Structures and reactivities of ethylene dication electrophiles

Article abstract of DOI:10.1021/ja953623z

1,2-Dicarbonyl compounds such as 2,3-butanedione reacted with benzene in the presence of a strong acid, trifluoromethanesulfonic acid, to give gem-diphenylated ketones in high yield. The monoxime derivative of 1,2-diones also reacted with benzene in the a

Full text of DOI:10.1021/ja953623z

6
220
J. Am. Chem. Soc. 1996, 118, 6220-6224
Structures and Reactivities of Ethylene Dication Electrophiles
Tomohiko Ohwada,* Takahisa Yamazaki, Takayoshi Suzuki, Shinichi Saito, and
Koichi Shudo
Contribution from the Faculty of Pharmaceutical Sciences, UniVersity of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113, Japan
X
ReceiVed October 30, 1995. ReVised Manuscript ReceiVed April 25, 1996
Abstract: 1,2-Dicarbonyl compounds such as 2,3-butanedione reacted with benzene in the presence of a strong
acid, trifluoromethanesulfonic acid, to give gem-diphenylated ketones in high yield. The monoxime derivative of
1
,2-diones also reacted with benzene in the acid, but slowly enough to allow discrimination of monophenylated and
diphenylated oximes. The reactive electrophiles in these Friedel-Crafts reactions are considered to be O,O-
diprotonated 1,2-dicarbonyl species and N,O-diprotonated ketoxime, i.e., related ethylene dications differentiated by
a single different heteroatom substituent, a hydroxy or a hydroxyamino group. The reactivity depends on the electron-
donating ability of substituents adjacent to the reaction center.
High-level theoretical calculations were successful in predict-
1,2-dicarbonyl species and N,O-diprotonated ketoxime, i.e.,
related ethylene dications differentiated by a single different
heteroatom substituent (a hydroxy or a hydroxyamino group)
adjacent to the reaction center.
ing the unusual anti-van’t Hoff perpendicular structure, rather
than planar geometry, of the ethylene dication (C2H42+). Ab
initio calculations also revealed that multiple substitution of the
1
2
a,3
ethylene dication with π-donating groups such as hydroxy,
fluoro,2 or amino groups tends to stabilize the planar geometry
relative to the perpendicular geometry. For example, in the
cases of fluoro and amino substitutions, the mono- and disub-
b
4
Results and Discussion
Acid-Catalyzed Reaction of 2,3-Butanedione. 2,3-Butane-
dione (1a) reacted with benzene in the presence of trifluo-
romethanesulfonic acid (TFSA) at 5 °C to give 3,3-diphenyl-
2-butanone (2a) in 94% yield (Scheme 1) (Table 1). The
product is probably formed through diphenylation of one of the
carbonyl groups. The reaction depends on the acidity of the
medium, as judged from the yield of the product. At H0 ) -8
to -9, where the medium is sufficiently acidic to monoprotonate
the dione (at least partially) to give the monocation 3 (Scheme
stituted dications C2H3X2 and 1,1-C2H2X2 take perpendicular
+
2+
structures, whereas the tri- and tetrasubstituted dications C2-
HX3 and C2X4 favor planar structures.1 Apart from the
2
+
2+
intriguing structural dichotomy, the ethylene dications can be
regarded as reactive electrophiles involving (at least formally)
5
,6
adjacent carbenium centers, i.e., gitonic superelectrophiles.
Ethylene dications substituted with two gem-diphenyl groups
3a,c
undergo facile electrocyclization to give fluorene derivatives.
9
,10
2
),
essentially no reaction takes place. But, an increase of
However, the electrophilic behavior of ethylene dications in
7
the acidity to -11 (H0) gave 2a in a yield of 62%, and the
conversion was quantitative in 50% TFSA-50% TFA (H0 )
Friedel-Crafts reactions is still little understood. Diprotonation
1
2
of 1,2-diones in strong acids has been shown to afford O ,O -
-
12). In TFSA the reaction is very rapid. The heterogeneity
diprotonated 1,2-diones, i.e., 1,2-dihydroxyethylene dications
11
8
of the reaction precludes quantitative kinetic studies. However
this acidity-dependent behavior of the reaction implies that an
additional protonation is required to increase the electrophilicity
4
, and therefore we chose to examine the Friedel-Crafts
reaction of 1,2-diones 1 and some derivatives with benzene in
the presence of trifluoromethanesulfonic acid (TFSA). While
,2-diones react readily with two benzenes to give gem-diphenyl
ketones 2, an oxime derivative of a 1,2-dione reacts slowly,
allowing discrimination of monophenylation and diphenylation.
