Novel extensions of the tert-amino effect: Formation of phenanthridines and diarene-fused azocines from ortho-ortho′-functionalized biaryls

Article abstract of DOI:10.1055/s-0028-1083537

Phenanthridines and azocines fused to two benzene rings or one benzene and one pyridazinone ring, which are otherwise difficult to access, were prepared via two new extensions of the tert-amino effect. The synthetic pathway includes three steps: i) Suzuki reaction of an ortho-functionalized phenylboronic acid with ortho-disubstituted benzenes or pyridazinones; ii) the Knoevenagel condensation reaction of the biaryl aldehydes formed with active methylene compounds to obtain vinyl derivatives or, through their cyclization, phenanthridines via a tert-amino effect; and iii) thermal isomerization of vinyl or phenanthridinium compounds to fused azocines via another type of tert-amino effect. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0028-1083537

2846
LETTER
Novel Extensions of the tert-Amino Effect: Formation of Phenanthridines and
Diarene-Fused Azocines from ortho-ortho′-Functionalized Biaryls
F
ormationof Phen
g
anthridines
a
n
ndDiarene-
F
e
usedAzoc
s
ines Polonka-Bálint,a Caterina Saraceno,a Krisztina Ludányi,b Attila Bényei,c Péter Mátyus*a
a
Department of Organic Chemistry, Semmelweis University, Hőgyes Endre u. 7., 1092 Budapest, Hungary
Fax +36(1)2170851; E-mail: peter.matyus@szerves.sote.hu
b
c
Department of Pharmacy, Semmelweis University, Hőgyes Endre u. 7., 1092 Budapest, Hungary
Institute of Physical Chemistry, University of Debrecen, Egyetem tér 1., 4010 Debrecen, Hungary
Received 27 June 2008
This work is kindly dedicated to Prof. László Tőke on the occasion of his 75th birthday.
attempted formylation of para-substituted tert-anilines
Abstract: Phenanthridines and azocines fused to two benzene rings
or one benzene and one pyridazinone ring, which are otherwise dif-
ficult to access, were prepared via two new extensions of the tert-
amino effect. The synthetic pathway includes three steps: i) Suzuki
with N-methylformanilide led to the formation of diazo-
cines via hydrogen migration, followed by C–C bond for-
mation between a-carbon of the tert-amino group and the
reaction of an ortho-functionalized phenylboronic acid with ortho- phenyl ring of the formanilide moiety.5
disubstituted benzenes or pyridazinones; ii) the Knoevenagel con-
As a continuation of our studies, we set out to extend the
densation reaction of the biaryl aldehydes formed with active meth-
ylene compounds to obtain vinyl derivatives or, through their
cyclization, phenanthridines via a tert-amino effect; and iii) thermal
isomerization of vinyl or phenanthridinium compounds to fused
azocines via another type of tert-amino effect.
scope of the tert-amino effect to biaryl, triaryl, and fused
systems containing the tert-amino and vinyl groups in the
ortho positions of aryl rings connected, directly or at-
tached to another aryl ring or in peri positions of a fused
ring system, respectively. The isomerization of such com-
pounds might provide facile routes to fused medium and
macrocyclic ring systems that are otherwise difficult to
access. This paper reports on our attempts to synthesize
2-(2-vinylphenyl)-tert-anilines and their pyridazinone an-
alogues and to isomerize them to biologically interesting
fused azocines with some structural resemblance to the
naturally occurring anticancer allocolchicine (1) and
allocolchicinoid derivatives 2–4 (Figure 1).6
Key words: tert-amino effect, dibenzazocines, pyridazinobenzazo-
cines, nonbonding interactions, Suzuki reaction
The term ‘tert-amino effect’ was originally coined by
Meth-Cohn and Suschitzky to describe the thermal ring-
closure reactions of tertiary anilines containing an unsat-
