Base-induced rearrangement of tritylamines to imines: discovery and investigation of the mechanism

Article abstract of DOI:10.1016/j.tet.2007.03.055

An unexpected compound, the aniline derived benzophenone imine, was isolated when tritylamine was treated with n-BuLi and alkyl halides, during the formation of N-alkyl tritylamines, in the process of preparing primary amines. A nucleophilic attack of the nitrogen anion of tritylamide on the adjacent C-bonded phenyl, either substituted or not, involving a bridging anionic intermediate, is proposed for this base-induced tritylamine rearrangement to produce the corresponding imine. Electron-withdrawing groups in the aromatic ring, favoring the negative charge development, affect the relative migratory tendencies.

Full text of DOI:10.1016/j.tet.2007.03.055

Tetrahedron 63 (2007) 4284–4289
Base-induced rearrangement of tritylamines to imines:
discovery and investigation of the mechanism
*
Vassiliki Theodorou, Konstantinos Skobridis and Aris Karkatsoulis
Department of Chemistry, Section of Organic Chemistry and Biochemistry, University of Ioannina,
GR-451 10 Ioannina, Greece
Received 17 November 2006; revised 13 February 2007; accepted 8 March 2007
Available online 13 March 2007
Abstract—An unexpected compound, the aniline derived benzophenone imine, was isolated when tritylamine was treated with n-BuLi and
alkyl halides, during the formation of N-alkyl tritylamines, in the process of preparing primary amines. A nucleophilic attack of the nitrogen
anion of tritylamide on the adjacent C-bonded phenyl, either substituted or not, involving a bridging anionic intermediate, is proposed for this
base-induced tritylamine rearrangement to produce the corresponding imine. Electron-withdrawing groups in the aromatic ring, favoring the
negative charge development, affect the relative migratory tendencies.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
Due to the high symmetry of the bulky trityl group, most of
the N-alkylated tritylamines, obtained almost quantitatively
in the reaction mixture, are easily separated and purified by
recrystallization, while the byproducts can be easily sepa-
rated, recycled, and reused. Trityl moiety is a valuable
acid-labile sterically bulky protecting group for peptide, car-
bohydrate, and nucleotide chemistry, which can be selec-
tively removed.1 The extremely mild acidic conditions,
under which the N-trityl group can be removed, make trityl-
amine a very useful ammonia equivalent in synthesis.
The conversion of alkyl halides, alcohols or other similar
alkylating agents to their corresponding primary amines is
a very useful organic transformation. In the search for
a mild, simple, and convenient method for such a conversion,
we reported the application of triphenylmethylamine or
tritylamine1 (Ph3CNH2 or TrNH2) as an ammonia synthon.
Primary alkyl halides react smoothly in CH3CN solution
with 2 equiv of tritylamine to give N-tritylamines (TrNHR)
and the salt of TrNH2$HX. Alkyl tosylates react faster in
refluxing toluene, giving the desired TrNHR. Such amines
are also successfully derived from alkyl halides and lithium
tritylamide formed by the reaction of TrNH2 and n-BuLi in
THF (Scheme 1).
Recently, we have reported2 that during the preparation of
N-alkyl tritylamines by the reaction of lithium tritylamide
and primary alkyl halides in THF, an unexpected compound
was isolated, the imine Ph2C]NPh (benzophenone phenyl-
imine or benzophenone anil), formed by phenyl migration
and hydride loss from the tritylamide anion.
RX
Ph3CNH2
Ph3CNHR
Ph3CNH2 HX
RX
+
X: Br, CI, OTs
In view of these results, we tried to examine the mechanism
of this transformation. To our delight, we realized that the
formation of the imine was taking place regardless of the
addition, or not, of alkyl halide.
Ph CNHR LiX
+
3
THF
Ph3CNH2 n-BuLi
Ph3CNHLi
+
LiH
Ph2C=NPh +
RNH2 HCl Ph3COCH3
1. CF3COOH
Ph3CNHR
+
After that, we tried to explore the mechanistic details for the
above-mentioned base-induced rearrangement.
