Mesoionic carbenes: Reactions of 1, 3-diphenyltetrazol-5-ylidene with electron-deficient alkenes, and synthesis and catalytic activities of the (tetrazol-5-ylidene)rhodium(I) complexes

Article abstract of DOI:10.1002/jhet.61

The reactions of 1, 3-diphenyltetrazol-5-ylidene, a rare example of mesoionic carbenes, with electron- deficient unsaturated compounds were studied. The carbene reacted with dimethyl 1, 2, 4, 5-tetrazine-3, 6-dicarboxylate to give a 5-tetrazoliomethylide, together with hydrazine derivatives. The reaction with tetracyanoethylene gave another methylide in low yield. On the contrary, the reactions with weaker electrophiles, such as 3, 6-diphenyl-1, 2, 4, 5-tetrazine, fumalonitrile, N-phenylmaleimide, and dimethyl acetylenedicarboxylate, did not give any coupling products, but gave phenylated products and/or Michael addition products via the degradation of the 1, 3-diphenyltetrazole ring. Novel mesoionic mono- and bis(carbene)-rhodium(I) complexes were synthesized and the structures were characterized by X-ray crystallography. Their catalytic activities for the decarbonylative addition reaction of benzoyl chloride to ethynylbenzene were investigated.

Full text of DOI:10.1002/jhet.61

164
Mesoionic Carbenes: Reactions of 1,3-Diphenyltetrazol-5-ylidene
with Electron-Deficient Alkenes, and Synthesis and Catalytic
Activities of the (Tetrazol-5-ylidene)rhodium(I) Complexes
Vol 46
Shuki Araki,a* Keisaku Yokoi,a Ryosuke Sato,a
Tsunehisa Hirashita,a and Jun-Ichiro Setsuneb
aOmohi College, Graduate School of Engineering, Nagoya Institute of Technology,
Showa-ku, Nagoya 466-8555, Japan
bFaculty of Science, Department of Chemistry, Kobe University, Nada-ku, Kobe 657-8501, Japan
*E-mail: araki.shuki@nitech.ac.jp
Received June 24, 2008
DOI 10.1002/jhet.61
Published online 19 March 2009 in Wiley InterScience (www.interscience.wiley.com).
The reactions of 1,3-diphenyltetrazol-5-ylidene, a rare example of mesoionic carbenes, with electron-
deficient unsaturated compounds were studied. The carbene reacted with dimethyl 1,2,4,5-tetrazine-3,
6-dicarboxylate to give a 5-tetrazoliomethylide, together with hydrazine derivatives. The reaction with
tetracyanoethylene gave another methylide in low yield. On the contrary, the reactions with weaker
electrophiles, such as 3,6-diphenyl-1,2,4,5-tetrazine, fumalonitrile, N-phenylmaleimide, and dimethyl
acetylenedicarboxylate, did not give any coupling products, but gave phenylated products and/or
Michael addition products via the degradation of the 1,3-diphenyltetrazole ring. Novel mesoionic mono-
and bis(carbene)-rhodium(I) complexes were synthesized and the structures were characterized by X-ray
crystallography. Their catalytic activities for the decarbonylative addition reaction of benzoyl chloride
to ethynylbenzene were investigated.
J. Heterocyclic Chem., 46, 164 (2009).
INTRODUCTION
1b can also be generated by the reaction of 5-azido-1,3-
diphenyltetrazolium salt with sodium azide [5]. Carbene 1
is thermally labile; upon warming to room temperature, 1
undergoes a ring-opening to give 3-cyanotriazene. In the
case of 1b, further degradation and recombination reac-
tions occur spontaneously to give 4-(phenylazo)phenyl-
cyanamide (3) and phenylcyanamide (4) (Scheme 1) [6].
As an isomeric carbene of 1, 2,3-diphenyltetrazol-5-yli-
dene (5) is also known, which is generated similarly by
deprotonation of 2,3-diphenyltetrazolium salts [7]. This
mesoionic carbene is also unstable to give cyanoazimine 6
(Scheme 2). Although various stable NHCs have recently
been attracting increasing interest, only these two exam-
ples 1 and 5 of mesoionic carbenes are thus far
synthesized.
Since the Arduengo’s pioneering work on stable
heterocyclic carbenes, various heterocyclic carbenes,
including N-heterocyclic carbenes (NHCs) such as imida-
zol-2-ylidene and 1,2,4-triazol-3-ylidene, have hitherto
been prepared and their physical and chemical properties
have intensively been investigated [1]. Mesoionic com-
pounds are the unique family of heterocycles owing to
their interesting electronic nature; i.e., they cannot be for-
mulated satisfactorily by a single covalent or polar struc-
ture, but expressed as a resonance hybrid of a series of
dipolar canonical structures [2]. Mesoionic carbenes,
which are derived formally from mesoionic compounds by
a removal of the exo-cyclic atom(s), are also resonance
hybrids of several canonical structures. Contrary to the
extensive study on NHCs, mesoionic carbenes are scare-
cely studied. 1,3-Dimethyl- (1a) and 1,3-diphenyltetrazol-
5-ylidene (1b) are rare examples of such mesoionic car-
benes. These carbenes are easily prepared by deprotonation
of the corresponding 1,3-disubstituted tetrazolium salts
2a,b with a base at low temperature [3,4]. Alternatively,
C
V
2009 HeteroCorporation

March 2009 Mesoionic Carbenes: Reactions of 1,3-Diphenyltetrazol-5-ylidene with Electron-Deficient
Alkenes, and Synthesis and Catalytic Activities of the (Tetrazol-5-ylidene)rhodium(I) Complexes
165
Scheme 1
Scheme 2
was generated in situ from the tetrazolium salt 2b and
reacted with 8 under different reaction conditions, by
changing the solvent and the reaction temperature.
