Ruthenium complexes of κ(P)-and κ(P)-η6- coordinated KITPHOS monophosphines: Efficient catalysts for the direct ortho arylation of 2-phenylpyridine and N-phenylpyrazole with aryl chlorides

Article abstract of DOI:10.1021/om200843f

Thermolysis of the P-coordinated KITPHOS monophosphine complex [(p-cymene)RuCl₂(KITPHOS)] in chlorobenzene at 120 °C resulted in displacement of the p-cymene to afford [{κ(P)-η ⁶-KITPHOS} RuCl₂], the first example of a con

Full text of DOI:10.1021/om200843f

Article
pubs.acs.org/Organometallics
Ruthenium Complexes of κ(P)- and κ(P)-η6-Coordinated KITPHOS
Monophosphines: Efficient Catalysts for the Direct Ortho Arylation of
2-Phenylpyridine and N-Phenylpyrazole with Aryl Chlorides
Simon Doherty,* Julian G. Knight, Carrie R. Addyman, Catherine H. Smyth, Nicholas A. B. Ward,
and Ross W. Harrington
School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne NE1 7RU, U.K.
S
* Supporting Information
ABSTRACT: Thermolysis of the P-coordinated KITPHOS mono-
phosphine complex [(p-cymene)RuCl2(KITPHOS)] in chloroben-
zene at 120 °C resulted in displacement of the p-cymene to afford
[{κ(P)-η6-KITPHOS}RuCl2], the first example of a constrained-
geometry complex in which the κ(P)-bonded diphenylphosphino
group and the η6-coordinated proximal phenyl ring are connected
by an unsaturated two-carbon tether; both complexes form efficient
catalysts for the direct ortho arylation of 2-phenylpyridine and N-phenylpyrazole with a range of aryl chlorides.
rguably, since their introduction electron-rich biaryl
monophosphines such as 1 and 2 have evolved into one
We have also embraced this basic design concept and
recently introduced a new class of electron-rich biaryl-like
KITPHOS monophosphine (8),16 which bears a close
architectural similarity to Buchwald’s biaryl monophosphines
in that there is a PR2 group connected to a carbon−carbon
double bond, albeit part of an anthracene-derived bicyclic
framework, and a non-phosphine-containing proximal aryl ring,
which can be systematically modified as it is derived from a 1-
alkynylphosphine oxide. Gratifyingly, in addition to being an
architectural analogue, electron-rich KITPHOS monophos-
phines either rivaled or outperformed their biaryl-based
counterparts in palladium-catalyzed Suzuki−Miyaura and
Buchwald−Hartwig cross couplings16 as well as a host of
gold-catalyzed intramolecular cyclizations.17 The latter studies
also revealed that electrophilic gold(I) complexes of
diphenylphosphino-based KITPHOS monophosphines were
more efficient than their dicyclohexyl-based counterparts for
the 5-exo-dig cycloisomerization of a range of propargyl amides,
to afford the corresponding alkylidene oxazolines.17a Extending
this analogy further, diphenylphosphino-substituted KITPHOS
monophosphines would be potential surrogates for PPh2-based
biaryl monophosphines, a class of ligand that has received far
less attention than its dialkylphosphino counterpart (vide
supra), primarily because they are less electron-rich and lack
steric bulk, properties that are commonly considered necessary
to achieve efficient palladium-catalyzed C−C and C−
heteroatom bond formation.
A
of the most versatile and efficient classes of ligand for
transition-metal-catalyzed C−C and C−heteroatom bond
formation; examples include Suzuki−Miyaura,1 Negishi,2 and
Hiyama3 cross couplings and Buchwald−Hartwig amination,4
borylation,5 silylation,6 etherification,7 and direct arylation8 as
well as a diverse range of gold-catalyzed cyclizations and
cycloisomeriszations.9 The biaryl framework appears to be
integral to the success of these ligands and as such has been
used as a lead architecture for developing alternative systems
such as cataCXium P-ligand 3,10 indolyl-based monophosphines
4 and 5,11 BippyPhos 6,12 and most recently BI-DIME 7.13
Interestingly though, a PPh2-based biaryl-like phosphine has
recently been reported to outperform its bulkier, electron-rich
counterpart for a host of challenging cross couplings,14 and
electron-deficient triarylphosphines have been reported to be
highly effective ligands for the direct arylation of heterocycles
with aryl iodides, substrates that had proven to be particularly
unreactive under broadly applicable conditions.15 These
examples should encourage further investigations to explore
and develop the use of bulky electron-deficient phosphines in
palladium-catalyzed transformations.
