Electron-rich, bicyclic biaryl-like KITPHOS monophosphines via [4+2] cycloaddition between 1-alkynylphosphine oxides and anthracene: Highly efficient ligands for palladium-catalysed C-N and C-C bond formation

Article abstract of DOI:10.1002/adsc.200800307

Electron-rich, bicyclic biaryl-like KITPHOS monophosphines have been prepared via Diels-Alder cycloaddition between 1-alkynylphosphine oxides and anthracene in an operationally straightforward and highly modular synthetic protocol that will allow access to an architecturally and electronically diverse family of ligands. Palladium complexes of these ligands are highly efficient catalysts for the Buchwald-Hartwig amination and Suzuki-Miyaura coupling of a wide range of aryl chlorides, which for the vast majority of substrate combinations outperform their o-(dicyclohexylphosphino) biphenyl-based counterparts.

Full text of DOI:10.1002/adsc.200800307

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DOI: 10.1002/adsc.200800307
Electron-Rich, Bicyclic Biaryl-Like KITPHOS Monophosphines
via [4+2]Cycloaddition between 1-Alkynylphosphine Oxides and
Anthracene: Highly Efficient Ligands for Palladium-Catalysed
À
À
C N and C C Bond Formation
Simon Doherty,a,* Julian G. Knight,a,* Catherine H. Smyth,a,*
and Graeme A. Jorgensona
a
School of Natural Sciences, Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne, NE1 7RU, U.K.
Fax : (+44)-191-222-6929; e-mail: simon.doherty@ncl.ac.uk
Received: May 17, 2008; Published online: July 24, 2008
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: Electron-rich, bicyclic biaryl-like KIT-
PHOS monophosphines have been prepared via
Diels–Alder cycloaddition between 1-alkynylphos-
phine oxides and anthracene in an operationally
straightforward and highly modular synthetic proto-
col that will allow access to an architecturally and
electronically diverse family of ligands. Palladium
complexes of these ligands are highly efficient cata-
lysts for the Buchwald–Hartwig amination and
Suzuki–Miyaura coupling of a wide range of aryl
chlorides, which for the vast majority of substrate
combinations outperform their o-(dicyclohexylphos-
phino)biphenyl-based counterparts.
Figure 1. Biaryl monophosphines 1 and 2 and KITPHOS
monophosphines.
wide range of aryl chlorides, often at room tempera-
ture and at very low catalyst loadings.[3] The efficiency
of these catalyst systems has been attributed to a
combination of factors, the first of which is their
highly electron-rich character which facilitates oxida-
tive addition of less reactive aryl chlorides.[3a,b] Recent
studies also suggest that the steric bulk of these biaryl
phosphines is critical to achieving high activity by pro-
moting the formation of the monophosphine com-
plex.[4] Finally, computational and experimental stud-
ies reveal that a weak interaction between palladium
Keywords: aminations; aryl chlorides; biaryl-like
compounds; electron-rich species; phosphines;
Suzuki–Miyaura coupling
À
À
Palladium-catalysed C C and C heteroatom bond and the ortho-carbon atom of the non-phosphine-con-
formation has evolved into an exceptionally powerful taining ring of the ligand stabilises the catalyst when
tool which has found widespread use in many areas of the palladium is not involved in a step within the cat-
organic synthesis.[1] While the first catalysts for these alytic cycle.[5]
transformations were typically based on a source of
As palladium-catalysed aminations and cross-cou-
palladium and either a triarylphosphine or a chelating plings are pivotal to the synthesis of important inter-
diphosphine such as 2,2’-bis(diphenylphosphino)-1,1’- mediates and building blocks for the fine chemicals
binaphthyl (BINAP) or 1,1’-bis(diphenylphosphino)- industry there is immense interest in using this design
ferrocene (dppf), high temperatures were often re- concept to develop alternative ligands that are easy to
quired to achieve acceptable levels of efficiency and prepare, highly modular and cost effective.[6] Ideally,
they were generally unreactive towards aryl chloride these ligands should have a broad substrate scope, be
substrates.[2] However, recent advances in catalyst de- capable of activating chloride-based substrates under
velopment have led to the discovery that electron-rich mild conditions, facilitate the coupling of hindered
biaryl monophosphines such as 1 and 2, and their de- substrates and allow for scale-up and process develop-
rivatives (Figure 1), form highly efficient catalysts for ment. These monophosphines are typically prepared
À
À
À
C N as well as C C and C O bond formation with a either by modification of an existing biaryl precursor
Adv. Synth. Catal. 2008, 350, 1801 – 1806
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Simon Doherty et al.