These results are summarized in Table 1.
Three products 9, 10, and 11 have been isolated from
the reaction mixtures in varying yields, together with
small amounts of olate 7 and 4 (Scheme 3). Olate 7 is
considered to be formed by the reaction of 1b with
atmospheric oxygen [4]. The compound 9 has been
revealed to be a 1:1 adduct of 1b and 8 on the basis of
the MS and elemental analyses. A characteristic signal
at 78.7 ppm in the 13C NMR spectrum suggests that this
compound is a 5-tetrazoliomethylide derivative [9]. The
structure was finally comfirmed by X-ray chrystallogra-
phy (Fig. 1). The exo-cyclic methylide CACAN plane
and the tetrazolium ring share almost the same plane.
The phenyl ring at N3 lies in this plane; on the contrary,
the phenyl ring at N1 as well as the tetrazole ring
attached to the methylide carbon tilt considerably. The
Our previous work demonstrated that carbene 1b has
a nucleophilic nature and hence is best expressed by the
singlet structures A and B. For example, although the
attempted reactions with alkyl halides or carbonyl com-
pounds were unsuccessful, 1b reacts readily with nitro-
gen electrophiles such as benzenediazonium and azido-
tetrazolium salts [4,5]. Here, we report further reactions
of 1b with electron-deficient tetrazine and alkene elec-
trophiles. Furthermore, the synthesis and characteriza-
tion of stable rhodium(I) complexes of 1b are disclosed
as examples of transition-metal complexes of mesoionic
carbenes.
˚
pivotal carbonAcarbon bond length is 1.406(3) A, indi-
cating a mixed nature of single and double bonds. Com-
pound 10 has a molecular formula C19H16N6O4, which
corresponds to a loss of N2 from the sum of 1b and 8.
1H and 13C NMR data show that 10 has a p-substituted
benzene ring and two inequivalent CO2Me groups. The
structure was eventually deduced to be 4-(p-phenylazo)
phenylhydrazone of a pyrazolone derivative. Compound
11 has a structure similar to 10, bearing an unsubstituted
phenylhydrazone moiety. An attempted reaction of 11
with benzene-diazonium salt did not give 10. This fact
suggests that 10 might be formed via an intramolecular
migration of the phenylazo group. The most plausible
mechanism for the formation of compounds 9, 10, and
11 is shown in Scheme 4.
RESULTS AND DISCUSSION
A. Reaction with 1,2,4,5-tetrazines. It is reported
that 1,2,4-triazol-3-ylidene reacts with 3,6-diphenyl-
1,2,4,5-tetrazine to give a [4þ1] cycloaddition product
[8]. We attempted the reaction of 1b with 3,6-diphenyl-
1,2,4,5-tetrazine. However, no reaction occurred and
only the degradation products 3 and 4 were obtained.
Next, dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate 8, a
more electron-poor tetrazine bearing the electron-with-
drawing substituents, was employed. The carbene 1b
A nucleophilic attack of 1b to the electron-deficient
carbon of tetrazine 8 followed by a ring-contraction
Table 1
The reaction of mesoionic carbene 1b with 8.
Yield (%)
Entry
Temperature
Solvent
t (h)
9
10
11
7
4
1
2
3
ꢀ20ꢁC to r.t.
ꢀ20ꢁC to r.t.
ꢀ60ꢁC to r.t.
DMA
DMF
DMF
3
3
4
Trace
2a
15a
6a
7a
4a
24a
37a
22a
8
3
2
18
5
8
a Determined by 1H NMR.
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S. Araki, K. Yokoi, R. Sato, T. Hirashita, and J.-I. Setsune
Vol 46
Scheme 3
Scheme 4
furnishes 9. Compounds 10 and 11 are considered to be
formed via the spiro intermediate C which is akin to the
product of the reaction of 1,2,4-triazol-3-ylidene and
diphenyltetrazine [8]. The cleavage of the tetrazole ring
of C followed by an intramolecular migration of the
phenylazo group (aza para-Claisen rearrangement) gives
10, whereas a loss of the phenylazo group furnishes 11.
Now it has been found that carbene 1b reacts readily
with the electron-deficient unsaturated compound. Then,
the reactions of 1b with alkenes with electron-withdraw-
ing groups were next undertaken.
warmed to room temperature. The reaction gave a com-
plex mixture of products, from which only cyanomethy-
lide 12 was isolated as stable yellow crystals in low
yield (9%) after column chromatography. The formation
of 12 is rationalized by Scheme 5. The initially formed
Michael adduct is considered to undergo a cyano group
migration to furnish the fully conjugated methylide 12.
C. Reaction with N-phenylmaleimide, fumalonitrile,
and dimethyl acetylenedicarboxylate. The reaction of
mesoionic carbene 1b with N-phenylmaleimide at –60ꢁC
to room temperature gave phenylated maleimides 13
and 14 together with cyanamide 15 (Scheme 6). To
determine the origin of the phenyl group of 13 and 14,
B. Reaction with tetracyanoethylene. A mixture of
the tetrazolium salt 2b and tetracyanoethylene was
treated with DBU at –60ꢁC, and the mixture was
Scheme 5
Figure 1. Molecular structure of methylide 9. [Color figure can be
viewed in the online issue, which is available at www.interscience.
wiley.com.]
