1H-1,3-diazepines and ketenimines from cyanotetrazolopyridines

Article abstract of DOI:10.1071/CH08252

Cyano-substituted tetrazolo[1,5-a]pyridines/2-azidopyridines 8T and 15T undergo thermal ring opening to the azides 8A and 15A. Solution photolysis causes nitrogen elimination and ring expansion to 1,3-diazacyclohepta-1,2,4,6- tetraenes 10 and 17, which react with alcohols to afford 2-alkoxy-1H-1,3- diazepines, with secondary amines to 2-dialkylamino-5H-1,3-diazepines, and with water to 1,3-diazepin-2-ones (1214, 19, 21). Argon matrix photolysis of the azides affords the diazacycloheptatetraenes 10 and 17 as principal products together with ring-opened dicyanovinylketenimines 11 and 18. The matrix-isolated species were identified on the basis of comparison of the infrared spectra with those calculated at the B3LYP/631+G*level. CSIRO 2008.

Full text of DOI:10.1071/CH08252

Full Paper
CSIRO PUBLISHING
Aust. J. Chem. 2008, 61, 592–599
www.publish.csiro.au/journals/ajc
1H-1,3-Diazepines and Ketenimines from
Cyanotetrazolopyridines∗
Chris AddicottA and Curt WentrupA,B
AChemistry Building, School of Molecular and Microbial Sciences,
University of Queensland, Brisbane, QLD 4072, Australia.
BCorresponding author. Email: wentrup@uq.edu.au
Cyano-substituted tetrazolo[1,5-a]pyridines/2-azidopyridines 8T and 15T undergo thermal ring opening to the azides 8A
and 15A. Solution photolysis causes nitrogen elimination and ring expansion to 1,3-diazacyclohepta-1,2,4,6-tetraenes 10
and 17, which react with alcohols to afford 2-alkoxy-1H-1,3-diazepines, with secondary amines to 2-dialkylamino-5H-
1,3-diazepines, and with water to 1,3-diazepin-2-ones (12–14, 19, 21). Argon matrix photolysis of the azides affords the
diazacycloheptatetraenes 10 and 17 as principal products together with ring-opened dicyanovinylketenimines 11 and 18.
The matrix-isolated species were identified on the basis of comparison of the infrared spectra with those calculated at the
B3LYP/6−31+G∗ level.
Manuscript received: 11 June 2008.
Final version: 7 July 2008.
Introduction
products, 4 and 5, are believed to be formed via cleavage of the
N1–C2 bond in the 2-pyridylnitrenes to generate ring-opened,
transient cyanovinylnitrenes; an H-shift then generates keten-
imines 4, which tautomerize to the glutacononitriles 5 at elevated
temperatures. However, a detailed spectroscopic description of
the ketenimines and their rates of formation relative to 3 has
been lacking. Here we report the direct observation of cyclic
carbodiimides 3 and ring-opened ketenimines 4 as well as the
synthesis of 1,3-diazepines in the cyano-substituted series.
1,4-Diazepines are well known for their many pharmaceuti-
cal properties. In contrast, 1,3-diazepines are relatively little
known.[1] Some 1,3-diazepin-2-ones and other cyclic ureas
have received considerable attention recently as potential anti-
AIDS drugs.[2] In previous publications, we have described
the photolysis of variously substituted tetrazolo[1,5-a]pyridines
1T/2-azidopyridines 1A as a convenient method of synthesis
of 1,3-diazepines 6 and diazepinones 7.[3,4] The reaction pro-
ceeds via ring expansion of the first-formed 2-pyridylnitrenes
2 to 1,3-diazacyclohepta-1,2,4,6-tetraenes 3, which in several
cases have been characterized by matrix-isolation infrared (IR)
spectroscopy (Scheme 1).[3,5]
In several cases, the additional appearance of IR bands
near 2040 cm−1 has been ascribed to formation of keten-
imines 4 formed in a side reaction in the low-temperature
matrix photolyses.[3] Flash vacuum thermolysis (FVT) reactions
of tetrazolo/azidopyridines also produce 1,3-diazacyclohepta-
1,2,4,6-tetraenes 3, and here glutacononitriles 5 are often iso-
latedasminorby-productsin yieldsoftheorderof10%.[5] These
Results and Discussion
6-Cyanotetrazolo[1,5-a]pyridine exists in the tetrazole form 8T
in the solid state, but NMR spectroscopy demonstrated that this
tetrazole and the azide 8A coexist in a ratio of ∼2.2:1 in CDCl3
solution at room temperature. However, in [D6]DMSO only the
tetrazole form 8T was detectable in the 1H NMR spectrum. This
solvent effect on the azide–tetrazole equilibrium is common.[6]
Photolysis of tetrazole/azide 8T/8A in 1,4-dioxan solution
containing diisopropylamine afforded the 1H-1,3-diazepine 12a
(Nu = NPri2) as a red solid in 35% yield after chromatography
R
R
R
R
R
hν
NuH
∆
N
N
N
N
N
N3
N
N
N
N
N
H
N
Nu
6
H2O
1A
1T
2
3
R
R
R
∆
N
N
H
H
C
CN
CN CN
O
NH
5
7
4
Scheme 1. Overview of 2-pyridylnitrene chemistry.
∗ ThispaperisDiazepinesPartIV. ForPartIII, seeA. Reisinger, R. Koch, P.V. Bernhardt, C.Wentrup, Org. Biomol. Chem. 2004, 2, 1227. doi:10.1039/B317099C
© CSIRO 2008 10.1071/CH08252 0004-9425/08/080592

1H-1,3-Diazepines and Ketenimines
593
NC
N
NC
N
NC
N
NC
NC
NC
NuH
hν
∆
N
N
N
N
N
H
H
N3
N
N
N
Nu
Nu
N
N
12 a Nu = NPri
b Nu = NEt2
c Nu = OMe
d Nu = OEt
13
8A
8T
9
10
H2O
NC
e Nu = OPri
HN
HN
NC
NC
CN
C
CN
C
N
N
H
C
CN
C
CN
CN CN
s-Z-cis-11
H
NH
O
NH
14
s-E-cis-11
s-Z-trans-11
s-E-trans-11
Scheme 2. Reactions of 5-cyano-2-pyridylnitrene.