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or via addition of an aryl-Grignard reagent to a ben-
zyne intermediate.[3e,7] Recently though, cycloaddition
methodology has evolved as a potentially versatile ap-
proach for constructing the biaryl fragment in mono-
phosphines. For example, Diels–Alder cycloaddition
between 1-alkynylphosphine oxides and various 1,3-
dienes[8] as well as rhodium-catalysed [2+2+2]cyclo-
addition of tethered diynes with 1-alkynylphosphine
sulfides have both been used to prepare biaryl-based
monophosphines.[9] In this regard, we have recently
used the double Diels–Alder cycloaddition between
1,4-bis(diphenylphosphinoyl)buta-1,3-diyne and an-
thracene to construct the skeletal framework of the
biaryl-like diphosphine CATPHOS, which forms a
highly efficient catalyst for the Buchwald–Hartwig
amination of a wide range of aromatic bromides as
well as the a-arylation of ketones.[10] This approach
Scheme 1. Synthesis of electron-rich biaryl-like monophos-
phines 6a–c.
has now been extended to include the corresponding are currently underway to develop an alternative
[4+2]cycloaddition between 1-alkynylphosphine more efficient synthesis of 5c. Reduction of these
oxides and anthracene to generate an entirely new phosphine oxides was achieved by heating a toluene
family of electron-rich biaryl-like monophosphine, solution of 5a–c, trichlorosilane and triethylamine at
KITPHOS. These ligands form highly active catalysts 1108C for 48 h to afford the desired KITPHOS mono-
for the Buchwald–Hartwig amination and Suzuki– phosphines 6a–c (Scheme 1).[12]
Miyaura coupling of a range of aryl chlorides which,
Since palladium(0) complexes of electron-rich
for the vast majority of substrate combinations, out- biaryl monophosphines are highly efficient catalysts
perform their o-(dicyclohexylphosphino)biphenyl- for the amination of bromides, chlorides and trif-
based counterparts. Key features of these KITPHOS
lates[3b] as well as Suzuki coupling reactions[3c,e,13] of a
ACHTREUNG
monophosphines include their operationally straight- wide range of aryl and heteroaryl compounds, often
forward synthesis in good to excellent yield from in- at low catalyst loadings, we chose these two reactions
expensive starting materials and their highly modular with which to undertake a comparative study. Prelimi-
synthesis which allows the substitution pattern of the nary reactions focused on the amination of a range of
1-alkynylphosphine oxide aryl group to be systemati- aryl and heteroaryl bromides with primary and secon-
cally varied, the steric bulk and basicity of the phos- dary amines [Eq. (1)] using Pd(0)/6a (1.0 mol% Pd/
phino group to be fine-tuned, and the biaryl-like frag-
ment to be modified by performing the cycloaddition
with a substituted anthracene or its equivalent.
