New cleavage of the azirine ring by single electron transfer: The synthesis of 2H-imidazoles, pyridazines and pyrrolines

Article abstract of DOI:10.1002/1099-0690(200103)2001:6<1183::AID-EJOC1183>3.0.CO;2-G

Here we report the first example of the dimerisation of azirines to 2H-imidazoles or 3,5-disubstituted pyridazines from a reaction promoted by FeCl₂. An azirine complex with a radical structure is proposed as an intermediate. Cyclopropyl ketones and pyrrolines are isolated when the reaction is carried out in the presence of styrenes.

Full text of DOI:10.1002/1099-0690(200103)2001:6<1183::AID-EJOC1183>3.0.CO;2-G

FULL PAPER
New Cleavage of the Azirine Ring by Single Electron Transfer: The Synthesis
of 2H-Imidazoles, Pyridazines and Pyrrolines
Sergio Auricchio,*[a] Simona Grassi,[a] Luciana Malpezzi,[a][‡] Attilio Sarzi Sartori,[a] and
Ada M. Truscello[a]
Keywords: Nitrogen heterocycles / Cyclizations / Iron / Halides / Ketones / Electron transfer
Here we report the first example of the dimerisation of azir-
ines to 2H-imidazoles or 3,5-disubstituted pyridazines from a
reaction promoted by FeCl2. An azirine complex with a rad-
ical structure is proposed as an intermediate. Cyclopropyl ke-
tones and pyrrolines are isolated when the reaction is carried
out in the presence of styrenes.
Introduction
Results and Discussion
The cleavage of an azirine ring gives intermediates that
3-Aryl-2H-azirines (1a؊c) were treated with anhydrous
can lead to cycloaddition reactions. It is possible to obtain FeCl2 in dry acetonitrile at room temperature for 24 h. The
reactive intermediates by cleavage of each of the three reaction gave 2H-imidazoles 2, pyridazines 3 and pyrazines
bonds of azirine.[1] Photolysis and thermolysis, as well as 4, together with arylketones 5 and traces of α-chloroketones
the use of metal carbonyls, silver salts and acids have been 6 (Scheme 1).
applied to open the azirine ring, obtaining a lot of reactive
intermediates (e.g. nitrile ylides, vinyl nitrenes and various parison with an authentic sample, or on the basis of analyt-
organometallic complexes.)[1]
ical and spectral data. The molecular structure of the 2H-
In general, when a self-condensation occurs, the dimers imidazole 2a was confirmed by X-ray analysis (Figure 1).
obtained are derivatives of a pyrazine ring. In one case, a The distribution of products and yields for 2؊4 are listed
bromo-dimeric 2H-imidazole derivative from the reaction in Table 1.
The structures of the products were elucidated by com-
of azirine with CuBr2 was obtained. Although the interme-
The reaction affords 2H-imidazole 2 when performed in
diate was isolated, no mechanism for its formation was sug- dilute solution or pyridazine 3 in concentrated solution.
gested.[2]
These compounds were not obtained when methanol was
Recently, we proved that single electron transfer is a use- used as the solvent, where pyrazine 4 was the only dimer
ful way to open heterocyclic systems under very mild condi- present along with various unidentified compounds. When
tions.[3] Extending our study to azirines we found that iron FeCl2·4H2O was used instead of the anhydrous salt, a de-
dichloride is able to cleave the carbon-nitrogen single bond crease in the yield of 2H-imidazole 2 and an increase in the
in the azirine ring. The radical intermediate obtained reacts yield of by-products 5 and 6 was observed.
with: a) another molecule of azirine to give 2H-imidazoles
The structures of the products obtained (2؊6) in reac-
and 2,5-substituted pyridazines; b) styrene, if added as a tions with FeCl2 provide evidence that the azirine ring is
reagent, to give new intermolecular cycloadditions with the cleaved at the carbon-nitrogen single bond. This cleavage
formation of pyrrolidines.
can generate vinyl nitrenes as intermediates, and when it
occurs, only pyrazines were produced.[1]
Scheme 1. Products from reaction of azirines with FeCl2
It is possible that FeCl2 acts as a Lewis acid to promote
this ring cleavage. A reaction of azirine with TiCl4 as the
Lewis acid was previously reported[4] and again pyrazines
were obtained. In this case a zwitterionic intermediate was
[a]
Dipartimento di Chimica, Politecnico di Milano,
Via Mancinelli 7, 20131 Milano, Italy
[‡]
Crystal structure analysis.
