α-Substituted Benzylic Complexes of Palladium(II) as Precursors of Palladium Hydrides

Article abstract of DOI:10.1021/acs.organomet.8b00066

The adoption of a pseudoallylic (η³) form makes palladium benzylic derivatives a class of stabilized palladium alkyls that can undergo β-H elimination reactions in a more controlled way. α-(Pentafluorophenylmethyl)benzyl palladium complexes have been studied, and they decompose by β-H elimination to give palladium hydrides that, depending on the auxiliary ligands, can: (a) transmetalate to another palladium atom and, by reductive elimination, give hydrogenated products; this process is favored for a combination of bridging ligands (i.e., halogens) and low coordinating ligands. (b) Be used as a hydride source and get trapped by a diene to give palladium allylic derivatives. The presence of carbon monoxide does not induce a β-H elimination reaction, and only CO insertion into the Pd-benzyl bond to give acyl derivatives is observed.

Full text of DOI:10.1021/acs.organomet.8b00066

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
α‑Substituted Benzylic Complexes of Palladium(II) as Precursors of
Palladium Hydrides
Blanca Martín-Ruiz, Ignacio Perez-Ortega, and Ana C. Albeniz*
́ ́
IU CINQUIMA/Química Inorgan
́
ica, Universidad de Valladolid, E-47071 Valladolid, Spain
*
S Supporting Information
3
ABSTRACT: The adoption of a pseudoallylic (η ) form makes palladium
benzylic derivatives a class of stabilized palladium alkyls that can undergo β-H
elimination reactions in a more controlled way. α-(Pentafluorophenylmethyl)-
benzyl palladium complexes have been studied, and they decompose by β-H
elimination to give palladium hydrides that, depending on the auxiliary ligands,
can: (a) transmetalate to another palladium atom and, by reductive
elimination, give hydrogenated products; this process is favored for a
combination of bridging ligands (i.e., halogens) and low coordinating ligands.
(
b) Be used as a hydride source and get trapped by a diene to give palladium
allylic derivatives. The presence of carbon monoxide does not induce a β-H elimination reaction, and only CO insertion into the
Pd−benzyl bond to give acyl derivatives is observed.
INTRODUCTION
intermediate and, since it requires the return to the σ-benzylic
form, how it will be influenced by the ligands present in
solution and in the metal coordination sphere (Scheme 1). We
have previously prepared a variety of benzylic palladium
complexes with different ligands and an α-methylpentafluor-
■
The stabilization of palladium alkyls usually requires the
absence of hydrogens in the β position in order to avoid a β-H
elimination process that transforms the complex in a palladium
hydride and an olefin. Thus, alkyl groups bearing a
trisubstituted carbon, methyl, or conformationally rigid cyclic
derivatives (often metallacycles) are among the most common
organometallic palladium alkyl complexes. These groups are
impervious to β-H elimination, but they are also reluctant to
undergo other types of reactions involved in useful trans-
formations such as reductive elimination with another organic
5
ophenyl substituent (Scheme 2). These derivatives eventually
decompose by β-H elimination at very different rates, and they
are suitable model complexes to analyze this competing
reaction.
We have also been interested in exploring the ability of
benzylic complexes to behave as precursors of Pd-hydrides, the
byproducts of the β-H elimination reaction, and their
subsequent transformations. The interplay between the
1
fragment. Benzylic palladium complexes are weakly stabilized
alkyls due to the possibility of coordination of the aryl ring in a
3
3
pseudoallylic (η ) form (Scheme 1). Even when α-benzylic
stabilizing pseudoallylic (η ) form and the alkyl σ-benzylic
form (that restores the aromaticity of the aryl) slows down the
β-H elimination reaction, and this could be used as a controlled
source of Pd-H fragments. Also, the evolution of the generated
3
Scheme 1. η -Benzylic Stabilization and β-H Elimination in
Benzylic Palladium Complexes
[
PdHXL ] can produce unwanted side reactions. We describe
n
here a study of the decomposition of a number of benzylic
palladium complexes by β-H elimination and the fate of the
palladium hydrides that are produced.