Our interest in KITPHOS monophosphines 6a–c
began after recognising the close similarity of their
basic structural architecture with that of the dialkyl-
phosphanylbiphenyls developed by Buchwald and co- 2.5 mol% phosphine) in toluene at 808C with NaO-t-
workers. Monophosphines 6a–c were prepared from Bu, full details of which are given in Table 1. Under
the corresponding 1-alkynylphosphines oxides 3a–c in these conditions, Pd(0)/6a catalysed the amination of
three experimentally easy steps, according to the pro- a range of electron-deficient and electron-rich sub-
cedure outlined in Scheme 1. 1-Alkynylphosphine strates as well as sterically hindered 2,6-dimethylbro-
oxides 4a–c were prepared by reaction of chlorodicy- mobenzene, the latter achieving 70% conversion after
clohexylphosphine with the corresponding 1-lithio-1- 2 h compared with 50% when catalysed by Pd(0)/1, in
alkyne, generated either from the terminal alkyne the same time (entries 19–20). Notably, for the vast
(Y=H) or via desilylation (Y=SiMe3), followed by majority of substrate combinations tested Pd(0)/6a
oxidation prior to work-up and purification.[11] The either rivalled or outperformed its biphenyl counter-
biaryl-like skeleton was constructed via the Diels– part Pd(0)/1, in our laboratory under the same condi-
Alder reaction between 1-alkynylphosphine oxides tions. The difference in performance between the cat-
4a–c and anthracene, which gave 5a and b in excellent alysts based on 6a and 1 was most evident in the ami-
yield after purification. Unfortunately, competing N- nation of electron-deficient substrates particularly for
demethylation during the reaction between 4c and an- the reaction between 1-bromo-4-chlorobenzene and
thracene resulted in the formation of a significant aniline (entries 1 and 2). Bromopyridine substrates
amount of 4d and 5d as by-products and as a result 5c also proved to be effective coupling partners with the
could only be isolated in poor yield. Further studies amination between 2-bromopyridine and morpholine
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Electron-Rich, Bicyclic Biaryl-Like KITPHOS Monophosphines
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Table 1. Palladium-catalysed amination of aryl bromides using ligands 6a and 1.[a]
Entry
L
Aryl bromide
Amine
Time [h]
Conversion [%][b]
1
2
3
4
5
6
7
8
6a
1
6a
1
6a
1
6a
1
6a
1
6a
1
6a
1
6a
1
6a
1
6a
1
3
3
2
3
1
1
1
1
1
1
1
1
6
23
1
1
2
1
2
2
94
36
62
37
100
97
99
99
80
77
90
77
40
30
100
76
68
99
70
50
9
10
11
12
13
14
15
16
17
18
19
20
[a]
Reaction conditions: 1.0 equiv. of Ar-Br, 1.1 equiv. of amine, 1.4 equiv. of NaO-t-Bu, 0.5 mol% Pd
2ACHTRE(UNG dba)3, 2.5 mol% 6a or
1, toluene , 808C.
[b]
Conversions determined by GC analysis of the reaction mixture and based on aryl bromide. Average of three runs.
reaching completion within 1 h with Pd(0)/6a com- chlorobenzene and pyrrolidine which reached 97%
pared to only 76% conversion for its biphenyl coun- conversion with Pd(0)/6c compared with only 44% for
terpart (entries 15 and 16).
the Pd(0)/6a system. Similarly, Pd(0)/6c gave com-
Having firmly established the efficacy of KITPHOS plete conversion for the amination of 2,6-dimethyl-
monophosphine 6a in the palladium-catalysed amina- chlorobenzene with morpholine while Pd(0)/6a gave
tion of aryl bromides, catalyst testing was extended to only 52% conversion in the same time.
include chloride substrates. Gratifyingly, under our
The encouraging performance of Pd(0)/6a catalyst
conditions, Pd(0)/6a catalysed the amination of a for the amination of aryl chlorides prompted us to
range of aryl chlorides with a variety of amine cou- extend our studies to include Suzuki–Miyaura cou-
pling partners including cyclic secondary alkylamines, plings [Eq. (2)], a reaction that is efficiently catalysed
secondary anilines and primary alkylamines, in the
majority of cases giving good to excellent conversions
at 1008C with 1 mol% palladium (Table 2). As for the
aryl bromides described above, Pd(0)/6a consistently
outperformed its biaryl counterpart for all substrate
combinations tested, in some cases by a significant
margin. With the aim of developing a family of KIT- by systems based on either Pd2ACTHERNGU(dba)3 or PdACHTREUNG(OAc)2
PHOS monophosphines capable of transforming a and a biaryl monophosphine. Following a protocol de-
wide range of substrate combinations, the perfor- scribed by Buchwald, the reaction between phenyl-
mance of catalysts generated from 6a–c were com- boronic acid and a range of aryl chlorides was cata-
pared against the corresponding systems based on 1 lysed by PdACTHRE(UNG OAc)2/6a, b (Pd:L, 1:2.5) in either THF
and 2, for the amination of selected aryl chlorides. or toluene at 40–808C, details of which are presented
Table 3 reveals that the introduction of an ortho-OMe in Table 4. Under these conditions, good to excellent
or -NMe2 substituent results in a significant and in conversions were obtained for each substrate combi-
some cases substantial enhancement in catalyst per- nation, including those involving electron-donating
formance, with all four substrate combinations giving and heteroaryl partners. While activated aryl chlor-
markedly higher conversions for catalysts derived ides typically gave good conversions at 408C, elec-
from 6b and 6c compared to their unsubstituted coun- tron-rich and sterically demanding substrates required
terpart. The beneficial influence of the ortho substitu- slightly higher temperatures (60–808C) to reach com-
ent is particularly evident in the reaction between parable conversions. Based on these preliminary stud-
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Simon Doherty et al.