Reactive electrophiles in these Friedel-Crafts reactions have
been demonstrated by direct observation to be O,O-diprotonated
1
13
of the attacking cation 3. The H and C chemical shifts of
the species formed from 1a in TFSA-SbF5 (mole ratio 2.5:1)
correspond well to those of the O,O-diprotonated 2,3-butane-
dione 4 formed in FSO3H-SbF5 (see Tables 2 and 3).12
Therefore, the efficient conversion of 1a at high acidities is
consistent with the intervention of the O,O-diprotonated 2,3-
1
X
Abstract published in AdVance ACS Abstracts, June 15, 1996.
(1) Observation: Benoit, C.; Horsley, J. A. Mol. Phys. 1975, 30, 557.
(9) The acidity (H0) of the trifluoromethanesulfonic acid (TFSA)-
trifluoroacetic acid (TFA) system has been described: Saito, S.; Saito, S.;
Ohwada, T.; Shudo, K. Chem. Pharm. Bull. 1991, 39, 2718-2720. See
also: Saito, S.; Sato, Y.; Ohwada, T.; Shudo, K. J. Am. Chem. Soc. 1994,
116, 2312-2317, footnote 8.
(10) (a) Protonation of ketones: Paspaleev, E.; Kojucharova, A. Monatsch.
Chem. 1969, 100, 1213. (b) Protonation of imines: Childs, R. F.; Dickie,
B. D. J. Am. Chem. Soc. 1983, 105, 5041. (c) Protonation of oximes: Allen,
M.; Roberts, J. D. Can. J. Chem. 1981, 39, 451-458.
(11) The possible solubility increase of benzene in the case of the higher
acidity seems to affect the rate of reactions. However, the increase of
benzene may be caused by the production of benzenium ions which may
not be a substrate for the electrophile. Even if this was the case, the very
slow reaction in the medium which is acidic enough to monoprotonate the
substrate completely definitely eliminates the possibility of the participation
of the monoprotonated species.
(12) Though some difference in solvent effect may exist, the presence
of the same diprotonated species in TFSA is in accordance with the normal
acid-base equilibrium.
Theoretical calculations: Lammertsma, K.; Barzaghi, M.; Olah, G. A.;
Pople, J. A.; Kos, A. J.; Schleyer, P. v. R. J. Am. Chem. Soc. 1983, 105,
5
252.
2) (a) Koch, W.; Frenking, G.; Schwarz, H. Int. J. Mass Spectrom. Ion
(
Processes 1985, 63, 59-82. (b) Frenking, G.; Koch, W.; Schwarz, H. J.
Comput. Chem. 1986, 7, 406-416.
(
3) (a) Ohwada, T.; Shudo, K. J. Am. Chem. Soc. 1988, 110, 1862-
1
870. (b) Ohwada, T.; Shudo, K. J. Am. Chem. Soc. 1989, 111, 34-40. (c)
Ohwada, T.; Shudo, K. J. Org. Chem. 1989, 54, 5227-5237.
(4) Frenking, G. J. Am. Chem. Soc. 1991, 113, 2476-2481.
(5) Olah, G. A. Angew. Chem. Int. Ed. Engl. 1993, 32, 767-788.
(6) Olah, G. A.; Hartz, N.; Rasul, G.; Burrichter, A.; Prakash, G. K. S.
J. Am. Chem. Soc. 1995, 117, 6421-6427.
7) A part of the work has been communicated: Yamazaki, T.; Saito,
S.; Ohwada, T.; Shudo, K. Tetrahedron Lett. 1995, 36, 5749-5752.
8) Olah, G. A.; Calin, M. J. Am. Chem. Soc. 1968, 90, 4672-4675.
Olah, G. A.; Grant, J. L.; Westerman, P. W. J. Org. Chem. 1975, 40, 2102-
108.
(
(
2
S0002-7863(95)03623-7 CCC: $12.00 © 1996 American Chemical Society

Ethylene Dication Electrophiles
J. Am. Chem. Soc., Vol. 118, No. 26, 1996 6221
Scheme 1
yield of the diphenylated oxime 9 after 5 min (Scheme 3) (Table
1
6
1
).
The monophenyl product 8 was derived by the C1-
phenylation of the ketoxime 7a, and the diphenyl product 9 was
formed by double phenylation of the C1 position, as in the
reaction of 2,3-butanedione (1a). The isolated monophenyl
derivative 8 was converted to 9 under the same reaction
conditions.
