The curtius rearrangement of acyl azides revisited - Formation of cyanate (R-O-CN)

Article abstract of DOI:10.1002/ejoc.200500545

The Curtius rearrangement is a synthesis of isocyanates (R-N=C=O) by thermal or photochemical rearrangement of acyl acides and/or acylnitrenes. The photochemical rearrangement of benzoyl azide is now shown for the first time to produce a small amount of phenyl cyanate (Ph-O-CN) together with phenyl isocyanate. Wiley-VCH Verlag GmbH & Co. KGaA, 2005.

Full text of DOI:10.1002/ejoc.200500545

SHORT COMMUNICATION
The Curtius Rearrangement of Acyl Azides Revisited – Formation of Cyanate
(R–O–CN)
Curt Wentrup*[a] and Holger Bornemann[a]
Keywords: Azides / Reactive intermediates / Nitrenes / Isocyanates / Cyanates
The Curtius rearrangement is a synthesis of isocyanates (R–
N=C=O) by thermal or photochemical rearrangement of acyl
acides and/or acylnitrenes. The photochemical rearrange-
ment of benzoyl azide is now shown for the first time to pro-
duce a small amount of phenyl cyanate (Ph–O–CN) together
with phenyl isocyanate.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
Introduction
The Curtius rearrangement[1] of acyl azides R–CO–N3
(1) to isocyanates R–N=C=O (3) can take place either ther-
mally or photochemically.[2] There is general consensus that
the thermal Curtius rearrangement is a concerted reaction
of the azides, not involving acylnitrenes (2). It is usually
believed that the photochemical rearrangement can take
two distinct paths: (i) Concerted Curtius rearrangement to
isocyanate, and (ii) nitrene formation, which may or may
not lead to isocyanate.[2] Experiments[3] and calculations
indicate that acylnitrenes have singlet ground states. Calcu-
lations on the lowest singlet states of formylnitrene (2a),[3,4]
cyanocarbonylnitrene NC–CO–N (2b),[5] benzoylnitrene
(2c),[6] 2-naphthoylnitrene[6] and acetylnitrene[7] (2d) have
revealed structures that are intermediate between nitrenes
and oxazirenes with very long N–O bonds of the order of
1.73–1.82 Å at G2(MP2,SVP), BLYP/6-31G* and
CCSD(T)/cc-pVTZ levels of theory.
Moreover, the calculated NCO angle is nearly 90°. The
stabilization of the singlet state of the nitrene can be as-
cribed to the N–O bonding interaction.[4–7] Thus, the singlet
acylnitrenes may be regarded as resonance hybrids of ni-
trene and oxazirene structures (Scheme 1). Recent work by
Pritchina et al. revealed that Ar matrix photolysis of ben-
zoyl azide PhCON3 leads to phenyl isocyanate PhNCO by
two paths: (i) Concerted, and (ii) via benzoylnitrene Ph–
CO–N, which was observed by IR spectroscopy.[8]
Scheme 1.
Results and Discussion
Photolysis of benzoyl azide isolated in Ar matrix at 12 K
at 308 nm afforded the IR spectrum shown in Figure 1 (full
IR spectra are shown in the Supporting Information; for
details see also the footnote on the first page of this article).
It is readily seen that small amounts of phenyl cyanate (4c)
are formed together with the main product, phenyl isocya-
nate, PhNCO (3c). Phenyl cyanate (4c) is identified by its
prominent C–O–C stretch at 1193 cm–1; all the other bands
of PhOCN were also identified.
In contrast, flash vacuum thermolysis (FVT) of the azide
afforded only the isocyanate PhNCO (3c) (FVT is not nec-
essary in order to produce the isocyanate; this takes place
already on mild heating of the azide). In order to examine
the photochemical rearrangement of benzoylnitrene (2c)
itself, the matrix-isolated azide was irradiated at 254 nm un-
til all the azide had been consumed (5–20 min depending
on the concentration and amount of azide). This affords a
mixture of benzoylnitrene (2c),[8] phenyl isocyanate (3c) and
phenyl cyanate (4c). Further photolysis of this mixture at
308 nm for 5 min caused transformation of the benzoylni-
We have a long-standing interest in alkyl and aryl cya-
nates,[9] and we wondered why apparently nobody had ever
considered the formation of cyanates in the Curtius re-
arrangement. Therefore, we have reexamined the photo-
chemical Curtius rearrangement of benzoyl azide (1c).
