NMR Studies of the Species Present in Cross-Coupling Catalysis Systems Involving Pd(η3-1-Ph-C3H4) (η5-C5H5).

Article abstract of DOI:10.1021/om500618e

The compounds Pd(η³-1-Ph-C₃H₄) (η⁵-C₅H₅) (I), Pd₂(dba) ₃ (II), Pd(OAc)₂ (III), and [Pd(η³-1-Ph- C₃H₄)Cl]₂ (IV) are frequently utilized as catalyst precursors for a variety of cross-coupling processes, including Suzuki-Miyaura, Heck-Mizoroki, Sonogashira, and Buchwald-Hartwig reactions. In the preceding paper in this issue, we assess and compare catalyst systems based on I-IV activated with PBu^t₃, XPhos, and/or Mor-Dalphos for the prototypical Buchwald-Hartwig amination reactions of 4-bromo- and 4-chloroanisole with morpholine, noting several apparent incongruities which seem to indicate mechanistic dissimilarities for various reactant/precatalyst combinations. In this paper we investigate by NMR spectroscopy the solution chemistry of I and IV with PBu^t₃, XPhos, and Mor-Dalphos, noting similarities and differences in the respective abilities of these precursor-ligand combinations to generate palladium(0) catalyst systems. We find inter alia that steric requirements prevent Xphos and Mor-Dalphos from forming 2:1 palladium(0) complexes and, surprisingly, that 1:1 palladium(0) complexes of Xphos and Mor-Dalphos are unstable with respect to dissociation to free ligand and palladium metal. In other words, these two ligands and, by implication, other sterically demanding phosphine ligands do not form palladium(0) compounds.

Full text of DOI:10.1021/om500618e

Article
pubs.acs.org/Organometallics
NMR Studies of the Species Present in Cross-Coupling Catalysis
3
5
3
Systems Involving Pd(η ‑1-Ph‑C H )(η ‑C H ) and [Pd(η ‑1-
3
4
5 5
t
Ph‑C H )Cl] Activated by PBu , XPhos, and Mor-Dalphos:
3
4
2
3
Nonexistence of Pd(XPhos) and Pd(Mor-Dalphos) (n = 1, 2) at
n
n
Moderate Temperatures
Sogol Borjian and Michael C. Baird*
Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
3
5
ABSTRACT: The compounds Pd(η -1-Ph-C H )(η -C H ) (I), Pd (dba)
3
(
3
4
5
5
2
3
II), Pd(OAc) (III), and [Pd(η -1-Ph-C H )Cl] (IV) are frequently utilized
2 3 4 2
as catalyst precursors for a variety of cross-coupling processes, including
Suzuki−Miyaura, Heck−Mizoroki, Sonogashira, and Buchwald−Hartwig
reactions. In the preceding paper in this issue, we assess and compare catalyst
t
systems based on I−IV activated with PBu , XPhos, and/or Mor-Dalphos for the prototypical Buchwald−Hartwig amination
3
reactions of 4-bromo- and 4-chloroanisole with morpholine, noting several apparent incongruities which seem to indicate
mechanistic dissimilarities for various reactant/precatalyst combinations. In this paper we investigate by NMR spectroscopy the
t
solution chemistry of I and IV with PBu , XPhos, and Mor-Dalphos, noting similarities and differences in the respective abilities
3
of these precursor−ligand combinations to generate palladium(0) catalyst systems. We find inter alia that steric requirements
prevent Xphos and Mor-Dalphos from forming 2:1 palladium(0) complexes and, surprisingly, that 1:1 palladium(0) complexes of
Xphos and Mor-Dalphos are unstable with respect to dissociation to free ligand and palladium metal. In other words, these two
ligands and, by implication, other sterically demanding phosphine ligands do not form palladium(0) compounds.