Eur. J. Org. Chem. 2001, 1183Ϫ1187
WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001
1434Ϫ193X/01/0306Ϫ1183 $ 17.50ϩ.50/0
1183

S. Auricchio, S. Grassi, L. Malpezzi, A. S. Sartori, A. M. Truscello
FULL PAPER
Products 2, 3 and 5 can be formed by 9, as shown in
Scheme 3.
Figure 1. Molecular structure of 2a with the atom numbering
scheme; displacement ellipsoids correspond to 30% probability; H
atoms are shown as spheres of arbitrary radius
[a]
Table 1. Yields (%) of products from reaction of 1 with FeCl2
Scheme 3. Proposed mechanism for the reaction azirine-FeCl2
1
Ar
Solvent
2
3
4
The radical structure 9 can react by routes a or b:
(a) addition to azirine 1 to give 10, followed by cyclization
to the bicyclic radical 11 which in turn gives 3 by carbon-
nitrogen bond cleavage, loss of hydrogen and hydrolysis;
(b) abstraction of a hydrogen atom from the reaction me-
dium or reduction by FeCl2, to give complex 12, followed
by addition to azirine 1 to give intermediate 13 and cycliza-
tion to compound 14, which can then afford 2 by loss of a
hydrogen atom and hydrolysis. Route b is preferred in dilute
solution, where the rate of addition between 9 and azirine
becomes lower and the amount of FeCl2 in solution is
higher.
In order to find experimental evidence for the radical in-
termediate we performed the reaction in the presence of
styrene or α-methylstyrene. The reaction of azirine 1a with
an excess of styrene 15a or α-methylstyrene 15b in the pres-
ence of iron dichloride gave pyrrolines 16, phenyl (trans-2-
phenylcyclopropyl) ketones 17 and hydroxy ketones 18 to-
gether with acetophenone 5a, phenacyl chloride 6a, 2H-im-
idazole 2a and pyridazine 3a (Scheme 4).
a
a
a
b
c
C6H5
acetonitrile[b]
acetonitrile[c]
methanol[c]
32
5
Ϫ
20
10
2
4
C6H5
30
Ϫ
10
26
traces
23
C6H5
4F-C6H4
4CH3-C6H4
acetonitrile[b]
acetonitrile[b]
Ϫ
4
[a] The reactions were performed with an equimolar amount of an-
hydrous FeCl2 (heterogeneous phase). Ϫ 0.1 . Ϫ 0.4 .
[b]
[c]
proposed. We verified the reactivity of Lewis acids towards
azirines using FeCl3 and AlCl3. The product obtained was
the pyrazine 4, while no traces of 2H-imidazole 2 or pyrida-
zine 3 were found.
We think that iron dichloride is able to induce the forma-
tion of a radical intermediate as in the reaction of isox-
azoles.[3] The initial step in the azirine-FeCl2 reaction is
probably an N-donor complexation that gives intermediate
7, followed by carbon-nitrogen cleavage to give the zwitter-
ionic structure 8 or radical structure 9 by single electron
transfer (Scheme 2).
Pyrrolines 16a؊b and compounds 17b[15] and 18b were
characterised for the first time, whereas the structures of
the cyclopropyl derivative 17a and of the hydroxy ketone
18a were assigned by comparison with literature data.
When the reaction was carried out with FeCl2·4H2O a large
increase in the yield of hydroxy ketones 18 was obtained
(Table 2).
The stereoselective formation of 17 is in agreement with
a radical mechanism as the formation of cyclopropyl deriv-
atives in radical reactions is well known.[5] Radical 9 un-
dergoes attack by styrene to give the tautomeric interme-
diates 19a and 19b (Scheme 5).
