Characterization, reactivity, and potential catalytic intermediacy of a cyclometalated tri-tert -butylphosphine palladium acetate complex

Article abstract of DOI:10.1021/om2006936

Palladium acetate and tri-tert-butylphosphine react at room temperature via C-H activation of a tert-butyl group to form the novel palladium(II) complex [(P^tBu₃)Pd(CH₂C(CH₃) ₂P^tBu₂)(OAc). This cyclometalated complex can be reduced to Pd(P^tBu₃)₂ by either heat or hydrogen, but is resistant to reduction by alkoxide bases and amines. As a result of the existence of this cyclometalated complex, the production of catalytically active palladium(0) species generated in situ from Pd(OAc) ₂ and tri-tert-butylphosphine is diminished, as evident by the catalytic inactivity of this complex. Additionally, datively bound tri-tert-butylphosphine is easily displaced by amines and less basic phosphines to form a series of novel cyclometalated palladium(II) complexes.

Full text of DOI:10.1021/om2006936

ARTICLE
pubs.acs.org/Organometallics
Characterization, Reactivity, and Potential Catalytic Intermediacy of a
Cyclometalated Tri-tert-butylphosphine Palladium Acetate Complex
William H. Henderson, Jimmy M. Alvarez, Chad C. Eichman, and James P. Stambuli*
Department of Chemistry, The Ohio State University, 100 W. 18th Avenue, Columbus, Ohio 43210, United States
S Supporting Information
b
ABSTRACT: Palladium acetate and tri-tert-butylphosphine
react at room temperature via CÀH activation of a tert-butyl
group to form the novel palladium(II) complex [(PtBu3)Pd-
(CH2C(CH3)2PtBu2)(OAc)]. This cyclometalated complex
can be reduced to Pd(PtBu3)2 by either heat or hydrogen, but is resistant to reduction by alkoxide bases and amines. As a result
of the existence of this cyclometalated complex, the production of catalytically active palladium(0) species generated in situ from
Pd(OAc)2 and tri-tert-butylphosphine is diminished, as evident by the catalytic inactivity of this complex. Additionally, datively
bound tri-tert-butylphosphine is easily displaced by amines and less basic phosphines to form a series of novel cyclometalated
palladium(II) complexes.
’ INTRODUCTION
Pd(OAc)2 because one can imagine that a mixture of Pd(dba)2
and externally added phosphine react together and the weaker dba
ligand is replaced at the metal by the phosphine. The second dba
ligand can then be displaced by another phosphine or by the
incoming substrate.15 Moreover, the oxidation state at palladium
goes unchanged upon addition of ligand, and once the catalyst is
formed, oxidative addition can occur, resulting in a change in
oxidation state from Pd(0) to Pd(II). The formation of palladium
catalysts from Pd(OAc)2 is not as straightforward in this regard.
From a structural standpoint, palladium acetate is well-defined;16
however, the oxidation state of Pd(OAc)2 is Pd(II), and the mode
of reduction to Pd(0) upon addition of phosphine is still unclear in
many cases.17 Typically, no intermediate palladium species are
isolated during these reductions, and studies relating to the reduc-
tion of Pd(OAc)2/PtBu3 combinations have not been reported.
If one charges 5 mol % of Pd(OAc)2 and a corresponding ligand
into a cross-coupling reaction, the intermediate reactions that follow
are unlikely to produce the active catalytic species with quantitative
conversion. Therefore, the identification of intermediate complexes
during these transformations is important to understand how to
more cleanly generate a complex that better resembles the active
catalyst,18 while avoiding any potential reactions that unproduc-
tively consume Pd(OAc)2 and ligand. The identification and
isolation of these complexes may help to design catalysts that allow
a decreased catalyst loading and increased catalyst turnover. We
now report the isolation and reactivity of one of these complexes: a
cyclometalated complex formed from the facile CÀH insertion of
Pd(OAc)2 with PtBu3 at room temperature (Scheme 1).
