Ring opening of 1-azabicyclo[1.1.0]butanes with hydrazoic acid - A facile access to N-unsubstituted azetidin-3- amines

Article abstract of DOI:10.1002/hlca.200590130

Sterically congested 1-azabicyclo[1.1.0]butanes 1 add hydrazoic acid smoothly at 0-5°, giving 3-azidoazetidines 2 in good to excellent yields. After hydrogenolysis over Pd/C catalyst, compounds 2 were converted into N-unsubstituted azetidin-3-amines 4. At

Full text of DOI:10.1002/hlca.200590130

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Helvetica Chimica Acta Vol. 88 (2005)
Ring Opening of 1-Azabicyclo[1.1.0]butanes with Hydrazoic Acid
a Facile Access to N-Unsubstituted Azetidin-3-Amines
by Grzegorz Mloston¬* and Ma¯gorzata Celeda
¬
¬
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University of Ëodz, Department of Organic and Applied Chemistry, Narutowicza 68, PL-90-136 Ëodz
(phone: ‡ 4842635-57-61; fax: ‡ 4842678-16-09; e-mail: gmloston@uni.lodz.pl)
Dedicated to Professor Rolf Huisgen on the occasion of his 85th birthday
Sterically congested 1-azabicyclo[1.1.0]butanes
1 add hydrazoic acid smoothly at 0 58, giving 3-
azidoazetidines 2 in good to excellent yields. After hydrogenolysis over Pd/C catalyst, compounds 2 were
converted into N-unsubstituted azetidin-3-amines 4. Attempted reduction of 2a with Raney-Ni led to a mixture
of the expected azetidin-3-amine 4a and the ring-enlarged 2,5-dihydro-1H-imidazole derivative 5.
1. Introduction. Azetidines are four-membered nitrogen heterocycles that are of
great interest for fundamental research and practical applications [1]. These hetero-
cycles show remarkable biological activity, and were found as a central structural
moiety in some naturally occurring compounds [2]. Recently, a growing number of
reports describing new applications of azetidine derivatives as antiviral and/or
anticancer pharmaceuticals [3], plant-growth regulators [4], as well as energetic
materials [5] were observed.
Azetidin-3-amines are less well-known for their practical applications, as there is no
general method known for the preparation of these versatile building blocks [6].
However, one report deserves special attention. A few years ago, a French group
described a multi-step procedure based on a modified Strecker reaction with Me3SiCN
and dibenzylamine, leading to 3-phenylazetidin-3-amine, which is required for the
preparation of some antibacterial quinolones [7]. The isolated product could not be
obtained in pure form, and the authors claimed to have prepared the hydrochloride in
only ca. 95% purity.
In the last two decades, diverse conversions of strained 1-azabicyclo[1.1.0]butanes
of the type 1 with electrophilic reagents such as HF [8], methyl azido- or chloroformate
[9], thioacetic acid [10], as well as dichlorocarbene [11] have been described. In the
present paper, we present a straightforward method for the smooth transformation of
relatively easily available 1-azabicyclo[1.1.0]butanes 1a d into azetidin-3-amines 4 via
the intermediate 3-azidoazetidines 2 (see Scheme 1 below).
2. Results and Discussion. Hydrazoic acid (HN3) is well-known as a useful reagent
that has been widely applied for the synthesis of amino compounds. Its reactions with
small-ring heterocycles, however, were only scarcely studied, and the literature reports
are limited to the ring-opening of some oxirane derivatives [12]. On the other hand, the
ring-opening reactions of strained 1-azabicyclo[1.1.0]butanes with acidic agents, aimed
¹ 2005 Verlag Helvetica Chimica Acta AG, Z¸rich

