Troeger's base derived phosphanes for Suzuki-Miyaura and Buchwald-Hartwig cross-coupling reactions

Article abstract of DOI:10.1002/ejoc.201300519

A library of bis(phosphane) ligands based on the 2,8-substituted Troeger's base scaffold was synthesized through an efficient two-step protocol by starting from a commercially available aniline. The new ligands proved their efficiency in palladium-catalyzed Suzuki-Miyaura cross-coupling reactions and Buchwald-Hartwig aminations of aryl chlorides and bromides. Copyright

Full text of DOI:10.1002/ejoc.201300519

Job/Unit: O30519
/KAP1
Date: 05-06-13 10:22:57
Pages: 6
SHORT COMMUNICATION
Tröger’s Base P Ligands
ˇ
R. Pereira, J. Cvengros* .................... 1–6
Tröger’s Base Derived Phosphanes for
Suzuki–Miyaura and Buchwald–Hartwig
Cross-Coupling Reactions
Easily accessible bisphosphane ligands
based on the 2,8-substituted Tröger’s base
scaffold are synthesized and effectively ap-
plied in palladium-catalyzed C–C and C–
N bond-forming reactions of aryl chlorides
and bromides.
Keywords: Ligand design / Phosphane li-
gands
/
Cross-coupling
/
Palladium
/
Homogeneous catalysis
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Job/Unit: O30519
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Date: 05-06-13 10:22:57
Pages: 6
R. Pereira, J. Cvengrosˇ
SHORT COMMUNICATION
DOI: 10.1002/ejoc.201300519
Tröger’s Base Derived Phosphanes for Suzuki–Miyaura and Buchwald–Hartwig
Cross-Coupling Reactions
[a]
[a]
ˇ
Raul Pereira and Ján Cvengros*
Keywords: Ligand design / Phosphane ligands / Cross-coupling / Palladium / Homogeneous catalysis
A library of bis(phosphane) ligands based on the 2,8-substi-
tuted Tröger’s base scaffold was synthesized through an ef-
ficient two-step protocol by starting from a commercially
available aniline. The new ligands proved their efficiency in
palladium-catalyzed Suzuki–Miyaura cross-coupling reac-
tions and Buchwald–Hartwig aminations of aryl chlorides
and bromides.
hibited, and this gives rise to two enantiomers that are con-
figurationally stable and that can be separated at room tem-
perature. We recently exploited the 4,10-disubstituted
Tröger’s base scaffold as a structural backbone for a new
class of ligands.[8] X-ray crystallography and NMR spec-
troscopy revealed that such structures behave as doubly bi-
dentate ligands coordinating two metal atoms within a sin-
gle ligand molecule. The corresponding phosphane ligands
were catalytically active in Suzuki–Miyaura cross-coupling
reactions of aryl bromides. However, any attempt to employ
aryl chlorides proved fruitless. Herein, we provide a solu-
tion to this issue and present a concise strategy for the syn-
thesis of 2,8-bis(phosphane) Tröger’s base derived ligands
that can effectively involve aryl chlorides in palladium-cata-
lyzed C–C and C–N bond-forming reactions.
Introduction
Palladium-catalyzed cross-coupling reactions between
organometallic reagents and organic electrophiles have de-
veloped over the years into a sophisticated synthetic tool.[1]
This methodology helped chemists to access complex archi-
tectures ranging from natural products and medicinal com-
pounds to those in materials chemistry.[2] Organic phos-
phane ligands typically play a prominent role in these trans-
formations.[3] The ever-increasing demand to access new
and more challenging targets has advanced the field of li-
gand design. Thus, the phosphane family of ligands con-
sisting of simple representatives, such as 1,1Ј-bis(diphenyl-
phosphany)ferrocene (dppf) and PPh3, which were fre-
quently used at the infancy of palladium-catalyzed cross-
coupling reactions, recently expanded with the addition of
tailor-made members. The development of meticulously de-
signed electron-rich bulky phosphanes[4] allowed for the
sluggish reactivity of aryl chlorides and sterically hindered
coupling partners[5] to be addressed and also expanded the
Results and Discussion
Tröger’s base derived bis(phosphanes) 2a–c were ef-
substrate scope to form bonds other than carbon–carbon, ficiently synthesized by starting from rac-2,8-dibromo-4,10-
most notably carbon–nitrogen bonds.[6] dimethyl Tröger’s base analogue 1 (Scheme 1).[9] The lith-
Despite significant achievements, the quest for alterna- ium/bromine exchange on 1 followed by quenching with
tive motifs in ligand design should not be abated if a rea- chlorodiphenylphosphane or chlorodicyclohexylphosphane
sonable balance between the complexity of the ligand and afforded bis(phosphanes) 2a and 2c. Whereas 2a was iso-
its performance is achieved. Tröger’s base (2,8-dimethyl- lated in 75% yield without any issues, the purification of 2c
6,12-dihydro-5,11-methanodibenzo[b,f][1,5]diazocine) is a proved troublesome. Therefore, it was directly protected as
dissymmetric molecule belonging to the C2 point group.[7] its BH3 adduct 2cЈ in 74% overall yield to prevent oxidation
Probably the most interesting feature of Tröger’s base is that during workup and purification. Adduct 2cЈ was then read-
pyramidal inversion at the nitrogen atom is structurally pro- ily deprotected, typically immediately prior to its use in ca-
talysis, with morpholine to give 2c in 70% yield. In the case
of tert-butyl derivative 2b, transmetalation to organo-
cuprate[10] and an elevated reaction temperature were neces-
sary to introduce the di-tert-butylphosphane moiety effec-
tively. Omission of CuCl resulted in no product formation.