The benzylic palladium complexes used in this study are
shown in Scheme 2. All of them were synthesized from the
dimeric complex 1 upon addition of the suitable amount of
5
,6
ligand. The rich coordination chemistry of these derivatives
3
substitutents (R) provide hydrogens in a β position, the η3
allows forming η -benzylic and σ-benzylic complexes, as well as
tetra- or pentacoordinated compounds. A rich fluxional
behavior is also observed in some cases that involve ligand
exchange as well as η -σ benzylic exchange. As a result of these
manifold situations, the stability of the complexes is quite
different in solution, but all of them decompose cleanly by β-H
bonding mode makes the attainment of a cis Pd−β-H
arrangement difficult, preventing elimination, and stabilizing
3
5
2
the hydrocarbyl moiety. Yet, this group is reactive enough to
participate in other bond-forming processes and benzyl
palladium derivatives are involved in many catalytic reactions
3
,4
that use styrene or benzylic halides as substrates.
β-H
elimination is a competing reaction and it would be interesting
Received: January 31, 2018
to find out how efficiently this process can destroy a Pd-benzyl
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00066
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Organometallics
Article
Scheme 2. Benzylic Palladium Complexes Used in This
Work
benzylic palladium complex and the decomposition products 9
1
9
and 10 were determined according to the intensity of their
NMR resonances (C F group). Half-lives of the complexes in
F
6
5
solution and/or the time for complete decomposition are given,
and they show large differences in stability, clearly related to the
availability of an empty coordination site required for β-H
elimination, as shown in Scheme 1. It is noteworthy that, as
decomposition takes place, free ligand is released in solution
and this slows the decomposition down; this is the reason for
the long times required for total decomposition when
compared to the half-lives of some of the complexes.
The cationic derivatives, with a more electrophilic metal
center, are less prone to lose a ligand and create a vacant
coordination site, so, in general, they are more stable than their
neutral counterparts (cf. decomposition times for entries 2 and
1
0, 5 and 11, 13, and 17 in Table 1). Besides, an electron poor
metal center does not favor the cleavage of the C−H bond,
once the agostic interaction has been established (Scheme 1).
The complexes bearing chelating ligands, more difficult to
dissociate, are more stable that those with monodentate ligands
(
see entries 3−6 vs 7−9, Table 1). The dissociation of the
bromo ligand is also a possible pathway, and in fact, a distinct,
but moderate, decrease in the amount of decomposed complex
is observed for 2c (L = dppe) when bromide is added (after 3
days, 24% of 2c remains in solution vs 38% when 1 equiv of
bromide per palladium is added).
elimination to give pentafluorostilbene 9 (the trans isomer is
always the major one) and a palladium hydride that has been
detected in some cases. This hydride either decomposes or is
transferred to another complex molecule to give, by reductive
elimination, the reduction product 10 (eq 1).
The pentacoordinated complexes 8 decompose fast even at
low temperature (entries 16 and 17, Table 1). Both are
fluxional and the dynamic behavior of complex 8i indicates a
5
fast ligand exchange above 253 K. This increases the chances
of creating a vacant coordination site for β-H elimination. For
complexes 8i and 8j, both products of β-H elimination, 9 and
We have studied this hydride transfer before using complex 1,
and the proposed route is depicted in Scheme 3. A radical
1
the corresponding [PdHBrL ], could be detected by H NMR:
2
2
[
(
PdHBr(PPh ) ] (δ = −11.82 t, J
= 9.5 Hz); [PdHBr-
3
2
P‑H
7
AsPh ) ] (δ = −14.37, s).
