Synthesis, properties, and oxidizing ability of areno-annulated 1,3-dimethyl-10-phenylcyclohepta[4,5]pyrrolo[2,3-d]pyrimidine-2,4(1,3H) -dionylium ions

Article abstract of DOI:10.1021/jo051777j

Novel areno-annulated 1,3-dimethyl-10-phenylcyclohepta[4,5]pyrrolo[2,3-d] pyrimidine-2,4(1,3H)-dionylium ions 12a,b⁺·BF₄ ^- and 16a⁺-BF₄^- were synthesized by three-step reactions, starting f

Full text of DOI:10.1021/jo051777j

Synthesis, Properties, and Oxidizing Ability of Areno-Annulated
1,3-Dimethyl-10-phenylcyclohepta[4,5]pyrrolo[2,3-d]pyrimidine-
2,4(1,3H)-dionylium Ions
Shin-ichi Naya, Hiroshi Ohtoshi, and Makoto Nitta*
Department of Chemistry, Faculty of Science and Engineering, Waseda UniVersity, Shinjuku-ku,
Tokyo 169-8555, Japan
nitta@waseda.jp
ReceiVed August 23, 2005
Novel areno-annulated 1,3-dimethyl-10-phenylcyclohepta[4,5]pyrrolo[2,3-d]pyrimidine-2,4(1,3H)-dio-
-
-
nylium ions 12a,b+‚BF4 and 16a+‚BF4 were synthesized by three-step reactions, starting from the
reactions of benzo[b]tropone and naphtho[2,3-d]tropone with 6-anilino-1,3-dimethyluracil. Structural
characteristics of 12a,b+ and 16a+ were clarified on inspection of the UV-vis and NMR spectral data
as well as by X-ray crystal analyses. The stability of cations 12a,b+ and 16a+ is expressed by the pKR+
values which were determined spectrophotometrically as the values of ca. 0.5-9.0. The pKR+ value of
the naphtho[b]tropylium ion 4+ was clarified to be much lower, at <0. The electrochemical reduction of
12a,b+ and 16a+ exhibited reduction potentials at -0.46 to -0.67 (V vs Ag/AgNO3) upon cyclic
voltammetry (CV). The reduction potentials of the benzotropylium ion and cation 4+ were -0.26 and
-0.09 V, respectively. In a search for reactivity, reactions of 12a,b+‚BF4- with some nucleophiles, hydride
-
and diethylamine, were carried out. Although the reactions of 12a+‚BF4 afforded C11 adduct 19 as a
single product, the addition reactions of 12b+‚BF4- proceeded at both C9 and C11. The attempted reduction
of methyl benzoylformate by using 21 was carried out unsuccessfully. The photoinduced oxidation reaction
of 12a,b+‚BF4 and 16a+‚BF4 toward some amines under aerobic conditions was carried out to give
the corresponding imines (isolated by converting to the corresponding 2,4-dinitrophenylhydrazones) with
the recycling number of 3.6-21.7.
-
-
Introduction
benzene rings. These long-known substances have attracted the
wide interest of both theoretical and experimental scientists.2
Among these substances, pentacene has received much attention
as an active semiconducting material in field-effect transistors
because of its unusually high charge-carrier mobility.3 On the
other hand, helicenes have also attracted general interest because
of their repeating structural motif of ortho-fused aromatic rings
The concept of aromaticity is fundamental to rationalize and
understand the structure and reactivity of many known chemical
species.1 In the quest for a better understanding of this concept,
the study of compounds with condensed benzene rings has
occupied many organic and theoretical chemists. Polyacenes are
polycyclic aromatic hydrocarbons consisting of linearly fused
(2) (a) Clar, E. Polycyclic Hydrocarbons; Academic Press: London,
1964; Vols. 1 and 2. (b) Geerts, Y.; Kla¨rner, G.; Mu¨llen, K. In Electronic
Materials: The Oligomer Approach; Mu¨llen, K., Wagner, G., Eds.; Wiley-
VCH: Weinheim, Germany, 1998; 48.
* Corresponding author. Telephone: +81-(0)3-5286-3236. Fax: +81-(0)3-
3208-2735.
(1) Balaban, A. T.; Oniciu, D. C.; Katritzky, A. R. Chem. ReV. 2004,
104, 2777.
10.1021/jo051777j CCC: $33.50 © 2006 American Chemical Society
Published on Web 12/01/2005
176
J. Org. Chem. 2006, 71, 176-184

Synthesis and Properties of Areno-Annulated Ions
SCHEME 1a
FIGURE 1. Annulated tropylium ions.
that result in a unique combination of the following features:
helical chirality, inherently strong chromophores, and the
possibility for steric/electronic interactions between overlapping
rings.4 Thus, the π-conjugation mode in polycyclic conjugated
π-systems containing more than one (4n + 2) conjugation loop
is an important subject from both theoretical and experimental
viewpoints. The combination of more than one π-system can
endow the original π-system with new properties. Previously,
we have studied the synthesis and structural and chemical
properties of catacondensed aromatic π-systems, azuleno[1,2-
a]azulene derivatives5 and azuleno-annulated 1,6-methano[10]-
annulene derivatives.6 We have now focused our attention on
the cycloheptatrienylium (tropylium) ion 1+‚BF4 (Figure 1),
-
which has fair thermodynamic stability (pKR+ ) 3.9).7 Although
the tropylium ion 1+ is stabilized by annulation with five-
membered aromatic heterocycles, as demonstrated by cations
2a+ and 2b+ (2a+ pKR+ ) 6.7; 2b+ pKR+ ) 6.0),8,9 tropylium
ions annulated with benzene, 3+, have been appreciably
destabilized (pKR+ ) 1.6).10 In addition, because the synthesis
of naphtho-annulated tropylium ion 4+ has been reported,11 its
stability and electrochemical properties are also interesting.
a
Reagents and conditions: (i) (a) LiAlH4, Et2O, room temperature, 0.5
h; (b) 60% aqueous HClO4, Ac2O, 0 °C, 20 min; (ii) NaH, CH3CN, -40
°C, 20 h; (iii) NaBH4, CeCl3, molecular sieve of 0.4 nm, EtOH, reflux, 4
h; (iv) p-TsOH, molecular sieve of 0.4 nm, CH3CN, room temperature, 6
h; (v) (a) DDQ, CHCl3, reflux, 16 h; (b) 42% aqueous HBF4, Ac2O, 0 °C,
1 h.
as their novel photoinduced autorecycling oxidizing reactions
toward some alcohols and amines.15 In this context, we have
On the other hand, we have reported the synthesis, properties,
and reactivity of 1,3-dimethylcyclohepta[4,5]pyrrolo[2,3-d]-
pyrimidine-2,4(1,3H)-dionylium ion 5a,b+‚BF4-12 and its furan
recently reported the synthesis, properties, and oxidizing ability
of 6+‚BF4
.