In analogy to the previous ligand, 2b was directly treated
with BH3·THF and purified as borane adduct 2bЈ (88%).
Deprotection with morpholine yielded 2b in 80% yield. Ac-
[a] Department of Chemistry and Applied Biosciences, Swiss
Federal Institute of Chemistry, ETH Zurich,
8093 Zurich, Switzerland
Fax: +41-44-632-1310
E-mail: cvengros@inorg.chem.ethz.ch
Homepage: http://www.tognigroup.ethz.ch/Independent_
Researcher
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300519.
2
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Tröger’s Base Derived Phosphanes for Cross-Coupling Reactions
Scheme 1. Synthesis of ancillary phosphane ligands based on a Tröger’s base scaffold.
cording to our observations, bis(phosphanes) 2b and 2c are turned out to be the least active in terms of its substrate
bench-stable in the solid state, but their sensitivity towards scope, and its use was limited to aryl bromides.
oxygen increases in solution. Their ease of preparation from
Subsequently, we turned our attention to catalytic C–N
cheap starting materials and the high modularity of the pre- bond-forming reactions. On the basis of our results from
sented strategy bodes well for further development and tun- the Suzuki–Miyaura coupling reaction, we tested most-po-
ing of this family.
tent ligand 2b in Buchwald–Hartwig cross-coupling reac-
With a small library of bis(phosphanes) in hand, their tions of aryl bromides and chlorides with morpholine. We
efficiency was first tested in Suzuki–Miyaura cross-coupling were delighted to observe that the combination of 2b with
reactions. A brief screening of reaction conditions revealed Pd(OAc)2 in refluxing toluene with sodium tert-butoxide as
that the use of Pd(OAc)2 as the palladium precursor, potas- the base cleanly yielded the anticipated products in high
sium phosphate (tribasic) as the base, and toluene as the yields (Table 2). Also in this case, ortho substituents did not
solvent were critical for an efficient outcome of the coupling hamper the outcome of the reaction. Only in the presence
reaction.[11]
of strongly electron-donating groups on the aryl chloride
As summarized in Table 1 the palladium complexes with were lower yields obtained (Table 2, Entries 6 and 7).
ligands 2a–c efficiently catalyzed the cross-coupling of aryl
bromides and chlorides with arylboronic acids in short
periods of time. The palladium/ligand ratio turned out to
be crucial (Table 1, Entry 5). Although 1-chloro-2-methyl-
benzene (3e) could be coupled with phenylboronic acid (4a)
Conclusions
to provide the product in 80% yield if equimolar amounts
We have developed an efficient two-step synthetic route
of Pd(OAc)2 and 2b were used, an excess amount of palla- to novel C2-symmetric bis(phosphane) ligands based on a
dium slowed down the reaction, and an excess amount of Tröger’s base scaffold. The synthesis is highly modular, as
the ligand almost completely deactivated the catalyst. Sub- the phosphane moiety is introduced in the last step. We
stituents in the ortho position were tolerated both on the have shown that these ligands form a versatile catalytic sys-
haloarene and on the boronic acid (Table 1, Entries 2–5). tem with Pd(OAc)2 for Suzuki–Miyaura cross-coupling and
Moreover, the presence of electron-withdrawing groups at Buchwald–Hartwig amination reactions of aryl bromides
aryl chlorides is not necessary (Table 1, Entries 5 and 9). and chlorides. Intense efforts are currently dedicated to the
On the basis of the results obtained, tBu ligand 2b was iden- clarification of the coordination properties of the reported
tified to be the most active followed by cyclohexyl (Cy) ana- ligands. Additionally, we are focusing on their preparation
logue 2c, and both ligands were capable of effecting the in enantiomerically pure form for applications in asymmet-
cross-coupling of aryl chlorides. Bis(diphenylphosphane) 2a ric catalysis. The results will be reported in due course.
