Ring transformation of cyclohepta[b]pyrimido[5,4-d]furan-8(7H),10(9H)- dionylium ion to the corresponding pyrrole derivatives via troponeimine intermediates: Photo-induced autorecycling oxidizing reactions of some amines

Article abstract of DOI:10.1016/j.tet.2004.07.063

Ring transformation of 7,9-dimethylcyclohepta[b]pyrimido[5,4-d]furan- 8(7H),10(9H)-dionylium tetrafluoroborate 4⁺·BF ₄^- to 7,9-dimethylcyclohepta[b]pyrimido[5,4-d]pyrrrole- 8(7H),10(9H)-dionylium tetrafluoroborate 6a-d⁺·BF ₄^- consists of the reaction of 4⁺· BF₄^- with amines and subsequent exchange of the counter-ion using aq. HBF₄. Reactions of 4⁺·BF ₄^- with aniline and 4-substituted anilines afforded the corresponding pyrrole derivatives 6a-c⁺·BF₄ ^- directly in good yields. On the other hand, reaction of 4 ⁺·BF₄^- with benzylamine gave the troponeimine intermediate 9, which was not converted to 6d⁺· BF₄^- and reverted to 4⁺·BF ₄^- by adding HBF₄; however, it was converted to 6d⁺·BF₄^- upon treatment with (COCl)₂ or SOCl₂, followed by exchange of the counter-ion. In a search for the characteristics of 9, inspection and comparison of the X-ray crystal analyses, NMR and UV-vis spectra, and CV measurement of 9 and N,N-disubstituted troponeimine derivatives 12 were carried out to suggest the remarkable structure of 12 having ionic C-O bonding between the imine-carbon atom and the oxygen atom of the barbituric acid moiety in the solid state. Thus, characteristics of 9 were ascribed to the sterically hindered and favorable conformation of N-protonated troponeimine intermediates. Furthermore, novel photo-induced oxidation reactions of a series of 4⁺·BF ₄^-, 5⁺·BF₄^-, and 6a,e⁺·BF₄^- towards some amines under aerobic conditions were carried out to give the corresponding imines in 455-8362% yields [based on compounds 4⁺, 5⁺, and 6a,e ⁺], suggesting the oxidation reaction occurs in an autorecycling process. Mechanistic aspects of the amine-oxidation reaction are also postulated.

Full text of DOI:10.1016/j.tet.2004.07.063

Tetrahedron 60 (2004) 9139–9148
Ring transformation of
cyclohepta[b]pyrimido[5,4-d]furan-8(7H),10(9H)-dionylium ion
to the corresponding pyrrole derivatives via troponeimine
intermediates: photo-induced autorecycling oxidizing reactions
of some amines
*
Shin-ichi Naya and Makoto Nitta
Department of Chemistry, School of Science and Engineering, Waseda University, Shinjuku-ku, Tokyo 169-8555, Japan
Received 7 July 2004; revised 26 July 2004; accepted 26 July 2004
Available online 26 August 2004
Abstract—Ring transformation of 7,9-dimethylcyclohepta[b]pyrimido[5,4-d]furan- 8(7H),10(9H)-dionylium tetrafluoroborate 4C$BF4K to
7,9-dimethylcyclohepta[b]pyrimido[5,4-d]pyrrrole-8(7H),10(9H)-dionylium tetrafluoroborate 6a–dC$BF4K consists of the reaction of
4C$BFK4 with amines and subsequent exchange of the counter-ion using aq. HBF4. Reactions of 4C$BFK4 with aniline and 4-substituted
anilines afforded the corresponding pyrrole derivatives 6a–cC$BF4K directly in good yields. On the other hand, reaction of 4C$BF4K with
benzylamine gave the troponeimine intermediate 9, which was not converted to 6dC$BF4K and reverted to 4C$BFK4 by adding HBF4;
however, it was converted to 6dC$BF4K upon treatment with (COCl)2 or SOCl2, followed by exchange of the counter-ion. In a search for
the characteristics of 9, inspection and comparison of the X-ray crystal analyses, NMR and UV–vis spectra, and CV measurement of 9 and
N,N-disubstituted troponeimine derivatives 12 were carried out to suggest the remarkable structure of 12 having ionic C–O bonding between
the imine–carbon atom and the oxygen atom of the barbituric acid moiety in the solid state. Thus, characteristics of 9 were ascribed to the
sterically hindered and favorable conformation of N-protonated troponeimine intermediates. Furthermore, novel photo-induced oxidation
reactions of a series of 4C$BF4K, 5C$BF4K, and 6a,eC$BFK4 towards some amines under aerobic conditions were carried out to give the
corresponding imines in 455–8362% yields [based on compounds 4C, 5C, and 6a,eC], suggesting the oxidation reaction occurs in an
autorecycling process. Mechanistic aspects of the amine-oxidation reaction are also postulated.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
in the hope of gaining mechanistic insight into flavin-
catalyzed reactions. In this relation, 5-deaza-10-oxaflavin
1b (2H-chromeno[2,3-d]pyrimidine-2,4(3H)-dione),7 in
Flavins are known to play an important role as cofactors in a
wide variety of biological redox reactions.1 Dehydrogena-
tion reactions represent a major category of processes
mediated by a subclass of flavoenzymes known as oxidases.
Included in this group are the oxidative transformations of
alcohols to carbonyl compounds, of amines to imines, and
of fatty acid esters to their a,b-unsaturated analogs.2 The
flavin-redox systems have been investigated extensively
through synthetic model systems and theoretical calcu-
lations.3 Among these, 5-deazaflavin 1a (Fig. 1) has been
studied extensively in both enzymatic4 and model systems,5,6
Keywords: 7,9-Dimethylcyclohepta[b]pyrimido[5,4-d]furan-8(7H),10(9H)-
dionylium tetrafluoroborate; 7,9-Dimethylcyclohepta[b]pyrimido[5,4-d]pyr-
rol-8(7H),10(9H)-dionylium tetrafluoroborate; Ring-transformation; Photo-
induced oxidation reaction.