3
2
Scheme 3. Decomposition of 1 by β-H Elimination and
Subsequent Hydride Transfer
Besides the different stability discussed above, Table 1 also
shows the occurrence of hydride transfer between palladium
atoms for each particular complex, using the parameter of
efficiency of the transfer E = [10]/[9] (last column). If all the
Pd-H generated is transferred to another palladium atom
through an intermediate analogous to C in Scheme 3,
equimolar amounts of 9 and 10 are formed and E = 1. If no
transfer occurs, E = 0. Our previous study on the
decomposition of complex 1 ruled out the protonation of a
6
Pd-benzyl as a source of 10. However, the presence of extra
ligands, as in the derivatives shown here, makes the palladium
complexes more electron-rich and, therefore, more prone to
undergo protonation. For this reason, we followed up the
decomposition of complex 2a, with the strong donor PMe as a
3
ligand, in the presence of D O. No deuterium incorporation in
2
1
0 was observed, indicating that protonation of the benzylic-
reaction or a benzyl protonation was ruled out, and an
intermolecular hydride transfer between palladium atoms aided
by a halogen bridge (hydride transmetalation) is the most
plausible pathway (C, Scheme 3). In this work, we show now
how this process is influenced by the presence of additional
palladium moiety is not responsible of the formation of the
saturated compound.
Hydride transfer between palladium atoms is aided by the
presence of a bridging ligand that anchors both palladium
centers. Bromide is playing that role for the complexes studied,
and in the absence of Br or other ligand capable of acting as a
bridge, the hydride transfer observed is very small or
nonexistent (entries 10, 11, 13, Table 1). In the case of
6
ligands in the metal coordination sphere.
RESULTS AND DISCUSSION
■
Decomposition of the Benzylic Complexes. Table 1
collects the decomposition data for some of the complexes
prepared. Solutions of the complexes in CDCl or CD CN
complexes of composition [PdRBrL ] (R = H, benzyl), three
2
possible dimeric intermediates can be proposed for the
exchange (D−F, Scheme 4). The triply bridged intermediate
D has been proposed in the exchange of aryl groups between
3
3
were monitorized by NMR. The amounts of the remaining
B
DOI: 10.1021/acs.organomet.8b00066
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Article
a
Table 1. Decomposition Data for the Benzylic Complexes in Scheme 2
b
c
d
d
d
e
entry
1
[Pd]
ligand
Pd:L
T (K)
t1/2
t
trans-9 (%)
cis-9 (%)
10 (%)
E
1
293
293
283
293
293
293
293
293
293
293
293
283
293
293
293
273
273
7 days
1 day
4 h
46
45.3
49.9
69.7
50.2
55.4
63.3
43.4
53.3
74.7
74.9
52.1
100
53.5
68
33
0.72
0.74
0.78
0.44
0.72
0.71
0.3
f
2
2h
2a
2b
2d
2f
2c
2e
2g
3h
3d
4
CD CN
excess
1:2
12
42.7
38.7
30.3
39.0
40.8
20.2
5.4
3
3
4
5
6
7
8
PMe3
P(OMe)3
py
1:2
3 h
1:2
10 h
4.1
tht
1:2
30 min
36 h
3.5 h
48 h
72 h
40 h
1.5 h
16h
17 h
dppe
1:1
10 days
1 day
9 days
30 days
30 days
15 h
4.0
bipy
1:1
31.3
0.07
0.62
0.07
0.06
0.92
0
9
f
(PhSCH2)2
1:1
33.6
6.7
10
11
12
13
14
15
16
17
CD CN
excess
1:4
18.6
7.7
3
f
py
5.2
tht
1:1
47.9
5j
6
AsPh3
Br
1:2
2 days
3 h
1:5
46.3
31
0.86
0.45
0.04
0.77
7
PPh3
PPh3
AsPh3
1:1
10 min
10 min
30 min
8i
8j
1:2
96
4
1:2
48.7
37.3
a
19
The disappearance of the complexes and the byproduct distribution was analyzed by integration of the F NMR resonances of the decomposition
b
c
mixtures. CDCl as solvent unless otherwise noted. Time needed for decomposition of about 50% of the complex. Time for complete
decomposition. Percentage of products observed after complete decomposition (time t, in the preceding column); small amounts of other
3
d
unidentified byproducts (each of them less than 5%) were observed when the sum of percentages listed for 9 and 10 is less than 100, with the
e
f
exception of entry 1 where 21% of PhCHBrCH C F was observed as reported before (ref 6). E = [10]/[9]. CD CN as solvent.
