Highly efficient ligands for the palladium-assisted double N-arylation of primary amines for one-sep construction of carbazoles

Article abstract of DOI:10.1002/adsc.200900846

A highly efficient one-pot synthesis of carbazoles via palladium-catalyzed double N-arylation of primary amines with 2,2'-dihalobiphenyls is described using a catalyst system comprised of tris(dibenzylideneacetone)dipalladium(O) (Pd₂dba₃) and the proazaphosphatrane P(i-BuNCH ₂CH₂)₃N (8) or its derivative (t-Bu) ₂P=N-P(iBuNCH₂CH₂)₃N (9a) as the ligand. The process is effective for double N-arylation of 2,2'-biphenyl dibromide, diiodide, and even dichloride with a variety of primary amines including neutral, electronrich, electron-deficient, and sterically hindered anilines as well as aliphatic amines.

Full text of DOI:10.1002/adsc.200900846

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DOI: 10.1002/adsc.200900846
Highly Efficient Ligands for the Palladium-Assisted Double
N-Arylation of Primary Amines for One-Sep Construction of
Carbazoles
Yibo Zhoua and John G. Verkadea,*
a
Department of Chemistry, Gilman Hall, Iowa State University, Ames, IA 50011, U.S.A.
Fax : (+1)-515-294-0105; phone: (+1)-515-294-5023; e-mail: jverkade@iastate.edu
Received: December 6, 2009; Published online: March 4, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900846.
Abstract: A highly efficient one-pot synthesis of
carbazoles via palladium-catalyzed double N-aryla-
tion of primary amines with 2,2’-dihalobiphenyls is
described using a catalyst system comprised of tris-
A
(Pd2dba3)
and the proazaphosphatrane P(i-BuNCH2CH2)3N
ꢀ
(8)
or
its
derivative
(t-Bu)2P=N P(i-
BuNCH2CH2)3N (9a) as the ligand. The process is
effective for double N-arylation of 2,2’-biphenyl di-
bromide, diiodide, and even dichloride with a varie-
ty of primary amines including neutral, electron-
rich, electron-deficient, and sterically hindered ani-
lines as well as aliphatic amines.
Scheme 1. Pd-catalyzed double N-arlyation of primary
amines with 2,2’-dihalobiphenyls.
ꢀ
Keywords: biaryls; carbazoles; C N coupling; het-
erocycles; palladium
Pd-catalyzed double arylation of primary amines (6 in
Scheme 1) to synthesize 7 in this Scheme. Their ap-
proach was an important extension of the Buchwald–
Hartwig N-arylation reaction,[8] and it proved to be an
excellent protocol for construction of a variety of sub-
Carbazole alkaloids possess a wide range of biological stituted carbazoles in one-step. This novel method led
behaviors including antitumor, psychotropic, anti-in- to the successful synthesis of the carbazole-based nat-
flammatory, antihistaminic and antibiotic activities.[1] ural products mukonine[6a] and murrastifoline.[7] How-
Carbazole derivatives are also widely useful owing to ever, these methodologies are generally plagued by
their photorefractive, photoconductive, hole-trans- the requirement for high catalyst loadings and harsh
porting, and light-emitting properties.[2] The unique reaction conditions. Typically, 10–20 mol% of Pd cata-
importance of these organic materials has attracted lyst and 30 mol% of a phosphine ligand [(t-Bu)3P, [6]
considerable interest in the efficient synthesis of car- dialkylphosphinobiaryls 1–3[6] or Xantphos 4[7]] were
bazoles,[3] and although many useful syntheses have employed at 80–1208C, despite the observation that
been developed, there are few examples of the effi- these ligands have been reported to be excellent for
cient construction of carbazoles in a single step under N-arylation.[9,10] In addition, low yields were reported
mild conditions. The Pd(0)-catalyzed intramolecular when aliphatic primary amines (such as benzylamine
[4]
ꢀ
C H activation of 2-amino-2’-halodiphenyl and the or n-octylamine) were employed as the substrate. Fur-
Pd(II)-mediated oxidative cyclization of diaryl- thermore, no examples involving the less reactive sub-
ACHTUNGTRENNUNG
amines[5] have been reported to be useful methods for strate dichlorobiphenyl have been reported.
constructing carbazoles, but only moderate yields
Previous work from our group has established that
were generally obtained. Very recently, Nozaki[6] and the commercially available proazaphosphatrane 8[11]
Chida[7] independently utilized biphenyls possessing bearing isobutyl groups on the PN3 nitrogens and its
Br, I or OTf functionalities (5 in Scheme 1) for the derivative (t-Bu)2P N=P(i-BuNCH2CH2)3N) 9a[12]
ꢀ
616
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2010, 352, 616 – 620