2. HCl / CH3OH
Scheme 1. The synthesis of primary amines using tritylamine.1
Previous reports related to this type of rearrangements have
appeared in the literature. The well-known Stieglitz rear-
rangement3 of N-substituted amines involves the migration
of an aryl group from carbon to nitrogen through a bridging
cationic intermediate to form the corresponding imine. The
base-promoted rearrangement of tertiary benzylic amines,
demonstrated by Eisch et al.,4 involves an intramolecular
Keywords: Tritylamide; Tritylamine; Imine; Aryl or phenyl migration;
Hydride loss; Base-induced rearrangement; Bridged anionic intermediate.
* Corresponding author. Tel.: +30 26510 98591; fax: +30 26510 98682;
e-mail: vtheodor@cc.uoi.gr
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.03.055

V. Theodorou et al. / Tetrahedron 63 (2007) 4284–4289
4285
Ar3C NHY
Ar2C N Ar
Y
= -N2, -OPCl4, -OCOR, -Pd(OAc)3, -OSO2Ar, etc.
1. n-BuLi
Ph2CH
NHPh
PhCH2
NPh2
2. H2O
Li
CO2
Li/THF
-60 °C
Ph2C
CH2Ph
Ph2C
CH2Ph
Ph3C
CH2Cl
Ph3C
CH2Li
COOH
Scheme 2. Related 1,2-intramolecular aryl rearrangements.
shift of a phenyl or aryl group from nitrogen to carbon
through a bridging anionic intermediate, leading to the for-
mation of benzhydrylamines. Alternatively, the rearrange-
ment of triphenylethyl lithium, described by Grovenstein
and Williams,5 takes place via a phenyl migration from car-
bon to carbon and a resonance stabilized carbanion. Confir-
mation of these results was provided with the formation of
the subsequent addition product, through carboxylation to
the corresponding propanoic acid (Scheme 2).
monosubstituted tritylamines. Substituents of p-OCH3, p-
CH3, and p-CF3 were chosen, because of their compatibility
with the strong basic and nucleophilic reaction conditions
and because they provide varying electronic demands. Fur-
thermore, the electron-withdrawing p-Br was attempted,
even though it is not compatible with the reaction conditions,
since halogen–metal exchange is also carried out in the pres-
ence of the butyllithium reagent.6
2.2. Preparation of the tritylamines and imines
We are not aware, until now, of any reports on analogous re-
actions or related systems undergoing base-promoted phenyl
migration and hydride loss.
The p-monosubstituted tritylamines were synthesized by the
reaction of ammonia and the corresponding tritylchloride,
according to a modification of the published procedures.1,3c
The synthesis of the tritylchlorides was achieved by
a Grignard reaction of the appropriate benzophenone and
phenyl magnesium bromide to the respective trityl alcohol,
followed by reaction of the alcohol with thionyl chloride
to the corresponding tritylchloride (Scheme 3, Eq. 1).
Herein, we reveal our preliminary efforts in this area,1,2
demonstrating the ability of the tritylamides to undergo
intramolecular rearrangement, and we propose a mechanism
for this rearrangement on the basis of the electronic pro-
perties of the migrating group and trapping experiments.
Evidence was provided for the preferential migration,
suggesting the presence of an anionic bridged interme-
diate. The migrating group moves without its electron pair,
after a nucleophilic attack of the nitrogen anion on an
adjacent C-bonded phenyl or aryl, forming the cyclic inter-
mediate.
In addition, to characterize the products of the various trityl-
amines rearrangement, it was necessary to synthesize all the
potentially formed ketimines, according to the published
methods,7–9 as samples of comparison, in order to analyze
1
them directly by H NMR. They were prepared from the
appropriate benzophenone and aniline in the presence of
molecular sieves7 (Scheme 3, Eq. 2). After completion of
the reaction and purification by column chromatography on
alumina, the imines, as a mixture of E and Z geometric iso-
mers, were isolated as yellow oils in relatively low yields
(65–75%). This is due to their instability8 to both silica and
alumina.