2
6
5
3
Scheme 4. Plausible Dimeric Intermediates Involved in the
Hydride Transfer between Palladium Atoms
As can be seen in Table 1, the hydride transfer mechanism
can be quite efficient, reaching high values of E (see, for
example, entries 12 and 14, Table 1), which implies that almost
all the Pd-H produced by β-H elimination is consumed in the
reduction of the remaining Pd-R moieties. In a catalytic
reaction, this means that, if a substituted benzylic halide is used
as reactant, a dehalogenation reaction can be a competing
process. A Heck-type reaction with styrene can also produce a
formal hydrogenation of the product through this mechanism;
thus, this pathway could be responsible for the sometimes
observed saturated byproducts, or plays a role in the
mechanism of the reductive Heck-type reactions where
palladium hydrides are formed in the presence of suitable
9
hydrogen donors. According to our results, this inter-
palladium transfer can be minimized by avoiding the presence
of halides in solution and using a higher L:Pd ratio and better
coordinating ligands, provided the intended catalytic reaction is
not drastically slowed down by such a ligand choice.
palladium atoms where two of the bridging ligands are aryl
groups. In our case, only the hydride is an electron deficient
8
Reactions with CO. β-H elimination does not take place in
ligand, and it generally forms a stronger bridge than an aryl
ring. For this reason, a triple bridge is probably not necessary
for an efficient transfer and presumably the doubly bridging
species E or F are stable enough to support the exchange. In
any case, the dissociation of a ligand is required and strongly
coordinating species may prevent the transfer. This is observed
10
the presence of carbon monoxide. The reaction of CO with
complex 1 leads to the insertion of carbon monoxide into the
Pd−benzyl bond to give complex 11 as a white solid of low
solubility, and the formation of stilbene 9 is not observed
(
Scheme 5). The structure of complex 11 is proposed based on
1
19
the H and F NMR spectra as well as the IR spectrum where
when comparing complexes 8i (L = PPh , E = 0.04) and 8j (L
3
strong absorptions corresponding to the acyl fragment (1783−
=
AsPh , E = 0.77) where virtually no transfer occurs for the
−1
−1
3
1
727 cm ) and the coordinated CO (2112 cm ) were
stronger phosphine ligand but the value of E is high for the less
coordinating arsine (entries 16 and 17, Table 1). When the
ratio L:Pd decreases, the hydride transfer is more effective (cf.
entries 15 and 16, 6 and 12, Table 1). A comparison of the E
value for complexes 2c, 2e, and 2g with chelating ligands shows
that a higher value is observed for the more flexible ditioether,
followed by dppe, whereas virtually no transfer occurs for the
rigid bipy ligand (entries 7−9, Table 1); thus, either partial
decoordination of the ligand or a better accommodation to the
pentacoordinated structure of the dimeric intermediate in the
transfer (as in F, for example) must be favorable for 2g.
observed. Complex 11 undergoes a partial loss of CO at room
Scheme 5. Carbonylation Reaction of Complex 1
C
DOI: 10.1021/acs.organomet.8b00066
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Article
temperature, changing from white to yellow; the process is
reversible and the white solid is regenerated in the presence of
CO. Pd(II) complexes with coordinated CO are not very
common, but they are relevant in the Pd-catalyzed copoly-
merization of olefins and CO as well other carbonylation
reactions catalyzed by palladium. Some of them have been
(2L = Br, n = 0, Scheme 6), the exocyclic, less substituted
double bond of limonene inserts into the Pd−H bond and the
stereroselective cis-palladium migration occurs with retention of
14
the original stereochemistry of R-(+)-limonene to give 13.
