Synthesis and reactions of the first cyclopentadienyl isonitriles

Article abstract of DOI:10.1016/S0040-4039(03)00784-6

6-Azidofulvenes without a further substituent in the exocyclic position were transformed into new 1-isocyano- and 2-isocyanocyclopenta-1,3-dienes by photolysis in methanol. These novel functionalized cyclopentadienes are useful building blocks, e.g. as dienes in Diels-Alder reactions.

Full text of DOI:10.1016/S0040-4039(03)00784-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 3781–3783
Synthesis and reactions of the first cyclopentadienyl
isonitrilesꢀ,ꢀꢀ
Klaus Banert,* Frank Ko¨hler and Barbara Meier
Chemnitz University of Technology, Institute of Chemistry, Strasse der Nationen 62, D-09111 Chemnitz, Germany
Received 25 February 2003; accepted 18 March 2003
Dedicated to Professor Klaus Hafner on the occasion of his 75th birthday
Abstract—6-Azidofulvenes without a further substituent in the exocyclic position were transformed into new 1-isocyano- and
2-isocyanocyclopenta-1,3-dienes by photolysis in methanol. These novel functionalized cyclopentadienes are useful building
blocks, e.g. as dienes in Diels–Alder reactions. © 2003 Elsevier Science Ltd. All rights reserved.
Cyclopentadiene and its derivatives prove to be very
important building blocks in organic synthesis mainly
because of their high reactivity in Diels–Alder reactions
and because of the easiness of their deprotonation to
form cyclopentadienylides. These compounds can be
used in nucleophilic substitution or addition reactions
as well as in the generation of metallocenes and other
metal complexes. However, access to cyclopentadienes
bearing a functional group directly bound to the five-
membered ring is rather rare.1 For example, iso-
cyanocyclopentadienes are completely unknown to the
best of our knowledge. We report here on the first
synthesis of cyclopentadienyl isonitriles starting from
6-azidofulvenes 1a,b (Scheme 1).
termed azaspiroconjugation.2 Whereas irradiation of
1a3,4 in several aprotic solvents at −40 to −60°C gave
2a quantitatively without any successive reaction,2 we
have observed now that photolysis of 1a in methanol
at −50°C surprisingly led to
a mixture of the
isonitriles5 3a (68% yield) and 4a (29%). When the
latter transformation was monitored by 1H NMR
spectroscopy, the signals of the heterocycle 2a could
not be detected. Heating diluted solutions of 1a or 2a
in chloroform at 50°C furnished the same isonitriles in
44% or 87% yield, respectively. Flash vacuum
pyrolysis6 at 270°C was also a method to transform 1a
or 2a into 3a/4a. But the yield of the isonitriles was
limited
to
22–28%
because
the
secondary
rearrangement7 to give the known nitrile 5a8 could not
be suppressed. Flash vacuum pyrolysis of 1a, 2a, or
3a/4a at higher temperatures (420–500°C) led only to
5a (52–79% yield).
Recently, we thermolyzed or photolyzed solutions of
1a–c to get excellent yields of the spirocyclic azirines
2a–c, which showed a new type of homoconjugation
Scheme 1.
Keywords: azides; fulvenes; cyclopentadienes; isocyanides; Diels–Alder reactions.
ꢀ Part 15 of the series ‘Reactions of Unsaturated Azides’. For Part 14, see Ref. 2.
ꢀꢀ Supplementary data associated with this article can be found at doi:10.1016/S0040-4039(03)00784-6
* Corresponding author. Fax: 49 (0) 371/531-1839; e-mail: klaus.banert@chemie.tu-chemnitz.de
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00784-6