Conversion [%][b]
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Table 2. Palladium-catalysed amination of aryl chlorides using ligands 6a and 1.[a]
Entry
L
Aryl chloride
Amine
Time [h]
1
2
3
4
6a
1
6a
1
24
24
24
24
6
71
59
48
32
5
6a
98
6
1
6
89
7
8
6a
1
2
2
>99
95
9
6a
1
6a
1
6a
1
6a
1
6a
1
6a
1
2
2
2
6
1
1
1
1
6
6
6
6
96
69
10
11
12
13
14
15
16
17
18
19
20
100
23
100
100
93
10
96
40
52
nBuNH2
47
[a]
Reaction conditions: 1.0 equiv. of Ar-Cl, 1.1 equiv. of amine, 1.4 equiv. of NaO-t-Bu, 0.5 mol% Pd
1, toluene, 1008C.
Conversions determined by GC analysis of the reaction mixture and based on aryl chloride. Average of three runs.
2ACHTRE(UNG dba)3, 2.5 mol% 6a or
[b]
Table 3. Comparison of the palladium-catalysed amination of aryl chlorides using ligands 6a–c[a]
Entry
L
Aryl chloride
Amine
Time [h]
Conversion [%][b]
1
2
3
5
6
7
9
10
11
13
14
15
6a
6b
6c
6a
6b
6c
6a
6b
6c
6a
6b
6c
6
6
6
1
1
1
1
1
1
6
6
6
44
62
97
72
87
77
64
76
73
52
89
100
[a]
Reaction conditions: 1.0 equiv. of Ar-Cl, 1.1 equiv. of amine, 1.4 equiv. of NaO-t-Bu, 0.5 mol% Pd
toluene, 1008C.
Conversions determined by GC analysis of the reaction mixture and based on aryl chloride. Average of three runs.
2ACHTRE(UNG dba)3, 2.5 mol% 6a–c,
[b]
ies a moderately hindered, more challenging combina- nations while 6b is more efficient for the less hindered
tion was examined; the reaction between 2-methyl- substrates.
phenylboronic acid and 1-chloro-2,6-dimethylbenzene.
In conclusion, we have developed a modular and
Gratifyingly, Pd(0)/6a catalysed this reaction at 808C tunable family of biaryl-like monophosphines KIT-
to give 85% conversion after 16 h, which is a signifi- PHOS that are operationally straightforward to pre-
cant improvement on the corresponding system de- pare, relatively inexpensive and air-stable and which
rived from 1 which required 17 h to reach 88% con- form highly active catalysts for the amination and
version at 1008C.[3c] Within this limited study KIT- Suzuki coupling of a range of aryl chlorides. Compa-
PHOS monophosphine 6a appears to be the ligand of rative catalyst testing revealed that for the majority of
choice for sterically more demanding substrate combi- substrate combinations tested this new system either
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Electron-Rich, Bicyclic Biaryl-Like KITPHOS Monophosphines
Table 4. Palladium-catalysed Suzuki–Miyaura coupling of aryl chlorides using ligands 6a and 6b.[a,b]
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Entry
L
Aryl chloride
Boronic acid
Time [h]
Temperature [8C]
Conversion [%][c]
1[a]
6a
6b
6a
6b
6a
6b
6a
6b
6a
6b
6a
6b
6
6
6
6
6
6
6
6
6
6
6
6
80
80
80
80
40
40
40
40
50
50
60
60
84
98
84
88
61
100
87
>99
96
100
83
2[a]
3[a]
4[a]
5[b]
6[b]
7[b]
8[b]
9[b]
10[b]
11[a]
12[a]
78
13[b]
6a
6b
6
6
50
50
>99
96
14[b]
15[b]
16[b]
6a
6b
6
6
40
40
>99
>99
17[b]
6a
6b
6
6
50
50
98
100
18[b]
19[a]
20[a]
6a
6b
16
16
80
80
85
52
[a]
Reaction conditions: 1.0 equiv. of Ar-Cl, 1.5 equiv. of ArB(OH)2, 2.0 equiv. of K3PO4, 1.0 mol% Pd
or 6b, toluene (2 mL).