Table 1. Acid-Catalyzed Reactions of 1,2-Diones and Their
Derivatives with Benzene
In a weaker acid, 50% (w/w) TFSA-50% (w/w) TFA (H0
) -12), the ketoxime 7a provided the monophenyl product 8
in 78% yield after 20 h at 5 °C. The use of a smaller amount
of the acid catalyst (10 equiv) has an effect similar to that of
the reduction of the acidity of the acid. In the presence of 10
equiv of TFSA, the ketoxime 7a gave the monophenyl product
temp
(°C) time
monophenyl diphenyl
substrate
acida
product
product
1
1
1
1
1
1
7
7
7
7
7
8
8
a
a
a
a
b
c
a
a
a
a
a
6% TFSA-94% TFA
20% TFSA-80% TFA
50%TFSA-50% TFA
TFSA
TFSA
TFSA
TFSA
TFSA
TFSA
5
5
5
5
5
5
5
5
5
5
5
5
5
20 min
20 min
20 min
5 min
5 min
60 min
5 min
2a (0%)
2a (62%)
2a (94%)
2a (94%)
8
in 94% yield after 2.5 h. In TFSA-TFA or with a small
excess (10 equiv) of TFSA, the concentration of the conjugate
base (trifluoroacetate in the former case or trifluoromethane-
sulfonate anions in the latter case) would increase, and the base
would catalyze the proton elimination of the intermediate
dication 13 to give the monophenyl ene oxime 8 (see Scheme
2b (72%)
2c (34%)b
8 (58%) 9 (37%)
145 min 8 (50%) 9 (39%)
72 h
20 h
145 min 8 (94%) 9 (0%)
75 h
8 (6%)
9 (75%)
8 (78%) 9 (0%)
50%TFSA-50% TFA
TFSA (10 equiv)
TFSA
4). The long reaction time required to complete the monophe-
9 (64%)c
9 (85%)
nylation is in contrast to the very rapid monophenylation reaction
(within 20 min) of the 1,2-dione 1a even in a weaker acid, 50%
(w/w) TFSA-50% (w/w) TFA. Therefore, the monophenyla-
tion of the ketoxime 7a is essentially slow, but is accelerated
10% SbF5-90% TFSA
15 min
a
b
One hundred equivalents of the acid was used. 2-Phenyl-1-
c
indanone (36%). Recovery of 8 (11%).
+
by the increase of the acidity to that of TFSA. The pKBH of
1
7
2
,3-butanedione monoxime was reported to -3.49. In 50%
butanedione (4a), i.e., 1,2-dihydroxybutenium dication, as a
genuine electrophile. Phenylation can take place through this
TFA-50% TFSA (H0 ) -12) the monoxime is completely
monoprotonated without doubt.
1,2-dihydroxybutane dication (4a), resulting in the formation
of a ketol intermediate, 5a. Although we could not detect the
monophenylated intermediate 5a even at a short reaction time
in the case of 1a, a dehydroxylated derivative (1,2-diphenyl-
The phenylation of the monophenyl oxime 8 to give the
diphenylated oxime 9 was also slow, as judged from the fact
that an extended reaction time (145 min) did not significantly
change the yields of 8 and 9 from 7a in TFSA (50% and 39%,
respectively). A much longer time (72 h, optimal) is required
to complete the conversion of 8 to 9 in TFSA. Slow conversion
of 8 to 9 was also observed in the reaction of the isolated 8
with benzene in TFSA, giving the diphenylated 9 in 64% yield
after 75 h.
2
-propen-1-one) of the monophenylated intermediate was
isolated in the case of 1-phenyl-1,2-propanedione (1c), sup-
porting the intermediate formation of the ketol 5. This ketol 5
is postulated to ionize to another ethylene dication, 6a, stabilized
by the phenyl and hydroxy groups, in a strongly acidic medium
(
Scheme 2). It is known that related ethylene dications are
3a
formed from ketol precursors. The dication 6a is likewise
very active and can alkylate a second benzene to give 2a.
Another possible mechanism would be a pinacol-pinacolone
rearrangement after diphenylation at the two individual carbonyl
Similar ethylene dications 11a and 13, corresponding to the
dications 4 and 6 of the 1,2-diones, are presumed to participate
in the reaction. The reactions of these dications 11a and 13
are slower than those of 1,2-diones. As judging from the higher
basicity of the imino nitrogen atom of the oxime group
1
3,14
groups,
but the behavior of methyl pyruvate (1b) excluded
this alternative: this keto ester 1b reacted similarly with benzene
to give methyl 2,2-diphenylpropionate (2b). The ester group
survived the reaction, and the product was formed rapidly even
at lower temperature (-40 °C in methylene chloride as a
cosolvent). A similar diphenylation reaction of the ketoxime
also excluded this mechanism.
