Domino reactions initiated by intramolecular hydride transfers from tri(di)arylmethane fragments to ketenimine and carbodiimide functions

Article abstract of DOI:10.1039/c0ob00193g

The ability of triarylmethane and diarylmethane fragments to behave as hydride donors participating in thermal [1,5]-H shift/6π-ERC tandem processes involving ketenimine and carbodiimide functions is disclosed. C-Alkyl-C-phenyl ketenimines N-substituted by a triarylmethane substructure convert into a variety of 3,3,4,4-tetrasubstituted-3,4-dihydroquinolines, as structurally related carbodiimides transform into 3,4,4-trisubstituted-3,4- dihydroquinazolines via transient ortho-azaxylylenes. The first step of these one-pot conversions, the [1,5]-H shift, is considered to be a hydride migration on the basis of the known hydricity of the tri(di)arylmethane fragment and the electrophilicity of the central heterocumulenic carbon atom, whereas the final electrocyclization involves the formation of a sterically congested C-C or C-N bond. In the cases of C,C-diphenyl substituted triarylmethane-ketenimines the usual 6π-ERC becomes prohibited by the presence of two phenyl rings at each end of the azatrienic system. This situation opens new reaction channels: (a) following the initial hydride shift, the tandem sequence continues with an alternative electrocyclization mode to give 9,10-dihydroacridines, (b) the full sequence is initiated by a rare 1,5 migration of an electron-rich aryl group, followed by a 6π-ERC which leads to 2-aryl-3,4-dihydroquinolines, or (c) a different [1,5]-H shift/6π-ERC sequence involving the initial migration of a hydrogen atom from a methyl group at the ortho position to the nitrogen atom of the ketenimine function. Diarylmethane-ketenimines bearing a methyl group at the benzylic carbon atom experience a tandem double [1,5]-H shift, the first one being the usual benzylic hydride transfer whereas the second one involves the methyl group at the initial benzylic carbon atom, the reaction products being 2-aminostyrenes. Diarylmethane-ketenimines lacking such a methyl group convert into 3,4-dihydroquinolines by the habitual tandem [1,5]-H shift/6π-ERC processes.

Full text of DOI:10.1039/c0ob00193g

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Domino reactions initiated by intramolecular hydride transfers from
tri(di)arylmethane fragments to ketenimine and carbodiimide functions†‡
Mateo Alajarin,*a Baltasar Bonillo,a Maria-Mar Ortin,a Pilar Sanchez-Andrada,a Angel Vidal*a and
Raul-Angel Orenesb
Received 1st June 2010, Accepted 14th July 2010
DOI: 10.1039/c0ob00193g
The ability of triarylmethane and diarylmethane fragments to behave as hydride donors participating in
thermal [1,5]-H shift/6p-ERC tandem processes involving ketenimine and carbodiimide functions is
disclosed. C-Alkyl-C-phenyl ketenimines N-substituted by a triarylmethane substructure convert into a
variety of 3,3,4,4-tetrasubstituted-3,4-dihydroquinolines, as structurally related carbodiimides
transform into 3,4,4-trisubstituted-3,4-dihydroquinazolines via transient ortho-azaxylylenes. The first
step of these one-pot conversions, the [1,5]-H shift, is considered to be a hydride migration on the basis
of the known hydricity of the tri(di)arylmethane fragment and the electrophilicity of the central
heterocumulenic carbon atom, whereas the final electrocyclization involves the formation of a sterically
congested C–C or C–N bond. In the cases of C,C-diphenyl substituted triarylmethane-ketenimines the
usual 6p-ERC becomes prohibited by the presence of two phenyl rings at each end of the azatrienic
system. This situation opens new reaction channels: (a) following the initial hydride shift, the tandem
sequence continues with an alternative electrocyclization mode to give 9,10-dihydroacridines, (b) the
full sequence is initiated by a rare 1,5 migration of an electron-rich aryl group, followed by a 6p-ERC
which leads to 2-aryl-3,4-dihydroquinolines, or (c) a different [1,5]-H shift/6p-ERC sequence involving
the initial migration of a hydrogen atom from a methyl group at the ortho position to the nitrogen atom
of the ketenimine function. Diarylmethane-ketenimines bearing a methyl group at the benzylic carbon
atom experience a tandem double [1,5]-H shift, the first one being the usual benzylic hydride transfer
whereas the second one involves the methyl group at the initial benzylic carbon atom, the reaction
products being 2-aminostyrenes. Diarylmethane-ketenimines lacking such a methyl group convert into
3,4-dihydroquinolines by the habitual tandem [1,5]-H shift/6p-ERC processes.
and 1,3-dithiolane-ketenimines 1 (X = S; Y = CR2R3) transform,
Introduction
under mild thermal treatment, into spiroquinolines 3 (X = O,
S; Y = CR2R3) and spiroquinazolines 3 (X = O; Y = NAr),
C–H Bond functionalization, the direct and selective replacement
of C–H bonds with new C–C or C–heteroatom bonds, is an
important and long-standing goal in organic chemistry.1 A series
of transformations for C–H bond functionalization initiated by an
intramolecular hydride transfer and followed by cyclization of the
resulting intermediates have been recently disclosed.2 Such hydride
via a tandem sequence consisting of a [1,5]-H shift followed by
a 6p electrocyclic ring closure (6p-ERC) (Scheme 1).4 On the
other hand, when 1,3-oxathiolane-ketenimines 4 (Y = CR2R3)
and 1,3-oxathiolane-carbodiimides 4 (Y = NAr) were submitted
to mild heating the 2,1-benzisothiazol-3-ones 6 were formed, a
shift/cyclization sequences, serving for the efficient buildup of
transformation which seems to occur through a formal [1,5]-H
complex polycyclic systems, are in some sense related to those
shift/1,5 electrocyclization/[3+2] cycloreversion tandem process,
based upon the well-known tert-amino effect.3 In this context,
while investigating new reactions of nitrogenated heterocumu-
lenes, we recently found that 1,3-dioxolane-ketenimines 1 (X =
O; Y = CR2R3), 1,3-dioxolane-carbodiimides 1 (X = O; Y = NAr)
with the concomitant formation of ethylene (Scheme 1).5 The
[1,5]-H shifts from the acetalic functions to the electrophilic
central carbon atom of the ketenimine or carbodiimide moieties,
presumably providing the transient ortho-azaxylylenes 2 and
5, were characterized as intramolecular hydride transfers by
means of computational DFT studies. In fact, the computed
NBO analysis of heterocumulenes 1 and 4 shows the weakening
and polarization of the acetalic C–H bond by hyperconjugative
interactions of its s* C–H orbital with the lone pair electrons at
the vicinal heteroatoms. These results demonstrated the hydride
donor ability (hydricity) of the 2-monosubstituted 1,3-dioxolane,
1,3-oxathiolane and 1,3-dithiolane functions. Additionally, the
experimental work and the computational calculations showed
that the hydricity-imparting character of these acetalic func-
tions decreases in the order: 1,3-dioxolane > 1,3-oxathiolane >
1,3-dithiolane.
aDepartamento de Quimica Organica, Facultad de Quimica, Universidad de
Murcia, Campus de Espinardo, 30100, Murcia, Spain. E-mail: alajarin@
um.es, vidal@um.es; Fax: + 34-868 364149; Tel: + 34-868887497, + 34-
868887418
bServicio Universitario de Instrumentacion Cientifica, Universidad de Mur-
cia, Campus de Espinardo, 30100, Murcia, Spain. E-mail: raorenes@um.es
† Dedicated to Professor Jose´ Elguero with respect and admiration.
‡ Electronic supplementary information (ESI) available: Comprehensive
experimental section. 1H and 13C NMR spectra of compounds 11, 13, 18,
20, 22, 25, 27, 30, 32 and 34. 31P NMR spectra of compounds 11, 27 and
32. Cif files of 13e, 13k, 18a and 20b. CCDC reference numbers 779905–
779908. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c0ob00193g
4690 | Org. Biomol. Chem., 2010, 8, 4690–4700
This journal is
The Royal Society of Chemistry 2010
©

View Article Online
[1,5]-H shift/6p-ERC tandem processes closely related to those
described for compounds 1. As result of this experimental work
we here disclose that such triarylmethane-ketenimines converted
under thermal conditions into substituted quinolines or acridines
through tandem processes that involve as the first mechanistic step
either the expected [1,5]-H transfer or an unusual [1,5] sigmatropic
migration of an aryl group, and as the second step different
modes of electrocyclic ring closure in the resulting 3-azahexatriene
intermediates. The electrocyclization mode at this second step
seems to depend on the nature of the substituents at the aryl rings
(R1 and R2) and at the C-terminal carbon atom of the starting
ketenimine function (R3). In addition, we report that under
similar conditions triarylmethane-carbodiimides simply undergo
1,5 migration of the hydrogen at the sp3 methine carbon atom and
subsequent 6p-ERC to afford quinazolines.