urated ortho substituent with at least one heteroatom to af-
ford fused aza-ring systems incorporating a tert-amino
nitrogen.1 Subsequently, six types of t-amino effect (or
synonymously tert-amino effect) have been distin-
guished, based on the ring size and the mode of its forma-
tion.2
OMe
OMe
OMe
OMe
MeO
MeO
O
MeX
MeO
During the 1980s, the Reinhoudt group studied the ther-
mal isomerization of tert-anilines with an o-dicyanovinyl
substituent, which furnished dicyanotetrahydroquinolines
via a type 2 tert-amino effect. This type of transformation
has received considerable attention, in consequence of the
predictable stereo- and regiochemical features of the
isomerization.3 In our ongoing studies, started in the early
1990s, we have extended such isomerizations to pyri-
dazine analogues of tert-anilines, and we were able to
N
N
HN
N
N
O
2
1
OMe
OMe
OMe
OMe
MeO
MeO
MeO
MeO
NH
N
O
recognize
some
interesting
structure–reactivity
relationships4 and overcome the limitations of the solvent-
phased reactions through applications of microwave and/
or solvent-free conditions.4e,f
4
3
Figure 1 Allocolchicine (1) and its derivatives 2–4 (X = O or S)
Surprisingly, possible extensions of the tert-amino effect
to suitably tailored polyaryl systems in order to obtain me-
dium-sized or large rings have hardly been explored. One
unexpected finding was made by Meth-Cohn, when the
It was considered that appropriately ortho,ortho′-func-
tionalized biphenyl or phenylpyridazine derivatives could
be synthesized in the most straightforward way via the
Knoevenagel condensation of biaryl carbaldehydes 7a–c
and 12a–c. For the preparation of the required aldehydes,
aryl–aryl bond formation between two ortho-substituted
SYNLETT 2008, No. 18, pp 2846–2850
1
4
.1
1
.2
0
0
8
Advanced online publication: 15.10.2008
DOI: 10.1055/s-0028-1083537; Art ID: G22808ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Formation of Phenanthridines and Diarene-Fused Azocines
2847
Table 1 Notation of Basic Functional Groups R for All Schemes
Br
Br
CHO
i
ii
R1
R1
R1
or
N
NH2
N
N
R2
R1
N
R2
R2
R2
a
b
c
R1,R2 = (CH2)3
R1,R2 = (CH2)4
R1 = H, R2 = Me
5
6a 80%
6b 81%
7a 90%
7b 68%
7c 67%
B(OH)2
arenes, one of which possessing a formyl and the other a
dialkylamino group, was foreseen. Accordingly, we took
two different types of Suzuki reactions into account for
this step, using either o-formylboronic acid or o-(dialkyl-
amino)boronic acid.7 The choice of the required reagents
was based on their availability and reactivity. Aldehydes
7a,b and 12a–c were prepared from commercially avail-
able o-formylphenylboronic acid with easily obtainable
o-haloarylamines 6a,b8 and 11a–c9 (Tables 1 and 2), re-
spectively, whereas aldehyde 7c was synthesized through
the known reaction of o-(dimethylamino)phenyl-
boronic acid with o-bromobenzaldehyde.10 Each Su-
zuki reaction could be carried out smoothly in the pres-
ence of tetrakis(triphenylphosphine)palladium as catalyst
(Schemes 1 and 2). In the pyridazinone series, chloropy-
ridazinones were conveniently used instead of the less
readily available and more expensive bromo derivatives,
since the chloro atom has been found to be sufficiently re-
active for Suzuki coupling of pyridazinones, due to the
electron-withdrawing properties of the pyridazinone
ring.11
iii
iv
R1 = H
R1 = H
R2 = Me
R1
N
R2
R1
N
R
2 = Me
R2
8
9 37%
Scheme 1 Reagents and conditions: i) Br(CH2)nBr (n = 4 for 7a,
n = 5 for 7b), i-Pr2EtN, toluene, 110 °C, 15 h; ii) o-formylphenyl-
boronic acid, Pd(PPh3)4, aq 2 N Na2CO3, DME, 110 °C, argon, 29 h;
iii) a) TMEDA, Et2O; b) n-BuLi, 8 h; c) B(OMe)3, –70 °C, argon, 24
h; d) H2O, 1 h; iv) o-bromobenzaldehyde, Pd(PPh3)4, aq 2 N Na2CO3,
DME, 110 °C, argon, 48 h.
