Buchwald-Hartwig amination using Pd(i) dimer precatalysts supported by biaryl phosphine ligands

Article abstract of DOI:10.1039/c8dt00119g

We report the synthesis of air-stable Pd(i) dimer complexes featuring biaryl phosphine ligands. Catalytic experiments suggest that these complexes are competent precatalysts that can mediate cross-coupling amination reactions between aryl halides with bot

Full text of DOI:10.1039/c8dt00119g

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Kirlikovali, E. Cho, T. J. Downard, L. Grigoryan, Z. Han, S. Hong, D. Jung, J. C. Quintana, V. Reynoso, S.
Ro, Y. Shen, K. Swartz, E. Ter Sahakyan, A. I. Wixtrom, B. Yoshida and A. L. Rheingold, Dalton Trans., 2018,
DOI: 10.1039/C8DT00119G.
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1
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January 2016 Pages 1–398
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Efficient extraction of sulfate from water using
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Buchwald-Hartwig Amination Using Pd(I) Dimer Precatalysts
Supported by Biaryl Phosphine Ligands
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
a
a
Kent O. Kirlikovali, Eunho Cho, Tyler J. Downard, Lilit Grigoryan, Zheng Han, Sooji Hong,
a
a
a
a
a
a
Dahee Jung, Jason C. Quintana, Vanessa Reynoso, Sooihk Ro, Yi Shen, Kevin Swartz, Elizabeth
Ter Sahakyan, Alex I. Wixtrom, Brandon Yoshida, Arnold L. Rheingold, Alexander M.
DOI: 10.1039/x0xx00000x
a
a
a
b
a,c
www.rsc.org/
Spokoyny*
We report the synthesis of air-stable Pd(I) dimer complexes precatalysts are thought to either disproportionate to yield
featuring biaryl phosphine ligands. Catalytic experiments suggest both Pd(0) and Pd(II) species upon thermal activation, or form
that these complexes are comptent precatalysts that can mediate Pd(0) after reduction from nucleophile or base that are also
4
a
cross-coupling amination reactions between aryl halides with both present in the reaction mixture. Following the initial report of
t
aliphatic and aromatic amine nucleophiles. This work represents halide-bridged Pd(I) dimers Pd
5
2
(
ꢀ
an expansion of the air-stable precatalyst toolbox for Pd-catalyzed (Fig. 1), Hartwig and coworkers showed that these complexes
-X)
2 3 2
(P Bu ) (X = Br, I) in 1996
cross-coupling transformations.
Palladium-catalyzed cross-coupling
were effective precatalysts for both C–C and C–N cross-
6
coupling reactions. More recently, Schoenebeck and
reactions are
ubiquitous synthetic tools, providing practitioners with
colleagues have developed conditions for this precatalyst
system that are capable of chemoselective transformations
1
versatile methods to forge carbon–carbon and carbon–
2
heteroatom bonds. The development of air- and moisture-
7
with a wider breadth of substrates.
stable Pd(II) precatalysts that can quickly generate the
catalytically active Pd(0) species in situ has dramatically
improved the scope of transformations and increased the
efficiency of these reactions when compared to systems that
3
incorporate traditional palladium sources. Typically, Pd(II)
precatalysts are activated by a base (either nucleophile or
external base) followed by a reductive elimination to form the
catalytically active Pd(0) species and an organic-based auxiliary
3
a,b
3c-e
3f
(
e.g., carbazole,
pyridine,
allylic ethers,
and
3
g
benzylamines ) as a side product. A Pd-based precatalyst that
does not require external base or form side products would
result in fewer by-products formed en route to the catalytically
active Pd(0) species and overall greater operational simplicity.
A
lesser-studied class of air-stable, dinuclear Pd(I)
complexes have previously shown promise both as Fig. 1 Top: selected examples of Pd(I) dimer complexes as precatalysts. Bottom: Pd(I)
4
a-c,5-8
precatalysts for cross-coupling reactions
(Fig 1) as well as dimers synthesized in this work with biaryl phosphine ligands used (L1-L6).
a,e,f
4
active catalytic species in some transformations.
