Acceptorless, Reversible Dehydrogenation and Hydrogenation of N-Heterocycles with a Cobalt Pincer Catalyst

Article abstract of DOI:10.1021/acscatal.5b02002

Acceptorless, reversible dehydrogenation and hydrogenation reactions involving N-heterocycles are reported with a well-defined cobalt complex supported by an aminobis(phosphine) [PN(H)P] pincer ligand. Several N-heterocycle substrates have been evaluated under dehydrogenation and hydrogenation conditions. The cobalt-catalyzed amine dehydrogenation step, a key step in the dehydrogenation process, has been independently verified. Control studies with related cycloalkanes suggest that a direct acceptorless alkane dehydrogenation pathway is unlikely. The metal-ligand cooperativity is probed with the related [PN(Me)P] derivative of the cobalt catalyst. These results suggest a bifunctional dehydrogenation pathway and a nonbifunctional hydrogenation mechanism.

Full text of DOI:10.1021/acscatal.5b02002

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Article
Acceptorless, Reversible Dehydrogenation and
Hydrogenation of N-heterocycles with a Cobalt Pincer Catalyst
Ruibo Xu, Sumit Chakraborty, Hongmei Yuan, and William D. Jones
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02002 • Publication Date (Web): 15 Sep 2015
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Acceptorless, Reversible Dehydrogenation and Hydrogena-
tion of N-heterocycles with a Cobalt Pincer Catalyst
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†,‡,¶
†,¶
†
†,
Ruibo Xu,
Sumit Chakraborty, Hongmei Yuan and William D. Jones *
†
Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
School of Pharmacy, Huaihai Institute of Technology, Lianyungang, Jiangsu 222005, P.R. China
‡
cyclometalated iminoꢀiridium complexes as dehydrogenationꢀ
hydrogenation catalysts for a broad range of Nꢀheterocycles.
ABSTRACT: Acceptorless, reversible dehydrogenation and
hydrogenation reactions involving Nꢀheterocycles are reported
with a wellꢀdefined cobalt complex supported by a aminoꢀ
bis(phosphine) [PN(H)P] pincer ligand. Several Nꢀheterocycle
substrates have been evaluated under dehydrogenation and hydroꢀ
genation conditions. The cobaltꢀcatalyzed amine dehydrogenation
step, a key step in the dehydrogenation process, has been indeꢀ
pendently verified. Control studies with related cycloalkanes sugꢀ
gest that a direct acceptorless alkane dehydrogenation pathway is
unlikely. The metalꢀligand cooperativity is probed with the related
6
f,6h
Despite the progress with these iridiumꢀbased catalysts, there
is interest in the development of efficient homogeneous catalysts
containing sustainable transition metals such as iron, cobalt, and
nickel for these reactions. Our group has recently reported the first
iron catalyst, supported by an aminobis(phosphine) pincer ligand
i
iPr
(
HN[(CH ) P Pr ] (abbreviated as [PN(H)P] ligand), for the
2 2 2 2
8
reversible dehydrogenation/hydrogenation of Nꢀheterocycles. A
wide variety of Nꢀheterocycles including saturated and unsaturatꢀ
ed quinoline, imidazole, and pyridine derivatives were successfulꢀ
ly explored. The mechanism, highlighting the role of the metalꢀ
ligand cooperativity, has been investigated by experimental and
[PN(Me)P] derivative of the cobalt catalyst. These results suggest
a bifunctional dehydrogenation pathway and a nonꢀbifunctional
hydrogenation mechanism.
9
DFT studies.
In our continuous efforts to discover first row transitionꢀmetal
catalysts for these challenging transformations, we focused our
attention to the [PN(H)P]ꢀsupported cobalt complexes (Cy:
Keywords: acceptorless dehydrogenation, hydrogenation, cobalt
catalysis, Nꢀheterocycles, pincer ligands, H2 storage materials.