Results and discussion
The synthetic strategy for the preparation of the triarylmethane-
ketenimines and triarylmethane-carbodiimides starts with a
Friedel–Crafts acylation in which two equivalents of benzene
or a monosubstituted benzene react with one equivalent of
2-nitrobenzaldehyde 7a or 3-methyl-2-nitrobenzaldehyde 7b, to
provide nitrotriarylmethanes 8 (36–73%). These reactions were
run under different conditions depending on the nature of R2. In
the cases where R2 = H (8a,b) the acylation reaction was carried
out by treating benzene solutions of the 2-nitrobenzaldehydes 7
with anhydrous aluminium chloride.12 When R2 = N(CH3)2 (8c,d)
or N-morpholino (8e) mixtures of the 2-nitrobenzaldehydes 7 and
N,N-dimethylaniline or N-phenylmorpholine were treated with
anhydrous zinc chloride.13 The synthesis of the nitro derivatives
8f and 8g in which R2 = OCH3 was achieved by reaction of the
2-nitrobenzaldehydes 7 with anisole in the presence of aluminium
chloride.14 Next, the reduction of compounds 8 with molecular
hydrogen in the presence of Pd on activated charcoal, using ethanol
as solvent, yielded the anilines 9 (65–88%). The diazotation of
anilines 9 followed by the addition of sodium azide provided azides
10. For this latter conversion different reaction conditions were
also used in accordance with the nature of R2. For R2 = H or OCH3
the corresponding azides 10a,b,f,g were readily obtained (80–92%)
by sequential treatment of sulfuric acid/1,4-dioxane solutions of
amines 9a,b,f,g with sodium nitrite and sodium azide.13 For the
cases where R2 = N(CH3)2 or N-morpholino suspensions of the
amines 9c–e in 6 N hydrochloric acid were first treated with a
solution of sodium nitrite in water, and then with an aqueous
solution of sodium azide and sodium acetate (45–72%).15 The
imination reaction of triphenylphosphine with the azides 10,
in anhydrous diethyl ether at room temperature, provided the
triphenyliminophosphoranes 11 (86–99%) (Scheme 2).
Scheme 1 Proposed mechanisms for the conversions 1 → 3 and 4 → 6.
The main class of compounds possessing hydricity is
the metal hydrides.6 Nevertheless several types of organic
compounds are also considered to exhibit this property. For
example, NADH model compounds,7 triarylmethanes8 and
some 1,3-dinitrogenated heterocyclic systems, such as N,N-
dimethylbenzimidazoles,9
2-benzoyl-N,N-dimethylperhydro-
pyrimidine10 and orthoformamides,11 are generally appreciated as
good hydride donors.
Based on the known hydricity of the triarylmethanes, we
targeted the preparation of a series of these compounds bearing a
ketenimine or carbodiimide function linked by its nitrogen atom
to the ortho position of one of the three aryl groups (Fig. 1).
Our aim was to examine the viability of the [1,5]-H transfer
from the sp3 methine carbon to the electron-deficient central
carbon atom of the ketenimine or carbodiimide fragment, and
therefore the participation of this type of heterocumulene in
The aza-Wittig reaction of iminophosphoranes 11 with
ethylphenylketene and methylphenylketene, in dichloromethane
solution at room temperature, led to the formation of the
triarylmethane-ketenimines 12 (Scheme 3, Table 1), which were
separated from the triphenylphosphine oxide by column chro-
matography on a short pad of silica gel, checked by IR (strong
absorption near 2000 cm-1) and immediately submitted to the next
reaction step for preventing its partial hydrolysis.16 It is relevant
to note that compounds 12 bear a phenyl group and an ethyl
or methyl substituent at the sp2 terminal carbon atom of the
Fig.
1 General structure of triarylmethane-ketenimines and
triarylmethane-carbodiimides.
This journal is
The Royal Society of Chemistry 2010
Org. Biomol. Chem., 2010, 8, 4690–4700 | 4691
©

View Article Online
Table 1 Ketenimines 12 and Quinolines 13
12
Yield (%)
13
Yield (%)
entry
R1
R2
R3
a
b
c
d
e
f
g
h
i
j
k
l
m
n
H
H
CH3
CH3
H
H
H
H
H
CH3
CH2CH3
CH3
CH2CH3
CH3
CH2CH3
CH3
CH2CH3
CH3
CH2CH3
CH3
CH2CH3
CH3
CH2CH3
88
80
40
38
92
71
90
67
79
97
74
86
70
97
60
15
99
61
63
80
78
75
65
60
57
52
85
99
N(CH3)2
N(CH3)2
N(CH3)2
N(CH3)2
N-morpholino
N-morpholino
OCH3
OCH3
OCH3
OCH3
H
CH3
CH3
H
H
H
H
CH3
CH3
Scheme 2 Reagents and conditions: (a) R2 = H C6H6, AlCl3, 60 ◦C, 6 h;
◦
R2 = N(CH3)2, N-morpholino C6H5R2, ZnCl2, 80 C, 6 h; R2 = OCH3
C6H5OCH3, AlCl3, r.t., 12 h (36–73%). (b) H2, Pd/C, EtOH, r.t., 24 h
(65–88%). (c) R2 = H, OCH3 (1) NaNO2, H2SO4 (4 N)/1,4-dioxane, 0 ◦C,
30 min (2) NaN3, H2SO4 (4 N)/1,4-dioxane, r.t., 16 h; R2 = N(CH3)2,
N-morpholino (1) NaNO2, HCl/H2O, 0 ◦C, 30 min (2) NaN3, NaOAc,
HCl/H2O, r.t., 1 h (45–92%). (d) PPh3, Et2O, r.t., 16 h (86–99%).
Scheme 3 Reagents and conditions: (a) PhR3CCO, CH2Cl2, r.t., 30 min.
(b) Toluene or ortho-xylene, reflux, 5–144 h.
ketenimine function. When toluene or ortho-xylene solutions of
ketenimines 12 were heated at reflux temperature, during periods
of time ranging from 5 to 144 h, the 3-ethyl(methyl)-3,4,4-triaryl-
3,4-dihydroquinolines 13 were formed, and isolated in moderate
to good yields (Scheme 3, Table 1).
The structural determination of the 3-alkyl-3,4,4-triaryl-3,4-
dihydroquinolines 13 was carried out following their analytical and
spectral data. In their 1H NMR spectra the C(2)H proton resonates
as a singlet at d = 8.21–8.68 ppm. Relevant 13C NMR data of
compounds 13 are the chemical shifts of the quaternary carbon
C3 at d = 48.0–52.0 ppm, the quaternary carbon C4 at d = 59.7–
62.9 ppm and the methine C2 at d = 166.2–169.4 ppm. Moreover,
the structures of compounds 13e (R1 = H; R2 = N(CH3)2; R3 = CH3)
and 13k (R1 = H; R2 = OCH3; R3 = CH3) were determined by X-
ray crystal diffraction (see Electronic supplementary information).
The main feature of these two structures is the relatively long bond
distance of the C3–C4 bond (C8–C9 bond in the crystallographic
The transformation 12 → 13 should proceed by a mechanism
involving as the first step the cleavage of the C–H bond of
the triarylmethane fragment via a 1,5 hydride shift to give
the transient ortho-azaxylylene 14 (Scheme 4). Subsequent 6p
electrocyclic ring closure should afford the sterically congested
3,4-dihydroquinolines 13. The denotation of the first mechanistic
step as a hydride shift relies on the known hydricity of the
triarylmethane C–H bond and the electrophilicity of the central
ketenimine carbon atom.
The presence of a methyl substituent at the ortho position
to the keteniminic nitrogen atom (R1 = CH3) has a significant
influence on the reactivity of the triarylmethane-ketenimines 12: in
general, compounds 12 with R1 = CH3 afforded the corresponding
3,4-dihydroquinolines 13 in the best chemical yields and in the
shortest reaction times. This enhancement of reactivity could be
rationalized by considering that the presence of this methyl group
shifts the conformational equilibrium of compounds 12 to the
˚
˚
study), 1.598 A in compound 13e and 1.604 A in 13k. These long
distances are most probably due to the steric repulsion between
the bulky substituents at the C3 and C4 carbon atoms.17
4692 | Org. Biomol. Chem., 2010, 8, 4690–4700
This journal is
The Royal Society of Chemistry 2010
©

View Article Online
Scheme 5 Reagents and conditions: (a) Ph2CCO, CH2Cl2, r.t., 30 min.
Scheme 4 Proposed mechanism for the conversion 12 → 13.
Caryl–N rotational isomer that locates the electrophilic central
carbon atom of the ketenimine function close to the hydrogen
atom that is going to translate in the 1,5 hydride migration.18 In
addition, we have observed that the presence of strong electron-
releasing groups (R2 = N(CH3)2, N-morpholino and OCH3) at the
para position of the two other aryl rings facilitates the conversion
ofketenimines 12 by shortening the reaction time. These groups are
known to increase the hydricity of the triarylmethane C–H bond.8
In most of the cases, placing an ethyl group instead of a methyl one
at the terminal carbon atom of the ketenimine function results in
a lower yield of the tandem process converting ketenimines 12 into
quinolines 13, a fact that could be attributed to the slightly higher
steric hindrance that makes more difficult the electrocyclization
step.
We next approached the study of similar tandem processes in
which the future C3–C4 bond of the putative final quinolines is
more sterically congested by the presence of four aryl substituents
at the extremes of that bond. To this end we prepared the C,C-
diphenyl ketenimines 15 (60–92%) by reacting iminophosphoranes
11a–d and 11f,g with diphenylketene, in dichloromethane solution
at room temperature (Scheme 5).
When toluene solutions of ketenimines 15 were heated at reflux
temperature for several days these heterocumulenes remained
unaltered. Under more severe thermal conditions (ortho-xylene,
180 ◦C, sealed tube) and longer reaction times we were able
to achieve the transformation of 15 into different heterocyclic
compounds depending on the nature of the substituents R1 and
R2.
Scheme 6 Reagents and conditions: (a) ortho-Xylene, 180 ◦C, sealed tube,
5–7 d.
Thus, the thermal treatment of C,C-diphenyl ketenimines 15a
(R1 = R2 = H) and 15f (R1 = H; R2 = OCH3) under such reaction
conditions (5–7 days) yielded 9,10-dihydroacridines 18 (Scheme 6).
The structural determination of compounds 18 was achieved
following their spectral data, and confirmed by X-ray diffraction
of a monocrystal of compound 18a (R1 = R2 = H) (see Electronic
supplementary information).