Highly Efficient Ligands for the Palladium-Assisted Double N-Arylation of Primary Amines
Table 1. The double N-arylation of aniline 6a with 2,2’-di-
bromobiphenyl 5a.[a]
Entry Pd2dba3
(mol%)
Ligand Temp.
[oC]
Time
[h]
Yield
[%][b]
1
2
3
4
5
6
7
8
2.5
–
8
8
8
9a
9a
9a
9a
9b
9c
100
80
80
80
80
80
25
80
80
80
24
1
1
24
1
24
24
24
24
24
<2
98
98
67
98
0.25
0.125
0.05
0.125
0.05
0.5
0.5
0.25
0.25
(Scheme 1, both first synthesized in our laboratories)
are highly effective ligands for several palladium-cata-
lyzed cross-coupling reactions of aryl bromides, io-
dides and chlorides, such as Suzuki[12,13], Stille[14], a-ar-
ylation[15] and Buchwald–Hartwig amination reac-
tions.[16] We have suggested that the unusually high
activity of 8 and 9a as supporting ligands in Pd-cata-
lyzed coupling reactions is due primarily to their
steric bulk and the uniquely electron-rich nature of
their trivalent phosphorus atoms stemming from the
geometrically favorable arrangement of the three sp2
nitrogens contributing their unhybridized p electron
10
98
<2[c]
52
9
10
87
[a]
Pd2dba3:ligand=1:4, 0.5 mmol of 5a, 0.6 mmol of aniline,
1.5 mmol of NaO-t-Bu in 3 mL of toluene.
Isolated yields (average of 2 runs).
Cs2CO3 (3.0 equiv.) was used as the base in place of
NaO-t-Bu.
[b]
[c]
lone pair density to the phosphorus and the possibly Pd2dba3/9a in the presence of NaO-t-Bu as a base. As
temporary N!P transannulation donating the bridge- is evident from Table 2, a variety of anilines possess-
head nitrogenꢁs lone pair to the phosphorus in the ing electron-neutral and electron-rich (entries 1–8), as
proazaphosphatrane framework in each of these mol- well as sterically bulky groups (entries 6–11) were ef-
ecules.[12,17]
ficiently coupled with 5a, affording the desired carba-
We envisioned the possibility that the steric bulk zoles in excellent yields using both catalyst systems.
and electron-richness of 8 and 9 might also function Even the sterically encumbered 2,6-diisopropylaniline
well in facilitating Pd-catalyzed double N-arylation of 6g successfully coupled with 5a to afford impressive
primary amines with 2,2’-dihalobiphenyls for a one- yields (92–96%) of 7g with both catalyst systems (en-
pot synthesis of carbazoles (Scheme 1). Thus we first tries 9 and 10). Anilines with F or Cl functional
examined the reaction of 2,2’-dibromobiphenyl 5a groups also produced carbazoles in excellent yields
with aniline 6a for screening purposes (Table 1). A (entries 12–16). Generally, the use of ligand 8 re-
control experiment confirmed the need for a ligand quired lower catalyst loadings and shorter reaction
(entry 1). Both 8 and 9a proved to be effective li- times than 9a for the reaction to proceed to comple-
gands. Nearly quantitative yields of the carbazole 7a tion. However, the reactions of aliphatic primary
were obtained with 0.125 mol% of Pd2dba3 and amines and anilines with electron-withdrawing groups
0.5 mol% of ligand 8 or 9a in the presence of were more challenging for 8, providing only poor
3.0 equiv. of NaO-t-Bu in toluene at 808C for 1 hour yields (50–60%) of carbazoles even when a higher
(entries 3, 5). Even at room temperature, the reaction catalyst loading of 8 was employed (entries 17 and
gave rise to 7a in 98% yield in the presence of 18).
0.5 mol% of Pd2dba3 and 2 mol% of ligand 9a
It is noteworthy that at least 40-fold (and in some
(entry 7). The less sterically bulky analogous ligands cases 80-fold) lower catalyst loadings and much short-
9b and 9c gave 52% and 87% isolated yields of 7a, re- er reaction times are needed in our protocol than
spectively (entries 9, and 10). Ligand 8 appears to be with the Pd2dba3/ACHTNUGTRNENUG(t-Bu)3P, Pd2dba3/dialkylphosphine-
more efficient than 9a when catalyst loadings were biaryls 1–3, and Pd2dba3/Xanthphos 4 catalyst systems
decreased to 0.05 mol% of Pd2dba3 and 0.2 mol% of reported by Nozaki[6] and Chida,[7] respectively. For
ligand (entries 4 and 6). The use of Cs2CO3 as the example, 7a and 7b were synthesized using 5 mol% of
base failed to provide carbazole 7a (entry 8).
Pd2dba3 and 20 mol% of (t-Bu)3P at 808C for 24 h
Thus, the reactions of biphenyl dibromide 5a with whereby 86% and 92% of desired carbazoles were
substituted anilines and aliphatic amines 6 were ex- obtained, respectively.[6a] Using 10 mol% of Pd2dba3
amined using the two catalyst systems Pd2dba3/8 and and 30 mol% of 1 at 1208C for 24 h, produced an
Adv. Synth. Catal. 2010, 352, 616 – 620
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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617