R1
R2
CHO
Cl
Cl
N
R1
O
O
Cl
O
N
i
ii
N
R2
MeN
MeN
MeN
N
N
10
11a 77%
11b 77%
11c 77%
12a 65%
12b 61%
12c 68%
It is noteworthy that single-crystal X-ray diffraction of al-
dehyde 7b confirmed a short distance (2.835 Å)12 between
the nitrogen and carbonyl carbon, similarly as found for
aldehyde 7c (2.989 Å).10
Scheme 2 Reagents and conditions: i) base [pyrrolidine (for 11a),
piperidine (for 11b), N,N-dimethylamine–EtOH (for 11c)], EtOH–
H2O, 100 °C, 2 h; ii) o-formylphenylboronic acid, Pd(PPh3)4, aq 2 N
Na2CO3, DME, 110 °C, argon, 15 h.
We designed three series of vinyl derivatives, one series
with acyclic and two with cyclic vinyl substituents. First,
dicyanomethylene compounds were targeted as ‘gold
standard’.
Table 2 Notation of Basic Functional Groups A for All Schemes
A
A
A
A
–
A
A
CN
CN
CN
13a–c
17a–c
19a,c
23a
CN
O
O
O
14b
18a–c
20a
24a
15a,c
–O
O
O
O
O
NMe
NMe
NMe
NMe
16a–c
21a
O
O
–O
O
Synlett 2008, No. 18, 2846–2850 © Thieme Stuttgart · New York

2848
Á. Polonka-Bálint et al.
LETTER
The Knoevenagel condensations of aldehydes 7a–c and Supporting Information), indicating a nonbonding inter-
12a–c (Schemes 3– 5) with malononitrile took place action between the tertiary nitrogen and the a-vinylic car-
smoothly under mild conditions to afford the expected vi- bon.
nyl compounds 13a–c and 17a–c in good yields
(Schemes 3 and 5).
Next, two cyclic active methylene compounds, indane-
1,3-dione (ID) and N,N-dimethylbarbituric acid (DMB),
were reacted with biphenyl carbaldehydes to obtain vinyl
A
groups, in which the b-carbon was incorporated into a
A
ring. Such a structural motif was expected to lead to sig-
CHO
nificant acceleration of the isomerization reactions via a
R1
R1
type 2 tert-amino effect.4b–g The reactions of biphenyl car-
baldehydes 7a–c with ID and DMB in ethanol at room
temperature furnished the expected vinyl compound in
only one case: the reaction of piperidino derivative 7b
with ID gave 14b as an isolable product (Scheme 3); in all
the other cases, surprisingly, phenanthridinium com-
pounds 15a,c and 16a–c were isolated (Scheme 4).
N
N
R2
R2
7a–c
13a–c, 14b
7a
7b
7b
7c
13a 95%
13b 68%
14b 36%
13c 84%
The structures of 15 and 16 could be easily distinguished
from those of the vinyl compounds by NMR spectrosco-
py; in particular, the up-field shifts of the a-hydrogen and
a-carbon signals proved to be of diagnostic value (see
Supporting Information). The structure of 16c was con-
firmed by X-ray diffraction.12 The formation of this novel
type of compound can be explained by ring closure be-
tween the positively polarized carbon of the vinyl group
formed in the condensation reaction and the tert-amino ni-
trogen via a new type of tert-amino effect.
Scheme 3 Reagents and conditions: malononitrile (13a–c) or
1H-indene-1,3(2H)-dione (14b), EtOH, r.t., 24 h.
A
A
A
–
CHO
A
R1
+
N
R2
R1
R1
N
N
R2
R2
7a–c
15a,c, 16a–c
7a
15a 30%
16a 83%
16b 36%
15c 18%
16c 75%
The enhanced isomerization ability of indanedione and
barbituric acid derivatives versus malononitriles might be
related to stereoelectronic factors: the transition states of
15a,c and 16a–c are more favorable geometrically and
also because of the more efficient delocalization of the
negative charge developing at the vinyl carbon bearing the
two electron-withdrawing groups.