In
contrast to base-activated Pd(II) precatalysts, Pd(I) dimer
In contrast to the halide-bridged Pd(I) dimer complex,
Murahashi et al. reported the first homoleptic dinucelar Pd(I)
complex [Pd
interaction with no bridging ligands. Hor and coworkers then
demonstrated that the [SbF ] analogue of this dimer,
Pd MeCN ][SbF , was an active “ligandless” precatalyst for
2 6 4 2
MeCN ][BF ] that features a direct Pd–Pd
8
a
6
[
2
6
6 2
]
8
b
Suzuki-Miyaura cross-coupling reactions (Fig. 1). In fact, the
addition of phosphine ligands was found to enhance catalytic
performance, though no phosphine-ligated Pd complexes were
isolated in this study. Biaryl phosphine ligands have been
shown to improve reactivity and expand the substrate scope
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1
,2,3a,b,i,9
for many Pd-based catalytic transformations.
Based on behavior of the ligand (L4) in solution on the NMR timescale.
this reasoning, Christmann and colleagues went on to Steric bulk is increased even more with the replacement of
synthesize a Pd(I) dimer that incorporated a bulky biaryl dicyclohexylphosphine (L4) with di-tert-butylphosphine (L3).
t
10
31
phosphine, Pd
Pd(I) dimers were effective precatalysts for amination spectrum for
reactions of aryl halides; however, the synthesis of these bulk limiting the ability of the ligands to associate with the
2
X(ꢀ
-X)(
ꢀ
-P Bu
2
bph) (X = Cl, Br; Fig. 1). These Indeed, multiple resonances are observed in the P NMR
3
, potentially resulting from this increased steric
3
1
dimers is lengthy (48 hr reaction time), low-yielding (45-71%), Pd(I) centers. Variable temperature (VT) P NMR studies on
4
and not general as only one type of biaryl phosphine ligand in CD Cl reveal new resonances at -80°C, suggesting the
2
2
was used. Similarly, a Pd(I) dimer supported by two biaryl presence of multiple conformations due to limited (or slow)
phosphine ligands was reported and showed promise as a fluctionality at low temperature (Fig. S1). After warming this
precatalyst for C–C bond-forming reactions, however the sample back to 25°C, the broad resonances reappear, further
1
1
synthesis was not general for other Pd(I) dimers.
reinforcing the proposed fluctional behaviour. Due to low
in CD Cl solution on the
We were interested in expanding the class of Pd(I) dimers solubility and poor stability of
3
with biaryl phosphine ligands and studying their performance timescale required to acquire P NMR spectra (vide infra), VT
3
2
2
1
as precatalysts for cross-coupling reactions. To that end, we NMR studies were not possible for this sample. Attempts to
have developed a straightforward synthetic route that enables grow diffraction-quality crystals of
access to numerous Pd(I) dimer complexes via the suggested structures resulted in the formation of metallic Pd.
Pd MeCN ][BF precursor. Notably, this method can
3 and 4 to validate these
[
2
6
4 2
]
2+
2BF4
-
L
Pd
L
accommodate several biaryl ligands, and the desired Pd(I)
dimer complexes are formed quantitatively within minutes.
These Pd(I) dimers behave as excellent precatalysts for
Buchwald-Hartwig amination reactions, thus expanding the
range of available air-stable Pd-based precatalysts.
A
L
1
:1 MeCN:CH
hr, r.t.
2
Cl
2
Pd
2
(dba)
3
+ [Pd(MeCN)
4
][BF
4
]
2
L
Pd
L
1
L
L = MeCN
2+ 2BF
-
4
2 eq. L1-L6
Cl
2 2
5-30 min, r.t.
CH
R
R
Following the procedure reported by Murahashi and
P
Pd Pd
P
R
coworkers, [Pd
comproportionation reaction between Pd(0) and Pd(II)
Pd dba and [Pd(MeCN) ][BF , respectively) on a 0.5 gram
scale and served as a competent Pd(I) dimer precursor (Fig.
A). To generate Pd(I) dimer complexes , two equivalents of
a biaryl phosphine ligand (L1 L6) dissolved in CH Cl were
added to a slurry of [Pd MeCN ][BF in CH Cl and stirred at
2
MeCN
6
][BF
4
]
2
was synthesized by the
R
Air stable Pd(I)
dimer precatalysts
6
1-6 (72-99%)
RAryl groups are omitted for
clarity
(
2
3
4
4
]
2
X-Ray Structure
B
C
2
1-6
6
-
2
2
δ = 61.7 ppm
2
6
4
]
2
2
2
room temperature for up to 30 minutes (Fig. 2A). An
immediate color change was observed for all reactions,
suggesting rapid ligand exchange between biaryl phosphine
L6
δ = -9.1 ppm
3
1
1
P{ H} NMR
ligands L1-L6 and acetonitrile. Reaction progress can be easily
3
1
monitored via P NMR as there is a substantial downfield shift
upon coordination of the biaryl phosphine ligand to the metal
center (Fig. 2B). In some cases, prolonged reaction time results
in the formation of Pd-black so shorter reaction times were
8
0
60
40
20
0
ppm
Fig. 2 (A) Synthetic route to Pd(I) dimer complexes 1-6. Left: X-ray crystal structure of 6
drawn with 50% ellipsoid probability (CCDC 1812500). H atoms are omitted for clarity.