1
n the preceding paper in this issue, we compared the
Complementing the above and in order to assess the
possibilities that, as suggested in the literature, 1:1 complexes
3
5
I
efficacies of catalyst systems based on Pd(η -1-Ph-C H )(η -
C H ) (I), Pd (dba) (II), Pd(OAc) (III) and [Pd(η -1-Ph-
C H )Cl] (IV) activated by PBu , XPhos, and/or Mor-
3 4
3
may be more active, we also investigated catalyst systems
5
5
2
3
2
t
t
involving I and IV activated by PBu , Xphos, and Mor-Dalphos
3
4
2
3
3
1
t
Dalphos for the prototypical Buchwald−Hartwig amination2
reactions of 4-bromo- and 4-chloroanisole with morpholine.
We found, not surprisingly in view of previous studies of
Suzuki−Miyaura, Heck−Mizoroki, and Sonogashira cross-
with ligand:Pd ratios of 1:1. While the 1:1 PBu system
3
exhibited activities higher than those of the 2:1 systems, the 1:1
catalyst systems were generally less active.
In this paper, we investigate by NMR spectroscopy the
3
t
coupling reactions, that I is superior to II and III; however.
solution chemistry of I and IV with PBu , XPhos, and Mor-
3
IV, assessed for the first time utilizing our methodology,
Dalphos in an effort to better understand the apparent
mechanistic incongruities noted above.
presented apparent anomalies. Interestingly, comparisons of
t
PBu with Xphos and Mor-Dalphos (ligand:Pd ratios 2:1) as
3
activators for I and IV also revealed complications. The Xphos-
RESULTS AND DISCUSSION
■
t
based catalysts were comparable with the PBu -based systems
1,3a−c,e
3
We previously demonstrated that I
and its parent
for cross-coupling of 4-bromoanisole but were superior for 4-
chloroanisole cross-coupling and also produced higher
conversions with 4-chloro- than with 4-bromoanisole. The
Mor-Dalphos catalyst systems involving both I and IV were
decidedly inferior for the amination of 4-bromoanisole.
3
5
3g
compound, Pd(η -C H )(η -C H ), react with a variety of
3
5
5
5
phosphines L to form palladium(0) species of the type PdLn
eq 1).
(
Of distinct interest and utility, the products could be formed
very efficiently in the absence of competing ligands such as
anions and dba and could also in many cases be identified
Received: June 10, 2014
Published: July 31, 2014
©
2014 American Chemical Society
3936
dx.doi.org/10.1021/om500618e | Organometallics 2014, 33, 3936−3940

Organometallics
Article
1
31
unambiguously using H and P NMR spectroscopy. We were
We have also investigated reactions of I with Xphos and
t
t
thus able to show that Pd[PCy ] , Pd[PMeBu ] , Pd[PCy ]-
PMeBu ], and Pd[PBu ] can all be readily formed as in eq 1
and that Pd[PBu ] exhibits no inclination to increase its
coordination number. In contrast, we were also able to show
that Pd[PCy ] and Pd[PMeBu ] react readily with added
PCy3 or PMeBu to form homo- and heteroleptic 3:1
coordination compounds and thus, as anticipated, that ligand
Mor-Dalphos and of IV with PBu and Xphos. Reactions of I
3
2
2 2
3
3
t
t
[
with XPhos would not be anticipated to form 2:1 species
because of the enormous steric requirements of this ligand; its
cone angle is estimated to be ∼256°, much greater than that of
2
3 2
t
3
2
t
t
8
e.g. PBu (182°). However, complexes of biaryl ligands such
3
2
2 2
3
t
2
as XPhos are believed to be stabilized by π bonding of the non-
4
9
phosphorus-containing ring of the biaryl group, and thus a
steric requirements play a significant role in deciding the
maximum coordination numbers possible.
chelated 1:1 species Pd(XPhos) may be capable of existence.