Compound 17 arises from an intramolecular reaction be-
tween the benzylic radical and the carbon-carbon double
bond of the enaminic structure 19b, while pyrroline 16 is
Scheme 2. Formation of the azirine-FeCl2 complex
Structure 8 could justify the formation of the by-product formed from the intramolecular attack of the benzylic rad-
pyrazine 4 but not 2H-imidazoles 2 and pyridazines 3. ical on the CϭN in structure 19a. The hydroxy ketone 18
1184
Eur. J. Org. Chem. 2001, 1183Ϫ1187

Cleavage of the Azirine Ring by Single Electron Transfer
FULL PAPER
Scheme 4. Products from reaction of azirine 1a with styrene and α-methylstyrene
Table 2. Yields (%) of products from the reaction of 1a with 15a,b the imidazole CϭN bonds and are consistent with the
3
C
aromϪCϭNϪCsp sequence.[7]
R
Promoter
16
17
18
2a
3a
H
FeCl2
20
Ϫ
13
5
5
Ϫ
29
8
23
29
Ϫ
10
Ϫ
Conclusion
H
FeCl2·4H2O
FeCl2
11
9
10
Me
Me
9
FeCl2·4H2O
5
60
traces
This work has shown that iron dichloride is able to in-
duce ring opening in an aza-heterocyclic ring by single elec-
tron transfer.[3] This new type of bond cleavage of azirines
leads to a novel synthesis of 2H-imidazoles and pyridazines.
Furthermore, the complex azirine-FeCl2 could be a very in-
teresting synthon in cycloaddition reactions as shown in the
reaction with styrenes.
Experimental Section
NMR spectra were determined with TMS as internal standard on
a Bruker 250 MHz instrument. MS spectra were performed with a
Finnigan TSQ 70 instrument. Chromatographic separations were
performed using Merck Kieselgel 60. Melting points were deter-
mined with a Kofler apparatus and are uncorrected. GLC analyses
were performed with a capillary gas chromatograph equipped with
a DB-5 fused silica column (30 m ϫ 0.25 mm i.d., 0.25 µm film
thickness), PTV injector, flame ionisation detector, oven temper-
ature programme 50°(1) /10°/min 160°(1) /20°/min 280°(10). The
azirines were prepared from the corresponding azidostyrenes.[8]
Products given as traces were detected by GLC analyses.
Scheme 5. Proposed mechanism for the reaction with styrene
Reaction of 3-Phenyl-2H-azirine (1a) with FeCl2 (0.4
M): Anhydrous
comes from oxidation and reaction with traces of water that
are in the reaction medium.
iron dichloride (80 mesh; 0.635 g, 5.0 mmol) was added to a stirred
degassed solution of azirine 1a (0.58 g, 5.0 mmol) in anhydrous
CH3CN (12.5 mL) and the mixture stirred overnight. The solvent
was removed, the crude product dissolved in dichloromethane and
Structural Investigation
The X-ray single crystal analysis of 2a has been carried washed with an aqueous solution of HCl (0.1 ), aqueous
NaHCO3 and water. The residue was purified by column chromato-
graphy (eluent: n-hexane/ethyl acetate from 95:5 to 50:50) to give
acetophenone (5a), phenacyl chloride (6a) and 2,5-diphenylpyraz-
ine (4a) in trace amounts, 2-methyl-2,4-diphenyl-2H-imidazole (2a;
0.030 g, 5%), and 3,5-diphenylpyridazine (3a; 0.125 g, 30%).[10]
out in order to characterise the molecular structure of the
product unambiguously. A perspective view of the molecule
is presented in Figure 1. It is noticeable that the racemic
compound crystallises in a chiral space group with the two
enantiomers coexisting as independent molecules. These
molecules are correlated to each other by a noncrystallo-
graphic, and therefore approximate, centre of symmetry.
The present structure is one of the few compounds con-
taining the 2H-imidazole moiety reported in the Cambridge
Structural Database[6] and, to the best of our knowledge, it
is the first one containing the 2H-imidazole ring not fused
with other cyclic fragments. The 2H-imidazole rings are
2-Methyl-2,4-diphenyl-2H-imidazole (2a): Needles, m.p. (n-hexane)
1
89Ϫ91 °C. Ϫ H NMR (CDCl3): δ ϭ 1.85 (s, 3 H), 7.20Ϫ7.40 (m,
3 H), 7.44Ϫ7.58 (m, 3 H), 7.66Ϫ7.76 (m, 2 H), 8.00Ϫ8.10 (m, 2
H), 8.47 (s, 1 H). Ϫ 13C NMR (CDCl3): δ ϭ 27.0, 109.1, 126.9,
127.7, 128.3, 129.1, 131.1, 131.5, 140.1, 154.4, 163.8. Ϫ MS: m/z
(%) ϭ 234 (92) [Mϩ] 207 (100), 206 (40). Ϫ C16H14N2 (234.3):
calcd. C 82.02, H 6.02, N 11.96; found C 82.11, H 6.05, N 11.89.