Yibo Zhou and John G. Verkade
COMMUNICATIONS
Table 2. The double N-arylation of anilines 6 with 2,2’-dibro-
mobiphenyl 5a.[a]
Table 3. The double N-arylation of primary amines 6 with
2,2’-diiodobiphenyl 5b.[a]
Entry RNH2 Pd2dba3 (mol%) Time [h] Yield [%][b]
1
2
3
6l
6m
6n
5
0.5
3
48
40
24
34[c]
94
84[d]
Entry RNH2 Pd2dba3
(mol%)
Ligand Time
[h]
Yield
[%][b]
[a]
Pd2dba3:ligand=1:4, 0.5 mmol of 5b, 0.6 mmol of aniline,
1.5 mmol of NaO-t-Bu in 3 mL of toluene at 808C.
Isolated yields.
Cs2CO3 (3.0 equiv) was used as the base in place of NaO-
t-Bu.
[b]
[c]
1
2
3
4
5
6
7
8
6b
6b
6c
6c
6d
6e
6f
6f
6g
6g
6h
6i
6i
6j
6j
6k
6l
0.125
0.125
0.125
0.25 (0.125)
0.125
0.125
0.25
0.50
0.50
0.50
0.25
0.125
0.125
0.125
0.125
0.125
5
8
9a
8
4
6
4
98
97
97
[d]
9a
8
8
8
9a
8
9a
8
8
24 (24) 98 (44)
A 10% yield of the side product biphenyl was isolated.
24
15
30
24
24
48
20
2
98
99
98
97
96
92
99
98
Historically, aryl iodides have been less effective
ꢀ
substrates for Pd-catalyzed C N bond-forming amina-
tion processes than their bromide counter-parts be-
cause iodides are more prone to b-hydride elimina-
tion leading to formation of hydrodehalogenation
9
10
11
12
13
14
15
16
17
18
products,[18] even though they are more reactive in
9a
8
9a
1
4
6
99
95
96
[19]
ꢀ
Pd-catalyzed C C cross-coupling reactions. Howev-
er, the double N-arylation of long-chain aliphatic pri-
mary amines such as n-octylamine 6m with biphenyl
diiodide 5b proceeded smoothly to give the desired
carbazole product 7m in 94% isolated yield by using
5 mol% Pd2dba3 and 20 mol% ligand 8 (entry 2). By
contrast, under the same conditions, the reaction of
biphenyl dibromide 5a with 6m gave only a 50% yield
of 7m. Although the reaction did proceed to comple-
tion, hydrodehalogenation side products and other
uncharacterized compounds were detected by GC in
the reaction mixture (Table 2, entry 18). Importantly,
benzylamine 6n also successfully reacted with 5b to
provide N-benzyl-protected carbazole product 7n in
84% yield (entry 3). The major side product for this
8
8
8
4
40
6
97
60[c]
50[d]
6m
5
[a]
Pd2dba3:ligand=1:4, 0.5 mmol of 5a, 0.6 mmol of aniline,
1.5 mmol of NaO-t-Bu in 3 mL of toluene at 808C.
Isolated yields (average of 2 runs).
Cs2CO3 (3.0 equiv.) was used as the base in place of
NaO-t-Bu.
The reaction went to completion but hydrodehalogenat-
ed byproducts and other uncharacterized products were
observed.
[b]
[c]
[d]
85% yield of 7a.[7] By contrast, with our protocol in reaction was the biphenyl (10%) that was generated
which 0.125 mol% of Pd2dba3 and 0.5 mol% of either by hydrodehalogenation of 5b. The formation of the
ligand 8 or 9a are used, carbazoles 7a and 7b are pro- N-benzyl-protected carbazole is significant since most
ꢀ
vided in 97–98% yields at 808C in 1–6 h (Table 1, en- known natural carbazole alkaloids possess a free N
tries 3 and 5; Table 2, entries 1 and 2).
H moiety.[1] In the case of 3-nitroaniline 6l, biphenyl
To improve the yields for aliphatic primary amines diiodide 5b again provided a poor yield of product
and electron-deficient anilines, we investigated their (entry 1).
couplings with biphenyl diiodide 5b in the presence of
Recently, significant progress has been made in the
Pd2dba3 and the commercially available ligand 8 development of catalysts capable of utilizing unreac-
(Table 3). It is important to note that the N-alkylcar- tive aryl chlorides as substrates in Pd-catalyzed cross-
bazoles, which were prepared in poor yields under the coupling processes.[9b,10a,16b,20] To the best of our knowl-
conditions used by Nozaki[6] and Chida,[7] were ob- edge, however, there are no reports of the use of bi-
tained in good to excellent yields by employing diio- phenyl dichlorides for direct carbazole synthesis via a
dide 5b as the substrate and the catalyst system double N-arylation reaction of primary amines. En-
Pd2dba3/8 (entries 2 and 3).
couraged by our promising results with dibromide 5a
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Adv. Synth. Catal. 2010, 352, 616 – 620