compared with that of the oxygen atom of the carbonyl
10,17
group,
the ketoxime 7a is also diprotonated in TFSA to give
the N,O-diprotonated ketoxime 11a, probably through the
N-protonated monocation 10a. The dication 11a reacts with
benzene to give the hydroxy oxime 12, from which another
ethylene dication, 13, is generated. This dication 13 equilibrates
with the N-protonated ene oxime 8 (14) via elimination of a
methyl proton. The formation of the ene oxime 8a is consistent
with the intermediacy of the hydroxy oxime 12. The dication
13 can react with another benzene to give the diphenylated
oxime 9, like the dication 6a. The phenylation reaction of the
dication 11a is much slower than the corresponding reaction of
the dication 4a of the diketone 1a, as judged from the reaction
time. Involvement of the dicationic species 13 in this second
phenylation process was supported by the fact that the addition
of SbF5 to TFSA (10% SbF5-90% TFSA) significantly ac-
celerated the reaction of 8a to 9a, and the reaction was
completed in a very short time (15 min) (Table 1). Deuterium
exchange of methylene protons of the isolated ene oxime 8a in
TFSA-d definitely takes place, though it is slow even at ambient
1
5
Acid-Catalyzed Reaction of 2,3-Butanedione Monoxime.
2,3-Butanedione monoxime (7a) reacted with benzene in the
presence of a large amount of TFSA (100 equiv) to give the
monophenylated ene oxime 8 (58% yield), together with 37%
(
13) March, J. AdVanced Organic Chemistry, Reactions, Mechanisms,
and Structure, 4th ed; John Wiley & Sons: New York, 1992; pp 1072-
079 and references cited therein.
14) Probable pinacol intermediates will not give the ketol products in a
1
(
strong acid. The pinacol ionizes to the ethylene dications in TFSA and gives
another product: 1,1,2,2-tetraphenylethanediol gave 9,10-diphenylphenan-
threne in TFSA through a tetraphenylethylene dication intermediate (see
ref 3a,c). In the reactions of dicarbonyl compounds, such cyclized products
cannot be detected.
(15) The relative reactivity and regioselectivity data described in this
paper are very easily rationalized in terms of cation stability. Generally
more basic compounds form stabler cation species upon protonation.
+
Reported pKBH values of acetone, acetophenone, acetic acid (in place of
methyl acetate), and acetone oxime are -7.2, -6.17, -6.10, and -1.9,
respectively (Arnett, E. M. Prog. Phys. Org. Chem. 1963, 1, 223-403).
Thus, the cation centers of protonated species are stabilized in this order.
(16) 2,3-Butanedione monoxime was a single isomer with respect to the
oxime group, as judged from the NMR data.
(17) Hall, N. F. J. Am. Chem. Soc. 1930, 52, 5115-5128.

6
222 J. Am. Chem. Soc., Vol. 118, No. 26, 1996
Ohwada et al.
Scheme 2
Scheme 3
butanedione monoxime (11a). While the C1 carbon atom is
1
3
deshielded by 22 ppm in the C NMR spectrum as compared
with that of the neutral precursor, the C2 carbon atom is rather
shielded by 0.8 ppm, indicating an overwhelming predominance
of the imino resonance structure over the keto resonance
1
structure. In the H NMR spectrum, low-field shifts of two
methyl groups were observed, as compared with the methyl
groups of O-protonated acetone and N-protonated acetone
+
oxime. Two singlet absorptions at 14.49 (CdOH ) and 12.47
+
ppm (CdNH ) were also observed, though even in the TFSA-
temperature, indicating that the concentration of the equilibrating
cation 13 is low.
SbF5 (2.5:1) acid system at -30 °C, proton exchange with the
solvent began to cause merging of the former absorption
+
NMR Spectroscopic Observation of Dications. Direct
NMR spectroscopic observation of diprotonated 1,2-diketones
was accomplished in the so-called magic acid system (FSO3H-
(CdOH ) with the acid peak, giving a signal integration value
1
9
of less than one proton.
The acidity of TFSA-SBF5 (2.5:1) is stronger than -16 (H0
8
1
13
20
SbF5). On the basis of the coincidence of the H and
C
< -16) where the NMR study was performed and the
+
chemical shifts, 2,3-butanedione (1a) and benzil (1d) are also
diprotonated in TFSA-SbF5 (2.5:1). Their chemical shifts are
listed in Tables 2 and 3, together with the data for reference
dications were observed, so the second pKBH of 2,3-butane-
dione and 2,3-butanedione monoxime is around -17 to -19.