Loss of a hydride and subsequent phenyl rearrangement lead
to the formation of the imine. Readdition reactions with
CH3I and H2O or D2O excluded the possibility of an
intermediate carbanion, since no other products, except
TrNHCH3, were identified.
2. Results and discussion
2.3. Rearrangement reactions: analysis of products
2.1. Mechanism: substituent effects
The rearrangement reactions of tritylamines were readily
performed in THF at À75 ꢀC.
The principal mechanistic questions were (i) whether the
rearrangement is truly cationotropic/electrophilic (where
the migrating group moves without its electron pair) or,
maybe, anionotropic/nucleophilic (where the migrating
group moves with its electron pair), and (ii) what the
electronic requirements are for the migration of an aryl
group from the C to the N center. It was critical to learn,
also, whether an intramolecular 1,2-aryl shift occurs, by
way of a bridging anionic or cationic intermediate, or
whether an anionic process through a resonance stabilized
carbanion.2
A mixture of two structurally isomeric imines I (as a mixture
of E and Z isomers) and II is expected in all the rearrange-
ment reactions. The relative abundance of these two prod-
ucts is sensitive to the electronic properties of the phenyl
substituents (Scheme 3, Eq. 3).
We realized that the mixture of the expected isomeric imines
was efficiently hydrolyzed during the purification by column
chromatography on silica gel or alumina, in different rates,
and thus it was not possible to establish their real composi-
tion. Thus, in order to obtain the formed imines at their
representative composition, preparative chromatography
on alumina plates was immediately performed.
In order to determine the influence of the substituents on the
mechanism of the rearrangement, we examined a series of

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V. Theodorou et al. / Tetrahedron 63 (2007) 4284–4289
1. SOCl2
2. NH3
PhMgBr
ArCOPh
ArPh2C
ArPh2C OH
NH2
(1)
(2)
Ar: p-anisyl, p-tolyl, p-bromophenyl
Ar'
Ar
Ar
Molecular
sieves (4 Å)
Ar' NH2
C
N
+
+
C
N
Ar CO Ph
Ph
Ar'
Ph
Ar: p-anisyl, p-tolyl, p-bromophenyl, and Ar': phenyl
Ar: phenyl, and Ar': p-anisyl, p-tolyl, p-bromophenyl
Ar
Ph
Ar
Ph
n-BuLi
(3)
ArPh2C
ArPh2CNHLi
C
N
NH2
C N Ar
+
C
N
+
- LiH
Ph
Ph
Ph
Ph
Ar: p-anisyl, p-tolyl, p-bromophenyl
Ia (E)
Ib (Z)
II
Scheme 3. Synthesis of tritylamines and imines (Eqs. 1–3).
The products obtained from the rearrangement reactions
suggest the formation of special intermediates, as illustrated
in Scheme 4.
were observed, indicating the occurrence of the three iso-
mers (Ia, Ib, II) and confirming their identity (3.70, 3.75,
and 3.83 ppm, respectively). Relative integration of the
above three isomers gave a ratio of 2.1:1.2:1.
First of all, p-methoxytritylamine was examined, as p-OCH3
was considered to be a strong electron-donating group, com-
patible with the reaction conditions, so that a direct compar-
ison of the migratory tendencies of the unsubstituted phenyl
ring and the p-OCH3 substituted phenyl ring would be
possible. A mixture of two structural isomers I and II was
produced, in a ratio of w3:1, as estimated macroscopically
by the TLC picture.
With p-methyltritylamine and the weakly electron-donating
p-CH3 group, imine I was slightly favored, yielding a ratio of
imines: I:II of 2.1:1, while the two geometrical isomers E
1
and Z were 1:1, as revealed from the H NMR spectra of
the imine mixture, obtained through preparative chromato-
graphy. Thin layer chromatography does not distinguish
the two imines (I and II). Three signals for the p-tolyl methyl
protons were observed, indicating the occurrence of the
three isomers (Ia, Ib, II) and confirming their identity
(2.23, 2.29, and 2.39 ppm, respectively). Relative integra-
tion of the above three isomers gave a ratio of 1:1:0.95.