Interested in exploring the extent to which diphenylphos-
phino-based KITPHOS monophosphines could be surrogates
for conventional biaryl-like or triaryl monophosphines, we have
been investigating their ruthenium-based coordination chem-
Received: September 7, 2011
Published: October 14, 2011
© 2011 American Chemical Society
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Scheme 1
istry relevant to the direct arylation of (hetero)arenes and
discovered that the KITPHOS monophosphine 8 (R = Ph,
PPh2-(Ph)-KITPHOS) can coordinate either in a conventional
κ(P) manner or as a κ(P)-η6 eight-electron donor, tethered
through the proximal phenyl ring of the biaryl-like fragment to
form a constrained-geometry complex and that both form
efficient catalysts for the direct ortho arylation of 2-phenyl-
pyridine and N-phenylpyrazole. To this end, the ruthenium-
catalyzed arylation and alkylation of 2-aryl-substituted hetero-
(arenes) is rapidly emerging as a powerful and synthetically
versatile tool for C−H bond functionalization.18 While early
developments in this area involved ruthenium−arene precata-
lysts based on triphenylphosphine,19 air-stable secondary
phosphine oxides,20 electron-rich biaryl monophosphines,21
N-heterocyclic carbene-tethered arenes,22 and alkylidenes23
have proven to be efficient ligands for direct arylation and more
recently well-defined ligand-free carboxylato ruthenium(II)
systems have been shown to be competent catalysts; these
latter systems appear to operate via a concerted carboxylate-
assisted C−H activation.24
Figure 1. Molecular structure of [(p-cymene)Ru(κ(P)-{11-(diphenyl-
phosphino)-12-phenyl-9,10-dihydro-9,10-ethanoanthracene})Cl2] (9).
Hydrogen atoms and the chloroform molecule of crystallization have
been omitted for clarity. Ellipsoids are at the 40% probability level.
RESULTS AND DISCUSSION
■
ment at ruthenium. The p-cymene ring is essentially planar with
maximum and mean deviations from the least-squares mean
plane of the six carbon atoms of 0.0137(18) and −0.0028(19)
Å, respectively.
Addition of the KITPHOS monophosphine 8 (R = Ph) to a
dichloromethane solution of [(p-cymene)RuCl2]2 resulted in
25
cleavage of the dimer to afford [(p-cymene)Ru{PPh2-(Ph)-
KITPHOS}Cl2] (9), which was isolated in near-quantitative
yield as a pale orange solid (Scheme 1). Although 31P, 1H, and
13C NMR spectra, the electrospray mass spectrum, and
analytical data were all consistent with κ(P) coordination of
the KITPHOS monophosphine, the identity of 9 was
conclusively established by a single-crystal X-ray study; a
perspective view of the molecular structure is shown in Figure
1.