3,5-Diphenylpyridazine (3a):[9] 1H NMR (CDCl3): δ ϭ 7.45Ϫ7.64
(m, 6 H), 7.66Ϫ7.78 (m, 2 H), 7.98 (d, J ϭ 2 Hz, 1 H), 8.08Ϫ8.20
(m, 2 H), 9.40 (s, 1 H).
˚
planar within experimental error (r.m.s. deviation 0.003 A)
and make mean dihedral angles of 57.1(2)° and 11.6(3)°
with the C5ϪC10 and C11ϪC16 phenyl rings, respectively.
The C2ϭN1 and C3ϭN2 bond lengths [mean value
Reaction of 3-Phenyl-2H-azirine (1a) with FeCl2 (0.1
M): The same
˚
˚
1.287(7) A] are shorter than the average of 1.314(11) A for procedure as above was used but with 50 mL of solvent. Aceto-
Eur. J. Org. Chem. 2001, 1183Ϫ1187 1185

S. Auricchio, S. Grassi, L. Malpezzi, A. S. Sartori, A. M. Truscello
FULL PAPER
phenone (5a) and phenacyl chloride (6a) were obtained in trace azirine 1a (3.00 g, 26 mmol) in CH3CN (7 mL) at a rate of 2 mL
amounts along with 2,5-diphenylpyrazine (4a; 0.024 g, 4%),[10] 2-
methyl-2,4-diphenyl-2H-imidazole (2a; 0.19 g, 32%) and 3,5-di-
phenylpyridazine (3a; 0.012 g, 2%).
h
Ϫ1, and the mixture stirred overnight. The solvent was removed,
the crude product dissolved in dichloromethane and washed with
an aqueous solution of HCl (0.1 ), aqueous NaHCO3 and water.
The residue was purified by column chromatography (eluent: n-
hexane/ethyl acetate from 95:5 to 50:50) to give phenyl-(trans-2-
phenylcyclopropyl)methanone (17a; 0.27 g, 5%), acetophenone (5a;
0.12 g, 4%), 2,5-diphenylpyrroline (16a; 1.15 g, 20%), 2-methyl-2,4-
diphenyl-2H-imidazole (2a; 0.68 g, 23%) and 3,5-diphenylpyridaz-
ine (3a; 0.86 g, 29%).
Reaction of 3-(4-Fluorophenyl)-2H-azirine (1b) with FeCl2: Anhyd-
rous iron dichloride (80 mesh; 0.51 g, 4 mmol) was added to a
stirred, degassed solution of azirine 1b (0.50 g, 3.7 mmol) in anhyd-
rous CH3CN (37 mL), and the mixture stirred overnight. The solv-
ent was removed, the crude product dissolved in dichloromethane
and washed with an aqueous solution of HCl (0.1 ), aqueous
NaHCO3 and water. The residue was purified by column chroma-
tography (eluent: n-hexane/ethyl acetate from 95:5 to 50:50) to give
4-fluoroacetophenone (5b) in traces, 2,4-bis(4-fluorophenyl)-2-
methyl-2H-imidazole (2b; 0.10 g; 20%) and 3,5-bis(4-fluoro-
phenyl)pyridazine (3b; 0.05 g, 10%).
Phenyl-(trans-2-phenylcyclopropyl)methanone (17a):[13a][13b] Oil. Ϫ
1H NMR (CDCl3): δ ϭ 1.55 (m, 1 H), 1.92 (m, 1 H), 2.70 (m, 1
H), 2.90 (m, 1 H), 7.10Ϫ7.60 (m, 8 H), 7.99 (m, 2 H). Ϫ 13C NMR
(CDCl3): δ ϭ 19.2, 29.2, 29.9, 126.1, 126.5, 128.0, 128.5, 132.8,
137.6, 140.4, 198.4.
2,5-Diphenylpyrroline (16a): Oil. Ϫ 1H NMR (CDCl3): δ ϭ 1.89
(m, 1 H), 2.59 (m, 1 H), 2.90Ϫ3.30 (m, 2 H), 5.31 (m, 1 H),
7.18Ϫ7.50 (m, 8 H), 7.95 (m, 2 H). Ϫ 13C NMR (CDCl3): δ ϭ 32.3,
35.4, 75.8, 126.4, 126.7, 127.8, 128.3, 130.5, 134.3, 144.4, 173.6. Ϫ
MS: m/z ϭ 221 (49) [Mϩ], 220 (35), 193 (71), 192 (32), 165 (25),
118 (31), 117 (69), 115 (51), 89 (100).