31
1
(
B) P{ H} NMR data for L6 (bottom) and 6 (top), both in CD
dba , and Pd(II) is [Pd(MeCN) ][BF
2 2
Cl . (C) XPS data for
used to maximize yield (<10 minutes, see ESI for experimental compounds 1-6. Pd(0) is Pd
details). This is particularly noticeable in the synthesis of : Pd-
black forms within minutes of the addition of L5 to
Pd MeCN ][BF 2, and the isolated yield of after reacting for characterized via single crystal X-ray diffraction, confirming the
0 minutes is 35% (the yield increases to 91% with ~1 minute propsed dimeric structure (Fig. 1A and Fig. S2-S5). For all of
reaction time). In contrast, no Pd-black formation was these dimer structures, Pd(I)···Pd(I) distances range from
observed during the synthesis of , and the isolated yield was 2.702–2.716 Å in the solid state, slightly greater than the sum
quantitative even when the reaction time was 30 minutes. of covalent radii for two Pd atoms (2.40 Å). To further probe
Upon completion, the crude reaction mixtures were filtered the oxidation state of Pd in
, we employed X-ray
through Celite and the filtrates dried in vacuo to offer photoelectron spectroscopy (XPS). As anticipated, the binding
2
3
4
4 2
] .
5
On the other hand, compounds 2, 5 and 6 could all be
[
2
6
4
]
5
1
6
1
2
1-6
crystalline solids with yields ranging from 72% to 99%.
Most compounds exhibit one singlet resonance in the
energies for Pd 3d electrons in
1-6 fall between the binding
3
1
P
energies for Pd 3d electrons in Pd(0) (Pd dba ) and Pd(II)
2
3
NMR, indicating
a symmetric structure and chemically ([PdMeCN ][BF ] ) standards, suggesting that both Pd atoms
4 4 2
equivalent P atoms. As functional group bulk at the para are likely in the +1 formal oxidation state (Fig. 2C and Table
position on the bottom ring of the biaryl phosphine ligand S2).
increases (L3 and L4), multiple P NMR resonances are
3
1
Compounds
1
-
6
are stable as solids indefinitely when
and exhibit long-term
in CH Cl were
observed. Specifically, two doublets are observed for
3
4
stored in air at 5°C; in contrast, only
5
6
(
δ
 39.26 and 23.15, J P-P = 84 Hz, see ESI), suggesting fluctional stability in CH Cl solution. Solutions of
1
-
6
2
2
2
2
2
| J. Name., 2012, 00, 1-3
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stored on the bench top under atmospheric conditions, and were sufficient for full conversion. The temperature was
within 3 days, metallic Pd films coated the vials containing 4, decreased from 100°C to 80°C during the subsequent
whereas and remained in solution with no observed optimization with no observed loss in yield, and inclusion of
changes in P NMR spectra after 1 week. Compounds and extra supporting ligand L6 was deemed unnecessary (entries 9
1-
5
6
3
1
3
4,
t
which contain ligands with the greatest amount of steric bulk and 10, respectively). NaO Bu was successfully substituted for
t
on the bottom aryl ring (L3 and L4, respectively), exhibit the KO Bu as the yield remained >99% for this experiment (entry
lowest stability in CH
films both start forming within 30 minutes after dissolution.