Highly Efficient Ligands for the Palladium-Assisted Double N-Arylation of Primary Amines
and diiodide 5b, we further extended our approach to base (entries 9 and 11). However, in contrast to the
the use of biphenyl dichloride 5c as the substrate. reaction of n-octylamine 6m with diiodobiphenyl 5b
Representative results are summarized in Table 4. to afford a 94% yield of carbazole 7m (Table 3,
The catalyst system Pd2dba3/9a proved to be superior entry 2), the reaction of 6m with dichlorobiphenyl 5c
to the Pd2dba3/8 system when 5c was the substrate was sluggish and provided only a low yield of 7m
(entries 1 and 11). As seen in Table 4, a catalyst (entry 10).
system of 0.5 mol% Pd2dba3 and 2 mol% of 9a in
In conclusion, we have described two highly active
combination with NaO-t-Bu as the base in toluene at catalyst systems consisting of a palladium source and
1008C allowed coupling of biphenyl dichloride 5c the commercially available ligand 8 or its derivative
with a wide array of amines, including electron-poor, 9a for a one-pot synthesis of carbazoles. Our process
electron-neutral, and electron-rich examples, afford- is effective for the double N-arylation of primary
ing the desired carbazoles in excellent yields. Sterical- amines (including neutral, electron-rich, electron-defi-
ly hindered primary amines are also well tolerated, cient, and sterically hindered anilines and aliphatic
often giving the desired products in excellent yields amines) with biphenyl dibromide, diiodide, as well as
(entries 3, 5–7). Surprisingly, the reactions of electron- dichloride. Our methodology employs the lowest pal-
deficient anilines 3-nitroaniline 6l and 4-trifluorome- ladium and ligand loadings reported to date for the
thylaniline 6o proceed to completion, giving good to synthesis of carbazoles, and our protocols operate in
excellent isolated yields of carbazole 7l (75%) and 7o shorter reaction times compared with those in previ-
(90%), respectively, when the Pd2dba3/9a catalyst ous reports.
system was employed and Cs2CO3 was used as the
Experimental Section
Table 4. The double N-arylation of primary amines 6 with
2,2’-dichlorobiphenyl 5c.[a]
General Procedure for the Double N-Arylation
Reaction
Argon was bubbled into a mixture of Pd2dba3, ligand (8 or
9), base (NaO-t-Bu or Cs2CO3, 1.5 mmol, 3.0 equiv.), 2,2’-di-
halogenbiphenyl (5, 0.5 mmol), and amine (6, 0.6 mmol) in
toluene (3 mL) for 5 min. The mixture was then heated at
80–1008C for 1–40 h (see Tables) and after cooling, the mix-
ture was filtered through a pad of silica gel (3 g, ethyl ace-
tate). The filtrate was concentrated to give a residue, which
was purified by column chromatography (silica gel, hexane/
ethyl acetate 100/1) to afford carbazole 7. Full experimental
Entry RNH2 Pd2dba3 (mol%) Time [h] Yield [%][b]
details and characterization data are given in the Supporting
Information.
1
2
3
4
5
6
7
8
6a
6b
6c
6d
6e
6f
6g
6i
0.50
0.50
0.50
0.50
0.50
0.50
0.50
0.50
5
24
24
24
24
24
30
30
24
40
40
24
95 (61)[c]
95
95
94
94
Acknowledgements
92
94
95
We thank the National Science Foundation for support of this
work with a grant (0750463).
19
10
11
6l
6m
6o
75[d] (92)[e]
36
5
5
90[d] (88)[f]
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[a]
Pd2dba3:ligand 9a=1:4, 0.5 mmol of 5c, 0.6 mmol of
amine, 1.5 mmol of NaO-t-Bu in 3 mL of toluene at
1008C.
Isolated yields (average of 2 runs).
0.5 mol% Pd2dba3, 2 mol% ligand 8, 3.0 equiv. of NaO-t-
Bu, 1008C, 24 h. A 26% yield of monoarylation product
was obtained.
Cs2CO3 (3.0 equiv.) was used as the base in place of
NaO-t-Bu.
GC yield.
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Adv. Synth. Catal. 2010, 352, 616 – 620
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
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Yibo Zhou and John G. Verkade
COMMUNICATIONS
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