2,4-Bis(4-fluorophenyl)-2-methyl-2H-imidazole (2b): Colourless
solid m.p. (n-hexane) 100Ϫ103 °C. Ϫ H NMR (CDCl3): δ ϭ 1.80
1
(s, 3 H), 6.95Ϫ7.08 (m, 2 H), 7.14Ϫ7.28 (m, 2 H), 7.60Ϫ7.76 (m,
2 H), 8.00Ϫ8.12 (m, 2 H), 8.43 (s, 1 H). Ϫ 13C NMR (CDCl3): δ ϭ
27.2, 108.7, 115.1 (d, J ϭ 20 Hz), 116.3 (d, J ϭ 22 Hz), 127.2 (d,
J ϭ 4 Hz), 128.7 (d, J ϭ 7 Hz), 130.5 (d, J ϭ 7 Hz), 135.9 (d, J ϭ
4 Hz), 154.1, 162.2 (d, J ϭ 246 Hz), 162.7, 164.8 (d, J ϭ 252 Hz).
Ϫ MS: m/z (%) ϭ 270 (80) [Mϩ], 243 (96), 242 (26), 122 (100), 121
(71), 107 (28), 101 (34), 96 (47), 94 (25). Ϫ C16H12F2N2 (270.3):
calcd. C 71.10, H 4.48, N 10.36; found C 71.18, H 4.53, N 10.25.
Reaction of 3-Phenyl-2H-azirine (1a) with Styrene in the Presence
of Hydrated FeCl2: To a stirred degassed mixture of styrene (15 mL,
131 mmol), and anhydrous iron dichloride (5.60 g, 28 mmol) in
CH3CN (15 mL) kept at 45 °C, was added dropwise phenylazirine
1a (3.00 g, 26 mmol) in CH3CN (7 mL) at a rate of 2 mL hϪ1, and
the mixture stirred overnight. The solvent was removed, the crude
product dissolved in dichloromethane and washed with an aqueous
solution of HCl (0.1 ), aqueous NaHCO3 and water. The residue
was purified by column chromatography (eluent: n-hexane/ethyl
acetate form 95:5 to 50:50) to give phenyl-(trans-2-methyl-2-phenyl-
cyclopropyl)methanone (17a; 0.63 g, 11%), acetophenone (5a; 0.39
g, 13%), a 2:1 mixture (0.50 g) of acetophenone (5a) and phenacyl
chloride (6a), 2-methyl-2,4-diphenyl-2H-imidazole (2a; 0.30 g,
10%), and 4-hydroxy-1,4-diphenyl-butan-1-one (18a; 1.80 g, 29%).
3,5-Bis(4-fluorophenyl)pyridazine (3b): M.p. (MeOH) 220Ϫ224 °C.
Ϫ
1H NMR (CDCl3): δ ϭ 7.16Ϫ7.36 (m, 4 H), 7.64Ϫ7.80 (m, 2
H), 7.92 (d, J ϭ 2 Hz, 1 H), 8.04Ϫ8.20 (m, 2 H), 9.35 (s, 1 H). Ϫ
MS: m/z (%) ϭ 268 (100) [Mϩ], 120 (12). Ϫ C16H10F2N2 (268.3):
calcd. C 71.64, H 3.76, N 10.44; found C 71.73, H 3.81, N 10.37.
Reaction of 3-(p-Tolylphenyl)-2H-azirine (1c) with FeCl2: Anhyd-
rous iron dichloride (80 mesh; 0.49 g, 3.8 mmol) was added to a
stirred degassed solution of azirine 1c (0.50 g, 3.8 mmol) in anhyd-
rous CH3CN (38 mL) and the mixture stirred overnight. The solv-
ent was removed, the crude product dissolved in dichloromethane
and washed with an aqueous solution of HCl (0.1 ), aqueous
NaHCO3 and water. The residue was purified by column chroma-
tography (eluent: n-hexane/ethyl acetate from 95:5 to 50:50) to give
4-methylacetophenone (5c) in trace amounts, 2,5-di-p-tolyl-pyraz-
ine[11] (4c; 0.02 g, 4%), 2-methyl-2,4-di-p-tolyl-2H-imidazole (2c;
0.05 g, 10%) and 3,5-di-p-tolylpyridazine (3c; 0.13 g, 26%).