The formation of dihydroacridines 18 could be explained by
assuming the initial formation of the ortho-azaxylylene inter-
mediates 16 via a 1,5 hydride shift. The further cyclization
of these intermediates through a 6p electrocyclic ring closure
involving the N3–C4–C5–C6–C7–C8 1-azatriene fragment should
afford the final species 18. The alternative 6p-electrocyclization
of the ortho-azaxylylenes 16 that would involve the closure of the
This journal is
The Royal Society of Chemistry 2010
Org. Biomol. Chem., 2010, 8, 4690–4700 | 4693
©

View Article Online
C1–C2–N3–C4–C5–C6 3-azatriene system via C1–C6 bond for-
mation, that was initially expected and is similar to those presented
above, is probably made difficult by the presence of two phenyl
rings at each end (C1 and C6) of the azatrienic system, which
should result in a severe steric congestion at the transition state of
such an electrocyclization thus preventing its occurrence.19
In the cases of the C,C-diphenyl ketenimines 15c and 15d, where
the R2 substituent is a dimethylamino group, the reaction products
obtained from their thermal treatment were the 2-aryl substituted
3,4-dihydroquinolines 20 (Scheme 7).
(Scheme 8). The spectral data of quinolines 22 are fully consistent
with their proposed structures.
Scheme 8 Reagents and conditions: (a) ortho-Xylene, 180 ◦C, sealed tube,
14–21 d.
The conversions 15 → 22 should be initiated by a 1,5 migration
of one of the hydrogen atoms of the methyl group to give the
transient ortho-azaxylylenes 21, which undergo subsequent 6p-
electrocyclization to quinolines 22 (Scheme 8). Amongst the
various [1,5]-H shift/6p-ERC tandem processes that could un-
dergo ketenimines 15b and 15g the one leading to 22 became
the most viable reaction pathway, probably because it involves
a facile 6p-electrocyclization step via a non congested transition
state as the 3-azatriene intermediate experiencing the cyclization
is unsubstituted at one of its carbon termini. Although the
alternative [1,5]-H shift/6p-ERC sequences initiated by the [1,5]-
H transfer from the methine carbon atom of the triarylmethane
fragment should reasonably have a quite lower activation energy
for this first step, when compared with the [1,5]-H shift from the
methyl group, the further putative electrocyclizations must have
notably higher activation barriers, instead.
Scheme 7 Reagents and conditions: (a) ortho-Xylene, 180 ◦C, sealed tube,
1–2 d.
Quinolines 20 were characterized following their spectral data,
1
IR, H and 13C NMR and mass spectrometry, and additionally
by an X-ray crystal structure of example 20b (R1 = CH3) (see
Electronic supplementary information). In the solid state the C3–
C4 bond distance of compound 20b (C7–C8 in the crystallographic
3
3
˚
numbering) is also larger (1.588 A) than a standard Csp –Csp
bond, and slightly shorter than those shown by quinolines 13e
and 13k.
The formation of compounds 20 is apparently explained as
occurring by a tandem process in which the first step is the rare 1,5
migration of one electron-rich 4-dimethylaminophenyl group20 to
the central carbon atom of the ketenimine function leading to the
transient ortho-azaxylylene 19, followed by the habitual 6p-ERC
process.
C,C-Diphenyl ketenimines 15b (R1 = CH3; R2 = H) and 15g
(R1 = CH3; R2 = OCH3), both bearing a methyl group in the ortho
position to the keteniminic nitrogen atom (R1 = CH3), when heated
under the standard reaction conditions for extended periods of
time (14–21 days), converted into the new 4-unsubstituted 3,4-
dihydroquinolines 22a (R2 = H) and 22b (R2 = OCH3), respectively
The ability of triarylmethane fragments as hydride donors
participating in [1,5]-H shift/6p-ERC tandem processes involving
carbodiimide functions as the acceptor units was also evalu-
ated. The reaction of iminophosphoranes 11c,d,f,g with aryliso-
cyanates, in dichloromethane solution at room temperature, gave
triarylmethane-carbodiimides 23. The heating at 180 ◦C in a sealed
tube of ortho-xylene solutions of carbodiimides 23, during periods
of time ranging from 2 to 5 days, yielded the 3,4,4-triaryl-3,4-
dihydroquinazolines 25 (Scheme 9, Table 2).
The structural characterization of quinazolines 25 relies on their
spectroscopic data. Their IR spectra show an absorption band at
1606–1614 cm-1 attributed to the vibration of the N–C bond. In
4694 | Org. Biomol. Chem., 2010, 8, 4690–4700
This journal is
The Royal Society of Chemistry 2010
©

View Article Online
Table 2 Triarylmethane-carbodiimides 23 and quinazolines 25
23
Yield (%)
25
Yield (%)
entry
R1
R2
R3
a
b
c
d
e
H
N(CH3)2
N(CH3)2
N(CH3)2
OCH3
Br
Cl
CH3
Cl
Br
53
60
90
45
56
82
84
76
78
61
CH3
CH3
H
CH3
OCH3
Scheme 9 Reagents and conditions: (a) 4-R3-C6H4NCO, CH2Cl2, r.t.,
30 min. (b) ortho-Xylene, 180 ◦C, sealed tube, 2–5 d.
1
their H NMR spectra the signal of the C(2)H proton appears
as a singlet at d = 7.64–7.73 ppm. Their 13C NMR spectra show
the signals of the aliphatic quaternary carbon atom C4 and the
methine carbon C2 at d = 71.8–71.9 ppm and d = 146.3–147.4 ppm,
respectively.
Obviously, the formation of the dihydroquinazolines 25 from
carbodiimides 23 can be rationalized as resulting from a [1,5]-H
shift/6p-ERC sequence that involves the diazatriene intermediates
24.
It is worth noting that the thermal activation of triarylmethane-
carbodiimides 23 yields a single type of products, the quinazolines
25, in contrast with the variety of heterocyclic compounds formed
in the thermal cyclization of C,C-diphenyl ketenimines 15. This
difference is explained by the relatively easy electrocyclization of
the 1,3-diazatriene fragment of intermediates 24, which should
occur through a less sterically congested transition state than
the similar process of the C1–C2–N3–C4–C5–C6 3-azatriene
fragment in species 16. Moreover, it is conceivable that the lone
pair at the terminal nitrogen atom of the 1,3-diazatriene moiety of
24 may also assist the 6p-ERC step by conferring a certain degree
of pseudopericyclic character to its transition state.21
At this point, we were also interested in exploring whether
the C–H bond of a diarylmethane fragment would be similarly
activated as a formal hydride donor in related tandem processes.
With this aim and following the usual sequence of reactions, as
shown in Scheme 10, we prepared diarylmethane-ketenimines 28
Scheme 10 Reagents and conditions: (a) (1) NaNO2, H2SO4/H2O, 0 ◦C,
30 min (2) NaN3, H2SO4/H2O, r.t., 16 h. (b) PPh3, Et2O, r.t., 16 h. (c)
Ph2CCO, CH2Cl2, r.t., 30 min. (d) ortho-Xylene, reflux, 3 h. (e) PhR3CCO,
CH2Cl2, r.t., 30 min. (f) Toluene, 160 ◦C, sealed tube, 36 h.
and 33a,b, the first one bearing a methyl group at the benzylic
carbon atom, and the second one lacking this substituent.
The heating of ketenimine 28 in refluxing toluene provided the
substituted 2-aminostyrene 30, isolated as the unique reaction
product in 91% yield. In contrast, the thermal activation of
ketenimines 33 in toluene solution at 160 ◦C, in a sealed tube,
led to the quinolines 34 (Scheme 10). Quinoline 34b, in which
R3 is a methyl group was obtained as a pair of cis and trans
diastereoisomers in a ratio close to 1 : 1.5.
The conversion 28 → 30 probably proceeds by a tandem double
[1,5]-H shift, the first one being the usual benzylic hydride transfer
This journal is
The Royal Society of Chemistry 2010
Org. Biomol. Chem., 2010, 8, 4690–4700 | 4695
©

View Article Online
whereas the second one involves the migration of one of the methyl
protons towards the nitrogen atom of the intermediate ortho-
azaxylylene 29. This result demonstrated the ability of –(Ar)2C–H
hydrogen atoms to engage in intramolecular hydride shifts but
also reveals a limitation of the tandem [1,5]-H shift/6p-ERC
methodology here developed for the preparation of polynuclear
heterocycles: a primary (or secondary) alkyl group placed at the
benzylic carbon atom from which the hydride departs in the
first step interrupts the habitual tandem sequence at the 6p-ERC
stage, as their H atoms become involved in a competitive, and
clearly more successful, [1,5]-H shift for the final recovery of
the aromaticity at the benzene nucleus of the ortho-azaxylylene.
Notwithstanding, in the absence of such an alkyl group the
habitual tandem [1,5]-H shift/6p-ERC sequence is fully operative
when starting from diarylmethane hydride-donor fragments, as
the transformation of ketenimines 33 into quinolines 34 clearly
proves.
the chemical shifts for 13C are reported relative to the resonance
of CDCl3 d = 77.1 ppm. J values are given in Hz.
Preparation of the quinolines 13
To a solution of iminophosphorane 11 (1 mmol) in anhydrous
dichloromethane (15 mL) ethylphenylketene (0.15 g, 1 mmol) or
methylphenylketene (0.13 g, 1 mmol) in the same solvent (5 mL)
was added. The reaction mixture was stirred at room temperature
for 30 min. Then, the solvent was removed under reduced pressure
and the resulting crude material was chromatographed on a silica
gel column, using hexanes/diethyl ether (9 : 1, v/v) as eluent, to
give the corresponding ketenimine 12.
A solution of the ketenimine 12 (0.5 mmol) in anhydrous toluene
(20 mL) was heated at reflux temperature under an atmosphere
of nitrogen for 5–144 h. After cooling at room temperature, the
solvent was removed under reduced pressure and the resulting
material was purified by column chromatography on silica gel.