We turned our attention to the use of the synthesized Pd(I) there was a precipitous drop in the observed yield when 4-
dimers as air-stable precatalysts for Buchwald-Hartwig iodotoluene was employed (entry 13). This is consistent with
2
Cl
2
solution as Pd-black and metallic Pd 11). No significant change in performance was observed when
varying the halide from bromide to chloride (entry 12), but
1-6
amination reactions. For the initial optimization studies, 4- previously observed trends in Pd-catalyzed cross-coupling with
bromotoluene and morpholine were chosen as a reagent systems featuring electron-rich biaryl ligands where
1
3
system (Table 1). We first evaluated
1
-
6
as precatalysts at 0.5 transmetallation is a rate-determining step. Finally, the use
L6) using of precatalyst under the optimized conditions still results in
mol% loading with 0.5 mol% corresponding ligand (L1
t
KO Bu as the base (entries 1-6). These Pd(I) dimer complexes GC-MS yields >99% (entry 14).
are thought to disproportionate to form a Pd(II) species and a
-
5
1
1
R'
N
catalytically active Pd(0) species after thermal activation;
A
X
Pd(I) dimer (0.5-1.0 mol%)
base, solvent
R'
N
R''
R
+
R''
H
R
therefore, the role of the base in this system is limited to
deprotonating the amine adduct of palladium that forms
during the transmetallation step.
80-100°C, 1-3 hr.
X = Cl, Br, I
Secondary Amines:
O
O
CH
3
O
O
N
N
Table 1 Optimization of reaction conditions for 7.
N
N
7
H
3
C
92% (X = Br)
4% (X = Cl)
4% (X = I)
0.5 mol% 6, 80°C, 1 hr.)(0.5 mol% 6, 80°C, 1hr.) (0.5 mol% 6, 80°C, 1 hr.) (0.5 mol% 4, 100°C, 1 hr.)
3
F C
9
2
8
9
N
10
73% (X = Br)
77% (X = Br)
70% (X = Br)
(
Primary Amines:
CH
3
H
N
H
N
H
3
C
H
N
H
N
N
H
3
C
11
12
CH
98% (X = Br)
3
13
14
Pd(I)
86% (X = Br)
94% (X = Br)
41% (X = Br)
a
(1.0 mol% 6, 80°C, 1 hr.) (0.5 mol% 6, 80°C, 1 hr.) (1.0 mol% 6, 100°C, 3 hr.) (1.0 mol% 4, 100°C, 3 hr.)
Precat.
(mol%)
1, 0.5
2, 0.5
3, 0.5
4, 0.5
5, 0.5
6, 0.5
6, 1.0
6, 0.5
6, 0.5
6, 0.5
6, 0.5
6, 0.5
6, 0.5
5, 0.5
L
Temp
(°C)
100
100
100
100
100
100
100
100
80
Time
(min.)
30
Yield
(%)
46
Entry
(mol %)
L1, 0.5
L2, 0.5
L3, 0.5
L4, 0.5
L5, 0.5
L6, 0.5
L6, 0.5
L6, 0.5
L6, 0.5
–
Base
X
Aromatic Amines:
t
H
N
CH
3
H
N
H
H
N
1
2
3
4
5
6
7
8
9
KO Bu
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
O
N
O
O
O
t
KO Bu
30
25
H
3
C
F
3
C
N
t
KO Bu
30
12
O
O
O
O
1
5
16
17
42% (X = Br)
18
t
98% (X = Br)
98% (X = Br)
40% (X = Br)
KO Bu
30
45
(
0.5 mol% 6, 80°C, 1 hr.) (0.5 mol% 6, 80°C, 1 hr.)
(0.5 mol% 6, 80°C, 1 hr.) (0.5 mol% 4,100°C, 1.5 hr.)
t
KO Bu
30
60
t
B
Br
O
KO Bu
30
64
Br
4 or 6 (0.5 mol%)
Bu (1.2-1.4 eq), dioxane
0°C-100°C, 1-2 hr.
NaO
t
N
t
+
HN
O
+
KO Bu
30
>99
>99
>99
>99
>99
>99
N
X
8
Cl
X
t
Cl
X
O
KO Bu
60
1
.2-1.4 eq.
19: X = N; 99% (73%)
not observed
X = N, CH
t
1.0 eq.
X = N, CH
KO Bu
60
(0.5 mol% 4, 100°C, 2 hr.)
2
0: X = CH; 99% (92%)
t
1
1
1
1
1
0
1
2
3
4
KO Bu
80
60
(0.5 mol% 6, 80°C, 1 hr.)
t
–
NaO Bu Br
80
60
Fig. 3 (A) Substrate scope for the amination of aryl and heterocyclic halides. Conditions:
t
–
NaO Bu Cl
80
60
t
Aryl halide (2.5 mmol), amine (3.5 mmol), NaO Bu (3.5 mmol), Pd(I) dimer precatalyst
t
b
–
NaO Bu
I
80
60
40
(0.5-1.0 mol%), dioxane (2.5 mL), 80-100°C, 1-3 hr. Isolated yields represent the
average of two runs. (B) Amination of mixed-halide substrates demonstrates selectivity
for bromide.
t
–
NaO Bu Br
80
60
>99
Pd(I) dimers 1-6 were initially screened to find the most optimal precatalyst.