We attempted to determine their ratio using the chemical
shifts of the methoxy protons. 1H NMR analysis of the imine
mixture, after purification with preparative chromatography
on alumina, revealed that imine I was about three times as
much as imine II, showing that phenyl ring rearrangement
was favored over the p-OCH3 substituted phenyl ring.
Obviously, migration of one of the two phenyls was faster,
compared with the p-OCH3 substituted phenyl ring, due to
the greater stabilization of the bridged anionic intermediate
in the first case. Imine I exists as a mixture of two geomet-
rical isomers E and Z (Ia and Ib), in a ratio of 1.7:1, as deter-
mined by 1H NMR, while imine II exists only in one isomer.
Three signals for the aromatic p-methoxy group protons
In the case of p-bromotritylamine, which is not consistent
with the reaction conditions, the expected debromination
product6 aniline derived benzophenone imine, Ph2C]NPh
(1H NMR spectrum identical with that of authentic sample),
was isolated, at the same time with the p-bromoaniline
derived benzophenone imine. However, the aniline derived
bromobenzophenone imine was not detected by the 1H
NMR analysis. It seems that, only the p-bromophenyl group
was rearranged, since p-Br stabilizes the intermediate
R
R
Li
Li
Li
Li
Ar
and/or
Ar
Ph
Ph
Ph
Ph
+
C
N
C
N
+
C
N
C
N
Ph
H
H
Ph
H
H
R: OCH3, CH3, Br
Ar
Ar
Ph
N
Ph
Ar
C
N
Ph
C
+
C N
- LiH
Ph
Ph
Ph
Ia
Ib
II
D2O
Li
C
Li
C
Ar
Ph
Ph
H
Ph
Ph
Ar
H
+
N
N
CH3I
Scheme 4. Proposed mechanism for the imines formation.

V. Theodorou et al. / Tetrahedron 63 (2007) 4284–4289
4287
anionic species. It is not certain whether the bromine to
proton exchange occurs before or after the rearrangement.
Confirmation of these results was provided by a direct at-
tempt of readdition reactions. A rearranged carbanion could
be trapped by any relatively good electrophile. When the
reaction was quenched with D2O, after a short period of
time, no deuterated products corresponding to addition reac-
tion have been identified. Addition of CH3I to the reaction
mixture did not give any other product, but the expected
N-methyl tritylamine, TrNHMe.1 This negative trapping
result excludes the occurrence of any intermediate carbanion
and supports the proposed mechanism.
Finally, our attempts to obtain the rearrangement products of
p-trifluoromethyltritylamine were not successful. With the
strongly electron-withdrawing p-CF3 phenyl substituent,
we failed to isolate any imine, but we obtained benzophe-
none and trifluoromethylaniline, which are the hydrolysis
products of p-trifluoromethylaniline derived benzophenone
imine, together with unreacted tritylamine (w40%) and other
not identified minor byproducts (<5%). The above result
indicates that only the p-substituted phenyl group was rear-
ranged, as p-CF3 is strongly stabilizing the intermediate an-
ion. Moreover, it seems that the p-trifluoromethyl imines are
very unstable and are easily decomposed to their hydrolysis
products. Due to their great instability,8 we failed to prepare
pure p-CF3-substituted imines, by the reaction of the respec-
tive benzophenone and aniline over molecular sieves.
While the mechanism depicted in Scheme 4 is consistent
with experimental evidence, further theoretical and experi-
mental exploration of other mechanistic possibilities for
this type of migrations is continuing.
4. Experimental
4.1. General
3. Conclusion
All solvents were dried and distilled by standard methods.