The properties and reactivity of compound
- 16
- 13
- 14
and thiophene analogues 5c+‚BF4
and 5d+‚BF4
as well
-
6+‚BF4 were much perturbed by the benzo annulation on
5c+‚BF4-. Thus, the areno annulation onto 5a+‚BF4- is a very
interesting project from the viewpoint of exploration of novel
functions. In this study, we have investigated the synthesis of
novel cations 12a,b+‚BF4- (Scheme 1) and 16a+‚BF4- (Scheme
2), which are derived from annulation of 5a+ with benzene and
naphthalene rings. Their structural characteristics were deduced
on the basis of the UV-vis and NMR spectral data as well as
by X-ray crystal analyses. The stability and electrochemical
(3) (a) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002,
14, 99. (b) Hegmann, F. A.; Tykwinski, R. R.; Lui, K. P. H.; Bullock, J.
E.; Anthony, J. E. Phys. ReV. Lett. 2002, 89, 227403-1/4. (c) Meng, H.;
Bendikov, M.; Mitchell, G.; Helgeson, R.; Wudl, F.; Bao, Z.; Siegrist, T.;
Kloc, C.; Chen, C.-H. AdV. Mater. 2003, 15, 1090.
(4) Martin, R. H. Angew. Chem., Int. Ed. Engl. 1974, 13, 649.
(5) (a) Nitta, M.; Iino, Y.; Sugiyama, T.; Akaogi, A. Tetrahedron Lett.
1993, 34, 831. (b) Nitta, M.; Iino, Y.; Sugiyama, T.; Toyota, A. Tetrahedron
Lett. 1993, 34, 835. (c) Nitta, M.; Nishimura, K.; Iino, Y. Tetrahedron Lett.
1993, 34, 2157.
(6) (a) Nitta, M.; Kawaji, H.; Kanomata, N. Tetrahedron Lett. 1992, 33,
251. (b) Ito, K.; Kawaji, H.; Nitta, M. Tetrahedron Lett. 1994, 35, 2561.
(7) Okamoto, K.; Takeuchi, K.; Komatsu, K.; Kubota, Y.; Ohara, R.;
Arima, M.; Takahashi, K.; Waki, Y.; Shirai, S. Tetrahedron 1983, 39, 4011
and references therein.
(8) Kato, M.; Kobayashi H.; Miwa, T. Tetrahedron Lett. 1980, 21, 3375.
(9) Turnbo, R. G.; Sullivan, D. L.; Pettit, R. J. Am. Chem. Soc. 1964,
86, 5630.
properties of 12a,b+‚BF4 and 16a+‚BF4 as well as those of
-
-
naphtho[b]tropylium ion 4+‚ClO4 were demonstrated. In a
-
search for reactivity, reactions of 12a,b+‚BF4 with some
-
nucleophiles, hydride and diethylamine, were carried out. The
photoinduced oxidizing reaction of 12a,b+‚BF4- and 16a+‚BF4
-
(13) (a) Naya, S.; Miyama, H.; Yasu, K.; Takayasu, T.; Nitta, M.
Tetrahedron 2003, 59, 1811. (b) Naya, S.; Nitta, M. Tetrahedron 2003, 59,
3709.
(10) Rennhard, H. H.; Heilbronner, E.; Eschenmoser, A. Chem. Ind. 1955,
415.
(11) (a) Komatsu, K.; Tsuji, R.; Takeuchi, K. Tetrahedron Lett. 1989,
30, 4689. (b) Gerson, F.; Huber, W.; MGllen, K. HelV. Chim. Acta 1981,
64, 2766.
(14) Naya, S.; Miyama, H.; Yasu, K.; Takayasu, T.; Nitta, M. Tetrahe-
dron 2003, 59, 4929.
(15) Naya, S.; Nitta, M. Tetrahedron 2004, 60, 9139.
(16) Naya, S.; Tokunaka, T.; Nitta, M. J. Org. Chem. 2003, 68, 9317.
(12) Naya, S.; Nitta, M. Tetrahedron 2003, 59, 7291.
J. Org. Chem, Vol. 71, No. 1, 2006 177

Naya et al.
SCHEME 2 a
in the presence of p-TsOH at room temperature for 6 h afforded
a mixture of 9a and 9b in almost quantitative yield (99%). The
ratio of 9a and 9b was determined to be 1:5 from the 1H NMR
spectrum of the mixture. Thus, the synthesis of desired
compounds 9a,b was achieved without isolation of unstable
cation 3+. Because compounds 9a,b were unstable on SiO2 and
Al2O3, these regioisomers could not be separated, and thus, the
mixture of 9a and 9b was used for further reaction. A heating
of the mixture of 9a and 9b with DDQ in CHCl3 under reflux
for 16 h and a subsequent anion exchange reaction using
-
aqueous HBF4 in Ac2O afforded a mixture of 12a+‚BF4 and
-
-
12b+‚BF4 in a moderate yield. Separation of 12a+‚BF4 and
12b+‚BF4 was accomplished by fractional recrystallization
-
-
from CH3CN/AcOEt to give pure samples of 12a+‚BF4 and
12b+‚BF4-. Although the ratio of starting 9a and 9b was 1:5,
-
-
the yields of products 12a+‚BF4 and 12b+‚BF4 were 30%
and 8%, respectively. This fact suggests that the rearrangement
from 9b to 9a would occur under the reaction conditions (vide
infra).
The synthesis of naphtho-annulated derivative 16a+‚BF4- was
accomplished by a similar route starting from naphtho[2,3-d]-
tropone 1320 (Scheme 2). The reduction of 13 with NaBH4 in
EtOH in the presence of CeCl3 and molecular sieves (0.4 nm)
was carried out to give 14 in 95% yield as a single product. In
addition, the treatment of 14 with 70% aqueous HClO4 in Ac2O
afforded naphtho[b]tropylium ion 4+‚ClO4- in 64% yield. Thus,
the reduction of tropone derivatives using NaBH4 in the presence
of CeCl3 would provide a useful method for the generation of
unstable tropylium ions. The reaction of 8 with 14 in CH3CN
in the presence of p-TsOH at room temperature for 3 h afforded
a mixture of 15a and 15b in almost quantitative yield (99%).
