[2 π + 2 σ] Type cycloaddition reactions of iminotropone derivatives with naphtho[b]cyclopropene to form cyclic amine compounds

Article abstract of DOI:10.3987/COM-01-9416

Reactions of iminotropone derivatives with naphtho[b]cyclopropene under the presence of a catalytic amount of AgBF₄ afforded cyclic amine derivatives via [2 π + 2 σ] type cycloaddition reactions. On the other hand, a reaction using a tropone hydrazone derivative without a catalysis formed a substituted hydrazone via a σ-bond rupture of the cyclopropene ring.

Full text of DOI:10.3987/COM-01-9416

HETEROCYCLES, Vol. 57, No. 2, 2002, pp. 235-240, Received, 5th December, 2001
[2 π + 2 σ ] TYPE CYCLOADDITION REACTIONS OF
IMINOTROPONE DERIVATIVES
WITH NAPHTHO[b]CYCLO-
PROPENE TO FORM CYCLIC AMINE COMPOUNDS
Katsuhiro Saito,* Katsuhiko Ono, Narie Ito, Naoe Tada, and Shinichi
Ando
Department of Applied Chemistry, Nagoya Institute of Technology,
Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan
Abstract-Reactions of iminotropone derivatives with naphtho-
[b]cyclopropene under the presence of a catalytic amount of AgBF4
afforded cyclic amine derivatives via [2π+ 2σ] type cycloaddition
reactions. On the other hand, a reaction using a tropone hydrazone
derivative without a catalysis formed a substituted hydrazone via a
σ-bond rupture of the cyclopropene ring.
While naphtho[b]cyclopropene (1) has highly distorted three-membered ring, it is a fairly
1
stable colorless crystalline material. The reactivity of 1 toward cycloaddition reactions
has been increasingly investigated to find an existence of two types of reaction paths, a
2
3
π-type path and aσ-type path. In the former reaction path 1 reacts with it’s π-bond,
and in the latter, 1 ruptures the σ-bond in the three-membered ring moiety, respectively.
Recently, a carbenic reaction was proposed as a possibility of the third type of reaction
4
path.
Although an enormous amount of papers have been published on the chemistry of tropone,
5
researches on the reactivity of iminotropones are few in numbers. However, considering
the differences in chemical behaviors between tropones and iminotropones, and the
possibility to synthesize cyclic amine compounds, the study on iminotropones seems to be
worthwhile. Herein, we report on the reactions of 1 with iminotropones.

An anhydrous benzene solution of 1 with two-equimolar amount of N-(p-methylphenyl)-
°
iminotropone (2a) was stirred at 0 C for 30 min under a nitrogen stream. After
evaporation of the solvent, the residue was chromatographed on silica gel to give the
recovery of 1 in 92% yield. No obvious addition product was detected. The reaction was
6
fairly improved by an addition of a catalytic amount of AgBF4. Thus, a benzene solution
of 1 and two equimolar amount of 2a and 1 mol % of AgBF4 was reacted under the same
reaction conditions as above. The usual treatment of the reaction mixture gave a cyclic
7
amine derivative (3a) and an amine derivative (4a) in 12% and 15% yields, respectively.
R
N
+
N R
1
2
3
+
a: R = p-MeC6H4
b: R = p-MeOC6H4
c: R = p-BrC6H4
d: R = p-ClC6H4
NHR
4
Figure 1
°
Table 1. Conditions of the cycloaddition reactions at 0 C
Molar Ratio
(1) : (2)
Additive
AgBF4
Reaction
Time
Yield (%)
Run
Solvent
(3a)
(4a)
1
2
3
1 : 2
1 : 2
1 : 2
benzene
benzene
CHCl3
30 min
30 min
30 min
Recovery of (1) :92
1 mol %
1 mol %
12
15
34
×
4
5
1 : 2
1 : 2
1 mol %
3 mol %
toluene
60 min
30 min
8
24
45
benzene
×

The amount of AgBF4 used influenced on the product yields, i.e., the use of 3 mol % of
AgBF4, in stead of 1 mol %, promoted the yield of 4a to 45% but on the contrary, that of 3a
was diminished to 0%. The product yields of the reactions with 2a under various reaction
conditions are summarized in Table 1. The best result was achieved under the conditions
of the run 2. The product yields of the reactions with several kinds of iminotropones
(2a-d) under the presence of 1 mol % of AgBF4 are summarized in Table 2.
Table 2. Yields of products (3a-d) and (4a-d)
Compounds
a: R = p-MeC6H4 b: R = p-MeOC6H4 c: R = p-BrC6H4
d: R = p-ClC6H4
3
4
12%
15%
5%
0%
0%
19%
30%
44%
R
N
R
N
AgBF4
2
CH2
Ag
Ag
1
5
6
R
N
N R
7
3
Figure 2
The formation of 3 is considered to proceed as follows according to research reports of
1,6
benzocyclopropene and 1.
An electrophilic cleavage at the three-membered ring of 1 is
carried out by a catalytic amount of Ag(I) ion giving an intermediate (5). A nucleophilic
attack of iminotropone to the cationic carbon atom of 5 may afford a spiro-type
8
intermediate (7) via a cyclization process of 6 as well as a reaction of 1 with tropone. This
compound easily provides the final product (3) through a 1,2-shift of the nitrogen atom.

