Ylide-Functionalized Phosphine (YPhos)-Palladium Catalysts: Selective Monoarylation of Alkyl Ketones with Aryl Chlorides

Article abstract of DOI:10.1021/acs.orglett.9b02830

Ylide-functionalized phosphine (YPhos) ligands allow the palladium-catalyzed α-arylation of alkyl ketones with aryl chlorides with record setting activity. Using a cyclohexyl-substituted YPhos ligand, a wide range of challenging ketone substrates was efficiently and selectively monoarylated under mild conditions. A newly designed YPhos ligand bearing tert-butyl groups on the coordinating phosphorus atom is already active at room temperature. The synthetic potential was demonstrated by gram-scale reactions and the succinct synthesis of ?-caprolactone derivatives.

Full text of DOI:10.1021/acs.orglett.9b02830

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Ylide-Functionalized Phosphine (YPhos)−Palladium Catalysts:
Selective Monoarylation of Alkyl Ketones with Aryl Chlorides
Xiao-Qiang Hu,§,† Dominik Lichte,† Ilja Rodstein,‡ Philip Weber,† Ann-Katrin Seitz,† Thorsten Scherpf,‡
Viktoria H. Gessner,*,‡ and Lukas J. Gooßen*,†
†
̈
̈
Faculty of Chemistry and Biochemistry, Evonik Chair of Organic Chemistry, Ruhr-Universitat Bochum, Universitatsstr. 150, 44801
Bochum, Germany
‡
̈
̈
Faculty of Chemistry and Biochemistry, Chair of Inorganic Chemistry II, Ruhr-Universitat Bochum, Universitatsstr. 150, 44801
Bochum, Germany
S
* Supporting Information
ABSTRACT: Ylide-functionalized phosphine (YPhos) ligands
allow the palladium-catalyzed α-arylation of alkyl ketones with
aryl chlorides with record setting activity. Using a cyclohexyl-
substituted YPhos ligand, a wide range of challenging ketone
substrates was efficiently and selectively monoarylated under mild
conditions. A newly designed YPhos ligand bearing tert-butyl
groups on the coordinating phosphorus atom is already active at
room temperature. The synthetic potential was demonstrated by gram-scale reactions and the succinct synthesis of ε-
caprolactone derivatives.
In this context, the coupling of cyclohexanone to aryl
chlorides is the ultimate performance test for any α-arylation
catalyst. The efficient monoarylation of cyclohexanones under
mild conditions has so far been reported only with aryl
bromides or imidazolyl sulfonates.4c,5a,17 In the case of aryl
chlorides, Buchwald reported a 4.9:1 mono-to-diarylation
selectivity and 70% yield for cyclohexanone with 2-
dicyclohexylphosphino-2′-methylbiphenyl at 80 °C,5b Beller
reported a high selectivity but only 38% yield using n-
butylbis(1-adamantyl)phosphine (nBuPAd2) at 100 °C,7 and
Nolan reported a high yield but moderate selectivity using a
bulky NHC ligand in refluxing THF.18a However, the catalysts
were tested only for cyclohexanone itself, not for substituted
derivatives or other cyclic ketones, and mostly only in
combination with simple chlorobenzene (Scheme 1b).
After many years of optimization, the mature ligand
structures described above set a standard that is hard to
match with any conceptually distinct ligand type. Nevertheless,
NHCs and phosphines seem to reach their inherent perform-
ance limits, and innovative concepts are vital for further
performance leaps.
he synthesis of α-aryl ketones is of substantial interest in
1
Tpharmaceutical research and natural product synthesis.
Semmelhack et al. pioneered the transition-metal-catalyzed α-
arylations of ketones in their synthesis of cephalotaxinone.2
Efficient catalysts were later developed featuring electron-rich,
sterically hindered phosphines or N-heterocyclic carbenes
(NHCs) in combination with palladium or nickel catalysts.3
Owing to contributions, for example, by Hartwig,4 Buchwald,5
Miura,6 Beller,7 and Nolan,8 the reaction concept was
advanced to a stage that is close to synthetic maturity. The
key advantage of Pd/Ni systems is that they permit the use of
inexpensive aryl chlorides. Aryl bromides or iodides can also be
converted with copper catalysts,9 and highly reactive aryl
substrates, such as hypervalent iodine reagents,10 benzynes,11
diazonium salts,12 nitroarenes,13 are amenable to metal-free
arylation strategies.
