Alumina-mediated dealkylative dimerization of 4-aminobenzyl esters

Article abstract of DOI:10.1016/j.tet.2007.02.094

Treatment of 4-aminobenzyl esters with Al₂O₃ in DCM at rt afforded the dealkylative dimerized 4,4′-diamino-diphenylmethanes in satisfactory yield. The reaction may proceed via a quinone methide iminium ion intermediate, which may then undergo Michael type addition followed by retro-aldol extrusion of a formaldehyde species.

Full text of DOI:10.1016/j.tet.2007.02.094

Tetrahedron 63 (2007) 6051–6055
Alumina-mediated dealkylative dimerization of
4-aminobenzyl esters
*
Yu-Ying Lai, Nai-Ti Lin, Yi-Hung Liu, Yu Wang and Tien-Yau Luh
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
Received 13 January 2007; revised 15 February 2007; accepted 21 February 2007
Available online 27 February 2007
Abstract—Treatment of 4-aminobenzyl esters with Al2O3 in DCM at rt afforded the dealkylative dimerized 4,40-diamino-diphenylmethanes
in satisfactory yield. The reaction may proceed via a quinone methide iminium ion intermediate, which may then undergo Michael type
addition followed by retro-aldol extrusion of a formaldehyde species.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
iminium ion intermediate, which would react with methox-
ide nucleophile in the medium. It is interesting to note that
quinone methide derivatives are important intermediates in
DNA alkylation and DNA cross-linking reactions.12 In order
to understand the generality of this novel polymeric system,
we have synthesized various kinds of monomers having
a range of different linkers to connect the two norbornene
units.
Diarylmethane derivatives are useful building blocks for the
design of supramolecular structures,1 polymer synthesis,2 as
well as biologically active compounds.3 The preparation of
diarylmethanes can be achieved by numerous procedures
such as Friedel–Crafts reaction,4 cross coupling reactions,5
reduction of the corresponding diaryl ketones6 or alcohols,7
or disproportionation reactions.8 Reaction of N,N-dimethyl-
4-toluidine-N-oxide 1 in trifluoroacetic acid–triflic acid
gives diarylmethane 3 as the side product (Scheme 1).9 A
quinone methide iminium ion intermediate 2 was suggested.
During the course of this investigation, we have synthesized
bisnorbornene 7 using a diester linkage to tether two amino-
benzyl moieties. Attempts to purify 7 by column chromato-
graphy on neutral alumina or silica gel were unsuccessful. A
new dimeric diarylmethane 8 was obtained. The structure of
8 was unambiguously proved by X-ray crystallography
(Fig. 1 and Table 1).
We recently reported the first helical DNA-like double
stranded polymer 5 by ring opening metathesis polymeriza-
tion10 of a bisnorbornene derivative 4.11 Because of the pres-
ence of the relatively labile aminobenzyl fragment in 5, the
corresponding single stranded polymer 6 is easily obtained
by hydrolysis (Scheme 2).11 As can be seen from Scheme
2, the methyl ether 6 is obtained exclusively. Presumably,
an intermediate similar to 2 would be formed during the
course of hydrolysis. The chemistry shown in Scheme 2
might be a unique process involving a quinone methide
The isolation of 8 was somewhat striking. Presumably, the
benzylic C–O bond would be extremely labile and cleaved
readily under chromatographic separation conditions to give
quinone methide iminium ion intermediate 9. In the absence
of other nucleophiles, Michael type addition of another
aminobenzyl moiety to 9 would yield 10, which would
Me
N Me
Me
Me
O
NMe2
N
TFA
+
TfOH
Me2N
NMe2
CH2OH
Me
1
2
3
Scheme 1.
* Corresponding author. Tel.: +886 223636288; fax: +886 223644971; e-mail: tyluh@ntu.edu.tw
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.02.094

6052
Y.-Y. Lai et al. / Tetrahedron 63 (2007) 6051–6055
N
N
O
O
O
O
Fe
Fe
(Cy3P)2RuCl=CHPh
O
O
O
O
N
N
5
4
NaOH
MeOH
OH
O
OMe
N
Fe
O
HO
6
Scheme 2.
Figure 1. ORTEP structure of 8.
N
O
N
O
O
O
N
N
7
8
then undergo a retro-aldol extrusion of the formaldehyde
species to generate 8 (Scheme 3).
EtOAc was used as the reaction medium. When the reaction
was carried out in MeOH, the solvent may serve as the
nucleophile to quench the quinone methide iminium ion
intermediate leading to the corresponding methoxy ether
13 in 99% yield (Table 2).
It is envisaged that the reaction might be considered to be
a general reaction for 4-aminobenzyl esters. Thus, upon
treatment with Al2O3 in DCM at rt, the crude 12, prepared
from the reaction of 11 and Ac2O, afforded 8 in 40% yield.
When pure 12 was employed, 8 was isolated in 62% yield.
Several different conditions have been tried and the results
are outlined in Table 2. It is interesting to note that DCM
at rt was the best condition for this transformation. At reflux-
ing temperature, a significant amount of side products was
observed and 8 was obtained in 50% yield. When the reac-
tion was carried out in polar solvents such as THF or
DMF, starting compound 12 was almost completely recov-
ered (>90). It is noteworthy that 8 was obtained in 10% yield
together with a mixture of unidentified products when
N
X
N
N
R
O
OR
N
O
11 R = CH2OH
12 R = CH2OAc
13 R = CH2OMe
14 R = CHO
15 R = H
16 R = Ac
17
18 X = OH
19 X = OMe
20 X = Me