CN
CN
N3
CN
N
CN
NuH
∆
hν
CN
N
N
N
N
N
N
H
N
N
Nu
N
N
19
15A
15T
16
17
a Nu ϭ OMe
b Nu ϭ NPri2
H2O
HN
N
N
N
CN
CN
CN
C
CN
CN
HN
N
N
C
H
H
NMe2
NH
O
NC
s-E-18
21
20
s-Z-18
Scheme 3. Reactions of 3-cyano-2-pyridylnitrene.
(Scheme 2). Similar photolysis in 1,4-dioxan/diethylamine
afforded the 1H-diazepine 12b (Nu = NEt2) in 47% yield. 1,3-
Diazepines can potentially exist in 1H and 5H forms; the
1H forms are favoured thermodynamically for 2-alkoxy-1,3-
diazepines, and the 5H forms for 2-amino-1,3-diazepines, but
the 1H forms are always formed first and are isolable in both
the alkoxy- and the amino-substituted series.[3,4a] Furthermore,
unsymmetrical 1,3-diazepines can exist in interconverting 1H
and 3H forms.[3] The 3H-isomers 13a,b were not observed by
NMR spectroscopy in the present work.
nucleophilic dimethylamine caused ring-opening of the pyridine
ring in a reaction in the dark at room temperature to afford the
dienyltetrazole 20.[7]
4-Cyano-1,3-diazepin-2-one 21 was prepared by photolyzing
15T in wet acetonitrile. Column chromatography and sublima-
tion yielded the yellow compound 21 in 20% yield.
For the characterization of the diazepines by NMR spectro-
scopy, it is advantageous to use [D6]acetone and especially
[D6]DMSO, where signals are better resolved (although still
somewhat broad) and coupling information can be obtained.This
is attributed to their higher polarity helping to disrupt hydrogen
bonding between diazepine molecules. Such disruption prevents
formation of a diazepine molecular network with intermolecu-
lar hydrogen exchange responsible for line broadening in NMR
spectra.
Irradiation of a methanol/1,4-dioxan solution of 8T/8A for
2.5 h followed by chromatography furnished the 2-methoxy
derivative 12c (Nu = OMe) as a red solid in 35% yield. Here,
both the 1H form 12c and the 3H form 13c were present, in an
1
∼3:1 ratio as determined from the H and 13C NMR spectra.
Using ethanol under similar reaction conditions afforded a mix-
ture of 12d and 13d (Nu = OEt) in 50% yield. The isopropyl
derivatives 12e/13e (Nu = OPri) were prepared by photolysis
of 8T in 1,4-dioxan/isopropyl alcohol. Again, the NMR spectra
demonstrated the presence of both the 1H-isomer 12e and the
3H-isomer 13e, in a ratio of ∼2:1. The alkoxydiazepines 12/13
are red solids, which can be stored below 0◦C. They are easily
purified by sublimation.
Matrix Isolation
Argon matrix isolation studies were carried out in order to iden-
tify the 1,3-diazacyclohepta-1,2,4,6-tetraene intermediates. For
this purpose, 8T was sublimed at 70◦C in a steady stream of Ar
and condensed onto a CsI deposition window maintained at 25 K
to form a matrix. After cooling of the matrix to 7 K, the IR spec-
trum showed absorptions assigned to the azide 8A, the strongest
of which were the azide symmetrical (1288 cm−1) and antisym-
metrical (2126, 2136, 2153, and 2164 cm−1) bond stretches (see
Fig. S1 in theAccessory Publication).The simplicity of the spec-
trum suggested that 8T was not present, and this was confirmed
by co-subliming 8T with Ar through an oven held at ∼300◦C,
whichresultedintheexclusivedepositionof 8A.Theexperimen-
tal spectrum of 8A so obtained matches the calculated spectrum
very accurately (see Fig. S1). Comparison with the IR spectrum
Photolysis of 8T in water/1,4-dioxan followed by chromato-
graphy on alumina afforded a yellow solid insoluble in CDCl3
and identified as the cyclic urea 14 by its spectroscopic and
microanalytical data.
Photolysis of the isomeric tetrazole 15T in methanol/dioxan
afforded the 1H-2-methoxy-1,3-diazepine 19a (Scheme 3). It
was clear from the 1H NMR spectrum in CDCl3 that the
3H isomer of 19a was absent. The 2-diisopropylamino-1,3-
diazepine 19b was obtained with diisopropylamine, but the more

594
C. Addicott and C. Wentrup
(a)
1986
H2O
1975
(b)
4000
3000
2000
1500
1000
500 400
Fig. 1. (a) Infrared (IR) spectrum obtained after 3 min of UV irradiation of 8A (abscissa in wavenumbers,
ordinate in arbitrary absorbance units). The bands marked with wavenumbers are assigned to 10. Additional
bands at 3350, 2060, 2041, 757, and 683 cm−1 are marked with black dots and assigned (in part) to
ketenimine 11. (b) Calculated IR spectrum of 10 at the B3LYP/6−31+G∗ level (wavenumbers scaled by
0.9613).
of solid tetrazole 8T (see Experimental) confirmed the absence
of this compound. The nitrile group absorptions of 8A (2238,
2244 cm−1) were very weak.A weak overtone/combination band
at 2285 cm−1 accompanied them.
830 and 698 cm−1) (Fig. S3).The appearance of new ketenimine
bands may be due to either the formation of new conformers
of ketenimine 11 or the creation of new photo-induced matrix
sites.
The matrix was photolyzed at λ > 260 nm and monitored
by periodically recording IR spectra. After 1 min, a new band
at 1986 cm−1 (with shoulder at 1994 cm−1) appeared, whose
intensity increased on further irradiation. This absorption was
assigned to carbodiimide 10 (Scheme 2). A total of 3 min of
irradiation caused complete destruction of 8A and left a matrix
composed largely of 10 (Fig. 1). Cyclic carbodiimides isolated
The slow formation of ketenimine products and the disap-
pearance of 10 can be interpreted in terms of ring-opening of
nitrene 9, which is expected to exist in photoequilibrium with
10.[8,9] Several other such ring-opening reactions have been
reported recently,[3,9] and are particularly well documented in
the 1-isoquinolylnitrene[9] and 2-quinazolylnitrene systems.[10]
The occurrence of unassigned bands in the 2040 cm−1 region
in the matrix photolyses of other pyridylnitrenes[5a] can be
explained analogously. Thus, the ring opening of nitrene 9 can
give rise to several conformers of the dicyanovinylketenimine
11 (Scheme 2). The calculated IR spectra of the conformers of
11 are all very similar, and it is not possible to determine which
conformers are present (see Fig. S2 in the Accessory Publica-
tion). The general appearance of these spectra is similar to those
of other, related cumulenes.[9,11]
inAr matrices are known to absorb strongly near 2000 cm−1 [3–5]
.
