Palladium-catalyzed reaction of aryl iodides with ortho-bromoanilines and norbornene/norbornadiene: Unexpected formation of dibenzoazepine derivatives

Article abstract of DOI:10.1002/anie.201104363

Expecting the unexpected: The title reaction leads to satisfactory yields of dihydrodibenzoazepines 1-a from norbornene. The dibenzoazepines 2 can also be accessed from compounds of type 1-b when norbornadiene is used as a reactant. Theoretical studies show that the reaction represents a chelation-driven deviation from the usual selectivity observed in the presence of ortho-substituents on the aryl iodide.

Full text of DOI:10.1002/anie.201104363

DOI: 10.1002/anie.201104363
Heterocycles
Palladium-Catalyzed Reaction of Aryl Iodides with ortho-
Bromoanilines and Norbornene/Norbornadiene: Unexpected
Formation of Dibenzoazepine Derivatives**
Nicola Della Ca’, Giovanni Maestri, Max Malacria, Etienne Derat,* and Marta Catellani*
Herein we report a new palladium-catalyzed reaction that
leads to dibenzoazepine derivatives when starting with aryl
iodides, ortho-bromoanilines, and either norbornene or
norbornadiene. The resulting products arise from a dramatic
deviation from the usually observed carbon–carbon bond
formation sequence.[1] According to our general methodology,
which allows the selective synthesis of unsymmetrical biaryl
units starting from ortho-substituted aryl iodides and aryl
bromides under the joint catalytic action of palladium and
Scheme 1. Palladium-catalyzed reactions of ortho-substituted aryl
norbornene,[2] the ortho substituent in the aryl iodide ensures
ꢀ
iodides and ortho-bromoanilines: norbornene (ZZ=CH2 CH2) or
Csp2 Csp2 rather than Csp2 Csp3 coupling.[3] In the accompany-
ing paper,[1] however, it has been shown that certain chelating
groups are able to offset the ortho effect and direct migration
of the palladium-bonded aryl towards the cycloaliphatic
carbon atom. In relation to this possibility we attempted to
use ortho-bromoaniline and found that the resulting product
indeed implied aryl–norbornyl or aryl–norbornadienyl cou-
pling. To our surprise, however, a subsequent cyclization led
to an unexpected nitrogen-containing seven-membered ring
(3; Scheme 1). The result is remarkable when compared to
our recent report on the formation of N-substituted carba-
zoles, which involves aryl–aryl coupling in accordance with
the ortho effect when using the same reagents having
acetylated or sulfonylated amino groups (Scheme 1, dashed
arrow).[4] As shown, the use of norbornene with 1 and 2 leads
to the dihydrodibenzoazepine derivatives 3, whereas with
=
ꢀ
ꢀ
norbornadiene (ZZ=CH CH) incorporation into dihydrodibenzo-
azepine or dibenzoazepine derivatives, respectively. The dashed arrow
indicates previously reported carbazole synthesis. DMF=N,N’-dime-
thylformamide.
the selective synthesis of these seven-membered rings con-
tinues to represent an important and challenging goal. The
methodology proposed herein is based on the ordered
assembly of three simple and readily available components
under mild catalytic conditions. It affords satisfactory yields
and offers a significant alternative to previously reported
procedures.
The addition of triarylphosphines to the palladium
catalyst was crucial to the success of the reaction and the
best one proved to be PPh3 when used in a 2.5:1 molar ratio
relative to Pd(OAc)2. Among the bases examined, Cs2CO3
gave the highest yields. Thus, by heating ortho-iodotoluene
(1) and ortho-bromoaniline (2) with norbornene in the
presence of Pd(OAc)2, PPh3, and Cs2CO3 in DMF at 1058C
for 6 hours under nitrogen, the compound 3a (ZZ = CH2-
CH2; R1 = Me; R2, R3 = H) was isolated in 74% yield
(Table 1, entry 1). The use of norbornadiene in place of
norbornene leads to similar results to give compounds 3
norbornadiene
a subsequent retro-Diels–Alder reaction
affords the dibenzoazepines 4.
