Non-redox metal ions can promote Wacker-type oxidations even better than copper(II): A new opportunity in catalyst design

Article abstract of DOI:10.1039/c5dt02612a

In Wacker oxidation and inspired Pd(ii)/Cu(ii)-catalyzed C-H activations, copper(ii) is believed to serve in re-oxidizing of Pd(0) in the catalytic cycle. Herein we report that non-redox metal ions like Sc(iii) can promote Wacker-type oxidations even better than Cu(ii); both Sc(iii) and Cu(ii) can greatly promote Pd(ii)-catalyzed olefin isomerization in which the redox properties of Cu(ii) are not essential, indicating that the Lewis acid properties of Cu(ii) can play a significant role in Pd(ii)-catalyzed C-H activations in addition to its redox properties. Characterization of catalysts using UV-Vis and NMR indicated that adding Sc(OTf)₃ to the acetonitrile solution of Pd(OAc)₂ generates a new Pd(ii)/Sc(iii) bimetallic complex having a diacetate bridge which serves as the key active species for Wacker-type oxidation and olefin isomerization. Linkage of trivalent Sc(iii) to the Pd(ii) species makes it more electron-deficient, thus facilitating the coordination of olefin to the Pd(ii) cation. Due to the improved electron transfer from olefin to the Pd(ii) cation, it benefits the nucleophilic attack of water on the olefinic double bond, leading to efficient olefin oxidation. The presence of excess Sc(iii) prevents the palladium(0) black formation, which has been rationalized by the formation of the Sc(iii)...H-Pd(ii) intermediate. This intermediate inhibits the reductive elimination of the H-Pd(ii) bond, and facilitates the oxygen insertion to form the HOO-Pd(ii) intermediate, and thus avoids the formation of the inactive palladium(0) black. The Lewis acid promoted Wacker-type oxidation and olefin isomerization demonstrated here may open up a new opportunity in catalyst design for versatile C-H activations.

Full text of DOI:10.1039/c5dt02612a

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ARTICLE
Non-redox metal ions can promote Wacker-type
oxidations even better than copper(II): a new
opportunity in catalyst design
Cite this: DOI: 10.1039/x0xx00000x
Shuhao Qin†, Lei Dong†, Zhuqi Chen, Sicheng Zhang, Guochuan Yin*
Received 00th January 2012,
Accepted 00th January 2012
In Wacker oxidation and inspired Pd(II)/Cu(II)ꢀcatalyzed CꢀH activations, copper(II) is
believed to serve reꢀoxidizing of Pd(0) in the catalytic cycle. Herein we report that nonꢀredox
metal ions like Sc(III) can promote Wackerꢀtype oxidations even better than Cu(II), both
Sc(III) and Cu(II) can greatly promote Pd(II)ꢀcatalyzed olefin isomerization in which the redox
properties of Cu(II) are not essential, indicating that the Lewis acid properties of Cu(II) can
play significant role in Pd(II)ꢀcatalyzed CꢀH activations in addition to its redox properties.
Characterizations of catalysts with UVꢀVis and NMR indicated that adding Sc(OTf)3 to the
acetonitrile solution of Pd(OAc)2 generates a new Pd(II)/Sc(III) bimetallic complex having
diacetate bridge which serves as the key active species for Wackerꢀtype oxidation and olefin
isomerization. Linkage of trivalent Sc(III) to the Pd(II) species makes it more electronꢀ
deficient, thus facilitates the coordination of olefin to the Pd(II) cation. Due to the improved
electron transfer from olefin to the Pd(II) cation, it benefits the nucleophilic attack of water on
the olefinic double bond, leading to efficient olefin oxidation. The presence of excess Sc(III)
prevents the palladium(0) black formation, which has been rationalized by the formation of the
Sc(III)•••HꢀPd(II) intermediate. This intermediate inhibits the reductive elimination of the Hꢀ
Pd(II) bond, and facilitates the oxygen insertion to form the HOOꢀPd(II) intermediate, thus
avoids the formation of the inactive palladium(0) black. The Lewis acid promoted Wackerꢀtype
oxidation and olefin isomerization demonstrated here may open up a new opportunity in
catalyst design for versatile CꢀH activations.
DOI: 10.1039/x0xx00000x
www.rsc.org/
、
[(HMPA)2CuCl2(PdCl2)2]n,
(HMPA:
Introduction
hexamethylphosphoramide) were isolated by Hosokawa with
Xꢀray identifications.8,9 In investigating the kinetics of Wacker
oxidation, Sigman also observed that the concentration of Cu(II)
can significantly affect the reaction order on olefin, thus
suggested that Cu(II) plays a complex role in Wacker
chemistry.10 However, due to the mask of its redox properties,
the alternative roles of Cu(II) in Wackerꢀtype oxidations have
not been clearly evidenced.