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R. Pereira, J. Cvengrosˇ
SHORT COMMUNICATION
Table 1. Suzuki–Miyaura cross-coupling.[a]
Table 2. Amination of aryl bromides and chlorides.[a]
[a] Reactions conditions: aryl halide (1.0 mmol), morpholine
(1.2 mmol), NaOtBu (1.2 mmol), Pd(OAc)2 (1.5 mol-%), ligand
(1.5 mol-%), toluene (2.5 mL), 110 °C, 15 h, argon.
Experimental Section
rac-2,8-Bis(diphenylphosphanyl)-4,10-dimethyl-6,12-dihydro-5,11-
methanodibenzo[b,f][1,5]diazocine (2a): THF (16 mL) was added to
a Schlenk flask charged with 1 (0.82 g, 2 mmol) under argon. The
mixture was cooled to –78 °C, and nBuLi (1.6 m in hexane, 2.8 mL,
4.4 mmol, 2.2 equiv.) was added dropwise to form a pale-yellow
[a] Reactions conditions: aryl halide (1.0 mmol), arylboronic acid
(1.5 mmol), K3PO4 (2.0 mmol), Pd(OAc)2 (1.5 mol-%), ligand
(1.5 mol-%), toluene (2.5 mL), argon. [b] Yield of isolated product
after column chromatography. [c] Ligand/Pd = 1:2. [d] Ligand/Pd
= 2:1. [e] In xylene at 130 °C.
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Tröger’s Base Derived Phosphanes for Cross-Coupling Reactions
clear solution. After 10 min, chlorodiphenylphosphane (0.78 mL,
4.4 mmol, 2.2 equiv.) was added in one portion. Stirring was con-
tinued at ambient temperature overnight. A saturated aqueous catalysis. H NMR (300 MHz, CD2Cl2): δ = 1.12 [d, J = 11.4 Hz,
give 2b (197 mg, 80%) as a white solid, which was briefly checked
1
by H NMR and 31P NMR spectroscopy and immediately used in
1
3
3
solution of NH4Cl (20 mL) was added, and the mixture was ex-
tracted with CH2Cl2 (3ϫ 20 mL). The combined organic layers
were washed with brine, dried with MgSO4, and concentrated un-
der reduced pressure. The crude product was purified by flash col-
umn chromatography [SiO2 (40 g), ethyl acetate/hexane (1:10 to
20:80) containing 0.1% Et3N] to give 2a (747 mg, 75%) as a white
18 H, (CH3)3], 1.13 [d, J = 11.7 Hz, 18 H, (CH3)3], 2.39 (s, 6 H,
2
CH3), 4.02 (d, J = 16.9 Hz, 2 H, CH2N), 4.26 (s, 2 H, NCH2N),
3
3
4.58 (d, J = 16.8 Hz, 2 H, CH2N), 7.07 (d, JH,P = 7.6 Hz, 2 H,
Ar-H), 7.35 (d, JH,P = 5.2 Hz, 2 H, Ar-H) ppm. 31P NMR
3
(121.5 MHz, CD2Cl2): δ = 38.5 ppm.