Cy
cyclohexyl) for the purpose of comparing the activity of cobalt to
that of iron. Hanson and coworkers have used these cobalt comꢀ
plexes (preformed or generated inꢀsitu) as efficient catalysts for
the dehydrogenation of alcohols and hydrogenation of olefins,
aldehydes, ketones, and imines (Figure 1). It is noteworthy that
primary amines have been used as trapping reagents in the cobaltꢀ
Acceptorless catalytic dehydrogenation and hydrogenation of
Nꢀheterocycles are one of the most atomꢀefficient ways to proꢀ
duce quinoline and tetrahydroquinoline derivatives that are an
integral part of numerous pharmaceuticals and bioactive moleꢀ
10
1
cules. In addition, these reactions are also applicable to the field
2
of liquid organicꢀhydrogenꢀstorageꢀmaterials and commercial
catalyzed dehydrogenation of primary alcohols to generate imines
3
10b
fuel cells. Acceptorless dehydrogenation of Nꢀheterocycles is a
from the corresponding Schiffꢀbase reaction.
Interestingly,
thermodynamically uphill process at ambient conditions. Howevꢀ
er, experimental and computational studies have demonstrated
that the presence of the nitrogen atom in these heterocycles deꢀ
under the catalytic conditions (at 120 °C), no products arising
from the dehydrogenation of the primary amine was observed. We
questioned if it is possible to dehydrogenate amines using this
cobalt pincer complex.
o
creases the ꢁH of the reaction when compared to regular cycloꢀ
4
alkanes. As H2 gas is released during this process, the
Previous work
de/hydrogenation equilibrium can be altered by removing H from
OH
R'
O
NR"2
R'
2
[1]
"R NH
2
4c
5
BArF
4
the system. Although several heterogeneous and few homogeꢀ
R
H
2
R
R' R' = H
R
H
N
6
ꢀ8
neous catalysts have been reported to carry out either acceptorꢀ
less dehydrogenation or hydrogenation of Nꢀheterocycles, examꢀ
ples of a single metal catalyst for both the reactions are extremely
R1
R1
Cy P
2
Co PCy2
in-situ
[1]
R2
R3
R2
R3
2 3
CH SiMe
5
h,6a,6i,8
O
N
OH
R2
HN
R1
rare in the literature.
Fujita and coworkers first reported a wellꢀdefined homogeneꢀ
ous iridium catalyst, supported by nonꢀinnocent 2ꢀ
hydroxypyridine ancillary ligand, for the dehydrogenation of tetꢀ
H2
1
R1
R2
R1
R2
R1
R2
a
This work
6
a
[
1], acceptorless
rahydroquinoline derivatives under mild conditions. DFT studies
have indicated a ligandꢀpromoted hydrogen abstraction from the
substrate without changing the formal oxidation state of the iridiꢀ
R
R
N
[1], H
2
N
H
6c
um(III) center. Remarkably, the same iridium complex can also
Figure 1. Past and Current Applications of the Cobalt Pincer
Catalyst (1).
catalyze the hydrogenation of unsaturated quinoline derivatives
under an atmospheric pressure of H . More recently, Fujita et al.
2
Encouraged by the critical role of the [PN(H)P] ligand in the
have reported similar iridium catalysts bearing a bipyridonate
ligand for the perdehydrogenation and perhydrogenation of fused
8,9
iron system and superior activities of these cobalt complexes in
10
6
i
dehydrogenation and hydrogenation reactions, we became interꢀ
ested to investigate catalytic dehydrogenation and hydrogenation
of Nꢀheterocycles, containing a cyclic secondary amine moiety,
bicyclic Nꢀheterocycles that have more H content per molecule.
2
In addition to these studies, Xiao and coworkers have developed
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with the [PN(H)P]ꢀsupported cobalt complexes. Herein we report
for the first time that the preformed cobalt complex,
Table 1. Cobalt-Catalyzed Acceptorless Dehydrogenation
of N-Heterocycles.
a
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Cy
F
[{ PN(H)P}Co(CH SiMe )]BAr (1), can effectively catalyze
2
3
4
both the acceptorless dehydrogenation and hydrogenation of difꢀ
ferent Nꢀheterocycles under relatively mild conditions. Prelimiꢀ
nary mechanistic results indicate a metalꢀligand cooperative
pathway for the dehydrogenation process and a nonꢀcooperative
process for the hydrogenation reaction.
entry
1
substrate
product
time
conv.
