Z-Selective alkyne semi-hydrogenation catalysed by piano-stool N-heterocyclic carbene iron complexes

Article abstract of DOI:10.1039/c8cy00681d

NHC iron(ii) piano-stool complexes catalyse the selective semi-hydrogenation of alkynes to alkenes using silanes as reducing agents. Aromatic terminal alkynes are converted to styrenes without over-reduction to ethylbenzene derivatives. Furthermore, internal aryl alkynes afford cis-alkenes with excellent Z-selectivity.

Full text of DOI:10.1039/c8cy00681d

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Z-selective Alkyne Semi-Hydrogenation Catalysed by Piano-Stool
N-Heterocyclic Carbene Iron Complexes
Chloe Johnsona and Martin Albrecht*,a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
NHC iron(II) piano-stool complexes catalyse the selective semi- the E/Z selectivities observed by this method have so far been
hydrogenation of alkynes to alkenes using silanes as reducing highly substrate and silane dependent, and are therefore not
agents. Aromatic terminal alkynes are converted to styrenes broadly applicable.12 Using a similar approach, Beller and
without over-reduction to ethylbenzene derivatives. Furthermore, coworkers reported in 2012
a
promising transfer
internal aryl alkynes afford cis-alkenes with excellent Z-selectivity. hydrogenation protocol based on formic acid as a hydrogen
source and an iron phopsphine catalyst, which was selective
The selective semi-hydrogenation of alkynes to alkenes is a
valuable transformation in organic chemistry. The alkene
products have utility in a wide range of industrially relevant
processes, for example, polymerisation1 and olefin metathesis2
reactions. Semi-hydrogenation has been challenging since the
alkene intermediate is generally a much better substrate for
hydrogenation than the alkyne. This reactivity difference has
long made it difficult to stop the reaction at the alkene stage,
and instead exhaustive over-reduction to the alkane was
observed with most catalysts. With the discovery of Lindlar’s
catalyst,3 however, alkyne semi-hydrogenation became
feasible. Ever since, a variety of homogeneous catalysts were
developed based on precious metals such as Pd,4 Ru,5 Rh6 and
Ir.7 More recently, semi-hydrogenation catalysts which exploit
cheaper and more Earth-abundant base metals were
introduced, and remarkable catalytic activity was
accomplished with systems containing Co,8 Ni,9 and Cu10.
However, efficient and selective semi-hydrogenation catalysts
based on iron, arguably the most abundant, least toxic, and
cheapest transition metal, have been elusive. Homo- and
heterogeneous Fe(0) systems are potent alkyne reduction
catalysts,11 though they typically display low selectivity for
semi-hydrogenation and generate fully saturated alkane
products.
for terminal alkynes and resulted in the exclusive formation of
alkene products.13
Recently we developed a series of triazolylidene iron(II)
piano-stool complexes which efficiently catalyse the
hydrosilylation of aldehydes and ketones.14 Herein we show
that in the presence of hydrosilanes, these complexes provide
access to catalysts with excellent semi-hydrogenation
selectivity with a variety of alkynes, both in terms of chemo-
and stereoselectivity. The complexes produce exclusively the
alkene product with excellent Z-selectivity when disubstituted
alkynes were utilised as substrates.
Based on the established activity of carbene-containing
piano-stool iron complexes,15 the known14,16 NHC iron
complexes 1–3 (Figure 1) were evaluated for their activity in
the semi-hydrogenation of terminal alkynes. We used
phenylacetylene as model substrate and methyldiethoxysilane
as hydrogen source. Excellent conversions were achieved for
all catalysts after 24 h with a 7 mol% catalyst loading when the
reaction was performed in 1,2-dichloroethane at 60 °C.
Triazolylidene complexes 1a,b containing diaryl wingtip groups
performed almost identically (entries 1,2 Table 1), reaching
appreciable conversions of 62% and 63% respectively after 4 h.
While catalyst 1c showed higher activity after
4
h
(74%
Since many of these catalysts operate under high H2
pressures, a viable strategy to avoid over-reduction involves
non-toxic and easy to handle hydrogen sources. While a
hydrosilylation-desilylation approach satisfies these criteria,
I
Mes
Mes
Mes
N
Fe
R
N
Fe
N
Fe
N
CO
N
CO
CO
N
I
I
N
N
N
Mes
3
1a (R = Mes)
1b (R = Ph)
1c (R = n-Bu)
2
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Figure 1 Triazolylidene and imidazolylidene iron(II) piano-stool semi-hydrogenation
89% conversion after 8 h). Other chlorinated solvents were
less suitable and reactions proceeded slower in 1,2-
dichlorobenzene (DCB, 66% conversion after 8 h, entry 2),
while CHCl3 severely inhibited the reaction (entry 3). In the
latter case,
precatalysts.
Table 1 Catalytic semi-hydrogenation of phenylacetylene using catalysts 1-3.a
[Fe], 7 mol%,
Me(EtO)2SiH,
Table 2 Optimisation of phenylacetylene semi-hydrogenation catalysed by 1b.a
1,2-DCE, 60 ο
C
1b, 7 mol%,
silane,
entry
catalyst
conversion /%b
2 h
42
41
38
27
27
4 h
62
63
74
36
64
24 h
100
100
93
solvent, 60 ο
C
1
2
3
4
5
1a
1b
1c
2
entry
silane
solvent
conversion /%b
2 h
33
23
22
41
16
0
8 h
89
66
23
95
28
3
24 h
100
100
23
1c
2
3
4
5
6
7
Me(EtO)2SiH
Me(EtO)2SiH
Me(EtO)2SiH
Me(EtO)2SiH
