The first direct observation of an allylic [3,3] sigmatropic cyanate-isocyanate rearrangement

Article abstract of DOI:10.1016/S0040-4039(01)01212-6

Evidence is presented that the [3,3] sigmatropic rearrangement of simple allyl cyanates to give allyl isocyanates proceeds much more rapidly than the analogous reaction of propargyl cyanates. Nevertheless, a substituted allyl cyanate is isolated for the first time, and the activation parameters of its [3,3] sigmatropic isomerization are measured.

Full text of DOI:10.1016/S0040-4039(01)01212-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 6133–6135
The first direct observation of an allylic [3,3] sigmatropic
cyanate–isocyanate rearrangement†,‡
Klaus Banert* and Antje Melzer
Chemnitz University of Technology, Institute of Chemistry, Strasse der Nationen 62, D-09111 Chemnitz, Germany
Received 24 June 2001; accepted 4 July 2001
Abstract—Evidence is presented that the [3,3] sigmatropic rearrangement of simple allyl cyanates to give allyl isocyanates proceeds
much more rapidly than the analogous reaction of propargyl cyanates. Nevertheless, a substituted allyl cyanate is isolated for the
first time, and the activation parameters of its [3,3] sigmatropic isomerization are measured. © 2001 Elsevier Science Ltd. All
rights reserved.
Allyl cyanates 1 are considered to be possible interme-
diates in several reactions leading to allyl isocyanates 6
via [3,3] sigmatropic isomerization (Scheme 1). How-
ever, direct proof of the presence of an allyl cyanate 1
is not yet available. Although the precursors 2,2 3,2 4,3
and 54 led to moderate to excellent yields of the iso-
cyanates 6 or their trapping products 7 (e.g. Nu=OR
or NR2), the postulated intermediates 1 could not be
observed directly. The sequence 5“1“6“7 introduced
by the group of Ichikawa4a–g involves the stereospecific
transfer of chirality and proves to be a useful method to
prepare optically active allyl amine derivatives 7 if
starting materials 5 are derived from chiral allylic
alcohols.
We treated the allylic alcohols 13 with sodium hydride
followed by chlorothiatriazole7 to produce the com-
pounds 148 in a convenient one-step procedure (Scheme
3).9 In solution, 14 decomposed even at room tempera
Recently, we succeeded in the first direct observation of
a [3,3] sigmatropic cyanate“isocyanate rearrangement
by NMR spectroscopic monitoring of the conversion
9“10“11 (Scheme 2).5 The maximum proportion of
the short-lived, quasi-stationary intermediate 10 in the
reaction mixture was only 5%, however, in the presence
of H2S 10 could be effectively trapped to yield 12. The
transformation 8“9“10“11 is the first systematic
route to prepare allenyl isocyanates.6 In this paper, we
present our attempts to observe and to isolate an allylic
cyanate of type 1 for the first time and to investigate its
[3,3] sigmatropic rearrangement to isocyanate 6.
Keywords: dienes; cyanates; rearrangements; allenes; isocyanates.
* Corresponding author. Fax: 49 (0) 371/531-1839; e-mail: klaus.
banert@chemie.tu-chemnitz.de
†
See Ref. 1.
‡
This paper is dedicated to Professor Henri Patin.
Scheme 1. Reactions postulated to run via allyl cyanate 1.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01212-6