(%)
yield
(%)
In order to determine the optimum conditions for catalysis, we
studied catalytic dehydrogenation of 1,2,3,4ꢀtetrahydroquinaldine
(a) in the presence of complex 1 (5ꢀ20 mM) under various condiꢀ
tions (Table Sꢀ1 in the Supporting Information). Carrying out the
catalytic reaction between 100–120 °C using relatively lowꢀ
boiling solvents such as THF and toluene did not produce any of
the desired unsaturated product. In contrast, when the same reacꢀ
tion mixture was heated to 150 °C in highꢀboiling pꢀxylene solꢀ
vent, ~83% of a was converted to form quinaldine (a′) as the sole
b
c
(d)
4
83
79
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
d
2
4
4
70
65
97
11
dehydrogenation product after four days. No partially dehydroꢀ
genated species was observed during this process when the reacꢀ
tion was monitored by gas chromatography at intermediate times.
When the concentration of the catalyst was doubled, a′ was
formed in almost quantitative amounts (91%) at 150 °C. Control
experiments without the catalyst did not yield any dehydrogenaꢀ
tion product. Furthermore, the homogeneous nature of this system
was indicated by the reproducible kinetic profile without an inꢀ
duction period (See the Supporting Information, Figure S4).
We next investigated the scope of Nꢀheterocycle substrates
3
4
5
100
4
100
98
(100 mM) in the dehydrogenation reaction with 10 mol% of 1 (10
4
3
0
ꢀ
mM) in pꢀxylene at 150 °C (Table 1). Under these catalytic condiꢀ
tions, several Nꢀheterocyclic compounds (a–d) including sixꢀ
membered
tetrahydroquinoline
and
fiveꢀmembered
2ꢀ
N
methylindoline were successfully dehydrogenated to produce
respective aromatic products (a′–d′) with almost quantitative GC
conversions and good GC yields. 1,2,3,4ꢀtetrahydroquinoline
reacted much more slowly in the presence of 10 mol% of catalyst
and afforded only 24% conversion after four days. In marked
contrast to other Nꢀheterocycle substrates, dehydrogenation of
d,e
6
100
96
N
f'
a
Conditions: 1 (10 ꢂmol, 10 mM), Nꢀheterocyclic substrate
(
0.1 mmol, 100 mM), pꢀxylene (1 mL), heated at 150 °C in a 500
2
,6ꢀdimethylpiperidine (e) did not afford any of the desired prodꢀ
b
mL roundꢀbottom ampule fitted with a Kontes valve. Converꢀ
sions were calculated from the relative peak area integrations the
reactant and product in the GC spectra. Yields were determined
by GC using dodecane as the internal standard. 20 mol% of 1
was used. With 10 mol% of 1, 74% conversion was observed
uct under these conditions. Hazari and coworkers have recently
reported accelerated rate effects of adding a Lewis acid catalyst in
c
1
2
the dehydrogenation of formic acid and methanol. Inspired by
d
these studies, we attempted to carry out the dehydrogenation of e
e
in the presence of Lewis acids such as LiBF and B(C F ) . Unꢀ
4
6
5 3
after 4 days.
fortunately, these additives had no effect on this reaction and no
dehydrogenation activity was observed. A much less bulky subꢀ
strate piperidine could not be dehydrogenated as well under these
lyst, after the first catalytic run with substrate a (with 20 mol% 1),
a fresh batch of substrate was introduced into the system and the
reaction was carried out for four more days at 150 °C. In this seꢀ
cond run, 84% of a was converted to a ′, indicating that the cataꢀ
lyst decomposition is minimal.
Once the protocol for the cobaltꢀcatalyzed dehydrogenation of
Nꢀheterocycles was demonstrated, we became interested to carry
out the microscopic reverse of the dehydrogenation process with
the same cobalt catalyst 1. As mentioned earlier, it is rare to find a
single transition metal catalyst that can perform both dehydroꢀ
iPr
conditions. In comparison, the related [PN(H)P] iron complex
catalyzes the complete dehydrogenation of 2,6ꢀdimethylpiperidine
8
to afford 2,6ꢀlutidine under similar conditions. Catalytic dehyꢀ
drogenation of 1,2,3,4ꢀtetrahydroquinoxaline (f), containing two
amine moieties in the same molecule, proceeded smoothly to
yield the desired quinoxaline derivative as the sole product. It is
interesting to note that the formation of the quinoline moieties
does not inhibit the catalysis by binding to the cobalt center
through the basic Nꢀatoms. Although unclear at this point, the
genation
heterocycles.
