Mild and homogeneous cobalt-catalyzed hydrogenation of C=C, C=O, and C=N bonds

Article abstract of DOI:10.1002/anie.201206051

A cationic cobalt(II)-alkyl complex is an effective precatalyst for hydrogenation of alkenes, aldehydes, ketones, and imines under mild conditions (1-4 atm H₂; see scheme). The catalyst shows a high functional-group tolerance across a broad range of substrates. Experiments suggest that the active catalytic species is a cobalt(II)-hydride complex. Copyright

Full text of DOI:10.1002/anie.201206051

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Angewandte
Communications
DOI: 10.1002/anie.201206051
Cobalt Catalysts
=
Mild and Homogeneous Cobalt-Catalyzed Hydrogenation of C C,
=
=
C O, and C N Bonds**
Guoqi Zhang, Brian L. Scott, and Susan K. Hanson*
The replacement of precious-metal catalysts with cheap and
We synthesized cobalt(II) complexes of the tridentate
abundant metals is a major goal of sustainable chemistry.[1]
Hydrogenation catalysts have diverse and widespread appli-
cations, including the production of biorenewable chemicals
and fuels, commodity chemicals, and the synthesis of fine
chemicals and pharmaceuticals.[2–4] Homogeneous rhodium,
ruthenium, and iridium catalysts are also of critical impor-
tance in asymmetric hydrogenation.[5] Despite significant
recent advances, the design of earth-abundant-metal hydro-
genation catalysts has lagged behind, perhaps because of the
tendency of 3d metals to engage in one-electron or radical
chemistry. Several iron catalysts have been developed for the
hydrogenation of ketones or alkenes, but they are typically
chemoselective, reducing only one class of substrate.[6–9]
Furthermore, iron catalysts are often quite sensitive to
additional oxygen- and nitrogen-containing functional
groups and water.[10]
ligand bis[2-(dicyclohexylphosphino)ethyl]amine (PNHPCy)
(Scheme 1). Previous work by Fryzuk et al. had shown that
a related pincer amidodiphosphine ligand stabilized a square-
planar 15-electron d7-cobalt(II)–alkyl complex,[19] and we
There is growing evidence that cobalt complexes can be
effective catalysts for homogeneous hydrogenation. Cobalt(I)
complexes, such as [Co(H)(CO)4] and [Co(H)(CO)(PnBu3)3],
are known to catalyze the hydrogenation of alkenes and
arenes under hydroformylation conditions (> 1208C,
> 30 atm H2/CO).[11–13] Diiminopyridine cobalt complexes
and the dinitrogen complex [Co(H)(N2)(PPh3)3] catalyze
olefin hydrogenation at room temperature,[14,15] and an
asymmetric hydrogenation of substituted styrenes was
recently developed.[16] However, prior examples of cobalt
hydrogenation catalysts have been quite limited in substrate
scope, and nearly all have involved cobalt(I).[17,18] Herein, we
report a cobalt-based catalytic system for the homogeneous
hydrogenation of alkenes, aldehydes, ketones, and imines.
The hydrogenation reactions take place under very mild
conditions and require no base additives. The ability to
hydrogenate multiple classes of substrates and broad func-
tional-group tolerance make this cobalt system a significant
advance over previously reported earth-abundant-metal
hydrogenation catalysts.
Scheme 1. a) Complexes 1 and 2. b) X-ray structure of complex 1 (ther-
mal ellipsoids at 50% probability, hydrogen atoms omitted for clarity).