The development and application of palladium-catalyzed
cross-coupling reactions is a dominant area of research in
synthetic chemistry. The utility of these classes of reactions
was punctuated in 2010 with the awarding of the Nobel Prize in
Chemistry to three pioneers in cross-coupling chemistry, Richard
Heck, Ei-ichi Negishi, and Akira Suzuki.1 At the forefront of the
advancement of these reactions was the discovery that the
combination of palladium salts and bulky, electron-rich trialk-
ylphosphines created highly unsaturated and reactive catalysts.2
One of the first widely applicable bulky, electron-rich ligands was
tri-tert-butylphosphine (PtBu3). The increased activity of PtBu3 is
highlighted by its ability to form stable unsaturated T-shaped
monomeric palladium complexes, which are proposed as catalytic
intermediates in many reactions.3 In 1997, the Tosoh Corporation
reported that Pd(dba)2 (dba = dibenzylideneacetone) or Pd-
(OAc)2, when used with PtBu3, effectively catalyzes the amination
of aryl halides with piperazines.4 More recently Hartwig and co-
workers have expanded the scope of palladium-catalyzed reactions
usingPtBu3 in arylaminations5 aswellas α-arylations of carbonyls6
and nitriles.7 Additionally, Fu and co-workers have shown that
PtBu3 and a palladium salt effectively catalyze SuzukiÀMiyaura,8
Negishi,9 Heck,10 Sonogashira,11 and Stille12 cross-coupling reac-
tions with aryl bromides and aryl chlorides.13
Most of the catalytic cycles proposed in palladium-catalyzed
cross-coupling reactions are believed to operate on a Pd(0)/Pd(II)
platform. The most popular starting palladium precatalysts are
Pd(dba)2 and Pd(OAc)2, as they are readily available or easily
prepared, and are stable to air and moisture. The widespread use of
Pd(dba)2 as a precatalyst is interesting because of the ability of dba
to deleteriously affect a reaction outcome.14 Moreover, the actual
composition of Pd(dba)2 is subject to preparatory method of
choice, can vary from batch to batch, and is difficult to quantify
its purity spectroscopically. However, Pd(dba)2 provides a more
direct route to generating a desired palladium catalyst than
’ RESULTS AND DISCUSSION
While repeating a procedure using a catalyst formed from five
equivalents of PtBu3 and one equivalent of Pd(OAc)2, employing
Received: July 27, 2011
Published: August 25, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/om2006936 Organometallics 2011, 30, 5038–5044
|

Organometallics
ARTICLE
Scheme 1
Scheme 2
Figure 2. ORTEP diagram of complex 1. Thermal ellipsoids are set at
50% probability. Selected bond lengths [Å] and angles [deg]: PdÀ
P1 2.2795(7); PdÀC2 2.066(3); PdÀO1 2.156(2); C2ÀPdÀO1
162.35(11); P1ÀPdÀP2 167.01(3); C1ÀP1ÀPd 88.32 (11).
provide insight into how Pd(OAc)2 is reduced to Pd(0) in cross-
coupling reactions that employ PtBu3 as a ligand.
Figure 1. Variable-temperature 31P NMR spectra of palladacycle 1: (a)
33 °C, (b) 16 °C, (c) 6 °C, (d) À14 °C.
The seemingly complex 31P NMR spectrum is caused by the
equilibrium of 1 and known dimeric complex 2 (Scheme 2).7,20
At À14 °C the broad resonances in the 31P NMR and 1H NMR
spectra coalesce to form two sharp, distinct signals that corre-
spond to datively bound PtBu3 at 68.1 ppm and the cyclometa-
lated PtBu3, which appears at À8.0 ppm (Figure 1). If one
equivalent of PtBu3 is added to Pd(OAc)2, the known cyclome-
talated palladium dimer 2 is formed in situ and is identified by a
broad resonance at À8.9 ppm. Addition of another equivalent of
PtBu3 causes dimer 2 to dissociate, forming complex 1. Any
additional PtBu3 remains as free ligand exchanging with bound
PtBu3, noted by the broad signal at 63.1 ppm. Complex 1 formed
directly from Pd(OAc)2 and PtBu3 contained coordinated acetic
acid and was used for subsequent studies; however, complex 1
less the coordinated acetic acid could be prepared from isolated
complex 2 and PtBu3.
a smaller amount of expensive PtBu3 was desired. Moreover, it
was unclear what the identity of the product of this reaction
would be. Initially, one would expect Pd(PtBu3)2 to somehow
form. The formation of this Pd(0) complex could arise from the
generation of Pd[(PtBu3)2(OAc)2], which further reacts to form
Pd(PtBu3)2. However, the existence of metal complexes contain-
ing two bulky PtBu3 ligands on a single palladium center is rare.19
A quick investigation of this reaction by 31P NMR spectroscopy
did not show any evidence of the diacetate complex or Pd-
(PtBu3)2. A large signal corresponding to uncoordinated PtBu3 at
63.1 ppm was observed along with broad signals near 68.1 and
À8.0 ppm. The downfield resonances nearer to uncomplexed
PtBu3 are in the area of datively bound PtBu3, while PtBu3
resonances upfield from 0 ppm typically correspond to cyclo-
metalated phosphines. Therefore, it was believed that there were
at least two distinct complexes present in this reaction.