and hydrogenation
reactions involving Nꢀ
Cy
8,13
steric and electronic properties of the [PN(H)P] ligand might
We performed catalytic hydrogenation of quiꢀ
play a crucial role to prevent catalyst inhibition by substrate or
product coordination.
naldine (a′) under various conditions (see Table S2 in the Supꢀ
porting Information). We observed that a′ could be quantitatively
converted to a in the presence of 5 or 10 mol% of 1, 10–20 atm of
To confirm the identity of the released gas from the dehydroꢀ
genation reaction, the volatiles were vacuum transferred from the
reaction vessel to a sealed J. Young NMR tube containing a
H pressure at 120 °C in THF. Remarkably, almost quantitative
2
(90%) hydrogenation of quinaldine b′ could also be achieved with
1
chilled solution of C D . Recording the H NMR spectrum of the
H pressure as low as 5 atm.
6
6
2
resulting solution showed a singlet resonance at δ4.47 which is
Having established the best hydrogenation conditions (5
characteristic for dihydrogen. Additionally, the released H gas
mol% of 1, 10 atm H ), we next planned to expand the substrate
2
2
was also detected by gas chromatography (See the Supporting
Information, Figure S2). In order to test the reusability of the cataꢀ
scope of this reaction (Table 2). For quinoline derivatives (entries
1, 2, and 6), quantitative conversions were observed in two days
2
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to produce the respective products (entries 1, 2, 6). In all of these
process and this observation is also consistent with the observed
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
quinoline substrates, the aromatic phenyl ring remained intact
after the reaction. Surprisingly, under optimum catalytic condiꢀ
tions, only 11% of isoquinoline (c′) was converted to the correꢀ
sponding saturated compound. Doubling both the catalyst concenꢀ
inactivity of e′ under this condition.
Experimental support for the first amine dehydrogenation step
to release the first equivalent of dihydrogen comes from the sucꢀ
cessful dehydrogenation reaction observed for 1,2,3,4ꢀ
tetrahydroquinoxaline (f) by complex 1 under standard catalytic
conditions (Figure 2, eq 1). Monitoring this reaction periodically
by gas chromatography and comparing the relative rates of dehyꢀ
drogenation with 1,2,3,4ꢀtetrahydroquinoline indicates that the
rate of the first amineꢀdehydrogenation step is comparable, and
that it is likely to be slower than the second dehydrogenation step
(Figure 2), since no partially dehydrogenated product was obꢀ
served at any given time during catalysis.
tration and the H pressure increased the conversion to 56% (entry
2
3
). In the case of the 2ꢀmethylindole (entry 4), a much lower conꢀ
version (24%) was observed as well. Furthermore, attempts to
hydrogenate 2,6ꢀlutidine (e′, entry 5) proved unsuccessful even at
a much higher temperature (150 °C) and with a higher pressure of
H (20 atm).
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
a
Table 2. Hydrogenation of Unsaturated N-Heterocycles.
entry
1
substrate
product
time
conv.
(%)
yield
(%)
b
c
(d)
100
80
2
100
98
60
40
1
,2,3,4-tetrahydroquinoxaline
,2,3,4-tetrahydroquinoline
2
2
2
2
100
56
99
53
19
20
0
1
d
0
20
40
60
80
100
3
N
time (h)
c'
Figure 2. Relative Rates of Dehydrogenation (with 10 mol% 1).
In order to release the second equivalent of H from the subꢀ
strate, two CꢀH bonds need to be broken. This could happen by
two different pathways: (a) a direct alkane dehydrogenation by
cobalt from the partially oxidized substrate or (b) isomerization of
the initially formed C=N to a C=C, followed by a second dehyꢀ
drogenation from the secondary amine fragment. To test this
hypothesis, we carried out dehydrogenation reactions of the folꢀ
d
4
24
2
14
5
6
2
2
0
ꢀ
6e
lowing
substrates
with
catalyst
1:
(i)
1,2,3,4ꢀ
tetrahydronaphthalene and (ii) 1,2ꢀdihydronaphthalene. For these
first two substrates (without any Nꢀatom), no dehydrogenation
activity was observed, suggesting the direct alkane dehydrogenaꢀ
tion pathway is unlikely (eq 2).