NMR experiments involving the reaction of I with XPhos
We also demonstrated that while treatment of I with 3 molar
equiv of PMePh2 or PPh3 in toluene generates the
were carried out in toluene-d , solutions of XPhos and I (1:1
8
1
31
and 2:1 molar ratios) being monitored by H and P NMR
1
corresponding tris complexes PdL , treatment with only 2
spectroscopy. At both molar ratios, H NMR spectra indicated
3
molar equiv in the same solvent and in the temperature range
that no reaction had occurred within 30 min at room
temperature, the singlet Cp resonance of I remaining
prominent. However, the resonances of I had almost
completely disappeared within 1 h at 75 °C and resonances
of the reductive elimination product, C H , had grown in,
5
0−75 °C results in the formation of palladium metal in
3b
addition to the tris complexes. The reaction sequence shown
in eq 2 seems implied. Thus, while the smaller phosphines can
stabilize Pd(0) as three-coordinate (and four-coordinate)
14
14
4
complexes, as has long been known, the corresponding 2:1
much as in eq 1; in addition, palladium metal powder had
3
1
complexes are, at moderate temperatures, unstable with respect
to the disproportionation shown in the second step of eq 1.
precipitated. Surprisingly. the only strong resonance in the
P
spectra was that of free XPhos, although a number of very weak
resonances of unidentifiable palladium(0) species in the range δ
3
3
3
Pd(η ‐1‐R‐C H )(η ‐C H ) + 6L
3
4
5
5
2
0−85 were also observed (Figure 1) .
R = H, Ph
−
3C H R
8 9
⎯
⎯⎯⎯⎯⎯⎯⎯ ⎯→ 3PdL → Pd + 2PdL
(2)
2
3
To our knowledge, the instability of 2:1 complexes of smaller
phosphines has not previously been recognized, although their
roles as cross-coupling catalysts have been considered
5
seriously. However, energies for the first dissociation reaction
shown in eq 3 for compounds of the types PdL are calculated
2
5
to be 19−33 kcal/mol, depending on the phosphines
considered and the computational methodologies employed.
Less is known about the corresponding Pd−P bond
dissociation energies for the resulting 1:1 species PdL, but
the Pd−P bond lengths of PdL are calculated to be shorter than
5
those of PdL by ∼0.1 Å and thus the bonds are presumably
2
somewhat stronger than 19−33 kcal/mol.
Figure 1. 31P NMR spectra showing the progress of the reaction of I
with XPhos (1:1 molar ratio) (a) at room temperature immediately
after mixing and (b) after 1 h at 75 °C.
PdL → PdL + L
(3)
2
Since the enthalpy of atomization of palladium metal is ∼91
6
kcal/mol while the decomposition reaction of eq 4 involves a
positive entropy change, it follows that all Pd(0) precatalysts of
the type PdLn must be thermodynamically unstable with
respect to dissociation to palladium metal and free phosphine,
as in eq 4. As to why steric bulk might stabilize bis-phosphine
complexes, we suggest tentatively that the more sterically
demanding ligands prevent the close approach of Pd atoms
which might be necessary to permit aggregation to metal
particles. We note that the behavior noted here resembles that
Incidentally, the palladium metal powder which precipitated
was found not to function as a catalyst for amination cross-
coupling reactions; thus, traces of palladium metal do not
contribute inadvertently to any of the catalytic processes
described in this and previous work, as might otherwise be
10
anticipated.
Similar experiments were carried out in toluene-d utilizing
8
3
5
3g
observed with Pd(η -C H )(η -C H ) which, on heating in
Mor-Dalphos and, although the reactions of I with this ligand
were slightly slower than those with XPhos, the result was
much the same; precipitation of palladium metal left the free
ligand as the only major phosphorus-containing product.
Compound I is stable for at least 10 h under these
conditions, and thus it is clear that both XPhos and Mor-
Dalphos react with I in solution to induce reductive elimination
of C H but that the resulting palladium-containing products
3
5
5
5
the absence of potential ligands, reductively eliminates C H −
5
5
7
C H and deposits palladium metal.