Thus, in TFSA (H0 ) -14) where the reactions are performed,
18
-3
-5
compounds (O-protonated ketones (acetone and acetophenone)
and N-protonated oximes (acetone oxime and acetophenone
10 -10 formation of the dication is estimated, which is
sufficient for the reaction to proceed. The Zucker-Hammett
criteria indicate that even the small concentration of the active
species warrants the linearity between the acidity and rate (and
1
0c
oxime)).
The dication formed from benzil is stable and did
not react with benzene at 0 °C. A related stable species is
formed from benzil monoxime (7b) in TFSA and TFSA-SbF5
2
1
therefore the yield).
(
2.5:1 mole ratio) at -30 °C, and it was assigned as the N,O-
Conclusion
diprotonated benzil monoxime (11b), i.e., a hydroxyamino-
substituted ethylene dication (Scheme 4). In the H NMR
1
We have directly observed the dications (discrete gitonic
superelectrophiles) formed from 1,2-diones and their derivatives
in Friedel-Crafts reactions. These are analogous to the
spectra in TFSA-SbF5 (2.5:1 mole ratio), three sets of singlet
absorptions were observed at 14.76, 13.44, and 10.67 ppm,
+
+
which can reasonably be assigned to a CdOH , a CdNH ,
and an oxime OH group, respectively.18 In the C NMR
spectra, the chemical shifts of the C1 and C2 carbons seemed to
be unperturbed or even slightly shielded upon diprotonation,
as compared with the neutral precursors. O-protonation of
acetophenone and N-protonation of acetophenone oxime resulted
in a large deshielding by 21 and 18 ppm, respectively, at the
trigonal carbons. The “high-field” shifts of the adjacent
electron-deficient centers suggest a strong contribution of the
22a
previously reported O,O-diprotonated nitronaphthalenes,
13
22b
O,O-diprotonated nitrostyrenes, and gem-diphenylethylene
3a,c
dications.
Several recent studies have indicated a nonplanar
structure of tetrasubstituted ethylene dications, presumably
owing to steric repulsion: an X-ray structure analysis showed
that the tetra-p-anisylethylene dication has a 41° twist around
23
the central C-C bond, and the dibromo and dichloro salts of
the tetrakis(dimethylamino)ethylene dication have twisting
angles around the C-C bond of 76° (chloro salt) and 67° (bromo
+
+
protonated carbonyl (CdOH ) and protonated imino (CdNH )
24
salt). This nonplanar geometry is not rigid in solution. For
+
resonance structures rather than the hydroxycarbenium (C sOH)
and aminocarbenium (C sNH) centers. A similar counterpois-
+
(19) This spectroscopic investigation of diprotonated 2,3-butanedione
monooxime in TFSA-SbF5 suggested a predominant hydroxyiminium
resonance structure, which is consistent with the observed reactivity as an
electrophile (see ref 15).
(20) Olah, G. A.; Prakash, G. K. S.; Sommer, J. Super Acids; Wiley-
Interscience: New York, 1985.
ing effect of the adjacent carbenium centers is also observed in
1
3
the C NMR spectra of diprotonated 2,3-butanedione (1a) (in
TFSA-SbF5 (2.5:1)) and benzil (1d) (in TFSA), where the low-
field shifts of the carbonyl carbons are much smaller (6.2 and
(21) (a) Sato Y.; Yato, M.; Ohwada, T.; Saito, S.; Shudo, K. J. Am.
2.3 ppm) than the corresponding low-field shifts of the carbonyl
Chem. Soc. 1995, 117, 3037-3043. (b) Saito, S.; Sato, Y.; Ohwada, T.;
Shudo, K. J. Am. Chem. Soc. 1994, 116, 2312-2317. (c) Ohwada, T. ReV.
Heteroatom Chem. 1995, 12, 179-209.
carbons of acetone (41.1 ppm, TFSA-SbF5 (2.5:1)) and
acetophenone (21.0 ppm, TFSA) upon O-monoprotonation.
Finally, we also prepared a solution of the ion from 2,3-
butanedione monoxime (7a) in TFSA-SbF5 (2.5:1) at -30 °C.
The observed species was assigned as N,O-diprotonated 2,3-
(
22) (a) Ohta, T.; Shudo, K.; Okamoto, T. Tetrahedron Lett. 1984, 325-
3
28. (b) Ohwada, T.; Ohta, T.; Shudo, K. J. Am. Chem. Soc. 1986, 108,
3029-3032.
(23) Baenziger, N. C.; Buckles, R. E.; Simpson, T. D. J. Am. Chem.
Soc. 1967, 89, 3405.
(
18) Brookhart, M.; Levy, G. C.; Winstein, S. J. Am. Chem. Soc. 1967,
(24) Bock, H.; Ruppert, K.; Merzweiler, K.; Fenske, D.; Goesmann, H.