An X-ray crystal structure of complex 1 was obtained
(Figure 2). The geometry about the palladium center is distorted
square planar with C(2)ÀPdÀO(1) and P(1)ÀPdÀP(2) bond
angles of 162.3° and 167.0°, respectively. The cyclometalated
phosphine has a bite angle of 68.3°, slightly more strained than
observed in the previously reported X-ray crystal structure of the
cyclometalated dimer bis(μ-chloro)bis[2-(di-tert-butylphosphino)-
2-methylpropyl]dipalladium,21 which has a bite angle of
70.0°. The PdÀC(2) and PdÀP(1) bond lengths are 2.066
Repeating the reaction using two equivalents of PtBu3 pro-
duced the same resonances in the 31P NMR spectrum, less the
free phosphine peak at 63.1 ppm. Crystallization of the crude
reaction mixture from pentane unexpectedly provided the single
novel complex [(PtBu3)Pd(CH2C(CH3)2PtBu2)(OAc)]HOAc
(1), in 81% yield, resulting from insertion into the CÀH bond of
PtBu3 and formation of AcOH. Because complex 1 was formed
from mixing PtBu3 and Pd(OAc)2 at room temperature, it
functions either as a precatalyst to the active catalytic species
or as a product of the catalyst deactivation pathway and may
ꢀ
ꢀ
and 2.279 Å, respectively, compared to 2.052 and 2.209 Å in bis
(μ-chloro)bis[2-(di-tert-butylphosphino)-2-methylpropyl]dipalladium.
ꢀ
The PdÀO(1) bond is 2.156 Å, slightly elongated compared to
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Organometallics
ARTICLE
Scheme 3
Table 1. Aryl Aminations of 2-Bromotoluene Using 1, 2, and
Pd(PtBu3)2
the known HerrmannÀBeller palladacycle with bridging acet-
ꢀ 22
entry
catalyst
temp (°C)
time (h)
yielda
ates, which have PdÀO bond distances of 2.147 and 2.111 Å.
In order to better understand how complex 1 may function as
a catalyst precursor, we attempted to affect its reduction to Pd(0)
by adding bases typically utilized in cross-coupling reactions. In
THF at 23 °C, K2CO3, Cs2CO3, K3PO4, and NaOAc do not
promote reduction to Pd(0) as observed by 31P NMR spectros-
copy. Interestingly, the addition of NaOtBu created a new
unidentifiable complex with a sharp 31P NMR signal at À2.4
ppm and free PtBu3. All attempts to isolate this complex were
unsuccessful. This complex could potentially be formed via tert-
butoxide coordinating to palladium, displacing either the acetate,
PtBu3, or both.
1
2
Pd(OAc)2/PtBu3
complex 1
complex 2
Pd(PtBu3)2
complex 1
complex 2
Pd(PtBu3)2
complex 1
complex 2
Pd(PtBu3)2
23
23
6
81%b
6%
6
3
23
6
8%
4
23
6
6%
5
60
3
34%
33%
30%
20%
24%
19%
6
60
3
7
60
3
8
110
110
110
15 min
15 min
15 min
9
10
a All yields are an average of two 0.25 mmol reactions. Yields were
determined by 1H NMR using 1,3,5-trimethoxybenzene as an internal
standard. b Reported literature yield.5
Complex 1 was then submitted directly to a hydrogen atmo-
sphere, since hydrogenations have been reported with Pd(OAc)2
and excess PtBu3 in the presence of formic acid in THF.23
Interestingly, when complex 1 is stirred in either THF or toluene
at 23 °C under a hydrogen atmosphere, it was cleanly converted
to Pd(PtBu3)2 in 12 h. Further investigations into the reactivity of
complex 1 showed that in the presence of O2 and air the complex
decomposes, forming tri-tert-butylphosphine oxide and minor
amounts of unidentified products. Not surprisingly, complex 1 is
unreactive toward aryl bromides and aryl iodides at room
temperature.
unsuccessful, and likely this unidentified complex plays an
important role in generating the active catalyst. If five equivalents
of NaOtBu are added to complex 1 or complex 2, a resonance at
À2.4 ppm appears, indicating formation of the unknown com-
plex. Surprisingly, if Pd(OAc)2, 0.8 equivalent of PtBu3, and five
equivalents of NaOtBu are stirred together in toluene, a reso-
nance at À2.4 ppm appears as well as a much smaller resonance
at 85.7 ppm corresponding to Pd(PtBu3)2. If the complex at À2.4
ppm was generated in situ by addition of 2.2 equivalents of
NaOtBu to complex 2 in toluene and submitted to the amination
conditions at 23 °C, N,N-dibutyl-o-toluidine was produced in a
30% yield.