Helvetica Chimica Acta Vol. 88 (2005)
1659
Scheme 1
at the preparation of azetidine derivatives, can be smoothly performed [13]. In known
cases, strong acids of the type HX (X ˆ F, Cl) or sulfuric acid were applied to complete
the conversion [8][14][15].
In a typical experiment, the addition of a diluted solution of 1 to a cooled solution of
hydrazoic acid in CH2Cl2 or benzene resulted in instantaneous formation of 3-
azidoazetidines 2 and, unlike reported reactions with strong acids [14], no formation of
polymeric material was observed (Scheme 1). After typical aqueous workup, the oily
products 2 were purified by chromatography on silica gel without noticeable
decomposition. The IR spectra of the isolated products revealed the presence of N3
and NH functional groups by absorption bands located at 2080 and ca. 3210 cmÀ1,
respectively. 3-Azidoazetidines 2 obtained in these reactions are fairly stable
compounds, which could be stored in a refrigerator for several days. However, at
room temperature, they decomposed after a few hours to a mixture of polymeric
products.
The attempted reduction of 2a with Raney-Ni in boiling EtOH resulted in the
formation of a mixture of azetidin-3-amine 4a and 2,5-dihydro-1H-imidazole 5, which
were separated by column chromatography (Scheme 2). Based on the 1H-NMR
spectrum of the crude mixture of products, a 4a/5 ratio of 55 :45 was established.
Apparently, the reduction of the azido group with Raney-Ni occurs via nitrene 3 as a
reactive intermediate, which subsequently is either reduced to azetidin-3-amine 4a or
undergoes a competitive ring expansion to the isolated 2,5-dihydro-1H-imidazole 5. It
is noteworthy that the same ring expansion was observed in the gas phase during flash
vacuum pyrolysis of 2a, and, in this case, 5 was found as the major component of the
pyrolysate collected on a cold finger [9].
For comparison, reduction of the previously described 3-azidoazetidine-1-carbox-
ylate 2d [9] with Raney-Ni afforded 3-aminoazetidine-1-carboxylate 4d as the sole
product; in this case, ring enlargement was not observed (Scheme 3).
In order to avoid the undesirable reaction leading to 2,5-dihydro-1H-imidazoles of
type 5 during reduction of 2a with Raney-Ni, other reducing agents were tested
(LiAlH4 or H2). Best results were achieved with H2 over Pd/C. In this case, facile
reactions led smoothly to azetidin-3-amines 4b d in good yields (Scheme 4). The
structures of the products were established by means of spectroscopic methods. Thus,
absorption bands characteristic for primary amino groups appeared in the IR spectra in
the region of 3000 3300 cmÀ1. On the other hand, the initially observed strong
absorption of an N3 group at 2080 cmÀ1 disappeared completely, which clearly indicated
1
complete consumption of the starting material. In the H-NMR spectrum of 4b, the

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Helvetica Chimica Acta Vol. 88 (2005)
Scheme 2
Scheme 3
signals of CH2(2) and CH2(4) appeared as an AB-system (3.68 and 3.94 ppm) shifted
downfield in comparison with the starting 1b.
Isolation of the pure 3-aminoazetidine-1-carboxylate 4d was easily achieved by
crystallization, but all attempts to purify the oily azetidin-3-amines 4b and 4c by means
of chromatographic methods (column or plates) failed. For this reason, alcoholic
solutions of 4b and 4c were treated with 2.5 mol-equiv. of HBr (Scheme 4). After
cooling, crystalline dihydrobromides were obtained from EtOH diluted with Et2O. The
analogous treatment of 4d with HBr afforded the monohydrobromide exclusively.
Scheme 4

