Cobalt(II)-catalysed transfer hydrogenation of olefins

Article abstract of DOI:10.1039/c6ra02021f

Catalytic transfer hydrogenation of olefins by isopropanol is achieved using an earth-abundant metal cobalt(ii) complex based on a pincer-type PNP ligand. A range of olefins including aromatic and aliphatic alkenes as well as internal and cyclic alkenes have been transfer hydrogenated in good to excellent yields. The catalyst also showed good functional group and water tolerance to olefin transfer hydrogenation reactions.

Full text of DOI:10.1039/c6ra02021f

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Cobalt(II)-catalysed transfer hydrogenation of
olefins†
Cite this: RSC Adv., 2016, 6, 22419
*
Guoqi Zhang, Zhiwei Yin and Jiawen Tan
Received 22nd January 2016
Accepted 18th February 2016
DOI: 10.1039/c6ra02021f
www.rsc.org/advances
Catalytic transfer hydrogenation of olefins by isopropanol is achieved catalysts were reported for the asymmetric TH of amine-
using an earth-abundant metal cobalt(II) complex based on a pincer- functionalized alkenes15 and a Fe(BF4)2/P4 system for the
type PNP ligand. A range of olefins including aromatic and aliphatic transfer semihydrogenation of terminal alkynes,16 except for
alkenes as well as internal and cyclic alkenes have been transfer heterogeneous nickel nanoparticle catalysts that have been
hydrogenated in good to excellent yields. The catalyst also showed extensively studied.17 Molecular cobalt catalysts capable of
good functional group and water tolerance to olefin transfer hydro- reducing such substrates under homogeneous TH conditions
genation reactions.
remain unprecedented, although cobalt-catalysed hydrogena-
tion by H2 source has been previously reported by the Chirik
and Peters groups.18 It is, therefore, quite desirable to develop
new catalysts based on earth abundant metal cobalt for the TH
of olens.
Previously, we have reported on the discovery of an ionic
cobalt(II) complex [(PNHPCy)Co(CH2SiMe3)][BArF4] (1, Scheme 1)
built on a pincer-type PNP ligand that is efficient hydrogenation
catalyst for a broad range of polar and non-polar double bonds
under very mild conditions.19 The ability of this catalyst in
Catalytic transfer hydrogenation (TH) that utilizes a mild and
safe hydrogen source other than hydrogen gas provides
a versatile and alternative way to reduce diverse multiple bonds,
typically including polar C]O and C]N bonds.1 Amongst the
many methods for catalytic TH reaction developed thus far,
transition metal catalytic TH is found to be the most efficient
and practically applicable.2 Although precious metal Ru, Ir and
Rh catalysts have been extensively explored and great advance
has been achieved in terms of high turnover frequencies and
efficacy,3–6 catalysts involving earth-abundant metals such as
iron, cobalt and nickel are relatively less developed.7 It has been
realized in recent years that it would be urgent to replace
precious metal catalysts by abundant elements towards more
practical and widespread industrial applications of TH in the
future,1,8–12 as the high expense and scarcity of precious metals
will largely limit their employment in sustainable chemical
transformations.
Whereas most of reported TH catalysts were effective for
polar double bonds, TH reactions of compounds that contain
non-polar carbon–carbon multiple bonds were little explored.13
Compared to hydrogenation by H2, TH of alkenes and alkynes is
more challenging, and known catalysts for such substrates
involve mainly precious metal Ru, Rh and Pd complexes.13,14 To
the best of our knowledge, only a few homogeneous nickel
Department of Sciences, John Jay College and Ph.D. Program in Chemistry, The
Graduate Center of the City University of New York, New York, 10019 NY, USA.
E-mail: guzhang@jjay.cuny.edu
† Electronic supplementary information (ESI) available: Detailed experimental
procedures and spectroscopic data. See DOI: 10.1039/c6ra02021f
Scheme
hydrogenation.
1 Cobalt complexes 1–6 studied for alkene transfer
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carrying out effective “acceptorless” dehydrogenation was catalytic activity of related cobalt complexes in the TH of
further established.19–23 Subsequently, homogeneous TH of styrene, 3–6 were then examined for reactions under the same
a variety of polar double bonds including C]O and C]N bonds conditions (entries 6–9). It was observed that changing the
by the same cobalt catalyst was investigated using isopropanol substituents (from cyclohexyl to phenyl) on phosphorus atoms
as a hydrogen source.24 In this work, we noticed that except for of the ligand has little inuence on the catalytic activity, while
simple ketones, two a,b-unsaturated ketones were fully hydro- a great reactivity difference was previously revealed for the H2
genated to afford saturated alcohols, indicating the potential of hydrogenation reactions.19 However, the counter anion in the
this cobalt catalyst in reducing non-polar double bonds. Herein, cobalt catalyst was crucial for high reactivity, as replacing the
À
we report for the rst time a cobalt-catalysed TH of olens by BArF À with BPh4 (in 4) drastically decreased the yield of eth-
4
isopropanol under homogeneous conditions.