The ratio of 15a and 15b was determined to be 1:7 from the 1H
NMR spectrum of the mixture. Heating of the mixture of 15a,b
and DDQ in CHCl3 under reflux for 16 h and a subsequent
anion exchange reaction using aqueous HBF4 in Ac2O afforded
16a+‚BF4-, which is expected from 15a, in a modest yield
(19%) as a single product, and generation of 16b+ was not
observed. Apparently, rearrangement from 15b to 15a would
occur under the reaction conditions. Because 15b could be
isolated by recrystallization of a mixture of 15a and 15b, thermal
reactions of 15b in the absence or presence of CHCl3 were
monitored by 1H NMR spectroscopy in acid-free CDCl3
prepared by passing through basic Al2O3. By heating at 60 °C
for 6 h, the reaction of 15b in the absence of CHCl3 afforded
a mixture of 8, 15a, and 15b in a ratio of 0.01:0.04:1.00. In
contrast, the reaction in the presence of CHCl3 afforded a
mixture of 8, 15a, and 15b in a ratio of 0.03:1.40:1.00,
suggesting that the acid-catalyzed rearrangement from 15b to
15a and decomposition would occur. Consequently, the method
for synthesizing 16a+‚BF4- was modified as follows: A solution
of 15a and 15b in CHCl3 was heated at 60 °C for 6 h, followed
by addition of DDQ. The resulted mixture was heated under
reflux for 16 h, and a subsequent anion exchange reaction using
a
Reagents and conditions: (i) NaBH4, CeCl3, molecular sieve of 0.4
nm, EtOH, reflux, 5 h; (ii) 60% aqueous HClO4, Ac2O, 0 °C, 20 min; (iii)
p-TsOH, molecular sieve of 0.4 nm, CH3CN, room temperature, 3 h; (iv)
(a) CHCl3, reflux, 6 h; (b) DDQ, CHCl3, reflux, 16 h; (b) 42% aqueous
HBF4, Ac2O, 0 °C, 1 h; (v) CHCl3, CD3CN, 60 °C, 6 h.
toward some amines was investigated as well. We report here
the results in detail.
Results and Discussion
Synthesis. The synthesis of 12a+‚BF4- and 12b+‚BF4- was
accomplished by three-step reactions starting from benzo[b]-
tropone 7 (Scheme 1). According to the reported procedure,17
the benzotropylium cation 3+‚ClO4 was prepared by the
-
reduction of benzo[b]tropone 718 with LiAlH4 and subsequent
-
dehydroxylation by HClO4. A reaction of 3+‚ClO4 with 1,3-
dimethyl-6-phenylaminouracil 8 in CH3CN in the presence of
NaH at -40 °C for 20 h gave a mixture of 9a and 9b in a good
combined yield (82%). The ratio of 9a and 9b was determined
1
to be 1:5 from the H NMR spectrum of the mixture. Because
the cation 3+‚BF4- is unstable (pKR+ ) 1.6),10 the stray water
in the solvent causes lowering of the yield of 9a,b. Conse-
quently, an alternative route via 5-ethoxy-5H-benzocycloheptene
11b was studied. Previously, the synthesis of 11b from 7 via
hydrazone 10 has been reported;19 however, the yield of 10 was
modest (50%) and the reaction of 10 afforded a mixture of 11a
and 11b. Thus, as the modified procedure, the one-step reduction
of 7 with NaBH4 in EtOH in the presence of CeCl3 and
molecular sieves (0.4 nm) was carried out to give 11b in 94%
yield. It is noteworthy that 11b was a single product, and no
generation of 11a was observed. The result is probably due to
the more stable nature of 11b, suggested by the calculated heat
of formation (vide infra). A reaction of 8 with 11b in CH3CN
aqueous HBF4 in Ac2O afforded 16a+‚BF4 in a better yield
-
(25%). To clarify the product ratios in the reduction of 7 and
13 as well as in the substitution reaction of 11b and 14, MO
calculations of 9a,b and 15a,b as well as of MeO-substituted
model compounds 17a,b and 18a,b were carried out using the
AM1 method (MOPAC97),21 and their heat of formation is
summarized in Figure 2. Compounds 17a and 18a are less stable
(17) Rennhard, H. H.; Modica, G. Di; Simon, W.; Heilbronner, E.;
Eschenmoser, A. HelV. Chim. 1957, 22, 230.
(18) Buchanan, G. L.; Lockhart, D. R. J. Chem. Soc. 1959, 3586.
(19) Kirmse, W.; Sluma, H. D. J. Org. Chem. 1988, 53, 763.
(20) Hayakawa, K.; Hori, N.; Kanematsu, K. Chem. Pharm. Bull. 1983,
31, 1809.
178 J. Org. Chem., Vol. 71, No. 1, 2006

Synthesis and Properties of Areno-Annulated Ions
TABLE 1.
λ
max, pKR+ Values, and Reduction Potentialsa of
Cations 2+, 3+, 12a,b+, and 16a,b+b and Reference Compounds 1+
and 5a+
reduction potential
compd
λ
max/nm
pKR+
(E1red/V)
12a+
12b+
16a+
1+ c
457
491
467
273
426
593
414
9.0
ca. 0.5
6.3
3.9
1.6
-0.67
-0.49
-0.46
-0.51
-0.26
-0.09
-0.84
3+ d
4+ e
< 0
10.9
5a+ f
-
a V vs Ag/AgNO3: cathodic peak potential. b Salts 12a,b+‚BF4 and
-
-
-
16a+‚BF4 as well as salts 3+‚ClO4 and 4+‚ClO4 were used for the
measurement. c Ref 7. d Ref 10. e Ref 11. f Ref 12.
FIGURE 2. Calculated heat of formation (kcal/mol).
FIGURE 4. Chemical shifts and coupling constants of 12a,b+‚BF4
,
-
FIGURE 3. UV-vis spectra of 5a+‚BF4-, 12a,b+‚BF4-, and 16a+‚BF4-
in CH3CN.
-
16a+‚BF4-, 5a+‚BF4-, 3+‚ClO4-, and 4+‚ClO4
.
become longer in the order 5a+ < 12a+ < 16a+, suggesting
the elongated π-conjugation (Table 1). In addition, λmax of 12b+
shows a large red-shift by 77 and 34 nm, as compared with
those of 5a+ and 12a+, respectively. The feature would be
explained by considering the raised HOMO of 12b+ due to the
large deformation of the π-system from planarity (vide infra).
than 17b and 18b, respectively. In addition, the acid-catalyzed
rearrangement from 17a to 17b has been reported.22 Thus,
exclusive generation of 11b and 14 would be rationalized from
the thermodynamic stability and possible rearrangement in the
presence of CeCl3 under thermal conditions. In contrast, the
reactions of 11b and 14 with 8 proceeded under relatively low
temperature, and thus, the reactions seem to proceed kinetically.
Concerning uracil adducts 9a,b and 15a,b, compounds 9a and
15a are more stable than 9b and 15b, respectively, and thus,
the rearrangement from 9b to 9a as well as that from 15b to
15a would proceed under acidic and thermal conditions.
The H NMR spectra of 12a,b+‚BF4 and 16a+‚BF4 are
noteworthy because the chemical shifts of fused aromatic ring
systems are quite useful in determining structural properties such
as diatropicity and bond alternation. Unambiguous proton
1
-
-
1
assignments were made by analyzing H NMR, H-H COSY,
and NOE spectra. The chemical shift of N1Me and selected
-
-
Properties. Compounds 12a,b+‚BF4 and 16a+‚BF4 were
fully characterized on the basis of the 1H NMR, 13C NMR, IR,
UV-vis, and mass spectral data, as well as by elemental
analyses and X-ray analyses. Mass spectra of 12a,b+‚BF4- and
16a+‚BF4- exhibited the correct M+-BF4- ion peak, which is
indicative of the cationic nature of the compound. The charac-
-
coupling constants of the peripheral protons of 12a,b+‚BF4
and 16a+‚BF4 are shown in Figure 4, together with those
-
- 10
- 11
of the reference compounds 3+‚ClO4
,
4+‚ClO4
,
and
- 12
5a+‚BF4
.