Despite the many advances made in this field,14 the coupling
of small, unbranched alkyl ketones to aryl chlorides still
remains a challenge, and even the most sophisticated catalyst
systems reach their performance limit (Scheme 1a). The main
obstacle is the particularly slow reductive elimination of aryl
enolates from Pd for sterically unhindered ketones.15 The
monoarylation products are inherently more reactive because
the additional aryl group facilitates the reductive elimination
step. Moreover, the additional aryl group increases the acidity
of any remaining α-C−H bonds, so that predominantly the
product molecules are deprotonated when using equimolar
amounts of base. The resulting formation of mixtures of
polyarylated products is a particular problem when elevated
temperatures are required to convert unreactive aryl chloride
substrates.5b,16
We recently disclosed a new class of ylide-functionalized
monophosphine ligands (YPhos).19 The ylide group induces
donor qualities that match or even exceed those of NHCs.
Metal−YPhos complexes are easily generated from bench-
stable phosphonium salts, which, in turn, are accessible in great
structural diversity from simple precursors. The first catalytic
applications of YPhos in Au-catalyzed hydroamination and
hydration reactions19b as well as Pd-catalyzed Buchwald−
Received: August 9, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02830
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Letter
a
Scheme 1. Transition-Metal-Catalyzed α-Arylation of Alkyl
Table 1. Optimization of the Arylation Reaction
Ketones with Aryl Chlorides
entry
catalyst
ligand
base
3aa (%)
1
2
3
4
5
6
7
8
Pd(COD)Cl2
Pd(COD)Cl2
Pd(COD)Cl2
Pd(COD)Cl2
Pd(COD)Cl2
Pd(COD)Cl2
Pd(COD)Cl2
Pd2dba3
L3
L4
L5
L6
L7
L1
L2
L1
L1
L1
L1
L1
L1
L1
L1
L2
L2
L2
L1
L2
L1
L2
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
NaOtBu
Cs2CO3
K3PO4
NaOH
NaOMe
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
n.d.
n.d.
n.d.
3
37
73
61
71
56
75 (68)
14
31
30
14
4
66
68
63
64
95
54
9
9
Pd(OAc)2
10
11
12
13
14
15
16
17
18
19
20
21
22
Pd(COD)Cl2
Pd(COD)Cl2
Pd(COD)Cl2
Pd(COD)Cl2
Pd(COD)Cl2
Pd(COD)Cl2
Pd(COD)Cl2
Pd2dba3
Pd2dba3
Pd(COD)Cl2
Pd2dba3
Pd(COD)Cl2
Pd2dba3
b
b
b
bc
,
d
bd
,
Hartwig aminations20 showcase the potential of this ligand
class.
e
be
,
The unusual activity of the catalysts can be explained by the
high donor strength of the YPhos ligands. With YPhos−Pd
complexes, oxidative additions of aryl chlorides have been
found to be facile even at low temperatures.20 In addition, for
YPhos derivatives with bulky substituents, highly reactive
monoligated Pd complexes are preferentially formed. These
properties gave us confidence that YPhos catalysts would
enable the α-arylation of carbonyl compounds with aryl
chlorides, whereas the modular tunability of the 3D ligand
structure would facilitate the design of selective monoarylation
catalysts (Scheme 1c).
a
Reaction conditions: 2.0 mmol 1a, 1.0 mmol 2a, 1 mol % [Pd], 2
mol % ligand, 1.5 equiv base, 2.0 mL of THF, 60 °C, 16 h. Yields were
determined by gas chromatography (GC) analysis using n-tetradecane
as an internal standard. Isolated yields are in parentheses. Reaction at
rt. Methoxycyclopentane (CPME) was used as solvent. 4-
b
c
d
e
Bromotoluene was used. 4-Iodotoluene was used.
(triclyclohexyl)ethylphosphonium iodide (Figure 1) and
isolated as a colorless solid in an overall yield of 60%. L2 is
To benchmark our YPhos ligands against the best known
catalysts, we chose the challenging arylation of cyclohexanone
1a with 4-chlorotoluene 2a as a test reaction. The temperature
was set to only 60 °C at a catalyst loading of 1 mol % (Table
1). At most, trace amounts of product were observed when
employing Pd(COD)Cl2 together with the benchmark ligands
nBuPAd2, BINAP, dppf, and Xantphos (entries 1−4). Solely 2-
di-tert-butylphosphino-2′-(N,N-dimethylamino)biphenyl (tBu-
DavePhos) gave the monoarylation product 3aa in 37% yield
Cy
(entry 5). To our delight, our YPhos ligand YMePCy2 (L1)
was more active than these state-of-the-art systems under the
same conditions. With L1, 3aa was obtained in >70% yield
along with only traces (<5%) of diarylation product (entries 6,
8, and 10).