4-Hydroxy-1,4-diphenylbutan-1-one (18a):[14a] M.p. 93Ϫ94 °C,
1
(ref.[14b] 95 °C). Ϫ H NMR (CDCl3): δ ϭ 2.20 (m, 2 H), 3.11 (t,
J ϭ 7 Hz, 2 H), 4.83 (m, 1 H), 7.20Ϫ7.60 (m, 8 H), 7.94 (m, 2 H).
Ϫ
13C NMR (CDCl3): δ ϭ 33.0, 34.6, 73.3, 125.7, 127.4, 128.0,
128.3, 128.4, 133.0, 136.7, 144.3, 200.5.
Reaction of 3-Phenyl-2H-azirine (1a) with α-Methylstyrene in the
Presence of Anhydrous FeCl2: To a stirred degassed mixture of α-
methylstyrene (11 mL, 85 mmol), and anhydrous iron dichloride
(2.40 g, 19 mmol) in anhydrous CH3CN (7 mL) kept at 45 °C, was
added dropwise phenylazirine 1a (2.00 g, 17 mmol) in CH3CN
(8 mL) at a rate of 2 mL hϪ1, and the mixture stirred overnight.
The solvent was removed, the crude product dissolved in dichloro-
methane and washed with an aqueous solution of HCl (0.1 ),
aqueous NaHCO3 and water. The residue was purified by column
chromatography (eluent: n-hexane/ethyl acetate from 95:5 to 50:50)
to give phenyl-(trans-2-methyl-2-phenyl-cyclopropyl)methanone[15]
(17b; 0.38 g, 9%), 2-methyl-2,5-diphenylpyrroline (16b; 0.52 g,
13%), acetophenone (5a; 0.36 g, 17%), 2-methyl-2,4-diphenyl-2H-
imidazole (2a; 0.18 g, 9%), 3,5-diphenyl-pyridazine (3a; 0.21 g,
10%) and 4-hydroxy-4-methyl-1,4-diphenylbutan-1-one (18b;
0.34 g, 8%).
1
2-Methyl-2,4-di-p-tolyl-2H-imidazole (2c): Viscous oil Ϫ H NMR
(CDCl3): δ ϭ 1.82 (s, 3 H), 2.32 (s, 3 H), 2.42 (s, 3 H), 7.15 (d, J ϭ
8 Hz, 2 H), 7.30 (d, J ϭ 8 Hz, 2 H), 7.58 (d, J ϭ 8 Hz, 2 H), 7.94
(d, J ϭ 8 Hz, 2 H), 8.43 (s, 1 H) Ϫ 13C NMR (CDCl3): δ ϭ 21.0,
21.6, 26.9, 108.9, 126.8, 128.3, 129.0, 129.7, 137.3, 137.4, 141.8,
154.3, 163.6. Ϫ MS: m/z (%) ϭ 263 (23) [Mϩ ϩ 1], 262 (100) [Mϩ],
235 (75).
3,5-Di-p-tolylpyridazine (3c): White solid, m.p. (n-hexane/AcOEt)
201Ϫ204 °C (ref.[12] 199Ϫ200 °C) Ϫ H NMR (CDCl3): δ ϭ 2.45
1
(s, 6 H), 7.36 (d, J ϭ 8 Hz, 2 H), 7.37 (d, J ϭ 8 Hz, 2 H), 7.64 (d,
J ϭ 8 Hz, 2 H), 7.96 (d, J ϭ 2 Hz, 2 H), 8.04 (d, J ϭ 8 Hz, 2 H),
9.37 (d, J ϭ 2 Hz, 1 H) Ϫ 13C NMR (CDCl3): δ ϭ 21.3, 21.4,
120.5, 127.0, 127.1, 129.8, 130.3, 131.9, 133.6, 139.0, 140.3, 140.4,
148.1, 159.3.
Reaction of 3-Phenyl-2H-azirine (1a) with Styrene in the Presence
of FeCl2: To a stirred degassed mixture of styrene (15 mL,
131 mmol) and anhydrous iron dichloride (3.56 g, 28 mmol) in an-
hydrous CH3CN (15 mL) kept at 45 °C was added dropwise phenyl-
Phenyl(trans-2-methyl-2-phenylcyclopropyl)methanone (17b):[15] Oil.
Ϫ H NMR (CDCl3): δ ϭ 1.44 (s, 3 H), 1.61 (dd, J ϭ 5 and 8 Hz,
1 H), 1.88 (dd, J ϭ 5 and 6 Hz, 1 H), 2.91 (dd, J ϭ 6 and 8 Hz, 1
H), 7.20Ϫ7.60 (m, 8 H), 7.99 (m, 2 H).