* Corresponding author. Tel.: C81-352863236; fax: C81-332082735;
e-mail: nitta@waseda.jp
Figure 1.
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.07.063

9140
S. Naya, M. Nitta / Tetrahedron 60 (2004) 9139–9148
which the nitrogen atom is replaced by an oxygen, has also
been synthesized and found to possess a strong function to
oxidize alcohols to the corresponding carbonyl compounds.
On the basis of the above observations, we have previously
studied the preparation of 6-substituted 9-methyl-
cyclohepta[b]pyrimido[5,4-d]pyrrole-8(6H),10(9H)-diones
(2a,b)8 and 9-methylcyclohepta[b]pyrimido[5,4-d]furan-
8,10(9H)-diones (3),9 which are structural isomers of 5-
deazaflavin 1a and 5-deaza-10-oxaflavin 1b. Furthermore,
we have recently reported the synthesis, properties, and reac-
tivity of 7,9-dimethylcyclohepta[b]pyrimido[5,4-d]furan-
8(7H),10(9H)-dionylium tetrafluoroborate (4C$BF4K)11,12
and its sulfur and nitrogen analogues 5C$BF4K and
6a,eC$BF4K.13,14 In addition, novel photo-induced autorecy-
cling oxidizing reactions of 4C$BF4K, 5C$BF4K, 6a,eC$BFK4
toward some alcohols are studied as well.12–14 Thus, the
uracil-annulated heteroazulenes such as 4C$BF4K, 5C$BF4K,
6a,eC$BFK4 are very interesting from the viewpoint of
exploration of novel functions. Through these studies, we
have accomplished a ring-transformation of 3 to 2a,b,10 which
are clarified to have oxidizing ability toward some amines: the
reaction of 3 with some amines undergoes ring-opening
reaction of furan to give troponimine intermediates which
undergo thermal ring-closure to give 2.
Figure 2. Numbering is shown according to the ORTEP drawing of 12.
of isolated troponeimine intermediate 9 (Scheme 1) and
N,N-disubstituted troponeimine 12 (Fig. 2) were investi-
gated by inspection of their X-ray crystal analyses and
spectral data including CV measurements. The remarkable
structure of 12 having ionic C–O bonding between the
imine-carbon atom and the oxygen atom of the barbituric
acid moiety in the solid state was clarified. Thus,
characteristics of 9 are rationalized by postulating sterically
hindered and favorable conformation of the N-protonated
troponeimine intermediates. Furthermore, the oxidizing
ability of a series of 4C$BF4K, 5C$BF4K, 6a,eC$BFK4
toward some amines was studied as well. Furthermore,
mechanistic aspects of the amine-oxidation reaction are also
postulated. We report herein the results in detail.
2. Results and discussion
2.1. Ring-transformation
From this viewpoint, we studied the ring transformation of
4C$BFK4 to the corresponding pyrrole derivatives 6a–
dC$BFK4 . In order to clarify the reactivity of the
troponeimine intermediate, the detailed structural features
As the ring transformation of 3a to 2a,b, we have reported
that the thermal reaction of troponeimine 7, obtained by the
reaction of 3 with benzylamine, gives neutral pyrrole
derivative 2a (Scheme 1).10 Thus, a reaction of 4C$BF4K
with PhNH2 was carried out to give the corresponding
troponeimine 8a, which is labile and easily cyclizes at room
temperature to give 6aC. Thus, a thermal reaction of
4C$BF4K with PhNH2 was carried out to give 6aC$BF4K
quantitatively (Scheme 1, Table 1, entry 1). In order to
elucidate the generality of this method, reactions of
4C$BFK4 with 4-substituted anilines were carried out
(Table 1). The reactions of 4C$BF4K with 4-MeOC6H4NH2
and 4-ClC6H4NH2 afforded the corresponding 6b,cC$BF4K
in quantitative yields, respectively (entries 2 and 3). On the
reaction of 4C$BF4K with 4-NCC6H4NH2, which has a
strong electron-withdrawing substituent, addition reaction
was not observed in the 1H NMR monitoring during
prolonged reaction time, and the starting materials were
recovered quantitatively (cf. entry 4).
On the other hand, we have recently reported that the
reaction of 4C$BF4K with benzylamine gives troponeimine
9 (Scheme 1).12 While the reaction of 9 with HBF4
regenerates 4C$BF4K (Scheme 1), thermal reaction of 9
resulted in the formation of a complicated mixture, and the
expected compound 6dC$BF4K was not obtained.12 Regard-
ing the X-ray analysis of compounds 6a,eC$BF4K, large
steric hindrance between the N6-substituent and N7Me has
been suggested.14 Thus, the possible steric hindrance
between the large benzyl group and the NMe group in 9
seems to inhibit the cyclization. Thus, in order to
accomplish the ring-transformation of 4C$BF4K to
6dC$BFK4 , inhibition of the nucleophilic attack of oxygen
of the barbituric acid moiety as well as acceleration of the
Scheme 1. Reagents and conditions: (i) 1,4-dioxane, reflux, 5 h; (ii) (a)
CH3CN, reflux, 3 h, (b) 42% aq HBF4, Ac2O, 0 8C, 1 h; (iii) 42% aq. HBF4,
Ac2O, 0 8C, 1 h.

S. Naya, M. Nitta / Tetrahedron 60 (2004) 9139–9148
Table 1. Results for the reaction of 4C$BFK4 with aniline and 4-substituted anilines
9141
Entry
4
C$BFK4
Aniline
Timea/h
Product
Yieldb/%
1
2
3
4
4
4
4
4
C$BFK4
C$BFK4
C$BFK4
C$BFK4
PhNH2
3
3
3
24
6aC$BFK4
6bC$BFK4
6cC$BFK4
Nonec
100
100
100
—
4-MeOC6H4NH2
4-ClC6H4NH2
4-NCC6H4NH2
a
Reaction was carried out in CH3CN solution under reflux.