The calculated IR spectrum of 10 at the B3LYP/6−31+G∗ level
is in very good agreement with the experimental spectrum,
leaving little doubt that this is the main product of photolysis
(Fig. 1). The strongest calculated bands at 2241, 1975, 1010,
954, 785, and 508 cm−1 correlate well with the experimen-
tal values at 2221, 2234, 1986, 1019, 967, and 532 cm−1. The
experimental spectrum in Fig. 1 also reveals that one or more
additional photoproducts are formed, with principal absorptions
at 3350, 2060, 2041, 757, and 683 cm−1. Absorptions in the
3350 and 2040 cm−1 regions are indicative of a ketenimine
moiety, C=C=NH. This product or product mixture formed
more slowly than 10, as documented by plotting the intensi-
ties of all peaks as a function of time (see Figs S1–S4 in the
Accessory Publication). Moreover, extensive photolysis of the
matrix for 60 min at 7 K resulted in the complete disappear-
ance of 10 and development of additional bands in the NH,
CN, and CCN regions (2249, 2245, 2070, 2054, 2047, and
2041 cm−1) and in the fingerprint region (several bands between
The matrix isolation of 8-cyanotetrazolo[1,5-a]pyridine/
2-azidopyridine 15T/15A proceeded in a similar manner. Sub-
limation at 100◦C in a stream of Ar and deposition on the CsI
window at 25 K resulted in a mixture of 15T and 15A. The
azide photolyzed more rapidly than the tetrazole, thus allowing
an assignment of IR bands to each compound (Fig. S5). Sublima-
tion of 15T through the FVT oven maintained at 290◦C afforded
exclusively the azide 15A, which exhibits strong IR absorptions
at 1425 (symmetrical N3 stretch), 2140, and 2153 cm−1 (anti-
symmetric N3 stretches), a weak CN band at 2234 cm−1 and a
weak overtone/combination band at 2289 cm−1
.

1H-1,3-Diazepines and Ketenimines
595
1986
(a)
1971
(b)
4000
3000
2000
1500
1000
500 400
Fig. 2. (a) Infrared (IR) spectrum obtained after 7.5 min of UV irradiation of 15A (abscissa in wavenum-
bers; ordinate in arbitrary absorbance units). The bands marked with wavenumbers are assigned to 17.
Additional bands at 2042, 2052, 2229, 3326, and 3349 cm−1 marked with black dots are assigned (in part)
to ketenimine 18. (b) Calculated IR spectrum of 17 at the B3LYP/6−31+G∗ level (wavenumbers scaled by
0.9613).
Photolysis of the matrix of 15A with light of λ > 260 nm
resulted in the disappearance of the IR bands of 15A in the
course of 7.5 min to be replaced with two medium to strong
bands at 1986 and 2042 cm−1 (Fig. 2a). The 1986 cm−1 band
is assigned to carbodiimide 17, and the bands at 2234, 1272,
1141, 1003, 931, 785, 548, and 528 cm−1 also belong to this
species. There is very good agreement with the calculated bands
at 2243, 1971, 1110, 993, 922, and 773, 528, and 513 cm−1
(Fig. 2b).Another group of bands at 2042, 2052, 2229, 3326, and
3349 cm−1 as well as two clusters of bands at 740–775 cm−1 and
805–850 cm−1 belong to different photoproducts that form more
slowly according to the kinetic monitoring of the growth of all
bands (Figs S6–S9). These bands are assigned to the conformers
of the ring-opened ketenimine 18 because of the good agreement
with the calculated IR spectra (Fig. S7).
When the matrix was irradiated further, carbodiimide 17
slowly disappeared. The bands assigned to ketenimine 18 ini-
tially maintained their intensities, and then slowly decreased. A
new nitrile band at 2240 cm−1 grew during this time. In the fin-
gerprint region, some new, weak bands appeared between 680
and 710 cm−1 (Fig. S8).The deterioration of the matrix on exten-
sive photolysis due to line broadening and a decreased signal to
noise ratio made a structural assignment of the secondary prod-
ucts impossible, but because it is known that NH-ketenimines
rearrange very easily to nitriles,[11] and glutacononitriles (2-
propene-1,3-dicarbonitriles 5) are formed as byproducts in the
thermal reactions of 2-pyridylnitrenes,[5] the disappearance of
the ketenimine and appearance of a new nitrile function is not
surprising.
Conclusions
The 6- and 8-cyanotetrazolo[1,5-a]pyridines 8T and 15T
undergo valence tautomerization to the corresponding azides 8A
and 15A on mild thermolysis (FVT at 100–250◦C). The azides
photolyze to 2-pyridylnitrenes 9 and 16, which undergo effi-
cient ring expansion to 1,3-diazacyclohepta-1,2,4,6-tetraenes 10
and 17. The latter are readily observable by matrix-isolation
IR spectroscopy and trappable in solution with nucleophiles to
produce 1H-1,3-diazepines 12, 13, and 19 and 1,3-diazepin-
2-ones 14 and 21. Slower-forming photolysis products are
assigned as the various conformers of the open-chain keten-
imines 11 and 18, which are assumed to be formed by ring
opening of the 2-pyridylnitrenes in photoequilibrium with the
1,3-diazacyclohepta-1,2,4,6-tetraenes.