Secondary and tertiary amines are known to promote the
reduction of Pd(OAc)2 to Pd(0) when used in conjunction with
alkoxide bases.17c Indeed, when Pd(OAc)2, 0.8 equivalent of
PtBu3, five equivalents of NaOtBu, and HNBu2 are stirred in
toluene, the only phosphine resonance present is at 85.7 ppm,
indicating formation of Pd(PtBu3)2. If five equivalents of NaOt-
Bu, HNBu2, and either complex 1 or complex 2 are stirred
together in toluene, initially, only a very small amount of
Pd(PtBu3)2 is seen along with a large signal at À2.4 ppm. After
18 h at room temperature, complex 1 showed only a small
amount of Pd(PtBu3)2 by 31P NMR spectroscopy, while complex
2 continues to slowly form Pd(PtBu3)2. The low yields of
arylamines combined with the ability of complex 1 to withstand
conditions that would typically reduce Pd(II) sources to Pd(0)
lead to the conclusion that complex 1 is not an intermediate that
leads to formation of the active catalyst under these conditions.
Instead, complex 1 is likely a product along the catalyst decom-
position pathway.
Since many reactions containing Pd(OAc)2/PtBu3 combina-
tions require elevated temperatures, we postulated that thermal
decomposition to Pd(0) might be possible. Complex 1 is stable
in toluene at 23 °C for days; however, when heated to 90 °C, it
decomposes within 12 h to form Pd(PtBu3)2. If alternatively
prepared Pd(PtBu3)2 was stirred with four equivalents of AcOH
at 23 °C, complex 1 was not observed.
To study the reactivity of complex 1 in cross-coupling reac-
tions, we submitted 1 to previously reported reaction conditions
that do not require elevated temperatures, thus preventing
thermal decomposition to Pd(0) complexes. Hartwig has shown
that Pd(OAc)2 and 0.8 equivalent of PtBu3 catalyze the amina-
tion of ortho-substituted aryl bromides at room temperature.5
We submitted complex 1, cyclometalated dimer 2, and Pd(PtBu3)2
to the reported reaction conditions using 2-bromotoluene and
dibutylamine (Table 1). Surprisingly, all three complexes gave
poor yields of N,N-dibutyl-o-toluidine. These reactions were
then repeated at 60 and 110 °C. At 60 °C, the reaction proceeds
in higher yields and faster reaction times for all complexes;
however, none of the complexes catalyzed the reaction in yields
comparable to reported conditions. At 110 °C, yields and
reaction times suffer, perhaps due to rapid catalyst decomposi-
tion. It is possible that the active catalyst contains fewer
equivalents of PtBu3 compared to Pd since a higher metal to
ligand ratio was more competent at catalyzing these reactions.
To identify the active catalyst in Hartwig’s aryl amination
reactions, Pd(OAc)2 and PtBu3 (0.8 equiv) were stirred in
toluene for 1 h. The 31P NMR spectrum showed an unidentified
resonance at À11.3 ppm and resonances corresponding to 2.