Optimization of the reaction conditions proceeded with 6 (optimized conditions
a
b
in entry 11). Yield determined using GC-MS analysis. Full conversion of 4-
iodotoluene was observed by GC-MS.
The optimized conditions presented in entry 11 were
adopted for further experiments to expand the substrate
scope. Precatalyst
bromides and multiple types of nucleophiles, including
secondary amines 10), primary amines 11 14), and
aromatic amines (15 18, Fig. 3A). Reactions between 4-
bromotoluene and both cyclohexylamine and 3,5-
dimethoxyaniline ( and 11, respectively) proceeded with 0.5-
.0 mol% loading of . Using this catalytic system, aryl
6 was found to work with a range of aryl
After reacting for 30 minutes at 100°C in 1,4-dioxane,
offered the highest GC-MS yields of product (60% and 64%,
respectively). Due to the relative ease of synthesis of
5 and
6
7
(
7
-
(
-
6
-
compared to
forward with
5
(vide supra), we therefore decided to move
as the model Pd(I) dimer precatalyst. Full
6
7
conversion of 4-bromotoluene was achieved both when the
precatalyst loading was doubled to 1.0 mol% (entry 7) and
when the reaction time was increased to 1 hour (entry 8),
demonstrating that increasing the catalyst loading was not
necessary as lower catalyst loadings with longer reaction times
1
6
bromides with electron-withdrawing trifluoromethyl groups in
the para- position were found to work with secondary amines
and anilines (9, 17). Reactions with ortho-substituted aryl
bromides proceeded efficiently with all three classes of
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amines, and products were isolated in yields of up to 98% (
8
,
2
3
P. Ruiz-Castillo, S. L. Buchwald, Chem. Rev., 2016, 116
2564-12649 (and references within).
(a) M. R. Biscoe, B. P. Fors, S. L. Buchwald, J. Am. Chem. Soc.
008, 130, 6686-6687; (b) N. C. Bruno, N. Niljianskul, S. L.
Buchwald, J. Org. Chem., 2014, 79, 4161-4166; (c) C. J.
O’brien, E. A. B. Kantchev, C. Valente, N. Hadei, G. A. Chass,
A. Lough, A. C. Hopkinson, M. G. Organ, Chem. Eur. J., 2006,
12, 4743-4748; (d) M. G. Organ, S. Avola, I. Dubovyk, N.
Hadei, E. A. B. Kantchev, C. J. O’Brien, C. Valente, Chem. Eur.
J., 2006, 12, 4749-4755; (e) M. G. Organ, S. Çalimsiz, M.
,
1
12,
16). In all cases, reactions involving a heterocyclic
electrophile (3-bromopyridine) proceeded sluggishly with GC-
MS yields <30% when the standard reaction conditions were
2
employed with
proved competent for the transformation between all three
classes of amines and 3-bromopyridine 10 14 18),
6 as a precatalyst. However, precatalyst 4
(
,
,
demonstrating the potential utility of this class of Pd(I) dimer
precatalysts. This observation is consistent with previous
reports that suggest the increased bulk of L4 leads to greater
Sayah, K. H. Hoi, A. J. Lough, Angew. Chem. Int. Ed., 2009, 48
,
2383-2387; (f) U. Christmann, R. Vilar, Angew. Chem. Int. Ed.,
2005, 44, 366-374; (g) R. B. Bedford, C. S. J. Cazin, S. J. Coles,
T. Gelbrich, P. N. Horton, M. B. Hursthouse, M. E. Light,
Organometallics, 2003, 22, 987-999; (h) N. Marion, S. P.
Nolan, Acc. Chem. Res., 2008, 41, 1440-1449; (i) N. Hazari, P.
9
c,e
efficiency of aminations of heteroaryl halides.