Computational Method
The GAUSSIAN 98 suite of programs was used to simu-
late IR spectra.[12] Geometry optimizations were performed at
either B3LYP/6−31+G∗ or B3LYP/6−311++G∗∗ levels. The
B3LYP formulation of density functional theory corresponds

596
C. Addicott and C. Wentrup
to Becke’s three-parameter exchange functional in combi-
nation with the Lee–Yang–Parr correctional functional.[13]
A frequency scaling factor of 0.9613 was used for both
B3LYP/6−31G∗ and B3LYP/6−31+G∗.[14] Frequencies calcu-
lated at the B3LYP/6−311++G∗∗ level were scaled by 0.97.[15]
The temperature used was 298 K. Standard orientations, ener-
gies, and IR spectra are presented in the Accessory Publication.
suspension of sodium azide (7.06 g, 0.109 mol) in 50% aque-
ous ethanol (38 mL). Aqueous HCl (10%, 27 mL) was added,
and the solution was refluxed for 24 h. Cooling the solution to
room temperature afforded crystals of crude 15T that were col-
lectedonaBüchnerfunnelandpurifiedbysublimation(13 Pa, oil
bath 140◦C). Yield 6.33 g (81%). Mp 185–188◦C (dec.) [lit.[17]
183◦C]. m/z 145 (M+•), 117 (–N2), 90 (–N2, –HCN), 63 (–N2,
–2HCN). νmax(KBr)/cm−1 3123m, 3103s, 3087s, 3046m, 2237s,
1623vs, 1568m, 1498vs, 1422m, 1373vs, 1347s, 1281m, 1245m,
1161m, 1109m, 1098m, 1059m, 1020s, 997s, 883s, 817s, 805s,
Experimental
General procedures for matrix isolation experiments have been
published.[5,9,10] Ar matrices were deposited onto a CsI disk,
usually at 25 K, and IR spectra were recorded at 7–10 K. Ar
was ultra high purity grade (99.999%). The Ar flow rate was
maintainedat100 Pa. Inallmatrixexperiments, abedofAr(200–
300 Pa) was laid on the deposition window before sublimation
of the desired tetrazole. On completion of sublimation, another
200–300 PaAr was used to coat the matrix. Photolyses were per-
formed using either a Gräntzel (Karlsruhe) 350W low-pressure
Hg lamp (λ 200–254 nm) or a Hanovia 1000W high-pressure
broadband Hg–Xe lamp (λ > 260 nm) equipped with a water
filter. IR spectra were recorded at 1-cm−1 resolution.
All solution trapping experiments used the aforementioned
1000W high-pressure Hg–Xe lamp. Reactions were conducted
in quartz vessels with magnetic stirring. Photolysis solvents
were distilled over appropriate drying agents, except acetonitrile,
which was distilled once from Na2CO3. Before photolysis, solu-
tions were purged with N2 for ∼2 h. Reaction progress was
mainly monitored by gas chromatography-mass spectrometry
and TLC. Alumina 90 (Merck) for column chromatography
of diazepines was deactivated before use by stirring in a 1:1
methanol/water solution overnight, then drying on a Büchner
funnel. Silica gel 100 (Merck) was used as supplied. TLC was
performed on silica gel 60 or neutral alumina plates.
3
764vs, 663s, 639w. δH ([D6]DMSO, 400 MHz) 7.59 (t, J 7.2,
3
4
1H, H6), 8.58 (dd, J 7.2, J 0.94, 1H, H7, or H5), 9.61 (dd,
3J 7.0, 4J 0.9, 1H, H5, or H7). δH ([D6]DMSO, 100 MHz) 99.5
(C8), 113.7 (CN), 116.9 (C6 or C7), 131.4 (C7 or C6), 141.2
(C5), 147.3 (C8a).
Solution Photolysis and Trapping Studies
5-Cyano-2-diisopropylamino-1H-1,3-diazepine 12a
A solution of 8T (0.15 g, 1.03 mmol) in 1,4-dioxan (100 mL)
containing diisopropylamine (15 mL) was irradiated for 4.5 h.
The solution turned dark red-brown. Evaporation of excess sol-
vent gave a red oil, which was chromatographed on neutral
alumina with 1:1 diethyl ether/hexane. A red-brown band was
eluted, which on evaporation of the solvent gave the red solid
diazepine.This was recrystallized by dissolving in ether and then
adding hexane dropwise until precipitation commenced. Yield
80 mg (35%). Mp 126–127◦C. Calc. for C12H18N4: C 66.0, H
8.3, N 25.7. Found: C 66.2, H 8.6, N 25.5%. m/z 218 (M+•), 203
(–Me), 175 (–propyl radical), 148, 132. νmax(KBr)/cm−1 3418m,
2979w, 2196s, 1665w, 1544vs, 1458w, 1396w, 1371w, 1341m,
1315w, 1274m, 1161w, 1123w, 1008w, 958w, 845w, 750w, 713w.
δH (CDCl3, 400 MHz) 1.25 (d, 3J 6.9, 12H, 4 Me), 4.01 (br d, 3J
5.2, 1H, H1), 4.10(septet, 3J6.9, 2H, Pri), 5.41(t, 3J6.3, 1H, H7),
5.5 (d, 3J 6.68, 1H, H6), 7.22 (s, 1H, H4). δC (CDCl3, 100 MHz)
21.0 (Me), 47.5 (Pri), 94.8 (C5), 118.1 (C6), 121.1 (CN), 126.2
(C7), 151.5 (C2), 154.8 (C4). δH ([D63]acetone, 400 MHz) 1.27
6-Cyanotetrazolo[1,5-a]pyridine 8T
2-Chloro-5-cyanopyridine (3.18 g, 23 mmol) was added to 66%
aqueous ethanol (37 mL) and the mixture combined with a sus-
pension of sodium azide (3.06 g, 47 mmol) in 50% aqueous
ethanol (17 mL). Aqueous HCl (10%, 12 mL) was added, and
the solution was refluxed for 24 h. Cooling the solution to room
temperature and then in a refrigerator (2◦C) for several hours
afforded crystals of the crude tetrazole, which were collected
on a Büchner funnel. The light brown 8T so obtained was puri-
fied by sublimation (13 Pa, oil bath 90◦C). Yield 2.29 g (69%).
Mp 96–98◦C [lit.[16] 90◦C]. m/z 145 (M+•), 117 (–N2), 90 (–N2
andHCN), 63(–N2 and2HCN). νmax(KBr)/cm−1 3140m, 3090s,
3043m, 2248s, 1761w, 1658w, 1634vs, 1505vs, 1490vs, 1426s,
1361m, 1334s, 1260s, 1203s, 1149s, 1106m, 1065s, 991s, 885s,
828vs, 751m, 612vs. δH (CDCl3, 400 MHz) tetrazole 7.79 (dd, 3J
9.4, 4J 1.5, 1H, H7), 8.2 (dd, 3J 9.4, 5J 1.2, 1H, H8), 9.28 (t, aver-
age 4J, 5J 1.2, 1H, H5); azide 6.87 (dd, 3J 8.5, 5J 0.8, 1H, H8),
7.83 (dd, 3J 8.5, 4J 2.3, 1H, H7), 8.60 (dd, 4J 2.3, 5J 0.80, 1H,
H5), ratio tetrazole:azide 2.26:1. δC (CDCl3, 100 MHz) 103.6,
105.5, 114.1, 114.2, 116.5, 117.5, 131.4, 132.2, 141.4, 148.3,
3
(d, J 6.9, 12H, 4 Me), 4.09 (septet, J 6.8, 2H, Pri), 5.17 (d,
3
3J 7.0, 1H, H6), 5.58 (br d, J 5.0, 1H, H7), 5.60 (br s, 1H,
H1), 7.05 (s, 1H, H4). δC ([D6]acetone, 100 MHz) 21.1 (Me),
48.3 (Pri), 96.2 (C5), 114.7 (C6), 121.4 (CN), 130.5 (C7), 154.4
(C2), 156.2 (C4).
