Formation of keto-carbeniums from 1,2,2-triarylacenaphthen-1-ols by breaking a long C-C bond: Unusual alcohol protonolysis accompanied by formal hydride abstraction

Article abstract of DOI:10.1016/S0040-4039(03)01459-X

Oxidation of 1,2,2-tris(4-dimethylaminophenyl)- and 2,2-bis(4-dimethylaminophenyl)-1-phenylacenaphthen-1-ols 1a,b with I₂ induced the C₁-C₂ bond fission to give 8-aroylnaphthalen-1-yl-bis(4-dimethylaminophenyl)carbenium derivatives 2a,b⁺, the intramolecular Lewis acid-base pairs. Treatment of 1 with HBF₄ did not induce the expected C-OH bond heterolysis but caused fission of CO-H and C₁-C₂ bonds to give exactly the same carbenium 2⁺.

Full text of DOI:10.1016/S0040-4039(03)01459-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 6095–6098
Formation of keto-carbeniums from
1,2,2-triarylacenaphthen-1-ols by breaking a long CꢀC bond:
unusual alcohol protonolysis accompanied by
formal hydride abstractionꢀ
Takanori Suzuki,* Takayuki Nagasu, Hidetoshi Kawai, Kenshu Fujiwara and Takashi Tsuji
Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
Received 16 May 2003; revised 7 June 2003; accepted 11 June 2003
Abstract—Oxidation of 1,2,2-tris(4-dimethylaminophenyl)- and 2,2-bis(4-dimethylaminophenyl)-1-phenylacenaphthen-1-ols 1a,b
with I2 induced the C1ꢀC2 bond fission to give 8-aroylnaphthalen-1-yl-bis(4-dimethylaminophenyl)carbenium derivatives 2a,b+, the
intramolecular Lewis acid–base pairs. Treatment of 1 with HBF4 did not induce the expected CꢀOH bond heterolysis but caused
fission of COꢀH and C1ꢀC2 bonds to give exactly the same carbenium 2+.
© 2003 Elsevier Ltd. All rights reserved.
,
Covalent bond lengths such as 1.54 A for Csp3ꢀC are
aromatization8 product 3a7 (yield 37%) whose struc-
ture was confirmed by X-ray analysis.9 Such an anoma-
lous addition is resulted from the steric hindrance
around the carbonyl group in the starting acenaph-
thone. By using PhLi, 1b7 was similarly obtained in
46% yield. Detailed geometrical data for the triarylace-
naphthenol skeleton were determined by X-ray
analysis9 on 1b at −100°C (Fig. 1). The notable feature
3
basic parameters in chemistry, and large devisaptions
from the standard values are diagnostic of novel steric
and electronic features as exemplified by tetra-
arylbenzocyclobutenes with a CꢀC bond length of 1.72
1
2,3
,
A. Beside elucidating the origin of bond elongation,
exploring novel reactivities involving such weakened
bonds is necessary to shed light on the boundary
between long bonds and short nonbonds.4 In our con-
tinuing studies on the oxidative cleavage of polyaryl-
ated long CꢀC bonds upon electron-transfer (ET),5 we
have found that the title acenaphthenols 1a,b undergo
CꢀC bond fission not only upon 2e-oxidation but also
upon treatment with HBF4 (Scheme 1). The latter
reaction is quite unusual because the Brønsted acid
induces formal hydride abstraction from the hydroxy
group to give keto-carbeniums 2a,b+. We report here
the details of these novel reactions along with precise
geometrical features of the reactant and products deter-
mined by low-temperature X-ray analyses.
is that the polyarylated C1ꢀC2 bond is much longer
10
,
,
[1.648 (4) A] than the standard value (1.54 A). Such
Reaction of 2,2-bis(4-dimethylaminophenyl)acena-
phthen-1-one6 with 4-Me2NC6H4Li in THF gave 1a7 in
29% yield along with the conjugated addition–
Keywords: long bond; electron transfer; protonolysis; dye; peri-inter-
action.
ꢀ Supplementary data associated with this article can be found at
doi:10.1016/S0040-4039(03)01459-X
* Corresponding author. Tel.: +49-11-706-2714; fax: +49-11-706-2714;
e-mail: tak@sci.hokudai.ac.jp
Scheme 1.
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01459-X

6096
T. Suzuki et al. / Tetrahedron Letters 44 (2003) 6095–6098
accompanied by C1ꢀC2 bond fission and deprotonation,
resulting in the formation of stable I3 salts7 of keto-
−
carbeniums 2a+ (yield 81%) and 2b+ (yield 72%), respec-
tively. Their structures are assigned on the basis of
spectral data, and finally confirmed by the X-ray
analysis9 on 2a+I3 salt at −120°C.