Angew. Chem., Int. Ed. Engl. 1989, 28, 1684-1685.
8
9, 1735.

Ethylene Dication Electrophiles
J. Am. Chem. Soc., Vol. 118, No. 26, 1996 6223
Scheme 4
Table 2. 1H NMR Spectroscopic Data of Ionsa
substrate
acid
temp (°C)
δ(NH/OH)
NDb
δ(H
o
)
δ(H
m
)
δ(H
p
)
δ(CH
4.33
4.0
3
)
1
1
a
TFSA-SbF
5
-30
-60
-30
-30
c
FSO3H-SbF
5
ND
d
11.65 (dOH)e
d
TFSA
8.64 (d, 7.32)
9.57 (br d)
.79 (br d)
8.17 (t, 7.33)
8.72 (br t)
8.54 (br t)
8.55 (t, nd)
9.27 (br t)
TFSA-SbF
TFSA-SbF
TFSA
5
ND
8
7
a
5
-30
-30
-30
14.49 (dOH)e
4.17, 3.58
1
2.47 (dNH)
7
b
12.66 (dNH)
8.48 (d, 7.8)
.38 (t, 7.8)
9.54 (br d)
8.81 (br d)
8.64 (br d)
7.96 (t, 7.3)
8.25 (t, nd)
8.19 (t)
9.16 (br t)
8.38 (nd)
8
TFSA-SbF
5
14.73 (dOH)
8.64 (br t)
8.48 (br t)
8.38 (nd)
1
1
3.44 (dNH)
0.67 (OH)
8
.38 (nd)
acetone
TFSA
TFSA-SBF
TFA
TFSA
TFSA-SbF
TFSA
TFSA-SbF
TFSA
-30
-30
-10
-30
-30
-30
-30
-30
ND
14.76 (dOH)
ND
3.26
3.61, 3.55
2.41
3.51
3.84
2.67, 2.64
3.00, 2.95
3.29
5
acetophenone
13.80 (dOH)e
13.32 (dOH)
11.56 (dNH)
11.47 (dNH)
12.06 (dNH)
11.47 (dNH)
8.70 (d, 7.81)
9.06 (br d)
7.97 (t, 7.81)
8.30 (br t)
8.37 (t, 7.32)
8.71 (br t)
5
acetone oxime
5
acetophenone oxime
8.18 (d, 7.33)
7.96 (d, 7.33)
8.08 (t, 6.84)
7.85 (t, 7.33)
8.27 (t, nd)
8.02 (t, 7.33)
TFSA-SbF
5
-30
3.10
a
Coupling modes and coupling constants in hertz are shown in parentheses, d ) doublet, t ) triplet, br d ) broad doublet, br t ) broad triplet,
nd ) not determined. ND ) not detected. Reference 8a. d Reference 3c. e Signal integration values were less than expected.
b
c
example, the O-protonated[(p-methoxyphenyl)carbonyl]bis(p-
methoxyphenyl)methyl dication (akin to 6) exhibited equivalent
gem-aromatic rings of the dication in the H NMR spectra,
dicular) of these dications may determine the electronic nature
of these Friedel-Crafts electrophiles. Futher studies will
address this possibility with the assistance of theoretical
calculations.
1
supporting an intermediate structure between the perpendicular
and planar geometries, with equilibration between possible
3
a
conformations by rotation about the central C-C bond.
Therefore, although tetrasubstitution, as in the dications 4, 6,
Experimental Section
General Methods. All the melting points were measured with a
Yanagimoto hot-stage melting point apparatus (MP-500) and are
uncorrected. Proton (400 MHz) and carbon (100 MHz) NMR spectra
2
5
1
1, and 13, of course, exhibited different steric effects, the
much attenuated reactivity of the dication formed from 2,3-
butanedione monoxime (7a), as compared with the 1,2-dicar-
bonyl counterpart 1a, can be interpreted in terms of the divergent
electron-donating ability of a substitent, rather than the size of
the substitents. The geometrical preference (planar vs perpen-
1
were measured on a JEOL GX-400 NMR spectrometer with TMS ( H)
and the middle peak of CDCl
in CDCl as the solvent. Coupling constants are given in hertz. Flash
column chromatography was performed on silica gel (Kieselgel 60,
(77.0 ppm, 13C) as internal references
3
3
2
30-400 mesh, Merck) with the specified solvent.26 Combustion
(
25) Suzuki, T.; Shiohara, H.; Monobe, M.; Sakimura, T.; Tanaka, S.;
Yamashita, Y.; Miyashi, T. Angew. Chem., Int. Ed. Engl. 1992, 31, 455-
58. Malandra, J. L.; Mills, N. S.; Kadlecek, D. E.; Lowery, J. A. J. Am.
Chem. Soc. 1994, 116, 11622-11623.
analyses were carried out in the microanalytical laboratory of this
4
(26) Still, W. C.; Kahn, M.; Mitra, A. J. Org. Chem. 1978, 43, 2923-
2925.