Tetrahydrofuran was purified by fresh distillation over so-
dium/benzophenone. Commercially available reagents
were used without further purification. Analytical thin layer
chromatography and preparative scale chromatography were
performed on silica gel 60 F254 (Merck) and aluminum oxide
150 F254 neutral (Typ T, Merck), respectively. Column chro-
matography was performed either on silica gel (230–400
mesh) or on aluminum oxide neutral (type 507 C, Fluka).
1H NMR spectra were recorded on Bruker AMX 400 and
AMX 250 spectrometers (400 MHz and 250 MHz, respec-
tively), at ambient temperature using tetramethylsilane
(TMS) as an internal standard and deuterated chloroform
as the solvent. Chemical shifts are reported in ppm (d) refer-
enced to TMS.
From all these results, a cationotropic or electrophilic rear-
rangement of tritylamines upon treatment with n-BuLi is
suggested. The base-promoted 1,2-intramolecular phenyl
migration from the benzylic carbon of the tritylamide to
the nitrogen occurs via a bridging anionic intermediate.
Loss of a hydride and simultaneous aryl rearrangement leads
to the formation of imines. The driving force for the phenyl
migration and the hydride abstraction seems to be the release
of steric strain from tritylamine relative to the imine.
The nitrogen anion of tritylamide attacks the adjacent aryl,
affording, preferably, the better stabilized anion. The devel-
opment of the negative charge on the migrating aryl group is
influenced by the electronic nature of the substituents, being
favored with electron-withdrawing groups (Br, CF3). On the
other hand, the electron-donating p-OCH3 group does not fa-
vor the development of the negative charge and phenyl mi-
gration is preferred. The weakly electron-donating p-CH3
group seems to slightly affect the composition of the reaction
products (Table 1). This substituent effect can be explained
by consideration of the mechanism of Scheme 4, excluding
the rearrangement through a cationic intermediate. The rel-
ative amounts of imines I and II are dependent on the elec-
tron-withdrawing nature of the migrating aryl. The overall
order of migratory tendencies (migratory aptitudes) is con-
sidered to be the following: CF3>Br>HwCH3>OCH3
(Table 1).
4.2. General synthetic procedures
4.2.1. Synthesis of p-monosubstituted tritylcarbinols. To
a solution of phenyl magnesium bromide (11 mmol) in
30 mL of diethyl ether, prepared from equivalent quantities
of bromobenzene and magnesium turnings, a solution of the
monosubstituted benzophenone (10 mmol) in 20 mL of
diethyl ether was added dropwise under an atmosphere of
argon over a period of several minutes. Spontaneous boiling
started and the reaction mixture, after being refluxed for 1 h,
was diluted with additional 20 mL of diethyl ether and left
under stirring at room temperature for about 2 h. At this
point, the mixture was cooled in ice and quenched with a sat-
urated aqueous solution of NH4Cl (10 mL). The organic
phase was separated, washed with H2O (2Â25 mL), dried
over Na2SO4, evaporated, and the residue was recrystallized
from ether/hexane. Total yields: 65–90% colorless solids.
Their identity was confirmed by comparing their melting
points with those of the literature10 (98–99, 73–74, and
78–79 ꢀC for the p-Br, p-CH3, and p-OCH3 carbinol deriva-
tives, respectively).
Table 1. Reaction of n-BuLi with ArPh2C–NH2 (Eq. 3)
Amine (Ar)
Relative yielda II:I
M.A.b
p-Anisyl
p-Tolyl
p-Bromophenyl
p-Trifluromethylphenyl
1:3.34
1:2.1
0.60
0.95
1:n.d.c
1:n.d.
[1
([1)d
The ratios of I and II were obtained from the imine mixture by 1H NMR
analysis.
M.A (migratory aptitudes) were calculated on the basis of the statistical
preference factor of 2 for the phenyl migration versus the p-substituted
phenyl: II:I/2.
a
b
4.2.2. Synthesis of p-monosubstituted tritylchlorides. To
a solution of the p-substituted tritylcarbinol (10 mmol) in
50 mL of anhydrous CCl4, freshly distilled thionyl chloride
(4 mL, 55 mmol) was added under an atmosphere of argon
c
Not detected (n.d.).