The corrected and precise proton assignment of
-
1
4+‚ClO4 was made by analyzing H NMR and H-H COSY
spectra in this study. The proton signal of N1Me of 12a,b+‚BF4
-
-
teristic absorption band for the counterion BF4 was observed
and 16a+‚BF4 (12a+‚BF4-, δ 3.14; 12b+‚BF4-, δ 3.17;
-
at 1084 cm-1 in the IR spectra. UV-vis spectra of 12a,b+ and
16a+‚BF4-, δ 3.15) is remarkably shifted to a higher field as
compared with that of N1Me of 5b+‚BF4- (δ 3.94),12 suggesting
16a+ in acetonitrile are shown in Figure 3, together with that
of 5a+. The longest wavelength absorption maxima (λmax
)
that N1Me of 12a,b+‚BF4 and 16a+‚BF4 is located within
the shielding region of the phenyl group. This feature is similar
to that of 5a+‚BF4- and is confirmed by X-ray structure analysis
-
-
(21) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J.
Am. Chem. Soc. 1985, 107, 3902. Dewar, M. J. S.; Zoebisch, E. G.
THEOCHEM 1988, 180, 1.
-
(vide infra). Concerning the seven-membered ring, 12a+‚BF4
(22) Bertelli, D. J.; Rossiter, W. J. Tetrahedron 1968, 24, 609.
[J5,6 ) 11.5 Hz], 12b+‚BF4- [J9,10 ) 10.5 Hz, J10,11 ) 9.5 Hz],
J. Org. Chem, Vol. 71, No. 1, 2006 179

Naya et al.
FIGURE 5. ORTEP drawings of 12a,b+‚BF4- with a thermal ellipsoid plot (50% probability). Selected bond lengths (Å) of 12a+‚BF4-: N1-C1,
1.355; N1-C4, 1.414; C1-C2, 1.395; C2-C3, 1.393; C3-C4, 1.441; C4-C5, 1.365; C5-C6, 1.421; C6-C7, 1.425; C7-C8, 1.439; C8-C9,
1.353; C3-C9, 1.417; C6-C10, 1.427; C10-C11, 1.360; C11-C12, 1.404; C12-C13, 1.369; C7-C13, 1.415. Selected bond lengths (Å) of
12b+‚BF4-: N1-C1, 1.354; N1-C4, 1.411; C1-C2, 1.417; C2-C3, 1.416; C3-C4, 1.442; C4-C5, 1.374; C5-C6, 1.403; C6-C7, 1.363; C7-
C8, 1.431; C8-C9, 1.442; C3-C9, 1.450; C8-C10, 1.429; C10-C11, 1.365; C11-C12, 1.409; C12-C13, 1.378; C9-C13, 1.414.
and 16a+‚BF4- [J5,6 ) 11.5 Hz] show larger bond alternations
as compared with that of 5a+‚BF4-. In addition, the value of
the coupling constant of the C9-C10-C11 moiety of 12b+‚BF4-
much shorter than those of C5-C6, C7-C8, and C3-C9.
Similarly, in cation 12b+‚BF4-, the bond lengths of C4-C5
and C6-C7 are much shorter than those of C5-C6, C7-C8,
and C3-C9. Furthermore, in both cations of 12a,b+‚BF4-, the
bond lengths of C10-C11 and C12-C13 are shorter than that
of C11-C12. These facts suggest the existence of bond
alternation, as shown in the canonical structure of 12a,b+-B
(Figure 6). In addition, the differences in bond lengths of C10-
- 18
- 18
is similar to those of 3+‚ClO4
and 4+‚ClO4
, suggesting
the existence of the bond fixation of the seven-membered ring
of 12a,b+‚BF4- and 16a+‚BF4- due to the benzo and naphtho
-
annulation. The benzo moiety of 12a+‚BF4 [J7,8 ) 8.0 Hz,
J8,9 ) 7.0 Hz, J9,10 ) 8.0 Hz] and the naphtho moiety of
16a+‚BF4- [J8,9 ) 8.0 Hz, J9,10 ) 6.5 Hz, J10,11 ) 8.0 Hz] show
relatively smaller bond alternations as compared with those of
C11-C12 in 12a,b+‚BF4 are relatively smaller as compared
-
with those in the seven-membered ring, suggesting the existence
of a small contribution of the canonical structures of 12a,b+-
C. Furthermore, the differences in bond lengths of C10-C11-
-
3+‚ClO4 and 4+‚ClO4-, respectively. In contrast, the benzo
moiety of 12b+‚BF4 [J5,6 ) 8.5 Hz, J6,7 ) 7.0 Hz, J7,8 ) 8.5
-
C12 in cation 12a+‚BF4 are smaller as compared with those
-
Hz] shows a larger bond alternation as compared with those of
-
-
12a+‚BF4 and 16a+‚BF4 and the coupling constants are
in 12b+‚BF4-. Thus, a contribution of the canonical structure
of 12a+-C would be larger than that of 12b+-C. The features
-
similar to those of 3+‚ClO4
.
agree with the structures of 12a,b+‚BF4 on the basis of
-
Single crystals of 12a,b+‚BF4 were obtained by recrystal-
-
1
inspection of the H NMR.
lization from CH3CN/AcOEt. Thus, X-ray crystal analyses of
-
12a,b+‚BF4 were performed, and the ORTEP drawings are
The affinity of the carbocation toward hydroxide ions
expressed by the pKR+ value is the most common criterion of
carbocation stability.24 The pKR+ values of cations 12a,b+ and
16a+ as well as that of naphtho[b]tropylium ion 4+ were
determined spectrophotometrically in buffer solutions prepared
in 50% aqueous CH3CN and are summarized in Table 1, along
with those of the reference compounds 1+,7 3+,10 and 5a+.12
Because a sharp titration curve for the neutralization of cation
12b+ was not obtained, the pKR+ value was estimated to be ca.
0.5. In addition, the reaction of cation 4+ with hydroxide in
all-buffer solutions proceeded completely and the peak of cation
4+ was not observed in all-buffer solutions; thus, the pKR+ value
shown in Figure 5.23 The counteranion (BF4-) in 12a,b+‚BF4
-
is omitted for clarity. The planes of the phenyl groups in
-
12a,b+‚BF4 are twisted at 77.8° and 85.9° against the plane
of the pyrrole ring, respectively. Although the π-system of
12a+‚BF4 has a nearly planar structure, the π-system of
-
12b+‚BF4 shows large deformation from planarity like a
-
helicene because of the steric repulsion between the C13H and
C17dO2 double bonds. In both cations of 12a,b+‚BF4-, the
bond length of N1-C1 is shorter than that of N1-C4,
suggesting that the former bond has a larger bond order. In
cation 12a+‚BF4-, the bond lengths of C4-C5 and C8-C9 are
(23) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, M.; Giaco-
vazzo, C.; Guagliardi, A.; Polidori, G. J. Appl. Crystallogr. 1994, 27, 435.