1
rac-2,8-Bis(dicyclohexylphosphanyl)-4,10-dimethyl-6,12-dihydro-
5,11-methanodibenzo[b,f][1,5]diazocine (2c): THF (8 mL) was added
to a 50 mL Schlenk flask charged with 1 (0.4 g, 1.0 mmol) under
argon. The mixture was cooled to –78 °C, and nBuLi (1.6 m in hex-
ane, 1.5 mL, 2.4 mmol, 2.4 equiv.) was added dropwise to form a
pale-yellow clear solution. After 10 min, a solution of chlorodicy-
clohexylphosphane (560 mg, 2.4 mmol) in THF (5 mL) was used
to quench the lithiated species. The reaction mixture was allowed
to reach room temperature overnight. It was then cooled to 0 °C,
and BH3·THF (1 m in THF, 10 mL, 10 mmol) was added. The reac-
tion mixture was allowed to reach room temperature, and it was
then stirred for an additional 6 h. The reaction mixture was then
poured into a saturated aqueous solution of NH4Cl (20 mL) under
argon, and the resulting mixture was stirred for 5 min and then
extracted with CH2Cl2 (3ϫ 50 mL). The combined organic layers
were dried with MgSO4 and concentrated. The crude product was
purified by flash column chromatography [SiO2 (30 g), hexane/ethyl
acetate (4:1 to 3:1) containing 0.1% Et3N] to give borane-protected
phosphane 2cЈ (495 mg, 74 %) as a white solid. 1H NMR
(300 MHz, CD2Cl2): δ = –0.26–0.94 [br., 6 H, (BH3)2], 0.99–1.39
[m, 20 H, (C6H11)2], 1.41–2.08 [m, 24 H, (C6H11)2], 2.43 (s, 6 H,
solid. H NMR (300 MHz, CD2Cl2): δ = 2.30 (s, 6 H, CH3), 3.92
2
2
(d, J = 17.0 Hz, 2 H, CH2N), 4.26 (s, 2 H, NCH2N), 4.50 (d, J
3
= 16.9 Hz, 2 H, CH2N), 6.75 (d, JH,P = 7.9 Hz, 2 H, Ar-H), 7.02
3
(d, JH,P = 7.9 Hz, 2 H, Ar-H), 7.29 (m, 20 H, Ar-H) ppm. 13C
NMR (75 MHz, CD2Cl2): δ = 17.5 (Ar-CH3), 55.4 (CH2N), 67.9
(NCH2N), 128.8, 128.9, 129.0, 129.1, 130.6 (d, JC,P = 21.4 Hz),
132.1 (d, JC,P = 10.3 Hz), 133.9, 134.1 (d, JC,P = 19.6 Hz), 134.8
(d, JC,P = 20.2 Hz), 138.28 (d, JC,P = 11.4 Hz), 138.31 (d, JC,P
=
11.3 Hz), 147.71 ppm. 31P NMR (121.5 MHz, CD2Cl2): δ =
–5.8 ppm. HRMS (ESI): calcd. for C41H37N2P2 [M + H]+ 619.2426;
found 619.2430.
rac-4,10-Bis(di-tert-butylphosphanyl)-2,8-dimethyl-6,12-dihydro-
5,11-methanodibenzo[b,f][1,5]diazocine (2b): THF (8 mL) was added
to a Schlenk flask charged with 1 (0.41 g, 1 mmol) under argon.
The mixture was cooled to –78 °C, and nBuLi (1.6 m in hexane,
1.5 mL, 2.4 mmol, 2.4 equiv.) was added dropwise to form a pale-
yellow clear solution. After 5 min, the flask was opened, CuCl
(0.198 g, 2.0 mmol) was quickly added, and the resulting suspen-
sion was stirred for 5 min after which di-tert-butylphosphane
(0.44 g, 2.4 mmol, 2.2 equiv.) was added in one portion. The re-
sulting mixture was warmed to ambient temperature within 60 min.
Stirring was continued at 70 °C for 36 h. The mixture was then
cooled to 0 °C, and BH3·THF (1 m in THF, 10 mL, 10 mmol) was
added. The reaction mixture was allowed to reach room tempera-
ture, and it was then stirred for an additional 6 h. The reaction
mixture was poured into a saturated aqueous solution of NH4Cl
(25 mL) under argon, and the resulting mixture was stirred for
5 min and then extracted with CH2Cl2 (3ϫ 75 mL). The combined
organic layers were filtered through a Whatman filter paper, dried
with MgSO4, and concentrated. The crude product was purified by
flash column chromatography [SiO2 (30 g), hexane/ethyl acetate
(4:1) containing 0.1% Et3N] to give borane-protected phosphane
2bЈ (500 mg, 88%) as a white solid. 1H NMR (400 MHz, CD2Cl2):
δ = 0.22–0.99 [br., 6 H, (BH3)2], 1.25 [d, 3J = 12.8 Hz, 18 H,
2
CH3), 4.09 (d, J = 17.0 Hz, 2 H, CH2N), 4.28 (s, 2 H, NCH2N),
4.63 (d, 2J = 17.0 Hz, 2 H, CH2N), 7.16 (d, 3JH,P = 9.2 Hz, 2 H, Ar-
3
H), 7.28 (d, JH,P = 8.2 Hz, 2 H, Ar-H) ppm. 13C NMR (75 MHz,
CD2Cl2): δ = 17.7 (Ar-CH3), 26.5, 26.7, 26.8, 26.9, 27.0, 27.1, 27.19,
27.22, 27.24, 27.33 27.36, 27.4, 31.6 (d, JC,P = 34.2 Hz), 31.9 (d,
JC,P = 33.9 Hz), 55.1 (CH2N), 67.5 (NCH2N), 120.6 (d, JC,P
=
49.4 Hz), 128.7 (d, JC,P = 10.4 Hz), 131.0 (d, JC,P = 10.3 Hz), 133.4
(d, JC,P = 5.7 Hz), 133.9 (d, JC,P = 8.6 Hz), 149.6 ppm. 31P NMR
(121.5 MHz, CD2Cl2): δ = 24.8 (br. s) ppm. HRMS (ESI): calcd.