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

March 2009 Mesoionic Carbenes: Reactions of 1,3-Diphenyltetrazol-5-ylidene with Electron-Deficient
Alkenes, and Synthesis and Catalytic Activities of the (Tetrazol-5-ylidene)rhodium(I) Complexes
167
Scheme 6
Scheme 8
1-(p-tolyl)-3-phenyltetrazolium 2b0 was employed in
place of 2b. The reaction gave 13 and 14, and the p-tol-
ylcyanamide adduct 150. p-Tolylcyanamide 40 itself was
also isolated (Scheme 7). On the basis of these results, a
plausible reaction mechanism is illustrated in Scheme 8.
Carbene 1b decomposes thermally to give benzendiazo-
nium cation and phenylcyanamide anion; the former is
considered to give phenyl radical [10] which reacts with
N-phenylmaleimide to furnish the phenylated com-
pounds 13 and 14. The phenylcyanamide anion gives
the adduct 15 via the Michael addition to N-phenyl-
maleimide.
The reactions of tetrazolium salt 2b with bis(1,5-
cyclooctadiene)-l,l0-dichlorodirhodium (20) in DMF
mediated by DBU were performed under various condi-
tions. Two products, namely, mono- 21 and bis(tetrazol-
5-ylidene)rhodium(I) complexes 22, were isolated as sta-
ble crystalline solids (Scheme 10, Table 2). When an
excess tetrazolium salt 2b was used 22 was formed
exclusively. Obviously, the cationic bis-carbene complex
22 is formed via the neutral mono-carbene complex 21.
Indeed, the reaction of 21 with 2b/DBU gave 22.
The molecular structures of the rhodium(I) complexes
21 and 22 were analyzed by X-ray crystallography
(Figs. 2 and 3). In the square planner complex 21, the
tetrazol-5-ylidene ring is twisted relative to the coordi-
nation plane. The bond angle C(carbene)-Rh-Cl is
The reaction of 1b with fumalonitrile also gave the
phenylation products 16, 17, and 18 in low yields, to-
gether with the degradation products 3 and 4. The reac-
tion with dimethyl acetylenedicarboxylate gave the addi-
tion product 19 in 29% yield via a similar mechanism to
the N-phenylmaleimide case (Scheme 9).
ꢁ
˚
90.59(8) . The C(carbene)-Rh length [2.017(3) A] is
almost coincident to that of (imidazol-2-ylidene)rhodium
D. Preparation and characterization of rhodium(I)
complexes of 1,3-diphenyltetrazol-5-ylidene. It is well
known that carbenes can be stabilized by complexation
with metals. Indeed, a large number of main group
metal as well as transition metal complexes of NHCs
have hitherto been prepared and characterized. Of such
NHCs, imidazol-2-ylidene [11] and 1,2,4-triazol-3-yli-
dene [12] are most extensively studied, and some imida-
zol-2-ylidene-metal complexes have been used as effec-
tive catalysts in organic syntheses [13]. On the other
hand, very few examples of mesoionic carbene-metal
complexes, such as mercury(II) and palladium(II) com-
plexes, have been synthesized by us [4]. Here, we
describe the preparation and characterization of rho-
dium(I) complexes of 1b as a new entry of mesoionic
carbene-metal complexes.
˚
complex [11] [2.023(2) A] and somewhat longer than
that of (1,2,4-triazol-3-ylidene)rhodium complex [12(a)]
˚
[2.004 (7) A]. The cationic bis(tetrazol-5-ylidene) com-
plex 22 is also square planner. The bond angle C(car-
bene)-Rh-C(carbene) is 90.47(13)ꢁ. The C(carbene)-Rh
˚
lengths [2.039(3) and 2.031(3) A] are longer than that
of 21, whereas they are shorter than the averaged length
˚
(2.047 A) of the corresponding (imidazol-2-ylidene)rho-
dium complex [11].
To evaluate the catalytic activities, the complexes 21
and 22 were subjected to the decarbonylative addition
reaction of benzoyl chloride to ethynylbenzene. The
reaction is known to give (Z)-1-chloro-1,2-diphenyle-
thene (23) by the catalysis of the (cod)2Rh2Cl2 (20)
Scheme 9
Scheme 7
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168
S. Araki, K. Yokoi, R. Sato, T. Hirashita, and J.-I. Setsune
Vol 46
Scheme 10
[14]. The mono(tetrazol-5-ylidene)Rh complex 21
(10 mol%) was found to be as effective as 20 to give 23
in 61% yield (Scheme 11). On the contrary, the bis
(tetrazol-5-ylidene) complex 22 is less reactive to give
lower yield (22%) of 23, probably owing to the steric
crowding around the rhodium atom.
Figure 2. Molecular structure of the (tetrazol-5-ylidene)rhodium
complex 21.
or a Varian Gemini 300 (300 MHz), and 13C NMR spectra
were obtained using a Varian Gemini-200 (50 MHz). Chemical
shifts are recorded in ppm downfield from tetramethylsilane.
J values are given in Hz. Mass spectra were taken with a Hita-
chi M-2000 spectrometer (EI, 70 eV). For TLC, Merck Silica
gel 60 F254 Plate and Fuji Silysia Chemical Ltd. NH TLC
Plate were used. For column chromatography, Merck Silica
gel 60 (0.063–0.200 mm) and Fuji Silysia Chemical Ltd. Chro-
matorex NH Chromatography Silica Gel (100–200 mesh) were
used.
Although no problems were hitherto encountered, it should
be noted that polyaza compounds may in general be explosive
and should be handled with due care.
Reaction with dimethyl 1,2,4,5-tetrazine-3,6-dicarboxy-
late (8).