Encouraged by the high activity of L1, we turned our
attention toward possible room-temperature applications. We
hypothesized that an even more electron-donating ligand
would allow for a further increase in activity and thus also
operate at room temperature. Hence, the tert-butyl analogue of
L1, CyYMePtBu2 (L2), was synthesized in one step starting from
Figure 1. Synthesis and structures of L2 and L2·Pd(dba).
characterized by two doublets in the 31P{1H} nuclear magnetic
resonance (NMR) spectrum with coupling constants of 146.9
Hz and is, with a buried volume (%Vbur) of 51.3%, sterically
more demanding than L1 (%Vbur = 45.2) and most common
phosphine ligands.19b
As hypothesized, L2 gave exclusively high yields of the
desired monoarylation product, even at room temperature,
B
DOI: 10.1021/acs.orglett.9b02830
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Scheme 2. Scope of Ketones and Aryl Chlorides
a
Reaction conditions: Method A: 2.0 mmol 1, 1.0 mmol 2, 1 mol % Pd(COD)Cl2, 2 mol % L1, 1.5 equiv NaOtBu, 2.0 mL of THF, 60 °C, 16 h,
isolated yields. Method B: 2.0 mmol 1, 1.0 mmol 2, 0.5 mol % Pd2(dba)3, 2 mol % L2, 1.5 equiv NaOtBu, 2.0 mL of THF, rt, 16 h, isolated yields.
b
c
d
e
f
Mono/diarylation 17:1. 10 mmol 1j, 1 mol % Pd2dba3, 1.5 equiv LiOtBu. 80 °C. 1.5 mmol 1, 0.5 mol % Pd(COD)Cl2, 1 mol % L1 at rt. 1 mol
g
h
% Pd(COD)Cl2, 2 mol % L1 was used, GC yield. Reaction at 60 °C. GC yield.
when using Pd2dba3 as the Pd source and NaOtBu as the base
(entry 17). This sets a new record for the coupling of the
challenging cyclohexanone substrate with an aryl chloride. To
the best of our knowledge, no efficient room-temperature
coupling has yet been accomplished for this substrate
combination with any other catalyst. At elevated temperatures,
L2 gave slightly lower yields than L1 due to competing catalyst
decomposition. Methoxycyclopentane (CPME), a “green”
alternative to THF, is also an effective solvent (entry 18).
We then tested the optimized protocols for other halides. 4-
Bromotoluene gave a moderate yield with L1 (entry 19) but a
near-quantitative yield with L2 (entry 20). Interestingly, 4-
iodotoluene gave unsatisfactory yields under these conditions
(entries 21 and 22).
The reaction with 1 mol % of Pd reached an average
turnover frequency (TOF) of 4.7 h−1 for L1 and an average
TOF of 4.3 h−1 for L2. At a catalyst loading of 0.5 mol % Pd,
turnover numbers (TONs) of 110 for L1 and 106 for L2 were
reached within 24 h. At 0.25 mol % Pd loading, a TON of 185
was recorded for L1.
groups of the phosphonium moiety is observed. This suggests
the presence of a stabilizing agostic interaction in the complex
analogous to L1. In THF solution, L2·Pd(dba) exhibits
remarkable stability, showing no decomposition reactions
within 1 week. The complex was isolated in 63% yield and
characterized by NMR spectroscopy. L2·Pd(dba) is charac-
terized by two doublets in the 31P{1H} NMR spectrum at 30.7
and 53.4 ppm (2JPP = 85.2 Hz).
To probe the scope of the new catalyst class, we used both
Cy
the robust YMePCy2-based protocol A (1 mol % Pd(COD)-
Cl2, 2 mol % CyYMePCy2, 1.5 equiv NaOtBu, 2.0 mL of THF at
60 °C) and the highly efficient but sensitive CyYMePtBu2-based
protocol B (0.5 mol % Pd2dba3, 2 mol % CyYMePtBu2, 1.5 equiv
NaOtBu, 2.0 mL of THF at rt). As shown in Scheme 2, a wide
range of alkyl and aryl ketones were selectively monoarylated
in the reaction with p-chlorotoluene in moderate to good yield.