Helvetica Chimica Acta Vol. 88 (2005)
1661
3. Conclusions.
Strained 1-azabicyclo[1.1.0]butanes 1 react smoothly with
hydrazoic acid at low temperature to 3-azidoazetidines 2 in basically quantitative
yields. Upon hydrogenolysis with H2 over Pd/C, these products can be cleanly
converted into N-unsubstituted azetidin-3-amines 4, which are valuable building blocks
in organic synthesis. The presented method is based on the application of relatively
easily available 1-azabicyclo[1.1.0]butanes 1, and is more efficient than other
procedures recommended for preparation of 4 [3][7]. Reduction of 2a with Raney-
Ni was proposed to occur via a nitrene intermediate 3, and yielded a mixture of the
expected 3-amino derivative 4a and the ring expanded 2,5-dihydro-1H-imidazole 5.
The authors thank Professor Alan P. Marchand and Professor R. Bartnik for suggestions and helpful
discussions. G. M. thanks Professor J. Warkentin for his comments. Financial support of the Polish State
Committee for Scientific Research (Grant No. 3T09A04) is gratefully acknowledged.
Experimental Part
General. Melting points were determined in a capillary using a MelTemp2 apparatus; uncorrected. IR
Spectra were recorded on a NEXUS FT-IR spectrophotometer (KBr pellets or neat). 1H- and 13C-NMR Spectra
were recorded on a Varian Gemini 200 spectrometer, and Me4Si was used as an internal standard. 13C-NMR
Assignments were made on the basis of DEPTexperiments. Mass spectra (EI-MS) were recorded on a MAT-112
spectrometer (15 or 70 eV). Elemental analyses were performed by the Microanalytical Laboratory of the
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Polish Academy of Science (CBMM Ëodz).
Starting Materials. 2,2-Dimethyl-3-phenyl-1-azabicyclo[1.1.0]butane (1a), 3-phenyl-1-azabicyclo[1.1.0]bu-
tane (1b) [14], and 3-ethyl-1-azabicyclo[1.1.0]butane (1c) [16] were prepared according to described procedures.
Hydrazoic acid (HN3) was prepared in CH2Cl2 soln. according to [17], and the crude soln. was used without
distillation. Raney-Ni was freshly prepared following the literature protocol [18]. Methyl 3-azido-3-phenyl-
azetidine-1-carboxylate (2d) was prepared from 1b and methyl azidoformate at r.t., according to [9].
Reactions of 1-Azabicyclo[1.1.0]butanes 1a c with Hydrazoic Acid. The appropriate 1 (4 mmol) was
dissolved in 1 ml of CH2Cl2 and cooled to 0 58 in a H2O/ice bath. A cold soln. of HN3 in CH2Cl2 was added
dropwise with stirring, until TLC revealed consumption of the starting material. The mixture was extracted with
a 1% soln. of NaOH, saturated with NaCl, before the org. layer was separated and dried (MgSO4). The solvent
was evaporated in vacuo without heating, and the residual oily products were identified by 1H-NMR
spectroscopy. Analytically pure products were obtained after PLC (SiO2; CH2Cl2/MeOH 5 :1): Rf 0.2 (2a), 0.5
(2b), and 0.6( 2c), resp.
3-Azido-2,2-dimethyl-3-phenylazetidine (2a). With 1a (636.9 mg); yield 470 mg (58%, after chromatog-
raphy). Colorless thick oil, decomposes slowly during storage at r.t. IR (film): 3210m (br., NH), 2920s, 2830m,
2100vs (N3), 1500m, 1460m, 1360m, 1265vs, 1160m, 1050m, 910m, 770s, 730s, 705vs. 1H-NMR (CDCl3): 0.97 (s,
Me); 1.45 (s, Me); 2.69 (br. s, NH); 3.68, 4.23 (AB, J ˆ 9.2, CH2(4)); 7.21 7.45 (m, 5 arom. H). 13C-NMR
(CDCl3): 25.4 (q, Me); 26 .8 (q, Me); 49.9 (t, CH2); 67.4 (s, C(2)); 73.0 (s, C(3)); 126.5, 128.2, 128.6 (3d, 5 arom.
‡
C); 137.6( s, arom. C). EI-MS: 202 (0.3, M ), 173 (1), 145 (2), 117 (15), 77 (16), 43 (100). Anal. calc. for
C11H14N4 (202.26): C 65.32, H 6.98; found: C 65.12, H 7.12.
3-Azido-3-phenylazetidine (2b). With 1b (524.7 mg); yield 530 mg (76%, after chromatography). Colorless
liquid, changed to yellow on standing at r.t. IR (film) 3210m (NH), 2090s, 2020s , 2100vs (N3), 1500m, 1450m,
1340m, 1260s, 770s, 715vs. 1H-NMR (CDCl3): 2.08 (br. s, NH); 3.97 (br. s, CH2(2), CH2(4)); 7.40 (br. s, 5 arom.
H). 13C-NMR (CDCl3): 58.2 (t, C(2), C(4)); 66.9 (s, C(3)); 125.4, 128.1, 128.8 (3d, 5 arom. C); 140.2 (s, arom. C).
‡
‡
EI-MS: 174 (0.2, M ), 159 (3, [M À NH] ), 144 (3), 117 (8), 104 (17), 77 (12), 71 (100). Anal. calc. for C9H10N4
(174.20): C 62.05, H 5.79; found: C 62.36, H 5.58.
3-Azido-3-ethylazetidine (2c). With 1c (332.5 mg); yield 320 mg (63%, after chromatography). Colorless
liquid, changed to yellow on standing at r.t. IR (film): 3210m (NH), 2090s (N3), 1560m, 1400s, 1260s. 1H-NMR
(CDCl3): 0.96( t, J ˆ 7.3, Me); 1.86( q, J ˆ 7.3, CH2); 1.89 (br. s, NH); 3.49, 3.75 (AB, J ˆ 9.5, CH2(2), CH2(4)).
‡
13C-NMR (CDCl3): 8.0 (q, Me); 30.0 (t, CH2); 56.0 (t, C(2), C(4)); 65.0 (s, C(3)). EI-MS: 127 (12, [M ‡ 1] ), 97
(7), 84 (21), 69 (36), 68 (32), 55 (19), 54 (98), 42 (100). Anal. calc. for C5H10N4 (126.16): C 47.60, H 7.99; found:
C 47.90, H 7.73.