Isolated yield.
Reaction did not proceed and the starting materials were recovered quantitatively.
b
c
room temperature to give 6dC$BF4K quantitatively
(Table 2, entry 4).
Compound 6aC$BF4K was identified on the basis of a
comparison of the physical data with those of the authentic
specimen.14 In addition, new compounds 6b–dC$BFK4 were
fully characterized on the basis of the 1H and 13C NMR, IR,
and mass spectral data as well as elemental analysis.
Furthermore, the CV measurement of 6b–dC$BF4K in
CH3CN exhibited irreversible reduction waves (E1red),
which are summarized in Table 3, together with those of
the reference compounds. The irreversible nature is
probably due to the formation of a radical species and its
dimerization, as reported to be a typical property of uracil-
annulated heteroazulenylium ions 4C$BFK4 ,12 5C$BF4K,13
and 6a,eC$BF4K.14 While the E1red of 6dC$BFK4 is similar
to that of 6aC$BF4K (K0.84 V),14 the E1red of 6b,cC$BFK4
are less negative than that of 6aC$BF4K. This feature is
rationalized on the basis of the electron-withdrawing
property of MeO- and Cl-substituted phenyl groups in
6b,cC$BFK4 , in which the substituted phenyl groups
experience steric hindrance with the N7Me group and
would twist against the plane of the heteroazulene unit.
Scheme 2. Reagents and conditions: (i) (COCl)2 or SOCl2, conditions
described in Table 2; (ii) 42% aq. HBF4, Ac2O, 0 8C, 1 h.
nucleophilic attack of nitrogen of the troponeimine moiety
is necessary (vide infra). Thus, upon treatment with (COCl)2
and SOCl2, the hydroxyl group of 9 was expected to
convert to a leaving group X, which can not attack the
troponeimine moiety (Scheme 2). Under both conditions,
the reaction of 9 with (COCl)2 afforded a mixture of
6dC$BFK4 and 4C$BF4K (Table 2, entries 1–3). In
contrast, the reaction of 9 with SOCl2 proceeded at
2.2. Structural properties of troponeimines
We have recently reported that the reaction of 4C$BF4K
with diethylamine gives the troponeimine 12 (Fig. 2).12
Thus, the detailed characteristics of 9 and 12 are interesting
Table 2. Results for the reaction of imine 9 with (COCl)2 and SOCl2
Run
Additive
Solvent
Conditions
Product (yielda/%)
1
2
3
4
(COCl)2
(COCl)2
(COCl)2
SOCl2
(CH2Cl)2
(CH2Cl)2
None
Room temperature, 1 h
70 8C, 1 h
Reflux, 3 h
6dC$BFK4 (26), 4C$BFK4 (74)b
6dC$BFK4 (25), 4C$BFK4 (75)b
6dC$BFK4 (78), 4C$BFK4 (22)b
6dC$BFK4 (100)
None
Room temperature, 10 min
a
Isolated yield.
Product ratio was calculated by H NMR spectroscopy.
b
1
Table 3. Redox potentials of 6b–dC$BF4K, 9, and 12 and reference compounds 4C$BF4K, 5C$BF4K, and 6a,eC$BFK4
Compound
Redox potentiala
Compound
Redox potentiala
E1red
E1ox
E1red
E1ox
4
5
C$BFK4 b
C$BFK4 b
K0.58
K0.53
K0.84
K0.80
K0.76
—
—
—
—
—
6dC$BFK4
6eC$BFK4 c
K0.84
K0.87
—
—
6aC$BFK4 d
6bC$BFK4
6cC$BFK4
9
12
K1.40
K1.85
C0.63
C0.41
a
V vs. Ag/AgNO3; cathodic and anodic peak potentials.
b
c
d
Ref. 10.
Ref. 15.
Ref. 13.

9142
S. Naya, M. Nitta / Tetrahedron 60 (2004) 9139–9148
structure, while the troponeimine moiety has a boat shape,
but the deformation from planarity is small. Furthermore,
the troponeimine moiety of 9 shows a large bond
alternation. In addition, the atomic distances of O1–C6
˚
and N3–C9 are 3.039 and 3.023 A, respectively (Table 4),
and the dihedral angle of C6–C7–C8–C9 is 57.38. On the
other hand, the troponeimine moiety of 12 shows a large
bond alternation as shown in the canonical structure 12-A
1
(Fig. 2, Table 4), which is supported by the H and 13C
NMR study.12 While the barbituric acid moiety of 12 has a
nearly planar structure, the troponeimine moiety is highly
distorted to a boat shape. A remarkable feature is the short
˚
atomic distance of O1–C6 (2.362 A), which is larger than a
typical O–C covalent bond (1.43 A),16 but it is considerably
˚
16
˚
shorter than the sum of the van der Waals radii (3.25 A).
This is probably due to the interaction between the O1 and
the C6; however, the sum of the bond angles of N3–C6–C5,
N3–C6–C7, and C5–C6–C7 is 359.18, and thus, the C6
carbon atom exists as sp2 hybridization. Furthermore, the
˚
short bond length of the N3–C6 (1.304 A) suggests its
double bond character, and the bond length of the O1–C9 is
similar to that of the O3–C13. These features support that
the interaction of the O1–C6 is an ionic bonding, and not a
covalent bonding in the solid state. The remarkable structure
of 12 seems close to the transition state of the intramolecular
nucleophilic addition of the O1 atom reverting to the furan-
ring of 4C: intermediate 10C, generated by protonation of 9,
may have a structure similar to that of 12, and thus, the
acidic reaction of 9 using aq. HBF4 easily regenerate
4C$BFK4 (Scheme 1). Thus, in order to accomplish the ring
transformation of 4C$BF4K to 6dC$BF4K, (COCl)2 or
SOCl2 is required for cyclization of 9 to 6dC$BF4K (vide
supra).