Figure 1 clearly shows that the ruthenium atom adopts a
pseudo-octahedral geometry with the p-cymene, two chloride
ligands, and the κ(P)-KITPHOS monophosphine completing
the coordination sphere. The Ru−C(arene) bond lengths fall
between 2.144(3) and 2.243(3) Å, and the range of Ru−
C(arene) distances of 2.145(2)−2.242(2) Å is within the range
reported for related complexes such as [(p-cymene)Ru-
(PPh2py)Cl2] (2.214(2) Å),26 [(p-cymene)Ru(PPh2H)Cl2]
(2.198(4) Å),26 [(p-cymene)Ru(dppv)Cl2] (2.21(3) Å),27
and [(p-cymene)Ru(PPh3)Cl2] (2.218(2) Å).28 The Ru(1)−
P(1) bond length of 2.3527(6) Å is unexceptional and is similar
to those in [(p-cymene)Ru{o-(diphenylphosphino)aniline)}-
Cl2] (2.376(2) Å),29 [(p-cymene)Ru(PPh3)Cl2] (2.3438(6)
Å),2 8 and [(p-cymene)Ru{(1,2-diphenylvinyl)-
diphenylphosphine}Cl2] (2.3632(11) Å)30 as are the Ru(1)−
Cl(1) and Ru(1)−Cl(2) bond lengths of 2.4038(6) and
2.4106(6) Å, respectively. The Cl(1)−Ru(1)−Cl(2) angle of
89.60(2)° and the P(1)−Ru(1)−Cl(1) and P(1)−Ru(1)−
Cl(2) angles of 87.08(2) and 89.34(2)°, respectively, are close
to those in [(p-cymene)Ru{(1,2-diphenylvinyl)-
diphenylphosphine}Cl2],30 suggesting a similar steric environ-
Interestingly, the 31P NMR spectrum of the solution isolated
after crystallization of 9 contained two signals, one at δ 23.9
corresponding to 9 and a low-field signal at δ 49.6, which was
not present prior to crystallization. Reasoning that 9 was slowly
interconverting to a thermodynamically more stable product, a
chlorobenzene solution was heated at 120 °C for approximately
16 h; after this time analysis of the reaction mixture by 31P
NMR spectroscopy revealed that 9 had cleanly and
quantitatively converted into a single product, 10, correspond-
ing to the species at δ 49.6. Our formulation that 10 was the
product of an intramolecular displacement of the p-cymene
fragment by the proximal phenyl ring of the KITPHOS
monophosphine was initially based on the absence of any
distinctive signals associated with the methyl and methine
groups of the p-cymene together with three high-field
multiplets, a triplet of doublets at δ 6.30, a triplet at δ 6.05,
and a doublet at δ 4.98, which are diagnostic of the η6-
coordinated proximal phenyl ring of the biaryl-like fragment;
two additional doublets at δ 5.44 and 5.13, each of intensity 1H,
belong to the protons attached to the bridgehead carbon atoms
of the bicyclic framework. A single-crystal X-ray structure
determination of 10 was undertaken in order to unequivocally
establish its identity and to determine the influence of κ(P)-η6
coordination on the metal−phosphine bonding as well as the
coordination geometry at the metal, by comparison with related
systems. A perspective view of the molecular structure of 10 is
shown in Figure 2. The molecular structure shows that the
ruthenium atom adopts the expected three-legged piano-stool
coordination and confirms that 8a is coordinated as an eight-
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Article
remaining five Ru(1)−C bonds (2.128(2)−2.220(2) Å).
Similar bond length patterns have previously been reported
for related arene ruthenium(II) complexes of tertiary mono-
phosphines35 and attributed to the bond-lengthening trans
effect of the tertiary phosphine ligand.
With the intention of comparing the performance of catalysts
generated from ruthenium(II) complexes of κ(P)- and κ(P):η6-
coordinated monophosphines in the direct ortho arylation of 2-
phenyl-substituted N-heterocycles (vide infra), it was also
necessary to prepare a benchmark or reference precatalyst
based on a KITPHOS monophosphine that could not form a
constrained-geometry complex. For this, we chose 12, since it
lacks a proximal phenyl ring on the biaryl-like fragment and as
such will only coordinate to ruthenium as a κ(P) donor; in this
regard 12 can be described as a triaryl-like phosphine.
Monophosphine 12 was prepared by following the procedure
previously outlined for 8, based on the Diels−Alder cyclo-
addition between diphenylethynylphosphine oxide and anthra-
cene. Unfortunately, reduction of the resulting oxide proved to
be problematic, as the use of conventional conditions (110 °C,
trichlorosilane/NEt3) gave a multitude of products, of which
the desired phosphine was only a minor component.