5-Cyano-2-diethylamino-1H-1,3-diazepine 12b
A solution of diethylamine (2 mL) in 1,4-dioxan (100 mL) was
purged with N2 for 0.5 h. Tetrazole 8T (0.13 g, 0.90 mmol)
was added and irradiation commenced immediately at room
temperature. Photolysis was continued for 4.5 h. Evaporation
of excess solvent gave a red-brown oil, which was chro-
matographed on neutral alumina with 1:1 diethyl ether/hexane.
An orange band was eluted, which on evaporation of the
solvent gave the orange, solid diazepine. Yield 0.08 g (47%).
Recrystallization from diethyl ether/hexane gave analytically
pure red needles. Mp 109–110◦C. Calc. for C10H14N4: C
63.1, H 7.4, N 29.5. Found: C 63.2, H 7.6, N 29.3%.
m/z 190 (M+•), 175 (–Me), 161 (–Et), 134, 118, 106,
93. νmax(KBr)/cm−1 3362s, 2979m, 2936m, 2198vs, 1652m,
1582vs, 1539vs, 1464m, 1436m, 1418m, 1394s, 1359s, 1333vs,
1317s, 1280s, 1266m, 1227w, 1178m, 1080m, 1013m, 925m,
883m, 786w, 752w, 704m, 650w, 550w, 513w. δH ([D6]acetone,
400 MHz) 1.16 (t, 3J 7.1, 6H, 2 Me), 3.42 (q, 3J 7.1, 4H,
2 –CH2–), 4.98 (d, 3J 7.4, 1H, H6), 5.52 (br d, 3J 5.6, 1H, H7),
6.16 (br s, 1H, H1), 6.94 (s, 1H, H4). δC ([D6]acetone, 100 MHz)
3
152.5, 158.2. δH ([D6]DMSO, 400 MHz) 8.14 (d, J 9.2, 1H,
H7, or H8), 8.38 (d, 3J 9.2, 1H, H8, or H7), 10.24 (s, 1H, H5). δC
([D6]DMSO, 100 MHz) 102.5 (C6), 115.4 (CN), 116.4, 133.4,
134.1, 148.3 (C8a).
8-Cyanotetrazolo[1,5-a]pyridine 15T
2-Chloro-3-cyanopyridine (7.43 g, 0.054 mol) was added to 66%
aqueous ethanol (87 mL) and the mixture combined with a

1H-1,3-Diazepines and Ketenimines
597
13.6 (Me), 44.0 (CH2), 96.0 (C5), 112.6 (C6), 121.4 (CN), 130.7
(C7), 155.8 (C2), 156.7 (C4).
νmax(KBr)/cm−1 3329s, 2992m, 2215s, 1663s, 1607s, 1574s,
1470m, 1436m, 1415m, 1385w, 1373w, 1353m, 1336m, 1273s,
1247s, 1221s, 1176m, 1141w, 1103m, 945m, 924w, 914m, 905w,
844m, 795m, 734m, 714m, 664w, 563w, 521w. δH ([D6]DMSO,
5-Cyano-2-methoxy-1H-1,3-diazepine 12c/
5-Cyano-2-methoxy-3H-1,3-diazepine 13c
3
400 MHz) 1H isomer 12e 1.19 (d, J 6.2, 6H, 2 Me), 4.76 (d,
3
3
3J 7.9, 1H, H6), 4.81 (septet, J 6.2, 1H, Pri), 5.33 (t, J 7.1,
1H, H7), 6.71 (s, 1H, H4), 7.23 (d, 3J 5.2, 1H, H1); 3H isomer
13e (isopropyl group signals masked) 4.90 (d, 3J 7.4, 1H, H6),
A solution of tetrazole 8T (0.11 g, 0.76 mmol), 1,4-dioxan
(70 mL), and methanol (10 mL) was irradiated for 2.5 h while
cooling with ice–water. Evaporation of excess solvent gave a
red-brown oil, which was chromatographed on neutral alumina
with 1:1 ether/hexane. The red solid diazepine was collected and
dried under vacuum.Yield 0.04 g (35%). Mp 79–80◦C. Calc. for
C7H7N3O: C 56.4, H 4.7, N 28.2. Found: C 56.4, H 4.7, N 28.0%.
m/z 149 (M+•), 134 (–Me), 106, 92. νmax(KBr)/cm−1 3326s,
3053w, 2949m, 2525w, 2296w, 2211s, 2048w, 1675s, 1618s,
1587s, 1461s, 1414s, 1344s, 1259s, 1226s, 1172m, 1068s, 986s,
939m, 925m, 906s, 782m, 715s, 666w. δH (CDCl3, 400 MHz)
1H isomer 12c 3.72 (s, 3H, OMe), 4.75 (br s, 1H, H1), 4.88 (ddd,
3J 7.7, 4J 1.7, 4J 1.1, 1H, H6), 5.36 (ddd, 3J 7.7, 3J 6.5, 5J 1.1,
1H, H7), 6.75 (t, average 4J, 5J 1.1, 1H, H4); 3H isomer 13c 3.7
(s, 3H, OMe), 5.02 (br s, 1H, H3), 5.05 (d, 3J 8.28, 1H, H6), 6.03
3
3
3
5.87 (d, J 7.5, 1H, H7), 6.17 (d, J 5.9, 1H, H4), 7.71 (d, J
4.4, 1H, H3); ratio of 1H isomer to 3H isomer was ∼2–2.5:1. δC
([D6]DMSO, 100 MHz) 1H isomer 12e 21.3 (Me), 72.3 (Pri),
101.7 (C5), 107.5 (C6), 118.9 (CN), 134.7 (C7), 153.4 (C4),
158.8 (C2); 3H isomer 13e 21.3 (Me), 71.6 (Pri), 94.9 (C5),
112.8 (C6), 118.4 (CN), 139.8 (7), 148.1 (C4), 153.9 (C2).