Dibenzoazepines as well as dihydrodibenzoazepines are
important building blocks for the synthesis of antidepressant
and antiepileptic drugs.[5] Various methods to synthesize these
compounds have been reported in the literature.[6] Never-
theless, the development of new and efficient procedures for
=
(ZZ = CH CH), which then undergo a retro-Diels–Alder
[*] Dr. N. Della Ca’, Prof. M. Catellani
reaction that easily occurs with norbornadiene at 1308C and
allows the direct synthesis of dibenzoazepines 4 in one pot
(Table 2).
Dipartimento di Chimica Organica e Industriale and CIRCC
Universitꢀ degli Studi di Parma
Parco Area delle Scienze, 17/A, 43124 Parma (Italy)
E-mail: marta.catellani@unipr.it
It can be observed from the results in Table 1 that a
variety of substituents can be present on different positions of
the aryl iodide and bromide. As expected on the basis of
previous knowledge,[2a,3] even aryl iodides not bearing ortho
substituents react with ortho-bromoaniline to give the desired
products 3 (Table 1, entries 16–19). However, yields are not as
good as those for aryl iodides bearing ortho-substituents,
which under the reaction conditions strongly reduce the
formation of by-products.[7] 4-Chloro-2-bromoaniline
(entries 5, 11, and 15), behaves better than other substituted
Dr. G. Maestri, Prof. M. Malacria, Dr. E. Derat
UPMC Univ Paris 06, Institut Parisien de Chimie Molꢁculaire
(UMR CNRS 7201), 4 place Jussieu, C. 229, 75005 Paris (France)
E-mail: etienne.derat@upmc.fr
Homepage: http://www.ipcm.fr/
[**] This work was supported by MIUR (PRIN 2008A7P7YJ). The NMR
instrumentation was provided by Centro Interdipartimentale del-
l’Universitꢀ di Parma.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201104363.
Angew. Chem. Int. Ed. 2011, 50, 12257 –12261
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
12257

Communications
Table 1: One-pot synthesis of dihydrodibenzoazepine derivatives: cis,exo-
1,2,3,4,4a,13b-hexahydro-1,4-methano-9H-tribenzo[b,f]azepines.[a]
Table 2: Synthesis of 5H-dibenzo[b,f]azepines.[a]
Entry
Product
t
[h]
Yield
[%][b]
1
2
3
4
5
6
3a: R2, R3 =H
6
6
21
6
24
16
74
67
76
76
81
77
3b: R2 =7-Me, R3 =H
3c: R2 =7-OMe, R3 =H
3d: R2 =6-CO2Me, R3 =H
3e: R2 =H, R3 =Cl
3 f: R2 =H, R3 =F
Entry
Product
Yield [%][b]
1
2
3
4
5
6
7
4a: R2, R3 =H
61
66
63
66
72
78
72
4b: R2 =H; R3 =7-Me
4c: R2 =3-Me; R3 =H
4d:R2 =2-CO2Me, R3 =H
4e: R2 =3-OMe, R3 =H
4 f:R2 =2-CO2Me;R3 =8-Cl
4g: R2 =2,3-OMe; R3 =H
7
8
9
10
11
12[c]
13
3g: R2, R3 =H
18
44
72
28
24
98
30
85
88
74
83
90
56
82
3h: R2 =7-OMe, R3 =H
3i: R2 =H, R3 =11-Me
3j: R2 =H, R3 =12-Me
3k: R2 =H, R3 =12-Cl
3l: R2 =H, R3 =11-F
3m: R2 =H, R3 =12-F
8[c]
4h: R3 =H
4i: R3 =Me
4j: R3 =Cl
4k: R3 =F
59
65
74
66
9[c]
10[c]
11[c]
14
15
3n: R3 =H
3o: R3 =Cl
20
24
79
94
12
13
14
4l: R3 =H
70
65
50
4m: R3 =9-Me
4n; R3 =10-Cl
16
3p: R2 =H
20
64
24
21
46
72
60
64
17[d]
18
3q: R2 =6-CO2Me
3r: R2 =7-Me
3s: R2 =7-OMe
19
15[d]
16
4o; R2 =H
45
58
54
4p: R2 =1-Et
4q: R2 =3-F
17[d]
[a] Reaction conditions: aryl iodide (1.1 equiv), aryl bromide (1.0 equiv),
norbornene (1.2 equiv), Pd(OAc)2 (5 mol%), PPh3 (12.5%), Cs2CO3
(2.25 equiv) in DMF at 1058C under N2, for the time needed for
palladium black precipitation; 0.2ꢂ10ꢀ2 mmol Pd(OAc)2/mL DMF.