Only the hydrolysis products of the respective imine II were detected.
d

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V. Theodorou et al. / Tetrahedron 63 (2007) 4284–4289
and the mixture was stirred at room temperature for 5 h.11
After that, the mixture was concentrated in vacuo to give
the corresponding chlorides as white solids, which were
used without further purification.
alumina, using the eluent dichloromethane/hexane (1:1),
the imines were isolated as yellow oils. Their 1H NMR spec-
tra were acquired and determined as a mixture of E and Z
geometric isomers.
4.2.3. Synthesis of p-monosubstituted tritylamines. To
a solution of the p-substituted tritylchloride (10 mmol) in
15 mL CH2Cl2, ammonium hydroxide (15 mL, 28% in wa-
ter) was added and the mixture was stirred vigorously for
about two days. After that, it was extracted with CH2Cl2
(2Â20 mL) and washed with water (2Â25 mL). The organic
layer was concentrated in vacuo and the resulting amine was
purified by column chromatography on silica using CH2Cl2
4.4.1. Benzophenone phenylimine (or benzophenone
anil) Ph2C]N–Ph. H NMR (250 MHz, CDCl3) d 6.72
(2H, dd, ArH), 6.92 (1H, dt, ArH), 7.10–7.27 (7H, m,
ArH), 7.37–7.48 (3H, m, ArH), 7.75 (2H, dd, ArH) (lit.2).
1
4.4.2. p-Bromobenzophenone phenylimine Br–C6H4–
PhC]N–Ph. H NMR (400 MHz, CDCl3) d 6.69 (2H, dd,
1
ArH), 6.89 (1H, m, ArH), 6.95 (2H, d, ArH), 7.05–7.24
(5H, m, ArH), 7.34–7.50 (3H, m, ArH), 7.59 (1H, m,
ArH), 7.69 (1H, m, ArH) (lit.3e,12a).
1
as eluent (yield: 70–80%). The H NMR spectra of the
respective compound confirmed its identity.3c
4.2.3.1. p-Bromophenyldiphenylmethylamine p–Br–
C6H4–Ph2C–NH2. 1H NMR (250 MHz, CDCl3) d 2.05
(2H, s, NH2), 7.19 (2H, d, ArH), 7.29 (10H, m, ArH), 7.44
(2H, d, ArH); white solid.
4.4.3. Benzophenone p-bromophenylimine Ph2C]N–
C6H4–Br. 1H NMR (400 MHz, CDCl3) d 6.60 (2H, dt,
ArH), 7.10 (2H, dd, ArH), 7.23–7.31 (5H, m, ArH), 7.38–
7.42 (2H, t, ArH), 7.46–7.51 (1H, m, ArH), 7.74 (2H, dd,
ArH) (lit.6).
4.2.3.2. Diphenyl-p-tolylmethylamine p-CH3–C6H4–
Ph2C–NH2. H NMR (250 MHz, CDCl3) d 2.17 (2H, s,
NH2), 2.33 (3H, s, CH3), 7.13 (4H, m, ArH), 7.26 (10H,
m, ArH); oil.
1
4.4.4. p-Methylbenzophenone phenylimine CH3–C6H4–
PhC]N–Ph. 1H NMR (250 MHz, CDCl3) d 2.29 and
2.39 (3H, s, CH3, as a mixture of E/Z isomers, w1:1),
6.72 (2H, dt, ArH), 6.88–7.26 (9H, m, ArH), 7.36–7.47
(1H, m, ArH), 7.64 (1H, d, ArH), 7.72–7.75 (1H, m, ArH)
(lit.9a).
4.2.3.3. p-Anisyldiphenylmethylamine p-CH3O–
C6H4–Ph2C–NH2. 1H NMR (250 MHz, CDCl3) d 2.19
(2H, s, NH2), 3.75 (3H, s, OCH3), 6.78 (2H, d, ArH), 7.13
(2H, d, ArH), 7.23 (10H, m, ArH); oil.