−
As shown in Figure 2, the Ar2C+-type carbenium unit
and the carbonyl group at the 1,8-positions of naphtha-
lene nucleus are rotated in the same direction [59.4 (5)°
and 58.6 (4)°, respectively] in 2a+, thus furnishing the
face-to-face overlap of these moieties. The methylium
,
carbon is located 2.73 A above the carbonyl plane, and
separated by only 2.94 and 2.93 A from the O and C
,
atoms of the carbonyl group, respectively. This is the
shortest carbeniumꢀcarbonyl contact in crystal ever
reported. This structure is best described as a p-com-
plex of Lewis acid and base (Form A in Scheme 2),
suggesting the presence of peri-interaction13 between
the two moieties. Owing to such short-nonbonded
interaction, treatment of 2a+I3 with NaBH4 in EtOH
−
gave not only the triarylmethane derivative 4a (yield
28%) but also the cyclic isomer 5a (yield 21%), whose
structural assignment was unambiguously carried out
by X-ray analyses.9 The former is the usual hydride
adduct (leuco-base) of triarylmethylium dyes,14 whereas
the latter is the product through the hydride addition
on the s-complex (Form B) or the oxonium (Form C)
generated from the p-complex, Form A. Although the
presence of peri-interaction in 2+ is not so clear by
comparisons of spectral data (2a+: umax 630 nm in
MeCN; lc 179 ppm in CDCl3) with the reference
carbenium without aroyl group [bis(4-dimethyl-
aminophenyl)-1-naphthylcarbenium: 627 nm; 175 ppm],
p-acidity of 2a+ (Ered −0.65 V) is much reduced than the
reference carbenium (−0.56 V) due to the interaction
with the aroyl p-base. Such a difference indicates that
through-space interaction15 stabilizes the carbenium
unit in 2+.
Figure 1. Molecular structure of 1b determined by X-ray
analysis.
The most striking reactivity of the acenaphthenols 1 is
the acid-assisted long CꢀC bond fission to give the same
stabilized keto-carbeniums 2+. Upon treatment of 1a
Figure 2. Molecular structure of 2a+ determined by X-ray
−
analysis of I3 salt.
expansion can be accounted for by steric repul-
sion between aryl substituents,3 which is inevitable
due to the rigid five-membered-ring structure
with small torsion angles [C8aꢀC1ꢀC2ꢀC2a: 17.0
(3)°; CAr1ꢀC1ꢀC2ꢀCAr2: 22.2 (3)°; OꢀC1ꢀC2ꢀCAr2%: 27.9
(3)°].
According to the voltammetric analyses, acenaph-
thenols 1 are strong electron donors [Eox: +0.42 V for
1a and +0.53 V for 1b]11 due to multiple substitution of
the electron-donating aryl groups, and irreversibility of
the oxidation waves suggests the facile fission of the
weakened bond followed by ET.12 Upon treatment with
3 equiv. of iodine, 1a and 1b underwent 2e-oxidation
Scheme 2.

T. Suzuki et al. / Tetrahedron Letters 44 (2003) 6095–6098
6097
with 1.2 equiv. of HBF4 in THF at room temperature,
64, 3102; (b) Baldridge, K. K.; Kasahara, Y.; Ogawa, K.;
Siegel, J. S.; Tanaka, K.; Toda, F. J. Am. Chem. Soc.
1998, 120, 6167.
2a+BF4 salt was isolated in 87% yield without giving
−
the expected carbenium 6a+ by CꢀOH heterolysis.14
Similarly, 2b+BF4 was obtained by acid treatment of
2. (a) Choi, C. H.; Kertesz, M. Chem. Commun. 1997, 2199;
(b) Bettinger, H. F.; Schleyer, P. v. R.; Schaefer, H. F.,
III Chem. Commun. 1998, 769.
3. (a) Baldridge, K. K.; Battersby, T. R.; Clark, R. V.;
Siegel, J. S. J. Am. Chem. Soc. 1997, 119, 7048; (b)
Suzuki, T.; Ono, K.; Nishida, J.; Takahashi, H.; Tsuji, T.
J. Org. Chem. 2000, 65, 4944; (c) Suzuki, T.; Ono, K.;
Kawai, H.; Tsuji, T. J. Chem. Soc., Perkin Trans. 2 2001,
1798.
