample: 60 62 8).
General Procedures for the Reaction between Bis(trimethylsilyl)amines 48a 48c and Nitroso Compounds
49a 49c.
Procedure A. A soln. of amine 48a 48c in THF was added dropwise to a cold (À 708) suspension of nitroso
derivative 49a 49c and a catalytic amount of Bu4NF on SiO2 in THF (same volume). The mixture was warmed
to 208, stirred for 12 h, and concentrated invacuo . The residue was purified (CC; PhMe/hexane 0:1 ! 1:0).
Procedure B. Same as Procedure A except that Bu4NF on SiO2 was replaced by 1 equiv. of MeOK, and the
starting temp. was 08.
Reactionbetween 48a and 49a. According to Procedure A, 48a (263 ml, 295 mg, 934 mmol), 49a (100 mg,
934 mmol), and Bu4NF on SiO2 (42 mg, 47 mmol) in THF (6 ml) gave 1-(4-bromophenyl)-2-phenyldiazene (51a;
15 mg, 6%), 1-(4-bromophenyl)-2-phenyldiazene 2-oxide (52a; 55 mg, 21%), 50a (95 mg, 58%), and 4-
bromoaniline (90mg, 56%).
Data of 51a: Orange solid. M.p. 86 888 ([47]: 88 908). 1H-NMR (CDCl3, 300 MHz): 7.51 (m, 3 H); 7.65
(m, 2 H); 7.80( m, 2 H); 7.92 (m, 2 H).
1
Data of 52a: Yellow solid. M.p. 72 738 ([48]: 73 748). H-NMR (CDCl3, 300 MHz): 7.55 (m, 5 H); 8.08
(m, 2 H); 8.29 (m, 2 H).
According to Procedure B, 49a (100 mg, 934 mmol), 48a (263 ml, 295 mg, 934 mmol), and MeOK (66 mg,
934 mmol) in THF (5 ml) provided 51a (50mg, 20%) and 50a (50mg, 58%).
Reactionbetween 48a and 49b. According to Procedure A, 49b (200 mg, 2.30 mmol), 48a (726 mg,
2.30mmol), and Bu 4NF on SiO2 (210mg, 230 mmol) in THF (6 ml) gave, after purification (CC; PhMe/hexane
2 :8, then AcOEt 100%), 1-(4-bromophenyl)-2-(tert-butyl)diazene (51b; 170mg, 61%) and 4-bromoaniline
(10mg, 30%).
Data of 51b: Yellow, volatile oil. IR (neat): 830s, 1009s, 1066s, 1362s, 1471s, 1524s, 1579s, 1587s, 1651w,
1905w, 2866m, 2929s, 2971s. 1H-NMR (CDCl3, 300 MHz): 1.32 (s, 9 H); 7.53 (m, 2 H); 7.58 (m, 2 H). 13C-NMR
‡
‡
(CDCl3, 75 MHz): 26.8; 67.9; 123.5; 124.1; 132.0; 151.0. EI-MS: 242 ( M ). HR-EI-MS: 240.0254 (M ,
C10H13N2Br ; calc. 240.0262).
‡
Reactionbetween 48a and 49c. According to Procedure B, 49c (158 mg, 886 mmol), 48a (250 ml, 280mg,
886 mmol), and MeOK (62 mg, 886 mmol) in THF (4 ml) yielded, after purification (CC; AcOEt/PhMe/hexane
gradient 0:0:1 ! 1:0:0), 4-nitroso-3-[(4-bromophenyl)amino]-N,N-diethyl-4-nitrosoaniline (53; 20mg, 6%),
1-(4-bromophenyl)-2-[4-(diethylamino)phenyl]diazene 2-oxide (52c; 50mg, 16%), 1,2-bis[4-(diethylamino)-
phenyl]diazene 1-oxide (50c; 100 mg, 33%) 49c (40mg, 25%), and 4-bromoaniline (40mg, 26%).
Data of 53: Dark green solid. M.p. 155 1578. IR (CH2Cl2): 1140s, 1263s, 1331m, 1492s, 1527m, 1551m,
1620s, 2978w. 1H-NMR (CDCl3, 300 MHz): 1.22 (t, J ˆ 7.2, 6 H); 3.41 (q, J ˆ 7.2, 4 H); 5.97 (d, J ˆ 2.5, 1 H); 6.46
(dd, J ˆ 9.6, 2.5, 1 H); 7.17 (d, J ˆ 6.9, 2 H); 7.49 (d, J ˆ 6.9, 2 H); 8.18 (d, J ˆ 9.6, 1 H). 13C-NMR (CDCl3,
75 MHz): 12.9; 45.3; 88.4; 106.5; 118.2; 125.9; 132.4; 137.3; 138.6; 142.2; 151.5; 154.1. HR-MALDI-MS: 348.0710
‡
‡
(MH , C16H19BrN3O ; calc. 348.0711).