CONCLUSION
The nucleophilic nature of 1,3-diphenyltetrazol-5-yli-
dene 1b was demonstrated in the reactions with the elec-
tron-deficient tetrazine and alkene to yield the new 5-tet-
razoliomethylide compounds 9 and 12. In the cases of
weak electrophiles, the degradation of the tetrazole ring
took place and the phenylated products were obtained.
Two (tetrazol-5-ylidene)rhodium(I) complexes 21 and 22
were synthesized via a nucleophilic displacement of the
chlorine atoms of 20 with 1b. Catalytic activities of 21
and 22 were investigated for the chloroarylation of ethy-
nylbenzene. Further catalytic properties of these new
rhodium complexes are now under way.
Typical procedure. To a solution of 2b (1.6 g, 5.0 mmol)
and 8 (0.99 g, 5.0 mmol) in dry DMF (20 mL), DBU
(0.75 mL, 5.0 mmol) was added at ꢀ60ꢁC, and the mixture
was warmed gradually to room temperature during a period of
4 h (Table 1, entry 3). Water was added and the precipitated
EXPERIMENTAL
Melting points were measured with a hot-stage apparatus
and are uncorrected. Elemental analyses were carried with a
Perkin–Elmer 2400 II CHNS/O. IR spectra were taken as KBr
discs on a JASCO A-102 spectrometer. Electronic spectra
were measured on a U-3500 spectrophotometer. 1H NMR
spectra were obtained using a Varian Gemini 200 (200 MHz)
Table 2
The reaction of tetrazolium salt 2b with 20.
Yield (%)
Entry 2b (mmol) 20 (mmol)
Conditions
21
22
1
2
3
4
5
0.50
0.25
0.25
0.25
0.50
0.25
0.25
0.25
0.25
0.13
ꢀ60ꢁC, 3 h
12 31
30 13
31 26
53 17
0ꢁC, 3 h
ꢀ60ꢁC, 3 h
ꢀ60ꢁC, 25 h
Figure 3. Molecular structure of the bis(tetrazol-5-ylidene)rhodium
complex 22. [Color figure can be viewed in the online issue, which is
available at www.interscience.wiley.com.]
ꢀ60ꢁC, 18 h Trace 99
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March 2009 Mesoionic Carbenes: Reactions of 1,3-Diphenyltetrazol-5-ylidene with Electron-Deficient
Alkenes, and Synthesis and Catalytic Activities of the (Tetrazol-5-ylidene)rhodium(I) Complexes
169
Scheme 11
129.2 (m-Ph), 135.8, 136.1 (i-Ph, C¼¼N), 152.1, 155.6, 162.7,
165.7 (CO2Me ꢂ 2, CACO2Me ꢂ 2); uv/vis (MeCN): kmax
(log e) ¼ 362.0 (3.44), 292.0 (4.21), 261 (3.96), 228 nm
(3.97); ms (EI, 70 eV): m/z 289 (Mþþ1, 18), 288 (Mþ, 100),
229 (41), 170 (77), 144 (82), 117 (72), 77 (73). Anal. Calcd.
for C13H12N4O4 (288.3): C, 54.16; H, 4.20; N, 19.44. Found
C, 54.00; H, 4.13; N, 19.01.
solid was filtered off. The products were extracted from the fil-
trate with dichloro-methane. The extracts were dried over an-
hydrous Na2SO4 and the solvent was removed under reduced
pressure. The residue (reddish brown solid, 1.0 g) was chroma-
tographed (SiO2/CH2Cl2:EtOAc gradient and MeOH) to give 4
(49 mg, 8%), 7 (26 mg, 2%), and a mixture of 9, 10, and 11
(0.72 g). On the basis of the 1H NMR analysis, the yields
were estimated to be 9 (15%), 10 (4%), and 11 (22%). Pure
samples of 9, 10, and 11 were obtained after repeated column
chromatography. The entries 1 and 2 were similarly carried
out by changing the solvent and the reaction temperature. The
results are summarized in Table 1.
Reaction with tetracyanoethylene. DBU (0.15 mL, 1.0
mmol) was added to a mixture of 2b (0.31 g, 1.0 mmol) and
tetracyanoethylene (0.13 g, 1.0 mmol) in DMF (3.0 mL) at
–60ꢁC. The mixture was gradually warmed to room tempera-
ture and allowed to stand at room temperature for 3 h. Water
was added and the black solid deposited was collected by fil-
tration (0.33 g). The solid was chromatographed (SiO2/
CH2Cl2-MeCN gradient and MeOH) to give 12 (32 mg, 9%),
7 (59 mg, 25%), together with two unknown compounds.
1H NMR and 13C NMR spectra of the unknown compounds
are very similar to those of 2. However, the structures of these
products could not be determined.
(Ethoxycarbonyl)(5-ethoxycarbonyltetrazol-2-yl)-a-(1,3-
diphenyl-5-tetrazolio)methylide (9). Yellow crystals, Mp.