Finally, reactions between morpholine and both 2-chloro-
-bromopyridine and 1-bromo-4-chlorobenzene were
5
performed to test halide selectivity and confirm that these
R. Melvin, M. Beromi, Nat. Rev. Chem., 2017, 1, 0025.
(a) I. G. Powers, C. Uyeda, ACS Catal., 2017, 7, 936-958; (b)
transformations proceed via cross-coupling and not through a
4
nucleophilic aromatic substitution (S
N
Ar) pathway (Fig. 3B). In
N. Hazari, D. P. Hruszkewycz, Chem. Soc. Rev., 2016, 45
871-2899; (c) R. S. Paton, J. M. Brown, Angew. Chem. Int.
Ed., 2012, 51, 10448-10450; (d) S. Lin, D. E. Herbert, A.
Velian, M. W. Day, T. Agapie, J. Am. Chem. Soc., 2013, 135
5830-15840; (e) D. P. Hruszkewycz, J. Wu, N. Hazari, C. D.
,
2
both reactions, full conversion of the electrophile was
achieved within 2 hours, but no chloro-substituted product
could be detected by GC-MS. In fact, only the desired products
were observed by GC-MS and isolated in 73% yield (19) and
,
1
Incarvito, J. Am. Chem. Soc., 2011, 133, 3280-3283; (f) R. K.
Das, B. Saha, S. M. W. Rahaman, J. K. Bera, Chem. Eur. J.,
92% yield (20), confirming the selectivity of this class of
2
010, 16, 14459-14468.
precatalysts for bromide. Moreover, this selectivity potentially
enables the use of orthogonal methods that are reactive
5
6
7
R. Vilar, D. M. P. Mingos, C. J. Cardin, J. Chem. Soc., Dalton
Trans., 1996, 4313-4314.
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Ed., 2002, 41, 4746-4748.
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1
4
towards chlorides, enhancing the prospective scope of
transformations available with this precatalyst system.
In conclusion, we have synthesized and characterized a
series of air-stable Pd(I) dimer complexes incorporating biaryl
phosphine ligands. A comproportionation reaction between
commercially-available Pd(0) and Pd(II) sources followed by
the addition of biaryl phosphine ligands results in a nearly
quantitative strategy towards the synthesis of these
complexes. Single crystal X-ray diffraction studies on several
Pd(I) dimers supported by the biaryl phosphine ligands confirm
the proposed dimeric structure of these complexes in the solid
state, and XPS data suggests the +1 formal oxidation state of
the Pd centers in each dimer complex. Synthesized Pd(I)
dimers can be used as precatalysts for Buchwald-Hartwig
amination cross-coupling reactions. Overall, this work presents
a direct and simple route to a potentially large class of air-
stable Pd(I) dimer compounds which, by further tailoring the
,
8
9
5
696.
(a) E. Jacobsen, Adv. Synth. Catal., 2015, 357, 2173-2174; (b)
E. R. Strieter, D. G. Blackmond, S. L. Buchwald, J. Am. Chem.
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R. E. Tundel, T. Ikawa, R. A. Altman, S. L. Buchwald, Angew.
Chem. Int. Ed., 2006, 45, 6523-6527.
biaryl phosphine ligand, can be potentially used for further 10 U. Christmann, R. Vilar, A. J. P. White, D. J. Williams, Chem.
1
5
cross-couplings and other transformations.
16
Commun., 2004, 1294-1295.
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Acknowledgements
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Organometallics, 2004, 23, 1533-1541; (c) T. D. Sheppard,
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, 170-173; (b) A. H. Roy, J. F. Hartwig,
We would like to acknowledge Mr. Rafal Dziedzic and Dr. Julia
Stauber for assistance with mass spectrometry measurements.
7
1
1
4 M. H. Keylor, Z. L. Niemeyer, M. S. Sigman, K. L. Tan, J. Am.
Chem. Soc., 2017, 139, 10613-10616.
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Conflicts of interest
There are no conflicts to declare.
1
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V. Ozerov, J. Am. Chem. Soc., 2007, 129, 10318-10319; (b) R.
Huacuja, D. J. Graham, C. M. Fafard, C.-H. Chen, B. M.
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Notes and references
nd
Metal Catalyzed Cross-Coupling Reactions, 2 Ed. Wiley,
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008 (and references within).
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| J. Name., 2012, 00, 1-3
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Page 5 of 5
Dalton Transactions
R'
N
Br
R'
H N
R''
[
Pd]
Het
R"
+
Het
2+
[
Pd] =
R
R
P
Pd Pd
P
R
R
Air stable Pd(I) dimer precatalysts