When the cyclization process is not performed after the nucleophilic attack of iminotropone
to 5, compound (6) may exist as a tropylium ion form in a flask. The intermediate (6) is
hydrolyzed during working-up to afford compound (4).
R
N
+
2
CH2
Ag
hydrolysis
N
R
NHR
Ag
6
5
4
Figure 3
The reaction of 1 with tropone tosylhydrazone (8) without any catalyst provided an
9
N,N-doubly substituted hydrazone derivative (9) in 15% yield. The reaction is considered
to proceed through a nucleophilic attack of the amine nitrogen atom of 8 to the positively
charged saturated carbon atom of 1. Considering the charge densities on the amine
nitrogen atom and the imine nitrogen atom of 8 (Figure 5), this reaction mechanism can be
considered to be a result of a competition on the nucleophilicity of the nitrogen atoms.
H
N N
Ts
δ+
+
N N Ts
Yield 15%
1
8
9
Figure 4
O
H
N N S
O
Me
-0.07
-0.42
charge densities
Figure 5

In summery, we investigated a nucleophilic reaction of iminotropone (2) to 1 with an Ag(I)
catalyst and a nucleophilic reaction of 8 to 1 without a catalyst. The former reaction
afforded [2π+ 2σ] type cycloadducts (3) by the following cyclization.
REFERENCES AND NOTES
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1
7.
H NMR spectral data for the cycloadducts are as follows.
1
3a: H NMR (CDCl3), δ: 2.27 (s, 3 H, Me), 3.61 (d, 1 H, Jef = 5.4 Hz, He), 4.61 (d, 1 H, Jab
= 14.8 Hz, Ha), 4.67 (d, 1 H, Jab = 14.8 Hz, Hb), 5.62 (dd, 1H, Jef = 5.4, Jfg = 9.4 Hz, Hf),
6.31 (dd, 1H, Jgh = 4.9, Jfg = 9.4 Hz, Hg), 6.88 (m, 2 H, Hh,i), 6.93 (d, 2 H, Jcd = 8.8 Hz,
Hd), 7.00 (d, 2 H, Jcd = 8.8 Hz, Hc), 7.02 (d, 1 H, Jij = 5.8 Hz, Hj), 7.39-7.46 (m, 2 H), 7.69
(s, 1 H), 7.76-7.81 (m, 2 H), 8.10 (s, 1 H).
1
3b: H NMR (CDCl3), δ: 3.76 (s, 3 H, OMe), 3.55 (d, 1 H, Jef = 5.2 Hz, He), 4.46 (d, 1 H,
Jab = 14.7 Hz, Ha), 4.61 (d, 1 H Jab = 14.7 Hz, Hb), 5.61 (dd, 1 H, Jef = 5.2 Hz, Jfg = 9.8
Hz, Hf), 6.29 (dd, 1 H, Jgh = 5.5, Jfg = 9.8 Hz, Hg), 6.82-6.86 (m, 2 H, Hh,i), 6.84 (d, 2 H,
Jcd = 9.1 Hz, Hd), 6.99 (d, 2 H, Jcd = 9.1 Hz, Hc), 7.02 (m, 1 H, Hj), 7.41-7.45 (m, 2 H),
7.67 (s, 1 H), 7.76-7.81 (m, 2 H), 8.09 (s, 1 H).
Hd
Hc
Hb
R
Ha
He
N
Hf
Hg
Hj
Hh
Hi
3a : R = Me
3b : R = OMe
8. K. Saito, S. Ando, and Y. Kondo, Heterocycles, 2000, 53, 2601.
1
9.
H NMR spectral data for 9 is as fowllows.
1
H NMR (CDCl3), δ: 2.46 (s, 3 H), 4.34 (br s, 2 H), 6.25 (dd, 1 H, J = 7.7, 15.3 Hz), 6.28
(d, 1 H, J = 11.0 Hz), 6.34 (dd, 1 H, J = 8.0, 16.0 Hz), 6.48 (dd, 1 H, J = 7.6, 12.2 Hz),
6.55 (dd, 1 H, J = 2.5, 12.4 Hz), 7.34-7.44 (m, 5 H), 7.49 (dd, 1H, J = 1.7, 8.4 Hz), 7.62 (s,
1 H), 7.68-7.80 (m, 5 H).