1.0 equiv. of Ar-Cl, 1.5 equiv. of ArB(OH)2, 3.0 equiv. of KF, 1.0 mol% PdACHTRE(UGN OAc)2, 2.5 mol% 6a or 6b, THF (2 mL).
ACHTRE(UNG OAc)2, 2.5 mol% 6a
[b]
[c]
Conversions determined by GC analysis of the reaction mixture and based on aryl chloride. Average of three runs.
General Experimental Procedure for the Reduction
of Monophosphine Oxides 5a–c
competes with or outperforms its well-established
biaryl monophosphine counterpart. Further studies
are currently underway to (i) optimise catalyst perfor-
mance, (ii) explore the substrate scope and applica-
tions of this new class of phosphine, particularly with
respect to more challenging transformations, (iii) de-
velop a structure-activity relationship by varying the
substitution pattern of the biaryl-like aryl ring, the
nature of the anthracene fragment and the phosphino
group and (iv) develop a chiral version of KITPHOS
for use in asymmetric catalysis.
A
flame-dried Schlenk flask was charged with 5a–c
(1.34 mmol), toluene (25 mL), and triethylamine (7.5 mL,
53.6 mmol). Trichlorosilane (1.35 mL, 13.4 mmol) was added
slowly and the mixture heated at 1108C for 3 days. The reac-
tion mixture was diluted with diethyl ether (20 mL) and
added slowly to a mixture of ice (10 g) and 20% aqueous
NaOH (20 mL). After stirring vigorously at room tempera-
ture for 30 min, the organic layer was removed and the
aqueous phase extracted with diethyl ether (3”30 mL). The
organic fractions were combined, washed with saturated
NaHCO3 (2”20 mL), water (2”20 mL) and brine (2”
20 mL), dried over MgSO4, filtered and the solvent removed
under vacuum. The product was purified by column chroma-
tography eluting with hexane/ethyl acetate (9:1) to afford
6a–c as spectroscopically pure white solids in 69% (0.44 g,
6a), 67% (0.455 g, 6b) and 65% (0.45 g, 6c) yield. In each
case a spectroscopically and analytically pure sample was
obtained by slow diffusion of a chloroform solution layered
with methanol at room temperature.
Experimental Section
General Experimental Procedure for the Synthesis of
Monophosphine Oxides 5a–c
1-Alkynylphosphine oxide 4a–c (4.00 mmol) and anthracene
(1.07 g, 6.00 mmol) were mixed in a flask which was gradual-
ly heated to 2208C using a Woodꢁs metal bath. The tempera-
ture was then lowered to 2008C and the mixture heated for
a further 12 h. The resulting dark solid residue was purified
by column chromatography eluting with CH2Cl2/ethyl ace-
tate (3:2) to afford 5a–c as off-white solids in 80% (1.57 g,
5a), 83% (1.73, 5b) and 24% (0.51 g, 5c) yield.
Supporting Information
General comments, synthesis and characterisation data for
alkynylphosphine oxides 4a–c, synthesis and characterisation
data for monophosphine oxides 5a–c, reduction of biaryl di-
phosphines 5a–c and characterisation data for 6a–c, general
procedure for the amination of aryl halides, general proce-
Adv. Synth. Catal. 2008, 350, 1801 – 1806
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Simon Doherty et al.
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dures for the Suzuki–Miyaura coupling of aryl chlorides, and
1H, 31P and 13C NMR spectra for compounds 4a–c, 5a–c and
6a–c are available as Supporting Information.
Walker, J. R. Martinelli, S. L. Buchwald, J. Am. Chem.
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7235.
Acknowledgements
We gratefully acknowledge the EPSRC for funding (CHS,
Newcastle University) and Johnson Matthey for generous
loans of palladium salts.
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