1
On the other hand, in the H NMR spectrum of 12, the
signals of N1Me and N3Me appear equivalent (d 3.34) at
room temperature.12 They appear as two sharp singlets at
low temperature (K90 8C), while the signals of the seven-
membered ring show no appreciable change. Thus, rapid
rotation around the C7–C8 bond of 12 (Fig. 2) clearly occurs
on the NMR time scale at room temperature in solution.
Through variable temperature 1H NMR measurement of 12,
the coalescence temperature was determined to be 261 K,
and the chemical shift difference between N1Me and N3Me
was 50.6 Hz. Consequently, rotational barrier (DG‡) around
the C7–C8 bond was determined to be 12.75 kcal molK1. In
the 13C NMR spectrum of 12 at K90 8C, the signals of two
carbonyl-carbons C9 and C13 appear as two sharp signals
(dC 160.3 and 161.5), which is not changed from the
Figure 3. ORTEP drawing of 9 and 12 with thermal ellipsoid plot (50%
probability).
in the view of clarifying the unreactive nature of 9 in the
present ring transformation. Single crystals of 9 and 12 were
obtained by recrystallization from CHCl3 and AcOEt,
respectively, and their ORTEP drawings are shown in
Figure 3. The barbituric acid moiety of 9 has a nearly planar
Table 4. Bond lengths and atomic distance of 9 and 12 obtained by X-ray structure analysis
Bonda
Bonda
˚
Bond length/A
˚
Bond length/A
9
12
9
12
O1–C6b
N3–C9b
C1–C2
C2–C3
C3–C4
C4–C5
C5–C6
C6–C7
3.039(3)
3.023(3)
1.429(4)
1.348(4)
1.419(4)
1.354(4)
1.436(3)
1.459(4)
2.362(3)
C1–C7
N3–C6
C7–C8
C8–C9
C8–C13
O1–C9
O3–C13
1.372(4)
1.331(4)
1.479(4)
1.408(4)
1.421(4)
1.242(3)
1.243(3)
1.375(3)
3.300(3)
1.444(3)
1.369(4)
1.429(4)
1.359(4)
1.457(3)
1.486(3)
1.305(3)
1.448(3)
1.414(3)
1.425(3)
1.243(3)
1.239(3)
a
The numbering is shown in Figure 3.
Atomic distance.
b

S. Naya, M. Nitta / Tetrahedron 60 (2004) 9139–9148
9143
2.3. Autorecycling oxidation
We have previously reported that compounds 4C$BF4K,
5C$BFK4 , and 6a,eC$BFK4 undergo autorecycling oxidation
of some alcohols to give the corresponding carbonyl
compounds under photo-irradiation.12–14 In this context
and in a search for other functions, we examined the
oxidation of some amines by using 4C$BF4K and
6a,eC$BFK4 as well as sulfur analogue 5C$BFK4 under
aerobic and photo-irradiation conditions (RPR-100, 350 nm
lamps). We found that a series of compounds 4C$BF4K,
5C$BFK4 , and 6a,eC$BFK4 have an oxidizing ability toward
some amines to give the corresponding imines. Imines 20
are produced at first; and they react with other amines to
result in the formation of R1R2C]N–CHR1R2 (21)
(Scheme 4). The results are summarized in Table 5. Direct
irradiation of the amines in the absence of 4C$BF4K,
5C$BFK4 , and 6a,eC$BFK4 (named ‘blank’) gives the
imines in low to modest yields. Thus, the yields of imines
are calculated by subtraction of the ‘blank’ yield from the
yields obtained in the presence of 4C$BF4K, 5C$BF4K, and
6a,eC$BFK4 . More than 100% yields are obtained [based
on compounds 4C$BF4K, 5C$BF4K, and 6a,eC$BF4K]
(Table 5), and thus, autorecycling oxidation clearly
proceeds; however, cyclohexylamine was not oxidized
(Table 5, entries 26–29). In order to clarify the details of
the oxidizing reaction, time dependency of the yields of the
imines was investigated as summarized in Table 5 (entries
1–16) and Figure 5. Concerning the oxidizing reaction using
4C$BFK4 , the yield of benzaldimine was increased simply
as the irradiation time was prolonged to 8 h. After
irradiation for 12 and 16 h, the yield of benzaldimine is
not so increased, suggesting plausible decomposition of
4C$BFK4 . A Similar feature is observed in the oxidizing
reaction by using 5C$BF4K, and 6a,eC$BFK4 . The photo-
irradiation of CD3CN solution of 4C$BF4K, 5C$BF4K, and
6a,eC$BFK4 in the absence of amine under aerobic
conditions, decomposition of 4C$BF4K, 5C$BF4K, and
Figure 4. UV–vis spectra of 9 and 12 in CH3CN.
equivalent signal at room temperature (dC 162.1).12 In
addition, the chemical shift of the iminium-carbon C6 at K
90 8C (dC 172.6) is similar to the corresponding signal at
room temperature (dC 174.4),12 thus, the interaction
between the carbonyl oxygen atom O1 and the C6 would
not be large in solution.
The UV–vis spectral data of 9 and 12 in CH3CN are
summarized in Figure 4. The spectra of 9 and 12 show
remarkable difference, and the longest wavelength absorption
maximum of 12 shows a blue-shift by 122 nm, as compared
with that of 9. Furthermore, the reduction and oxidation
potentials of 9 and 12 in CH3CN were determined by cyclic
voltammetry (CV), and each reduction wave and oxidation
wave are irreversible. While the reduction potentials (E1red) of
9and12areK1.40andK1.85 V,respectively, theiroxidation
potentials (E1ox) are C0.63 and C0.41 V, respectively. Thus,
the values (E1red and E1ox) of 12 are more negative than those
of 9. After the first cycle of CV measurement of 12, another
reversible reduction wave was recorded at C0.10 V, which is
probably the reduction wave of14, generated bycyclization of
13 under CV measurement (Scheme 3). After generation of 14
and subsequent measurement in the limited range of K0.20
andC0.40 V, decay of this wave was observed due to the slow
ring-opening reaction of 15 giving 12. In contrast, neutral
imine 9 does not exhibit behavior similar to that of 12.