3-Methyl-3,4,4-triphenyl-3,4-dihydroquinoline 13a. Yield
=
0.11 g, 60%; mp 194–195 ◦C (colourless prisms from diethyl ether–
hexane); nmax(Nujol)/cm-1 1633; dH(300 MHz; CDCl3) 1.83 (3 H,
s), 6.61 (2 H, d, J 7.5), 6.90 (4 H, t, J 7.2), 6.98–7.06 (4 H, m), 7.10–
7.21 (6 H, m), 7.45 (1 H, dd, J 7.5 and 1.2), 7.55–7.58 (2 H, m), 8.21
(1 H, s); dC(75 MHz; CDCl3) 20.2, 48.5 (s), 61.4 (s), 126.2, 126.3,
126.6, 126.7, 127.2, 127.5, 127.6, 127.9, 128.4, 128.6, 129.2, 131.8,
135.1 (s), 138.2 (s), 140.8 (s), 141.7 (s), 146.1 (s), 167.8; HRMS
(ESI): Calcd for C28H24N [M + H]+ 374.1903, found 374.1907.
Conclusions
We here disclosed a novel synthetic protocol for convert-
ing C-alkyl-C-phenyl ketenimines N-substituted by triaryl-
methane substructures into a variety of 3,3,4,4-tetrasubstituted-
3,4-dihydroquinolines, which is also valid for transforming
structurally related carbodiimides into 3,4,4-trisubstituted-3,4-
dihydroquinazolines. This protocol is based on the hydride donor
ability of the triarylmethane C–H fragment which, by thermal
activation in solution, promotes a tandem [1,5]-H shift/6p-ERC
sequence leading from the heterocumulenes to the heterocyclic
products. In the first step, a hydrogen atom translates from the ben-
zylic position to the central carbon atom of the heterocumulenic
function. The last mechanistic step, the 6p electrocyclization, is
also noteworthy as it implies the formation of sterically congested
C–C or C–N bonds. Only when the new putative C–C bond would
be tetraaryl substituted the usual 6p-ERC is prevented and the tan-
dem sequence continues with a second 6p-electrocyclization mode
leading to 9,10-dihydroacridines. Alternatively, the full sequence
either becomes a new tandem process of the type [1,5]-Ar shift/6p-
ERC which leads to 2-aryl substituted 3,4-dihydroquinolines
initiated by the rare migration of an electron-rich aryl group, or
is replaced by another [1,5]-H shift/6p-ERC process involving the
initial migration of a hydrogen atom from the methyl group ortho
to the N atom of the ketenimine. We have also demonstrated
the ability of diarylmethane fragments for promoting similar
transformations and the interruption of the final electrocyclization
in the cases where a methyl group is placed at the benzylic H-donor
carbon atom. New investigations are currently underway in our
laboratories aimed to extend the applicability of this synthetic
methodology by replacing the acceptor heterocumulene units by
other functions bearing electrophilic carbon atoms.
3-Ethyl-3,4,4-triphenyl-3,4-dihydroquinoline 13b. Yield
=
0.03 g, 15%; yellow oil; nmax(neat)/cm-1 1625; dH(300 MHz;
CDCl3) 0.80 (3 H, t, J 7.2), 2.00–2.10 (1 H, m), 2.53–2.62 (1 H,
m), 6.61 (2 H, d, J 7.6), 6.93–7.25 (14 H, m), 7.41–7.46 (3 H,
m), 8.63 (1 H, s); dC(75 MHz; CDCl3) 8.8, 24.8, 51.6 (s), 62.6 (s),
126.5, 126.6, 126.7, 126.8, 127.2, 127.3, 127.4, 127.8, 128.3, 129.0,
130.4, 130.5, 131.7, 135.2 (s), 135.3 (s), 141.8 (s), 142.3 (s), 144.0
(s), 167.7; HRMS (ESI): Calcd for C29H26N [M + H]+ 388.2060,
found 388.2066.
3,8-Dimethyl-3,4,4-triphenyl-3,4-dihydroquinoline 13c. Yield =
0.19 g, 99%; mp 195–196 ◦C (colourless prisms from diethyl ether);
n
max(Nujol)/cm-1 1633; dH(400 MHz; CDCl3) 1.81 (3 H, s), 2.56 (3
H, s), 6.65 (2 H, d, J 10.0), 6.86–7.04 (10 H, m), 7.09–7.24 (4 H, m),
7.54–7.57 (2 H, m), 8.25 (1 H, s); dC(100 MHz; CDCl3) 18.5, 20.6,
48.0 (s), 61.8 (s), 126.3, 126.6, 126.7, 127.1, 127.4, 127.5, 128.0,
128.8, 128.9, 132.1, 135.0 (s), 135.9 (s), 138.5 (s), 140.1 (s), 141.4
(s), 146.2 (s), 167.4; HRMS (ESI): Calcd for C29H26N [M + H]+
388.2060, found 388.2066.
3-Ethyl-8-methyl-3,4,4-triphenyl-3,4-dihydroquinoline
13d.
Yield = 0.12 g, 61%; mp 145–147 ◦C (colourless prisms from
diethyl ether); nmax(Nujol)/cm-1 1628; dH(400 MHz; CDCl3) 0.78
(3 H, t, J 9.6), 1.96–2.08 (1 H, m), 2.54 (3 H, s), 2.58–2.63 (1 H,
m), 6.57 (2 H, d, J 9.0), 6.91–7.20 (14 H, m), 7.43 (2 H, d, J 9.0),
8.68 (1 H, s); dC(100 MHz; CDCl3) 8.80, 18.4, 24.9, 50.9 (s), 62.9
(s), 126.4, 126.5, 126.7, 126.8, 126.9, 127.1, 127.2, 127.3, 129.0,
130.4, 130.6, 131.9, 134.9 (s), 135.5 (s), 140.6 (s), 142.0 (s), 144.2
(s), 166.2; HRMS (ESI): Calcd for C30H28N [M + H]+ 402.2216,
found 402.2219.
Experimental
All melting points are uncorrected. Infrared (IR) spectra were
recorded neat or as Nujol emulsions. 1H NMR spectra were
recorded in CDCl3 at 300 or 400 MHz. 13C NMR spectra were
recorded in CDCl3 at 75 or 100 MHz. The chemical shifts are
expressed in ppm, relative to Me4Si at d = 0.00 ppm for 1H, while
4,4-Bis[4-(N,N-dimethylamino)phenyl]-3-methyl-3-phenyl-3,4-
dihydroquinoline 13e. Yield = 0.14 g, 63%; mp 190–191 ◦C
(colourless prisms from diethyl ether); nmax(Nujol)/cm-1 1613;
4696 | Org. Biomol. Chem., 2010, 8, 4690–4700
This journal is The Royal Society of Chemistry 2010
©

View Article Online
dH(300 MHz; CDCl3) 1.77 (3 H, s), 2.90 (12 H, s), 6.37 (2 H,
d, J 9.0), 6.50 (2 H, d, J 9.0), 6.65 (2 H, d, J 8.1), 6.72 (2 H, d,
J 8.1), 6.93 (2 H, t, J 7.5), 6.99–7.04 (2 H, m), 7.10–7.20 (2 H,
m), 7.37–7.42 (3 H, m), 8.23 (1 H, s); dC(75 MHz; CDCl3) 20.6,
40.3, 40.7, 49.0 (s), 59.9 (s), 110.5, 111.2, 126.4, 126.6, 127.3, 127.8,
128.2, 129.0, 129.8, 132.6, 134.0 (s), 136.6 (s), 139.1 (s), 141.8 (s),
148.8 (s), 148.9 (s), 169.4 (s); HRMS (ESI): Calcd for C32H34N3
[M + H]+ 460.2747, found 460.2751.
from diethyl ether); nmax(Nujol)/cm-1 1713 and 1608; dH(400 MHz;
CDCl3) 0.76 (3 H, t, J 6.8), 1.94–2.04 (1 H, m), 2.45–2.52 (1 H,
m), 3.07–3.16 (8 H, m), 3.81–3.86 (8 H, m), 6.54 (2 H, d, J 8.8),
6.67–6.72 (4 H, m), 6.80 (2 H, d, J 7.6), 7.00–7.11 (4 H, m), 7.15–
7.21 (2 H, m), 7.29 (2 H, d, J 8.8), 7.41 (1 H, d, J 7.6), 8.60 (1
H, s); dC(100 MHz; CDCl3) 8.4, 22.0, 48.3, 48.5, 51.4 (s), 60.6 (s),
66.4, 66.5, 112.7, 113.1, 126.1, 126.5, 126.8, 127.2, 127.6, 128.4,
130.1, 131.2, 131.8, 132.7 (s), 134.1 (s), 135.4 (s), 135.7 (s), 141.8
(s), 148.9 (s), 149.0 (s), 167.7; HRMS (ESI): Calcd for C37H40N3O2
[M + H]+ 558.3115, found 558.3122.
4,4-Bis[4-(N ,N -dimethylamino)phenyl]-3-ethyl-3-phenyl-
3,4-dihydroquinoline 13f. Yield = 0.19 g, 80%; mp 171–172 ◦C
(colourless prisms from diethyl ether); nmax(Nujol)/cm-1 1610;
dH(400 MHz; CDCl3) 0.74 (3 H, t, J 7.3), 1.98–2.07 (1 H, m),
2.45–2.54 (1 H, m), 2.85 (6 H, s), 2.91 (6 H, s), 6.37 (2 H, d, J
9.1), 6.53 (2 H, d, J 9.1), 6.70 (2 H, d, J 7.7), 6.76 (2 H, d, J 7.7),
7.00–7.09 (4 H, m), 7.14–7.20 (2 H, m), 7.24–7.26 (2 H, m), 7.40
(1 H, dd, J 7.6 and 1.3), 8.63 (1 H, s); dC(100 MHz; CDCl3) 9.0,
25.6, 40.4, 40.5, 52.0 (s), 60.9 (s), 110.5, 110.8, 126.4, 126.7, 127.1,
127.5, 128.0, 128.9, 129.9 (s), 130.6, 131.3 (s), 131.7, 132.2, 136.4
(s), 136.5 (s), 142.3 (s), 148.9 (s), 149.0 (s), 168.5; HRMS (ESI):
Calcd for C33H36N3 [M + H]+ 474.2904, found 474.2910.