[b] Yield of isolated product. [c] Conversion of 1 (R1 =OMe) and 2
(R3 =5-F) is 75 and 69%, respectively. [d] K2CO3 as base; o-iodoaniline in
place of o-bromoaniline.
[a] Reaction conditions as in Table 1 using norbornadiene (2.0 equiv) in
place of norbornene at 1058C (24 h) and then at 1308C (12–14 h).
[b] Yield of isolated product. [c] K2CO3 as base. [d] Bromobenzene and
4-fluorobromobenzene in place of the corresponding iodides.
or unsubstituted ortho-bromoanilines. NHMe as well as
acetylated and sulfonylated NH anilines prevent the forma-
tion of 3, with the former being unreactive and the latter
leading to carbazole (Scheme 1).[4] The substituent effect, in
both the iodides and bromides, is not easy to rationalize
because more than one step can be affected by each
substituent.
The results in Table 2 show a similar trend to that seen in
Table 1, but yields are lower and a higher ratio of norborna-
diene to palladium is needed because of the higher reactivity
of norbornadiene. The higher reactivity of norbornadiene
leads to by-products by reaction with aryl iodides according to
previously described pathways.[2a,7] Some reactions shown in
Table 2 (entries 8, 10, and 11) were carried out for 24 hours at
1058C without additional heating to 1308C. Compounds 3h,
The reaction pathway, proposed on the basis of both
experimental and DFT calculations, is depicted in Scheme 2
for an ortho-substituted aryl iodide and ortho-bromoaniline.
As previously shown through stoichiometric reactions, the
formation of the palladacycle 7[8] readily takes place through:
a) oxidative addition of the aryl iodide to [Pd0L2];[9] b) nor-
bornene or norbornadiene stereoselective insertion to give
the cis,exo-palladacycle precursor 6;[10] c) intramolecular C
ꢀ
H bond activation.[11] Oxidative addition of ortho-bromoani-
line to palladacycle 7 affords 8.[12] Our proposal of a PdIV
intermediate has recently been substantiated by Vicente and
co-workers who isolated and characterized a PdIV aryl
complex, which was obtained by oxidative addition of the
chelating ortho-iodobenzoate to PdII.[12e] The newly formed
ꢀ
complex 8 undergoes reductive elimination by Csp2 Csp3
=
3j, and 3k (ZZ = CH CH) were isolated in 45, 61, and 40%
coupling to give 9. This step is in contrast to the aryl group
ꢀ
yield, respectively, together with their corresponding diben-
zoazepines 4h, 4j and 4k in 20, 20, and 33% yield,
respectively. The retro-Diels–Alder reaction of compound
migration to the aryl site of the palladacycle—Csp2 Csp2
coupling to give the intermediate 11—which is invariably
observed in the absence of the amino group when the
palladacycle contains an ortho substituent (R1 ¼ H; see below
for a proposed rationalization of this behavior). The PdII
complex 9 thus formed contains the two aryl groups in a
cis,exo arrangement, which forces them into a close prox-
1
2
3
=
3h (ZZ = CH CH; R = OMe, R , R = H) was carried out
separately in DMFat 1308C for 14 hours to give product 4h in
almost quantitative yield.