179–184ꢁC (CH2Cl2/hexane); ir (KBr): 1744, 1680, 1552,
1490, 1380, 1362, 1346, 1238, 1224, 1186, 1118, 1072, 1040,
1022, 996, 898, 816, 758, 696, 682, 670/cm; 1H NMR (200
MHz, deuteriochloroform): d 3.58 (s, 3H, Me), 3.99 (s, 3H,
Me), 7.30–7.41 (m, 5H, Ph), 7.61–7.68 (m, 3H, Ph), 8.19 (dd,
2H, J ¼ 8.1 Hz, J ¼ 1.7 Hz, Ph); 13C NMR (50 MHz, deuter-
iochloroform): d 50.6 (Me), 52.8 (Me), 78.7 (Cꢀ), 120.7
(o-Ph), 124.8 (o-Ph), 129.1 (m-Ph), 129.9 (m-Ph), 130.6
(p-Ph), 132.6 (p-Ph), 133.2 (i-Ph), 135.1 (i-Ph); 156.5, 157.6,
158.0, 163.4 (CO2Me ꢂ 2, C ¼ N, Cþ); uv/vis (MeCN): kmax
(log e) ¼ 392.0 (3.16), 279.0 nm (4.48). ms (EI, 70 eV): m/z
393 (32, Mþþ1-N2), 293 (33), 248 (100), 231 (62), 219 (26),
169 (72), 131 (88), 119 (97), 101 (21). Anal. Calcd. for
C19H16N8O4 (420.4): C, 54.28; H, 3.84; N, 26.66. Found C,
53.94; H, 3.76; N, 26.39.
Dimethyl 4-(4-phenylazophenylhydrazono)-4H-pyrazole-
3,5-dicarboxylate (10). Orange crystals, Mp. 191–192.5ꢁC
(MeOH). ir (KBr): 2935, 2855, 1750, 1700, 1614, 1598, 1570,
1530, 1444, 1406, 1364, 1286, 1226, 1202, 1174, 1102, 1080,
1036, 844, 796, 770, 720, 684/cm. 1H NMR (200 MHz, deu-
teriochloroform): d 4.10 (s, 3H, Me), 4.15 (s, 3H, Me), 7.50–
7.54 (m, 3H, Ph), 7.91–8.02 (m, 6H, Ph), 10.70 (bs, 1H, NH).
13C NMR (50 MHz, deuteriochloroform): d 53.7 (Me), 53.9
(Me), 121.8 (o-Ph), 122.8, 124.0 (Ph), 129.0 (m-Ph), 131.0 (p-
Ph), 136.0 138.6 (i-Ph, C¼¼N), 149.6, 152.0, 152.5, 155.5,
162.6, 165.7 (CACO2Me ꢂ 2, CO2Me ꢂ 2, CAN¼¼NAC); uv/
vis (MeCN): kmax (log e) ¼ 449.0 (3.36), 353.0 (4.53), 288.0
(4.07), 235.0 nm (4.22); ms (EI, 70 eV): m/z 393 (Mþþ1, 17),
392 (Mþ, 69), 288 (19), 287 (100), 143 (10), 77 (56). Anal.
Calcd. for C19H16N6O4 0.5(CH3OH) (408.4): C, 57.73;
H, 4.04; N, 20.58. Found C, 57.36; H, 4.04; N, 20.65.
(Cyano)(tricyanomethyl)-a-(1,3-diphenyl-5-tetrazolio)-
methylide (12). Yellow crystals, Mp. 153–157ꢁC (acetone/
hexane); ir (KBr): 2940, 2180, 1592, 1560, 1486, 1466, 1456,
1364, 1342, 1292, 1268, 1176, 1072, 998, 842, 760, 720, 710,
1
688/cm; H NMR (200 MHz, deuteriochloroform): d 7.64–7.73
(m, 8H, Ph), 8.18 (dd, 2H, J1 ¼ 8.0 Hz, J2 ¼ 1.5 Hz, Ph). 13C
NMR (50 MHz, deuteriochloroform): d 30.5 [AC(CN)3], 38.6
(Cꢀ), 107.4 (CN ꢂ 3), 115.9 (CN), 120.6 (o-Ph), 126.4 (o-Ph),
129.8 (m-Ph), 130.4 (m-Ph), 131.2 (i-Ph), 132.6 (p-Ph), 133.2
(p-Ph), 134.7 (i-Ph), 157.2 (Cþ); uv/vis (MeCN): kmax (log e)
¼ 379.0 (3.68), 276.0 nm (4.31); ms (EI, 70 eV): m/z 351
(Mþþ1, 3), 350 (Mþ, 10), 324 (9), 270 (10), 245 (3), 194
(60), 179 (40), 152 (23), 128 (100), 118 (67), 103 (32). Anal.
Calcd. for C19H10N8 (350.35): C, 65.13; H, 2.88; N, 31.99.
Found C, 64.94; H, 2.99; N, 32.18.
Reaction with N-phenylmaleimide. To a mixture of 2b
(0.31 g, 1.0 mmol) and N-phenylmaleimide (0.17 g, 1.0 mmol)
in DMF (3.0 mL) was added DBU (0.15 mL, 1.0 mmol) at
–60ꢁC, and the mixture was gradually warmed to room tem-
perature in a period of 1.5 h, and then further stirred at room
temperature for 1.5 h. Water was added and the products were
extracted with dichloromethane. The extracts were dried over
anhydrous Na2SO4 and the solvent was removed under reduced
pressure. The residue (0.32 g) was chromatographed (SiO2/
CH2Cl2:hexane ¼1:1 to MeOH) to give 13 (37 mg, 11%), 14
(22 mg, 9%), 15 (57 mg, 20%) and 7 (4 mg, 2%).
1,3,4-Triphenyl-1H-pyrrole-2,5-dione (13). Yellow crys-
tals, Mp. 174–176ꢁC (EtOH) (lit Mp. 175ꢁC) [15].
1,3-Diphenyl-1H-pyrrole-2,5-dione (14). Pale yellow crys-
tals, Mp. 115–120ꢁC (EtOH) (lit Mp. 117ꢁC) [16].
(2,5-Dioxo-1-phenylpyrrolidin-3-yl)phenylcyanamide (15).