Recently, nonꢀredox metal ions functioning as Lewis acid
and promoting high oxidation state metal ion mediated
oxidations has attracted much attention. Adding these nonꢀ
redox metal ions to the synthetic transition metal complexes at
high oxidation state can substantially accelerate their oxidations
in electron transfer, oxygenation, and/or hydrogen
abstraction.11ꢀ14 The promotional effect was attributed to the
linkage of a Lewis acid to the active intermediate, which
positively shifts its redox potential, thus improves the oxidizing
Wacker oxidation is a classic process in homogeneous catalysis
which transforms ethylene to acetaldehyde by aerobic
oxidation.1 In this reaction, PdCl2 is the catalyst responsible for
olefin oxidation, and CuCl2 is believed to promote reꢀoxidation
of the reduced Pd(0) back to the active Pd(II) species, after
which the reduced Cu(I) is feasibly oxidized by O2 to complete
the catalytic cycle. Up to now, Wackerꢀtype oxidations have
been widely applied in olefin functionalization,2ꢀ6 and inspired
Pd(II)ꢀcatalyzed CꢀH activations with Cu(II) as the
stoichiometric oxidant or cocatalyst have been extensively
explored in organic synthesis.7 Although above mechanism has
been widely accepted, there still exist arguments including the
assumption that the active Pd(II) species is dimeric rather than
monomeric, or the suspicion that Cu(II) cation is linked to the
Pd(II) species, and several Pd(II)/Cu(II) bimetallic clusters
including [(PdCl2)2CuCl2(DMF)4]n and Pd6Cu4Cl12O4(HMPA)4,
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VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C5DT02612A
ability of the active metal ion. Inspired by these studies and our
General procedure for catalytic olefin oxidation by
earlier works in elucidating the reactivity relationships of Pd(OAc)2 with Sc(OTf)3 In a typical experiment, Pd(OAc)2
different metal intermediates in oxidation,15 we explored Lewis (4.5 mg, 0.02 mmol) and Sc(OTf)3 (19.7 mg, 0.04 mmol) were
acid promoted catalytic oxygenation with manganese dissolved in 5 mL of acetonitrile in a glass tube, after stirred for
complexes, dioxygen activation by iron complexes and 10 min, 1ꢀhexene (0. 25 mL, 2 mmol) and 0.2 mL of water
selective benzene hydroxylation by PdII(bpym) (bpym: 2,2’ꢀ were added. The glass tube was put into a 50 mL stainless
pyrimidine) catalyst.16 In particular, nonꢀredox metal ion, Al3+, autoclave which was then charged with 20 atm of oxygen. Next,
o
can promote PdII(bpym) catalyzed selective hydroxylation of the reaction solution was magnetically stirred at 80 C in oil
benzene to phenol, whereas PdII(bpym) alone is inactive. The bath for 2 h. After the reaction, the autoclave was cooled to
promotional effect was attributed to the ligation of Al3+ to the room temperature and carefully depressurized to normal
two remote nitrogen atoms of the PdII(bpym) complex, which pressure. The product analysis was performed by GC using the
positively shifts the redox potential of the Pd(II) cation in the internal standard method. Control experiments including using
PdII(bpym) complex, thus improves its capability in CꢀH Pd(OAc)2, or Sc(OTf)3 alone, or Pd(OAc)2 plus Cu(OTf)2 as
activation.
catalyst were carried out in parallel.
Herein, we demonstrate that certain nonꢀredox metal ions like
General procedure for catalytic kinetics of styrene
Sc(III) can promote Pd(II)ꢀcatalyzed Wackerꢀtype oxidations oxidation by Pd(OAc)2 with Sc(OTf)3
Pd(OAc)2
(4.5
even better than Cu(II), and both nonꢀredox metal ions and mg, 0.02 mmol) and Sc(OTf)3 (19.7 mg, 0.04 mmol) were
Cu(II) can promote Pd(II)ꢀcatalyzed olefin isomerization in dissolved in 5 mL of acetonitrile in a glass tube, after stirred for
which the redox properties of Cu(II) is not essential, suggesting 10 min, styrene (42.0 mg, 0.4 mmol) and 0.2 mL of water were
that Cu(II) may play multiple roles in Wackerꢀtype oxidations added. The glass tube was connected with O2 balloon. Then, the
o
in addition to oxidize Pd(0) as described in textbooks. Taken reaction solution was magnetically stirred at 55 C in oil bath
together with previous findings in literatures,11ꢀ14,16 this work for 10 h. The product analysis was performed by GC using the
has not only evidenced that the Lewis properties of the Cu(II) internal standard method at set intervals. Control experiments
cation can play significant role in Wackerꢀtype oxidations, but using Pd(OAc)2 plus Cu(OTf)2 or Pd(OAc)2 alone as catalyst
also implicated that these nonꢀredox metal ions can play were carried out in parallel.
significant roles in versatile transition metal ion mediated
General procedure for catalytic isomerization of
homogeneous catalysis, for example, CꢀH activation, thus terminal olefin by Pd(OAc)2 with Sc(OTf)3 In 0.5 mL of
provides new opportunities for catalyst design which was not acetonitrileꢀd3 containing 4 mM Pd(OAc)2 and 8 mM Sc(OTf)3,
fully recognized before.