4,4-Bis(4-methoxyphenyl)-3-methyl-3-phenyl-3,4-dihydroquino-
line 13k. Yield = 0.12 g, 57%; mp 168–170 ◦C (colourless prisms
from diethyl ether); nmax(Nujol)/cm-1 1606; dH(400 MHz; CDCl3)
1.78 (3 H, s), 3.72 (3 H, s), 3.75 (3 H, s), 6.54 (2 H, d, J 9.0), 6.62
(2 H, d, J 7.7), 6.67 (2 H, d, J 9.0), 6.78 (2 H, d, J 8.4), 6.93 (2
H, t, J 7.7), 7.01–7.06 (2 H, m), 7.15–7.19 (2 H, m), 7.43–7.46
(3 H, m), 8.21 (1 H, s); dC(100 MHz; CDCl3) 20.5, 48.8 (s), 55.2,
55.3, 60.2 (s), 111.8, 112.9, 126.7, 127.1, 127.6, 128.0, 128.5, 128.8,
128.9, 130.0, 132.9, 133.1 (s), 136.0 (s), 138.3 (s), 138.6 (s), 141.8
(s), 158.0 (s), 158.1 (s), 169.0; HRMS (ESI): Calcd for C30H28NO2
[M + H]+ 434.2115, found 434.2119.
4,4-Bis[4-(N ,N -dimethylamino)phenyl]-3,8-dimethyl-3-phe-
nyl-3,4-dihydroquinoline 13g. Yield = 0.18 g, 78%; mp 170–171 ◦C
(colourless prisms from diethyl ether); nmax(Nujol)/cm-1 1610;
dH(400 MHz; CDCl3) 1.75 (3 H, s), 2.54 (3 H, s), 2.87 (6 H,
s), 2.88 (6 H, s), 6.36 (2 H, d, J 8.8), 6.48–6.51 (2 H, m), 6.62 (2
H, d, J 7.6), 6.71 (2 H, d, J 8.8), 6.88–7.04 (6 H, m), 7.37–7.39 (2
H, m), 8.27 (1 H, s); dC(100 MHz; CDCl3) 18.5, 20.9, 40.4, 40.7,
48.3 (s), 60.3 (s), 110.5, 111.2, 126.4, 126.8, 127.1, 127.3, 128.4,
129.1, 129.6 (s), 130.1, 132.7, 134.0 (s), 135.4 (s), 136.4 (s), 139.3
(s), 140.1 (s), 148.8 (s), 148.9 (s), 167.9; HRMS (ESI): Calcd for
C33H36N3 [M + H]+ 474.2904, found 474.2909.
3-Ethyl-4, 4-bis(4-methoxyphenyl)-3-phenyl-3, 4-dihydro-
quinoline 13l. Yield = 0.12 g, 52%; mp 187–188 ◦C (colourless
prisms from diethyl ether); nmax(Nujol)/cm-1 1606; dH(400 MHz;
CDCl3) 0.77 (3 H, t, J 7.3), 1.96–2.05 (1 H, m), 2.45–2.55 (1 H,
m), 3.73 (3 H, s), 3.78 (3 H, s), 6.55 (2 H, d, J 9.1), 6.67 (2 H,
d, J 7.7), 6.71 (2 H, d, J 9.1), 6.82 (2 H, d, J 7.7), 7.01–7.05 (2
H, m), 7.07–7.10 (2 H, m), 7.15 (1 H, dd, J 7.8 and 1.3), 7.19 (1
H, td, J 7.4 and 1.4), 7.31 (2 H, d, J 8.5 Hz), 7.42 (1 H, dd, J
7.7 and 1.2), 8.63 (1 H, s); dC(100 MHz; CDCl3) 8.9, 25.4, 51.8
(s), 55.2, 55.3, 61.2 (s), 111.8, 112.3, 126.7, 127.1, 127.3, 127.8,
128.2, 128.8, 130.4, 131.8, 132.6, 134.0, 135.7 (s), 135.8 (s), 135.9
(s), 142.2 (s), 158.1 (s), 158.3 (s), 168.1; HRMS (ESI): Calcd for
C31H30NO2 [M + H]+ 448.2271, found 448.2273.
4,4-Bis[4-(N,N-dimethylamino)phenyl]-3-ethyl-8-methyl-3-phe-
nyl-3,4-dihydroquinoline 13h. Yield = 0.18 g, 75%; mp 165–167 ◦C
(colourless prisms from diethyl ether); nmax(Nujol)/cm-1 1610;
dH(300 MHz; CDCl3) 0.72 (3 H, t, J 7.2), 1.94–2.06 (1 H, m),
2.45–2.50 (1 H, m), 2.53 (3 H, s), 2.86 (6 H, s), 2.92 (6 H, s), 6.36
(2 H, d, J 9.0), 6.53 (2 H, d, J 9.0), 6.67 (2 H, d, J 8.1), 6.76 (2
H, d, J 8.1), 6.91–7.08 (6 H, m), 7.23–7.26 (2 H, m), 8.68 (1 H,
s); dC(75 MHz; CDCl3) 15.4, 18.4, 25.7, 40.5, 40.6, 51.4 (s), 61.2
(s), 110.4, 110.7, 126.3, 126.7, 126.9, 127.1, 128.4, 130.2 (s), 130.6,
131.7 (s), 131.8, 132.4, 135.1 (s), 136.2 (s), 136.8 (s), 140.6 (s), 148.9
(s), 149.0 (s), 167.0; HRMS (ESI): Calcd for C34H38N3 [M + H]+
488.3060, found 488.3065.
4,4-Bis(4-methoxyphenyl)-3,8-dimethyl-3-phenyl-3,4-dihydro-
quinoline 13m. Yield = 0.19 g, 85%; mp 158–160 ◦C (colourless
prisms from diethyl ether); nmax(Nujol)/cm-1 1629; dH(300 MHz;
CDCl3) 1.76 (3 H, s), 2.55 (3 H, s), 3.72 (3 H, s), 3.74 (3 H, s),
6.53 (2 H, d, J 8.4), 6.59 (2 H, d, J 7.8), 6.67 (2 H, d, J 8.7), 6.76
(2 H, d, J 7.8), 6.92–7.02 (6 H, m), 7.44 (2 H, d, J 8.4), 8.26 (1
H, s); dC(75 MHz; CDCl3) 18.5, 20.7, 48.1 (s), 55.1, 55.2, 60.5 (s),
111.6, 112.7, 126.6, 126.7, 127.3, 127.5, 128.8, 128.9, 130.1, 133.0,
133.5 (s), 135.7 (s), 135.8 (s), 138.2 (s), 138.7 (s), 140.0 (s), 157.9
(s), 158.0 (s), 167.6; HRMS (ESI): Calcd for C31H30NO2 [M + H]+
448.2271, found 448.2276.
3-Methyl-4,4-bis[4-(N-morpholino)phenyl]-3-phenyl-3,4-dihy-
droquinoline 13i. Yield = 0.18 g, 65%; mp 221–222 ◦C (orange
prisms from diethyl ether); nmax(Nujol)/cm-1 1608 and 1512;
dH(400 MHz; CDCl3) 1.79 (3 H, s), 3.06–3.09 (8 H, m), 3.79–
3.85 (8 H, m), 6.55 (2 H, d, J 8.4), 6.65–6.67 (4 H, m), 6.78 (2 H,
d, J 8.4), 6.93 (2 H, t, J 7.6), 7.03 (2 H, t, J 8.4), 7.17 (2 H, t, J
7.6), 7.45 (3 H, m), 8.23 (1 H, s); dC(100 MHz; CDCl3) 20.1, 48.2,
48.4 (s), 48.8, 59.7 (s), 66.4, 66.5, 112.9, 113.6, 126.2, 126.5, 127.0,
127.5, 127.9, 128.3 (s), 128.4, 128.5, 129.3, 131.8 (s), 132.2, 135.6
(s), 136.7 (s), 138.3 (s), 141.4 (s), 148.9 (s), 149.1 (s), 168.6; HRMS
(ESI): Calcd for C36H38N3O2 [M + H]+ 544.2959, found 544.2963.
3-Ethyl-4,4-bis(4-methoxyphenyl)-8-methyl-3-phenyl-3,4-dihy-
droquinoline 13n. Yield
n
= 0.23 g, 99%; colourless oil;
max(neat)/cm-1 1606; dH(400 MHz; CDCl3) 0.75 (3 H, t, J 7.2),
1.99 (1 H, m), 2.49 (1 H, m), 2.53 (3 H, s), 3.71 (3 H, s), 3.76 (3 H,
s), 6.54 (2 H, d, J 8.8), 6.64 (2 H, d, J 7.6), 6.70 (2 H, d, J 8.8), 6.81
(2 H, d, J 8.0), 6.95–7.09 (6 H, m), 7.31 (2 H, d, J 8.0), 8.67 (1 H, s);
dC(100 MHz; CDCl3) 8.9, 18.3, 25.5, 51.1 (s), 55.1, 55.2, 61.4 (s),
111.7, 112.2, 126.5, 126.6, 127.1, 127.3, 128.9, 130.4, 131.9, 132.7,
134.2 (s), 135.4 (s), 135.5 (s), 135.9 (s), 136.2 (s), 140.5 (s), 158.0
(s), 158.2 (s), 166.6; HRMS (ESI): Calcd for C32H32NO2 [M + H]+
462.2428, found 462.2436.