[a] Chemistry Building, School of Molecular and Microbial Sci-
ences, The University of Queensland,
Brisbane Qld 4072, Australia
E-mail: wentrup@uq.edu.au
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
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DOI: 10.1002/ejoc.200500545
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C. Wentrup, H. Bornemann
SHORT COMMUNICATION
Figure 1. Ar matrix spectra at 12 K. Top: authentic sample of phenyl cyanate PhOCN; middle: phenyl isocyanate PhNCO from FVT of
PhCON3 at 450 °C; bottom: photolysis of PhCON3 at 254 nm (15 min) and at 308 nm (5 min).
trene to products, producing a spectrum similar to Figure 1, to PhN+· at m/z 91, whereas for PhOCN the strongest peak
whereby the amount of isocyanate had increased strongly, is due to Ph+ at m/z 77. The spectra are shown in the sup-
and the amount of cyanate had increased slightly. Thus, the porting information.
nitrene 2c undergoes photo-rearrangement to isocyanate
and cyanate; most likely these two products are also formed 1193 cm–1 in the matrix IR spectrum of Pritchina, Bally, et
in a direct photo-reaction of the azide.
al.[3] This was ascribed to an artefact due to the spectral
It is interesting to note that a peak is also present at
In a control experiment phenyl isocyanate (3c) was irra- subtraction but is undoubtedly due to phenyl cyanate (4c),
diated in an Ar matrix at 254 nm for 90 min, followed by formed by photolysis of the nitrene/oxazirene 2c.
308 nm for 15 min. No isomerization to phenyl cyanate (4c)
Theoretical calculations were carried out in an effort to
was observed at either wavelength. In order to prove beyond determine the differences in activation barriers for cyanate
doubt that phenyl cyanate (3c) is really formed, the product and isocyanate formation. Previous efforts to describe the
of the photolysis was rinsed from the cryostat with dichlo- HCNO energy surface[4,11] demonstrate that this is not a
romethane after warming to room temperature, and the re- trivial task. Fueno et al.[4a] reported a very small barrier for
sulting solution was examined by GC-MS. Direct compari- the isomerization of 2a to HNCO (TS1a) and a 16.5 kcal/
son with authentic samples of phenyl cyanate and phenyl mol higher transition state for the isomerization of singlet
isocyanate proved the formation of the cyanate and isocya- oxazirene (2a) (1AЈ) to HOCN (TS2a) using CASSCF(6,10)
nate in relative yields of 1:85 (uncorrected relative peak and MRD-CI/6-31G(d,p) calculations. HOCN was found
areas). This experiment was repeated several times with the to be 21 kcal/mol less stable than HNCO at this computa-
same results. The mass spectra of phenyl cyanate and tional level.
phenyl isocyanate are very characteristic and easily distin-
Morokuma et al.[4b] failed to locate TS1a on the singlet
guished.[10] For example, PhNCO gives a strong peak due surface at the B3LYP/6-311G(d,p) level, but they located
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Eur. J. Org. Chem. 2005, 4521–4524

Curtius Rearrangement of Acyl Azides Revisited
SHORT COMMUNICATION
TS2a lying only 0.7 kcal/mol above the oxazirene structure to cyanate HOCN and PhOCN + N2 with barriers of less
2a. Note however that the structure of their TS2a corre- than 1 kcal/mol. We do not believe these carbene intermedi-
sponds very closely to our TS3a (HOCN Ǟ HNCO), which ates are realistic; concerted rearrangement on the excited
lies 5.5 kcal/mol higher than 2a at the B3LYP/6-31G(d,p) state energy surfaces of the azides is more likely.
level.