1
1186
Eur. J. Org. Chem. 2001, 1183Ϫ1187

Cleavage of the Azirine Ring by Single Electron Transfer
FULL PAPER
2-Methyl-2,5-diphenylpyrroline (16b): Oil. Ϫ 1H NMR (CDCl3):
δ ϭ 1.64 (s, 3 H), 2.25 (m, 2 H), 2.85Ϫ3.20 (m, 2 H), 7.10Ϫ7.60
SHELXL-97.[17] In each cycle of refinement, with anisotropic tem-
perature factors for all non-H atoms, 168 parameters were refined.
(m, 8 H), 7.95 (m, 2 H). Ϫ 13C NMR (CDCl3): δ ϭ 29.5, 35.0, The final stage of refinement converged to R ϭ 0.059 for 1615
38.0, 78.1, 125.2, 126.1, 127.8, 128.1, 128.4, 130.4, 134.6, 149.1,
observed reflections, with I Ն 2σ(I), and R ϭ 0.139 for all data.
171.1 Ϫ MS: m/z (%) ϭ 235 (11) [Mϩ], 220 (110), 207 (51), 206 The mean shift/error was 0.005 and the goodness of fit, S, was
(59), 131 (26), 117 (38), 115 (68).
0.990. The final difference map showed a maximum and minimum
residual peaks of 0.16 and Ϫ0.18 e AϪ3, respectively.
˚
4-Hydroxy-4-methyl-1,4-diphenylbutan-1-one (18b): Oil Ϫ 1H NMR
(CDCl3): δ ϭ 1.58 (s, 3 H), 2.25 (m, 2 H), 2.70Ϫ3.20 (m, 2 H),
7.10Ϫ7.55 (m, 8 H), 7.82 (m, 2 H). Ϫ 13C NMR (CDCl3): δ ϭ
31.0, 33.6, 37.6, 73.8, 124.7, 126.4, 127.9, 128.1, 128.3, 132.9, 136.7,
147.1, 201.1 Ϫ MS: m/z (%) ϭ 254 (1) [Mϩ], 237 (100), 134 (27),
133 (30), 121 (36), 105 (33).
Acknowledgments
The authors wish to thank MURST ‘‘Programma di ricerca scient-
ifica di rilevante interesse nazionale’’ for financial support.
Reaction of 3-Phenyl-2H-azirine (1a) with α-Methylstyrene in the
Presence of Hydrated FeCl2: To a stirred degassed mixture of α-
methylstyrene (17 mL, 130 mmol), and hydrated iron dichloride
(5.8 g, 29 mmol) in CH3CN (15 mL) kept at 45 °C, was added drop-
wise phenylazirine 1a (3.00 g, 26 mmol) in CH3CN (7 mL) at a rate
of 2 mL hϪ1, and the mixture stirred overnight. The solvent was
removed, the crude product dissolved in dichloromethane and
washed with an aqueous solution of HCl (0.1 ), aqueous
NaHCO3 and water. The residue was purified by column chromato-
graphy (eluent: n-hexane/ethyl acetate) to give phenyl-(trans-2-
methyl-2-phenylcyclopropyl)methanone (17b; 0.32 g, 5%), 2-
methyl-2,5-diphenylpyrroline (16b; 0.32 g, 5%), acetophenone (5a;
0.37 g, 12%), a 2:1 mixture (0.26 g) of acetophenone (5a) and phen-
acyl chloride (6a), traces of 2-methyl-2,4-diphenyl-2H-imidazole
(2a), and 4-hydroxy-4-methyl-1,4-diphenylbutan-1-one (18b;
3.85 g, 60%).
[1]
V. Nair Azirines, in Small Ring Heterocycles (Ed.: A. Hassner),
Part I, (The Chemistry of Heterocyclic Compounds, Series Eds.:
A. Weissberger; E. C. Taylor), Interscience, New York, 1983.
[2]
K. Hayashi, K. Isomura, H. Taniguchi, Chem. Lett. 1975,
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[3]
S. Auricchio, A. Bini, E. Pastormerlo, A. M. Truscello, Tetra-
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H. Alper, J. E. Pickett, S. Wallowitz, J. Am. Chem. Soc. 1977,
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[5]
A. Citterio, R. Sebastiano, M. Nicolini, Tetrahedron 1993, 49,
7743Ϫ7760 and references cited therein.