(24) Freedman, H. H. In Carbonium Ions; Olah, G. A., Schleyer, P.,
Eds.; Wiley-Insterscience: New York, 1973.
180 J. Org. Chem., Vol. 71, No. 1, 2006

Synthesis and Properties of Areno-Annulated Ions
FIGURE 7. Plot of pKR+ values against E1red of 5a+‚BF4-, 12a,b+‚BF4-
,
and 16a+‚BF4
.
-
and -106.89, respectively;12 thus, the slope of the regression
line of cations 5a+, 12a+, and 16a+ is smaller than that of
cations 5a-d+. Furthermore, the value is smaller than 1.0,
suggesting that the more stable cation gives a less stable radical
in a single-electron reduction of 5a+, 12a+, and 16a+.25,26 The
plot of 12b+ is placed at a far position from the regression line
because of its very small pKR+ value, suggesting that the relation
between E1red and pKR+ values is different from that of cations
5a+, 12a+, and 16a+. The feature would be attributable to the
deformation of 12b+ from planarity arising from the steric
repulsion: because the steric repulsion is reduced by the
incorporation of OH-, equilibrium between cation 12b+ and
the corresponding alcohol would favor the alcohol. Thus, the
pKR+ value of 12b+ would become smaller as compared with
that expected for E1red. This feature may also be ascribed to
the instability of the corresponding alcohol under the conditions
of the pKR+ measurement. Immediate (5 s) acidification of an
alkaline solution (ca. pH 14) of 12b+ with TFA regenerated
the absorption maxima of the cations in the visible regions in
91% yield.
FIGURE 6. Resonance contributions for tropylium ions 12a+ and
12b+.
of cation 4+ was estimated to be <0. The pKR+ values are
smaller, in the order 5a+ (pKR+ ) 10.9) > 12a+ (pKR+ ) 9.0)
> 16a+ (pKR+ ) 6.3), suggesting that the perturbation derived
from the naphtho annulation is larger as compared with that
from the benzo annulation. The feature is similar to those of
cations 1+-4+, and thus, cation 4+ would have a very small
pKR+ value. In contrast, the pKR+ value of cation 12b+ (pKR+
) ca. 0.5) is much smaller.
The reduction potentials of cations 12a,b+ and 16a+ as well
as those of cations 3+ and 4+ were determined by cyclic
voltammentry (CV) in CH3CN. The reduction waves were
irreversible under the conditions of the CV measurements, and
thus, the peak potentials are summarized in Table 1, together
with those of the reference compounds 1+ 7 and 5a+.12 The
irreversible nature of 12a,b+ and 16a+ as well as that of cations
3+ and 4+ is probably due to the formation of a radical species
and its dimerization, which is reported to be a typical property
of uracil-annulated heteroazulenylium ions such as 5a-c+.12-14
Reactivitiy. To clarify the reactivity, the reactions of
12a,b+‚BF4- with some nucleophiles, NaBH4 and diethylamine,
were investigated. Although the reduction of 5a-d+‚BF4- with
NaBH4 proceeded at C5, C7, and C9 to give mixtures of three
regioisomers,12-14 the reduction of 12a+‚BF4- occurred at C11
to give a single product 19 in moderate yield (Scheme 3).
-
Compound 19 was oxidized by DDQ to regenerate 12a+‚BF4
in good yield. On the other hand, a reaction of 12b+‚BF4- with
NaBH4 was carried out in an NMR tube to give a mixture of
C9 adduct 20 and C11 adduct 21 in a ratio of 2:3. The formation
of 19-21 would be ascribed to the stability of the closed
pyrrolopyrimidine and benzene rings. The formation of the C11
adduct which predominates over the C9 adduct is similar to
The E1red values are less negative in the order 5a+ > 12a+
>
12b+ > 16a+. Similarly, the E1red values are less negative in
the order 1+ > 3+ > 4+, supporting that the perturbation derived
from the naphtho annulation is larger as compared with that
from the benzo annulation.
the reaction of 5a+‚BF4 with NaBH4, in which the ratio of
-
The pKR+ values of cations 5a+, 12a,b+, and 16a+ are plotted
against the E1red values of these cations (Figure 7). The units
of E1red and pKR+ values were converted to kJ/mol [-96.5 ×
E1red/V and 5.7 × (pKR+ - 14)].12,25,26 A good linear correlation
line for 5a+, 12a+, and 16a+ was obtained, and the slope and
y-intercept of this regression line were 0.72 and -75.439,
respectively (correlation coefficient ) 0.999). We have previ-
ously reported a similar correlation line obtained for cations
5a-d+, and the slope and y-intercept were obtained as 1.09
the C9 adduct was larger than those of the C5 and C7 adducts.
In addition, compound 19 was used for the reduction of ethyl
benzoylformate 24; however, the reduction did not proceed and
starting compounds 19 and 24 were recovered quantitatively.
The low reactivity of 19 would be attributable to the lower
stability of the corresponding cation 12a+‚BF4-, and thus, benzo
and naphtho annulations were not effective for the reducing
ability.27 Furthermore, reactions of 12a,b+‚BF4 with diethy-
-
lamine in NMR tubes were carried out. The diethylamine
addition of 12a+‚BF4- occurred at C11 to afford 22, similar to
(25) Naya, S.; Nitta, M. J. Chem. Soc., Perkin Trans. 2 2000, 2427.
(26) Naya, S.; Sakakibara, T.; Nitta, M. J. Chem. Soc., Perkin Trans. 2
2001, 1032.
(27) Naya, S.; Nishimura, J.; Nitta, M. J. Org. Chem., in press.
J. Org. Chem, Vol. 71, No. 1, 2006 181

Naya et al.
SCHEME 3 a
SCHEME 4a
a
Reagents and conditions: (i) hν, aerobic, CH3CN, room temperature,
1 h.
TABLE 2. Autorecycling Oxidation of Some Amines by
-
-
12a,b+‚BF4 and 16a+‚BF4 under Photoirradiationa
run
compd
amine
yield/µmolb,c recycling no.d
-
1
2
3
4
5
6
7
8
9
12a+‚BF4-
12a+‚BF4-
12a+‚BF4-
PhCH2NH2
PhCH(Me)NH2
hexylamine
83.9
50.0
17.9
0.0
108.4
52.9
42.9
18.0
88.1
67.3
79.5
29.5
16.8
10.0
3.6
a
12a+‚BF4- cyclohexylamine
12b+‚BF4- PhCH2NH2
12b+‚BF4- PhCH(Me)NH2
12b+‚BF4- hexylamine
0
Reagents and conditions: (i) NaBH4, CH3CN, room temperature, 2 h;
21.7
10.6
8.6
(ii) NaBH4, CD3CN, room temperature, 2 h; (iii) (a) DDQ, CH2Cl2, room
temperature, 2 h; (b) 42% aqueous HBF4, Ac2O, 0 °C, 1 h; (iv) diethylamine,
CD3CN, room temperature, 30 s; (v) 42% aqueous HBF4, Ac2O, 0 °C, 1 h;
(vi) Mg(ClO4)2, CHCl3, CH3CN, 60 °C, 4 days.