Data of 52c: Orange solid. M.p. 147 1498. IR (CH2Cl2): 822m, 1183m, 1515s, 1598s, 2976m. 1H-NMR
(CDCl3, 300 MHz): 1.22 (t, J ˆ 7.2, 6 H); 3.44 (q, J ˆ 7.2, 4 H); 6.64 (d, J ˆ 9.6, 2 H); 7.55 (d, J ˆ 9.0, 2 H); 7.97
(d, J ˆ 9.0, 2 H); 8.15 (d, J ˆ 9.6, 2 H). 13C-NMR (CDCl3, 75 MHz): 12.4; 44.6; 110.1; 121.6; 124.0; 126.7; 131.6;
‡
‡
136.4; 143.4; 150.1. HR-MALDI-MS: 348.0741 (MH , C16H19BrN3O ; calc. 348.0711).
1
Data of 50c: Brown solid. M.p. 156 1588 ([49]: 156 1578). H-NMR (CDCl3, 300 MHz): 1.20 (t, J ˆ 7.2,
12 H); 3.41 (q, J ˆ 7.2, 8 H); 6.64 (d, J ˆ 9.6, 2 H); 6.68 (d, J ˆ 9.3, 2 H); 8.12 (d, J ˆ 9.3, 2 H); 8.27 (d, J ˆ 9.6,
2 H). 13C-NMR (CDCl3, 75 MHz): 12.6; 12.7; 44.5; 44.7; 109.9; 110.1; 110.5; 123.3; 128.1; 134.1; 149.1; 150.1.
Reactionbetween 48b and 49a. According to Procedure A, 48b (508 mg, 1.87 mmol), 49a (200 mg,
1.87 mmol), and Bu4NF on SiO2 (85 mg, 93 mmol) in THF (10ml) gave 1-(3-chlorophenyl)-2-phenyldiazene
(51d; 50mg, 12%), 1-(3-chlorophenyl)-2-phenyldiazene 2-oxide (52d; 70mg, 16%), 50a (30mg, 16%), and 3-
chloroaniline (30mg, 25%).

Helvetica Chimica Acta Vol. 86 (2003)
3115
Data of 51d: Bright orange solid. M.p. 68 708 ([50]: 68 698). 1H-NMR (CDCl3, 300 MHz): 7.47 (m, 2 H);
7.52 (m, 3 H); 7.83 (m, 1 H); 7.92 (m, 3 H); 13C-NMR (CDCl3, 75 MHz): 121.6; 122.2; 122.9; 129.0; 130.1; 130.5;
131.3; 135.0; 152.1; 153.2.
Data of 52d: Orange solid. M.p. 53 558 ([47]: 53 548). 1H-NMR (CDCl3, 300 MHz): 7.42 (m, 2 H); 7.56
(m, 3 H); 7.98 (ddd, J ˆ 7.5, 1.5, 1.2, 1 H); 8.25 (dd, J ˆ 1.5, 1.2, 1 H); 8.29 (m, 2 H). 13C-NMR (CDCl3, 75 MHz):
122.2; 123.7; 125.1; 128.7; 129.3; 129.5; 131.8; 134.1; 144.7; 147.9.
Reactionbetween 48c and 49a. According to Procedure B, 49a (123 mg, 1.15 mmol), 48c (280mg,
1.15 mmol), and MeOK (81 mg, 1.15 mmol) in THF (6 ml) gave 50a (80mg, 70%).
Reactionbetween 48c and 49c. According to Procedure B, 49c (205 mg, 1.15 mmol), 48c (280mg,
1.15 mmol), and MeOK (81 mg, 1.15 mmol) in THF (6 ml) gave 50c (120mg, 61%).
Hexacarbonyl{m2-[(1,2-h :1,2-h)-1-(tert-butyl)-2-nitrosoacetylene]}dicobalt (58). Isoamyl nitrite (740 ml,
645 mg, 5.51 mmol) and Me3SiCl (700 ml, 599 mg, 5.51 mmol) were heated for 30min at 40 8 under a gentle
stream of Ar, and the brown NOCl gas that evolved was collected in a flask cooled to À 788. A soln. of 54 [51]
(500 mg, 1.38 mmol) in CH2Cl2 (20ml) at À 708 was then transferred into the cold NOCl, and a green color
appeared immediately. After stirring for 15 min at this temp., a soln. of [Co2(CO)8] (521 ml, 942 mg, 2.76 mmol)
in CH2Cl2 (10ml) was added slowly, leading to immediate gas evolution. The mixture was stirred for 2 h while
warming from À 708 to 208, and was then filtered over Celite and concentrated invacuo to yield 400 mg of a dark
brown solid. Purification (CC; hexane) afforded 58 (270mg, 49%). Brown solid that slowly decomposes in soln.
1
M.p. 80 83 8(dec.). IR (CCl4): 1456w, 2020s, 2950w. H-NMR (CDCl3, 300 MHz): 1.3 (br. s, 9 H). Due to the
instability of the soln., an acceptable 13C-NMR spectrum could not be recorded.
Dr. Carlo Thilgen is gratefully acknowledged for his help with the nomenclature. We thank the ETH
Research Council and the Fonds der Chemischen Industrie, Germany, for their support of this work.
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Received May 7, 2003