Two crystallographically independent molecules were present in the
unit cell for complex 1, one of which is shown above. Selected bond
distances [ꢀ] and angles [8]: Co1–N1 1.880(3), Co1–P1 2.202(1), Co1–
P2 2.204(1), Co1–C29 2.005(3); N1-Co1-C29 172.2(1), N1-Co1-P1
84.9(1), C29-Co1-P1 95.4(1), N1-Co1-P2 84.6(1), C29-Co1-P2 95.7(1),
P1-Co1-P2 168.7(4).
were interested in exploring the reactivity of this type of
unusual odd-electron cobalt–alkyl species. Reaction of
PNHPCy with [(pyr)2Co(CH2SiMe3)2][20] (pyr= pyridine)
afforded the new cobalt(II)–alkyl complex [(PNPCy)Co-
(CH2SiMe3)] (1) as dark-yellow crystals. In the solid state,
paramagnetic complex 1 has a square-planar geometry, and
solution-state magnetic-moment measurements are also con-
sistent with a square-planar low-spin d7 configuration (meff
=
2.2 mB).[21,22] The solution-state magnetic moment of complex
1 is quite similar to that of the related square-planar complex
[(N(SiMe2CH2PPh2)2)Co(CH2SiMe3)] (2.1 mB), which was
reported by Fryzuk et al.[19]
[*] Dr. G. Zhang, Dr. B. L. Scott, Dr. S. K. Hanson
Chemistry and Materials Physics Applications Divisions
Los Alamos National Laboratory
MS J582, Los Alamos, NM 87545 (USA)
E-mail: skhanson@lanl.gov
When one equivalent of the acid H[BArF ]·(Et2O)2
4
(BArF4 = B(3,5-(CF3)2C6H3)4) was added to a solution of 1 in
[**] This work was supported by Los Alamos National Laboratory LDRD
Early Career Award (20110537ER).
[D8]THF,
a
new paramagnetic product, [(PNHPCy)Co-
(CH2SiMe3)]BArF4 (2), was detected by 1H NMR spectrosco-
py in 90% yield (integration against an internal standard).
Complex 2 was isolated and characterized by NMR and IR
spectroscopy, X-ray crystallography, and elemental analysis.
Supporting information for this article, including experimental
details, synthetic procedures, and crystallographic and spectral
data, is available on the WWW under http://dx.doi.org/10.1002/
anie.201206051.
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Table 1: Cobalt-catalyzed alkene hydrogenation.[a]
The 1H NMR spectrum of 2 showed a broad signal at
À20.88 ppm, corresponding to the Si(CH3)3 protons on the
À
alkyl ligand, and the IR spectrum of 2 showed an N H stretch
at 3147 cmÀ1. In the solid state, complex 2 has a distorted
square-planar geometry.
Entry
Substrate
Product
t
[h]
Yield[b]
[%]
Initially, we tested the reactivity of cobalt complexes 1 and
2 with H2, evaluating the catalytic hydrogenation of styrene.
Only very slow hydrogenation occurred using complex
1 (2 mol%), with around 2% conversion of styrene to
ethylbenzene observed after 24 hours at 608C under H2
(1 atm). In contrast, complex 2 was a highly active pre-
catalyst. Complete conversion (50 turnovers) was observed
within two hours at room temperature under just 1 atm
of H2 using 2 (2 mol%, generated in situ from 1 and
1
2
3
4
24
24
24
24
100
100
99
100
H[BArF ]·(Et2O)2). Using an isolated sample of complex 2
4
for the hydrogenation of styrene afforded identical results as
when complex 2 was generated in situ. The hydrogenation of
styrene with precatalyst 2 was unaffected by the addition of
excess Hg metal, which is consistent with an active homoge-
neous catalyst. With a lower catalyst loading (0.05 mol%
5
6
24
24
99
100
7
8
24
24
99
100
1 and 0.05 mol% H[BArF ]·(Et2O)2), a total of 1100 turnovers
4
were observed after 24 hours at room temperature (1 atm H2).