100
98
N
H
Ph
g
d,e
7
2
98
?
a
Conditions: 1 (10 μmol, 10 mM), substrate (0.2 mmol, 200
b
To elucidate the involvement of the NꢀH moiety on the pincer
mM), 2 mL THF. pH = 10 atm. Conversions were calculated
from the relative peak area integrations in the GC spectra. Yields
2
c
ligand in the catalysis, the reactivities of
1
and
Cy
F
10c
[ {PN(Me)P}Co(CH SiMe )]BAr (1-Me)
were compared
2
3
4
were determined by GC using dodecane as the internal standard.
d
e
(Scheme 1). Hanson and coworkers have reported that complex 1-
Me displayed similar catalytic activity as 1 in the dehydrogenaꢀ
1
0 mol%, 20 atm H . 150 °C.
2
10c
Recently, Fujita and coworkers have reported perhydrogenaꢀ
tion of 1ꢀphenylethanol, but the same methyl derivative was
found to be an inactive catalyst for the hydrogenation of ketones
tion of fused bicyclic Nꢀheterocycles with an iridium bipyridinone
6i
complex with a high H pressure (70 atm). The fully hydrogenꢀ
(with 1 atm H pressure). In stark contrast, we observed that while
2
2
ated bicyclic product carries more hydrogen per molecule and
therefore is an attractive hydrogen storage material. Inspired by
this finding, we planned to test perhydrogenation of 1,5ꢀ
naphthyridine (h′) with cobalt catalyst 1. When the reaction was
the dehydrogenation of 1,2,3,4ꢀtetrahydroquinaldine was comꢀ
pletely inhibited in the presence of the 1-Me catalyst, hydrogenaꢀ
tion of quinaldine proceeded smoothly (~80% conversion) to
produce the desired product under standard catalytic conditions
(Scheme 1). The completely opposite reactivity of 1-Me catalyst
in the amine dehydrogenation reaction points toward an energetiꢀ
cally and/or a mechanistically different scenario for the alcoꢀ
hol/ketone case. It is also possible that the differences in the cataꢀ
performed in the presence of 20 atm of H , h′ was almost quantiꢀ
2
tatively converted (98%) and h was observed as the only hydroꢀ
genation product. One of the pyridine rings remained intact in this
3
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lytic conditions, especially the H pressure, between the two sysꢀ
2
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
tems allow for the variation in the mechanisms. Nevertheless,
results shown in Scheme 1 suggest that the cobalt catalyst 1 dehyꢀ
drogenates Nꢀheterocycles in a cooperative fashion involving the
NꢀH moiety on the pincer ligand. On the contrary, the hydrogenaꢀ
tion reaction can proceed without the involvement of the NꢀH
group.
Environ. Sci. 2011, 4, 2767ꢀ2773. (e) Armaroli, N.; Balzani, V.
ChemSusChem 2011, 4, 21ꢀ36. (f) Fukuzumi, S.; Suenobu, T. Dalton
Trans. 2013, 42, 18ꢀ28.
(3) (a) Nawar, S.; Huskins, B; Aziz, M. MRS Proceedings 2013, 1491,
mrsf12ꢀ1491ꢀc08ꢀ09. doi:10.1557/opl.2012.1737. (b) Nagao, M.; Kobaꢀ
yashi, K.; Yamamoto, Y. Hibino, T. J. Electrochem. Soc. 2015, 162,
F410ꢀF418.
Scheme 1. Probing Metal-Ligand Cooperativity.
(4) (a) Crabtree, R. H. Energy Environ. Sci. 2008, 1, 134ꢀ138 and refs.
therein. (b) Jessop, P. Nat. Chem. 2009, 1, 350ꢀ351 and refs. therein. (c)
Watson, L. A.; Eisenstein, O. J. J. Chem. Educ. 2002, 79, 1269ꢀ1277. (d)
Clot, E.; Eisenstein, O.; Crabtree, R. H. Chem. Commun. 2007, 2231ꢀ
2233. (e) Moores, A.; Poyatos, M.; Luo, Y.; Crabtree, R. H. New J. Chem.
2006, 30, 1675ꢀ1678.
[
1-Me](10 mol%)
+
2 H
2
(4)
(5)
BArF
4
1
50 oC, 4 d
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
N
H
N
Me
N
p-xylene
0
% conversion
Cy
2
P
Co PCy
CH SiMe
1-Me
2
(5) For heterogeneous systems see: (a) Adkins, H.; Lundsted, L. G. J.
