Zirconium complexes supported by a ferrocene-based ligand as redox switches for hydroamination reactions

Article abstract of DOI:10.1039/c9cc01076a

The synthesis of (thiolfan?)Zr(NEt₂)₂ (thiolfan? = 1,1′-bis(2,4-di-tert-butyl-6-thiophenoxy)ferrocene) and its catalytic activity for intramolecular hydroamination are reported. In situ oxidation and reduction of the metal complex re

Full text of DOI:10.1039/c9cc01076a

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
=
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3
N@D5h-C80
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DOI: 10.1039/C9CC01076A
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Zirconium Complexes Supported by a Ferrocene-Based Ligand as
Redox Switches for Hydroamination Reactions
a
a,b
c
a,*
Yi Shen, Scott M. Shepard, Christopher J. Reed, and Paula L. Diaconescu
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
2 2
The synthesis of (thiolfan*)Zr(NEt ) (thiolfan*= 1, 1'-bis(2,4-di-tert-
Wrighton and coworkers, 1995
butyl-6-thiophenoxy)ferrocene) and its catalytic activity for
intramolecular hydroamination are reported. In situ oxidation and
reduction of the metal complex results in reactivity towards
2
+
+
PPh2
RhSn
PPh2
oxidized form:
PPh2
RhSn
PPh2
e-
+
-
Co
Co
e-
2 2
different substrates. The reduced form of (thiolfan*)Zr(NEt )
catalyzes hydroamination reactions of primary aminoalkenes,
whereas the oxidized form catalyzes hydroamination reactions of
secondary aminoalkenes.
reduced form:
better hydrogenation
catalyst
better hydrosilylation
catalyst
Fig. 1 The first reported example of a redox switchable catalyst.
Redox-switchable catalysis is a field of growing importance, in
which a redox-active functionality is incorporated within the
Since then, several groups have become interested in applying
framework of a metal complex.1-7 Historically, the oxidation or redox switchable catalysis to organic synthesis using homogeneous
3
4-54
reduction of a transition metal catalyst results in a change in the catalysts.
These efforts fall into two categories: (1) different
metal’s oxidation state; this may result in compound degradation or reaction rates for the two catalytically distinct states that apply to a
loss of catalytic activity. As an alternative, the integration of a redox- single reaction (including on/off switches); (2) different reaction
active ligand into a metal complex provides a controllable way of rates for the two catalytically distinct states that apply to distinct
tuning its electronic properties, potentially inducing selectivity or reactions but without orthogonality, i.e., each catalytic state is active
new catalytic activity with changing the redox state.5-7 The activity of
in both reactions but with different rates. The first category contains
3
4, 35, 41, 47
ring closing metathesis,
polymerization, oxidation of dihydrogen,
ring-opening metathesis
a metal complex supported by a redox-active ligand can be
influenced in situ by the redox event occurring at the ligand via a
chemical or electrochemical process. Redox switchable catalysis has
been applied successfully to the ring-opening polymerization of cyclic
4
8
36, 37
oxazoline formation,
4
3, 49, 50
furan formation, phenol formation,
cyclization of alkynes with
5
2
46
51, 52
42
45
furans, hydroalkoxylation, hydroamination,
hydrosilylation,
3
9
8
-21
5, 22-26
acetonitrile dissociation, Kumada coupling, Buchwald-Hartwig
esters and ethers,
and to olefin polymerization.
4
4
53
cross-coupling,
Mitsunobu reaction,
and allylic alcohol
Polymerization is so far the only area of redox switchable catalysis in
which two orthogonal reactions could be coupled, resulting in the
5
4
isomerization. The only example in the second category is the
comparative study of hydrogenation/hydrosilylation reported by
9
-12, 14-16, 19, 20, 27-32
synthesis of block copolymers.
3
3
Wrighton and coworkers and discussed above. Other examples
similar to the second category but using a different type of external
trigger are: ring closing metathesis and ring opening metathesis
The first example using a metallocene redox switch in order to
influence catalytic activity was reported by Wrighton and co-workers
3
3
in 1995 (Figure 1). In that seminal work, the authors showed that a
rhodium complex containing cobaltocene (reduced form) is a better
catalyst for the hydrogenation of olefins than the corresponding
compound incorporating cobaltocenium (oxidized form). The reverse
trend was observed in the hydrosilylation of acetone. It is important
to note that the two states of the catalytically active species did not
achieve an on/off switch for either reaction.
polymerization studied with
complex, and a hydrogenation and a dehydrogenative coupling
performed by an acid-base triggered ruthenium complex. The last
example is the only reported instance in which the two catalytic
cycles were combined for assisted tandem catalysis.
a
photoswitchable ruthenium
4
0
5
5
5
6-58
Although examples of tandem catalysis
are numerous, the
5
9-
number of examples of assisted tandem catalysis is relatively small,
7
7
likely because of issues associated with reagent/substrate
compatibility and the complexity in designing such a single
‘multitasking’ catalyst. As such, switchable catalysis may provide a
general method for obtaining complex products. The first step in
achieving assisted tandem switchable catalysis is to demonstrate
a.