We have now used the same diene to analyze how the presence
of additional ligands influences the efficiency of the Pd-H trap
by a diene.
As can be seen in Table 2 (entry 2), the presence of an
equimolar amount of triphenylphosphine is enough to
effectively block the coordination and insertion of a double
bond of the diene into the Pd-H one. A higher amount of PPh3
11
characterized in the course of those studies. The addition of
bipy to 11 led to a more soluble complex 12 that shows a
characteristic acyl 13C NMR resonance at 230.5 ppm. The fact
that insertion of carbon monoxide into Pd−benzyl bonds is
faster than β-H elimination makes carbonylation reactions of
substituted benzylic derivatives possible, including copolymer-
12
or AsPh produces the same result (entries 3, 4, Table 2).
3
ization of styrene and CO. The preferred insertion of CO vs
2
Complexes 7 and 8 decompose by β-H elimination, but the
transfer of hydride to the diene is null. Still, the presence of the
diene significantly reduces the inter-palladium H-transfer to
give 10 and the E factor is reduced to 0.27 and 0.04 for
complexes 7 and 8j (vs 0.45 and 0.77 without diene, Table 1).
This means that the formation of the required dimeric
halogen-bridged species (C, Scheme 3) is disfavored in the
presence of the diene but the insertion of the latter may not be
observed if the required cis-hydride alkene arrangement is not
favored. The same negligible H-transfer to 4-vinylcyclohexene,
bearing an exocyclic terminal double bond, was observed when
this diene was reacted with complexes 8i,j. The cationic
derivatives 5i,j do form a mixture of allylic derivatives from R-
(+)-limonene (13 and 14, Scheme 6) that result from insertion
of the exocyclic, less substituted double bond into the Pd-H
moiety. The presence of both isomeric complexes 13 and 14 is
caused by differences in the Pd-migration process. A straight Pd
migration to reach the endocyclic double bond of limonene
leads to 13; however, in the course of palladium migration, a
coordinated cyclohexadiene fragment (a terpinene) is formed
and if a double bond coordination switch from one to the other
occurs, followed by insertion of the initial endocyclic bond into
the Pd−H bond, the Pd migration changes its course and 14 is
palladium migration has also been observed before for η -σ
13
alkenyl complexes of palladium.
Trap of the Pd-H Generated from the Benzylic
Complexes with Dienes. In the absence of other reagents
in solution, the palladium hydride species, formed by β-H
elimination from the benzylic complexes in Scheme 2, either
decompose or transfer to another palladium benzyl to give the
reduction product 10 (Scheme 3 and Table 1). However, these
species can be trapped by insertion of an alkene into the Pd−H
bond, provided that the reactant alkene coordinates to the
metal better than stilbene 9 or other ligands present in solution.
This could be a re-entrance point of Pd(II) species in a catalytic
cycle. Also, the Pd-H species thus generated could be used as
reagents, and this strategy has been employed by us before to
synthesize enantiomerically pure allyls from R-(+)-limonene
and complex 1 (Scheme 6 and Table 2, entry 1). In this case
Scheme 6. Reactions with R-(+)-Limonene
15
formed (Scheme 6). This is not observed for the neutral
complex 1 where a fast, direct Pd migration is observed and 13
is formed selectively. About 60% of the generated Pd-H
generated from decomposition of the cationic complexes 5i,j is
trapped by R-(+)-limonene, a percentage that increases to 73%
for the naked palladium hydride derived from 1.