12b+‚BF4-
16a+‚BF4-
16a+‚BF4-
16a+‚BF4-
16a+‚BF4
cyclohexylamine
PhCH2NH2
PhCH(Me)NH2
hexylamine
3.6
17.6
13.5
15.9
5.9
10
11
12
the reaction with NaBH4. In contrast, the reaction of 12b+‚BF4
-
with diethylamine proceeded at only C9 to give 23 as a single
cyclohexylamine
product. The reaction of 5a+‚BF4- with diethylamine proceeded
-
a A CH3CN (16 mL) solution of compound 12a,b+‚BF4- or 16a+‚BF4
at only C7,16 and thus, the reactivity of 12b+‚BF4 would be
-
(5 µmol) and amines (2.5 mmol, 500 equiv) was irradiated by RPR-100
350 nm lamps under aerobic conditions for 16 h. b Isolated by converting
to the corresponding 2,4-dinitrophenylhydrazone. c The yield is calculated
by subtraction of the “blank” yield from the total yield. d Recycling number
similar to that of 5a+‚BF4-. Compounds 22 and 23 are stable
in CD3CN for a few days in the presence of diethylamine in
the dark; however, they decompose during concentration in
vacuo. Satisfactory 1H and 13C NMR were obtained for 22 and
23; however, HRMS of 22 and 23 gave only the (M-NEt2)+
peaks instead of the M+ peak. Upon treatment with 42%
aqueous HBF4 in Ac2O, compounds 22 and 23 regenerated
-
-
of 12a,b+‚BF4 and 16a+‚BF4
.
-
the presence of 12a,b+‚BF4 and 16a+‚BF4-. The recycling
numbers are more than one (Table 2), and thus, autorecycling
oxidation clearly proceeds; however, cyclohexylamine was not
12a,b+‚BF4 in good yields, respectively.
-
oxidized effectively by 12a+‚BF4 (Table 2, run 4). In the
-
Autorecycling Oxidation of Amines. We have previously
-
reported that compounds 5a-d+‚BF4 undergo autorecycling
oxidation of 1-phenylethylamine, hexylamine, and cyclohexy-
lamine, the yield of the imine became higher in the order
oxidation toward some alcohols and amines under photoirradia-
tion.12-15 In this context, we found that compounds 12a,b+‚BF4
-
12a+‚BF4 < 12b+‚BF4 < 16a+‚BF4-, and thus, cation
16a+‚BF4- would be more effective for the oxidation of amines.
We propose that the present autorecycling oxidation proceeds
via electron transfer from an amine to the excited compounds
-
-
and 16a+‚BF4 have oxidizing ability toward benzylamine,
-
1-phenylethylamine, hexylamine, and cyclohexylamine to give
the corresponding imines under aerobic and photoirradiation
conditions. Imine R1R2CdNH was produced at first; however,
it reacts with another amine to result in the formation of R1R2Cd
N-CHR1R2 (Scheme 4). Then, the reaction mixture was diluted
with ether and filtered, and the filtrate was treated with 2,4-
dinitrophenylhydrazine in 6% HCl to give 2,4-dinitrophenyl-
hydrazone of the corresponding carbonyl compound which is
generated by hydrolysis of imine. The results are summarized
in Table 2. Direct irradiation of the amines in the absence of
12a,b+‚BF4 and 16a+‚BF4-, as shown in Scheme 4.15,28 The
-
electron transfer from an amine to the excited state of
-
-
12a,b+‚BF4 and 16a+‚BF4 would occur to produce radicals
12a,b• and 16a• and a cation radical 26•+. On the other hand,
there is also a possibility that the homolysis of the amine
adducts, generated by the reaction of 12a,b+‚BF4- and 16a+‚BF4-
with amines, by photoirradiation would afford radicals 12a,b•
and 16a• directly. An electron transfer from radical species
12a,b• and 16a• to molecular oxygen may give the superoxide
anion radical and 12a,b+ and 16a+ because tropyl radical
12a,b+‚BF4 and 16a+‚BF4 (named “blank”) gives the cor-
responding carbonyl compounds in low to modest yields upon
similar treatments. Thus, the yields are calculated by subtraction
of the blank yield from the yield of the carbonyl compound in
-
-
(28) Naya, S.; Iida, Y.; Nitta, M. Tetrahedron 2004, 60, 459.
182 J. Org. Chem., Vol. 71, No. 1, 2006

Synthesis and Properties of Areno-Annulated Ions
under reflux for 16 h. After evaporation of the solvent, the residue
was dissolved in a mixture of Ac2O (25 mL) and 42% aqueous
HBF4 (5 mL) at 0 °C and the mixture was stirred for 1 h. To the
mixture was added Et2O (150 mL), and the precipitates were
collected by filtration and washed with Et2O to give a mixture of
derivatives are known to be oxidized readily by molecular
oxygen.29 Then, a proton transfer from cation radical 26•+ to
the superoxide anion radical may occur, followed by formation
of the products 28 and H2O2. Compound 28 reacts with excess
amine to give imine 29.
-
-
12a+‚BF4 and 12b+‚BF4-. The mixture of 12a+‚BF4 and
12b+‚BF4- was recrystallized from CH3CN/AcOEt to give crystals
of 12b+‚BF4- (59 mg, 8%) and filtrate containing 12a+‚BF4-. The
filtrate was concentrated in vacuo and recrystallized from CH3CN/
Summary
A convenient preparation of novel areno-annulated 1,3-
-
Et2O to give crystals of 12a+‚BF4 (220 mg, 30%).
dimethyl-10-phenylcyclohepta[4,5]pyrrolo[2,3-d]pyrimidine-2,4-
Synthesis of 14. A mixture of naphtho[2,3-d]tropone 13 (137
mg, 0.67 mmol) and CeCl3‚7H2O (322 mg, 0.86 mmol) in the
presence of molecular sieves (0.4 nm, 60 mg) in EtOH (35 mL)
was stirred at room temperature for 5 min. To the solution was
added NaBH4 (55.3 mg, 1.46 mmol), and the mixture was heated
under reflux for 5 h. To the resulted mixture was added saturated
aqueous NaCl, and the mixture was extracted with PhH. The extract
was dried over Na2SO4 and concentrated in vacuo to give 14 (149
mg, 95%).