Attempts to isolate the source of the signal at À11.3 ppm were
Because complex 1 is able to withstand reduction to Pd(0) in
the presence of amines, we chose to examine whether secondary
amines were unreactive toward complex 1 or potentially formed
new complexes, preventing reduction to Pd(0). The addition of
morpholine to complex 1 results in displacement of coordinated
PtBu3, forming a new cyclometalated complex rather than
a Pd(0) species. The morpholine-containing complex 3 was
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Scheme 4
Scheme 6
Scheme 7
resulting from CÀH activation are seen. Instead, previously
unreported Pd[(PtBu2Me)2(OAc)2] (6) is formed in 69% yield,
which may lend insight into the steric requirements for facile
CÀH activation. Indeed, some complexes containing bulky
PtBu3 show an agostic CÀH interaction that may allude to a
low barrier of activation toward cyclometalation.3 Additionally,
the inability of PtBu2Me to form stable cyclometalated com-
plexes with Pd(OAc)2 may explain enhanced yields in certain
cross-coupling reactions when PtBu2Me was used instead of
PtBu3.25c,d,26
Scheme 5
Cyclometalated complexes are reported in the literature and
occasionally lead to highly reactive catalysts, such as allylic
substitutions catalyzed by iridium and a cyclometalated phos-
phoramidite ligand.27 In contrast, the results provided herein
indicate that complex 1 is likely a catalyst decomposition product
at room temperature, unable to form Pd(0) species in catalyti-
cally useful amounts. However, prior to cyclometalation, Pd-
(OAc)2 in the presence of NaOtBu, dibutylamine, and PtBu3 is
readily reduced to Pd(0). Moreover, the stability of complex 1 is
demonstrated in its ability to withstand reduction to Pd(0) under
conditions that reduce Pd(OAc)2/PtBu3 and complex 2 to
Pd(PtBu3)2, a Pd(0) species.
The analysis and identification of catalyst decomposition
pathways is often overlooked and understudied. However, the
importance in identifying such species lies in the ability to
engineer long-lived catalysts that avoid rapid decomposition
pathways. For example, it is common in the literature to find
procedures in which a metal salt and ligand are premixed, prior to
the addition of substrate and other reactants, to effectively form a
catalyst precursor or the active catalyst.26,28 However, if Pd-
(OAc)2 and two equivalents of PtBu3 are premixed, reactions
may proceed slowly or not at all at room temperature, thus
requiring elevated temperatures in order to effectively catalyze
reactions. Procedurally, since the cyclometalation of Pd(OAc)2
and PtBu3 occurs at room temperature, it would be pertinent to
avoid combining Pd(OAc)2 and PtBu3 in solution without base
or a combination of amine and base present. Furthermore, the
stoichiometry of Pd(OAc)2 to PtBu3 must be considered. In
many cases the optimal stoichiometry of PtBu3 to Pd(OAc)2
is less than 2:1, with greater amounts of PtBu3 requiring higher
isolated by recrystallization from pentane in 58% yield. We
hypothesized that because of the bulkiness of PtBu3 at the
palladium center, this ligand could also be displaced by smaller,
less basic phosphines. Indeed, the addition of PPh3 to 1 readily
displaces PtBu3 after 2 h at 23 °C to form the PPh3-ligated
complex 4 in 93% yield. Spectroscopically, complex 4 is
well-defined in the 31P NMR spectrum, which shows two
sharp doublets at 23 °C. Other phosphines such as 1,2-bis-
(diphenylphosphino)ethane (dppe) are also able to displace
PtBu3, forming the corresponding complex 5 in 98% yield.
Complexes 3 and 4 are alternatively formed from the addition
of one equivalent of PtBu3 and one equivalent of PPh3 or
morpholine, respectively, in THF to a stirring solution of Pd-
(OAc)2. The addition of dppe and PtBu3 to Pd(OAc)2 primarily
forms previously reported (dppe)2Pd(OAc)2.24
Recently, PtBu2Me has seen expanded use as a ligand in cross-
coupling reactions.25 While PtBu2Me is similar to PtBu3 electro-
nically, it has a significantly smaller cone angle (161° for PtBu2Me
vs 182° for PtBu3), and when 2.3 equivalents are stirred with
Pd(OAc)2 in THF for 2 h, no cyclometalated complexes
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Organometallics
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temperatures.2,5,29 The ability of 1 to thermally decompose to a
zerovalent palladium species should also be noted. While triar-
ylphosphine ligands readily form Pd(0) complexes from Pd-
(OAc)2, even at room temperature, it would be worthwhile to
identify a bulky electron-rich trialkylphosphine ligand that would
readily form a zerovalent palladium complex from Pd(OAc)2
without the addition of external reagents.
1570, 1410, 1271, 1172, 1021, 806. 1H NMR (C7D8, 400 MHz): δ 13.85
(s, 1H), 2.05 (s, 6H), 1.47À1.12 (m, 53H). 31P NMR (C7D8, 162 MHz,
23 °C): δ 67.3, 67.2, 62.9, 6.9, 8.9. 31P NMR (C7D8, 162 MHz, À20 °C):
δ 66.3 (d, J = 354 Hz), À8.7 (d, J = 354 Hz). Anal. Calcd for
C28H60O4P2Pd: C, 53.45; H, 9.61. Found: C, 53.53; H, 9.52.