−
1b in 84% yield. By considering the fact that 1,1,2-
triphenylacenaphthen-1-ol 1c underwent the acid-cata-
lyzed substitution reactions via 6c+,16 not only the
expansion of the C1ꢀC2 bond by steric factors but also
the strong donating property of amino substituents on
aryl groups are responsible for this unusual behavior.
One plausible explanation may be that both factors
raise the orbital energy of C1ꢀC2 s bond, thus enhanc-
ing the affinity of this s bond toward proton although
the involvement of hydrogen-bridged17,18 carbenium 7+
in this protonolysis is still unclear. In summary, this is
one of the rare examples where Brønsted acid is proven
to act as an oxidant or to assist oxidation to give the
carbenium salt.19 Studies on the relationship between
this unusual alcohol protonolysis and the alkane pro-
tonolysis by superacid18,20 are now in progress.21
4. (a) Bell, P. C.; Wallis, J. D. Chem. Commun. 1999, 257;
(b) O’Leary, J.; Bell, P. C.; Wallis, J. D.; Schweizer, W.
B. J. Chem. Soc., Perkin Trans. 2 2001, 133.
5. (a) Suzuki, T.; Yamamoto, R.; Higuchi, H.; Hirota, E.;
Ohkita, M.; Tsuji, T. J. Chem. Soc., Perkin Trans. 2 2002,
1937; (b) Nishida, J.; Suzuki, T.; Ohkita, M.; Tsuji, T.
Angew. Chem., Int. Ed. 2001, 40, 3251; (c) Suzuki, T.;
Nisida, J.; Tsuji, T. Chem. Commun. 1998, 2193.
6. Zsuffa, M. Chem. Ber. 1910, 43, 2915.
1
7. For 1a: mp 172.0–174.0°C; H NMR (300 MHz, CDCl3)
l/ppm 7.76 (br. d, 1H, J=7.5 Hz), 7.69 (br. d, 1H,
J=7.5 Hz), 7.58 (dd, 1H, J=7.5, 7.5 Hz), 7.50 (dd, 1H,
J=7.5, 7.5 Hz), 7.34 (br. d, 1H, J=7.5 Hz), 7.20 (br. d,
1H, J=7.5 Hz), 7.10 (AA%XX%, 2H), 7.09 (AA%XX%, 2H),
6.64 (AA%XX%, 2H), 6.53 (AA%XX%, 2H), 6.41 (AA%XX%,
2H), 6.27 (AA%XX%, 2H), 3.26 (s, 1H), 2.86 (s, 6H), 2.81
(s, 6H), 2.74 (s, 6H); IR (KBr) w/cm−1 3556. For 1b: mp
1
180.0°C (decomp.); H NMR (300 MHz, CDCl3) l/ppm
7.78 (br. d, 1H, J=7.5 Hz), 7.72 (br. d, 1H, J=7.5 Hz),
7.58 (dd, 1H, J=7.5, 7.5 Hz), 7.53 (dd, 1H, J=7.5, 7.5
Hz), 7.32 (br. d, 1H, J=7.5 Hz), 7.23 (br. d, 1H, J=7.5
Hz), 7.16 (AA%XX%, 2H), 7.01 (AA%XX%, 2H), 6.84–6.74
(m, 5H), 6.50 (AA%XX%, 2H), 6.40 (AA%XX%, 2H), 3.48 (s,
1H), 2.85 (s, 6H), 2.81 (s, 6H); IR (KBr) w/cm−1 3520.
Supplementary material
Crystallographic data (excluding structure factors) for
the structures in this paper have been deposited with
the Cambridge Crystallographic Data Centre as supple-
mentary publication numbers CCDC 209572–209576.