Fortunately, optimum conditions were identified after
persistent modification and mild heating (45 °C) of a
toluene−THF solution of 11 in the presence of trichlorosilane
and triethyl phosphite eventually gave 12 in near-quantitative
Figure 2. Molecular structure of [Ru(κ(P)-η6-{11-(diphenylphosphi-
no)-12-phenyl-9,10-dihydro-9,10-ethanoanthracene})Cl2] (10). Hy-
drogen atoms and the chloroform molecule of crystallization have
been omitted for clarity. Ellipsoids are at the 40% probability level.
electron donor in a κ(P)-η6 manner, bonded through the
phosphorus atom and tethered by the proximal phenyl ring
attached to the two-carbon bridge of the bicyclic framework.
This is the first example of a KITPHOS monophosphine
coordinating in such a manner and is particularly unusual, since
it is the first example of a constrained-geometry complex in
which a κ(P)-bonded diphenylphosphino group and an η6-
coordinated arene are linked by an unsaturated two-carbon
tether; the overwhelming majority of related complexes
typically contain a three-carbon saturated tether. Faller has
reported that 2-dicyclohexylphosphino-2′-(N,N -
dimethylamino)biphenyl forms the related tethered ruthenium
half-sandwich complex [Ru{η6:η1-2-dicyclohexylphosphino-2′-
(N,N-dimethylamino)biphenyl}Cl2], with the η6-coordinated
dimethylamino-containing aryl ring connected to the κ(P)-
dicyclohexylphosphino group via the double bond of an arene
ring; interestingly, this complex was resolved into its planar
chiral enantiomers, which were used as catalyst precursors for a
number of asymmetric transformations.31 The six Ru−
C(arene) distances in 10 lie between 2.128(2) and 2.220(2)
Å, which are within the range reported for related complexes
such as [{η 6-C6H5CH2CH2CH2P(C6H5)2}RuCl2],32 [Ru-
{η6:η1-o-(C6H4)(CH2OH)(CH2CH2PPh2)}Cl2],33 and [Ru-
{(R)-η 1-PPh2(CH2)2CH(CH3)-η 6-C6H5}Cl2].34 While the
η6-aryl ring is close to planar, with a maximum deviation of
−0.0159 (16) Å from the least-squares mean plane, C(16)
bends quite markedly toward the metal center and lies 0.267(4)
Å below this plane; presumably the result of strain enforced by
only having a two-carbon tether linking the η6-arene to the
phosphine. The Ru(1)−P(1) distance of 2.3487(6) Å is similar
to that of 2.3527(6) Å in 9 and close to that of 2.3199(6) Å
found in [RuCl2{(R)-η1-PPh2(CH2)2CH(CH3)-η6-C6H5}].34
The P(1)−Ru(1)−Cl(1) and P(1)−Ru(1)−Cl(2) angles of
90.60(2) and 91.59(2)°, respectively, and the Cl(1)−Ru(1)−
Cl(2) angle of 86.77(2)° lie within a fairly narrow range and are
all surprisingly close to the corresponding angles in 9,
suggesting that constraining the coordinated phosphino group
and arene ring via a two-carbon unsaturated tether does not
have a marked influence on the coordination geometry at the
metal. The Ru−C bond that lies trans to the phosphorus atom
(Ru(1)−C(20) = 2.254(2) Å) is slightly longer than the
1
yield (Scheme 2). The H NMR spectrum of 12 contains a
Scheme 2
triplet of doublets at δ 6.81 associated with the vinylic proton,
while the bridgehead protons appear as a doublet at δ 5.15 and
a doublet of doublets at 5.0; the corresponding signals for the
vinylic and bridgehead carbon atoms appear at δ 146.7 and δ
55.2 and 52.5, respectively.