3-Ethyl-4,4-bis[4-(N-morpholino)phenyl]-3-phenyl-3,4-dihydro-
quinoline 13j. Yield = 0.17 g, 60%; mp 184–185 ◦C (orange prisms
This journal is
The Royal Society of Chemistry 2010
Org. Biomol. Chem., 2010, 8, 4690–4700 | 4697
©

View Article Online
Preparation of acridines 18, 2-arylquinolines 20 and quinolines 22
131.4, 134.8 (s), 142.3 (s), 142.4 (s), 142.5 (s), 149.5 (s), 150.5 (s),
168.6 (s); HRMS (ESI): Calcd for C38H38N3 [M + H]+ 536.3060,
found 536.3062.
To a solution of iminophosphorane 11 (1 mmol) in anhydrous
dichloromethane (15 mL) a solution of diphenylketene (0.19 g,
1 mmol) in the same solvent (5 mL) was added. The reaction
mixture was stirred at room temperature under nitrogen for 30 min.
Next the solvent was removed under reduced pressure and the
resulting material was purified by column chromatography on
silica gel, using hexanes/diethyl ether (9 : 1; v/v) as eluent, to
provide the expected ketenimine 15.
A deoxygenated solution of the ketenimine 15 (0.5 mmol) in
anhydrous ortho-xylene (20 mL) was heated in a sealed tube at
180 ◦C during 2–21 d. After cooling, the solvent was removed
under reduced pressure. The resulting material was purified by
column chromatography on silica gel.
3,3-Diphenyl-8-diphenylmethyl-3,4-dihydroquinoline
22a.
Yield = 0.11 g, 51%; mp 159–161 ◦C (colourless prisms
from diethyl ether); nmax(Nujol)/cm-1 1599, 1581 and 1493;
dH(400 MHz; CDCl3) 3.47 (2 H, s), 6.55 (1 H, s), 6.81 (1 H, d, J
7.6), 6.95 (1 H, d, J 7.6), 7.00–7.07 (9 H, m), 7.17–7.28 (12 H,
m), 8.22 (1 H, s); dC(100 MHz; CDCl3) 38.1, 48.7 (s), 50.4, 125.9,
126.1, 126.5 (s), 127.4, 127.7, 127.8, 128.1, 128.7, 129.8, 140.0
(s), 140.8 (s), 143.8 (s), 144.3 (s), 166.7; HRMS (ESI): Calcd for
C34H28N [M + H]+ 450.2216, found 450.2222.
8-Bis(4-methoxyphenyl)methyl-3,3-diphenyl-3,4-dihydroquino-
line 22b. Yield = 0.13 g, 50%; mp 147–149 ◦C (colourless prisms
from diethyl ether–hexane); nmax(Nujol)/cm-1 1610; dH(400 MHz;
CDCl3) 3.43 (2 H, s), 3.78 (6 H, s), 6.39 (1 H, s), 6.75–6.78 (4 H,
m), 6.90–6.95 (5 H, m), 6.99 (1 H, d, J 7.6), 7.01–7.04 (5 H, m),
7.16–7.24 (6 H, m), 8.21 (1 H, s); dC(100 MHz; CDCl3) 38.1, 48.6
(s), 48.7, 55.3, 113.5, 125.9, 126.4 (s), 126.7, 127.3, 127.8, 128.4,
128.5, 130.6, 136.9 (s), 140.6 (s), 140.7 (s), 143.8 (s), 157.7 (s),
166.6; HRMS (ESI): Calcd for C36H32NO2 [M + H]+ 510.2428,
found 510.2433.
10-(2,2-Diphenylethenyl)-9-phenyl-9,10-dihydroacridine
18a.
Yield = 0.21 g, 95%; mp 191–192 ◦C (colourless prisms from
diethyl ether); nmax(Nujol)/cm-1 1589; dH(300 MHz; CDCl3) 5.22
(1 H, s), 6.61 (1 H, s), 6.83–6.88 (4 H, m), 7.07–7.21 (14 H, m),
7.37–7.41 (3 H, m), 7.48–7.51 (2 H, m); dC(75 MHz; CDCl3) 47.9,
114.0, 121.5, 124.0, 124.7 (s), 126.1, 127.2, 127.5, 127.8, 128.2,
128.3, 128.5, 128.6, 129.0, 129.6, 137.7 (s), 140.1 (s), 145.5 (s),
147.3 (s); HRMS (ESI): Calcd for C33H26N [M + H]+ 436.2060,
found 436.2064.
Preparation of quinazolines 25
10-(2,2-Diphenylethenyl)-3-methoxy-9-(4-methoxyphenyl)-9,10-
dihydroacridine 18b. Yield = 0.15 g, 61%; mp 150–151 ◦C
(colourless prisms from diethyl ether); nmax(Nujol)/cm-1 1595 and
1508; dH(400 MHz; CDCl3) 3.72 (3 H, s), 3.73 (3 H, s), 5.12 (1
H, s), 6.42 (1 H, dd, J 8.4 and 2.8), 6.59 (1 H, s), 6.62–6.65 (2 H,
m), 6.74 (1 H, d, J 2.4), 6.76–6.78 (2 H, m), 6.85 (1 H, m), 6.98 (1
H, d, J 8.0), 7.08 (2 H, d, J 7.2), 7.10–7.21 (6 H, m), 7.38–7.41 (3
H, m), 7.46–7.49 (2 H, m); dC(100 MHz; CDCl3) 46.3, 55.3, 55.4,
100.5, 106.4, 113.9, 114.0, 117.8, 121.5, 123.9, 125.4 (s), 127.0,
127.8, 128.1, 128.2, 128.4, 128.5, 128.6, 129.0, 129.5, 130.2, 137.8
(s), 139.9 (s), 140.0 (s), 140.3 (s), 141.0 (s), 145.7 (s), 157.8 (s),
158.9 (s); HRMS (ESI): Calcd for C35H30NO2 [M + H]+ 496.2271,
found 496.2275.
To a solution of iminophosphorane 11 (1 mmol) in anhydrous
dichloromethane (15 mL) a solution of the aryl isocyanate
(1 mmol) in the same solvent (5 mL) was added. The reaction
mixture was stirred at room temperature for 30 min. The solvent
was removed under reduced pressure, and the oily residue was
chromatographed on a silica gel column using hexanes/diethyl
ether (7 : 3, v/v) as eluent to give the corresponding carbodiimide
23.
A solution of the carbodiimide◦23 (0.5 mmol) in anhydrous o-
xylene (20 mL) was heated a 180 C, in a sealed tube, for 2–5 d.
After cooling at room temperature, the solvent was removed under
reduced pressure and the resulting material was purified by silica
gel column chromatography.
2,4-Bis[4-(N,N -dimethylamino)phenyl]-3,3-diphenyl-3,4-dihy-
droquinoline 20a. Yield
n
=
0.13 g, 50%; colourless oil;
3-(4-Bromophenyl)-4,4-bis[4-(N,N-dimethylamino)phenyl]-3,4-
dihydroquinazoline 25a. Yield = 0.22 g, 82%; mp 193–194 ◦C
(colourless prisms from diethyl ether–hexane); nmax(Nujol)/cm-1
1608; dH(400 MHz; CDCl3) 2.89 (12 H, s), 6.54–6.56 (4 H, m),
6.76–6.79 (3 H, m), 6.98–7.01 (1 H, m), 7.13–7.24 (8 H, m), 7.64 (1
H, s); dC(100 MHz; CDCl3) 40.3, 71.9 (s), 111.3, 118.5 (s), 124.1,
125.3, 127.1, 127.2, 127.6, 129.0, 130.4, 131.2, 133.0 (s), 140.7 (s),
142.0 (s), 147.4, 149.3 (s); HRMS (ESI): Calcd for C30H30BrN4
[M + H]+ 525.1648, found 525.1655.
max(neat)/cm-1 1608; dH(400 MHz; CDCl3) 2.82 (6 H, s), 2.89
(6 H, s), 4.56 (1 H, s), 6.31 (2 H, d, J 7.6), 6.40–6.43 (4 H, m), 6.57
(2 H, d, J 8.8), 6.79 (2 H, t, J 8.4), 6.93–6.98 (3 H, m), 7.08–7.18 (2
H, m), 7.21–7.23 (2 H, m), 7.31 (2 H, d, J 9.2), 7.40 (1 H, d, J 8.0),
7.63 (2 H, d, J 8.0); dC(100 MHz; CDCl3) 40.1, 40.6, 54.6, 59.0
(s), 110.5, 112.2, 125.6, 125.9, 126.5, 126.7, 126.9, 127.2, 127.3 (s),
127.6, 127.9, 128.8 (s), 129.7, 131.1, 131.2, 131.4, 131.8, 142.0 (s),
142.1 (s), 144.1 (s), 149.6 (s), 150.7 (s), 171.7 (s); HRMS (ESI):
Calcd for C37H36N3 [M + H]+ 522.2904, found 522.2908.
3-(4-Chlorophenyl)-4,4-bis[4-(N,N -dimethylamino)phenyl]-8-
methyl-3,4-dihydroquinazoline 25b. Yield = 0.21 g, 84%; mp 230–
231 ◦C (colourless prisms from diethyl ether); nmax(Nujol)/cm-1
1614 and 1574; dH(300 MHz; CDCl3) 2.41 (3 H, s), 2.90 (12 H, s),
6.52–6.57 (4 H, m), 6.61 (1 H, d, J 7.8), 6.83–6.87 (2 H, m), 6.91 (1
H, d, J 7.8), 7.00–7.04 (3 H, m), 7.11–7.16 (4 H, m), 7.70 (1 H, s);
dC(75 MHz; CDCl3) 17.8, 40.4, 71.9 (s), 111.3, 124.7, 125.1, 126.6,
128.3, 129.0, 129.2 (s), 130.3 (s), 130.4, 131.9 (s), 133.0 (s), 139.1
(s), 141.7 (s), 146.5, 149.3 (s); HRMS (ESI): Calcd for C31H32ClN4
[M + H]+ 495.2310, found 495.2317.