ylbenzene. In addition, ionic cobalt complex 5 built on the N-
Initial reactivity tests were conducted by using styrene as methylated ligand displayed the same activity as cobalt 1.
a model compound and the cobalt complexes 1 and 2 However, when the Co(III) complex (6), an active reactive inter-
(Scheme 1) as catalysts, and the results are summarized in mediate isolated during alcohol dehydrogenation reaction, was
Table 1. Although 1 (2 mol%, generated in situ from equivalents employed for the TH of styrene, only moderate yield was ob-
of 2 and H[BArF4]$(Et2O)2) was known to readily hydrogenate tained, suggesting that Co(III) species was unlikely to be a cata-
a variety of alkenes at room temperature under 1 atm of dihy- lyst resting state in the TH process, consistent with the results
drogen, the TH test on styrene at 25 C by using isopropanol from TH of ketones catalysed by 6.24
ꢀ
(excess in THF) as a hydrogen source was unsuccessful (entry 1,
Further reaction screening was carried out by using different
Table 1). However, heating the reaction mixture to 80 ꢀC solvents (entries 10–13, Table 1). Generally, solvents with high
resulted in the reduction of styrene to ethylbenzene in 75% boiling point favored the reaction, furnishing the TH of styrene
yield (determined by GC analysis). To our delight, the reaction at 100 ꢀC in high yields, while the use of volatile solvents such as
ꢀ
was observed to be almost completed at 100 C, providing the acetonitrile and cyclohexane led to inferior results. The reaction
desired product in 97% yield (entries 2 and 3, Table 1). In also underwent smoothly under neat conditions without the
contrast, the neutral cobalt complex 2 was inactive to the TH of addition of extra solvents (entry 14, Table 1). Finally, we tested
styrene under the same condition, and no reaction was detected the feasibility of utilizing other alcohols as hydrogen sources. It
without using a cobalt catalyst (entries 4 and 5). To evaluate the is worth noting that 1-phenylethanol (1.1 equivalent) could also
act as a good hydrogen source for TH, stoichiometrically
transferring dihydrogen to styrene. However, ethanol was not
effective at all for the present TH reaction, despite it has found
applications as a suitable hydrogen source in rhodium(I)-cata-
lysed TH of both C]C and C]O bonds.14d
Table 1 Catalyst and condition screening for the transfer hydroge-
nation of styrenea
The nding of excellent catalytic activity of 1 in TH of styrene
under the above mentioned conditions encouraged us to extend
the substrate scope. Thus, a range of olens were tested for the
TH reactions under standard conditions (2 mol% 1,
ꢀ
Entry
Catalyst
Solvent
Yieldb (%)
isopropanol/THF, 100 C, 24 h), and the results are shown in
Table 2. Substituted styrene with either electron-withdrawing or
electron-donating group at the para-position was converted to
the corresponding substituted ethylbenzene compound in
quantitative yield (entries 2 and 3, Table 2). Likewise, the
internal alkene, b-methylstyrene also proceeded well, affording
propylbenzene in 98% yield (entry 4, Table 2). However, the
relatively bulky internal alkenes, trans- and cis-stilbenes showed
only moderate reactivity under the present TH conditions
(entries 5 and 6, Table 2). In these cases, the hydrogenated
product was observed aer 48 h in 35% and 24% yields,
1c
2d
3
4
5
6
7
8
9
10
11
12
13
14e
15f
16g
17h
1
1
1
2
—
3
4
5
6
1
1
1
1
1
1
1
1
THF
THF
THF
THF
THF
THF
THF
THF
0
75
97
0
0
95
45
96
63
98
96
81
77
98
99
0
THF
Toluene
1,4-Dioxane
CH3CN
Cyclohexane
—
THF
THF
respectively. In contrast,
a precious metal ruthenium/N-
heterocyclic carbene catalyst reported previously reduced both
stilbenes in only 7% yield under TH conditions.14b Next, several
aliphatic alkenes were also examined. Terminal alkenes such as
4-phenylbutene and 1-octene are suitable substrates for TH,
affording the corresponding products in excellent yields
(entries 7 and 8, Table 2). On the other hand, the TH with
internal alkenes including cyclooctene and norbornylene
exhibited equally high efficiency, producing both cyclic alkanes
in quantitative yields (entries 9 and 10, Table 2). Furthermore,
functional group-containing alkenes were also used to evaluate
THF
81
a
Conditions: styrene (0.5 mmol), i-PrOH (0.5 mL), cobalt catalyst (2
mol%), solvent (1.5 mL) sealed in a 100 mL Schlenk tube under N2,
b
100 ꢀC, 24 h. Determined by GC with hexamethylbenzene as internal
c
d
e
standard. Reaction run at 25 ꢀC. Reaction run at 80 ꢀC. Reaction
f
run under neat conditions (i-PrOH, 2 mL). 1-Phenylethanol (0.5
g
mmol) was used as hydrogen source. Ethanol (0.5 mL) was used as
hydrogen source. h 1 mol% of 1 was used.