Copies of the data can be obtained, free of charge, on
application to CCDC, 12 Union Road, Cambridge CB2
1EZ, UK [fax: +44 (0)-1223-336033 or e-mail:
deposit@ccdc.cam.ac.uk]. ORTEP drawings with atom
1
For 2a+I3−: mp 241.0–242.5°C (decomp.); H NMR (300
MHz, CDCl3) l/ppm 8.20 (br. d, 1H, J=7.5 Hz), 8.10
(br. d, 1H, J=7.5 Hz), 7.65 (dd, 1H, J=7.5, 7.5 Hz), 7.57
(dd, 1H, J=7.5, 7.5 Hz), 7.47 (br. d, 1H, J=7.5 Hz), 7.36
(br. d, 1H, J=7.5 Hz), 7.70–7.10 (br., 8H), 6.68 (br., 4H),
3.27 (s, 12H), 3.15 (br. s, 6H); IR (KBr) w/cm−1 1582. For
2b+I3−: mp 141.5–143.5°C (decomp.); 1H NMR (300
MHz, CDCl3) l/ppm 8.22 (dd, 1H, J=7.5, 1.5 Hz), 8.14
(dd, 1H, J=7.5, 1.5 Hz), 7.70 (dd, 1H, J=7.5, 7.5 Hz),
7.56 (dd, 1H, J=7.5, 7.5 Hz), 7.51–7.42 (m, 6H), 7.23
(dd, 1H, J=7.5, 7.5 Hz), 7.40–7.00 (br., 4H), 6.63
(AA%XX%, br., 4H), 3.27 (s, 12H); IR (KBr) w/cm−1 1580.
numbering schemes for 1b, 2a+I3 , 3a, 4a, and 5a were
submitted as electronic supplementary material (PDF).
−
Acknowledgements
1
For 3a: mp 250.0–255.0°C; H NMR (300 MHz, CDCl3)
This work was supported by the Ministry of Education,
Science, and Culture, Japan (No. 15350019 and
14654139). We thank Professor Tamotsu Inabe (Hok-
kaido University) for use of the X-ray structure analysis
system. MS spectra were measured by Mr. Kenji
Watanabe and Dr. Eri Fukushi at the GC–MS and
NMR Laboratory (Faculty of Agriculture, Hokkaido
University).
l/ppm 8.05 (br. d, 1H, J=7.5 Hz), 7.77 (br. d, 1H,
J=7.5 Hz), 7.73 (br. d, 1H, J=7.5 Hz), 7.66 (AA%XX%,
2H), 7.57 (dd, 1H, J=7.5, 7.5 Hz), 7.43 (br. d, 1H, J=7.5
Hz), 7.14 (AA%XX%, 4H), 6.80 (AA%XX%, 2H), 6.60
(AA%XX%, 4H), 3.01 (s, 6H), 2.88 (s, 12H); IR (KBr)
w/cm−1 1706. For 4a: mp 213.0–215.0°C (decomp.); 1H
NMR (300 MHz, CDCl3) l/ppm 7.92 (dd, 1H, J=7.5,
1.5 Hz), 7.78 (dd, 1H, J=7.5, 1.5 Hz), 7.40 (dd, 1H,
J=7.5, 7.5 Hz), 7.39 (dd, 1H, J=7.5, 7.5 Hz), 7.29 (dd,
1H, J=7.5, 1.5 Hz), 7.14 (dd, 1H, J=7.5, 1.5 Hz),
7.75–6.10 (br., 12H), 6.17 (s, 1H), 3.02 (s, 6H), 2.95–2.70
(br., 12H); IR (KBr) w/cm−1 1590. For 5a: mp 221.0–
222.5°C; 1H NMR (300 MHz, CDCl3) l/ppm 7.76 (dd,
1H, J=7.5, 1.5 Hz), 7.71 (br. d, 1H, J=7.5 Hz), 7.37 (dd,
References
1. (a) Toda, F.; Tanaka, K.; Watanabe, M.; Tamura, K.;
Miyahara, I.; Nakai, T.; Hirotsu, K. J. Org. Chem. 1999,

6098
T. Suzuki et al. / Tetrahedron Letters 44 (2003) 6095–6098
1H, J=7.5, 7.5 Hz), 7.32 (AA%XX%, 2H), 7.27 (dd, 1H,
R value is 0.062 for 3510 independent reflections with I
>3|I and 361 parameters.
10. Allen, F. H.; Kennard, O.; Watson, D. G. J. Chem. Soc.,
Perkin Trans. 2 1987, S1.
J=7.5, 7.5 Hz), 7.20 (AA%XX%, 2H), 7.02 (AA%XX%, 2H),
6.93 (dd, 1H, J=7.5, 1.5 Hz), 6.74 (AA%XX%, 2H), 6.74
(br. d, 1H, J=7.5 Hz), 6.64 (AA%XX%, 2H), 6.62
(AA%XX%, 2H), 5.70 (s, 1H), 2.96 (s, 6H), 2.94 (s, 6H),
2.90 (s, 6H); IR (KBr) w/cm−1 1614.
11. Oxidation potentials (Eox/V versus SCE) were measured
in MeCN containing 0.1 mol dm−3 Et4NClO4 with a scan
rate of 100 mV s−1. Dimethylaniline undergoes irre-
versible oxidation at +0.72 V under the same conditions.