Department of Chemistry & Biochemistry, University of California, Los Angeles
Current address: Department of Chemistry, Massachusetts Institute of Technology
Division of Chemistry & Chemical Engineering, California Institute of Technology
Corresponding author, email: pld@chem.ucla.edu
b.
c.
*
Electronic Supplementary Information (ESI) available: synthetic and catalytic data.
See DOI: 10.1039/x0xx00000x
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+
that the two states of a catalyst show complementary on/off redox event near 0 V versus the Fc/Fc couple that was attributed to
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reactivity for each reaction. In the case of ring opening the oxidation of the ferrocene backbone.DTOhIe: 1r0e.f1o0r3e9,/Ca9CnCar0r1o0w76eAr
polymerization, the two substrates are rather similar and follow region was chosen for the second scan from -0.5 to 0.5 V to isolate
similar mechanisms. Therefore, we decided to take an analogous the corresponding oxidation peak, observed at 0.12 V versus the
+
approach in order to develop applications to organic synthesis and Fc/Fc couple, but it still did not result in a reversible event (Figure
focus on hydroelementation reactions.46
S42), indicating that the species that is generated is not reduced back
Zirconium catalysts were reported to exhibit hydroamination to the starting compound.
selectivity in dependent on the charge of the complex and the type
of aminoalkene used. With few exceptions,78 neutral zirconium Table
1
Intramolecular hydroaminations catalysed by
a
compounds
exclusively
catalyse
reactions
of
primary (thiolfan*)Zr(NEt
2
)
2
.
7
9-84
aminoalkenes.
On the other hand, cationic zirconium complexes
Entry
Reduced State
Oxidized State
have displayed either no selectivity, where they catalyse both
Conversion Time
Conversion Time
8
5-88
primary and secondary aminoalkenes,
secondary aminoalkenes.
or selective catalysis for
H
8
5, 89
1
2
N
95%
0%
72 h
16 h
Ph Ph
1a
Ph
Ph
NH2
0%
20 h
20 h
Ph Ph
Ph PhH
1b
tBu
tBu
NH2
N
3
4
Ph Ph H
N
N
tBu
tBu
Ph
Ph
S
S
100%
O
[AcFc][BArF]
O
2a
2b
Primary
aminoalkene
Secondary
aminoalkene
Fe
Zr(NEt2)2
Fe
Zr(NEt2)2
TMS
S
O
S
O
5
6
Ph Ph H
N
N
0%
0%
24 h
18 h
Ph
Ph
tBu
tBu
TMS
Bn
3a
3b
0%
0%
30 h
20 h
tBu
tBu
N
NH
Ph
Ph
Bn
Ph
Ph
Red
Ox
7
8
Ph Ph H
N
N
Ph
Fig. 2 A redox-active zirconium complex catalyses hydroamination
4a
Ph 4b
reactions selectively.
Ac
9
Ph Ph H
N
0%
0%
21 h
20 h
N
Herein, we report the development of an [OSSO]-type zirconium
Ph
Ac
Ts
1
0
1
Ph 5b
0%
0%
24 h
21 h
complex, (thiolfan*)Zr(NEt
thiophenoxy) ferrocene), which can be switched in situ between a
neutral and cationic species when triggered by the
)
2 2
(thiolfan*= 1, 1'-bis (2,4-di-tert-butyl-6-
5a
Ts
1
a
Ph Ph H
N
N
Ph
Ph
oxidation/reduction of the ferrocene backbone. The two catalytic
states generated by electron transfer react with different types of
substrates.
12
6a
6b
1
3
4
Ph Ph
7a
N
99%
0%
4 h
NH2
Ph
2 2 4
The reaction of H (thiolfan*) with Zr(NEt ) results in the clean
1
Ph 7b
99%
99%
0%
2 h
1
7,24
formation of (thiolfan*)Zr(NEt
2
)
2
,
as confirmed by NMR
1
13
TMS
spectroscopy ( H NMR spectrum - Figure S1, C NMR spectrum -
Figure S2, and heteronuclear single quantum coherence, HSQC,
spectrum - Figure S3), and elemental analysis.