3
5
PdL → Pd + 2L
(4)
2
In complementary attempts to gain further information
about the nature of the palladium(0) species present in solution
prior to and during catalytic cross-coupling reactions, we have
14
14
1
31
now extended our earlier NMR investigations ( H, P) of the
are not stable with respect to palladium metal formation. As
with bis-phosphine complexes of small ligands, it seems that the
palladium atoms of the presumed PdXphos and PdMor-
Dalphos are sufficiently exposed that palladium atoms can react
and aggregate.
t
reaction of I with PBu . We find that, when it is reacted in a 1:1
3
t
molar ratio in toluene-d , half of the I reacts to give Pd[PBu ] ,
8
3 2
there being evidence of neither a 1:1 species nor the
precipitation of palladium metal.
3
937
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Organometallics
Article
3
Reactions of dimeric η -allylic species such as IV with tertiary
phosphine ligands have been much studied and frequently
result in cleavage of the chloro bridges and formation of
seems not to have been prepared previously but is a reasonable
product on the basis of eq 3. We also note that the relatively
low field 31P chemical shifts of the closely related Pd(η -
3
11
t
11f
11i
monomeric Pd(II) products as in eq 5. (Mor-DalPhos has
C H )Cl(PBu ) (δ 82.5 in CH Cl , δ 86.0 in CDCl ) and
3 5 3 2 2 3
11i
3
t
Pd(η -crotyl)Cl(PBu ) (δ 89.7 in CDCl ) are consistent
3
3
3
with our proposed identification of Pd(η -1-PhC H )Cl-
3
4
t
(
PBu ), and it is interesting to note that the resonance of
3
3
t
Pd(η -1-PhC H )Cl(PBu ) weakens over 1 h as that of
3
4
3
t
3
Pd[PBu ] increases in intensity. Thus, Pd(η -1-PhC H )Cl-
previously been shown to react readily with IV at room
temperature to cleave the chloride bridges and form both a
chloro-η -cinnamyl complex and a cationic η -cinnamyl
complex; in both, the Mor-DalPhos chelates via the phosphorus
and nitrogen atoms. ) In addition, palladium(0) compounds
can also be formed via reactions thought to involve
combination of the allylic moiety with a nucleophile, which
may be anionic or neutral as in eq 6. Formation of the
3 2
3
4
t
t
(
PBu ) may be an intermediate in the formation of Pd[PBu ] .
3
3
2
1
3
The resonance at δ 50.9 was very weak, largely because the
compound responsible for it precipitated from solution as
colorless crystals; it seemed, however, to be more soluble in
12
31
benzene-d , in which its P resonance became much stronger
6
at room temperature prior to precipitation. The compound
giving rise to the resonance appears not to have been described
in the literature but was, fortuitously, encountered during a
parallel line of inquiry in which the bromide salt of the
t
+
1
phosphonium cation [PhC H PBu ] was identified by H and
3
4
3
31
13
P NMR spectroscopy and X-ray crystallography. The
palladium(0) and allylic products can in principle involve
product formed here was separated from the NMR solution,
3
1
31
intermolecular nucleophilic attack on an η -allylic ligand or
washed with toluene, and dried. It exhibits H and P NMR
spectra (CD Cl ) essentially identical with those of
1
intramolecular reductive elimination from an η -allylic
2
2
11b,c,e,g−i
t
t
species.
In this investigation we have studied the reactions of IV with
[PhC H PBu ]Br and is identified as [PhC H PBu ]Cl, a
3 4 3 3 4 3
t
reasonable byproduct during the formation of Pd[PBu ] as in
3
2
t
t
1
31
PBu (PBu :Pd ratio 2:1) in toluene-d at 75 °C by H and P
eq 6.
3
3
8
NMR spectroscopy. Several products, in varying proprortions
Interestingly, two weak new doublets were also observed in
the initial room-temperature spectrum, the constituents of an
AB quartet at δ 85.3, 88.6 (JPP = 48.2 Hz). These resonances
weakened further over 1 h, being replaced by the constituents
depending on the time elapsed, were observed and identified
3
1
(
see Figure 2 for P NMR spectroscopic results), although
attempts to isolate them were unsuccessful.