The p-cymene ruthenium(II) complex 13 was prepared in
near-quantitative yield by treatment of [(p-cymene)RuCl2]2
with 12 at room temperature for 4 h. The 31P NMR spectrum
contains a resonance at δ 23.1, which is close to that of δ 23.9
reported above for 9. Crystals suitable for X-ray structure
analysis were grown to compare the key structural parameters
with those for the corresponding complexes of triphenylphos-
phine and the triaryl-like (1,2-diphenylvinyl)-
diphenylphosphine. A perspective view of the molecular
structure is shown in Figure 3. The molecular structure of 13
shows that the Ru−P and Ru−Cl bond lengths and the
associated bond angles at ruthenium are very close to those in
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Table 1. Ruthenium(II) Catalyzed Ortho Arylation of 2-
a
Phenylpyridine with Aryl Chlorides
monoarylation
bc
diarylation
precat.
b
,
entry
1
aryl chloride
C6H5
(%)
(%)
[(p-cymene)
59 (52)
6
RuCl2]2
2
C6H5
9
76 (72)
74 (69)
70 (62)
76 (73)
77 (72)
70 (66)
75 (71)
71 (68)
74 (67)
63 (60)
63 (58)
60 (55)
57 (54)
54 (50)
52 (48)
67 (64)
61 (54)
66 (58)
64 (61)
67 (62)
62 (55)
9
9
3
C6H5
10
13
9
4
C6H5
8
5
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeC6H4
8
6
10
13
9
8
7
9
8
10
9
9
4-MeC6H4
10
13
9
Figure 3. Molecular structure of [(p-cymene)Ru(κ(P)-{2-(diphenyl-
phosphino)-9,10-dihydro-9,10-ethanoanthracene})Cl2] (13). Hydro-
gen atoms have been omitted for clarity. Ellipsoids are at the 40%
probability level.
10
11
12
13
14
15
16
17
18
19
20
21
22
4-MeC6H4
9
4-MeC(O)C6H4
4-MeC(O)C6H4
4-MeC(O)C6H4
4-CNC6H4
4-CNC6H4
4-CNC6H4
3-Me2NC6H4
3-Me2NC6H4
3-Me2NC6H4
4-EtO2CC6H4
4-EtO2CC6H4
4-EtO2CC6H4
12
15
16
16
17
13
4
10
13
9
[(p-cymene)Ru(PPh3)Cl2]28 and [(p-cymene)Ru{(1,2-
diphenylvinyl)diphenylphosphine}Cl2].30
10
13
9
In an extension of our reasoning that the core structural
architecture of KITPHOS monophosphines resembles a biaryl
monophosphine, 12 is a potential surrogate for triphenylphos-
phine. As such, the direct arylation of 2-phenyl-substituted N-
heterocycles was considered an ideal benchmark transformation
with which to undertake a comparative study to investigate
whether the biaryl-like fragment influences catalyst perform-
ance, particularly since [(p-cymene)Ru(PPh3)Cl2] has already
been shown to form an efficient catalyst for ortho arylations.36
In addition, since Peris and co-workers have reported that the
arene ligand affects catalyst performance,37 a comparison
between the efficiency of catalysts generated from precursors
based on KITPHOS monophosphines 8 and 12 was considered
worthwhile, since the former coordinates in a κ(P)-η6 manner
while the latter can only coordinate as a κ(P) donor. Our
preliminary evaluation focused on the arylation of 2-phenyl-
pyridine with a range of aryl chlorides using 2 mol % of catalyst
precursor in 1-methyl-2-pyrrolidone (NMP) with potassium
carbonate as base, full details of which are provided in Table 1.
Comparative catalyst testing for the arylation of 2-phenyl-
pyridine with chlorobenzene revealed that catalysts generated
from 9 and 10 both outperformed that formed from [(p-
cymene)RuCl2]2, albeit marginally, the former giving con-
versions of 76 and 74%, respectively, to monophenylated
product, while the latter only reached 59% conversion in the
same time. Gratifyingly, using previously optimized condi-
tions,30 good conversions and isolated yields of monoarylated
product were obtained across a range of electron-poor as well
as electron-rich aryl chlorides; in each case a minor amount of
diarylated product was also obtained. Although slightly higher
conversions and yields of diarylated product could be obtained
by using an excess of aryl chloride and potassium carbonate, the
catalyst system remained selective for monoarylation; in this
regard, phosphine complexes of ruthenium(II) have previously
been reported to favor monoarylation over diarylation.39
However, the selectivity of direct arylation catalyzed by a
combination of RuCl3·xH2O and PPh3 has recently been shown
to be markedly dependent on the base, with K2CO3 favoring
10
13
9
6
6
3
10
13
5
5
a
Reaction conditions: 0.5 mmol of Ar−Cl, 0.5 mmol of 2-
phenylpyridine, 0.5 mmol of K2CO3, 2.5 mol of 9, 10, or 13, 2 mL
b
of NMP, 120 °C, 24 h. Conversions were determined by analysis of
the 1H NMR spectrum of the reaction mixture using bromomesitylene
c
as internal standard. Average of two runs. Isolated yield in
parentheses.