3782
K. Banert et al. / Tetrahedron Letters 44 (2003) 3781–3783
After photolysis of 1b in methanol at −50°C and flash
chromatography (SiO2, Et2O/hexane, 1:10) we obtained
a 1:1 mixture of 3b and 4b as a yellow oil (49% yield)9
and 5b as yellow crystals (22%, mp 103°C). Prolonged
irradiation of 1b in chloroform produced also 3b, 4b,
and 5b, but the yields were lower. On the other hand,
thermolysis of 1b in chloroform (66°C, 4 h) or flash
vacuum pyrolysis of 3b/4b at 420°C resulted only in 5b
formed with 44% or 61% yield, respectively. In all cases
of photolyses and thermolyses of 1b, the intermediate
2b could be observed when the transformations were
this equilibration, retro-Diels–Alder reaction of 11 per-
formed by sublimation at 150°C/10−3 torr gave only
TCNE and 5a instead of 2-cyanocyclopentadiene.
Because of steric hindrance, we observed no reaction of
3b or 4b with TCNE. However, the Diels–Alder prod-
ucts 12 and 13 were obtained in 73% yield on treatment
with the more reactive dienophile 4-phenyl-1,2,4-
triazolinedione.
At present, we are continuing the investigations of
other transformations starting with isocyanocyclo-
pentadienes.
1
monitored by H NMR spectroscopy.
The rearrangement of 2a,b to give 3a,b and 4a,b can be
explained by ring opening to nitrile ylides 6a,b followed
by tautomerism. Thus, photolysis or thermolysis of 2c
did not afford any isonitrile. Thermal ring cleavage of
2H-azirines to generate nitrile ylides is rare, however,
the negative charge is highly stabilized by the aromatic
cyclopentadienylide in the case of 6a,b.
Acknowledgements
We thank Professor Klaus Hafner, Technische Univer-
sita¨t Darmstadt, for helpful discussions. This research
was supported by the Fonds der Chemischen Industrie.
We wish to thank Dynamit Nobel GmbH, Leverkusen,
for providing chemicals.
We assume a rapid equilibration of 3a and 4a, and thus
we were not able to separate both compounds.
Although these isonitriles can only be handled in solu-
tion, treatment with an excess of tetracyanoethylene
(TCNE) in acetone at −25 to 20°C yielded quantita-
tively the Diels–Alder products 7 and 8 (Scheme 2). The
analogous reaction of 2a led to the relative stable
cycloadduct 9.10 Treatment of 5a with TCNE at −25°C
gave quantitatively a 7:1 mixture of 10 and 11, whereas
the same reaction at 20°C resulted in a 1:1 mixture of
10 and 11. On heating these mixtures to 50°C, 11 and
regenerated 5a were the only products observed. Thus,
the Diels–Alder reaction of 5a and TCNE is reversible
already at low temperature favoring the thermodynami-
cally more stable product 11. The latter is formed from
TCNE and 2-cyanocyclopentadiene generated in a
small concentration by equilibration with 5a. Due to
References
1. For a review, see: Ivchenko, N. B.; Ivchenko, P. V.;
Nifant’ev, I. E. Russ. J. Org. Chem. (Engl. Transl.) 2000,
36, 609–637.
2. Banert, K.; Ko¨hler, F.; Kowski, K.; Meier, B.; Mu¨ller,
B.; Rademacher, P. Chem. Eur. J. 2002, 8, 5089–5093.
3. For the synthesis of 1a, see: Hafner, K.; Bauer, W.;
Schultz, G. Angew. Chem. 1968, 88, 800–801; Angew.
Chem., Int. Ed. Engl. 1968, 7, 806–807.
4. Caution should be exercised during isolation of azides
which may be explosive.
5. Mixture 3a/4a: IR (CCl4): w˜ 2117 cm−1 (NC). GC–MS
m/z (%): 91 (100) [M+], 64 (91), 63 (40), 39 (45), 38 (33).
1
Compound 3a: H NMR (CDCl3): l 3.04 (q, J=1.5 Hz,
3
4
3
2H, H-5), 6.10 (ddt, J=5.7 Hz, J=1.5 Hz, J=1.5 Hz,
3
3
4
1H, H-4), 6.23 (ddt, J=5.7 Hz, J=2.3 Hz, J=1.5 Hz,
1H, H-3), 6.46 (m, 1H, H-2). 13C NMR (CDCl3): l 42.78
1
1
(t, J(C,H)=130.0 Hz), 116.80 (s), 130.99 (d, J(C,H)=169.7
Hz), 131.62 (2C, 2×d, 1J(C,H)=174.1 Hz), 166.82 (t,
1J(C, N)=5.9 Hz, NC). Compound 4a: 1H NMR
14
(CDCl3): l 2.93 (q, J=1.5 Hz, 2H, H-5), 6.23 (m, 1H,
3
4
3
H-3), 6.32 (ddt, J=5.4 Hz, J=2.4 Hz, J=1.5 Hz, 1H,
H-4), 6.36 (m, 1H, H-1). 13C NMR (CDCl3): l 40.09 (t,
1J(C,H)=128.5 Hz), 123.35 (s), 129.68 (d, 1J(C,H)=176.4
Hz), 130.14 (d, 1J(C,H)=176.6 Hz), 135.20 (d, J(C,H)
=
1
1
172.9 Hz), 164.74 (t, J(C, N)=6.6 Hz, NC). Assignments
of the H NMR signals of 3a and 4a are based on double
14
1
resonance, NOE, and LIS experiments.
6. For experimental details of flash vacuum pyrolyses, see:
Banert, K.; Hagedorn, M.; Mu¨ller, A. Eur. J. Org. Chem.
2001, 1089–1103.
7. For the thermal rearrangement of isonitriles to nitriles,
see: Ru¨chardt, C.; Schmittel, M.; Wolber, E. K. A. Chem.
Ber. 1992, 125, 525–531.
8. Chou, C.-H.; Wu, C.-C.; Chen, W.-K. Tetrahedron Lett.
1995, 36, 5065–5068 and references cited therein.
9. Mixture of 3b/4b: IR (CCl4): w˜ 2113 cm−1 (NC). GC–MS
m/z (%): 203 (9) [M+], 188 (12), 147 (11), 133 (9), 132
Scheme 2.

K. Banert et al. / Tetrahedron Letters 44 (2003) 3781–3783
3783
(77), 117 (10), 77 (12), 57 (100), 41 (48), 39 (19). Com-
4J=1.1 Hz, 1H, H-3). 13C NMR (CDCl3): l 29.47 (q),
1
pound 3b: H NMR (CDCl3): l 1.134 (s, 9H), 1.279 (s,
30.37 (q), 33.11 (s), 33.48 (s), 39.45 (t, C-5), 120.26 (t,
1J(C, N)=12 Hz, C-2), 122.74 (d, C-3), 150.02 (s), 156.23
9H), 3.16 (d, 4J=1.1 Hz, 2H, H-5), 6.02 (t, 4J=1.1 Hz, 1H,
14
(s), 166.32 (t, 1J(C, N)=5 Hz, NC). Assignments of the 1H
H-3). 13C NMR (CDCl3): l 29.15 (q), 30.42 (q), 33.07 (s),
14
1
NMR signals of 3b and 4b are based on NOE difference
spectra.
33.42 (s), 42.98 (t, C-5), 116.97 (t, J(C, N)=14 Hz, C-1),
14
123.66 (d, C-3), 151.87 (s), 156.62 (s), 167.83 (t, 1J(C, N)=5
14
1
10. Surprisingly, we were not able to get 9 by photolysis of the
Diels–Alder product easily available from 1a and TCNE.
Hz, NC). Compound 4b: H NMR (CDCl3): l 1.131 (s,
4
9H), 1.275 (s, 9H), 2.98 (d, J=1.1 Hz, 2H, H-5), 5.89 (t,