[(C4H9NO)Pd(CH2C(CH3)2PtBu2)(OAc)]HOAc (3). Method
A. To a 4 mL borosilicate glass vial charged with Pd(OAc)2 (92 mg,
0.40 mmol), tri-tert-butylphosphine (83 mg, 0.41 mmol, 1.0 equiv), and
morpholine (36 μL, 0.41 mmol, 1.0 equiv) was added THF (2 mL). The
reaction mixture was stirred at ambient temperature for 2 h. The solvent
was removed under reduced pressure, and the yellow solid dissolved in a
minimal amount of pentane. The solution was then filtered through a
pad of Celite and placed at À30 °C for 12 h. The product was isolated by
vacuum filtration, washing with cold pentane to give [(C4H9NO)-
Pd(CH2C(CH3)2PtBu2)(OAc)]HOAc (44 mg, 0.086 mmol, 21%) as
an off-white crystalline solid.
’ CONCLUSION
In conclusion we have demonstrated that Pd(OAc)2 and
PtBu3, which are commonly used in cross-coupling reactions,
form the novel palladacyclic complex 1 through a facile CÀH
insertion step at room temperature. The formation of this
complex prevents quantitative formation of the precatalyst to
the active Pd(0) species. This CÀH insertion is likely due to the
pronounced steric nature of PtBu3, as PtBu2Me, another electron-
rich trialkylphosphine, does not form cyclometalated products
with Pd(OAc)2 at room temperature. With respect to aminations
of ortho-substituted aryl bromides, palladacycle 1 is catalytically
active. However, 1 appears to move further away from the
predominant active catalytic species, as reactions conducted with
this complex are slower and significantly lower yielding than
reactions using an in situ generated catalyst from Pd(OAc)2 and
PtBu3. While complex 1 is stable to reduction in the presence of
amine and base, we have shown that it may be reduced to
Pd(PtBu3)2 with heat or hydrogen. The ability of 1 to form Pd(0)
complexes at elevated temperatures provides an avenue for entry
into general Pd(0)/Pd(II) catalysis. Typically, strongly coordi-
nating PtBu3 is easily displaced from palladacycle 1 by smaller,
more weakly binding nitrogen- and phosphorus-containing
ligands. Utilizing the ability to easily displace PtBu3, we have
prepared a series of novel cyclometalated complexes. Currently,
attempts to isolate and identify the aforementioned unidentified
complexes are underway in our laboratory.
[(C4H9NO)Pd(CH2C(CH3)2PtBu2)(OAc)]HOAc (3). Method
B. To a 4 mL borosilicate glass vial charged with complex 1 (70 mg,
0.11 mmol) were added morpholine (10 μL, 0.11 mmol, 1.1 equiv) and
THF (1 mL). The reaction mixture was stirred at ambient temperature
for 2 h. The solvent was removed under reduced pressure. The resulting
white solid was dissolved in pentane, and the solution was placed
at À30 °C for 12 h. The product was isolated by vacuum filtration,
washing with cold pentane to give [(C4H9NO)Pd(CH2C(CH3)2PtBu2)-
(OAc)]HOAc (34 mg, 0.07 mmol, 58%) as a white crystalline solid.
Additionally the morpholine complex was prepared less the coordi-
nated acetic acid as follows. [(C4H9NO)Pd(CH2C(CH3)2PtBu2)-
(OAc)]: To complex 2 (112 mg, 0.15 mmol) in THF (1 mL) was
added morpholine (26.0 μL, 0.30 mmol, 2 equiv). The reaction was
stirred for 2 h at ambient temperature. The solvent was removed under
reduced pressure, and the off-white solid dissolved in a minimal amount
of pentane. The crude solution was then filtered through a pad of Celite,
and the solution placed at À30 °C for 12 h. The product was isolated by
vacuum filtration, washing with cold pentane to give [(C4H9NO)-
Pd(CH2C(CH3)2PtBu2)(OAc)] (61 mg, 0.13 mmol, 44%) as a white
crystalline solid. FTIR (film, cmÀ1): 3014, 2960, 1580, 1385, 1179, 1105,
1040, 887, 662. 1H NMR (C6D6, 400 MHz): δ 3.46 (t, J = 4.4 Hz, 4H),
2.64 (s, 4H), 2.16 (s, 3H), 1.36 (d, J = 13.6 Hz, 18H), 1.24 (d, J = 13.6
Hz, 6H), 0.55 (d, J = 0.4 Hz, 2H). 31P NMR (C6D6, 162 MHz): δ À6.7;.