6a,eC$BFK4 was not observed as monitored by their H
1
NMR spectra. Thus, the decomposition of 4C$BF4K,
5C$BFK4 , and 6a,eC$BFK4 would occur in their oxidation
cycle.
Furthermore, in the oxidation of benzylamine by using
5C$BFK4 , and 6a,eC$BFK4 , the yield of the imine became
higher in the order 6eC$BF4K!6aC$BF4K!5C$BFK4
(Table 5, entries 5–16). This fact is probably ascribed to
the E1red values in the order 6eC$BF4K (K0.87 V)14O
6aC$BFK4 (K0.84 V)14O5C$BFK4 (K0.53 V).13 [The
reduction potentials of 5C$BF4K, and 6a,eC$BFK4 in the
ground state would be correlated with their LUMO’s, and
thus, the LUMO’s of these compounds would be lower in
the order 6eC$BFK4 O6aC$BF4KO5C$BFK4 . In the excited
state of these compounds, the electron-accepting orbital
would be the singly occupied HOMO’s. In as much as the
UV–vis spectra of these compounds are similar, and thus,
the energy level of HOMO’s of the compounds is
expected to be lower in the order 6eC$BF4KO6aC$
BFK4 O5C$BFK4 .] A similar tendency was observed in the
cases of the oxidation of 1-phenylethylamine and hexyl-
amine (Table 5, entries 19–21 and 23–25). Benzylamine and
hexylamine are oxidized more effectively by using
4C$BFK4 (Table 5, entries 4 and 22), which has a lower
Scheme 3.

9144
S. Naya, M. Nitta / Tetrahedron 60 (2004) 9139–9148
Scheme 4. Reagents and conditions: (i) hn, aerobic, CH3CN, room temperature.
oxidizing ability toward 1-phenylethylamine as compared to
5C$BFK4 (cf. entries 18 and 19). The feature is probably due
to the ring-opening reaction of 4C$BF4K with some amines.
However, in the oxidation of benzylamine by using isolated
9, the yield of the imine (643%) is lower than that by using
4C$BFK4 (8161%) (Table 5, entries 17 and 4). Thus, the
presence of HBF4 is necessary for the effective oxidizing
cycle, suggesting that the reaction of 9 with HBF4 would
regenerate 4C$BF4K in the oxidizing cycle.
In a search for the substituent effect of benzylamine toward
oxidation, the oxidation reactions of 4-substituted
Table 5. Autorecycling oxidation of some amines by 4C$BF4K–6a,eC$BF4K under photo-irradiationa
Entry
Compound
Amine
Time/h
Yieldb/%
Entry
Compound
Amine
Time/h
Yieldb/%
1
2
3
4
5
6
7
8
4
4
4
4
5
5
5
5
C$BFK4
C$BFK4
C$BFK4
C$BFK4
C$BFK4
C$BFK4
C$BFK4
C$BFK4
PhCH2NH2
PhCH2NH2
PhCH2NH2
PhCH2NH2
PhCH2NH2
PhCH2NH2
PhCH2NH2
PhCH2NH2
PhCH2NH2
PhCH2NH2
PhCH2NH2
PhCH2NH2
PhCH2NH2
PhCH2NH2
PhCH2NH2
PhCH2NH2
PhCH2NH2
4
8
12
16
4
4601
7559
7951
8161
3552
5076
6846
6993
1476
2552
4133
4175
455
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
4
5
C$BFK4
C$BFK4
PhCH(Me)NH2
PhCH(Me)NH2
PhCH(Me)NH2
PhCH(Me)NH2
Hexylamine
Hexylamine
Hexylamine
Hexylamine
Cyclohexylamine
Cyclohexylamine
Cyclohexylamine
Cyclohexylamine
4-MeOC6H4CH2NH2
4-MeC6H4CH2NH2
4-ClC6H4CH2NH2
4-PyCH2NH2
16
16
16
16
16
16
16
16
16
16
16
16
16
16
16
16
3947
7040
3533
2367
7464
4457
2557
1564
0c
6aC$BFK4
6eC$BFK4
4
5
C$BFK4
C$BFK4
8
12
16
4
6aC$BFK4
6eC$BFK4
9
6aC$BFK4
6aC$BFK4
6aC$BFK4
6aC$BFK4
6eC$BFK4
6eC$BFK4
6eC$BFK4
6eC$BFK4
9
4
5
C$BFK4
C$BFK4
10
11
12
13
14
15
16
17
8
0c
12
16
4
6aC$BFK4
6eC$BFK4
0c
0c
4
4
4
4
C$BFK4
C$BFK4
C$BFK4
C$BFK4
6278
7967
8362
6789
8
1385
2993
3000
643
12
16
16
Isolated by converting to the corresponding 2,4-dinitrophenylhydrazone.
a
CH3CN solution was irradiated by RPR-100, 350 nm lamps under aerobic conditions.
Based on 4C$BF4K, 5C$BF4K, and 6a,eC$BFK4 used; the yield is calculated by subtraction of the ‘blank’ yield from the total yield of carbonyl compound in
b
the presence of 4C$BF4K, 5C$BF4K, and 6a,eC$BF4K
The blank yield was higher than the yield in the presence of 4C$BF4K, 5C$BF4K, and 6a,eC$BFK4
.
c
.

S. Naya, M. Nitta / Tetrahedron 60 (2004) 9139–9148
9145
state of 4C, 5C, and 6a,bC would occur to produce radicals
17 and a cation radical 18. On the other hand, the amine-
adducts obtained by the reaction of 4C, 5C, and
6a,eC$BFK4 with amines, are stable in solution in the
dark.12–14 Thus, there is also a possibility that the homolysis
of 16 by photo irradiation would afford 17 and 18 directly.