0.02 mmol of 1ꢀhexene were added by syringe. Then, the
1
reaction mixture was monitored by H NMR at 25 oC, and
the yield of internal olefins was determined using TMS as an
internal standard. Control experiments using Pd(OAc)2 or
Sc(OTf)3 alone, or using Pd(OAc)2/Cu(OTf)2 as catalyst were
carried out in parallel.
Experimental
General information
All
chemical
reagents
are
commercially available and used without further purification.
Palladium(II) acetate (Pd(OAc)2) was purchased from
Strem Chemicals Inc. Copper(II) trifluoromethanesulfonate
(Cu(CF3SO3)2) and 1,4ꢀcyclohexadiene came from Alfa Aesar;
sodium trifluoromethanesulfonate (NaCF3SO3), magnesium(II)
trifluoromethanesulfonate (Mg(CF3SO3)2), 1ꢀhexene, 2ꢀ
hexanone, 1ꢀoctene, 4ꢀhexene, 2ꢀoctanone, 3ꢀoctanone,
cyclohexene and cyclopentene were purchased from Aladdin.
Scandium trifluoromethanesulfonate (Sc(CF3SO3)3) was
purchased from Shanghai Shaoyuan Co. Ltd. 3ꢀHexanone, 4ꢀ
octanone and transꢀ3ꢀhexene were purchased from TCI
General procedure for catalytic isomerization of 1,5-
cyclooctadiene by Pd(OAc)2 with Sc(OTf)3 In
a
typical
experiment, Pd(OAc)2 (4.5 mg, 0.02 mmol) and Sc(OTf)3 (19.7
mg, 0.04 mmol) were dissolved in 5 mL of acetonitrile in a
glass tube, after stirred for 10 min, 1,5ꢀcyclooctadiene (0.12 mL,
1 mmol) was added. The reaction solution was magnetically
o
stirred at 90 C in oil bath for 24 h. The yield was determined
by GC analysis using direct integral method. Control
experiments using Pd(OAc)2 plus Cu(OTf)2, and Pd(OAc)2,
Sc(OTf)3 or Cu(OTf)2 alone as catalyst were carried out in
parallel.
Shanghai,
Other
trifluoromethanesulfonates
including
Ca(CF3SO3)2, Zn(CF3SO3)2, Al(CF3SO3)3, Y(CF3SO3)3, and
Yb(CF3SO3)3 came from Shanghai Dibai Chemical Co. 1ꢀ
Dodecene, 1,5ꢀcyclooctadiene, and 4ꢀoctanone were purchased
from J&K Scientific Ltd. Toluene, dimethylsulfoxide,
acetonitrile and acetic acid were purchased from local
Sinopharm Chemical Reagent. UVꢀVis spectra were studied on
Analytikjena specord 205 UVꢀVis spectrometer, GCꢀMS
analysis was performed on an Agilent 7890A/5975C
spectrometer, and NMR analysis was performed on Bruker
AV400.
General procedure for catalytic kinetics of 1,5-
cyclooctadiene isomerization by Pd(OAc)2 with Sc(OTf)3
Pd(OAc)2 (4.5 mg, 0.02 mmol) and Sc(OTf)3 (19.7 mg, 0.04
mmol) were dissolved in 5 mL of acetonitrile in a glass tube,
after stirred for 10 min, 1,5ꢀcyclooctadiene (108.0 mg, 1 mmol)
was added. The reaction solution was magnetically stirred at 90
oC in oil bath. The yield was determined by GC analysis using
direct integral method at the set intervals. Control experiments
using Pd(OAc)2 plus Cu(OTf)2, or using Pd(OAc)2, Sc(OTf)3 or
Cu(OTf)2 alone as catalyst were carried out in parallel.