2,4-Bis[4-(N,N-dimethylamino)phenyl]-8-methyl-3,3-diphenyl-
3,4-dihydroquinoline 20b. Yield = 0.25 g, 93%; mp 234–235 ◦C
(colourless prisms from diethyl ether); nmax(Nujol)/cm-1 1610 and
1519; dH(300 MHz; CDCl3) 2.46 (3 H, s), 2.80 (6 H, s), 2.89 (6 H,
s), 4.51 (1 H, s), 6.32 (2 H, d, J 7.5), 6.41 (4 H, t, J 8.7), 6.54 (2 H,
d, J 8.7), 6.76–6.84 (4 H, m), 6.93–6.98 (2 H, m), 7.11–7.23 (3 H,
m), 7.40 (2 H, d, J 9.0), 7.61 (2 H, d, J 7.5); dC(75 MHz; CDCl3)
17.9, 40.2, 40.7, 55.0, 58.4 (s), 110.3, 112.3, 125.1, 125.6, 125.9,
126.2, 126.8, 127.7 (s), 127.8, 128.5, 129.2 (s), 129.7, 131.1, 131.3,
4698 | Org. Biomol. Chem., 2010, 8, 4690–4700
This journal is
The Royal Society of Chemistry 2010
©

View Article Online
4,4-Bis[4-(N,N -dimethylamino)phenyl]-8-methyl-3-(4-methyl-
phenyl)-3,4-dihydroquinazoline 25c. Yield = 0.18 g, 76%; mp 255–
257 ◦C (yellow prisms from diethyl ether); nmax(Nujol)/cm-1 1606;
dH(400 MHz; CDCl3) 2.19 (3 H, s), 2.42 (3 H, s), 2.88 (12 H, s),
6.53–6.56 (4 H, m), 6.59 (1 H, d, J 7.6), 6.79–6.81 (2 H, m), 6.85–
6.89 (3 H, m), 7.01 (1 H, d, J 7.6), 7.14–7.18 (4 H, m), 7.73 (1
H, s); dC(75 MHz; CDCl3) 17.9, 20.8, 40.5, 71.8 (s), 111.3, 124.3,
125.1, 125.5, 128.8, 128.9, 129.9 (s), 130.6, 131.7 (s), 133.0 (s),
134.4 (s), 139.5 (s), 140.6 (s), 147.3, 149.2 (s); HRMS (ESI): Calcd
for C32H35N4 [M + H]+ 475.2856, found 475.2860.
Preparation of quinolines 34
To solution of 1-phenylmethyl-2-triphenylphosphoranyl-
a
ideneaminobenzene 32 (0.5 g, 1.1 mmol) in anhydrous
dichloromethane (15 mL) a solution of diphenylketene (1.1 mmol)
or methylphenylketene (1.1 mmol) in the same solvent (5 mL)
was added. The reaction mixture was stirred at room temperature
under nitrogen for 30 min. Next the solvent was removed under
reduced pressure and the resulting material was purified by
column chromatography on silica gel, using hexanes/diethyl ether
(9.5 : 0.5; v/v) as eluent, to provide ketenimine 33a (91% yield) or
ketenimine 33b (70% yield), respectively.
A deoxygenated solution of ketenimine 33 (0.5 mmol) in
anhydrous toluene (20 mL) was heated in a sealed tube at 160 ◦C
during 36 h. After cooling, the solvent was removed under
reduced pressure. The resulting material was purified by column
chromatography on silica gel, using hexanes/diethyl ether (4 : 1)
as eluent.
3-(4-Chlorophenyl)-4,4-bis(4-methoxyphenyl)-3,4-dihydroquin-
azoline 25d. Yield = 0.18 g, 78%; mp 96–97 ◦C (colourless prisms
from diethyl ether–hexane); nmax(Nujol)/cm-1 1608; dH(300 MHz;
CDCl3) 3.71 (6 H, s), 6.72–6.82 (7 H, m), 7.01–7.06 (3 H, m),
7.19–7.26 (6 H, m), 7.66 (1 H, s); dC(75 MHz; CDCl3) 55.3,
71.9 (s), 113.2, 124.4, 125.7, 126.9, 127.0, 128.1, 128.5, 130.8,
131.1 (s), 132.3 (s), 133.4 (s), 140.6 (s), 141.1 (s), 147.4, 158.8
(s); HRMS (ESI): Calcd for C28H24ClN2O2 [M + H]+ 455.1521,
found 455.1527.
3,3,4-Triphenyl-3,4-dihydroquinoline 34a. Yield
97%; mp 192–194 ◦C (colourless prisms from diethyl ether);
max(Nujol)/cm-1 1614; dH(400 MHz; CDCl3) 4.75 (1 H, s), 6.56 (2
= 0.17 g,
n
3-(4-Bromophenyl)-4,4-bis(4-methoxyphenyl)-8-methyl-3,4-di-
hydroquinazoline 25e. Yield = 0.16 g, 61%; mp 83–85 ◦C
(colourless prisms from diethyl ether); nmax(Nujol)/cm-1 1608;
dH(400 MHz; CDCl3) 2.42 (3 H, s), 3.76 (6 H, s), 6.55 (1 H, d,
J 7.6), 6.73–6.77 (6 H, m), 6.93 (1 H, t, J 7.6), 7.06 (1 H, d, J 7.6),
7.17–7.22 (6 H, m), 7.70 (1 H, s); dC(100 MHz; CDCl3) 15.3, 55.3,
71.9 (s), 113.2, 118.6 (s), 124.9, 125.1, 126.8, 129.5, 130.8, 131.4,
132.3 (s), 132.4 (s), 133.4 (s), 139.1 (s), 141.8 (s), 146.3, 158.8 (s);
HRMS (ESI): Calcd for C29H26BrN2O2 [M + H]+ 513.1172, found
513.1174.
H, d, J 7.2), 6.76–6.78 (2 H, m), 6.92–7.00 (3 H, m), 7.03–7.06 (3
H, m), 7.21–7.23 (4 H, m), 7.25–7.29 (2 H, m), 7.30–7.34 (2 H, m),
7.45 (1 H, d, J 7.2), 8.54 (1 H, s); dC(100 MHz; CDCl3) 50.8, 54.2
(s), 126.3, 126.4, 127.0, 127.6, 127.7, 127.8, 127.9, 128.2, 128.4,
128.5, 128.9, 129.1, 130.1 (s), 139.6 (s), 141.8 (s), 143.1 (s), 144.3
(s), 165.5; HRMS (ESI): Calcd for C27H22N [M + H]+ 360.1747,
found 360.1751.
3-Methyl-3,4-diphenyl-3,4-dihydroquinoline 34b. Yield
=
0.10 g, 70%; colourless oil; nmax(neat)/cm-1 1622; dH(300 MHz;
CDCl3) 1.30 (3 H, s, minor isomer), 1.62 (3 H, s, major isomer),
4.02 (1 H, s, major isomer), 4.28 (1 H, s, minor isomer), 6.47–6.49
(3 H, m), 6.89–7.36 (23 H, m), 7.49 (1 H, d, J 7.5, minor isomer),
7.56 (1 H, d, J 7.8, major isomer), 7.90 (1 H, s, minor isomer),
8.05 (1 H, s, major isomer); dC(75 MHz; CDCl3) 20.5 (minor
isomer), 23.2 (major isomer), 45.4 (s, minor isomer), 46.3 (s,
major isomer), 54.0 (minor isomer), 54.7 (major isomer), 126.6,
126.9, 127.0, 127.1, 127.3, 127.4, 127.6, 127.7, 127.89, 127.94,
128.0, 128.1, 128.4, 128.5, 128.8, 129.6, 129.8, 138.5 (s), 139.1 (s),
140.7 (s), 142.1 (s), 142.2 (s), 144.1 (s), 168.8; HRMS (ESI): Calcd
for C22H20N [M + H]+ 298.1590, found 298.1594.
Preparation of 4-chloro-2-[1-(4-methoxyphenyl)vinyl]-N-(2,2-
diphenylvinyl)aniline 30
To a solution of iminophosphorane 27 (0.8 g, 1.5 mmol) in
anhydrous dichloromethane (15 mL) a solution of diphenylketene
(0.24 g, 1.5 mmol) in the same solvent (5 mL) was added. The
reaction mixture was stirred at room temperature under nitrogen
for 30 min. Next the solvent was removed under reduced pressure
and the resulting material was purified by column chromatography
on silica gel, using hexanes/diethyl ether (9.5 : 0.5; v/v) as eluent,
to provide the ketenimine 28 (81% yield).
A solution of the ketenimine 28 (0.33 g, 0.75 mmol) in anhydrous
ortho-xylene (20 mL) was heated at reflux temperature under
nitrogen for 3 h. After cooling, the solvent was removed under
reduced pressure and the resulting material was purified by column
chromatography on silica gel, using hexanes/diethyl ether (9 : 1,
v/v) as eluent.
Acknowledgements
This work was supported by the Ministerio de Ciencia e Inno-
vacion of Spain (Project CTQ2008-05827/BQU) and Fundacion
Seneca-CARM (Project 08661/PI/08). B. B. and M.-M. O. thank
Fundacion Seneca-CARM and Fundacion CajaMurcia for their
respective fellowships.