of another AB quartet at δ 84.1, 86.7 (J = 66.8 Hz). The two
PP
sets of AB quartets are reminiscent of 31P NMR spectra of syn
and anti dinuclear palladium(I) species of the types Pd L (μ-1-
2
2
3
b,g
14
PhC H )(μ-Cp) and Pd L (μ-1-PhC H )(μ-Cl), in which
3
4
2
2
3
4
the 3 P nuclei are nonequivalent because of the asymmetry of
1
the bridging cinnamyl ligand. The P−P coupling constants
observed for the latter compounds are similar to that observed
t
here, although no PBu analogue was reported. We therefore
3
attribute the two sets of resonances to syn and anti isomers of
t
Pd (PBu ) (μ-1-PhC H )(μ-Cl), although it is not clear how
2
3 2
3
4
this compound would form. Analogous di-μ-halo compounds of
the type Pd L (μ-X) are known to be formed and involved
2
2
2
15
during cross-coupling reactions.
We also briefly investigated the species formed on treating a
t
t
toluene-d solution of IV and PBu (PBu :Pd 2:1) with
8
3
3
t
NaOBu and morpholine, the reactants utilized during our
amination reactions. Interestingly, addition of 4 equiv of
NaOBu to the mixture of IV and PBu at room temperature
t
t
Figure 2. 31P NMR spectra showing the progress of the reaction of IV
3
t
31
3
with PBu 3 (P:Pd ratio 2:1) in toluene-d (a) at room temperature
resulted in the appearance of the P resonances of Pd(η -1-
8
t
t
immediately after mixing, (b) after 30 min at 75 °C, and (c) after 1 h
at 75 °C.
PhC H )Cl(PBu ) (weak) and Pd[PBu ] (moderate) and, in
3 4 3 3 2
t
addition, a relatively strong resonance of unreacted PBu . On
3
the subsequent addition of morpholine to the solution and
3
3
1
warming to 75 °C, the resonance of Pd(η -1-PhC H )Cl-
As is clear in Figure 2a, the P NMR spectrum of an initially
formed mixture at room temperature exhibited the resonances
3
4
t
31
(
PBu ) disappeared immediately and the major P resonance
observed was that of Pd[PBu ] , although a small amount of
free PBu remained. Inspection of the H NMR spectrum at
3
t
t
3b,g
t
of PBu and Pd[PBu ] at δ 62.2 and 85.0,
addition to a strong new singlet at δ 92.3 and weaker
resonances in the region δ 83−90 and at δ 50.9. Within 1 h, the
resonance of free PBu became very weak as the resonance of
Pd[PBu ] increased in intensity (Figure 2b,c).
Athough resonances in the H NMR spectrum overlapped
somewhat with resonances of other species, rendering assign-
respectively, in
3 2
3
3 2
t
1
3
this stage revealed the presence of N-cinnamylmorpholine and
cinnamyl-tert-butyl ether in a ∼5:1 ratio.
t
3
t
3
2
1
3
1
ments tenuous, the species with the P resonance at δ 92.3
3
t
appeared to be Pd(η -1-PhC H )Cl(PBu ). This compound
3
4
3
3
938
dx.doi.org/10.1021/om500618e | Organometallics 2014, 33, 3936−3940

Organometallics
Thus, in the absence of bases, Pd[PBu ] can be partially
Article
t
nor Mor-Dalphos can support Pd(0) as either 1:1 or 2:1
complexes at moderate tempertatures.
We have also investigated the products formed when IV is
3
2
t
formed by the coupling of cinnamyl and PBu ligands to form
3
t
+
the phosphonium cation [PhC H PBu ] . However, addition
of a combination of NaOBu and morpholine results rapidly in
the formation of Pd[PBu ] via the coupling of the allylic group
3
4
3
t
t
t
t
treated with PBu (PBu :Pd 2:1), NaOBu , and morpholine:
3 3
t
1
t
3
2
i.e., the mixture producing catalysis. On addition of NaOBu at
room temperature and prior to the disappearance of unreacted
with either an alkoxy or amido group.