6134
K. Banert, A. Melzer / Tetrahedron Letters 42 (2001) 6133–6135
To synthesize an allyl cyanate with substituents, which
retard the rapid [3,3] sigmatropic rearrangement, we
prepared the known11 ester 25 by treating the dicarban-
ion of ethyl acetoacetate with benzophenone, followed
by dehydration of the resulting tertiary alcohol with
concentrated sulfuric acid (Scheme 5). The yield of the
first step (79%) was significantly increased by use of
LDA (lithium diisopropylamide) in THF instead of
KNH2/NH3,11a while the second step led to unknown
2612 (27% after separation by flash chromatography) in
addition to 25 (55%). The reaction of 25 with ClCN/
NEt3 afforded the allyl cyanate 2713 (E/Z:1:10). The
structure of the main isomer of 27, which could be
isolated as a stable crystalline compound, was proved
by the spectroscopic data. This included 1H NMR
nuclear Overhauser enhancement (NOE) difference
spectra which assigned the Z configuration. On heating
in solution, both E-27 and Z-27 were irreversibly trans-
formed into isocyanate 28.14 Obviously, the rate of this
[3,3] sigmatropic rearrangement is strongly decreased
by the fact that conjugation of the CꢀC bonds with the
cyanato and the phenyl groups is present only in the
case of 27, whereas 28 possesses cumulated CꢀC bonds
and isolated isocyanato and phenyl groups. When the
Scheme 2. Generation, trapping, and rearrangement of
propargyl cyanate 10.
1
conversion Z-27“28 was followed by H NMR spec-
troscopy in the range of 70–120°C, the activation
parameters of this first-order reaction could be mea-
sured: log A=6.62 0.59, Ea=84.5 4.1 kJ mol−1,
DH298"=81.0 4.1 kJ mol−1, DS298"=−126.5 11
J
mol−1 K−1. The large negative activation entropy is
characteristic of the cyclic transition state of the con-
Scheme 3. Attempts to prove allyl cyanate 15.
ture almost quantitatively to the rearranged isocyanates
1
16. When the conversion 14“16 was followed by H
NMR spectroscopy, the intermediate 15 could not be
detected. In the case of 14a and 14b, which were
isolated as a very pure liquid and solid, respectively, the
1H NMR signals of 15a,b must be definitely smaller
than those of the 13C satellites of 14a,b. Thus, the
maximum proportion of the intermediates 15a,b should
be clearly less than 0.5%. The trapping experiment
failed since decomposition of 14a,b in the presence of
H2S produced 16a,b instead of 17a,b. All attempts to
generate higher proportions of the intermediates 15a or
15b by photolysis10 of 14a or 14b at low temperature
were without success.
We interpret these results by the assumption that the
allyl cyanates normally rearrange much more rapidly to
isocyanates than do propargyl cyanates. To test this
assumption, we transformed the alcohol 18 into thiatri-
azol 19 which should give rise to the cyanate 21
(Scheme 4). This short-lived intermediate could isomer-
ize by allylic or propargylic migration of the cyanato
group, however, only allyl isocyanate 22 (78%, E/Z=
4.5:1) and the heterocyclic product 20 (22%, E/Z=3:2)
were observed after decomposition of 19. Thus, the
rearrangement 21“23 is not able to compete with the
very rapid allylic shift 21“22. Because thiatriazolone
20 is stable at room temperature, it cannot be a precur-
sor for 22, as it was analogously discussed in the case of
other 5-allyloxy-1,2,3,4-thiatriazoles.2
Scheme 4. Generation and rearrangement of cyanate 21.