4-Chloro-2-[1-(4-methoxyphenyl)vinyl]-N -(2,2-diphenylvinyl)-
aniline 30. Yield = 0.30 g, 91%; orange oil; nmax(neat)/cm-1 1659;
dH(300 MHz; CDCl3) 3.79 (3 H, s), 5.07 (1 H, d, J 0.9), 5.53 (1
H, d, J 0.9), 6.10 (1 H, d, J 12.0), 6.71–6.75 (2 H, m), 6.88–6.96
(3 H, m), 6.99–7.04 (2 H, m), 7.08–7.13 (3 H, m), 7.18–7.25 (7 H,
m), 7.29–7.38 (1 H, m); dC(75 MHz; CDCl3) 55.3, 112.7, 114.1,
115.4, 118.4 (s), 123.9, 124.0 (s), 125.6, 126.2, 126.8, 127.6, 128.3,
128.8, 129.0, 129.8, 130.5, 130.7 (s), 137.9 (s), 138.7 (s), 141.6 (s),
144.5 (s), 159.8 (s); HRMS (ESI): Calcd for C29H25ClNO [M + H]+
438.1619, found 438.1619.
Notes and references
1 (a) K. Godula and D. Sames, Science, 2006, 312, 67–72; (b) For a series
of recent monographic reviews, see: Chem. Rev., 2010, Issue 2.
2 (a) I. D. Junberg, Y. Odabachian and F. Gagosz, J. Am. Chem. Soc.,
2010, 132, 3543–3552; (b) P. A. Vadola and D. Sames, J. Am. Chem.
Soc., 2009, 131, 16525–16528; (c) K. M. McQuaid, J. Z. Long and
D. Sames, Org. Lett., 2009, 11, 2972–2975; (d) K. M. McQuaid and
D. Sames, J. Am. Chem. Soc., 2009, 131, 402–403; (e) S. J. Pastine
and D. Sames, Org. Lett., 2005, 7, 5429–5431; (f) S. J. Pastine, K. M.
This journal is
The Royal Society of Chemistry 2010
Org. Biomol. Chem., 2010, 8, 4690–4700 | 4699
©

View Article Online
McQuaid and D. Sames, J. Am. Chem. Soc., 2005, 127, 12180–12181.
No substrate possessing a heterocumulenic functionality as hydride
acceptor was involved in these reports.
3 (a) S. Murarka, I. Deb, C. Zhang and D. Seidel, J. Am. Chem.
Soc., 2009, 131, 13226–13227; (b) C. Zhang, S. Murarka, S. and D.
Seidel, J. Org. Chem., 2009, 74, 419–422; (c) S. Murarka, C. Zhang,
M. D. Konieczynska and D. Seidel, Org. Lett., 2009, 11, 129–132, and
references cited in these reports.
11 P. Brunet and J. D. Wuest, J. Org. Chem., 1996, 61, 2020–2026 and
references cited therein.
12 H. E. Ungnade and E. W. Crandall, J. Am. Chem. Soc., 1949, 71, 2209–
2210.
13 K. Fukui, Y. Inamoto, H. Gitano and C. Nagata, J. Am. Chem. Soc.,
1959, 81, 5954–5957.
14 X. Wang, Y. Wang, D.-M. Du and J. Xu, J. Mol. Catal. A: Chem., 2006,
255, 31–35.
4 M. Alajarin, B. Bonillo, M.-M. Ortin, P. Sanchez-Andrada and A.
Vidal, Org. Lett., 2006, 8, 5645–5648.
15 R. N. Carde, G. Jones, W. H. McKinley and C. Price, J. Chem. Soc.,
Perkin Trans. 1, 1978, 1211–1218.
5 (a) M. Alajarin, B. Bonillo, P. Sanchez-Andrada, A. Vidal and D.
Bautista, Org. Lett., 2009, 11, 1365–1368; (b) M. Alajarin, B. Bonillo,
P. Sanchez-Andrada and A. Vidal, J. Org. Chem., 2010, 75, 3737–3750.
6 (a) C. Creutz and M. H. Chou, J. Am. Chem. Soc., 2009, 131, 2794–
2795; (b) M. R. Nimlos, C. H. Chang, C. J. Curtis, A. Miedaner, H. M.
Pilath and D. L. DuBois, Organometallics, 2008, 27, 2715–2722; (c) X.-
J. Qi, Y. Fu, L. Liu and Q.-X. Guo, Organometallics, 2007, 26, 4197–
4203; (d) W. W. Ellis, A. Miedaner, C. J. Curtis, D. H. Gibson and D. L.
DuBois, J. Am. Chem. Soc., 2002, 124, 1926–1932; (e) T.-Y. Cheng, B. S.
Brunschwig and R. M. Bullock, J. Am. Chem. Soc., 1998, 120, 13121–
13137; (f) T.-Y. Cheng and R. M. Bullock, Organometallics, 1995, 14,
4031–4033.
7 (a) D. Richter and H. Mayr, Angew. Chem., Int. Ed., 2009, 48, 1958–
1961; (b) R. Lavilla, Curr. Org. Chem., 2004, 8, 715–737; (c) X.-Q. Zhu,
H.-R. Li, Q. Li, T. Ai, J.-Y. Lu, Y. Yang and J.-P. Cheng, Chem.–Eur. J.,
2003, 9, 871–880; (d) S. Fukuzumi, Q. Inada and T. Suenobu, J. Am.
Chem. Soc., 2003, 125, 4808–4816; (e) X.-Q. Zhu, Y.-C. Liu and J.-P.
Cheng, J. Org. Chem., 1999, 64, 8980–8981.
8 (a) W. W. Ellis, J. W. Raebiger, C. J. Curtis, J. Bruno and D. L. DuBois,
J. Am. Chem. Soc., 2004, 126, 2738–2743; (b) W. W. Ellis, R. Ciancanelli,
S. M. Miller, J. W. Raebiger, M. R. DuBois and D. L. DuBois, J. Am.
Chem. Soc., 2003, 125, 12230–12236; (c) E.-U. Wu¨rthwein, G. Lang,
L. H. Schappele and H. Mayr, J. Am. Chem. Soc., 2002, 124, 4084–
4092; (d) X.-M. Zhang, J. W. Bruno and E. Enyinnaya, J. Org. Chem.,
1998, 63, 4671–4678; (e) K. L. Handoo, J.-P. Cheng and V. D. Parker,
J. Am. Chem. Soc., 1993, 115, 5067–5072; (f) J.-P. Cheng, K. L. Handoo
and V. D. Parker, J. Am. Chem. Soc., 1993, 115, 2655–2660.
9 (a) D. E. Schwarz, T. M. Cameron, P. J. Hay, B. L. Scott, W. Tumas
and D. L. Thorn, Chem. Commun., 2005, 5919–5921; (b) I.-S. Lee and
E. H. Jeoung, J. Org. Chem., 1998, 63, 7275–7279; (c) F. Montgrain,
S. M. Ramos and J. D. Wuest, J. Org. Chem., 1988, 53, 1489–1492.
10 X.-M. Zhang, J. W. Bruno and E. Enyinnaya, J. Org. Chem., 1998, 63,
4671–4678.
16 All the heterocumulenes, ketenimines and carbodiimides synthesized
in this work were similarly treated due to their hydrolytic sensitivity.
3
3
˚
17 The standard Csp –Csp bond distance is 1.54 A, see: F. H. Allen, D. G.
Kennard, L. Watson, A. G. Barmmer, A. G. Orpen and R. Taylor,
J. Chem. Soc., Perkin Trans. 2, 1987, S1–S19.
18 For a related example of enhancement of reactivity by an ortho
substituent, see: K. Mori, T. Kawasaki, S. Sueoka and T. Akiyama,
Org. Lett., 2010, 12, 1732–1735.
19 The results of a preliminary DFT computation study of the transfor-
mation 15a → 18a are fully consistent with the reaction path outlined
in Scheme 6. The alternative C1–C6 electrocyclization of azaxylylene
16 into the corresponding 3,3,4,4-tetraphenyl-3,4-dihydroquinoline has
been also calculated and shown to be non competitive, on kinetic and
thermodynamic grounds, with the experimentally-observed channel
leading to 18.
20 For an example of a 1,3 migration of a phenyl group in ketenimines,
see: (a) C. O. Kappe, G. Kollenz, R. Leung-Toung and C. Wentrup,
J. Chem. Soc., Chem. Commun., 1992, 487–488; (b) For some examples
of reactions involving 1,5-aryl shifts, see: Z.-Y. Chen and M.-J. Wu,
Org. Lett., 2005, 7, 475–477; (c) B. Schulte, R. Froehlich and A. Studer,
Tetrahedron, 2008, 64, 11852–11859; (d) A. Studer, M. Bossart and T.
Vasella, Org. Lett., 2000, 2, 985–988; (e) P. J. Battye and D. W. Jones,
J. Chem. Soc., Chem. Commun., 1984, 990–992; (f) P. T. Lansbury and
R. L. Letsinger, J. Am. Chem. Soc., 1959, 81, 940–943.
21 (a) A. R. De Lera and F. P. Cossio, Angew. Chem., Int. Ed., 2002, 41,
1150–1152; (b) J. Rodriguez-Otero and E. M. Cabaleiro-Largo, Chem.–
Eur. J., 2003, 9, 1837–1843; (c) M. Alajarin, P. Sanchez-Andrada, F. P.
Cossio, A. Arrieta and B. Lecea, J. Org. Chem., 2001, 66, 8470–8377;
(d) M. Alajarin, P. Sanchez-Andrada, A. Vidal and F. Tovar, J. Org.
Chem., 2005, 70, 1340–1349; (e) M. Alajarin, A. Vidal, P. Sanchez-
Andrada, F. Tovar and G. Ochoa, Org. Lett., 2000, 2, 965–968; (f) S.
Sakai, Theor. Chem. Acc., 2008, 120, 177–183; (g) J. A. Duncan, D. E. G.
Calkins and M. Chavartha, J. Am. Chem. Soc., 2008, 130, 6740–6748.
4700 | Org. Biomol. Chem., 2010, 8, 4690–4700
This journal is
The Royal Society of Chemistry 2010
©