Colourless crystals; Mp. 138.5–140.5ꢁC (acetone/hexane); ir
(KBr): 2220, 1720, 1596, 1498, 1396, 1380, 1370, 1250, 1186,
1
Dimethyl 4-(phenylhydrazono)-4H-pyrazole-3,5-dicarbox-
ylate (11). Blass crystals; Mp. 159.5–162ꢁC (MeOH); ir
(KBr): 3280–2800, 1740, 1698, 1618, 1578, 1528, 1496, 1450,
1366, 1338, 1292, 1230, 1208, 1178, 1076, 1034, 962, 850–
820, 772, 740, 730, 698/cm; 1H NMR (200 MHz, deuterio-
chloroform): d 4.07 (s, 3H, Me), 4.13 (s, 3H, Me), 7.20–7.27
(m, 1H, p-Ph), 7.40–7.48 (m, 2H, m-Ph), 7.77–7.81 (m, 2H o-
Ph), 10.47 (bs, 1H, NH); 13C NMR (50 MHz, deuteriochloro-
form): d 53.5 (Me), 53.8 (Me), 121.6 (o-Ph), 125.7 (p-Ph),
748, 700, 684/cm; H NMR (200 MHz, deuteriochloroform): d
3.25 (dd, J ¼ 18.4 Hz, J ¼ 6.3 Hz, 1H, trans-CH2), 3.46 (dd,
J ¼ 18.4 Hz, J ¼ 9.1 Hz, 1H, cis-CH2), 4.92 (dd, J¼9.1 Hz,
J ¼ 6.3 Hz, 1H, CH), 7.21–7.55 (m, 10H, Ph); 13C NMR
(50 MHz, deuteriochloroform): d 35.6 (CH2), 59.1 (CH), 112.7
(CN), 120.0 (o-Ph), 127.4 (p-Ph), 127.9 (o-Ph), 130.8 (p-Ph),
130.9 (m-Ph), 131.7 (m-Ph), 132.6 (i-NPh), 140.9 (i-NCNPh),
172.9, 173.2 (C¼¼O); ms (EI, 70 eV): m/z 292 (Mþþ1, 21),
291 (Mþ, 100), 173 (63), 144 (90), 129, (17), 118 (56), 104
Journal of Heterocyclic Chemistry
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170
S. Araki, K. Yokoi, R. Sato, T. Hirashita, and J.-I. Setsune
Vol 46
(70), 91 (65), 77 (81). Anal. Calcd. for C17H13N3O2 (291.304):
C, 70.09; H, 4.50; N, 14.43. Found C, 70.12; H, 4.51; N,
14.47.
The reaction of 3-phenyl-1-p-tolyltetrazolium salt 2b0 and
N-phenylmaleimide was similarly conducted as above to yield
13 (54 mg, 17%), 14 (26 mg, 10%), 150 (70 mg, 23%), and 40
(29 mg, 22%).
1436, 1270, 1222, 1066, 1014, 894, 774, 760, 692/cm; 1H
NMR (200 MHz, deuteriochloroform, E isomer): d 3.77
(s, 3H, Me), 3.81 (s, 3H, Me), 7.00 (s, 1H, ¼¼CHCO2Me),
7.06–7.11 (m, 2H, o-Ph), 7.16–7.23 (m, 1H, p-Ph), 7.34–7.42
(m, 2H, m-Ph); 13C NMR (50 MHz, CDCl3, E isomer): d 52.7
(Me), 53.6 (Me), 117.4 (o-Ph), 125.5, 126.1 (C¼¼CHCO2Me,
p-Ph), 129.7 (m-Ph), 136.3, 139.0 (C¼¼CH, i-Ph), 162.2
(CO2Me), 162.5 (CO2Me); uv/vis (MeCN, E isomer): kmax
(log e) ¼ 317.0 (3.37), 227 nm (4.23); ms (EI, 70 eV, E/Z
mixture): m/z (%): 261 (Mþþ1, 18), 260 (Mþ, 100), 245 (25),
229 (37), 201 (61), 200 (33), 169 (27), 157 (52), 142 (28), 115
(30). Anal. Calcd. for C13H12N2O4 (288.27): C, 59.99; H, 4.65;
N, 10.77. Found C, 60.30; H, 4.70; N, 10.88.
Reaction of tetrazolium salt 2b with bis(1,5-cycloocta-
diene)-l,l0-dichlorodirhodium (20). Typical procedure (Table
2, entry 1): A mixture of 2b (0.16 g, 0.50 mmol) and bis
(1,5-cyclooctadiene)-l,l0-dichlorodirhodium (20) (0.13 g,
0.25 mmol) in dry DMF (10 mL) was cooled to –60ꢁC. DBU
(75 lL, 0.5 mmol) was added and the mixture was stirred at
ꢀ60ꢁC for 3 h. Cold water was added and the precipitated
solid (0.13 g) was filtered. The filtrate was extracted with
CH2Cl2, washed with water, and dried (Na2SO4). The solvent
was evaporated and the residue (90 mg) was chromatographed
(SiO2/CH2Cl2! MeOH) to give bis(1,5-cyclooctadiene)-l,l0-
dichlorodirhodium (16 mg, 12% recovery), 21 (29 mg, 12%),
and 22 (16 mg). The precipitated solid was purified by column
chromatography (SiO2/CH2Cl2!MeOH) to give 22 (43 mg).
The total yield of 22 is 59 mg (31%). Other reactions
were conducted similarly, and the results are summarized in
Table 2.