Anal. Calcd for C18H38NO3PPd: C, 47.63; H, 8.44. Found: C, 46.71;
H, 7.51.
’ EXPERIMENTAL SECTION
General Methods. Unless stated otherwise, reactions were con-
ducted in 4 mL borosilicate glass vials under an inert atmosphere.
Complex 2 was prepared according to a known literature procedure.1
Palladium acetate was purified by recrystallization from hot benzene.
Morpholine was distilled over calcium hydride. As per literature
procedure, dibutylamine was used as received and not dried prior to
use in amination reactions.7 All other commercially obtained reagents
were used as received. Thin-layer chromatography (TLC) was con-
ducted with Sorbent Technologies silica gel UV254 precoated plates
(0.25 mm) and visualized using UV lamps and anisaldehyde staining.
1H and 31P NMR spectra were recorded on Bruker spectrometers and
are reported relative to solvent. IR spectra were recorded on a Perkin-
Elmer 1600 FT-IR spectrometer. Elemental analyses were obtained
from Robertson Microlit Laboratories.
[(PPh3)Pd(CH2C(CH3)2PtBu2)(OAc)]HOAc (4). Method A.
To a 4 mL borosilicate glass vial charged with Pd(OAc)2 (103 mg,
0.46 mmol), tri-tert-butylphosphine (93 mg, 0.46 mmol, 1.0 equiv), and
triphenylphosphine (126 mg, 0.48 mmol, 1.0 equiv) was added THF
(4 mL). The reaction mixture was stirred at ambient temperature for 2 h.
The solvent was removed under reduced pressure, and the yellow solid
dissolved in diethyl ether. The crude solution was then filtered through a
pad of Celite, and the solvent reduced until crystal formation began. The
solution was placed at À30 °C for 4 h; then pentane was added slowly to
the reaction mixture and the solution cooled at À30 °C for an additional
12 h to complete crystallization. The product was isolated by vacuum
filtration, washing with cold pentane to give [(PPh3)Pd(CH2C(CH3)2-
PtBu2)(OAc)]HOAc (268 mg, 0.389 mmol, 85%) as an off-white
crystalline solid.
[(PtBu3)Pd(CH2C(CH3)2PtBu2)(OAc)]HOAc (1). Tri-tert-butyl-
phosphine (PtBu3) (230 mg, 1.14 mmol) was added to a stirring solution
of Pd(OAc)2 (116 mg, 0.517 mmol) in THF (1 mL) at 23 °C. The
solution was stirred for 2 h. The solvent was removed under reduced
pressure, and the resulting pale yellow solid dissolved in a minimal
amount of pentane before filtering over Celite. The resulting solution
was reduced under vacuum until solid formation began and then placed
at À30 °C overnight to promote crystal formation. The product was
isolated by vacuum filtration, washing with cold pentane to give
[(PtBu3)Pd(CH2C(CH3)2PtBu2)(OAc)]HOAc (263 mg, 0.418 mmol,
81%) as pale yellow needles. FTIR (film, cmÀ1): 2900, 2478, 1736, 1707,
[(PPh3)Pd(CH2C(CH3)2PtBu2)(OAc)]HOAc (4). Method B.
To a 4 mL borosilicate glass vial charged with complex 1 (74.0 mg,
0.12 mmol) was added triphenylphosphine (35 mg, 0.13 mmol,
1.1 equiv) and THF (2 mL). The reaction mixture was stirred at ambient
temperature for 2 h. The solvent was removed under reduced pressure,
and the white solid dissolved in diethyl ether. The crude solution was
then filtered through a pad of Celite, and the solvent reduced until crystal
formation began. The solution was placed at À30 °C for 4 h; then
pentane was added slowly to the reaction mixture and the solution
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Organometallics
ARTICLE
cooled at À30 °C for an additional 12 h to complete crystallization.