5-Cyano-2,3-dihydro-1,3-diazepin-2-one 14
A solution of tetrazole 8T (0.05 g, 0.34 mmol), 1,4-dioxan
(40 mL), and water (10 mL) was irradiated for 1.5 h at room
temperature. The yellow solution was evaporated, and the dark
residue was chromatographed on neutral alumina using 2.5%
methanol/dichloromethane to elute a yellow band. The result-
ing yellow solid was purified by sublimation (13 Pa, oil bath
80–110◦C). Yield 5 mg (11%). Mp 138–140◦C (dec.). Calc.
for C6H5N3O: C 53.3, H 3.7, N 31.1. Found: C 53.2, H 3.8,
N 31.1%. m/z 135 (M+•), 119, 107, 93, 80. νmax(KBr)/cm−1
3569m, 3498m, 3287s, 3181m, 3009m, 2230m, 2216s, 1707vs,
1673vs, 1646vs, 1507w, 1473w, 1437w, 1392m, 1304m, 1258s,
1237s, 1222s, 1215s, 1115w, 1076w, 898w, 766m, 649w. δH
3
5
3
5
(dd, J 8.5, J 0.9, 1H, H7), 6.05 (dd, J 7.2, J 0.9, 1H, H4);
ratio 1H isomer to 3H isomer was ∼3:1. δC (CDCl3, 100 MHz)
1H isomer 12c 56.6 (OMe), 103.0 (C5), 109.5 (C6), 118.7 (CN),
132.2 (C7), 152.6 (C4), 158.2 (C2); 3H isomer 13c 56.3 (OMe),
98.2 (C5), 113.3 (C6), 117.8 (CN), 139.8 (C4 or C7), 144.3 (C7
or C4), 153.6 (C2).
5-Cyano-2-ethoxy-1H-1,3-diazepine 12d/
5-Cyano-2-ethoxy-3H-1,3-diazepine 13d
3
4
([D6]DMSO, 400 MHz) 4.76 (dt, J 8.9, J 1.1, 1H, H6), 5.45
(ddd, 3J 8.9, 3J 6.0, 5J 0.6, 1H, H7), 6.26 (dt, 3J 7.0, average 4J,
These compounds were prepared in the same manner as 12c/13c
and obtained as a red oil, which solidified after drying on a
vacuum pump and storing overnight in a freezer. Sublimation
(13 Pa, oilbath50◦C)gaveanalyticallypureproduct.Yield0.03 g
(50%). Mp 59–60◦C. Calc. for C8H9N3O: C 58.9, H 5.6, N 25.8.
Found: C 58.8, H 5.6, N 25.8%. m/z 163 (M+•), 148 (–Me),
135, 120, 107, 93, 80. νmax(KBr)/cm−1 3300s, 2994w, 2983w,
2210s, 1813w, 1678s, 1615s, 1585, 1469s, 1416m, 1393m,
1367w, 1342s, 1262s, 1231m, 1154w, 1069m, 1015m, 942w,
923w, 909w, 886m, 797m, 753m, 726m, 715m, 565w, 519w. δH
(CDCl3, 400 MHz) 1H isomer 12d 1.22 (t, 3J 7.1, 3H, Me), 4.12
(q, 3J 7.1, 2H, –CH2–), 4.82 (br s, 1H, H1), 4.86 (ddd, 3J 7.8, 4J
1.68, 4J 1.1, 1H, H6), 5.35 (ddd, 3J 7.8, 3J 6.5, 5J 0.8, 1H, H7),
6.73 (t, average 4J, 5J 1.0, 1H, H4), 3H isomer 13d 1.21 (t, 3J
7.2, 3H, Me), 4.07 (q, 3J 7.1, 2H, –CH2–), 5.02 (d, 3J 8.2, 1H,
H6), 5.17 (br s, 1H, H3), 6.01 (d, 3J 8.2, 1H, H7), 6.05 (d, 3J 7.2,
1H, H4); ratio 1H isomer to 3H isomer was ∼3:1. δC (CDCl3,
100 MHz) 1H isomer 12d 14.0 (Me), 65.8 (CH2), 102.6 (C5),
109.3 (C6), 118.8 (CN), 132.3 (C7), 152.8 (C4), 157.6 (C2); 3H
isomer 13d 14.0 (Me), 65.3 (CH2), 97.8 (C5), 113.1 (C6), 117.9
(CN), 140.0 (C4 or C7), 144.6 (C7 or C4), 153.0 (C2).
5J 0.8, 1H, H4), 8.10 (br d, J 4.9, 1H, H1, or H3), 8.53 (br d,
3
3J 6.0, 1H, H3, or H1). δC ([D6]DMSO, 100 MHz) 92.5 (C5),
104.5 (C6), 119.1 (CN), 127.3 (C7), 139.7 (C4), 159.0 (C2).
4-Cyano-2-methoxy-1H-1,3-diazepine 19a
A solution of tetrazole 15T (0.15 g, 1.03 mmol) in methanol
(100 mL) was photolyzed under N2 at room temperature for 3 h.
Evaporation of the red-brown solution left a brown solid, which
was chromatographed on neutral alumina with 25% hexane/
ether. The brown band was collected, and the solvent was
removed to give an orange solid further purified by sublima-
tion (13 Pa, oil bath 60◦C). A red solid was obtained; yield 15
mg (10%). Mp 85–86◦C. Calc. for C7H7N3O: C 56.4, H 4.7,
N 28.2. Found: C 56.5, H 4.8, N 28.2%. m/z 149 (M+•), 134
(–Me), 120, 106, 92, 79. νmax(KBr)/cm−1 3338s, 3045w, 3008w,
2960m, 2223m, 1854w, 1684s, 1635s, 1589m, 1456s, 1405m,
1361m, 1275s, 1251s, 1197m, 1159m, 1116m, 1018m, 1000w,
971m, 892m, 860m, 835m, 740m, 720m, 688m, 636m, 599w,
551w, 539w. δH (CDCl3, 400 MHz) 3.70 (s, 3H, OMe), 4.72 (br
s, 1H, H1), 4.84 (ddd, 3J 7.9, 3J 6.1, 4J 1.7, 1H, H6), 5.47 (dd,
3
3
3J 8.1, J 6.6, 1H, H7), 5.87 (d, J 6.1, 1H, H5). δC (CDCl3,
100 MHz) 56.6 (OMe), 109.9 (C6), 118.6 (C4 or CN), 123.7
(CN or C4), 130.0 (C5), 135.8 (C7), 158.4 (C2).