Encouraged by the observation of high activity under such
mild conditions, we explored the substrate scope of this
hydrogenation using cobalt precatalyst 2. The hydrogenation
of a variety of alkenes was tested by employing a combination
9
24
100
10
11
24
40
100
80
of 1 (2 mol%) and H[BArF ]·(Et2O)2 (2 mol%) in THF
4
(Table 1). Hydrogenation of terminal and internal alkenes
proceeded readily within 24 hours at room temperature with
excellent yields (Table 1, entries 5–11). Addition of D2 to
norbornylene occurred with syn stereochemistry, affording
exo,exo-2,3-d2-norbornane. Hydrogenation of (R)-(+)-limo-
nene occurred selectively at the terminal position and the
12
42
99
[a] Conditions: substrate (0.5 mmol) in THF (2 mL), H2 (1 atm), 258C.
[b] Yields determined by GC analysis.
=
internal trisubstituted C C bond was not hydrogenated
(Table 1, entry 11), even under 4 atm H2.[23] At room temper-
ature, hydrogenation of dihydrocarvone occurred only at the
entry 12). To gain insight into the chemoselectivity of the
cobalt catalysis, a competition experiment was performed, in
which a 1:1 mixture of styrene and benzaldehyde was
hydrogenated at room temperature under 1 atm of H2 using
2. Benzaldehyde was hydrogenated more rapidly, with
complete conversion of benzaldehyde and only 16% con-
version of styrene observed after 24 hours.[23]
The hydrogenation of imines is a valuable synthetic route
for the preparation of amines, and thus we tested in situ
generated 2 for the hydrogenation of several imines. Hydro-
genation of N-benzylidenebenzylamine proceeded under
4 atm H2 at 608C to afford dibenzylamine in high yield
(84% yield of isolated product) using 2 mol% of 2 (Table 2,
entry 13). N-Benzylidenemethylamine and N-benzylidene-
aniline were also hydrogenated by the cobalt catalyst
(Table 2, entries 14 and 15). Previous examples of cobalt-
catalyzed imine hydrogenation are scarce.[28]
Given the broad substrate scope, we performed further
experiments to assess the functional-group tolerance of the
catalytic cobalt system (Table 3). According to GC analysis, 4-
penten-1-ol was hydrogenated quantitatively within 24 hours
at room temperature (1 atm H2, Table 3, entry 3). Complex 2
catalyzed the hydrogenation of 4-pentenoic acid to afford
pentanoic acid (82% yield of isolated product) (Table 3,
entry 2). The cobalt catalyst was also active in the presence of
=
C C bond, affording 5-isopropyl-2-methylcyclohexanone in
99% yield (Table 1, entry 12).
There have been few previous reports of cobalt catalysts
capable of ketone or aldehyde hydrogenation. Aldehyde
hydrogenation has been observed as a side reaction in the
hydroformylation of olefins catalyzed by [Co2(CO)8], but
these reactions occur under extremely harsh conditions
(300 atm syngas, 1858C).[11,24,25] Ohgo and Weber developed
a cobalt–dioxime-based system that catalyzed the asymmetric
hydrogenation of benzil, but the substrate scope was limited
to selected 1,2-dicarbonyl compounds.[26,27] Thus, we were
surprised to find that cobalt precatalyst 2 was quite active for
hydrogenation of both aldehydes and ketones under mild
conditions.
Hydrogenation of acetophenone proceeded in nearly
quantitative yield under 1 atm of H2 (Table 2, entry 1).
Aliphatic ketones 2-hexanone and 2-indanone were reduced
in high yields at 608C (Table 2, entries 5 and 6). Benzaldehyde
and substituted benzaldehydes were also reduced in excellent
yields within 24 hours under 1 atm of H2 (Table 2, entries 8–
11). The unsubstituted aliphatic aldehyde 1-octanal was
hydrogenated more slowly, and 1-octanol was isolated in
92% yield after 64 hours at 608C under 4 atm of H2 (Table 2,
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[a]
Table 3: Functional-group tolerance of cobalt-catalyzed hydrogenation.[a]
=
=
Table 2: Cobalt-catalyzed hydrogenation of C O and C N bonds.