1
5
6
N
20 h
21 h
Ph Ph
H
Ph
Ph
N
1 hb
24 h
1
TMS
8a
8b
H
N
1
7
8
0%
NH2
1
9a
9b
N
1
9
0
0%
23 h
H
N
3 hb
2
99%
0%
10a
10b
H
N
60 hc
2
1
66%
Tol
11a
NH2
Tol
Tol
2
2
3
11b
N
21 h
2 2
Fig. 3 Cyclic voltammetry diagram of (thiolfan*)Zr(NEt ) recorded in
-
1
F
o-F
2
C
H
6 4
at 100 mV s with 0.1 M [TPA][BAr ] as the electrolyte and
2
Tol
0%
24 h
+
H
N
referenced to the Fc/Fc couple.
12a
12b
77%
15 hb
To investigate the redox properties of (thiolfan*)Zr(NEt
2
)
2
, cyclic
24
voltammetry experiments were performed in o-F C H using
2 6 4
a
2 2 6 6
Conditions: 10 mmol% (thiolfan*)Zr(NEt ) , 0.5 mL C D , 100 °C.
F
F
[
TPA][BAr ] ([TPA][BAr ] = tetrapropylammonium tetrakis(3,5-
Conversions were calculated from the corresponding NMR spectra
using hexamethylbenzene (HMB) as an internal standard, except for
entry 24, when the conversion was calculated by HRMS because of
overlapping peaks in the corresponding NMR spectra. The oxidized
state was generated in situ by the addition of 0.9 equiv. of
bis(trifluoromethyl)phenyl)borate) as an electrolyte, revealing two
irreversible oxidation events (Figure 3). The electrochemical
behaviour of (thiolfan*)Zr(NEt
2
)
2
contrasts that of the corresponding
t
alkoxide complex, (thiolfan*)Zr(O Bu)
2
, which can be reversibly
1
1
oxidized and reduced. Although a cyclic voltammetry study of
Ac
F
b
[
Fc][BAr ]. Reactions were carried out in a 4:1 by volume C
6
D
6
:o-
t
(
(
thiolfan*)Zr(O Bu)
2
11
was
not
reported,
the
titanium
C
C
F
6
H F
4 2
solution instead of C
6 6
D .
Reaction was carried out in o-
i
i
9
thiolfan*)Ti(O Pr)
2
and aluminium (thiolfan*)Al(O Pr) alkoxide
2
C
6 4
H .
complexes exhibit a reversible and quasi-reversible, respectively,
2
| J. Name., 2012, 00, 1-3
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with (thiolfan*)Zr(NEt
2
)
2
(10 mol%) before the addition of a
View Article Online
Ac
F
Besides studying the electrochemical oxidation of substrate, we ensured that all [ Fc][BAr ] waDsOcIo: n10s.u1m03e9d/Ct9oCfCo0rm107th6Ae
(
thiolfan*)Zr(NEt
2
)
2
, chemical oxidation studies were also carried corresponding oxidized species. The results reported in Table 1
Ac
F
out.
[ Fc][BAr ]
(acetylferrocenium
tetrakis(3,5- account for these modifications.
, AgBPh , and The reactivity studies show that (thiolfan*)Zr(NEt
were used as oxidants, and the addition of CoCp was the cyclization of the primary aminoalkene 1a in 72 h (Table 1, entry
bis(trifluoromethyl)phenyl)borate), AgOTf, AgBF
NOBF
4
4
2 2
) can catalyse
4
2
employed to reduce the chemically oxidized species. However, no 1), while the oxidized species showed no conversion after 20 h (Table
1
reversible oxidation/reduction was observed according to H NMR 1, entry 2). In contrast, (thiolfan*)Zr(NEt
2
)
2
showed no reactivity
spectra (Figure S4). Furthermore, a determination of the solution toward the secondary aminoalkene 2a (Table 1, entry 3), but the
state magnetic susceptibility using the Evans method9 of the oxidized species can catalyse the cyclization of 2a in 20 h (Table 1,
oxidized species was unsuccessful due to the low solubility of this entry 4). Similarly, a 66% conversion of 11a was achieved in the
0
2 2
compound in benzene at ambient temperature. Adding a few drops presence of (thiolfan*)Zr(NEt ) after 60 h in o-DFB (Table 1, entry
of o-difluorobenzene resulted in loss of signal. However, elemental 21), while the oxidized species showed no conversion after 21 h
F
analysis agrees with the formula [(thiolfan*)Zr(NEt
oxidized compound.
2
)
2
][BAr ] for the (Table 1, entry 22). (thiolfan*)Zr(NEt
2
)
2
showed no reactivity toward
12a (Table 1, entry 23), but 77% conversion was achieved in 15 h in
the presence of the oxidized species (Table 1, entry 24).