CONCLUSIONS
■
Benzylic palladium complexes with an α-substituent on the
benzyl can be prepared and used as precursors of palladium
hydrides since they decompose cleanly by β-H elimination. In
the absence of any competing substrate and provided a bridging
ligand such as a halide is available, hydride transfer occurs in a
dimer that leads, by reductive elimination, to the saturated
compound 10. The use of different combinations of ligands
leads to complexes with quite different stability toward β-H
elimination and to the occurrence of the subsequent hydride
Table 2. Data for the Reactions Depicted in Scheme 6
a
entry complex
L, n
ratio 13:14 H-transfer to diene (%)
b
1
2
3
4
5
6
1
Br, 0
Br, PPh , 0
1:0
73
0
7
3
8i
8j
5i
5j
Br, 2 PPh , 0
0
3
3
transfer. Maximum H-transfer is observed for either η - or σ-
Br, 2 AsPh , 0
0
3
benzylic complexes with a bromide and ligands of low or
moderate coordination ability completing the metal coordina-
2 PPh , 1
1:3.8
1.6:1
58
61
3
2 AsPh , 1
3
3
tion sphere. The absence of a bromide, as in the cationic η -
a
1
benzylic derivatives, completely halts the H-transfer. This
reduction pathway may play a role in the origin of saturated
byproducts in Pd-catalyzed transformations of benzyl pre-
cursors, either benzylhalides or styrene derivatives.
Calculated by integration in the crude H NMR spectra: % H-transfer
b
to diene = (13 + 14)/9 × 100. A small amount of an allyl complex
resulting from insertion of the endocyclic double bond is observed
(
2% of the allyl mixture; see ref 14).
D
DOI: 10.1021/acs.organomet.8b00066
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Article
EXPERIMENTAL SECTION
■
General Methods. 1H, 19F, 13C, and 31P NMR spectra were
recorded on Bruker AC-300, ARX-300, and AV-400 as well as Agilent
MR-500 instruments. Chemical shifts (in δ units, ppm) were
1
1
3j: H NMR (400.13 MHz, δ, CDCl ): 6.05 (d, J = 6.6 Hz, 1H,
3
1
13
19
referenced to Me Si ( H and C), CFCl ( F), and H PO (85%,
2
3
6
4
3
3
4
H ), 4.94 (d, J = 6.6 Hz, 1H, H ), 1.80 (m, 1H, H ), 1.66 (m, 1H,
3
1
P). Signal assignments were made with the aid of heteronuclear
6
5
1
4
H ′), 1.64 (m, 1H, H ), 1.39 (s, 3H, Me ), 1.28 (m, 1H, H ), 1.02 (m,
1
13
1
7
5
8
H− C HMQC and homonuclear H COSY experiments. C, H, and
1H, H ), 1.09 (m, 1H, H ′), 0.57 (d, J = 5.8 Hz, 3H, H ), 0.55 (d, J =
9
N elemental analyses were performed on a PerkinElmer 2400 CHN
microanalyzer. Solvents were dried using a solvent purification system
SPS PS-MD-5 or distilled from appropriate drying agents under
5.8 Hz, 3H, H ).
1
1
3
14j: H NMR (400.13 MHz, δ, CDCl ): 5.03 (bm, 2H, H , H ),
3
7
5
4
6
2.29 (m, 1H, H ), 2.25 (m, 1H, H ), 1.66 (m, 2H, H , H ), 1.25 (m,
6
4
6
8
9
nitrogen, prior to use. The benzylic complexes 1, and 2−8 were
2H, H ′, H ′), 1.03 (d, J = 6.8 Hz, 6H, H , H ), 0.75 (d, J = 6.3 Hz,
5
5 13
3
(
2
H, Me ). C NMR (100.613 MHz, δ, CDCl , allylic group): 133.3
3
prepared as previously reported.
2
1
3
4
6
5
7
8
9
C ), 89.3 (C , C ), 36.9 (C , C ), 29.8 (C ), 35.5 (C ), 22 (C , C ),
0.8 (Me ).
Decomposition Reactions. Solutions of 0.02 mmol of the
5
corresponding complex were prepared in a deuterated solvent (CDCl3
The reaction of 5i with R-(+)-limonene was carried out in the same
way. The ratio of complexes in the crude mixture was 13i (21%) and
or CD CN, 0.6 mL). For the less stable derivatives, the complexes
3
were prepared in situ by addition of the appropriate ligand to a
1
4i (79%).