Complex 21. Yellow crystals, Mp. 165ꢁC (EtOH); ir (KBr):
2940, 2860, 1488, 1362, 1230, 1140, 1014, 762, 680/cm;
1H NMR (200 MHz, deuteriochloroform): d 1.86–1.95 (m, 4H,
cod-CH2), 2.32–2.37 (m, 4H, cod-CH2), 3.35 (s, 2H, cod-CH),
5.17 (s, 2H, cod-CH), 7.59–7.67 (m, 6H, Ph), 8.21–8.26 (m,
2H, Ph), 8.74–8.78 (m, 2H, Ph); 13C NMR (50 MHz, deuterio-
chloroform): d 29.0, 32.7 (cod-CH2), 69.7 (d, J (103Rh-13C) ¼
14.3 Hz, cod-CH), 98.7 (d, J (103Rh-13C) ¼ 7.2 Hz, cod-CH),
120.6, 123.9 (o-Ph), 128.9, 129.7 (m-Ph), 130.2, 131.6 (p-Ph),
135.0, 135.9 (i-Ph), 187.7 (d, J (103Rh-13C) ¼ 51.6 Hz, Rh-C);
uv/vis (MeCN): kmax (log e) ¼ 280.5 (3.20), 384.0 nm (2.23).
Anal. Calcd. for C21H22ClN4Rh (468.77): C 53.80, H 4.69, N
11.95; found C 53.70, H 4.60, N 11.90.
(2,5-Dioxo-1-phenylpyrrolidin-3-yl)-p-tolylcyanamide (150).
Colourless crystals, Mp. 140–142ꢁC (acetone/hexane); ir
(KBr): 3075, 2935, 2855, 2210, 1716, 1512, 1398, 1370, 1250,
1176, 818, 810, 740, 700/cm; 1H NMR (200 MHz, deuterio-
chloroform): d 2.36 (s, 3H, Me), 3.22 (dd, J ¼ 18.5 Hz, J ¼
6.4 Hz, 1H, trans-CH2), 3.43 (dd, J ¼ 18.5 Hz, J ¼ 9.2 Hz,
1H, cis-CH2), 4.86 (dd, J ¼ 9.2 Hz, J ¼ 6.4 Hz, 1H, CH),
7.21–7.35 (m, 6H, Ph), 7.44–7.51 (m, 3H, Ph); 13C NMR (50
MHz, deuteriochloroform): d 20.7 (Me), 33.8 (CH2), 57.9
(CH), 111.6 (CN), 118.9 (Ph), 126.2 (Ph), 129.0 (p-Ph), 129.2
(Ph), 130.5 (Ph), 130.9 (i-NPh), 135.9, 136.6 (i-NCNPh,
CAMe), 171.3, 171.6 (C¼¼O); ms (EI, 70 eV): m/z 306
(Mþþ1, 12), 305 (Mþ, 55), 173 (100), 158 (42), 132 (9), 131
(90), 91 (78). Anal. Calcd. for C18H15N3O2 (305.33): C, 70.80;
H, 4.95; N, 13.77. Found C, 70.46; H, 4.78; N, 13.64.
p-Tolylcyanamide (40). Oil; ir (KBr): 3175–2900, 2215,
1
1704, 1614, 1594, 1514, 1398, 1310, 1282, 1248, 804/cm; H
NMR (200 MHz, deuteriochloroform): d 2.31 (s, 3H, Me),
6.31 (bs, 1H, NH), 6.91 (d, J ¼ 8.5 Hz, 2H, Ph), 7.13 (d, J ¼
8.5 Hz, 2H, Ph) [17].
Reaction with fumaronitrile. To a mixture of 2b (0.31 g,
1.0 mmol) and fumaronitrile (0.78 g, 1.0 mmol) in DMF (3.0
mL) was added DBU (0.15 mL, 1.0 mmol) at –60ꢁC, and the
mixture was gradually warmed to room temperature in a pe-
riod of 1.5 h, and then further stirred at room temperature for
1.5 h. Water was added and the products were extracted with
dichloromethane. The extracts were dried over anhydrous
Na2SO4 and the solvent was removed under reduced pressure.
The residue (0.23 g) was chromatographed (SiO2/CH2Cl2:
hexane ¼ 2:3!MeOH) to give 16 (21 mg, 9%), 17 (5 mg,
3%), 18 (11 mg, 7%) and 7 (18 mg, 8%). A mixture of 3 and
4 (42 mg, 3:4 ¼ 31:69) was also obtained.
2,3-Diphenylfumalonitrile (16). Colourless crystals, Mp.
157–159ꢁC (hexane) (lit Mp. 160–161ꢁC) [18].
2-Phenylmaleonitrile (17). Colourless crystals, Mp. 37–
43ꢁC (lit Mp. 40.5–41ꢁC) [18].
2-Phenylfumalonitrile (18). Colourless crystals, Mp. 86–
91ꢁC (hexane) (lit Mp. 87.5–88ꢁC) [18].
Reaction with dimethyl acetylenedicarboxylate. To a mix-
ture of 2b (0.31 g, 1.0 mmol) and dimethyl acetylene-dicar-
boxylate (DMAD) (0.25 mL, 2.0 mmol) in DMF (2.0 mL) was
added DBU (0.15 mL, 1.0 mmol) at –60ꢁC, and the mixture
was gradually warmed to room temperature in a period of 1.5
h, and then further stirred at room temperature for 1.5 h. Water
was added and the products were extracted with dichlorome-
thane. The extracts were dried over anhydrous Na2SO4 and the
solvent was removed under reduced pressure. The residue
(0.57 g) was chromatographed (SiO2/CH2Cl2-EtOAc gradient
and MeOH) to give 19 (75 mg, 29% yield, E/Z mixture ¼ 3:1)
and 7 (18 mg, 8% yield). DMAD was recovered (53 mg,
recovery 19%).