6
224 J. Am. Chem. Soc., Vol. 118, No. 26, 1996
Ohwada et al.
Table 3. 13C NMR Spectroscopic Data of Ions and Neutral Precursors
substrate
acid
temp (°C)
δ(C
1
)
δ(C
2
)
δ(Cipso
)
δ(C
o
)
δ(C
m
)
p
δ(C )
3
δ(CH )
1
1
a
TFSA-SbF
5
-30
-60
23
-20
-60
23
-30
23
-20
203.3
204.0
197.1
196.9
197.3
194.6
219.2
197.2
188.2
24.7
25.5
23.4
a
FSO
3
H-SbF
5
CDCl
3
b
d
TFSA
FSO
Ha
CDCl
TFSA-SbF
CDCl
TFSA
128.3
128.3
132.9
135.0
136.2
129.9
130.8
131.4
129.0
144.3
145.6
134.9
3
3
7
7
a
5
156.4
157.2
159.9
29.2, 13.2
25.1, 8.1
3
b
131.4
133.7
132.2
142.1
138.0
129.9
129.9
131.9
130.9
139.9
138.8
155.3
143.8
1
22.4
126.4
20.4
TFSA-SbF5
-20
192.4
157.2
1
1
1
33.6
33.3
CDCl
3
23
194.2
157.0
134.5
30.8
134.6
129.4
128.9
126.4
130.5
129.0
1
acetone
TFSA
-30
-30
23
-30
-30
23
-30
-30
23
241.9
248.0
206.9
179.2
181.1
155.6
219.1
219.0
198.1
174.5
156.0
25.5
30.8, 29.6
30.8
20.6, 17.0
20.7, 17.0
21.7, 14.9
24.1
24.1
26.5
15.4
12.3
TFSA-SBF
5
CDCl
3
acetone oxime
acetophenone
TFSA
TFSA-SbF
5
CDCl
3
TFSA
128.8
128.6
137.0
126.8
136.5
130.3
130.5
133.0
129.8
128.5
130.3
130.5
128.5
127.4
113.9
144.8
145.4
133.0
135.9
129.3
TFSA-SbF
5
CDCl
3
acetophenone oxime
TFSA
CDCl
-20
23
3
a
Reference 8b. b Reference 3c.
faculty. Trifluoromethanesulfonic acid (TFSA) was purchased from
Central Glass Co., Japan, and used after distillation under reduced
pressure with a Widmer condenser as described previously. Trifluo-
mg (33% yield) of 2-phenyl-1-indanone. Data for 2c: colorless needles;
mp 91-92 °C (recrystallized from ethanol); H NMR δ 7.52 (2H, d,
7.0), 7.4-7.3 (5H, m), 7.3-7.2 (8H, m), 2.03 (3H, s). Anal. Calcd
for C21H18O: C, 88.08; H, 6.34. Found: C, 87.80; H, 6.19. Data for
1
8
roacetic acid (TFA) was obtained from Wako Pure Chemical Co., Japan,
8
and was distilled at atmospheric pressure. Antimony pentafluoride
2-phenyl-1-indanone: colorless prisms; mp 74-74.5 °C (recrystallized
1
was purchased from Aldrich Chemical Co. and was used after
distillation under reduced pressure. 2,3-Butanedione (1a), methyl
pyruvate (1b), and 1-phenyl-1,2-propanedione (1c) were purchased from
Wako Pure Chemical Co., and were used after distillation under reduced
pressure. 2,3-Butanedione monoxime (7a) was obtained from Aldrich,
and was further purified by recrystallization.
from ethanol); H NMR δ 7.82 (1H, d, 7.3), 7.65 (1H, t, 7.5), 7.53
(1H, d, 7.7), 7.43 (1H. t, 7.3), 7.32 (2H, t, 7.3), 7.2 (1H, not
determined), 7.19 (2H, d, 7.0), 3.90 (1H, dd, 8.4, 4.0), 3.70 (1H, dd,
7.6, 4.4), 3.28 (1H, dd, 7.2, 4.0). Anal. Calcd for C H O: C, 86.51;
15
12
H, 5.81. Found: C, 86.26; H, 5.70.