t
3
t
We also investigated the reaction of IV with XPhos by
monitoring the solutions of XPhos and IV (2:1 molar ratio) in
PBu , a small amount of Pd(η -1-PhC H )Cl(PBu ) and
3
3
4
3
t
rather more Pd[PBu ] formed quickly. On the subsequent
addition of morpholine to the solution and warming to 75 °C,
3
2
1
31
toluene-d at room temperature and 75 °C by H and P NMR
8
spectroscopy. It was found that, at room temperature, about
3
t
the resonance of Pd(η -1-PhC H )Cl(PBu ) disappeared and
3
4
3
3
1
6
5% of the XPhos reacted immediately and a broad
P
31
t
the major P resonance observed was that of Pd[PBu ] .
3
2
1
resonance appeared at δ ∼56. At this point, the H NMR
spectrum showed that all of the IV had reacted and that a new
set of cinnamyl resonances had appeared at δ 5.52 (dt, J = 12.6,
1
Inspection of the H NMR spectrum at this stage revealed the
presence of N-cinnamylmorpholine and cinnamyl-tert-butyl
ether in a ratio of ∼4:3. These results are consistent with
9
.4 Hz), 4.96 (dd, J = 10, 12.6 Hz), 2.90 (br), and 2.23 (br).
To investigate the system further, H and P NMR spectra
3
palladium(0) formation via either direct attack at the η -1-
1
31
PhC H ligand by alkoxide or amide anions or reductive
elimination of η -1-PhC H with alkoxide or amide ligands.
3
4
were run at −70 °C and it was found that the broad resonance
1
3
4
at δ ∼56 had decoalesced to two resonances of unequal (∼4:3)
1
Our results in this investigation demonstrate the utility of I
not only for generating useful catalytic species but also for
making possible useful investigations of phosphine palladi-
um(0) complexes in the absence of other potential ligands,
anionic or neutral.
intensities at δ 54.7 (br) and 56.9 (vbr). The H resonances of
the cinnamyl ligand also decoalesced: that at δ 5.52 to
resonances at δ 5.43 and 5.60 and that at δ 4.96 to resonances
at δ 5.07 and 4.93. Apparent decoalescence of the two broad
resoances at δ 2.90 and 2.23 was more difficult to define
because of extensive overlap with other resonances. While firm
assignments are impossible at this stage, the spectral evidence is
EXPERIMENTAL SECTION
■
3
3
consistent with either [(η -cinnamyl)Pd(Xphos)]Cl or (η -
cinnamyl)Pd(Xphos)Cl, both consistent with eq 5.
General Procedures. All syntheses were carried out under a dry,
deoxygenated argon or nitrogen atmosphere with standard Schlenk
line techniques. Argon was deoxygenated by passage through a heated
column of BASF copper catalyst and then dried by passing through a
column of 4A molecular sieves. Solvents were dried by passage
through activated alumina and were then stored under an argon
atmosphere for a minimum of 24 h over 3A molecular sieves which
had been activated by heating at 225 °C and 10 Torr for several
days. Phosphines were purchased from Strem Chemicals and all other
compounds from Sigma-Aldrich. Johnson-Matthey generously pro-
vided us the PdCl . Handling and storage of air-sensitive compounds
were carried out in an MBraun Labmaster glovebox. NMR spectra
were recorded on a 500 MHz spectrometer, with H NMR data being
referenced to TMS via the residual proton signals of the deuterated
solvents. Compound I, Pd(η -1-PhC H )(η -C H ), and IV were
prepared as detailed in the literature.