monoarylation while Na2CO3 was selective for diarylation,
under the same conditions.38 Table 1 also shows that the
catalyst generated from 10 gave conversions to the mono- and
diarylated products similar to those generated from 9, for all
substrates examined, which could be taken as evidence for the
generation of a common active species; this is not entirely
surprising, since 9 cleanly and nearly quantitatively converts
into 10 upon prolonged heating in chlorobenzene at 120 °C.
To complete the comparison, the influence of the constrained-
geometry coordination sphere was investigated by comparing
the performance of the catalyst generated from 10 with that
generated from 13, since the latter lacks the proximal biaryl-like
phenyl ring and thus cannot form a constrained-geometry
complex. The conversions obtained with the catalyst generated
from 13 matches those achieved with 10, which may indicate
that constraining the phenyl ring and the diphenylphosphino
group by a tether does not have a marked affect on catalyst
efficiency or that the same active species is generated regardless
of the precatalyst. In addition, since the catalyst generated from
[(p-cymene)RuCl2]2 is consistently less efficient than that
generated from 9 and 10,39 albeit only marginally, it would be
reasonable to suggest that the phosphino group and/or the
arene ring may remain coordinated during catalysis. The
encouraging performance of catalysts generated from 9 and 10
for the ortho arylation of 2-phenylpyridine prompted us to
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extend our studies to include the arylation of N-phenylpyrazole.
As shown in Table 2, good conversions were obtained for a
and N-phenylpyrazole with a range of aryl chlorides, giving
comparable conversions and selectivities, while the catalyst
derived from 13 gave good conversions for both substrates but
slightly lower selectivities for N-phenylpyrazole across a range
of electrophiles. Even though the κ(P)- and κ(P)-η 6
coordination of KITPHOS monophosphines was not man-
ifested in catalyst performance for direct ortho arylation, we are
particularly excited and encouraged about the broader
applications of biaryl-like KITPHOS monophosphines in
ruthenium-based catalysis, since the introduction of a
substituent at the 2- or 5-position of the proximal biaryl-like
phenyl ring will render the corresponding constrained-
geometry complex chiral, which when resolved could be used
for Lewis acid catalyzed asymmetric C−C bond forming
reactions such as Diels−Alder and hetero Diels−Alder
cycloadditions or the Mukaiyama reaction. In this regard,
since KITPHOS monophosphines are constructed from a 1-
alkynylphosphine oxide and anthracene, it will be relatively
straightforward to vary the substitution pattern of the proximal
aryl ring in order to control the stereochemical environment
within the coordination sphere of the metal in order to achieve
efficient catalysis.
Table 2. Ruthenium(II) Catalyzed Ortho Arylation of N-
a
Phenylpyrazole with Aryl Chlorides
monoarylation
bc
diarylation
b
,
entry
1
aryl chloride
C6H5
precat.