TFSA-Catalyzed Reaction of 2,3-Butanedione Monoxime (7a)
with Benzene. A solution of the monoxime 7a (253 mg, 2.5 mL) in
dry benzene (2.7 mL, total amount of benzene was 75 mL, 30 equiv
with respect to 7a) was added in portions over 2 min to a well-stirred
mixture of dry benzene (4 mL) and trifluoromethanesulfonic acid
TFSA-Catalyzed Reaction of 2,3-Butanedione (1a) with Benzene.
A solution of 2,3-butanedione (1a) (215 mg, 2.5 mmol) in dry benzene
(
2.7 mL, total amount of benzene was 75 mmol, 30 equiv with respect
to 1a) was added in portions to a well-stirred mixture of dry benzene
4 mL) and trifluoromethanesulfonic acid (TFSA) (22.2 mL, 100 equiv)
(
(TFSA) (22.2 mL, 100 equiv) at 5 °C in an ice-water bath. The whole
over 2 min at 5 °C in an ice-water bath. The whole mixture was
stirred for 5 min, then poured into ice-water (300 mL), and extracted
with methylene chloride (three portions of 100 mL). The organic phase
mixture was stirred for 5 min, then poured into ice-water (300 mL),
and extracted with methylene chloride (two portions of 100 mL). The
organic phase was washed with brine (100 mL), dried over Na SO ,
2 4
was washed with brine (100 mL), dried over Na
to give the crude residue (570 mg), which was flash-chromatographed
ethyl acetate-n-hexane (1:50)) to give the diphenylated ketone 2a (525
2 4
SO , and evaporated
and evaporated to give a crude residue (456 mg), which was flash-
chromatographed (ethyl acetate-n-hexane (1:10)) to give the monophe-
nylated 8 (232 mg, 58% yield) and the diphenylated 9 (221 mg, 37%).
Data for 8: colorless cubes; mp 155.1-155.4 °C (recrystallized from
(
2
7
mg, 94% yield): colorless powder; mp 39.0-39.5 °C (recrystallized
1
from n-hexane); H NMR δ 7.36-7.18 (10H, m), 2.11 (s, 3H, OCCH
.87 (3H, s, CH ). Anal. Calcd for C16 16O: C, 85.68; H, 7.19.
Found: C, 85.92; H, 7.28.
3
),
28 1
n-hexane); H NMR δ 8.58 (1H, s, OH), 7.32-7.20 (10H, m), 1.91
1
3
H
3 3
(1H, s, CH ), 1.78 (3H, s, CH ). Anal. Calcd for C16H17NO: C, 80.30;
H, 7.16; N, 5.85. Found: C, 80.60; H, 7.40; N, 5.85. Data for 9:
colorless powder; mp 90.5-91 °C (recrystallized from n-hexane-
TFSA-Catalyzed Reaction of Methyl Pyruvate (1b) with Benzene.
The reaction of methyl pyruvate (1b) with benzene in the presence of
TFSA (100 equiv) was conducted at 5 °C in the same manner as
described for 1a to give a 72% yield of methyl 2-diphenylpropionate
29
1
CH
(
C
2
Cl
2
); H NMR δ 7.84 (1H, s, OH), 7.36-7.30 (5H, m), 5.58
). Anal. Calcd for
10
H11NO: C, 74.50; H, 6.88; N, 8.69. Found: C, 74.20; H, 6.95; N,
1H, s, methylene), 5.44 (1H, s), 2.08 (3H, s, CH
3
(2b) as a colorless oil after column chromatography (ethyl acetate-
8
.76.
n-hexane (1:15)). Satisfactory NMR and IR spectroscopic data were
obtained.
Preparation and NMR Studies of Ions in Acids. NMR spectra
of ions were measured on a JEOL GX 400 spectrometer equipped with
a variable-temperature apparatus. Measurements and calibration of
chemical shifts were conducted as previously described.3c
TFSA-Catalyzed Reaction of 1-Phenyl-1,2-propanedione (1c)
with Benzene. 1-Phenyl-1,2-propanedione (1c) (374 mg) was allowed
to react with benzene (total amount of benzene was 6.7 mL, 30 equiv)
in the presence of TFSA (100 equiv, 22.1 mL) at 5 °C. After 60 min,
aqueous workup as described above gave a crude product (660 mg),
which was flash-chromatographed (ethyl acetate-n-hexane (1:60)) to
give 241 mg (34% yield) of 1,2,2-triphenylpropan-1-one (2c) and 173
JA953623Z
(28) Schenk, C.; deBeer, T. J. Recl. TraV. Chim. Pays-Bas 1979, 98,
18-21.
(
29) Tiffeneau, M. M.; Levy, M. J. Bull. Soc. Chim. Fr. 1927, 41, 1362-
(27) Gras, G.; Giral, L. Bull. Soc. Chim. Fr. 1970, 1115-1121.
1362.