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Angew. Chem. Int. Ed. 2011, 50, 12257 –12261

Both the calculated structures 8 and 8a display chelation
by the aromatic amino group.[16] We calculated the energy
ꢀ
barriers for the four possible C C bond-forming reductive
eliminations from these two complexes since 8a might be
present in solution in thermodynamic equilibrium with 8. The
distortion of the octahedral structure, caused by the strained
chelating ring, favors reductive elimination from 8 to 13,
which eventually leads to 3 (Figure 1 and see Figure S3 in the
Supporting Information).
Scheme 2. Possible reaction pathway.
imity,[10,13] thus making possible the coordination of the NH2
group to palladium. The subsequent steps involve NH2
deprotonation by the base and final seven-membered ring
[14]
ꢀ
closure through C N coupling to give compound 3. This
pathway represents a deviation from the intramolecular
aromatic substitution leading to the six-membered ring
compound 10 as reported in the accompanying paper.[1]
IV
ꢀ
Csp2 N bond formation directly from the Pd complex 8
leading to 12, as modeled by DFT calculations is also
ꢀ
energetically precluded. Formation of the Csp2 Csp3 instead
Figure 1. Modeled pathways for ortho-bromoaniline.
ꢀ
of Csp2 Csp2 bond, as well as the seven-membered ring closure
mentioned above are key features of the mechanism.
It is worth noting in this context that addition of water has
been shown by Malacria and co-workers to counteract the
chelating effect of the CH2CONH2 functionality,[1] thus
A possible explanation of amine chelation and of the
unusual reactivity which arises from it can be found upon
considering the relative electronic densities of the three
palladium-bonded carbon atoms involved. In 8, the palla-
dium-bonded Csp2 of the aniline ring is the more electron rich
and the norbornyl Csp3 is the electrophile (summary of
calculated charges in Table S1 in the Supporting Informa-
tion). Frontier molecular orbital analysis of these complexes
strongly supports this conclusion. The HOMO around the
palladium bonded Csp2 of the aniline ring nicely matches the
LUMO around the Csp3 belonging to the metallacycle of
complex 8 (Figure 2).
ꢀ
driving the reaction back to the usual Csp2 Csp2 bond
formation. In our case water addition turns out not to be
able to cause a significant change of the reaction course.
Further insight into the nature of the intermediates of the
reaction comes from theoretical calculations carried out on
norbornane-containing palladacycles. On the basis of our
previous studies[3a] we anticipated that ortho-substituted aryl
halide/PdIV complexes would be formed. The calculated
barrier for the oxidative addition of ortho-bromoaniline to
deliver 8 (R = Me, ZZ = CH2-CH2) upon ligand displacement
It should be observed that potential chelating groups are
not always able to shift the reaction towards Csp2 Csp3 bond
is lower by 9.9 kcalmolꢀ1 in DG compared to an Csp2 Csp3
ꢀ
ꢀ
reductive elimination from a bimetallic intermediate alter-
native leading to PdIV (see Figures S1 and S2 in the Support-
ing Information). We also considered a possible chelation by
the ortho-arylbromide at the PdII stage and its effect on
oxidative addition. By displacing both ligands L from 7, the
barrier is similar (DDG =+ 3.0 kcalmolꢀ1 at 278 K, ꢀ0.7 kcal
molꢀ1 at 378 K). Upon facile ligand association at the PdIV
stage, this pathway leads to the same intermediate 8, which
contains the halide trans to the aryl ring of the metallacycle.
Higher barriers are obtained to form its isomeric complex 8a,
(DG =+ 4.5 kcalmolꢀ1), wherein the halide and the amino
group are inverted. The octahedral PdIV complexes obtained
so far by oxidative addition of methyl, allyl, or benzyl halide
from the metallacycle 7 in Scheme 2,[8a,15] have the halide
trans to the norbornyl ring as in 8a.