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5DT02612A
Results and discussion
conversion in 2 h, which is similar to the results by adding
Non-redox metal ion promoted olefin oxidations by Al(III) (Table 1, entries 8 vs 10). Notably, adding Cu(II) to the
Pd(OAc)2 Olefin oxidations were performed in acetonitrile Pd(II)/Sc(III) system decreases the conversion, and the yields
containing water (v/v, 5/0.2), oxygen was employed as oxidant, are lower than those by only using Pd(II)/Sc(III) catalyst (Table
and various nonꢀredox metal ions were tested to investigate 1, entry 11), clearly suggesting that reꢀoxidation of Pd(0) by
their influence on Pd(II)ꢀcatalyzed Wackerꢀtype oxidation. Cu(II) is not essential here. In addition, the catalytic kinetics of
Using 1ꢀhexene as substrate, we first found that nonꢀredox styrene oxidation using oxygen balloon also disclosed that
tervalent metal ions can greatly promote Pd(II)ꢀcatalyzed olefin adding Sc(III) displays a faster oxidation rate than Cu(II)
oxidation, and adding bivalent metal ions also slightly improves although their initial rates look similar. As shown in Figure 1,
the efficiency, while the catalytic efficiency of Pd(OAc)2 alone in the absence of Lewis acid, oxidation of styrene by Pd(OAc)2
is very poor (Table 1). In particular, adding 2 equiv. of alone is very sluggish, while adding Cu(II) or Sc(III) greatly
Sc(OTf)3 to Pd(OAc)2 achieves >99% conversion with 53.1% accelerates the oxidation, and Sc(III) demonstrates a relatively
o
yield of 2ꢀhexanone and 29.3% of 3ꢀhexanone in 2 h at 80 C higher promotional effect than Cu(II). Unambiguously, nonꢀ
by GC analysis of the reaction mixture. The loss of 1ꢀhexene redox metal ions like Sc(III) can greatly promote Pd(II)ꢀ
mass balance is due to its vaporizing into gas phase under the catalyzed Wackerꢀtype oxidations even better than Cu(II), and
reaction conditions, and formation of 3ꢀhexanone can be this challenges the long believed textbook role of Cu(II) in
attributed to the isomerization of 1ꢀhexene prior to Wackerꢀtype Wackerꢀtype cycles.
oxidation (vide infra). In control experiment, Sc(III) alone is
inactive for olefin conversion, which is consistent with its nonꢀ
Pd(OAc) + Sc(OTf)
2
3
100
80
60
40
20
0
Pd(OAc) + Cu(OTf)
redox properties. However, increasing the ratio of Sc(III)/Pd(II)
can substantially accelerate olefin oxidation and prohibit
palladium black formation (See Table S3). For examples, in
2
2
Pd(OAc)
2
o
oxidation of 1ꢀhexene at 60 C, adding 0.5 equiv. of Sc(OTf)3
to Pd(OAc)2 provided only 8.1% of 2ꢀhexanone and 2.0% of 3ꢀ
hexanone with 25.7% conversion in 2 h, and some palladium(0)
black was observed after the reaction, while adding 3 equiv. of
Sc(OTf)3 offers 53.3% and 28.8% of 2ꢀ and 3ꢀhexanone,
respectively, with 98.2% conversion under identical conditions,
and no palladium black formation was observed.
0
100
200
300
400
500
600
700
Time (min)
Table 1. Wackerꢀtype oxidation of 1ꢀhexene by Pd(OAc)2 in the presence of
nonꢀredox metal ionsa
Figure 1 Catalytic kinetics for styrene oxidation to acetophenone by Pd(II)/(Sc(III)
or Pd(II)/Cu(II) catalyst. Reaction conditions: CH3CN (5.0 mL), styrene (0.4 mmol),
Pd(OAc)2 (0.02 mmol), Sc(OTf)3 or Cu(OTf)2 (0.04 mmol), H2O (0.2 mL), O2
balloon, 55 oC.
Entries
Lewis acid
Conv.
(%)
13.7
14.8
Yield of 3ꢀ
hexanone (%)
trace
Yield of 2ꢀ
hexanone (%)
1.3
1
2b
3
ꢀ
Na+
Mg2+
Ca2+
Zn2+
Y3+
trace
1.8
Table 2. Wackerꢀtype oxidation of different olefins by Pd(OAc)2 in the
23.4
21.1
21.3
30.5
32.4
55.1
>99
trace
trace
trace
1.1
4.2
4.0
a
presence of Sc(OTf)3
4
5
6
7
8
9
Yield (%)b
3.8
Entri
es
1
Substrate
1 ꢀhexene
Time
(h)
2
Conv.
(%)b
99.8
Products
7.9
Yb3+
Al3+
Sc3+
2ꢀhexanone
3ꢀhexanone
53.1(28.3)
29.3 (11.3)
1.6
8.4
(62.0)
9.1
24.8
53.1
29.3
2
3
3 ꢀhexene
1 ꢀoctene
5
99.6
(65.4)
99.3
2ꢀhexanone
3ꢀhexanone
2ꢀoctanone
3ꢀoctanone
4ꢀoctanone
2ꢀoctanone
3ꢀoctanone
4ꢀoctanone
acetophenone
benzaldehyde
cyclohexanone
cyclohexenone
cyclopentanone
56.0 (27.1)
33.9 (13.4)
39.0 (16.1)
27.0 (10.2)
20.6 (7.2)
37.4 (21.4)
26.9 (15.2)
22.5 (12.6)
87.3 (49.9)
7.0 (5.8)
Cu2+
10
11c
62.0
86.0
11.3
22.9
28.3
47.6
Sc3++Cu2+
12
(48.2)
[a] Conditions: 1ꢀhexene (2 mmol), Pd(OAc)2 (0.02 mmol), trifluoromethane
sulfonate (0.04 mmol), acetonitrile (5 mL), H2O (0.2 mL), O2 (20 atm), 80
oC, 2 h; [b] NaOTf (0.12 mmol); [c] 0.04 mmol of Sc(OTf)3 and Cu(OTf)2
added.