Y.-Y. Lai et al. / Tetrahedron 63 (2007) 6051–6055
6053
Table 1. Crystal data and structure refinement for 8
In a similar manner, the reaction of 16 under similar condi-
tions in DCM gave dealkylative dimer 17 in 70% yield.
Attempts to use other substrates having para-hydroxy-,
methoxy- or alkyl-substituted benzyl esters 18–20 under
similar conditions were unsuccessful. Even in the presence of
a base (e.g., DBN), 18 gave a mixture of unidentified prod-
ucts, no desired diarylmethane being detected. Presumably,
the reaction may require a strong electron donating substit-
uent to facilitate the leaving of the carboxylate group leading
to quinone methide imminium ion intermediate. In addition,
the aromatic ring may not be nucleophilic enough to proceed
the Michael type addition reaction giving the corresponding
diarylmethane derivative.
Empirical formula
Formula weight
Temperature
C31H34N2
434.60
295(2) K
0.71073 A
Monoclinic
P2(1)/n
˚
Wavelength
Crystal system
Space group
Unit cell dimensions
ꢁ
˚
a¼19.0940(5) A, a¼90
ꢁ
˚
b¼6.19200(10) A, b¼94.9880(10)
ꢁ
˚
c¼19.9310(3) A, g¼90
3
˚
2347.52 (8) A
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Index ranges
Reflections collected
Independent reflections
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I>2sigma(I)]
R indices (all data)
Extinction coefficient
Largest diff. peak and hole
4
1.230 Mg/m3
0.071 mmꢀ1
936
0.30ꢂ0.25ꢂ0.15 mm3
2.05–27.47ꢁ
In summary, we have uncovered an unprecedented dealkyl-
ative dimerization of 4-aminobenzyl esters promoted by
Al2O3. The absence of other nucleophiles would be the pre-
requisiteforthisreaction. Thereactionfurnishesaconvenient
route for the synthesis of dianilinomethane derivatives.
Further applications of this protocol are in progress in our
laboratory.
ꢀ24ꢃhꢃ24, ꢀ8ꢃkꢃ8, ꢀ24ꢃlꢃ25
16,793
5337 [R(int)¼0.0369]
Semi-empirical from equivalents
0.990 and 0.974
Full-matrix least-squares on F2
5288/0/299
0.876
R1¼0.0485, wR2¼0.1403
R1¼0.0926, wR2¼0.1864
0.0143 (33)
2. Experimental section
ꢀ3
˚
0.212 and ꢀ0.191 eA
2.1. Bis-[4-(4-aza-tricyclo[5.2.1.02,6]dec-8-en-4-yl)]-
benzyl hexandioate (7)
N
To a DCM solution (60 mL) of 1113 (2.0 g, 8.3 mmol) and di-
methylaminopyridine (DMAP, 1.0 g, 8.3 mmol) was added
slowly adipoyl dichloride (0.75 g, 4.14 mmol) in DCM
(20 mL) and the mixture was stirred at rt for 24 h. Water
(100 mL) was added and the organic layer was separated
and washed with brine (50 mL), dried (MgSO4), and filtered.
The filtrate was evaporated in vacuo and the oily residue was
taken up in DCM/hexane and cooled to ꢀ78 ꢁC. Yellowish
solid was precipitated. After filtration, the solid was collected
and recrystallized from DCM/hexane to give 7; mp 113–
114 ꢁC (dec); 1H NMR (400 MHz, CDCl3) d 1.50 (d,
J¼8.2 Hz, 2H), 1.60 (d, J¼8.2 Hz, 2H), 1.62–1.64 (m, 4H),
2.29 (br s, 4H), 2.88–2.91 (m, 4H), 2.97 (br s, 4H), 3.06–
3.07 (m, 4H), 3.18–3.24 (m, 4H), 4.97 (s, 4H), 6.13 (s, 4H),
6.40 (d, J¼8.5 Hz, 4H), 7.18 (d, J¼8.5 Hz, 4H); 13C NMR
(100 MHz) d 24.5, 34.1, 45.6, 46.7, 50.7, 52.3, 66.9, 111.8,
122.5, 130.2, 135.9, 147.8, 173.5; IR (KBr) n 2965, 2935,
2848, 1720, 1613, 1526, 1480, 1375, 1260, 1184, 1155,
915, 800, 731; MS (70 eV, EI) m/z (%) 592 (10) [M+], 526
(8), 434 (10), 369 (25), 367 (8), 301 (5), 224 (45), 159 (24),
158 (100), 118 (16); HRMS (EI) (C38H44O4N2) calcd:
592.3301; found: 592.3315.
O
N
O
N
- RCO2
R
O
O
R
9
N
H2O
8
- CH2O, RCO2
O
O
N
R
10
Scheme 3.
Table 2. Reaction of 12 with Al2O3
Solvent
Temperature
%Yield
2.2. 4-(4-Aza-tricyclo[5.2.1.02,6]dec-8-en-4-yl)benzyl
acetate (12)
THF
rt
rt
rt
rt
0a
10b
2c
EtOAc
Toluene
DMF
DCM
DCM
MeOH
0a
To a solution of 11 (2.0 g, 8.3 mmol) and triethylamine
(3.5 mL, 25 mmol) in DCM (15 mL) was added dropwise
acetyl chloride (1.2 mL, 9.1 mmol) at 0 ꢁC. The mixture
was stirred at rt for 2 h, poured into water (100 mL), and ex-
tracted with DCM (50 mL). The organic layer was separated,
dried (MgSO4), filtered, and evaporatedinvacuo. Theresidue
was chromatographed on silica gel (hexane/EtOAc 4:1) to
afford 12 as a white solid (2.1 g, 92%); mp 85–87 ꢁC; H
NMR (400 MHz, CDCl3) d 1.51 (d, J¼8.2 Hz, 1H), 1.61
rt
62
50
0d
Reflux
rt
a
Starting material was recovered in more than 90%.
A mixture of unidentified products was obtained in addition to about 25%
of unreacted 12.
A mixture of unidentified products was obtained including a significant
amount of aldehyde 14.
Methoxymethyl derivative 13 was obtained in 99% yield.
b
c
d
1