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Reduction of 2a with Raney-Nickel. To a stirred soln. of 260 mg (1.5 mmol) of 2a in EtOH (10 ml) was
added a suspension of freshly prepared Raney-Ni in EtOH in small portions at r.t. Each portion of the Ni
initiated a vigorous, exothermic reaction, with evolution of N2. When the gas evolution had ceased, the mixture
was stirred for another 30 min, and the black suspension was filtered through paper to separate the black Ni
precipitate. The colorless soln. was evaporated in vacuo, and the residual thick oil was analyzed by 13C- and
1H-NMR spectroscopy. Products 4a and 5 in a 55 :45 ratio were the only components of the crude mixture.
Attempted separation of both compounds using chromatography resulted in decomposition of 4a, and only 5
could be obtained in analytically pure form.
2,2-Dimethyl-3-phenylazetidin-3-amine (4a). Identified spectroscopically as a crude product in the mixture
1
with 5; decomposed during attempted chromatographic workup. H-NMR (CDCl3): 0.93, 1.44 (2s, 2 Me); 2.42
(br. s, NH, NH2); 3.36, 4.23 (AB, J ˆ 9.2, CH2(4)); 7.20 7.30 (m, 5 arom. H). 13C-NMR (CDCl3): 24.0, 27.1 (2q,
2 Me); 54.7 (s, C(3)); 63.1 (s, C(2)); 67.2 (t, C(4)).
2,5-Dihydro-2,2-dimethyl-4-phenyl-1H-imidazole (5). Isolated after PLC (SiO2; CH2Cl2/MeOH 9 :1).
Yield: 65 mg (30%). Pale yellow thick oil, which partially solidified at r.t. IR (neat): 3200 (br., NH), 1620s
(CˆN), 1460s, 1225s, 1170s, 1080m, 1020m, 850m, 780s, 705s. 1H-NMR (CDCl3): 1.48 (s, 2 Me); 2.45 (br. s, NH);
4.22 (s, CH2(5)); 7.40 7.45 (m, 3 arom. H); 7.73 7.80 (m, 2 arom. H). 13C-NMR (CDCl3): 28.0 (q, 2 Me); 54.8 (t,
C(5)); 64.0 (s, C(2)); 91.1 (t, C(5)); 127.8, 128.6, 130.9 (3d, 5 arom. C); 132.6( s, arom. C); 168.0 (s, CˆN). EI-
‡
‡
‡
MS: 173 (2, [M À 1] ), 172 (18, [M À 2] ), 145 (95), 104 (92, [C6H5ÀCꢀNH] ), 103 (25), 77 (28), 69 (100).
Anal. calc. for C11H14N2 (174.24): C 75.82, H 8.10, N 16.08; found: C 75.68, H 7.90, N 16.18.
Reductions of 3-Azidoazetidines 2 with H2 over Pd/C. General Procedure. 3-Azidoazetidines 2b
d
(2.5 mmol) were dissolved in 10 ml of MeOH, and 200 mg of Pd/C were added to the soln. The black suspension
was stirred vigorously in a closed vessel filled with H2 under pressure. The progress of the reactions was
monitored by TLC (SiO2; CH2Cl2/MeOH 9 :1), and the reactions were stopped when no more starting materials
was detected in the mixture. The solns. were filtered through a paper filter, and evaporated in vacuo to give thick
colorless oils. The crude products were identified by their 1H-NMR and IR-spectra, and immediately converted
into the corresponding hydrobromides by a treatment with a dilute soln. of HBr in MeOH. Only 4d could be
isolated and purified chromatographically. Compounds 4b and 4c decomposed on SiO2 and could not be