An electron transfer from radical species 17 to a molecular
oxygen would regenerate 4C, 5C, and 6a,eC, and the
superoxide anion radical, since tropyl radical derivatives are
known to be readily oxidized by molecular oxygen.19 Then,
a proton-transfer from cation radical 18 to a superoxide
anion radical occurs to result in the formation of the
products 20 and H2O2 (Path A). Compound 20 reacts with
excess amine to give imine 21. Substituted benzylamine
having a more negative oxidation potential seems to favor
the electron transfer process from amine to the excited state
of 4C, 5C, and 6a,eC, but disfavors the proton transfer
process from the cation radical 18 to the superoxide anion
radical. On the contrary, substituted benzylamine having a
more positive oxidation potential disfavors the electron
transfer process from amine to the excited state of 4C, 5C,
and 6a,eC, while the proton transfer process from cation
radical 18 to the superoxide anion radical becomes more
favorable. As such, a sensitive balance between the electron
donor ability of amine and the proton donor ability of cation
radical 18 is required to achieve the efficient photo-induced
oxidation reaction of amines. There may be an alternative
mechanistic pathway (Path B), in which compound 22 and
the imine 20 are generated directly from 17 and 18; the
former compound is oxidized under aerobic and photo-
irradiation conditions to regenerate 4C, 5C, and 6a,eC.
Under aerobic and photo-irradiation conditions, the CD3CN
solution of 22 was easily oxidized to regenerate 4C, 5C, and
6a,eC quantitatively. Thus, autorecycling oxidation would
also be possible in this Path B. However, attempted
detection of compound 17 or its dimers or compound 22
was unsuccessful in the oxidation reaction of benzylamine
under degassed and photo-irradiation conditions (degassed
by freeze–pump–thaw cycles). Thus, we prefer the former
pathway (Path A).
Figure 5. Time dependency of autorecycling oxidation of benzylamine by
using 4C$BF4K, 5C$BF4K, and 6a,eC$BF4K
.
benzylamines and picolylamine were carried out by using
4C$BFK4 under aerobic and photo-irradiation conditions
(Table 5, entries 30–33). The yields of imines are plotted
against the Hammet constants sp17 of the substituents on the
phenyl group and 4-picolylamine in Figure 6. The plots
seem to show a maximum value, and the yield of photo-
induced oxidation of the benzylamines becomes low at
either the high value (sp 0.23, 4-ClC6H4CH2NH2) or the low
value (sp K0.27, 4-MeOC6H4CH2NH2). The yields of the
imine derived from 4-picolylamine, which corresponds to
the benzylamine having a strong electron-withdrawing
substituent, becomes low and may be close to the yield
expected from 4-nitrobenzylamine. Thus, the oxidizing
reaction by using 4C$BF4K becomes less effective for the
amines, which have both lower and higher oxidation
potential. This feature is similar to the cases of the photo-
induced oxidizing reaction of benzy alcohol by using a
flavin analogue18 and the case of photo-induced oxidizing
reaction of benzylamine by using 2a,b,10 and thus, it is
rationalized by the pathways via a tropyl radical intermedi-
ate (vide infra).
The mechanistic pathways for the present oxidation of
amines are depicted in Scheme 4 by using general
structures.18 The electron-transfer from amine to the excited
3. Conclusion
The ring transformation of 4C$BF4K to the corresponding
pyrrole derivatives 6a–dC$BF4K was accomplished.
Furthermore, the detailed characteristics of troponeimine
intermediates 9 and 12 were clarified by the inspection of
their X-ray crystal analyses, NMR, UV–vis spectra, and CV
measurements. Novel photo-induced oxidation reaction of
4C$BF4K, 5C$BF4K, and 6a,eC$BFK4 toward some amines
under aerobic conditions was clarified to give the corre-
sponding imines in more than 100% yield [based on
compounds 4C, 5C, and 6a,eC], suggesting the oxidation
reaction occurs in an autorecycling process. Mechanistic
aspects of the amine-oxidation are also postulated.
4. Experimental
4.1. General
Figure 6. The Hammet plot of autorecycling oxidation of 4-substituted
benzylamine by 4C$BF4K. (a: 4-MeOC6H4CH2NH2, b: 4-MeC6H4CH2-
NH2, c: PhCH2NH2, d: 4-ClC6H4CH2NH2, e: 4-PyCH2NH2).
IR spectra were recorded on a HORIBA FT-710

9146
S. Naya, M. Nitta / Tetrahedron 60 (2004) 9139–9148
spectrometer. Mass spectra and high-resolution mass
spectra were run on JMS-AUTOMASS 150 and JMS-
SX102A spectrometers. Unless otherwise specified, H and
reaction was completed, the mixture was concentrated in
vacuo. The resulting residue was dissolved in a mixture of
acetic anhydride (1 mL) and 42% aq HBF4 (0.2 mL) at 0 8C,
and the mixture was stirred for another 1 h. To the mixture
was added Et2O (10 mL) and the precipitates were collected
by filtration to give a mixture of 6dC$BF4K and 4C$BF4K
(Table 2, runs 1 and 2).
1
13C NMR spectra were recorded on JNM-lambda500 and
AVANCE600 spectrometers using CD3CN as a solvent, and
the chemical shifts are given relative to internal SiMe4
standard; J-values are given in Hz. Mps were recorded on a
Yamato MP-21 apparatus and were uncorrected.
4.4. Reaction of 9 with (COCl)2 without solvent
4.2. Ring transformation of 4C$BF4K to 6a–cC$BFK4
A solution of 9 (35 mg, 0.1 mmol) in (COCl)2 (2 mL) was
heated under reflux for 3 h. After the reaction was
completed, the mixture was concentrate in vacuo. The
resulting residue was dissolved in a mixture of acetic
anhydride (1 mL) and 42% aq HBF4 (0.2 mL) at 0 8C, and
the mixture was stirred for another 1 h. To the mixture was
added Et2O (10 mL) and the precipitates were collected by
filtration to give a mixture of 6dC$BF4K and 4C$BF4K
(Table 2, run 3).