[
1-Me](5 mol%)
2
3
+
2 H
2
Am. Chem. Soc. 1949, 71, 2964ꢀ2965. (b) Lu, S. M.; Wang, Y. Q.; Han, X.
W.; Zhou, Y. G. Chin. J. Catal. 2005, 26, 287ꢀ290. (c) Pez, G. P.; Scott,
A. R.; Cooper, A. C.; Cheng, H.; Wilhelm, F. C.; Abdourazak, A. H. U.S.
Patent 7351395 and 7429372, 2008. (d) Sotoodeh, F.; Smith, K. J. J.
Catal. 2011, 279, 36ꢀ47. (e) Sotoodeh, F.; Huberb, B. J. M.; Smith, K. J.
Appl. Catal., A, 2012, 420, 67ꢀ72. (f) Yang, M.; Han, C.; Ni, G.; Wu, J.;
Cheng, H. Intl. J. Hydrogen Energy 2012, 37, 12839ꢀ12845. (g) Wan, C.;
An, Y.; Chen, F.; Cheng, D.; Wu, F.; Xu, G. Intl. J. Hydrogen Energy
1
20 oC, 2 d
THF
N
10 atm
N
H
80% conversion
In summary, we have demonstrated reversible dehydrogenaꢀ
tion/hydrogenation of Nꢀheterocycles with an inexpensive and
earthꢀabundant molecular cobalt pincer catalyst in the absence of
an acceptor. Respective products from both dehydrogenation and
hydrogenation reactions were formed with high conversions and
yields. Substrateꢀdriven mechanistic studies support the initial
amineꢀdehydrogenation step and argue against a direct alkane
dehydrogenation step from the partially oxidized Nꢀheterocycles.
Replacement of the NꢀH moiety in the pincer ligand in 1 with Nꢀ
Me inhibited the dehydrogenation activity but retained its hydroꢀ
genation activity. These results differ from what have been obꢀ
served in the dehydrogenationꢀhydrogenation reactions involving
alcohols and ketones. Future efforts will be focused on pursuing
detailed mechanistic investigation to rationalize these differences
and these results will be reported in the future.
2
013, 38, 7065ꢀ7072. (h) Mikami, K.; Ebata, K.; Mistudome, T.;
Mizugaki, T.; Jitsukawa, K.; Kaneda, K. Heterocycles 2011, 82, 1371ꢀ
1377.
(6) For homogeneous dehydrogenation and hydrogenation of tetrahyꢀ
droquinolines and quinolines see: (a) Yamaguchi, R.; Ikeda, C.;
Takahashi, T.; Fujita, K.ꢀI. J. Am. Chem. Soc. 2009, 131, 8410ꢀ8412. (b)
Wang, Z.; Tonks, I.; Belli, J.; Jensen, C. M. J. Organomet. Chem. 2009,
694, 2854ꢀ2857. (c) Li, H.; Jiang, J.; Lu, G.; Huang, F.; Wang, Z.ꢀX.
Organometallics 2011, 30, 3131ꢀ3141. (d) Zhang, X.ꢀB.; Xi, Z. Phys.
Chem., Chem. Phys. 2011, 13, 3997ꢀ4004. (e) Dobereiner, G. E.; Nova,
A.; Schley, N. D.; Hazari, N.; Miller, S. J.; Eisenstein, O.; Crabtree, R. H.
J. Am. Chem. Soc. 2011, 133, 7547ꢀ7562. (f) Wu, J.; Talwar, D.; Johnston,
S.; Yan, M.; Xiao, J. Angew. Chem. Int. Ed. 2013, 52, 6983ꢀ6987. (g)
Luca, O.; Huang, D. L.; Takase, M. K.; Crabtree, R. H. New J. Chem.
ASSOCIATED CONTENT
2
013, 37, 3402ꢀ3405. (h) Wu, J.; Barnard, J. H.; Zhang, Y.; Talwar, D.;
Robertson, C. M.; Xiao, J. Chem. Commun. 2013, 49, 7052ꢀ7054. (i)
Fujita, K.ꢀI.; Tanaka, Y.; Kobayashi, M.; Yamaguchi, R. J. Am. Chem.