6054
Y.-Y. Lai et al. / Tetrahedron 63 (2007) 6051–6055
(d, J¼8.2 Hz, 1H), 2.04 (s, 3H), 2.84–2.87 (m, 2H), 2.88–
3.00 (m, 2H), 3.01–3.11 (m, 2H), 3.19–3.24 (m, 2H), 4.98
(s, 2H), 6.13 (s, 2H), 6.41 (d, J¼8.6 Hz, 2H), 7.20 (d, J¼
8.6 Hz, 2H); 13C NMR (100 MHz, CDCl3) d 21.3, 45.5, 46.6,
50.5, 52.1, 66.9, 111.6, 122.0, 130.1, 135.6, 147.4, 170.9.
washed with DCM/NEt3 (10:1, 10 mLꢂ2) and the filtrate
was concentrated in vacuo to give the residue, which was
chromatographed on silica gel (hexane/EtOAc 7:1) to afford
17 as a white solid (220 mg, 70%); mp 82–83 ꢁC (lit.14 86.5–
87 ꢁC); 1H NMR (400 MHz, CDCl3) d 2.87 (s, 12H), 3.79 (s,
2H), 6.66 (d, J¼8.8 Hz, 2H), 7.03 (d, J¼8.8 Hz, 2H); 13C
NMR (100 MHz, CDCl3) d 39.9, 40.9, 112.8, 129.2, 130.1,
148.8. HRMS (FAB+) (C17H22N2) calcd: 254.1783; found:
254.1780.
2.3. Bis-{4-(4-aza-tricyclo[5.2.1.02,6]dec-8-en-4-yl)-
phenyl}methane (8)
To a DCM solution (60 mL) of 1113 (2 g, 8.3 mmol) and
DMAP (1.0 g, 8.3 mmol) was added slowly acetyl chloride
(0.65 g, 8.3 mmol) in DCM (20 mL) and the mixture was
stirred at rt for 24 h. Water (100 mL) was added and the
organic layer was separated and washed with brine, dried
(MgSO4), and filtered. The solvent was evaporated in vacuo
and the oily residue was dissolved in DCM (40 mL). Neutral
alumina (45 g) was added and the mixture was stirred at rt for
8 h. After filtration, solvent was removed in vacuo and the
residue was recrystallized from DCM/hexane to give 8
Acknowledgements
This work is supported by the National Science Council and
the National Taiwan University.
Supplementary data
1
(0.72 g, 40%); mp 188–189 ꢁC (dec); H NMR (400 MHz,
Supplementary data associated with this article can be found
in the online version, at doi:10.1016/j.tet.2007.02.094.
CDCl3) d 1.50 (d, J¼8.2 Hz, 2H), 1.59 (d, J¼8.2 Hz, 2H),
2.85–2.88 (m, 4H), 2.95 (br s, 4H), 3.03–3.05 (m, 4H),
3.17–3.22 (m, 4H), 3.75 (s, 2H), 6.13 (m, 4H), 6.37 (d, J¼
8.5 Hz, 4H), 6.99 (d, J¼8.5 Hz, 4H); 13C NMR (100 MHz)
40.0, 45.5, 46.5, 50.7, 52.2, 111.9, 129.1, 129.3, 135.8,
146.0; IR (KBr) n 2964, 2942, 2864, 2833, 1688, 1615,
1518, 1473, 1432, 1369, 1341, 1201, 1186, 1134, 971, 904,
797, 776, 739, 717; MS (70 eV, EI) m/z (%) 434 (100)
[M+], 393 (18), 367 (82), 372 (13), 301 (52); HRMS
(FAB+) (C31H34N2) calcd: 434.2722; found: 434.2725.
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