-
-
(1,3H)-dionylium ions 12a,b+‚BF4 and 16a+‚BF4 was ac-
complished by three-step reactions starting from benzo[b]tropone
7 and naphtho[2,3-d]tropone 13 with 6-anilino-1,3-dimethylu-
racil 8. Structural characteristics of 12a,b+ and 16a+ were
clarified on inspection of the UV-vis and NMR spectral data
as well as by X-ray crystal analyses. The stability of cations
12a,b+ and 16a+ is expressed by the pKR+ values which were
determined spectrophotometrically as the values of ca. 0.5-
9.0. In addition, the pKR+ value of naphtho[b]tropylium ion 4+
was clarified to be much lower, at <0. The electrochemical
reduction of 12a,b+ and 16a+ as well as that of benzotropylium
ion 3+ and cation 4+ exhibited a reduction potential at -0.09
to -0.67 (V vs Ag/AgNO3) upon cyclic voltammetry (CV). The
reactivity of 12a,b+‚BF4- with some nucleophiles, hydride and
Synthesis of 4+‚ClO4-. A mixture of Ac2O (1 mL) and 60%
aqueous HClO4 (180 mg) was cooled to 0 °C. To the solution was
added slowly a solution of 14 (39.8 mg, 0.167 mmol) in Et2O (3
mL), and the mixture was stirred for 20 min. The generated
precipitates were collected by filtration under an N2 atmosphere to
-
give 4+‚ClO4 (31.1 mg, 64%).
Reaction of 8 with 14. To a mixture of 8 (23.1 mg, 0.1 mmol)
and 14 (23.6 mg, 0.1 mmol) in the presence of molecular sieves
(0.4 nm, 10 mg) in CH3CN (5 mL) was added p-TsOH‚2H2O (1.9
mg, 0.01 mmol), and the mixture was stirred at room temperature
for 3 h. To the mixture was added saturated aqueous NaCl, and
the mixture was extracted with CH2Cl2. The extract was dried over
Na2SO4 and concentrated in vacuo to give a mixture of 15a and
15b (48.5 mg, 99%, 15a/15b ) 1:7).
diethylamine, was clarified. Although the reactions of 12a+‚BF4
-
afforded C11 adduct 19 as a single product, the addition
-
reactions of 12b+‚BF4 proceeded at both C9 and C11. The
attempted reduction of methyl benzoylformate using 21 was
carried out unsuccessfully. The photoinduced oxidation reaction
of 12a,b+‚BF4 and 16a+‚BF4 toward some amines under
aerobic conditions was carried out to give the corresponding
imines with the recycling numbers of 3.6-21.7.
-
-
Synthesis of 16a+‚BF4-. To a stirred solution of a mixture of
15a and 15b (48.5 mg, 0.1 mmol) in CHCl3 (4 mL) was added
DDQ (45.4 mg, 0.2 mmol), and the mixture was heated under reflux
for 16 h. After evaporation of the solvent, the residue was dissolved
in a mixture of Ac2O (25 mL) and 42% aqueous HBF4 (5 mL) at
0 °C and the mixture was stirred for 1 h. To the mixture was added
Et2O (150 mL), and the precipitates were collected by filtration
Experimental Section
Reaction of 3+‚ClO4- with 8. To a stirred solution of 3+‚ClO4
-
(482 mg, 2 mmol) and 8 (462 mg, 2 mmol) in CH3CN (20 mL) at
-40 °C was added NaH (80 mg, 2 mmol), and the mixture was
stirred at -40 °C for 20 h. To the mixture was added saturated
aqueous NaCl, and the mixture was extracted with CH2Cl2. The
extract was dried over Na2SO4 and concentrated in vacuo to give
a mixture of 9a and 9b (609 mg, 82%) in a ratio of 1:5.
-
and washed with Et2O to give 16a+‚BF4 (9.8 mg, 19%).
1H NMR Monitoring of the Thermal Rearrangement of 15b.
A solution of compound 15b (4.2 mg, 0.01 mmol) in acid-free CD3-
CN (0.5 mL) in the absence and presence of CHCl3 (0.05 mL) was
heated at 60 °C in an NMR tube. After 6 h, the NMR measurement
was carried out to exhibit the formation of 8, 15a, and 15b in a
ratio of 0.03:1.40:1.00.
Synthesis of 11b. A mixture of benzo[b]tropone 7 (46.8 mg,
0.3 mmol) and CeCl3‚7H2O (145.3 mg, 0.39 mmol) in the presence
of molecular sieves (0.4 nm, 30 mg) in EtOH (14 mL) was stirred
at room temperature for 5 min. To the solution was added NaBH4
(24.9 mg, 0.66 mmol), and the mixture was heated under reflux
for 4 h. To the resulting mixture was added saturated aqueous NaCl,
and the mixture was extracted with PhH. The extract was dried
over Na2SO4 and concentrated in vacuo to give 11b (52.6 mg, 94%).
Reaction of 8 with 11b. To a mixture of 8 (62.2 mg, 0.27 mmol)
and 11b (52.6 mg, 0.28 mmol) in the presence of molecular sieves
(0.4 nm, 30 mg) in CH3CN (9 mL) was added p-TsOH‚2H2O (5.4
mg, 0.028 mmol), and the mixture was stirred at room temperature
for 6 h. To the mixture was added saturated aqueous NaCl, and
the mixture was extracted with CH2Cl2. The extract was dried over
Na2SO4 and concentrated in vacuo to give a mixture of 9a and 9b
(98.6 mg, 99%, 9a/ 9b ) 1:5).
Modified Procedure for Synthesis of 16a+‚BF4-. A solution
of a mixture of 15a and 15b (100 mg, 0.238 mmol) in CHCl3 (10
mL) containing HCl, which is generated by decomposition of
CHCl3, was heated under reflux for 6 h. To the resulted solution
was added DDQ (108 mg, 0.476 mmol), and the mixture was heated
under reflux for 16 h. After evaporation of the solvent, the residue
was dissolved in a mixture of Ac2O (25 mL) and 42% aqueous
HBF4 (5 mL) at 0 °C and the mixture was stirred for 1 h. To the
mixture was added Et2O (150 mL), and the precipitates were
-
collected by filtration and washed with Et2O to give 16a+‚BF4
(30.4 mg, 25%).
Determination of pKR+ Values of 12a,b+, 16a+, and 4+. Buffer
solutions of slightly different acidities were prepared by mixing
aqueous solutions of potassium hydrogen phthalate (0.1 M) and
HCl (0.1 M) (for pH 0.0-4.0), potassium hydrogen phthalate (0.1
M) and NaOH (0.1 M) (for pH 4.1-5.9), KH2PO4 (0.1 M) and
NaOH (0.1 M) (for pH 6.0-8.0), Na2B4O7 (0.025 M) and HCl (0.1
M) (for pH 8.2-9.0), and Na2B4O7 (0.025 M) and NaOH (0.1 M)
(for pH 9.2-10.8) in various portions. For the preparation of sample
solutions, 1 mL portions of the stock solution, prepared by
dissolving 6 mg of compounds 12a,b+‚BF4-, 16a+‚BF4-, and
4+‚ClO4- in CH3CN (20 mL), were diluted to 10 mL with the buffer
-
Synthesis of 12a+‚BF4 and 12b+‚BF4-. To a stirred solution
of a mixture of 9a and 9b (600 mg, 1.62 mmol) in CHCl3 (40 mL)
was added DDQ (735 mg, 3.24 mmol), and the mixture was heated
(29) (a) Jacobi, D.; Abraham, W.; Pischel, U.; Grubert, L.; Schnabel,
W. J. Chem. Soc., Perkin Trans. 2 1999, 1241. (b) Jacobi, D.; Abraham,
W.; Pischel, U.; Grubert, L.; Sto¨sser, R.; Schnabel, W. J. Chem. Soc., Perkin
Trans. 2 1999, 1695.