Figure 2. Frontier orbitals for the calculated complex 8. Orange arrows
indicate the carbon atoms involved in the cross-coupling.
Angew. Chem. Int. Ed. 2011, 50, 12257 –12261
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 12259

Communications
formation. For example, the acetylated ortho-bromoaniline
important class of dihydrodibenzoazepine and dibenzoaze-
pine derivatives starting from simple and commercially
available compounds. The process is based on the sequential
reaction of three components, an aryl iodide, a bromoaniline,
and either norbornene or norbornadiene in the presence of
palladium as a catalyst. Whereas the use of norbornene leads
to dihydrodibenzoazepine derivatives, norbornadiene allows
an additional step consisting of a retro-Diels–Alder reaction,
thus leading to the parent dibenzoazepines. Chelation of the
amino group to palladium plays a key role for the selective
formation of the dibenzoazepine product causing a deviation
ꢀ
leads to Csp2 Csp2 bond formation and the reaction product is a
carbazole, which results from elimination of norbornene—in
this case acting catalytically—and subsequent ring closure
[4]
ꢀ
with C N bond formation.
We calculated reductive eliminations from a PdIV complex
with an acetyl-protected amino group (see Figure S4 in the
Supporting Information). In this case, PdIV structures con-
verged to a minima in which chelation occurs to neither the
nitrogen nor the oxygen atom, and the lowest barrier is that
for the Csp2 Csp2 formation (by 1.6 kcalmolꢀ1 in DG). This
ꢀ
ꢀ
ꢀ
effect should be attributed to the steric hindrance of the acetyl
group and to the reduced electron density on the nitrogen
atom. As a result the structure of the palladium intermediate
approaches that of a trigonal bipyramid, which is more
reactive than the octahedral intermediate and leads to the
from the usual Csp2 Csp2 bond formation to a Csp2 Csp3 bond
formation. Theoretical calculations support the experimental
findings, thus giving a rationale for the interpretation of the
preferred reaction course.
Csp2 Csp2 bond formation as previously shown by us.[3a] On the
contrary, analogous to the unsubstituted NH2, the
ꢀ
Received: June 23, 2011
Revised: August 24, 2011
Published online: October 26, 2011
ꢀ
CH2CONH2 favors Csp2 Csp3 bond formation through a
distorted octahedral complex, as reported in the accompany-
ꢀ
Keywords: C C coupling · density functional calculations ·
heterocycles · homogeneous catalysis · palladium
ing paper,[1] while CONH2[17] as well as CH2NH2,[18] which do
.
ꢀ
not cause octahedral distortion, favor Csp2 Csp2 bond forma-
tion (see the Supporting Information).
Since alkali carbonates are needed to perform the
palladium-catalyzed process, calculations were also carried
out to ascertain whether carbonate coordination could offer
more favorable energetic pathways. In particular we used
DFT to test a different route to product 3 from the PdIV
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ꢀ
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ꢀ
followed by C C ring closure (Scheme 2). To this end, we
replaced the bromide with a bicarbonate anion. The energy
ꢀ
barrier to Csp2 N formation is, however, higher than that the
ꢀ
corresponding to Csp2 Csp3 formation (DG =+ 36.9 versus
+ 6.0 kcalmolꢀ1, respectively; see Figure S5 in the Supporting
Information). Upon deprotonation of the amino group by a
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32.8 kcalmolꢀ1), but still outmatched by the PdIV Csp2 Csp3
ꢀ
reductive elimination. Final ring closure to 3 displays a
barrier of + 21 kcalmolꢀ1, thus indicating that a Buchwald–
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the PdIV stage (see Figure S6 in the Supporting Information).
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ꢀ
chelating amino group determines Csp2 Csp3 coupling in
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ꢀ
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