4.4.5. Benzophenone p-tolylimine Ph2C]N–C6H4–CH3.
1H NMR (250 MHz, CDCl3) d 2.23 (3H, s, CH3), 6.60
(2H, d, ArH), 6.91 (2H, d, ArH), 7.08–7.12 (2H, m, ArH),
7.22–7.26 (3H, m, ArH), 7.34–7.44 (3H, m, ArH), 7.69–
7.72 (2H, m, ArH) (lit.9b,c).
4.2.3.4.
Diphenyl-p-trifluoromethylphenylmethyl-
amine p-CF3–C6H4–Ph2C–NH2. 1H NMR (250 MHz,
CDCl3) d 2.16 (2H, s, NH2), 7.12 (2H, d, ArH), 7.24 (12H,
m, ArH); oil.
4.4.6. p-Methoxybenzophenone phenylimine CH3O–
C6H4–PhC]N–Ph. H NMR (250 MHz, CDCl3) d 3.75
1
4.3. Rearrangement of tritylamines
and 3.83 (3H, s, OCH3 as a mixture of E/Z isomers,
w1:1.7), 6.68–7.47 (12H, m, ArH), 7.69–7.75 (2H, m,
ArH) (lit.3c,9e,12b).
To a solution of 1 mmol of the respective tritylamine in
10 mL of THF, cooled to about À75 ꢀC, 1 mmol of a solution
of n-BuLi in hexane (1.6 M) was added under an argon at-
mosphere. A deep red-colored solution was obtained. The
solution was left at this temperature for about 1 h, then the
solvent was removed, CH2Cl2 was added (20 mL), and the
solution filtered and concentrated. The crude mixture8 was
directly absorbed on neutral preparative alumina plates
and eluted with a mixture of dichloromethane/hexane
(1:1). The obtained 1H NMR spectra confirmed the identity
of the products, in each case, by comparison with the pure
synthesized compounds.
4.4.7. Benzophenone p-anisylimine Ph2C]N–C6H4–
OCH3. 1H NMR (250 MHz, CDCl3) d 3.70 (3H, s, OCH3),
6.69 (4H, ws, ArH), 7.10–7.14 (2H, m, ArH), 7.26–7.29
(3H, m, ArH), 7.38–7.45 (3H, m, ArH), 7.72–7.76 (2H, dt,
ArH) (lit.9d).
References and notes
1. Theodorou, V.; Ragoussis, V.; Strongilos, A.; Zelepos, E.;
Eleftheriou, A.; Dimitriou, M. Tetrahedron Lett. 2005, 46,
1357 and references cited therein.
2. Theodorou, V.; Skobridis, K. Tetrahedron Lett. 2005, 46,
5021.
(Yield:w50–60% of imines, hydrolysis products of the im-
ines about 5% and other, not identified, minor byproducts
<2%, as well as unreacted tritylamines).
4.4. Synthesis of imines
3. (a) Stieglitz, J.; Leech, P. N. J. Am. Chem. Soc. 1914, 36, 272;
(b) Stieglitz, J.; Vosburgh, J. J. Am. Chem. Soc. 1916, 38,
2081; (c) Sisti, A. J.; Milstein, S. R. J. Org. Chem. 1974,
39, 3932; (d) Hofmann, R. V.; Poelker, D. J. J. Org. Chem.
1979, 44, 2364; (e) Stagner, B. A. J. Am. Chem. Soc. 1916,
38, 2069.
To a solution of the respective benzophenone (10 mmol) in
ether or toluene (10 mL), aniline (11 mmol) was added.
˚
The reaction mixture was refluxed upon 5 g of 4 A molecular
sieves for 12–24 h. After completion of the reaction, moni-
tored by TLC, the solution was filtered and concentrated
in vacuo. After purification by column chromatography on
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