NMR Studies of the Reactions of I and IV with PBu , XPhos,
We favor slightly the former, ionic structure both for steric
reasons and because the decoalescence patterns observed in the
1
31
H and the P NMR spectra are reminiscent of similar
observations reported previously for three-coordinate com-
pounds of the type PdArXL, where L = Xphos and similar
−2
9
ligands. Here exchange between two or more conformations of
the coordinated ligand, possibly stabilized by arene−Pd
interactions, resulted in broad, averaged NMR spectra at
higher temperatures and decoalesced spectra at lower temper-
atures. In the three-coordinate complex [(cinnamyl)Pd-
2
1
(
Xphos)]Cl, the vacant site provides for arene−Pd interactions
analogous to those invoked for the compounds PdArXL. (We
3
5
3
4
5
5
3
b,16
note that IV reacts with Mor-Dalphos similarly, producing both
1
3
12
t
3
η and η species. )
3
5
Complementing the results reported here, we are also
and Mor-Dalphos. Utilizing Pd(η -1-PhC
3 4 5 5
H )(η -C H ) (I). For L:Pd
reactant ratios of 2:1, 0.0057 g of I (0.02 mmol) was combined with
.008 g of PBu (0.04 mmol), 0.019 g of XPhos (0.04 mmol), or 0.018
3
g of Mor-Dalphos (0.04 mmol); for L:Pd ratios of 1:1, 0.0057 g of I
(0.02 mmol) was combined with 0.004 g of PBu (0.02 mmol), 0.009
g of XPhos (0.02 mmol), or 0.009 g of Mor-Dalphos (0.02 mmol) in
1
31
investigating by H and P NMR spectroscopy the interesting
t
0
intermediate species formed when I and IV react with various
1
phosphines in the presence of excess bromobenzene. This
t
13
3
work will be published separately.
0
.6 mL of benzene-d or toluene-d in an NMR tube. The reaction
6 8
SUMMARY AND CONCLUSIONS
mixtures were shaken briefly, placed in an oil bath at 75 °C, and
monitored at 30 min intervals by H and P NMR spectroscopy (1D,
COSY, H− P HMBC) at 25 °C.
Utilizing [Pd(η -1-Ph-C
0.0052 g of IV (0.01 mmol) was combined with 0.008 g of PBu
mmol) or 0.019 g of XPhos (0.04 mmol); for L:Pd ratios of 1:1,
■
1
31
In an effort to extend our previous studies into the
palladium(0) species formed when I reacts with tertiary
phosphines, we have investigated by H and P NMR
spectroscopy the reactions of I with PBu , Xphos, and Mor-
Dalphos (1:1 and 2:1 molar ratios) in toluene at 75 °C. As
1
31
3
1
31
3 4 2
H )Cl] (IV). For L:Pd reactant ratios of 2:1,
t
t
(0.04
3
3
t
0
.0052 g of IV (0.01 mmol) was combined with 0.004 g of PBu (0.02
1,3
3
anticipated on the basis of previous work,
all induced
mmol) or 0.009 g of XPhos (0.02 mmol) in 0.6 mL of benzene-d or
toluene-d in an NMR tube. The reaction mixtures were shaken briefly,
6
reductive elimination of C H , showing that all had reacted
1
4
14
8
with the otherwise thermally stable I. However, while a
1
placed in an oil bath at 75 °C, and monitored at 30 min intervals by H
t
and 31P NMR spectroscopy (1D, COSY, H− P HMBC) at 25 °C.
1
31
deficiency of PBu resulted only in the formation of
3
t
Pd[PBu ] , the major products formed ultimately with Xphos
3
2
and Mor-Dalphos were, surprisingly, palladium metal and the
free ligand. Other phosphorus-containing products were
present at very low concentrations and presumably account
AUTHOR INFORMATION
■
Notes
1
for the catalysis previously observed, but clearly neither Xphos
The authors declare no competing financial interest.
3
939
dx.doi.org/10.1021/om500618e | Organometallics 2014, 33, 3936−3940

Organometallics
Article
(
(
8) Clavier, H.; Nolan, S. P. Chem. Commun. 2010, 46, 841.
9) Barder, T. E.; Biscoe, M. R.; Buchwald, S. L. Organometallics
ACKNOWLEDGMENTS
■
We are grateful to Johnson-Matthey for a generous loan of
2
007, 26, 2183.
(10) Deraedt, C.; Astruc, D. Acc. Chem. Res. 2014, 47, 494.
11) For a sampling of relevant papers, see: (a) Powell, J.; Shaw, B. L.
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PdCl and acknowledge financial support from the Natural
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from Queen’s University.
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