(%)
(%)
[(p-cymene)
46 (39)
17
RuCl2]2
2
C6H5
9
58 (53)
56 (50)
54 (49)
68 (62)
69 (66)
61 (57)
58 (26)
62 (55)
56 (53)
54 (49)
60 (57)
44 (40)
52 (49)
54 (51)
44 (43)
52 (49)
55 (52)
51 (47)
57 (54)
59 (56)
51 (49)
14
14
19
10
10
17
16
15
20
16
12
23
8
3
C6H5
10
13
9
4
C6H5
5
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeC6H4
6
10
13
9
7
8
9
4-MeC6H4
10
13
9
10
11
12
13
14
15
16
17
18
19
20
21
22
4-MeC6H4
4-MeC(O)C6H4
4-MeC(O)C6H4
4-MeC(O)C6H4
4-CNC6H4
4-CNC6H4
4-CNC6H4
3-Me2NC6H4
3-Me2NC6H4
3-Me2NC6H4
4-EtO2CC6H4
4-EtO2CC6H4
4-EtO2CC6H4
EXPERIMENTAL SECTION
■
10
13
9
General Procedure for the Ruthenium-Catalyzed Ortho
Arylation of 2-Phenyl-Substituted N-Heterocycles with Aryl
Chlorides. A flame-dried Schlenk flask charged with 2-phenyl-
pyridine (0.071 mL, 0.5 mmol), aryl chloride (0.5 mmol), catalyst
(2.5 mol %, 0.0125 mmol), potassium carbonate (0.138 g, 0.5 mmol),
and 1-methyl-2-pyrrolidinone (2 mL) was stirred and heated at 120 °C
for the allocated time. After the reaction mixture had cooled to room
temperature, bromomesitylene (0.076 mL, 0.5 mmol) was added as
internal standard and the resulting mixture quenched with water and
extracted with diethyl ether (3 × 10 mL). The organic layers were
combined, washed with brine, dried over magnesium sulfate,
concentrated under reduced pressure, and analyzed by 1H NMR
spectroscopy to determine the conversion before being purified by
column chromatography, with hexane/ethyl acetate (5/1 v/v) as
eluent. Known products were characterized by NMR spectroscopy and
mass spectrometry and unknown products by NMR spectroscopy,
high-resolution mass spectrometry, and elemental analysis.
10
13
9
7
20
14
12
19
11
8
10
13
9
10
13
18
a
Reaction conditions: 0.5 mmol of Ar−Cl, 0.5 mmol of N-
phenylpyrazole, 0.5 mmol of K2CO3, 2.5 mol of 9, 10, or 13, 2 mL
b
of NMP, 120 °C, 36 h. Conversions were determined by analysis of
the 1H NMR spectrum of the reaction mixture using bromomesitylene
c
as internal standard. Average of three runs. Isolated yield in
parentheses.
ASSOCIATED CONTENT
* Supporting Information
■
selection of aryl chlorides, although slightly longer reaction
times were required to reach similar levels of conversion in
comparison to the corresponding reaction with 2-phenyl-
pyridine. As described above, the catalysts generated from 9
and 10 are slightly more efficient than [(p-cymene)RuCl2]2,
with the former reaching conversions of 59 and 57% for the
monoarylated product, respectively, compared with 46% for the
latter. In contrast to the arylation of 2-phenylpyridine, the
catalyst derived from 13 was slightly less selective than the
corresponding systems generated from 9 and 10, giving good
conversions but more diarylated product in the same time.
In conclusion, the KITPHOS monophosphine 8 has been
shown to coordinate as a conventional κ(P) donor in [(p-
cymene)Ru{PPh2-(Ph)-KITPHOS}Cl2] (9) and in a κ(P)-η6
manner as an eight-electron donor in [Ru{κ(P)-η6-PPh2-(Ph)-
KITPHOS}Cl2] (10); the latter is the first example of a
constrained-geometry complex in which a κ(P)-bonded
diphenylphosphino group and an η6-coordinated proximal
phenyl ring are connected by an unsaturated two-carbon tether.
Comparative studies revealed that both 9 and 10 formed highly
efficient catalysts for the direct arylation of 2-phenylpyridine
S
Text giving full details of experimental procedures, character-
ization data for all new compounds, and details of catalyst
testing and,CIF files giving details of crystal data, structure
solution, and refinement, atomic coordinates, bond distances,
bond angles, and anisotropic displacement parameters for
compounds 9, 10, and 13. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: simon.doherty@newcastle.ac.uk.
■
ACKNOWLEDGMENTS
■
We gratefully acknowledge the EPSRC for studentships (C.H.S.
and N.A.B.W.) and an equipment grant and Johnson Matthey
for generous loans of ruthenium salts.
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