4
5
4 ꢀoctene
16
99.8
(60.5)
styrene
2
99.7
(60.4)
99.7
(62.9)
99.8
Because adding 6 equiv. of NaOTf, which contain the same
amount of OTfꢀ as those in 2 equiv. of Sc(OTf)3, does not
generate an obviously promotional effect (Table 1, entry 2), it
clearly suggests that the promotional effect originates from
nonꢀredox metal ions like Sc(III) rather than OTfꢀ anion. In [a] Conditions: olefin (2 mmol), Pd(OAc)2 (0.02 mmol), Sc(OTf)3 (0.04
comparison, adding 2 equiv. of Cu(OTf)2 yields 28.3% of 2ꢀ
6c
7c
cyclohexene
cyclopentene
16
16
86.3 (45.8)
7.4 (5.7)
80.2 (72.2)
(99.1)
mmol), acetonitrile (5 mL), H2O (0.2 mL), O2 (20 atm), 80 oC. [b] The data in
parentheses comes from control experiments using Cu(OTf)2 instead of
Sc(OTf)3. [c] olefin (1 mmol).
hexanone and 11.3% of 3ꢀhexanone with only 62.0%
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Catalytic oxidations of other olefins also clearly support that only 49.3% of internal olefins with 55.8% conversion under the
Sc(III) produces much more efficient substrate conversion than identical conditions. In the case of 1,5ꢀcyclooctadiene (1,5ꢀ
Cu(II) in most cases (Table 2). For example, in styrene COD), the promotional effects of Sc(III) and Cu(II) are
oxidation, Pd(II)/Sc(III) catalyst achieves >99% conversion comparable, providing 89.0% yield of 1,3ꢀcyclooctadiene (1,3ꢀ
with 87.3% yield of acetophenone and 7.4% yield of COD) with 4.7% of 1,4ꢀcyclooctadiene (1,4ꢀCOD), and 88.0%
benzaldehyde in 2 h at 80 oC, whereas Pd(II)/Cu(II) catalyst can of 1,3ꢀCOD with 4.1% of 1,4ꢀCOD, respectively. In control
only convert 60.4% of styrene under the identical conditions, experiments, neither Pd(OAc)2 nor Lewis acid alone is active
providing 49.9% and 5.8% yields of acetophenone and for olefin isomerization (Table S4ꢀS7), which is consistent with
benzaldehyde, respectively. Notably, unlike the olefin that, in literature, oxidation of terminal olefins with Pd(OAc)2
oxidations reported by Kaneda and others in which oxidation of alone generally provided methyl ketones with no isomerized
terminal olefin produced solely methyl ketone,2b,6d here, ketone formation,2b,6d thus confirms the critical role of Sc(III)
Pd(II)/Sc(III) catalyzed oxidations of terminal olefins provided and Cu(II) in Pd(II)ꢀcatalyzed olefin isomerization.
both methyl ketone and other internal ketones as those in Undoubtedly, the role of Cu(II) in Pd(II)ꢀcatalyzed olefin
oxidation of internal olefins. For example, in oxidation of 1ꢀ isomerization is distinctly different from that proposed of reꢀ
hexene, the yields of 2ꢀhexanone and 3ꢀhexanone are 53.1% oxidizing Pd(0) in Wackerꢀtype processes, because its redox
and 29.3%, respectively, while they are 56.0% and 33.9% for 3ꢀ properties are unnecessary here. As evidence, Sc(III), a nonꢀ
hexene (Table 2, entries 1 and 2), suggesting that a rapid olefin redox metal ion but a stronger Lewis acid than Cu(II),
isomerization occurs prior to oxidation, which makes 1ꢀhexene demonstrates a better promotional effect in most cases, and
and 3ꢀhexene to give similar product distributions.
other metal ions like Al(III), Mg(II) and Zn(II) can also
accelerate Pd(II)ꢀcatalyzed olefin isomerization (Table S7).
Table 3 The influence of Lewis acid on Pd(II)ꢀcatalyzed olefin
isomerization.a
Substrate
1ꢀhexene
Lewis acid
Time
Conv.