7a
7b
7c
7c
Scheme 4 Reagents and conditions: 1H-indene-1,3(2H)-dione
(15a,c) or N,N-dimethylbarbituric acid (16a–c), EtOH, r.t., 24 h.
A
In contrast with these findings, the reactions of pyridazi-
nylphenyl aldehydes 12a–c with ID at room temperature
in ethanol and in the presence of piperidine as catalyst,
gave vinyl compounds 18a–c (Scheme 5). This difference
in behavior could be well explained by the electron-with-
drawing property of the pyridazinone ring, which decreas-
es the nucleophilicity of the tert-amino nitrogen.
A
CHO
R1
R1
O
O
N
N
R2
R2
N
MeN
N
N
12a–c
17a–c, 18a–c
12a
12a
12b
12b
12c
12c
17a 87%
18a 63%
17b 84%
18b 65%
17c 96%
18c 63%
A search of the literature for similar phenomena revealed
that an attractive interaction and even bond formation be-
tween a dimethylamino group and an acceptor substituent
located in peri positions of disubstituted naphthalenes had
been identified in excellent studies by the Wallis group,
but no relationship to the tert-amino effect was indicat-
ed.13 With regard to these results together with the find-
ings of our own studies on series of naphthalenes and
triaryl systems,14 we now propose that the isomerizations
leading to phenanthridinium compounds 15a,c and 16a–c
and those of the peri-fused naphthalenes reported by the
Wallis group proceed via a novel tert-amino effect.
Scheme 5 Reagents and conditions: for 17: malononitrile, piperi-
dine, EtOH, r.t., 24 h; for 18: 1H-indene-1,3(2H)-dione–dioxane,
piperidine, EtOH, r.t., 48 h.
With a view to acquiring information on the geometric ar-
rangement of the functionalities located in the
ortho,ortho′positions, X-ray analysis was also carried out.
Suitable crystals for single-crystal X-ray diffraction were
obtained from 13a,12 and it was found that the pyrrolidino
and vinyl groups were located on the same side of the bi-
aryl system; moreover, the nitrogen was located at a rela-
tively short distance from the a-vinylic carbon (2.878 Å
for N1–C18; for the numbering of each compound, see
In the following series of experiments, biphenylvinyl and
phenanthridinium derivatives could be isomerized to
dibenzazocines via another novel type of tert-amino ef-
fect. When 13a,c, 15a and 16a were heated in dimethyl
sulfoxide solutions, 19a,c, 20a, and 21a, respectively,
Synlett 2008, No. 18, 2846–2850 © Thieme Stuttgart · New York

LETTER
Formation of Phenanthridines and Diarene-Fused Azocines
2849
were obtained (Scheme 6). These compounds could readi-
ly be distinguished from the starting materials via the
NMR data (see Supporting Information). The structure of
19a was also analyzed by X-ray diffraction.12 The phenan-
thridinium derivatives are presumably first transformed to
the vinyl compounds, which isomerize to the dibenzo-
fused system in two steps. In the rate-limiting first step, a
dipolar intermediate is formed via either antarafacial
[1,7]-hydrogen migration or, more probably, hydride an-
ion transfer, and in the next step the intermediate cyclizes
to the fused ring system (Scheme 7).
i
NC
N
CN
CN
CN
i
+
N
N
13a
19a 23%
22 35%
Scheme 8 Reagents and conditions: i) MW, neat, 300 mW, 100 °C,
argon, 30 min.
A
Importantly, 19a proved to be stable under the conditions
applied in the microwave reactor, suggesting that it is not
an intermediate in the transformation of 13a to 22.
A
A
A
R1
R1
Pyridazine analogues even behaved differently in their re-
actions in DMSO at 160 °C, the product type depending
on the substituents on the starting vinyl derivative. Com-
pounds 17a and 18a, containing a pyrrolidino group, gave
pyridazinazocines 23a (the structure of which was con-
firmed by X-ray diffraction)12 and 24a, whereas 17c and
18b,c containing a piperidino or dimethylamino substitu-
ent, afforded pyridazinoisoquinoline 25 (Scheme 9).