for C41H67B2N2P2 [M + H]+ 671.4974; found 671.4966.
Prior to the catalytic test, borane-protected phosphane 2cЈ (200 mg,
0.3 mmol) was placed into a Schlenk flask under argon. Degassed
morpholine (3 mL) was added, and the flask was sealed. The reac-
tion mixture was stirred at 100 °C for 3 h. It was then cooled to
room temperature, and the excess amount of morpholine was re-
moved under reduced pressure. The residue was treated with de-
gassed ethanol and cooled to –78 °C. The clear yellow supernatant
was carefully removed by cannula filtration under argon. Degassed
CH2Cl2 and silica (1 g) were added to the white precipitate, and
the volatiles were removed. The residue was placed at the top of a
short silica column (5 g) and eluted with hexane/ethyl acetate (1:2)
+ 0.1% Et3N to give 2c (138 mg, 70%) as a white solid, which
3
(CH3)3], 1.26 [d, J = 12.8 Hz, 18 H, (CH3)3], 2.44 (s, 6 H, CH3),
4.08 (d, 2J = 17.0 Hz, 2 H, CH2N), 4.27 (s, 2 H, NCH2N), 4.62 (d,
3
2J = 17.0 Hz, 2 H, CH2N), 7.45 (d, JH,P = 8.0 Hz, 2 H, Ar-H),
3
7.59 (d, JH,P = 9.7 Hz, 2 H, Ar-H) ppm. 13C NMR (101 MHz,
CD2Cl2): δ = 17.7 (Ar-CH3), 29.1 [d, JC,P = 1.8 Hz, C(CH3)3], 29.2
[d, JC,P = 1.8 Hz, C(CH3)3], 33.4 [d, JC,P = 27.1 Hz, C(CH3)3], 33.5
[d, JC,P = 27.1 Hz, C(CH3)3], 55.3 (CH2N), 67.5 (NCH2N), 122.2
(d, JC,P = 46.2 Hz), 128.3 (d, JC,P = 10.3 Hz), 132.5 (br. s), 133.3
(d, JC,P = 8.4 Hz), 135.3 (br. s), 149.4 (d, JC,P = 2.5 Hz) ppm. 31P
NMR (162.1 MHz, CD2Cl2): δ = 43.2 (d, JB,P = 66.3 Hz) ppm.
HRMS (ESI): calcd. for C38H59B2N2P2 [M + H]+ 567.4345; found
567.4343.
1
was briefly checked by H NMR and 31P NMR spectroscopy and
immediately used in catalysis. 1H NMR (300 MHz, CD2Cl2): δ =
0.86–1.32 [m, 22 H, (C6H11)2], 1.52–1.84 [m, 22 H, (C6H11)2], 2.39
(s, 6 H, CH3), 4.02 (d, 3J = 16.9 Hz, 2 H, CH2N), 4.26 (s, 2 H,
Prior to the catalytic test, borane-protected phosphane 2bЈ
(260 mg, 0.46 mmol) was placed into a Schlenk flask under argon.
Degassed morpholine (5 mL) was added, and the flask was sealed.
The reaction mixture was stirred at 100 °C for 3 h. It was then
cooled to room temperature, and the excess amount of morpholine
was removed under reduced pressure. The residue was treated with
degassed ethanol and cooled to –78 °C. The clear yellow superna-
tant was carefully removed by cannula filtration under argon to
3
NCH2N), 4.57 (d, 3J = 16.8 Hz, 2 H, CH2N), 6.87 (d, JP, H
=
7.4 Hz, 2 H, Ar-H), 7.12 (d, JP,H = 6.3 Hz, 2 H, Ar-H) ppm. 31P
3
NMR (121.5 MHz, CD2Cl2): δ = 2.5 ppm.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures, characterization data and copies of
the NMR spectra.
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R. Pereira, J. Cvengrosˇ
SHORT COMMUNICATION
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Received: April 10, 2013
Published Online: ᭿
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