F. H. Allen, O. Kennard, Chem. Des. Autom. News 1993, 8,
31Ϫ37.
[6]
[7]
F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G.
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A. G. Hortmann, D. A. Robertson, B. K. Gillard, J. Org.
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J. Sauer, D. K. Heldmann, Tetrahedron 1998, 54, 4297Ϫ4312.
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J. J. Armand, K. Chekir, J. Pinson, Can. J. Chem. 1974, 52,
Crystal Structure Analysis of 2a:[18] C16H14N2, Mr ϭ 234.3, ortho-
3971Ϫ3980.
[11]
H. Alper, M. Laplante, Tetrahedron 1978, 34, 625Ϫ626.
G. K. Almström, Ber. Dtsch. Chem. Ges. 1914, 47, 848Ϫ851.
rhombic, space group P212121, a ϭ 5.713(1), b ϭ 16.058(2), c ϭ
[12]
3
˚
˚
27.966(4) A, V ϭ 2565.4(7) A , Z ϭ 8 (two crystallographically
[13] [13a]
independent molecules in the unit cell), Dc ϭ 1.21 gcmϪ3, µ ϭ
L. A. Yanovskaya, Tetrahedron 1972, 28, 1565Ϫ1573. Ϫ
[13b]
A. Nelson, S. Warren, J. Chem. Soc., Perkin Trans. 1
0.559 mmϪ1, F(000) ϭ 992; λ ϭ 1.54179 A, room temperature. A
˚
1999, 3425Ϫ3433.
colourless crystal, suitable for X-ray analysis, with a platelet form
and approximate dimensions of 0.2 ϫ 0.3 ϫ 0.05 mm was obtained
by crystallisation from n-hexane. Intensity data were collected on
a Siemens P4 diffractometer with graphite monochromated Cu-Kα
radiation (λ ϭ 1.54179A°), using the θ/2θ scan technique. Unit cell
parameters were determined from 53 reflections in the range 16 Յ
2θ Յ 46°; a total of 3190 reflections (2967 unique, Rint ϭ 0.046)
were collected up to 130° in 2θ, with completeness to 2θ of 92.8%
and index range: Ϫ1 Յ h Ն 6, Ϫ1 Յ k Ն 18, Ϫ1 Յ l Ն 32. A
correction for Lorentz and polarisation effects was applied; no de-
cay correction was deemed necessary.
[14] [14a]
H. Fiesselmann, J. Ribka, Chem. Ber. 1956, 89, 40Ϫ51. Ϫ
[14b]
J. Bach, R. Berenguer, J. Gordi, M. Lopez, J. Manzanal, J.
Vilarrasa, Tetrahedron 1998, 54, 14947Ϫ14962.
[15]
Compound 17b is reported in literature in mixture with the cis
isomer in: O.G. Kulinchov, I. G. Tishchenko, S. V. Sviridov, J.
Org. Chem USSR (Engl. Transl.) 1986, 22, 1873Ϫ1878.
[16]
A. Altomare, M. C. Burla, M. Camalli, G. L. Cascarano, C.
Giacovazzo, A. Guagliardi, A. G. Moliteni, G. P. Polidori, R.
Spagna SIR97: A New Tool for Crystal Structure Determina-
tion and Refinement. J. Appl. Cryst. 1999, 32, 115Ϫ119.
[17]
G. M. Sheldrick, SHELXL-97. Release 97Ϫ2. Program for the
Refinement of Crystal Structures. 1999. University of
Göttingen, Germany.
[18]
Crystallographic data (excluding structure factors) for the
The structure was solved by direct methods using the SIR97 pro-
gram,[16] which revealed the position of all non-H atoms; all H
atoms were added at ideal calculated positions and refined using a
riding model. Because of the quite low reflection-to-parameter ra-
tio, due to the presence of two independent molecules in the unit
cell, the refinement, based on F2, was carried out separately on
each molecule by blocked full-matrix least-squares procedure, with
structure reported in this paper has been deposited with the
Cambridge Crystallographic Data Centre as supplementary
publication No. CCDC-137930. Copies of the data can be ob-
tained, free of charge, on application to the CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK [Fax: (internat.) ϩ44-1223/
336-033 or E-mail: deposit@ccdc.cam.ac.uk].
Received December 13, 1999
[O99672]
Eur. J. Org. Chem. 2001, 1183Ϫ1187
1187