To a solution of 4C$BF4K (99 mg, 0.3 mmol) in CH3CN
(10 mL) was added aniline or 4-substituted aniline
(1.2 mmol), and the mixture was heated under reflux until
the reaction was completed (Table 1). The mixture was
concentrate in vacuo, the resulting residue was dissolved in
a mixture of acetic anhydride (5 mL) and 42% aq HBF4
(1 mL) at 0 8C, and it was stirred for another 1 h. To the
mixture was added Et2O (50 mL) and the precipitates were
collected by filtration to give products 6a–cC$BF4K
(Table 1). Compound 6aC$BFK4 was identical with the
authentic specimen.14
4.5. Reaction of 9 with SOCl2
A solution of 9 (35 mg, 0.1 mmol) in SOCl2 (2 mL) was
heated under reflux for 3 h. After the reaction was
completed, the mixture was concentrate in vacuo. The
resulting residue was dissolved in a mixture of acetic
anhydride (1 mL) and 42% aq HBF4 (0.2 mL) at 0 8C, and
the mixture was stirred for another 1 h. To the mixture was
added Et2O (10 mL) and the precipitates were collected by
filtration to give a single product 6dC$BF4K (Table 2, run 5).
4.2.1. 7,9-Dimethyl-6-(40-methoxyphenyl)cyclohepta[b]-
pyrimido[5,4-d]pyrrole-8(7H), 10(9H)-dionylium tetra-
fluoroborate (6bC$BF4K). Orange prisms; mp 227–230 8C
1
dec (from CH3CN/Et2O); H NMR (500 MHz, CD3CN) d
3.17 (3H, s, 7-Me), 3.46 (3H, s, 09-Me), 3.95 3H, s, OMe),
7.27 0(2H, d, JZ9.0 Hz, Ph-30, 5 ), 7.59 (2H, d, JZ9.0 Hz,
Ph-2 , 60), 8.31 (1H, d, JZ10.2 Hz, H-5), 8.37 (1H, dd, JZ
10.2, 9.7 Hz, H-4), 8.56 (1H, dd, JZ9.7, 9.5 Hz, H-3), 8.63
(1H, dd, JZ10.3, 9.5 Hz, H-2), 9.94 (1H, d, JZ10.3 Hz,
H-1); 13C NMR (150.9 MHz, CD3CN) d 29.1, 33.5, 56.9,
99.8, 116.9, 126.3, 131.8, 134.1, 139.8, 141.2, 143.4, 143.6,
145.8, 151.1, 152.2, 153.5, 159.0, 163.5; IR (KBr) n 1717,
1675, 1084 cmK1; MS (FAB) m/z 348 (MC-BF4); HRMS
calcd for C20H18BF4N3O3: 348.1347 (MKBF4). Found:
348.1391 (MC-BF4). Anal. calcd for C20H18BF4N3O3: C,
55.20; H, 4.17; N, 9.66. Found: C, 54.95; H, 4.08; N, 9.43.
4.5.1. 7,9-Dimethyl-6-benzylcyclohepta[b]pyrimido
[5,4-d]pyrrole-8(7H),10(9H)-dionylium tetrafluoroborate
(6dC$BFK4 ). Orange prisms; mp 210–211 8C (from CH3CN/
1
Et2O); H NMR (600 MHz, CD3CN) d 3.45 (3H, s, 9-Me),
3.75 (3H, s, 7-Me), 6.12 (2H, s, CH2), 7.19–7.21 (2H, m,
m-Ph), 7.40–7.44 (3H, m, o,p-Ph), 8.44 (1H, dd, JZ10.2,
10.0 Hz, H-2), 8.57 (1H, dd, JZ10.0, 9.5 Hz, H-4), 8.62
(1H, dd, JZ10.2, 9.5 Hz, H-3), 8.76 (1H, d, JZ10.2 Hz,
H-5), 9.97 (1H, d, JZ10.2 Hz, H-1); 13C NMR (150.9 MHz,
CD3CN) d 29.0, 33.8, 51.2, 100.5, 126.4, 129.6, 130.3,
133.0, 134.9, 139.6, 141.1, 143.6, 143.9, 145.8, 149.4,
4.2.2. 6-(40-Chlorophenyl)-7,9-dimethylcyclohepta[b]-
pyrimido[5,4-d]pyrrole-8(7H), 10(9H)-dionylium tetra-
fluoroborate (6cC$BF4K). Greenish prisms; mp 266–
269 8C dec (from CH3CN/Et2O); 1H NMR (500 MHz,
CD3CN) d 3.17 (3H, s, 7-Me), 3.46 (3H, s, 9-Me), 7.70 (2H,
d, JZ8.8 Hz, Ph-20, 60), 7.80 (2H, d, JZ8.8 Hz, Ph-30, 50),
8.29 (1H, d, JZ10.2 Hz, H-5), 8.37 (1H, dd, JZ10.2,
9.8 Hz, H-4), 8.58 (1H, dd, JZ9.8, 9.7 Hz, H-3), 8.65 (1H,
dd, JZ10.2, 9.7 Hz, H-2), 9.95 (1H, d, JZ10.2 Hz, H-1);
13C NMR (150.9 MHz, CD3CN) d 29.2, 33.8, 100.0, 132.1,
132.3, 133.1, 134.2, 139.2, 139.9, 141.4, 143.7, 143.9,
146.1, 150.6, 152.0, 153.4, 159.0; IR (KBr) n 1722, 1675,
1084 cmK1; MS (FAB) m/z 352 (MCKBF4); HRMS calcd
for C19H15BClF4N3O2: 352.0853 (MKBF4). Found:
352.0858 (MCKBF4). Anal. calcd for C19H15BClF4N3O2:
C, 51.91; H, 3.44; N, 9.56. Found: C, 51.80; H, 3.38; N,
9.46.