(%)
98.2
55.8
Product yield
(%)
Internal olefins
(89.5)
Sc(OTf)3
Cu(OTf)2
6 min
6 min
Internal olefins
(49.3)
1ꢀoctene
1ꢀdodecene
1,5ꢀCODb
Sc(OTf)3
Cu(OTf)2
4 min
6 min
94.5
75.7
Internal olefins
(90.3)
Internal olefins
(63.2)
Internal olefins
84.4)
Internal olefins
(41.4)
1,3ꢀCOD (89.0)
1,4ꢀCOD (4.7)
1,3ꢀCOD (88.0)
1,4ꢀCOD (4.1)
Figure 2. The 1H NMR spectra of olefin isomerization in CD3CN at 25 oC. A: 1-
octene (40 mM); B: 1-octene (40 mM) with Pd(OAc)2 (4 mM), 1 h; C: 1-octene (40
mM) with Cu(OTf)2 (8 mM), 1 h; D: 1-octene (40 mM) with Sc(OTf)3 (8 mM), 1 h;
E: 1-octene (40 mM) with Pd(OAc)2 (4 mM) and Cu(OTf)2 (8 mM), 10 min; F: 1-
octene (40 mM) with Pd(OAc)2 (4 mM) and Sc(OTf)3 (8 mM), 4 min.
Sc(OTf)3
Cu(OTf)2
6 min
6 min
99.6
48.4
Sc(OTf)3
Cu(OTf)2
24 h
24 h
95.2
93.0
Non-redox metal ions promoted olefin isomerization by
Pd(OAc)2 Figure 2 demonstrates Pd(II)ꢀcatalyzed 1ꢀoctene
1
isomerization by H NMR detection in the presence/absence of
[a] Conditions: acetonitrileꢀd3 (0.5 mL), Pd(OAc)2 (4 mM), Lewis acid (8
1
mM), olefin (40 mM), reaction performed in NMR tube at 25 oC, yield
Sc(OTf)3 or Cu(OTf)2. The spectrum A shows the H NMR
1
chemical shifts of olefinic bond in 1ꢀoctone, in which the peaks determined by H NMR. [b] CH3CN (5 mL), Pd(OAc)2 (4 mM), Lewis acid
o
(8 mM), 1,5ꢀcyclooctadiene (0.2 M), reaction performed in oil bath at 90 C,
at 4.9ꢀ5.1 ppm represent the chemical shifts of the methylene
group in olefinic bond, while the peaks at 5.8ꢀ5.9 ppm represent
yield determined by GC analysis .
the chemical shifts of the internal HꢀC(R) unit of the olefinic
bond. Spectra B, C and D confirm that none of Pd(II), Cu(II)
and Sc(III) alone can efficiently isomerize 1ꢀoctene to internal
2.5
Pd(OAc) 1 mM
octene in 1 h at 25 oC. However, adding 2 equiv. of Cu(OTf)2 to
Pd(OAc)2 isomerizes most of 1ꢀoctene to internal octenes in 10
min which have the chemical shifts of olefinic bond between
5.3 and 5.5 ppm, while adding Sc(OTf)3 to Pd(OAc)2 achieves
complete isomerization in 4 min, much faster than Cu(II).
Similar promotional effects have also been observed in
isomerization of other olefins as shown in Table 3. In
isomerization of 1ꢀhexene, adding 2 equiv. of Sc(OTf)3 to
Pd(OAc)2 provided 89.5% of internal olefins with 98.2%
2
2.0
1.5
1.0
0.5
0.0
-0.5
Sc(OTf) 2 mM
3
Pd(OAc) 1mM
2
+ Sc(OTf) 2mM
3
200
300
400
500
600
nm
Figure 3. UV-Vis spectra of Pd(OAc)2 and Sc(OTf)3 with their mixture in
acetonitrile containing water (v/v, 0.5/0.02 mL).
o
conversion in 6 min at 25 C, while adding Cu(OTf)2 offered
4 | J. Name., 2012, 00, 1-3
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ARTICLE
DOI: 10.1039/C5DT02612A
Characterizations of Pd(II)/Sc(III) catalyst with Cd2+, Nd2+, Zn2+ and Ce4+, etc., to generate heterobimetallic
mechanistic proposal for olefin oxidation The
UVꢀVis palladium(II) salts with acetate bridge was widely observed,18
spectra of Pd(II)/Sc(III) catalyst are displayed in Figure 3. In and several Pd(II)/Cu(II) bimetallic clusters were also isolated
acetonitrile containing water (v/v, 0.5/0.02) which is the and well characterized by Hosokawa.9 Based on this
reaction solvent for olefin oxidation, Pd(OAc)2 demonstrates a information with our characterizations, the new species
yellowish colour, having an absorbance band around 400 nm generated here can be assigned to the bimetallic Pd(II)/Sc(III)
involving the ligandꢀtoꢀmetal charge transfer as expected to dimers having two acetate bridges with two free sites on Pd(II)
take place from acetate ligand to Pd(II) cation. Adding 2 equiv. for solvent or substrate coordination (Fig. 6). Replacing one
of Sc(OTf)3 to Pd(OAc)2 turns its original yellowish colour to bivalent Pd(II) by tervalent Sc(III) can account for the
pale yellow, and yields a new absorbance band with maximum downshifts of the chemical shift of the carbonyl and methyl
near 320 nm, indicating the formation of a new Pd(II) species. groups in acetate bridges, and the multiple methyl peaks in 13C
Similar Sc(OTf)3 concentration dependent shifts have been NMR can be putatively attributed to its different orientation
observed on the UVꢀVis and 1H NMR spectral features (Fig. S3 caused by bulky OTfꢀ groups ligated to Sc(III), but is not
and S4). In 1H NMR spectra (Fig. 4), Pd(OAc)2 alone in conclusive.