5-Cyano-2-isopropoxy-1H-1,3-diazepine 12e/
5-Cyano-2-isopropoxy-3H-1,3-diazepine 13e
A mixture of 8T (0.05 g, 0.34 mmol), 1,4-dioxan (45 mL), and
isopropyl alcohol (5 mL) was irradiated for 1.5 h at room tem-
perature. Evaporation of excess solvent left a brown oil, which
was chromatographed on neutral alumina with 1:1 ether/hexane
to afford the product as a red oil, which solidified after dry-
ing on a vacuum pump. Sublimation (13 Pa, oil bath 52◦C)
afforded an orange solid, yield 0.04 g (65%). Mp 70–71◦C.
Calc. for C9H11N3O: C 61.0, H 6.3, N 23.7. Found: C 60.9,
H 6.3, N 23.7%. m/z 177 (M+•), 162, 148, 135, 118, 107, 93.
4-Cyano-1,3-dihydro-1,3-diazepin-2-one 21
A solution of tetrazole 15T (0.11 g, 0.76 mmol) in acetonitrile
(90 mL) was irradiated under N2 at room temperature for 3 h.
The solvent was removed, and the solid material was subjected
to column chromatography on silica gel. The crude prod-
uct was eluted with 2.5% methanol/dichloromethane and then
chromatographed once more on silica gel, with dichloromethane

598
C. Addicott and C. Wentrup
as eluent. The orange-yellow band was slowly eluted, and evapo-
ration yielded a yellow-orange solid (40 mg). After sublimation
(13 Pa, oil bath 90–110◦C) a yellow solid was obtained, yield
0.02 g (20%). Mp 175–180◦C (dec.). Calc. for C6H5N3O: C
53.3, H 3.7, N 31.1. Found: C 53.3, H 3.7, N 30.9%. m/z 135
(M+•), 107, 92, 80. νmax(KBr)/cm−1 3268s, 3153s, 3016m,
2973m, 2214s, 1723s, 1659m, 1625s, 1501w, 1476w, 1425m,
1383m, 1317m, 1258m, 1162m, 1122w, 1015w, 922w, 871w,
848w, 776s, 617m, 602m, 580m, 538w, 500w. δH ([D6]DMSO,
400 MHz) 5.05 (ddd, 3J 8.6, 3J 6.3, 4J 1.1, 1H, H6), 5.86 (dd, 3J
8.6, 3J 6.3, 1H, H7), 6.09 (dd, 3J 6.1, 4J 1.5, 1H, H5), 8.28 (br d,
3J 4.3, 1H, H1), 8.47 (br s, 1H, H3). δC ([D6]DMSO, 100 MHz)
107.0 (C6), 108.7 (C4 or CN), 116.3 (CN or C4), 127.5 (C5),
133.7 (C7), 163.0 (C2).
The time interval was increased to 1 min between several spec-
tra and later to 5, 20, and 30 min. The area of many bands were
integrated and plotted against time (see Fig. S4). This anal-
ysis identified four groups of absorptions according to their
rate of formation: (i) 10 was the fastest formed, 500w, 532w,
554w, 556w, 796w, 967w, 1015w, 1017w, 1019w, 1986s, 1994m,
2234w cm−1; (ii) conformers of 11, 683w, 729w, 732w, 757w,
1187w, 2041w, 2060w, 2221w, 3349w cm−1; (iii) 483w, 629w,
688w, 695w, 698w, 703w, 923w, 1410w, 2047w, 2054w, 2070w,
3370w cm−1; (iv) after 17 h of irradiation, 589w, 657w, 830w,
972w, 2249w cm−1
.
2-Azido-3-cyanopyridine 15A
Tetrazole 15T was co-sublimed at 90–100◦C in a stream of Ar
through a 10-cm FVT quartz tube held at 290◦C for 25 min,
and the vapours were condensed onto a KBr window held at
25 K. The matrix was then cooled to 10 K. The IR spectrum
showed absorptions of 15A only: νmax(Ar)/cm−1 (10 K) 2289w,
2234w, 2183w, 2177w, 2153m, 2140s, 2130w, 1587w, 1573w,
1568w, 1455w, 1446w, 1425s, 1319w, 1314w, 1310w, 1292m,
1281w, 1271w, 1248w, 1194w, 1145w, 1098w, 862w, 802w,
800w, 797w, 761w, 651w, 583w, 547w. When 15T was sublimed
under the same conditions in the absence of the FVT oven, the
IR spectrum of the resulting matrix exhibited a mixture of 15T
and 15A. Thus, bands for 15T were identified: νmax(Ar)/cm−1
(10 K) 1626m, 1508w, 1500s, 1497s, 1372m, 1369m, 1349w,
1342w, 1265w, 1238w, 1151w, 1103w, 1053w, 1049w, 1008w,
987m, 983m, 883w, 789w, 758s, 757s, 663w, 591w, 451w. The
matrix containing 15A only was photolyzed with broadband
UV light for 7.5 min. This resulted in almost complete con-
version to carbodiimide 17 and other photoproducts assigned
as the conformers of 18: νmax(Ar)/cm−1 (10 K) 3349w, 3326w,
2234w, 2229w, 2222w, 2218w, 2104w, 2066w, 2052w, 2042m,
1989m, 1986s, 1581w, 1575w, 1416w, 1293w, 1287w, 1272w,
1242w, 1195w, 1171w, 1151w, 1141w, 1126w, 1005w, 1003w,
931w, 893w, 884w, 869w, 857w, 849w, 845w, 840w, 833w, 823w,
817w, 814w, 807w, 785w, 779w, 770w, 765w, 761w, 759w, 757w,
748w, 746w, 714w, 707w, 693w, 684w, 647w, 618w, 613w, 605w,
574w, 557w, 548w, 465w, 456w, 451w, 442w. After 2.5 h of
irradiation, 17 had nearly disappeared. Some of the minor photo-
product 18 remained. New bands were observed: νmax(Ar)/cm−1
(10 K) 3470w, 3461w, 3455w, 2240w, 2124w, 2066w, 2025w,
1398w, 1343w, 1249w, 1190w, 1187w, 1184w, 1179w, 1167w,
1050w, 935w, 795w, 792w, 708w, 696w, 663w, 644w,
625w, 465w.