152.1, 154.2, 158.8; IR (KBr) n 1717, 1675, 1084 cmK1
;
MS (FAB) m/z 332 (MCKBF4); HRMS calcd for
C20H18BF4N3O2: 332.1399 (MKBF4). Found: 332.1379
(MCKBF4). Anal. calcd for C20H18BF4N3O2C1/5H2O: C,
56.82; H, 4.39; N, 9.94. Found: C, 56.75; H, 4.42; N, 9.96.
4.6. X-ray structure determination of 9†
Reddish plate, C20H19N3O3C2CHCl3, MZ588.14, mono-
˚
˚
clinic, space group P21/c, aZ11.1393(4) A, bZ19.5525(8) A,
3
˚
˚
cZ11.6973(6) A, bZ94.569(2) 8, VZ2539.6(2) A , ZZ4,
DcZ1.538 g mLK1, crystal dimensions 0.50!0.40!
0.10 mm3. Data were measured on a Rigaku RAXIS-
RAPID radiation diffractomater with graphite monochro-
mated Mo Ka radiation. Total 24288 reflections were
collected, using the u–2q scan technique to a maximum
2q value of 55.08. The structure was solved by direct
methods and refined by a full-matrix least-squares method
using SIR92 structure analysis software,20 with 357
4.3. Reaction of 9 with (COCl)2 in (CH2CCl)2
To a solution of 9 (35 mg, 0.1 mmol) in (CH2Cl)2 (2 mL)
was added (COCl)2 (65 mg, 0.5 mmol), and the mixture was
stirred at room temperature or 70 8C for 1 h. After the
† CCDC reference number 243861.

S. Naya, M. Nitta / Tetrahedron 60 (2004) 9139–9148
9147
variables and 3617 observed reflections [IO3.00s(I)]. The
4.9. General procedure for the autorecycling oxidation
non-hydrogen atoms were refined anisotropically. The
weighting scheme wZ4F2o[0.500sc2(Fo)0.002Fo2]K1 gave
satisfactory agreement analysis. The final R and Rw values
were 0.0410 and 0.1220. The maximum peak and minimum
of amines catalyzed by 4C$BF4K, 5C$BF4K, and
6a,eC$BFK4 under photo-irradiation
A CH3CN (16 mL) solution of compounds 4C$BF4K,
5C$BF4K, and 6a,eC$BFK4 (0.005 mmol) and amines
(2.5 mmol, 500 equiv.) in a Pyrex tube was irradiated by
RPR-100, 350 nm lamps under aerobic conditions for the
period indicated in Table 5. The reaction mixture was
concentrated in vacuo and diluted with Et2O and filtered.
The 1H NMR spectra of the filtrates revealed the formation
of the imines (Table 5). The filtrate was treated with a
saturated solution of 2,4-dinitrophenylhydrazine in 6% HCl
to give 2,4-dinitrophenylhydrazone. The results are sum-
marized in Table 5.
K
3
˚
peak in the final difference map were 0.61 and K0.77 e /A ,
respectively.
4.7. X-ray structure determination of 12‡
Orange prism, C17H21N3O3, MZ315.37, orthorhombic,
˚
˚
˚
space group Pna21, aZ7.1520(2) A, bZ18.7731(5) A,
3
˚
cZ11.4415(3) A, VZ1536.20(7) A , ZZ4, DcZ
1.363 g mLK1, crystal dimensions 0.80!0.50!0.10 mm3.
Data were measured on a Rigaku RAXIS-RAPID radiation
diffractomater with graphite monochromated Mo Ka radi-
ation. Total 13,877 reflections were collected, using the
u–2q scan technique to a maximum 2q value of 55.08. The
structure was solved by direct methods and refined by a full-
matrix least-squares method using SIR92 structure analysis
software,20 with 230 variables and 1671 observed reflec-
tions [IO3.00s(I)]. The non-hydrogen atoms were refined
anisotropically. The weighting scheme wZ4F2o[0.500sc2
(Fo)C0.0030F2o]K1 gave satisfactory agreement analysis.
The final R and Rw values were 0.0440 and 0.1230. The
maximum peak and minimum peak in the final difference
Acknowledgements
Financial support from a Waseda University Grant for
Special Research Project and 21COE ‘Practical Nano-
chemistry’ from MEXT, Japan, is gratefully acknowledged.
We thank the Materials Characterization Central Labora-
tory, Waseda University, for technical assistance with the
spectral data, elemental analyses and X-ray analyses.
K
3
˚
map were 0.26 and K0.32 e /A , respectively.
4.7.1. Variable temperature NMR data of 12. Tempera-
ture: K90 8C (CD2Cl2): 1H NMR (500 MHz) d 1.02 (3H, t,
JZ7.2 Hz, CH3), 1.34 (3H, t, JZ7.2 Hz, CH3), 3.22 (3H, s,
NMe), 3.33 (3H, s, NMe), 3.44–3.53 (2H, m, CH2), 3.58–
3.65 (1H, m, CH2), 3.71–3.78 (1H, m, CH2), 6.76 (1H, dd,
JZ10.6, 7.2 Hz, H-3), 6.83 (1H, d, JZ11.6 Hz, H-5), 7.09
(1H, dd, JZ11.6, 7.2 Hz, H-4), 7.12 (1H, dd, JZ10.6,
8.4 Hz, H-2), 8.64 (1H, d, JZ8.4 Hz, H-1); 13C NMR
(125.7 MHz) d 10.7 (CH3), 11.3 (CH3), 26.9 (NCH3), 27.3
(NCH3), 44.0 (CH2), 46.0 (CH2), 86.2 (C-5), 113.1 (C-70),
122.7 (C-30), 124.4 (C-50), 126.0 (C-20), 129.8 (C-60), 133.4
(C-400), 151.9 (C-2), 160.3 (C-4 or 6), 161.5 (C-4 or 6), 172.6
(C-1 ).
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)
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