acetonitrileꢀd3 reveals a chemical shift of 1.965 ppm, assigning
to the methyl group of acetate in Pd3(OAc)6 trimer in which all
of the acetates serve as bridges like those in the literature.8b,17
Adding Sc(OTf)3 to the Pd(OAc)2 solution downshifts this
methyl group to 1.989 ppm, indicating a more electronꢀ
deficient group than bivalent Pd(II) has bound to acetate, while
in monomeric NaOAc or NH4OAc, the chemical shift of acetate
is upshifted, giving the chemical shifts of 1.817 and 1.814 ppm,
respectively, and it is 1.958 ppm for Sc(OAc)3. In 13C NMR
spectra (Fig. 5), Pd(OAc)2 in acetonitrileꢀd3 displays one
chemical shift at 21.15 ppm and another at 173.73 ppm,
corresponding to the methyl and carbonyl groups of acetate
bridges in Pd3(OAc)6, respectively. Adding Sc(OTf)3
downshifts the carbonyl group to 179.49 ppm which is still a Figure 5. 13C NMR spectra of Pd(OAc)2, Sc(OTf)3 and their mixture in acetonitrile
containing water-d (v/v, 0.5/0.02 mL).
singlet peak, while the methyl group also downshifts but
demonstrates multiple peaks between 21.44 and 22.24 ppm
1
which have been evidenced by Hꢀ13C HSQC NMR (see Fig.
S2). Since the carbonyl group of acetate is a singlet peak, it
apparently suggests that there is only one type of acetate in
solution, thereof, the multiple methyl peaks between 21.44 and
22.23 ppm could be attributed to its different orientation in the
new species.
Figure 6. Proposed bimetallic Pd(II)/Sc(III) core for olefin oxidation.
In Pd(II)/Sc(III)ꢀcatalyzed olefin oxidation, since Sc(III) is
redox inactive, it cannot reꢀoxidize Pd(0) back to the active
Pd(II) species; however, Sc(III) demonstrates a more efficient
promotional effect than Cu(II), and adding Cu(II) to
Pd(II)/Sc(III) system does not further accelerate olefin
oxidation (see Table 1, entry 11). These observations strongly
suggest that Pd(0) may not essentially occur in the catalytic
cycle. Taken together, an alternative Wackerꢀtype mechanism
was proposed for Pd(II)/Sc(III)ꢀcatalyzed olefin oxidation
Figure 4. 1H NMR spectra of Pd(OAc)2/Sc(OTf)3 with related compounds in
acetonitrile containing water-d (v/v, 0.5/0.02 mL).
(Scheme 1). In this mechanism, the key active species for olefin
activation is a bimetallic Pd(II)/Sc(III) species as shown in
Figure 6. The presence of Sc(III) makes Pd(II) more electronꢀ
deficient, thus improves its olefin oxidation and CꢀH activation
ability in two aspects: 1) improving the electron transfer from
olefin to Pd(II) cation, thus benefiting its attack by water, and
2) accelerating CꢀH activation by Pd(II) in βꢀhydrogen
elimination to form the enol intermediate. The enhanced CꢀH
1
Taken together, the downꢀshifts of the H and 13C NMR
peaks of Pd3(OAc)6 trimer by adding Sc(OTf)3 clearly indicate
that a more electronꢀdeficient group than Pd(II) has linked to
acetate. In literature, breakdown of trimeric Pd3(OAc)6 by
adding metal salts, including Ba2+, Sr2+, Ca2+, Mn2+, Co2+, Ni2+,
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DOI: 10.1039/C5DT02612A
activation capability of Pd(II) by Sc(III) has been evidenced in properties of Cu(II) plays significant role in Wackerꢀtype
olefin isomerization (vide supra), and similar improvement was oxidations in addition to its redox properties. The origin of the
also observed by adding Al(III) to PdII(bpym) in benzene promotional effect has been attributed to the formation of a
hydroxylation.16c More importantly, the presence of excess bimetallic Pd(II)/Sc(III) dimer which serves as the key active
Sc(III) has prevented palladium black formation (see Table S3), species in CꢀH activation. Linkage of Sc(III) cation to Pd(II)
indicating that it can prohibit the reductive elimination of Hꢀ through diacetate bridge makes Pd(II) more electronꢀdeficient,
Pd(II) intermediate in Wackerꢀtype cycle (Note: Sc(III) is redox thus improves its capability in olefin coordination and CꢀH
inactive). To rationalize this phenomenon, we propose the role activation, and it also benefits the nucleophilic attack of water
of excess Sc(III) as forming a Sc(III)•••HꢀPd(II) intermediate on olefin.