Matrix Isolation and Photolysis
2-Azido-5-cyanopyridine 8A
Tetrazole 8T was co-sublimed at 70◦C in a stream of Ar for
20 min, and the vapour was condensed onto a CsI window held
at 25 K. The Ar matrix so obtained was then cooled to 7 K.
The IR spectrum showed absorptions of 8A: νmax(Ar)/cm−1
2424w, 2285w, 2280w, 2244w, 2238w, 2177w, 2169w, 2164w,
2153m, 2136s, 2126m, 1596m, 1566w, 1557w, 1474m, 1443w,
1425w, 1391w, 1384w, 1382w, 1375w, 1373w, 1362w, 1317w,
1309w, 1302w, 1288s, 1271w, 1254w, 1246w, 1242w, 1239w,
1217w, 1209w, 1195w, 1154w, 1144w, 1128w, 1022w, 943w,
940w, 862w, 849w, 846w, 840w, 837w, 823w, 820w, 768w, 751w,
599w, 545w. Broadband UV irradiation (3 min) caused loss of
8A and generation of carbodiimide 10 and other photoprod-
ucts assigned as the conformers of 11: νmax(Ar)/cm−1 (7 K)
3350w, 2234w, 2221w, 2097w, 2060w, 2041w, 1994m, 1986s,
1597w, 1593w, 1531w, 1328w, 1323w, 1318w, 1298w, 1265w,
1262w, 1187w, 1152w, 1122w, 1019w, 1017w, 1015w, 998w,
994w, 974w, 967w, 954w, 951w, 928w, 924w, 913w, 887w, 883w,
863w, 818w, 802w, 800w, 796w, 757w, 740w, 737w, 732w, 729w,
698w, 695w, 692w, 683w, 556w, 554w, 532w, 530w, 500w. Pro-
longed irradiation caused the loss of 10 (see Figs S3 and S4).
The IR spectrum showed that some bands generated after 3 min
of photolysis grew stronger after 60 min of irradiation. There
were also some new bands: νmax(Ar)/cm−1 (7 K) 3475w, 3469w,
3370w, 2249w, 2245w, 2237w, 2150w, 2138w, 2129w, 2122w,
2116w, 2094w, 2079w, 2070w, 2054w, 2047w, 2041w, 1410w,
1173w, 1135w, 1085w, 1073w, 972w, 947w, 923w, 865w, 837w,
830w, 819w, 752w, 703w, 698w, 695w, 689w, 677w, 664w, 657w,
644w, 629w, 606w, 589w, 575w, 574w, 569w, 563w, 482w, 455w.
After 14 h of irradiation, the following weak bands were present:
νmax(Ar)/cm−1 (7 K) 2249w, 2245w, 1476w, 1452w, 1412w,
1368w, 1316w, 1302w, 1282w, 1236w, 1217w, 1189w, 1174w,
1135w, 1105w, 1085w, 972w, 957w, 906w, 865w, 851w, 844w,
835w, 830w, 818w, 815w, 811w, 809w, 758w, 752w, 747w, 745w,
743w, 740w, 727w, 677w, 668w, 657w, 634w, 589w, 563w, 498w,
496w.
Kinetic Monitoring of the Photolysis of
2-Azido-3-cyanopyridine 15A
Tetrazole 15T was co-sublimed with Ar at 100◦C through a
quartz FVT tube (300◦C) for 23 min, and the vapours were con-
densed on a CsI deposition window at 25 K. This resulted in an
Ar matrix of 15A. The matrix temperature was lowered to 7 K.
Broadband irradiation of 15A was commenced, and IR spectra
were recorded every 30 s for 6 min, then at 1-min intervals for the
next 5 min. Spectra were then recorded after 13, 16, and 20 min
of photolysis. All significant bands were integrated and plotted
against time (see Fig. S9). Two groups of bands were character-
ized (cm−1): (i) 17, 746w, 785w, 884w, 931w, 1003w, 1141w,
1242w, 1986s; and (ii) conformers of 18, 748w, 765w, 770w,
807w, 814w, 817w, 823w, 833w, 840w, 849w, 2042m, 2052w,
3349w.
Kinetic Monitoring of the Photolysis of
2-Azido-5-cyanopyridine 8A in Ar at 7 K
Tetrazole 8T was co-sublimed with Ar at 71–72◦C for 30 min
onto a deposition window at 25 K. The temperature of the Ar
matrix of 8A so obtained was then lowered to 7 K. Broadband
irradiation was commenced and IR spectra were recorded every
20 s for 5 min, then at 30-s intervals during the next 5 min. The
distance between the light source and matrix was kept constant.

1H-1,3-Diazepines and Ketenimines
599
Accessory Publication
[5] (a) R. A. Evans, M. W. Wong, C. Wentrup, J. Am. Chem. Soc. 1996,
118, 4009. doi:10.1021/JA9541645
(b) C. Wentrup, H.-W. Winter, J. Am. Chem. Soc. 1980, 102, 6159.
doi:10.1021/JA00539A039
[6] C. Wentrup, Tetrahedron 1970, 26, 4969. doi:10.1016/S0040-
4020(01)93149-7
Electronic supplementary information is available showing Ar
matrix IR spectra (Figs S1–S6) and computational data (Carte-
sian coordinates, absolute energies, IR, 1H, and 13C NMR
spectra) from the author or, until August 2013, the Australian
Journal of Chemistry.
[7] C. Addicott, C. Wentrup, ARKIVOC.
[8] D. Kvaskoff, P. Bednarek, L. George, P. Sreekumar, C. Wentrup,
J. Org. Chem. 2005, 70, 7947. doi:10.1021/JO050898G
[9] C. Addicott, A. Reisinger, C. Wentrup, J. Org. Chem. 2003, 68, 1470.
doi:10.1021/JO026439M
[10] D. Kvaskoff, P. Bednarek, L. George, K. Waich, C. Wentrup, J. Org.
Chem. 2006, 71, 4049. doi:10.1021/JO052541I
Acknowledgements
The present research was supported by the Australian Research Council.
We are indebted to the Centre for Computational Molecular Science at the
University of Queensland for access to supercomputing facilities.
[11] C. Addicott, M. W. Wong, C. Wentrup, J. Org. Chem. 2002, 67, 8538.
doi:10.1021/JO0256991
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