In conclusion, phenyl cyanate (4c) is formed in small
HOCN was found to be 28.7 kcal/mol less stable than amount along with the main product, phenyl isocyanate
HNCO at the B3LYP/6-311G(d,p) level. The triplet nitrene (3c) on photolysis of benzoyl azide (1c). Both cyanate and
2a (3AЈЈ) did rearrange to HNCO and HOCN.
isocyanate are formed on photolysis of the (singlet) nitrene/
Shapley and Backskay[4c] located TS1a (1AЈ oxazirene Ǟ oxazirine 2c at 308 nm. Both cyanate and isocyanate are
HNCO) lying 30 kcal/mol above oxazirene (2a) when using also formed on photolysis of the azide 1c at 254 nm; this
the G2 computational method, but a direct path from 2a to may be a direct reaction of an excited state of the azide, or
HOCN (TS2a) was not reported. HOCN was 25.4 kcal/mol it may be a rapid reaction via the nitrene. The thermal Curt-
less stable than HNCO, and the direct isomerization ius rearrangement produces only the isocyanate 3c. Further
HNCO Ǟ HOCN had a barrier (TS3a) of 88 kcal/mol experimental and theoretical work on the Curtius re-
(HOCN Ǟ HNCO has a barrier of 62.6 kcal/mol).
arrangement is envisaged, e. g. an examination of substitu-
We found TS1a (oxazirene Ǟ HNCO, 15.8 kcal/mol) by ent effects on the barrier heights for cyanate and isocyanate
using B3LYP/6-31G(d) for the parent compound 2a, but formation. No direct evidence for the formation of nitrile
TS2a (oxazirene Ǟ HOCN) was not located on the singlet oxides, RCNO, in the Curtius rearrangement has been
surface. At the HF/6-31G(d.p) level, TS1a and TS2a were found so far, although such a rearrangement cannot be ex-
located. At the CASSCF(8,8) level, TS1a was securely lo- cluded.[12]
cated with both Gaussian03 and Molpro; it is about
20 kcal/mol above 2a. When using CASSCF(12,12) this en-
Experimental Section
ergy difference increases to 25.5 kcal/mol, which is in better
agreement with the literature.[4c] As for TS2a, it appeared
to be ca. 6 kcal/mol above TS1a, but it is most likely that
all structures obtained for TS2a at CASSCF levels are arti-
facts, since in neither case could they be verified by IRC
calculations.
Benzoyl azide was prepared following the procedure of Barrett and
Porter,[13] and phenyl cyanate by the procedure of Murray and
Zweifel.[14]
Matrix Isolation Procedure: A gaseous mixture of argon and ben-
zoyl azide (ratio Ϸ 1000:1) is deposited slowly onto a CsBr window
at 20 K. The matrix is irradiates with a 254-nm low-pressure mer-
cury lamp (75 W; Graentzel, Karlsruhe, Germany) for 5 to 20 min
(depending on the amount of azide), which leads to the complete
decomposition of the azide and the formation of phenyl isocyanate,
phenyl cyanate and singlet benzoylnitrene. Prolonged photolysis for
2–5 min with 308 nm excimer lamp (50 mW/cm2; Ushio, Singapore)
results in larger amounts of phenyl isocyanate, while the amount
of phenyl cyanate increases slightly. Relevant IR spectra are shown
in the supporting information. GC/MS analysis: For GC/MS
analysis of the products of the matrix photolysis, the cold window
and the inner surfaces of the cryostat cold head are rinsed with
dichloromethane after warming to room temperature and exam-
ined on a 30 m capillary column DB-5ms, diameter 0.32 mm, sta-
tionary phase thickness 0.25 µm; helium flow rate 1.3 mL/min;
temperature program: 50 °C for 3 min, then 15 °C/min till 260 °C,
and maintain this temperature for 8 min; injector: 200 °C; detector:
250 °C. GC retention times and mass spectra:
Similar calculations were carried out for the phenyl de-
rivatives 2c, 3c and 4c and the corresponding transition
states TS1c, TS2c and TS3c. At the B3LYP/6-31G(d) level
we found TS1c (phenyloxazirene Ǟ PhNCO, 14.7 kcal/
mol), and TS2c (phenyloxazirene Ǟ PhOCN, 19.3 kcal/
mol). At the HF/6-31G(d.p) level, TS2c was at 20.5 kcal/
mol, but all efforts to locate TS1c failed at this level. At the
CASSCF(8,8) level, TS2c was 17.6 kcal/mol above TS1c.