recovered from the PLC plates. When stored at r.t., 4b and 4c decomposed slowly to give a mixture of
unidentified polymeric products.
3-Phenylazetidin-3-amine (4b). With 2b (436.5 mg); yield of crude product: 351 mg (95%), reaction time
1 h. Colorless thick oil. IR (film): 3200vs (br., NH, NH2). 1H-NMR (CDCl3): 2.50 (br. s, NH, NH2); 3.68, 3.94
(AB, J ˆ 8.0, CH2(2), CH2(4)); 7.16 7.53 ( m, 5 arom. H). Crude 4b was dissolved in MeOH and treated with a
slight excess of aq. HBr diluted with MeOH. The solvent was evaporated, and the residue was triturated with a
small amount of acetone to give 631 mg (82%) of pure, crystalline dihydrobromide 4b ¥ 2 HBr. M.p. 179 1828
(dec.). IR (KBr): 3300 2400vs (br.), 1480s, 1350m, 1305m, 950m, 910m, 780m, 705s. 1H-NMR (D2O): 5.25, 5.42
(2 br. s, CH2(2), CH2(4)); 9.80 (br. s, 5 arom. H). 13C-NMR (D2O): 57.5 (t, C(2), C(4)); 58.0 (s, C(3)); 130.5,
134.1, 134.9 (3d, 5 arom. C); 138.5 (s, arom. C). Anal. calc. for C9H14Br2N2 (310.03): C 34.87, H 4.55, N 9.04;
found: C 34.93, H 4.95, N 9.02.
3-Ethylazetidin-3-amine (4c). With 2c (315.4 mg); yield of crude product 250 mg (99%), reaction time 1 h.
Colorless thick oil. IR (neat): 3200vs (br., NH, NH2). 1H-NMR (CDCl3): 0.92 (t, Me); 1.73 (q, CH2); 3.30 (br. s,
CH2(2), CH2(4)); 3.57 (br. s, NH, NH2). Crude 4c was dissolved in 2 ml of MeOH and treated with a slight
excess of aq. HBr diluted with MeOH. Evaporation in vacuo afforded a thick oil, which was recrystallized from
EtOH/Et2O to give 478 mg (73%) of colorless crystals of dihydrobromide 4c ¥ 2 HBr. M.p. 166 1708 (dec.). IR
(KBr): 3300 2400vs (br.). 1H-NMR (D2O): 1.62 (t, Me); 2.6 8 q(, CH2); 4.83, 5.03 (AB, J ˆ 12.0, CH2(2),
CH2(4)). 13C-NMR (D2O): 8.3 (q, Me); 30.4 (t, CH2); 56.0 (t, C(2), C(4)); 57.2 (s, C(3)). Anal. calc. for
C5H14Br2N2 (261.99): C 22.92, H 5.39, N 10.69; found: C 22.61, H 5.42, N 10.50.
Methyl 3-Amino-3-phenylazetidine-1-carboxylate (4d). With 2d [9] (580.6mg); reaction time 30 min, yield
of crude product: 433 mg (84%), purified by crystallization from hexane. M.p. 67 698. IR (KBr): 3250m (NH2),
1700s (CˆO), 1460s, 1390s, 1120m. 1H-NMR (CDCl3): 1.95 (br. s, NH2); 3.72 (s, MeO); 4.08, 4.25 (AB, J ˆ 9.0,
CH2(2), CH2(4)); 7.25 7.58 (m, 5 arom. H). 13C-NMR (CDCl3): 52.4 (q, MeO); 54.4 (t, C(2), C(4)); 64.8 (s,
C(3)); 124.9, 126.5, 127.4 (3d, 5 arom. C); 144.5 (s, arom. C); 157.3 (s, CˆO). Anal. calc. for C11H14N2O2
(206.24): C 64.06, H 6.84, N 13.58; found: C 63.87, H 6.96, N 13.52.
Methyl 3-Amino-3-phenylazetidine-1-carboxylate Hydrobromide (4d ¥ HBr). From 4d, after treatment with
aq. HBr diluted with MeOH. Yield: 72%. M.p. 187 1908 (dec.). Anal. calc. for C11H15BrN2O2 (287.15): C 46.01,
H 5.27, N 9.76; found: C 46.06, H 5.45, N 9.64.

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ReceivedMarch 1, 2005