which weakens the HꢀPd(II) bond, thus facilitating dioxygen
In fact, nonꢀredox metal ions have been extensively
insertion to generate the HOOꢀPd(II) intermediate. Finally, employed as additives to modify the reactivity and improve the
ꢀ
releasing of HO2 reꢀgenerates the active Pd(II) species, thereof, stability of redox catalysts in heterogeneous catalysis,20 and
formation of Pd(0) does not necessarily occur in catalytic cycle. they also occur in certain active sites of redox enzymes like
In literature, dioxygen insertion of the HꢀPd(II) intermediate to Ca2+ in the oxygen evolution center of Photosystem II.21
generate the corresponding HOOꢀPd(II) bond have also been However, they have not been fully recognized in homogeneous
reported by Stahl and Goldberg, and the insertion pathways are oxidations, for example, in CꢀH activation. Recent findings
highly ligand dependent of the HꢀPd(II) intermediates.19 Here, have disclosed that these nonꢀredox metal ions can promote
formation of the Sc(III)•••HꢀPd(II) intermediate may prevent active metal oxo intermediates mediated stoichiometric and
the reduction elimination of the Pd(II) hydride intermediate to catalytic oxidations.11ꢀ14,16 Here, we further revealed that the
yield the inactive palladium(0) black, and meanwhile weaken Lewis acid properties of Cu(II) cation, as well as its redox
the HꢀPd(II) bond, thus facilitate the dioxygen insertion, properties, can play significant role in Wackerꢀtype oxidations,
leading to efficient Wackerꢀtype oxidation. However, it is also which appeals for the attention of the Lewis acid properties of
worth to mention that the reduced amount of Pd black with the Cu(II) cation in many Pd(II)ꢀcatalyzed CꢀH activations
Sc(III) is not known, since there is no obvious Pd(0) formation where Cu(II) salts were generally employed and recognized as
observed.
stoichiometric oxidant or cocatalyst for reꢀoxidizing of Pd(0)
back to the active Pd(II) species.7 Finally, this knowledge may
open up new opportunities in catalyst design for versatile
Pd(II)ꢀcatalyzed CꢀH activations in which nonꢀredox metal ions
as Lewis acid can play significant roles.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No 21303063 and 21273086). The GCꢀ
MS analysis was performed in the Analytical and Testing
Center of Huazhong University of Science and Technology.
Notes and references
School of Chemistry and Chemical Engineering, Huazhong University of
Science and Technology. Key Laboratory for LargeꢀFormat Battery
Materials and System, Ministry of Education. Luoyu Road 1037, Wuhan
430074, PR China. Eꢀmail: gyin@hust.edu.cn.
†These authors equally contribute to this work.
Scheme 1. Proposed mechanism for Sc(III) promoted Wacker-type oxidation.
Electronic Supplementary Information (ESI) available: Reaction
optimizations for olefin oxidations; GCꢀMS graphs of olefin oxidation
and isomerization products; UVꢀVis spectra of Pd(OAc)2/Al(OTf)3 and
Pd(OAc)2/Cu(OTf)2 in acetonitrile; 1Hꢀ13C HSQC spectra of
Pd(OAc)2/Sc(OTf)3 in CD3CN; 1H NMR kinetics of olefin isomerization.
See DOI: 10.1039/c000000x/
Conclusions and perspective
This work revealed that certain trivalent metal ions like Sc(III)
can substantially promote Pd(II)ꢀcatalyzed Wackerꢀtype
oxidation even better than Cu(II), and the promotional effect is
apparently Lewis acid strength dependent. Both Sc(III) and
Cu(II) can greatly promote Pd(II)ꢀcatalyzed olefin
isomerization, whereas Pd(II) alone is very sluggish. Because
the redox properties of Cu(II) is not essential in promoting
Pd(II)ꢀcatalyzed olefin isomerization, and stronger Lewis acid
like Sc(III) demonstrates more efficient promotional effect
than Cu(II) in most cases, it clearly indicates that the Lewis
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DOI: 10.1039/C5DT02612A
Graphical Abstract
Non-redox metal ions can accelerate Pd(II)-catalyzed Wacker-type oxidations better
than Cu(II), revealing a new role of Cu(II) in Wacker-type mechanism.
100
Pd(OAc)2+Sc(OTf)3
Pd(OAc)2+Cu(OTf)2
80
60
40
20
0
Pd(OAc)2
[O]
O2
O
R
R
0
100
200
300
400
500
600
700
min