The product was isolated by vacuum filtration, washing with cold
pentane to give [(PPh3)Pd(CH2C(CH3)2PtBu2)(OAc)]HOAc (75 mg,
0.11 mmol, 93%) as a white crystalline solid. FTIR (film, cmÀ1): 2963,
2904, 1712, 1602, 1552, 1434, 1255, 1094, 755, 697; 1H NMR (C6D6,
400 MHz): δ 14.21 (s, 1H), 7.86 (t, J = 8.8 Hz, 6H), 7.17À7.14 (m, 6H),
7.08À7.05 (m, 3H), 1.98 (s, 6H), 1.45 (d, J = 13.2 Hz, 18H), 1.22 (d, J =
12.8 Hz, 6H), 0.70 (t, J = 6.8 Hz, 2H). 31P NMR (C6D6, 162 MHz): δ
23.2 (d, J = 402 Hz), 6.0 (d, J = 402 Hz). Anal. Calcd for C34H48O4P2Pd:
C, 59.26; H, 7.02. Found: C, 58.98; H, 7.01.
General Procedure for the Reduction of Complexes Using
NaOtBu and HNBu2. A 4 mL borosilicate vial was charged with either
complex 1 (10 mg, 0.16 mmol), complex 2 (5.0 mg, 0.014 mmol Pd), or
a combination of Pd(OAc)2 (10 mg, 0.44 mmol) and PtBu3 (7.2 mg,
0.36 mmol). Sodium tert-butoxide (5 equiv) or sodium tert-butoxide
(5 equiv) and dibutylamine (5 equiv) were added followed by toluene
(1 mL). The resulting mixture was stirred for 1 h before being analyzed
by 31P NMR.
[(PPh2CH2)2Pd(CH2C(CH3)2PtBu2)(OAc)]HOAc (5). To
a
’ ASSOCIATED CONTENT
round-bottomed flask charged with complex 1 (92 mg, 0.15 mmol)
was added 1,2-bis(diphenylphosphino)ethane (60 mg, 0.15 mmol, 1.0
equiv) and THF (1 mL). The reaction was stirred at ambient tempera-
ture for 2 h. After 2 h the solvent was removed under vacuum, and the
resulting clear residue was taken up in a minimum amount of toluene,
layered with pentane, and placed at À30 °C for 12 h. The off-white
crystals were washed with pentane and dried under reduced pressure to
give [(Ph2PCH2)2Pd(CH2C(CH3)2PtBu2)(OAc)]HOAc (118 mg,
0.143 mmol, 98%) as off-white crystals. FTIR (film, cmÀ1): 3054,
2966, 2911, 1670, 1436, 1367, 1101, 748, 699, 647. 1H NMR (CD2Cl2,
400 MHz): δ 7.57À7.39 (m, 20H), 2.50À2.38 (m, 2H), 2.23À2.13 (m,
2H), 1.789 (s, 6H), 1.44 (d, J = 13.2), 1.21À1.12 (m, 20H). 31P NMR
(CD2Cl2, 162 MHz): δ 47.1 (dd, J = 360 Hz, J = 26 Hz), 41.6 (d, J =
26 Hz), À0.5 (d, J = 360 Hz). Anal. Calcd. for C42H57O4P3Pd: C, 61.13;
H, 6.96. Found: C, 61.25; H, 6.84.
S
Supporting Information. NMR spectra and X-ray crys-
b
tallographic data (in CIF format) are available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: stambuli@chemistry.ohio-state.edu.
’ ACKNOWLEDGMENT
This work was generously supported by The Ohio State
University. We thank Dr. Judith Gallucci (OSU) for the X-ray
crystal structure of complex 1.
Pd(PtBu2Me)2(OAc)2 (6). Di-tert-butylmethylphosphine (PtBu2Me)
(82 mg, 0.51 mmol, 2.3 equiv) was added to a stirring solution of
Pd(OAc)2 (50 mg, 0.22 mmol) in THF (1 mL) at 23 °C. The solution
was stirred for 2 h. The solvent was removed under reduced pressure, and
the resulting yellow solid was dissolved in a minimal amount of diethyl
ether, layered with pentane, and placed at À30 °C overnight to promote
crystal formation. The product was isolated by vacuum filtration, washing
with cold pentane to give Pd(PtBu2Me)2(OAc)2 (102 mg, 0.153 mmol,
69%) as a yellow crystalline solid. FTIR (film, cmÀ1): 2966, 2900, 1628,
1370, 1311, 1020, 884, 690. 1H NMR (C6D6, 400 MHz): δ 2.05 (s, 6H),
1.32 (t, J = 6.8 Hz, 32H), 0.89 (t, J = 3.2 Hz, 6H). 31P NMR (C6D6, 162
MHz): δ 28.1. Anal. Calcd for C22H48O4P2Pd: C, 48.49; H, 8.88. Found:
C, 48.45; H, 8.66.
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1
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