Entry
Substrate
Product
t [h]
Yield[b] [%]
1
24
99
2[c]
3
24
24
65
99 (82)[d]
Entry
Substrate
Product
t
Yield[f] [%]
[h] (NMR)[g] {GC}[h]
99
66
4[c]
1
24
43
89 (98)
86 (94)
5
24
99
2[b]
[a] Conditions: substrate (0.5 mmol) in THF (2 mL), 1 (2 mol%),
H[BArF ]·(Et2O)2 (2 mol%), H2 (1 atm), 258C. [b] Yields determined by
4
GC-MS analysis. [c] Reactions run at 608C under 1 atm of H2. [d] Yield of
isolated product.
3[b]
24
48
92 (98)
91 (99)
(2 mol%) for the hydrogenation of styrene in the presence of
intentionally added water (10 mol%). Although the cobalt
catalyst was inhibited by water, the hydrogenation reaction
still occurred, affording ethylbenzene in 99% yield after
24 hours at room temperature (Table 3, entry 5). The toler-
ance of the cobalt catalytic system to an array of functional
groups and added water is remarkable, and is much more
typical of precious-metal hydrogenation catalysts than those
based on earth-abundant metals.[29,30]
4[b]
5[b]
6[b]
7[c]
48
24
65
97 (99)
{100}
{99}
8
24
24
24
24
86 (92)
96 (100)
92 (98)
91 (99)
In light of the high versatility and activity of cobalt
precatalyst 2, we performed additional experiments to gain
insight into possible catalytic reaction mechanisms. The
reaction of paramagnetic cobalt(II)–alkyl complex 2 with H2
(1 atm) in [D8]THF was monitored by 1H NMR spectroscopy.
Within one hour at room temperature, signals corresponding
to complex 2 disappeared from the 1H NMR spectrum, along
with concomitant appearance of a new signal at 0 ppm,
corresponding to tetramethylsilane (TMS). Although the
solution had a yellow color that was consistent with the
presence of a homogeneous cobalt complex, signals that could
be attributed to a cobalt-containing product were not
9
10[d]
11[d]
12[e]
13[e]
14[e]
64
42
72
92 (100)
84 (89)
88 (98)
1
observed in the H NMR spectrum of the reaction mixture.
The magnetic moment of the solution was measured (Evans
method) to be meff ꢀ 2.7 mB, consistent with (a) paramagnetic
reaction product(s).
Several more experiments suggested that a cobalt(II)–
hydride complex 3 might be formed upon the reaction of
cobalt(II)–alkyl complex 2 with H2. A solution of complex 2 in
[D8]THF was treated with H2 (1 atm) for three hours,
affording tetramethylsilane and the NMR-silent cobalt-con-
taining product(s). Subsequent addition of CHCl3 (2 equiv)
caused an immediate color change from yellow to red, and
formation of CH2Cl2 was detected by 1H NMR spectroscopy.
The cobalt-containing product of this reaction was isolated in
77% yield and identified as the cobalt(II)–chloride complex
15[e]
48
65 (70)
[a] Conditions: substrate (0.5 mmol) in THF (2 mL), H2 (1 atm), 258C.
[b] Reactions run at 608C. [c] Reactions run at 258C under 4 atm of H2.
[d] Reactions run at 508C. [e] Reactions run at 608C under 4 atm of H2.
[f] Yields of isolated products. [g] Yields determined by NMR spectros-
copy. [h] Yields determined by GC analysis.
an amine, hydrogenating N-methyl-4-piperidone to N-
methyl-4-piperidinol in 66% yield, according to GC analysis
(Table 3, entry 4).
Water is a common adventitious impurity in reagents and
solvents, and thus we tested the activity of cobalt precatalyst 2
[(PNHPCy)Co(Cl)]BArF (4) by X-ray crystallography, IR
4
spectroscopy, and elemental analysis. The formation of
chloride complex 4 upon trapping with CHCl3 suggests that
the hydride complex [(PNHPCy)Co(H)]BArF (3) is formed
4
upon the reaction of 2 with H2 (Scheme 2a).
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Keywords: cobalt · homogeneous catalysis · hydrogenation ·
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