Thus, in spite of some quantitative differences and failure
to locate some transition states at all attempted levels of
theory, all the computations agree that the barrier for cya-
nate formation is significantly higher than the one for isocy-
anate formation. This explains the absence of cyanate for-
mation in the thermal Curtius rearrangement. It should be
pointed out that all the calculations pertain to the ground
state energy surface, whereas the photochemical rearrange-
ments are likely to take place on excited state surfaces.
TS3c, the transition state for isocyanate-cyanate re-
arrangement, was computed to lie close to TS2c in energy,
at 20.1 kcal/mol above the ozazirene structure 2c (94 kcal/
mol above the isocyanate PhNCO) at the B3LYP/6-31G(d)
level. The cyanate was 31 kcal/mol less stable than the iso-
cyanate at this level. Experimentally, it is known that alkyl
cyanates rearrange thermally to alkyl isocyanates, but aryl
cyanates do not rearrange to aryl isocyanates.[9]
Authentic samples: Phenyl cyanate: Rt = 7.28 min, MS [m/z (rel.
int. %)]: 120 (4), 119 (49), 92 (2), 91 (17), 78 (7), 77 (100), 75 (3),
74 (4), 65 (6), 64 (5), 63 (4), 51 (27), 50 (12). Phenyl isocyanate: Rt
= 6.18 min, MS [m/z (rel. int. %)]: 120 (9), 119 (100), 92 (4), 91
(43), 77 (1), 74 (3), 65 (6), 64 (28), 63 (12), 62 (4), 61 (2), 60(4),
52(3), 51 (6), 50 (6).
Samples collected after matrix isolation: Phenyl cyanate: Rt
=
7.28 min, MS [m/z (rel. int. %)]: 120 (9), 119 (56), 91 (14), 78 (13),
77 (100), 65 (11), 63 (8), 57 (47), 51 (53), 50 (24). Phenyl isocyanate:
Rt = 6.18 min, MS [m/z (rel. int. %)]: 120 (8), 119 (100), 92 (4), 91
(50), 90 (3), 77 (2), 74 (3), 65 (7), 64 (34), 63 (13), 62 (4), 59 (5), 51
(7), 50 (6). Integration of GC peak areas: First run (rel. integrated
area): PhOCN 0.48, PhNCO 41.19, ratio 1:85. Second run: PhOCN
0.20, PhNCO 17.13, ratio 1:86. Third run: PhOCN 0.25, PhNCO
During a search for a concerted pathway from the azide
to the cyanate, 1 Ǟ 4, we found instead a carbene interme-
diate, RO–C–N3, formed from the azide HCON3 (1a) with
a barrier of 81.4 kcal/mol, and from the azide PhCON3 (1c)
with a barrier of 95.9 kcal/mol. These carbenes decompose 21.00, ratio 1:84.
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C. Wentrup, H. Bornemann
SHORT COMMUNICATION
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[6] N. P. Gritsan, E. A. Pritchina, Mendeleev Comm. 2001, 94.
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Supporting Information (see footnote on the first page of this arti-
cle): Contains IR spectra of matrix photolysates of benzoyl azide
and of authentic PhOCN and PhNCO, GC-mass spectra of
PhOCN and PhNCO isolated from the matrix photolysates as well
as the mass spectra of authentic PhOCN and PhNCO, and tables
of calculated energies and Cartesian coordinates of ground state
and transition structures.
[8] E. A. Pritchina, N. P. Gritsan, A. Maltsev, T. Bally, T. Autrey,
Y. Liu, Y. Wang, J. P. Toscano, Phys. Chem. Chem. Phys. 2003,
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[9] a) K. A. Jensen, M. Due, A. Holm, C. Wentrup, Acta Chem.
Scand. 1966, 20, 2091; b) A. Holm, C. Wentrup, Acta Chem.
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20, 2128; d) C. Wentrup, B. Gerecht, D. Laqua, H. Briehl, H.-
W. Winter, H. P. Reisenauer, M. Winnewisser, J. Org. Chem.
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[10] K. A. Jensen, A. Holm, C. Wentrup, J. Møller, Acta Chem.
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Acknowledgments
This work was supported by the Australian Research Council and
the APAC National Facility Merit Allocation Scheme (computing).
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Published Online: September 19, 2005
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Eur. J. Org. Chem. 2005, 4521–4524