The main side reaction was protodechlorination, whereas
arylated aldol condensation products were observed as minor
components only in some cases (e.g., for 3aa 2-(1-cyclohexen-
1-yl)-6-(4-methylphenyl)-cyclohexanone), and diarylation oc-
curred only in traces. Remarkably, substituted cyclohexanones
were smoothly converted (3ba−ea). Cyclic ketones with larger
ring sizes all the way up to cyclopentadecanone also gave good
results (3fa−ia). After adapting the reaction conditions
(LiOtBu, 80 °C), even acetone, another challenging sub-
To gain insight into the structure of the Pd catalyst with L2,
we attempted its isolation. Fortunately, we were able to grow
single crystals of the Pd(dba) complex from a solution of the
ligand and Pd2dba3. In the structure (Figure 1), a short C−H···
Pd distance (Pd−C 2.974(4) Å) to one of the cyclohexyl
C
DOI: 10.1021/acs.orglett.9b02830
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Organic Letters
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strate,21 was smoothly converted to exclusively give the
monoarylation product 3ja in 46% isolated yield. Aryl and
heteroaryl ketones are also suitable substrates (3ka−ma).
Notably, phenyl ethyl ketone 1l was efficiently already
converted at room temperature with 0.5 mol % catalyst. This
compares favorably with literature protocols, which call for
elevated temperatures (50−130 °C) or higher catalyst
loadings.1b,22
the best yields at 60 °C. The tert-butyl analogue L2 is active
already at room temperature. It appears plausible that the
systematic variation of the ligand backbone will enable the
design of ligands that combine the high reactivity of L2 with
the robustness of L1.
ASSOCIATED CONTENT
* Supporting Information
■
S
The scope of the α-arylation of cyclohexanone with respect
to aryl chlorides is also outlined in Scheme 2. Aryl chlorides
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02830.
t
bearing electron-donating (Me, Bu, OMe) and electron-
withdrawing (F) substituents in the ortho-, meta-, and para-
positions were successfully converted (3ab−aj), and various
sensitive substituents were tolerated (3al,am). The successful
conversion of highly electron-rich aryl chlorides confirms that
the oxidative addition step is strongly facilitated by the YPhos
ligands. Sterically crowded ortho-substituted chloroarenes and
even 9-chloroanthracene were successfully converted, albeit in
only moderate yield (3ag,ah,ak). Heteroaryl chlorides also
gave good results (3an,ao,ap).
The arylation of phenyl ethyl ketone 1l is also broadly
applicable with regard to the aryl chloride coupling partner.
High yields are obtained, even when reducing the catalyst
loading to 0.5 mol %.
Overall, protocol A based on the more robust ligand L1
seems to be more generally applicable and higher yielding.
However, the activity of L2 at room temperature is certainly
impressive. Further ligand refinement is required to identify a
ligand that combines the record-setting activity of L2 with the
robustness of L1. The preparative utility of the new protocol
was demonstrated by gram-scale arylations forming 3aa and
3la (Scheme 3a,b). Scheme 3c demonstrates that a reaction
Experimental procedures, full analysis data for new
compounds, and copies of NMR spectra (PDF)
Accession Codes
CCDC 1920473−1920475 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: viktoria.gessner@rub.de.
*E-mail: lukas.goossen@rub.de.
ORCID
Lukas J. Gooßen: 0000-0002-2547-3037
Present Address
§X.-Q.H.: Key Laboratory of Catalysis and Materials Sciences
at the Ministry of Education, South-Central University for
Nationalities, Wuhan 430074, P. R. China.
Scheme 3. Synthetic Application of This Method
Author Contributions
All authors have given approval to the final version of the
manuscript.
Notes
The authors declare the following competing financial
interest(s): The authors hold patent WO2019030304 covering
YPhos ligands L1 and L2.
ACKNOWLEDGMENTS
■
Funded by the Deutsche Forschungsgemeinschaft (DFG,
German Research Foundation) under Germany’s Excellence
Strategy − EXC-2033 − Projektnummer 390677874, the
European Research Council (Starting Grant: YlideLigands
677749), and BMBF and the state of NRW (Center of
Solvation Science “ZEMOS”). We thank UMICORE for
̈
donating chemicals and Florian Papp and Julian Loffler for
technical support.
sequence consisting of an α-arylation and Baeyer−Villiger
oxidation gives easy access to ε-caprolactone derivatives such
as 4.23 This opens up a convenient entry to modified poly-ε-
caprolactone (PCL), a polymer with various pharmaceutical
and biomedical applications.24
In summary, ylide-functionalized phosphine ligands were
shown to be highly efficient in the Pd-catalyzed α-arylation of
ketones.
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