Complex 22. Yellow crystals, Mp. 152–154ꢁC (EtOH); ir
(KBr): 1590, 1492, 1468, 1358, 1310, 1230, 1124, 1096, 1038,
1
1010, 766, 684/cm; H NMR (200 MHz, deuteriochloroform):
d 2.22–2.44 (m, 8H, cod-CH2), 4.65 (s, 4H, cod-CH), 7.51–
7.63 (m, 12H, Ph), 7.96–8.01 (m, 4H, Ph), 8.40–8.45 (m, 4H,
Ph). 13C NMR (50 MHz, deuteriochloroform): d 31.1 (cod-
CH2), 90.7 (d, J (103Rh-13C) ¼ 8.0 Hz, cod-CH), 120.6, 123.6
(o-Ph), 129.3, 129.9 (m-Ph), 130.8, 132.0 (p-Ph), 134.7, 135.2
(i-Ph), 185.3 (d, J (103Rh-13C) ¼ 54.8 Hz, Rh-C); uv/vis
(MeCN): kmax (log e) ¼ 282.0 (3.37), 409.5 nm (2.21). Anal.
Calcd. for C34H32BF4N8Rh (742.01): C, 55.03; H, 4.31; N,
15.09. Found C, 54.86; H, 4.26; N, 14.96.
Reaction of tetrazolium salt 2b with complex 21. A mix-
ture of 2b (22 mg, 0.070 mmol) and 21 (33 mg, 0.070 mmol)
in dry DMF (1 mL) was cooled to –60ꢁC. DBU (11 lL, 0.07
mmol) was added and the mixture was stirred at –60ꢁC for 4
h. Cold water was added and the precipitated solid (0.16 g)
was filtered. The product was purified by column
Dimethyl 2-(phenylcyanamido)-2-butenedicarboxylate (19).
Pale yellow crystals; Mp. 77–85.5ꢁC (CH2Cl2/hexane); ir
(KBr, E/Z mixture): 2230, 1736, 1722, 1640, 1594, 1492,
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

March 2009 Mesoionic Carbenes: Reactions of 1,3-Diphenyltetrazol-5-ylidene with Electron-Deficient
Alkenes, and Synthesis and Catalytic Activities of the (Tetrazol-5-ylidene)rhodium(I) Complexes
171
[4] Araki, S.; Wanibe, Y.; Uno, F.; Morikawa, A.; Yamamoto,
K.; Chiba, K.; Butsugan, Y. Chem Ber 1993, 126, 1149.
[5] Araki, S.; Yamamoto, K.; Yagi, M.; Inoue, T.; Fukagawa,
H.; Hattori, H.; Yamamura, H.; Kawai, M.; Butsugan, Y. Eur J Org
Chem 1998, 121.
chromatography (SiO2/CH2Cl2) to give 22 (23 mg, 44%). A
small amount of complex 21 was recovered (11 mg, 33%).
Reaction of ethynylbenzene and benzoyl chloride cata-
lyzed by 21 or 22. A mixture of ethynylbenzene (0.33 mL,
3.0 mmol), benzoyl chloride (0.23 mL, 2.0 mmol), triphenyl-
phosphine (5.2 mg, 0.020 mmol), and 21 (9.4 mg, 0.020
mmol) in octane (5 mL) was heated overnight at 140ꢁC.
2-Methylnaphthalene (20 mg) was added as an internal stand-
ard, and the reaction mixture was analyzed by glc. The yield
of the product (23) was estimated to be 61%. The mixture was
chromatographed (SiO2/hexane) and 23 was isolated (0.18 g,
43%). The reaction with 22 was similarly carried out and the
yield of 23 was 22% (glc).
[6] The structure of 3 was previously reported errornously as 1-
cyano-1,3-diphenyltriazene [4].
[7] Lowack, R. H.; Weiss, R. J Am Chem Soc 1990, 112, 333.
[8] Enders, D.; Breuer, K.; Runsink, J.; Teles, J. H. Liebigs
Ann Chem 1996, 2019.
[9] For other 5-tetrazoliomethylides; see: (a) Araki, S.; Mizuya,
J.; Butsugan, Y. J Chem Soc Perkin Trans1,1985, 2439; (b) Araki, S.;
Kuzuya, M.; Hamada, K.; Nogura, M.; Ohata, N. Org Biomol Chem
2003, 1, 978.
X-ray crystallography. X-ray analysis of 9 was performed
on a sample recrystallized from CH2Cl2/hexane. Complex 21
was recrystallized from ethanol, and 22 from a mixture of
CH2Cl2:EtOAc:hexane ¼ 1:9:1.
[10] (a) Rieker, A. Inst Chim Belge 1971, 36, 1078; (b)
Cadogan, J. I. G.; Murray, C. D.; Sharp, J. T. J Chem Soc 1973, 16, 572.
¨
[11] Herrmann, W. A.; Elison, M.; Fischer, J.; Kocher, C.;
Artus, G. R. J. Chem Eur J 1996, 2, 772.
[12] (a) Ender, D.; Gielen, H. J Organomet Chem 2001, 70, 617;
(b) Buron, C.; Stelzig, L.; Guerret, O.; Gornitzka, H.; Romanenko, V.;
Bertrand, G. J Organomet Chem 2002, 640, 70.
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Journal of Heterocyclic Chemistry
DOI 10.1002/jhet