K. Banert, A. Melzer / Tetrahedron Letters 42 (2001) 6133–6135
6135
Banert, K. Methoden Org. Chem. (Houben-Weyl), 4th ed.,
1993; Vol. E15, pp. 3103–3105.
6. Review: Banert, K. Liebigs Ann./Recueil 1997, 2005–2018.
7. Lieber, E.; Lawyer, C. B.; Trivedi, J. P. J. Org. Chem.
1961, 26, 1644–1646.
8. For alternative syntheses of 14a and 14b, see Ref. 2.
9. For experimental procedures of this one-step synthesis
and the decomposition of thiatriazoles, see Ref. 5. Cau-
tion should be exercised during isolation of thiatriazoles
which may be explosive.
10. Compare to the photolysis of 5-phenylthiatriazole: Kir-
mse, W. Chem. Ber. 1960, 93, 2353–2356.
11. (a) Wolfe, J. F.; Harris, T. M.; Hauser, C. R. J. Org.
Chem. 1964, 29, 3249–3252; (b) Taylor, E. C.; Davies, H.
M. L. Tetrahedron Lett. 1983, 24, 5453–5456.
12. Compound 26: Yellow crystals, mp 96.5–97.5°C. 1H
NMR (CDCl3): l 1.41 (t, 3J=7 Hz, 3H, Me), 4.33 (q,
3J=7 Hz, 2H, CH2), 6.54 (s, 1H, CH-CO2), 7.21–7.75 (m,
10H). 13C NMR (CDCl3): l 14.36 (q, Me), 60.58 (t,
CH2), 113.59 (d), 120.54 (d), 120.99 (d), 123.67 (d),
126.40 (d), 127.53 (d), 128.60 (d), 128.88 (d), 129.48 (d),
134.60 (s), 137.75 (s), 142.00 (s), 151.19 (s), 151.28 (s),
166.31 (s, CꢀO). GC MS (70 eV) m/z (%): 276 (M+, 99),
247 (29), 231 (57), 202 (100), 191 (33), 101 (34). Anal.
calcd for C19H16O2: C, 82.58; H, 5.84. Found: C, 82.13;
H, 5.90.
13. Experimental procedure for 27: Cyanogen chloride (210
ml, 249 mg, 4.05 mmol) was added dropwise to a solution
of 25 (1.0 g, 3.4 mmol) in dry Et2O (30 ml), which was
stirred at −10°C. After dropwise addition of freshly dis-
tilled NEt3 (0.35 g, 3.5 mmol), the reaction mixture was
stirred for 3 h at 0°C. Then, the precipitate of ammonium
salt was separated and repeatedly extracted with Et2O.
The combined ether solutions were concentrated in vacuo
to give crude 27 with E/Z:1:10. After separation by
flash chromatography (SiO2, Et2O/hexane, 1:1), 0.67 g
(62%) Z-27 were isolated besides small amounts of E-27.
Compound Z-27: Yellowish crystals, mp 66–67°C. IR
Scheme 5. Synthesis and rearrangement of allyl cyanate 27.
certed reaction 27“28. If this transformation was car-
ried out at 80°C in chloroform instead of cyclohexane,
the rate was accelerated by a factor of 40.
At present, we are investigating whether the rearrange-
ment 27“28 can be generalized as a synthetic method
to produce functionalized allenes of type 28.
Acknowledgements
We thank Dynamit Nobel GmbH, Leverkusen, for
providing chemicals. Our work was also supported by
the Deutsche Forschungsgemeinschaft and the Fonds
der Chemischen Industrie.
(CCl4): 2273 (OCN), 1729 (CꢀO) cm−1
.
1H NMR
3
3
(CDCl3): l 1.25 (t, J=7 Hz, 3H, Me), 4.17 (q, J=7 Hz,
2H, CH2), 5.35 (s, 1H, H-2), 6.46 (s, 1H, H-4), 7.22–7.45
(m, 10H, Ph). Saturating the proton H-2 led to 5%
enhancement of the signal of H-4, which was measured
with the help of NOE difference spectra. 13C NMR
References
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(CDCl3): l 14.05 (q, J=124 Hz, Me), 60.98 (t, J=148
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Ph), 128.75 (d, Ph), 129.10 (d, p-Ph), 129.19 (d, Ph),
129.89 (d, p-Ph), 137.72 (i-Ph), 140.20 (i-Ph), 154.83 (s),
156.36 (s), 162.28 (s, C-1). Assignments were based on
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H, 5.46; N, 4.22.
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(CCl4): 2250 (NCO), 1970 (CꢀCꢀC), 1724 (CO2Et) cm−1
.
3
1H NMR (CDCl3): l 1.34 (t, J=7.1 Hz, 3H, Me), 4.25
4
4
(m, 2H, CH2), 5.82 (d, J=6 Hz, 1H), 6.30 (d, J=6 Hz,
1H), 7.31–7.52 (m, 10H, Ph). 13C NMR (CDCl3): l 14.23
(q, Me), 61.17 (t, CH2), 68.23 (s, C-5), 93.37 (d), 104.27
(d), 125.50 (s, NCO), 126.46 (d, Ph), 126.66 (d, Ph),
127.98 (d, p-Ph), 128.04 (d, p-Ph), 128.30 (d, Ph), 128.37
(d, Ph), 142.59 (s, i-Ph), 143.20 (s, i-Ph), 164.51 (s, C-1),
210.31 (s, C-3).