13i: H NMR (500.13 MHz, δ, CDCl
suspension of 1 at 223 K, and they were characterized, by comparison
1
2
5
3
): 6.01 (t, J = 7 Hz, 1H, H ),
with NMR literature data, before the decomposition follow up. The
3
6
6
4
(
0
3
4
.50 (t, J = 7 Hz, 1H, H ), 1.73 (m, 1H, H ), 1.51 (m, 1H, H ′), 1.49
evolution of the complexes was monitored by 19F, P, and H NMR at
the corresponding temperature (Table 1) until complete decom-
position occurred. Integration of the 19F NMR resonances (C6F5
group) of the benzylic palladium complex and derivatives 9 and 10
allows the determination of the percentage of decomposition and the
organic product distribution (Figure S1 in the Supporting
Information). These data are collected in Table 1. All the benzylic
palladium complexes in Table 1 and compounds 9 and 10 have been
31
1
1
5
4
7
s, 3H, Me ), 1.19 (m, 1H, H ), 1.03 (m, 1H, H ), 0.95 (m, 1H, H ),
5
8
.99 (m, 1H, H ′), 0.54 (d, J = 5.3 Hz, 3H, H ), 0.52 (d, J = 5.3 Hz,
9 31 1
H, H ). P{ H} NMR (161.976 MHz, δ, CDCl ): 26.29 (d, P , J =
3
A
0.5), 21.91 (d, P , J = 40.5).
B
1
1
3
1
4i: H NMR (400.13 MHz, δ, CDCl ): 4.6 (m, 2H, H , H ), 2.33
3
7
5
4
6
(
m, 1H, H ), 2.05 (m, 1H, H ), 1.2 (m, 2H, H , H ), 1.07 (d, J = 6.8
8
9
4
6
Hz, 6H, H , H ), 1.05 (m, 2H, H ′, H ′), 0.65 (d, J = 6.3 Hz, 3H,
5
31
1
13
Me ). P{ H} NMR (161.976 MHz, δ, CDCl ): 22.54 (s). C NMR
3
5,6
2
1
3
described before.
(100.613 MHz, δ, CDCl , allylic group): 135.7 (C ), 90.3 (C , C ),
3
4
6
5
7
8
9
5
Reactions with CO. Synthesis of 11. CO was bubbled through a
suspension of 1 (0.1 g, 0.109 mmol) in CH Cl (5 mL) for 5 min. The
36.0 (C , C ), 29.4 (C ), 35.3 (C ), 22.1 (C , C ), 20.9 (Me ).
Reaction of Complex 8j with R-(+)-Limonene. Complex 1 (15 mg,
2
2
initial yellow solid dissolved and then a greyish solid precipitated. This
suspension was cooled down to 273 K, and the solid was filtered,
washed with cold CH Cl (5 mL) and diethyl ether (5 mL), and air-
0.0163 mmol) and AsPh (20 mg, 0.0652 mmol) were placed in an
3
NMR tube. The solids were dissolved in 0.7 mL of dry CDCl at 213
3
K. R-(+)-Limonene (10.5 μL, 0.0652 mmol) was added to the orange
2
2
1
19
solution. The reaction was followed by H and F NMR at room
temperature. After 24 h, the complete decomposition of 8j to stilbene
9 was observed and no formation of any palladium-allyl complex.
dried (89.7 mg, 80% yield). Analysis calc. for C H Br F O Pd : C,
3
2
16
2
10
4
2
−1
3
2
7.42; H, 1.57; found: C, 37.02; H, 1.58. IR (Nujol mull): ν = (cm )
112 (CO), 1783, 1750, 1727, 1661 (CO acyl). 19F NMR (282 MHz,
δ, CDCl ): −162.56 (m, Fmeta), −155.98 (t, Fpara), −142.53 (m, Fortho).