Soc. 2014, 136, 4829ꢀ4832.
(7) For catalytic dehydrogenation of other Nꢀheterocycles see: (a) Tsuꢀ
ji, Y.; Kotachi, S.; Huh, K. T.; Watanabe, Y. J. Org. Chem. 1990, 55, 580ꢀ
Supporting Information
Experimental procedures and product characterization data are
included. This material is free of charge via the internet at
http://pubs.acs.org.
5
84. (b) Hara, T.; Mori, K.; Mizugaki, T.; Ebitani, K.; Kaneda, K. Tetraꢀ
AUTHOR INFORMATION
hedron Lett. 2003, 44, 6207ꢀ6210. (c) Dean, D.; Davis, B.; Jessop, P. G.
New J. Chem. 2011, 35, 417ꢀ422. (d) Wang, Z.; Belli, J.; Jensen, C. M.
Faraday Discuss. 2011, 151, 297ꢀ305. See also refs. 6b and 6i.
(8) Chakraborty, S.; Brennessel, W. W.; Jones, W. D. J. Am. Chem.
Soc. 2014, 136, 8564ꢀ8567.
Corresponding Author
jones@chem.rochester.edu
Notes
(9) Bellows, S.; Chakraborty, S.; Cundari, T.; Jones, W. D. Unꢀ
published results.
The authors declare no competing financial interest.
(10) (a) Zhang, G.; Scott, B. L.; Hanson, S. K. Angew. Chem. Int. Ed.
2
012, 51, 12102ꢀ12106. (b) Zhang, G.; Hanson, S. K. Org. Lett. 2013, 15,
Author Contributions
650ꢀ653. (c) Zhang, G.; Vasudevan, K. V.; Scott, B. L.; Hanson, S. K. J.
Am. Chem. Soc. 2013, 135, 8668ꢀ8681
¶
These authors contributed equally.
(11) Fujita and Jones both have observed similar sharp increase in the
ACKNOWLEDGMENTS
percent conversion when the solvent was switched from toluene to pꢀ
xylene. See reference 6a and 8.
(12) (a) Bielinski, E. A.; Lagaditis, P. O.; Zhang, Y.; Mercado, B. Q.;
Würtele, C.; Bernskoetter, W. H.; Hazari, N.; Schneider, S. J. Am. Chem.
Soc. 2014, 136, 10234ꢀ10237. (b) Bielinski, E. A.; Förster, M.; Zhang, Y.;
Bernsꢀ koetter, W. H.; Hazari, N.; Holthausen, M. C. ACS Catal. 2015, 5,
This work was funded by the Center for Electrocatalysis,
Transport Phenomena, and Materials (CETM) for Innovative
Energy Storage, an EFRC funded by the U.S. DOE (Award DEꢀ
SC0001055), the ESD NYSTAR program, and in part, by NSF
under the CCI Center for Enabling New Technologies through
Catalysis (CENTC), CHEꢀ1205189.
2
404ꢀ2415.
(13) For hydrogenation of Nꢀheterocycles see: (a) Dobereiner, G. E.;
Nova, A.; Schley, N. D.; Hazari, N.; Miller, S. J.; Eisenstein, O.; Crabtree,
R. H. J. Am. Chem. Soc. 2011, 133, 7547ꢀ7562 and references cited thereꢀ
in. (b) Wu, J.; Barnard, J. H.; Zhang, Y.; Talwar, D.; Robertson, C. M.;
Xiao, J. Chem. Commun. 2013, 49, 7052ꢀ7054 and references cited thereꢀ
in. For hydrogenation of imines see: Lagaditis, P. O.; Sues, P. E.; Sonnenꢀ
berg, J. F.; Wan, K.Y.; Lough, A.J.; Morris, R.H. J. Am. Chem. Soc. 2014,
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ACS Catalysis
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TOC Graphic for
Acceptorless, Reversible Dehydrogenation and Hydrogena-
tion of N-heterocycles with a Cobalt Pincer Catalyst
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,‡,¶
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†
†,
Ruibo Xu,
Sumit Chakraborty, Hongmei Yuan and William D. Jones *
2 H2
N
R
+ 2 H
2
N
R
H
reversible, acceptorless
BArF
H
N
4
Cy P
Co PCy2
CH SiMe
2
2
3
Cobalt Pincer Catalyst
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