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Table 2 Substrate scope of the transfer hydrogenation of olefinsa
TH did not inuence the conversion signicantly (entry 14,
Table 2), indicating the good stability of the cobalt complex
against moisture. The functional group and water tolerance of
cobalt 1 observed here is remarkable, as most of thus far re-
ported base metal catalysts suited for hydrogenation or TH
reactions showed either limited functional group tolerance or
water stability.10–12
Entry
Substrate
Product
Yieldb (%)
1
2
97
99
It was noticed that from the above results most of the alkene
substrates studied showed excellent reactivity under the present
TH conditions, similar to that observed under dihydrogen
hydrogenation conditions, however, only low TH efficiency was
revealed for the diaryl alkenes such as cis- and trans-stilbenes,
which were not studied yet for the hydrogenation by H2 by using
the same catalyst system.25 This urged us to further explore the
reactivity difference between hydrogenation and TH reactions
for relevant substrates. First, the hydrogenation reactions of
both cis- and trans-stilbene were examined in the presence of 1
under 4 atm H2 gas (Scheme 2) at 25 or 100 ꢀC, the optimal
catalytic conditions for a plethora of C]C, C]O and C]N
bond hydrogenation.19 However, no reduction products were
detected in both cases, indicating these olens are reluctant to
hydrogenation reaction, whereas they are moderately active for
TH (entries 5 and 6, Table 2). Second, the same hydrogenation
conditions were employed to the reduction of a related
substrate, diphenylacetylene containing an internal carbon–
carbon triple bond. Interestingly, the reaction was completed in
the presence of 1 atm H2 and at room temperature. Effective
semihydrogenation was observed (complete conversion to
alkenes) and the resulting products cis- and trans-stilbenes were
detected in a ratio of 34 : 66. Exactly same reactivity was also
observed when using di-p-tolylacetylene as a substrate. It is
worth mentioning that the semihydrogenation reaction here is
signicant although the stereoselectivity was modest. Such
semihydrogenation of internal alkynes have been only carried
out previously by using palladium complexes as catalysts, and
(Z)-selective alkenes were exclusively obtained as the major
products.13d In contrast, attempt to transfer hydrogenate
diphenylacetylene under standard TH conditions gave no
reduced product (Scheme 2). The different reactivity of cobalt
catalyst in hydrogenating or transfer hydrogenating alkynes will
deserve further investigations.25
3
4
99
98
5c
6c
35
24
7
8
99
98
9
99
10
11c
12
99
98
92
13
54
95
14
a
Conditions: substrate (0.5 mmol) and cobalt catalyst 1 (2 mol%, in situ
formed by mixing 2 mol% of 2 and H[BArF4]$(Et2O)2) in i-PrOH/THF (2
mL, 1 : 3 (v/v), 100 ꢀC, 24 h. b Yields of products were determined by GC
c
analysis using an internal standard method. Reaction run for 48 h.
the functional group tolerance of the present cobalt catalyst
under TH conditions (entries 11–13, Table 2). Interestingly, 3-
butenoic acid bearing a carboxy group proceeded well for
cobalt-catalysed TH, giving reduced product 1-butanoic acid in
98% yield. 5-Hexen-2-one bearing isolated C]C and C]O
bonds was also fully reduced to 2-hexanol in high yield.
However, the ester-containing internal alkene (dimethyl 4-
cyclohexene-1,2-dicarboxylate) was found to be more chal-
lenging, providing the selectively hydrogenated product in
moderate yield. Finally, it was found that cobalt catalyst 1 was
water-tolerable under the TH reaction conditions. The addition
Scheme 2 The comparison between hydrogenation and transfer
of H2O (10 mol%) into the standard reaction system for styrene hydrogenation of alkynes and stilbenes.
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In summary, we have reported an efficient cobalt-catalysed
TH of olens by using isopropanol as a hydrogen source. A
variety of substituted terminal and internal alkenes as well as
acid- and ester-functionalized alkenes have been successfully
hydrogenated in moderate to high yields. Good functional
group and water tolerance was disclosed. Different catalytic
reactivity of the cobalt complex was also observed for the
hydrogenation or TH of alkynes and stilbenes. This represents
the rst example of cobalt-catalysed transfer hydrogenation of
olens under homogeneous conditions. Further explorations
leading to more effective and less expensive catalyst systems
involving earth-abundant metals are currently underway.
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