3
1
H NMR (300 MHz, δCDCl ): 7.3−7.5 (m, 5H, Ph), 4.75 (m, X, ABX
ASSOCIATED CONTENT
3
■
α
system, JAX = 7.2 Hz, JBX = 7.8 Hz, 1H, H ), 3.45, 3.17 (m, AB, ABX
*
S
Supporting Information
β
β
system, JAB = 13.6 Hz, 2H, H , H ′).
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00066.
Synthesis of 12. To a suspension of 11 (0.0372 g, 0.036 mmol) in
CH Cl was added 2,2′-bipy (0.0113 g, 0.072 mmol). The resulting
2
2
yellow solution was filtered through activated charcoal and Kieselgur.
The solvent was partially evaporated until crystals appeared. After a
day at −20 °C, the yellow crystals were filtered and air-dried (0.0266 g,
Additional information for the decomposition of the
palladium benzylic complexes, and F NMR spectra of a
solution of complex 4 (PDF)
19
5
7% yield). Analysis calc. for C H BrF N OPd: C, 46.79; H, 2.82; N,
25 16 5 2
19
4
.36; found: C, 46.95; H, 2.75; N, 4.22. F NMR (282 MHz, δ,
CDCl ): −163.74 (m, Fmeta), −158.13 (t, Fpara), −141.87 (m, Fortho).
3
1
H NMR (300 MHz, δ, CDCl ): 9.0−6.9 (m, 13H, Ph, bipy), 5.31 (m,
AUTHOR INFORMATION
3
■
*
α
X, ABX system, JAX = 6.3 Hz, JBX = 9.8 Hz, 1H, H ), 3.59, 3.28 (m, AB,
ABX system, JAB = 14 Hz, 2H, H , H ′). C{ H} NMR (74.5 MHz, δ,
CDCl ): 230.51 (s, CO); 154.13, 152.25, 135.57 (s, bipy, Ph); 151,05,
Corresponding Author
E-mail: albeniz@qi.uva.es.
β
β
13
1
3
1
1
50.08, 139.13, 138.56, 129.21, 128.44, 127.26, 126.09, 125.89, 122.17,
ORCID
α
β
21.55 (s, bipy, Ph), 66.95 (s, 1CH, C ), 24.77 (s, 1CH , C ).
2
Blanca Martín-Ruiz: 0000-0003-0519-007X
Ana C. Albeniz: 0000-0002-4134-1333
́
Notes
Reactions with Dienes. Reaction of Complex 5j with R-
(+)-Limonene. Complex 5j (15 mg, 0.0139 mmol) was placed in an
NMR tube and dissolved in dry CDCl3 (0.7 mL) at 233 K. R-
+)-Limonene (4.5 μL, 0.0278 mmol) was added, and the reaction was
(
The authors declare no competing financial interest.
19
1
19
followed by F and H NMR at room temperature. After 24 h, the
F
NMR clearly shows the complete decomposition of the benzylic
complex and the formation of stilbene 9. The H NMR of the crude
ACKNOWLEDGMENTS
1
■
Financial support from the Spanish MINECO (SGPI, grant
CTQ2016-80913-P) is gratefully acknowledged. This work is
dedicated to Prof. Peter M. Maitlis on occasion of his 85th
birthday. We are indebted to him for his impressive
contribution to organometallic chemistry that has inspired
many of us.
mixture shows the formation of complexes 13j (62%) and 14j (38%).
The ratio (13j + 14j)/9 indicates a hydride transfer efficiency of 61%.
The dark mixture was treated with activated carbon and it was filtered
through Kieselgur. The orange solution was evaporated to dryness,
and the orange residue was washed with 2 × 2 mL of hexane. The solid
was dried and characterized by NMR.
E
DOI: 10.1021/acs.organomet.8b00066
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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DOI: 10.1021/acs.organomet.8b00066
Organometallics XXXX, XXX, XXX−XXX