12. Maslak, P.; Narvaez, J. N.; Vallombroso, T. M., Jr. J.
Am. Chem. Soc. 1995, 117, 12373.
8. Fuson, R. C.; Griffin, G. W. J. Am. Chem. Soc. 1957, 79,
1941.
(
9. Crystal data for 1b: C34H32N2O, M 484.64, triclinic, P1,
,
a=10.062 (4), b=11.677 (4), c=12.405 (4) A, h=72.69
3
,
13. (a) Cozzi, F.; Siegel, J. S. Pure Appl. Chem. 1995, 67, 683;
(b) Schweizer, W. B.; Procter, G.; Kaftory, M.; Dunitz, J.
D. Helv. Chem. Acta 1978, 61, 2783; (c) Balasubra-
maniyan, V. Chem. Rev. 1966, 66, 567.
(2), i=75.04 (2), k=70.45 (2)°, U=1290.8 (8) A , Dcalcd
(Z=2)=1.247 g cm−1, v=0.75 cm−1, T=173 K. The final
R value is 0.063 for 3210 independent reflections with I
>3|I and 334 parameters. For 2a+I3−: C36H36I3N3O, M
14. Muthyala, R.; Lau, X. Chemistry and Applications of
Leuco Dyes; Plenum Press: New York, 1997; p. 125.
15. (a) Komatsu, K.; Tsuji, R.; Inoue, Y.; Takeuchi, K.
Tetrahedron Lett. 1993, 34, 99; (b) Tsuji, R.; Komatsu,
K.; Inoue, Y.; Takeuchi, K. J. Org. Chem. 1992, 57, 636.
16. (a) Wittig, G.; Petri, H. Chem. Ber. 1935, 68, 924; (b)
Crawford, H. M. J. Org. Chem. 1963, 28, 3082.
17. Sun, F.; Sorensen, T. S. J. Am. Chem. Soc. 1993, 115, 77.
18. McMurry, J. E.; Lectka, T. Acc. Chem. Res. 1992, 25, 47.
19. (a) Sankararaman, S.; Lau, W.; Kochi, J. K. J. Chem.
Soc., Chem. Commun. 1991, 396; (b) Komatsu, K.; Aka-
matsu, H.; Aonuma, S.; Jinbu, Y.; Maekawa, N.;
Takeuchi, K. Tetrahedron 1991, 34, 6951.
907.41, monoclinic, P21/c, a=10.847 (5), b=20.532 (10),
3
,
,
c=15.410 (7) A, i=100.214 (9)°, U=3377 (2) A , Dcalcd
(Z=4)=1.784 g cm−1, v=28.10 cm−1, T=153 K. The
final R value is 0.067 for 2881 independent reflections
with I >3|I and 388 parameters. The bond distance for
,
CꢁO is 1.24(1) A. For 3a: C36H35N3O, M 525.69,
orthorhombic, P212121, a=11.236 (6), b=15.226 (9), c=
3
,
,
16.221 (9) A, U=2774 (2) A , Dcalcd (Z=4)=1.258 g
cm−1, v=0.76 cm−1, T=100 K. The final R value is 0.060
for 2286 independent reflections with I >3|I and 362
(
parameters. For 4a: C36H37N3O, M 527.71, triclinic, P1,
,
a=9.610 (8), b=10.826 (8), c=15.28 (1) A, h=99.817
3
,
(9), i=98.46 (1), k=108.93 (1)°, U=1446 (1) A , Dcalcd
(Z=2)=1.212 g cm−1, v=0.73 cm−1, T=153 K. The final
R value is 0.081 for 2779 independent reflections with I
>3|I and 361 parameters. The bond distance for CꢁO is
20. McMurry, J. E.; Lectka, T. J. Am. Chem. Soc. 1990, 112,
869.
21. Keto-carbenium 2a+ was also generated when 1a was
treated with CCl3COOH (1.1 equiv.) in chloroform. Pre-
liminary results show that production of 2a+ obeys the
similar kinetics under Ar and under air, suggesting that
oxygen does not play an important role in this reaction.
,
(
1.244(5) A. For 5a: C36H37N3O, M 527.71, triclinic, P1,
,
a=10.548 (4), b=11.934 (6), c=12.384 (5) A, h=86.20
3
,
(2), i=69.52 (2), k=78.39 (2)°, U=1430 (1) A , Dcalcd
(Z=2)=1.225 g cm−1, v=0.74 cm−1, T=153 K. The final