57Fe Mӧssbauer spectroscopic studies at 80 K, with no applied
magnetic field, indicate that both (thiolfan*)Zr(NEt
2
)
2
and Furthermore, the hydroamination of the secondary aminoalkyne 8a
F
[
(thiolfan*)Zr(NEt
2 2
) ][BAr ] contain a low-spin iron(II) (Figure S43). An (Table 1, entry 16) and secondary aminoalkene 10a (Table 1, entry
X-band electron paramagnetic resonance (EPR) spectrum acquired at 20) could only be catalysed by the oxidized catalyst. Unfortunately,
7 K indicates the presence of an organic radical (g = 2.006, Figure (thiolfan*)Zr(NEt was not active towards other secondary
S44). Furthermore, UV-Vis spectra (Figure S45) of aminoalkene substrates: no conversion was observed for substrates
(thiolfan*)Zr(NEt ][BAr ] are different than those of 3a-6a with either state of the catalyst (Table 1, entries 5-12).
thiolfan*)Zr(NEt
7
2 2
)
F
[
(
2 2
)
Ac
F
2
)
2
or [ Fc][BAr ] supporting the Mӧssbauer results However, the results obtained with 1a and 2a indicate that there is
F
that [(thiolfan*)Zr(NEt
2
)
2
][BAr ] does not contain an Fe(III) species, selectivity between primary and secondary aminoalkenes triggered
but the oxidation occurs on the organic portion of a ligand.
Nonetheless, these data are consistent with an oxidized species selectivity was lost with primary aminoalkynes: 7a cyclized in the
by the oxidation state of the zirconium catalyst. Unfortunately, the
Ac
F
being generated with [ Fc][BAr ], and it was reasoned that the presence of both the reduced and oxidized species (Table 1, entries
coordination of substrate may enhance the stability of the 13-14).
catalytically active species.
In our search for redox-switchable catalysts for organic
in transformations, (thiolfan*)Zr(NEt was successfully synthesized
catalyses the
2 2
) was added to a J-Young NMR tube with hydroamination of primary aminoalkenes, whereas an oxidized
To investigate the reactivity of (thiolfan*)Zr(NEt
intramolecular hydroamination reactions, a catalytic amount (10 and characterized. We found that (thiolfan*)Zr(NEt
mol%) of (thiolfan*)Zr(NEt
2
)
2
2 2
)
2 2
)
the aminoalkene/alkyne of interest and an internal standard, species generated in situ catalyses the hydroamination of secondary
hexamethylbenzene(HMB), and the reaction mixture was heated to aminoalkenes. The selectivity of the two oxidation states observed in
Ac
F
1
6 6
00 °C in C D . Control experiments were performed with [ Fc][BAr ] these reactions will form the basis of developing redox switchable
in order to determine if the oxidant can act as a catalyst for the catalysts for assisted tandem reactions.
reactions of interest by itself. No conversion of 1a (Table 1) to 1b was
observed when heated to 100 °C for 20 h in presence of catalytic
Ac
F
amount of [ Fc][BAr ] (Figure S34). However, full conversion of 2a Acknowledgments
(
Table 1) to 2b was observed under the same reaction condition
(
Figure S35). To understand further the reasons behind this
Ac
observation, several experiments using catalytic amounts of Fc, This work was supported by the National Science Foundation (Grant
F
F
FcPF
6
, NaBAr , or [Fc][BAr ] were performed with 2a. After 19 hours, CHE-1809116 to P. L. D. and CHE-1048804 for NMR spectroscopy)
Ac
no conversion was observed in the presence of Fc (Figure S36). and UCLA. Yi Shen is grateful for an INFEWS fellowship (NSF Grant
F
After 2 hours, a partial conversion (34%) was observed with NaBAr
DGE-1735325).
F
(Figure S37), a full conversion was observed with [Fc][BAr ] (Figure
S38), and no conversion was observed with FcPF
6
(Figure S39). The
1
peaks corresponding to acetylferrocene were observed in the H Conflicts of interest
Ac
F
NMR spectra of control reactions catalysed by [ Fc][BAr ], and a
There are no conflicts to declare.
1
peak corresponding to ferrocene was observed in the H NMR
F
spectra of control reactions catalysed by FcPF
6
and [Fc][BAr ]. A
Ac
F
possible explanation is that the aminoalkene reduced [ Fc][BAr ] to
acetylferrocene and the reaction was catalysed by a Brønsted acid.
9
1, References
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2
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4
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Ac
F
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F
F
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