Interestingly, in the reaction of 17b, besides pyridazi-
noisoquinoline 25, benzophthalazinone 26 could be iso-
lated (Scheme 10).15
N
N
R2
R2
13a,c
19a,c, 20a, 21a
A
A
A
–
A
R1
+
N
R1
N
R2
R2
15a,16a
A
13a
13c
15a
16a
19a 8 h, 94%
A
R1
19c 72 h, 23%
20a 30 min, 98%
21a 30 min, 28%
A
A
R1
or
O
O
N
O
N
Scheme 6 Reagents and conditions: for 19a, 20a, 21a: DMSO,
110 °C, argon; for 19c: DMSO, 160 °C, argon.
N
R2
R2
MeN
MeN
MeN
N
N
N
17a,c, 18a–c
23a, 24a
25
A
A
–
A
A
17a
18a
17c
18b
18c
23a 24 h, 38%
24a 6 h, 67%
25 96 h, 39%
25 48 h, 33%
25 48 h, 34%
Hb
Hb
[1,7]-H
Ha
Ha
R1
R1
+
N
H-transfer
••
N
R2
R2
Scheme 9 Reagents and conditions: DMSO, 160 °C, argon.
A
–
Ha
A
NC
CN
A
Ha
Hb
A
R1
Hb
+
+
N
R1
O
O
N
O
N
R2
R2
CN
N
MeN
MeN
MeN
N
N
N
Scheme 7
17b
25 30%
26 14%
Scheme 10 Reagents and conditions: DMSO, 160 °C, argon, 48 h.
In an attempt to accelerate the reaction, microwave heat-
ing was applied. Surprisingly, irradiation of neat 13a per-
mitted the isolation of two different fused ring systems:
dibenzazocine 19a, which is the main product under tradi-
tional heating, and phenanthridine (22) in 23% and 35%
yields, respectively (Scheme 8).
In conclusion, this paper has described two new exten-
sions of the tert-amino effect. Isomerization of o-(o-vinyl-
aryl)aryl-tert-amines may lead through nitrogen–carbon
or carbon–carbon bond formation to novel types of
phenanthridines or diarene-fused azocines, respectively.
The latter transformation may proceed through [1,7]-hy-
Synlett 2008, No. 18, 2846–2850 © Thieme Stuttgart · New York

2850
Á. Polonka-Bálint et al.
LETTER
Halász-Dajka, B.; Vo-Thanh, G.; Dehaen, W.; Van der
drogen migration or hydride transfer. The X-ray analysis
demonstrated that the tert-amino and vinyl groups of o-(o-
vinylaryl)aryl-tert-amines are located on the same face of
the biaryl system, and their distances from each other in-
dicate a weak interaction between them via a tert-amino
effect.
Eycken, J.; Mátyus, P.; Loupy, A.; Van der Eycken, E.
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Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
Acknowledgment
This work was supported by grants from the Hungarian Scientific
Research Fund (OTKA; K73389) and Ministry of Health (ETT-
508/2006). The authors gratefully thank Dr. Tamás Gáti for the as-
signment of NMR spectra, Prof. Mátyás Czugler for diffraction
measurement of a 13a sample. Use of X-ray diffraction facilities at
Texas A and M University, College Station (Bruker APEX diffrac-
tometer), and at Chemical Research Center of Hungarian Academy
of Sciences, Budapest (Rigaku R-axis Rapid diffractometer) is also
acknowledged.
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(12) The X-ray data are available from the Cambridge
Crystallographic Data Centre CCDC under the numbers
692718–692722 for 7b, 13a, 16c, 19a, and 23a, respectively.
Copies of the data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: +44(1223)336033 or e-mail: deposit @ccdc.cam.ac.uk.
The details of the X-ray structures will be published
elsewhere.
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(14) Dunkel, P.; Földi, Á. A.; Mátyus, P. unpublished results.
(15) The experimental details are available in the Supporting
Information.
Synlett 2008, No. 18, 2846–2850 © Thieme Stuttgart · New York

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