J. Org. Chem, Vol. 71, No. 1, 2006 183

Naya et al.
solution (8 mL) and CH3CN (1 mL). The UV-vis spectrum was
0.1 mmol) in CHCl3 (10 mL) and CH3CN (5 mL) was added 24
(17.8 mg, 0.1 mmol), and the mixture was heated at 60 °C for 4
days. The resulting mixture was extracted with CH2Cl2, and the
extract was dried over Na2SO4 and concentrated in vacuo to give
the starting materials 19 (36.9 mg, 100%) and 24 (17.8 mg, 100%).
-
recorded for each cation, 12a,b+‚BF4-, 16a+‚BF4-, and 4+‚ClO4
,
in 20 different buffer solutions. Immediately after recording the
spectrum, the pH of each solution was determined on a pH meter
calibrated with standard buffers. The observed absorbance at the
specific absorption wavelength (457 nm for 12a+‚BF4-; 491 nm
Reaction of 12a,b+‚BF4 with Diethylamine in CD3CN. To
-
-
for 12b+‚BF4-; 467 nm for 16a+‚BF4-; and 599 nm for 4+‚ClO4
)
each solution of 12a,b+‚BF4- (0.01 mmol) in CD3CN (0.5 mL) in
NMR tubes was added diethylamine (7.3 mg, 0.1 mmol). The NMR
measurements were carried out immediately (after ca. 30 s).
Reaction of 22 and 23 with HBF4. To each solution of 22 and
23, prepared by the reaction of 12a,b+‚BF4- (22.75 mg, 0.05 mmol)
with diethylamine (7.3 mg, 0.1 mmol) in CH3CN (20 mL), was
added a mixture of acetic anhydride (5 mL) and 42% aqueous HBF4
(1 mL) at 0 °C, and the mixture was stirred for 1 h. To the mixture
was added Et2O (50 mL), and the precipitate was collected by
filtration to give 12a,b+‚BF4- (12a+‚BF4- 16.3 mg, 72%; 12b+‚BF4-
15.9 mg, 70%).
of each cation, 12a,b+‚BF4-, 16a+‚BF4-, and 4+‚ClO4-, was
plotted against pH to give a classical titration curve, whose midpoint
was taken as the pKR+ value.
Cyclic Voltammetry of Cations 3+, 4+, 12a,b+, and 16a+. A
three-electrode cell was used, consisting of Pt working and counter
electrodes and a reference Ag/AgNO3 electrode. Nitrogen was
bubbled through a CH3CN solution (4 mL) of each cation,
3+‚ClO4-, 4+‚ClO4-, 12a,b+‚BF4-, and 16a+‚BF4 (0.5 mmol
-
dm-3), and Bu4NClO4 (0.1 mmol dm-3) to deaerate it. The
measurements were made at a scan rate of 0.1 V s-1. Immediately
after the measurements, ferrocene (0.1 mmol) (E1/2 ) +0.083) was
General Procedure for Autorecycling Oxidation of Amines
in the Presence of 12a,b+‚BF4 and 16a+‚BF4-. A CH3CN (16
-
added as the internal standard, and the observed peak potential was
-
-
corrected with reference to this standard. The cations 3+‚ClO4
,
mL) solution of compound 12a,b+‚BF4 (2.28 mg, 5 µmol) or
4+‚ClO4-, 16a,b+‚BF4-, and 20a+‚BF4- exhibited reduction waves,
16a+‚BF4 (2.53 mg, 5 µmol) and amines (2.5 mmol, 500 equiv)
-
and they are summarized in Table 1.
in a Pyrex tube was irradiated by RPR-100 350 nm lamps under
aerobic conditions for 16 h. The reaction mixture was concentrated
-
Reaction of 12a+‚BF4- with NaBH4. A solution of 12a+‚BF4
1
(220 mg, 0.48 mmol) and NaBH4 (54.4 mg, 1.44 mmol) in CH3-
CN (20 mL) was stirred at room temperature for 2 h. To the mixture
was added saturated aqueous NH4Cl solution, and the mixture was
extracted with CH2Cl2. The extract was dried over Na2SO4 and
concentrated in vacuo to give 19 (114 mg, 64%).
in vacuo, diluted with Et2O, and filtered. The H NMR spectra of
the filtrates revealed the formation of the corresponding imines.
The filtrate was treated with a saturated solution of 2,4-dinitro-
phenylhydrazine in 6% HCl to give 2,4-dinitrophenylhydrazone of
the corresponding carbonyl compounds. The results are summarized
in Table 2.
Oxidation of 19. To a stirred solution of 19 (36.9 mg, 0.1 mmol)
in CH2Cl2 (2 mL) was added DDQ (34.1 mg, 0.15 mmol), and the
mixture was stirred at room temperature for 2 h. After evaporation
of CH2Cl2, the residue was dissolved in a mixture of Ac2O (5 mL)
and 42% aqueous HBF4 (1 mL) at 0 °C, and the mixture was stirred
for another 1 h. To the mixture was added Et2O (30 mL), and the
precipitate was collected by filtration to give 12a+‚BF4- (40.4 mg,
89%).
Acknowledgment. Financial support from a Waseda Uni-
versity Grant for Special Research Project and 21COE “Practical
Nano-chemistry” from MEXT, Japan, is gratefully acknowl-
edged. We thank the Materials Characterization Central Labora-
tory, Waseda University, for technical assistance with the
spectral data, elemental analyses, and X-ray analyses.
Reduction of 12b+‚BF4 with NaBH4 in CD3CN. A solution
-
Supporting Information Available: Physical, analytical, and
spectroscopic data of 4+‚ClO4-, 9a,b, 12a,b+‚BF4-, 14, 15a,b,
of 12b+‚BF4 (4.55 mg, 0.01 mmol) and NaBH4 (1.13 mg, 0.03
-
mmol) in CD3OD (0.5 mL) in an NMR tube was shaken at room
1
-
16a+‚BF4-, and 19-23. H and 13C NMR spectra of 4+‚ClO4
,
1
temperature for 2 h, and the H NMR spectrum of the solution of
9a,b, 12a,b+‚BF4-, 14, 15a,b, 16a+‚BF4-, and 19-23. This
material is available free of charge via the Internet at http://
pubs.acs.org.
a mixture of 20 and 21 (20/21 ) 2:3) was recorded using Me4Si as
the internal standard.
Attempted Reduction of Ethyl Benzoylformate 24 Using 19.
To a solution of 19 (36.9 mg, 0.1 mmol) and Mg(ClO4)2 (22 mg,
JO051777J
184 J. Org. Chem., Vol. 71, No. 1, 2006