PMHS
THF
84
3
100
1,2-DCB
CHCl3
a
General conditions: Phenylacetylene (0.2 mmol), methyldiethoxysilane (0.4
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
100
43
mmol), [Fe] (14 μmol), C6Me6 (20 μmol; NMR internal standard) or hexadecane
b
(0.2 mmol; GC standard), 1,2-dichloroethane (DCE, 1.25 mL), 60 °C. Conversion
determined spectroscopically by 1H NMR or GC analysis.
Et3SiH
3
PhSiH3
34
64
100
conversion, entry 3), full conversion was not achieved after
24h and the reaction stopped conversion, entry 3), full
conversion was not achieved after 24 h and the reaction
a
General conditions: Phenylacetylene (0.2 mmol), hydrosilane (0.4 mmol), [Fe]
(14 μmol), C6Me6 (20 μmol; NMR internal standard) or hexadecane (0.2 mmol; GC
standard), solvent (1.25 mL), 60 °C; DCE dichloroethane, DCB
=
=
stopped at 93% completion. This reactivity suggests that the dichlorobenzene. b Conversion and yield determined spectroscopically by 1H NMR
analysis. c Determined also by GC.
decreased steric demand of the n-butyl substituents is
beneficial for the initial turnover rate, but results in a less
conversion reached only 23%, even after 24 h. The same
robust catalyst that gradually decomposes before all substrate
conversion was already recorded after 2 h and was identical to
has been consumed. Introducing a chelating pyridine moiety
reactions in DCB, suggesting that CHCl3 is deactivating the
(complex
2) decreased the catalytic activity significantly,
catalyst at early stages, presumably due to the acidic character
of chloroform, whereas DCB is not acidic and catalysis
proceeds beyond the initial 2 h. Irrespective of the solvent,
product selectivity towards styrene is exquisite and no over-
reduction was observed in any of these catalytic runs.
reaching only 36% conversion after 4 h (entry 4). The low
activity is presumably due to the necessity for the pyridine to
dissociate in order to allow for substrate binding. The
imidazolylidene complex
3
(entry 5) displayed slower
conversion after
2
h compared to the monodentate
Various silanes were evaluated (see Table 2) in order to
further optimise catalytic activity and also to investigate the
influence of the silane on the product selectivity.
Methyldiethoxysilane afforded the highest activity (entry 4),
while other monohydridosilanes gave significantly poorer
conversion. For example, cheap and easy to handle
polymethylhydrosiloxane (PMHS) decreased the activity by
about a factor of three (entry 5), while Et3SiH was essentially
inactive (entry 6). The usually more active trihydrosilane
PhSiH3 is also a suitable hydrogen source for the semi-
hydrogenation, though the activity is significantly lower when
compared to methyldiethoxysilane (64% vs. 95% conversion
after 8 h, entry 7). Since the choice of silane has a profound
effect on the rate of substrate conversion, it is likely that the
silane is involved in either a critical pre-equilibrium or in the
rate-limiting step. We note that the type of silane did not have
any effect on the product distribution. There was no evidence
of alkyne hydrosilylation or carbon-silicon bond formation, and
styrene was the only detectable product in the 1H NMR
spectrum.
triazolylidene complexes, however, the performance is similar
to that of 1a,b at later stages of the reaction. All complexes 1–
1
produced styrene as the only detectable product by H NMR
3
spectroscopy.‡
Overall, the activities within the mondentate series of
catalysts vary only subtly. We observed previously14 that the
electronic properties (Tolman electronic parameters and
oxidation potentials) of the monodentate triazolylidene and
imidazolylidene complexes are very similar (ν(CO) = 1936±3
cm−1; E1/2 = 0.38±0.04 V), which provides a rationale for the
essentially identical activity of complexes 1 and 3.
In an effort to identify the oxidizing equivalent in this
reduction reaction, crude reaction products were analysed by
29Si NMR spectroscopy. A strong signal at −49.8 ppm was
observed, consistent with the formation of [Me(EtO)2Si]2O
during the catalytic reaction.17
Further optimisation of the semi-hydrogenation and a
preliminary substrate scope was performed with complex 1b
because the synthesis of this complex is easy and cost-
effective. The catalytic reaction was monitored in a range of
different solvents (see Table 2). Reactions carried out in THF
(entry 1) and 1,2-dichloroethane (DCE, entry 4) resulted in very
similar activities, with DCE slightly outperforming THF (95% vs.
A
range of 4-substituted ethynylbenzenes were
hydrogenated with 1b as catalyst under the optimised reaction
conditions. Both electron donating OMe, and Me groups and
2 | J. Name., 2012, 00, 1-3
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electron withdrawing substituents (Br, CF3, CN) are compatible of internal alkynes was notably slower than that of
with the semi-hydrogenation protocol (Table 3), and neither phenylacetylene. Diphenylacetylene was converted the
dehalogenation nor nitrile reduction was observed. The fastest, reaching 88% conversion after 24
h (entry 1).
electronic influence of the para-substituent does not have a Substituting one aromatic moiety for an n-butyl group resulted
profound effect on the reaction rate of alkyne reduction, in comparable initial activity (14% vs. 19% after 2 h), but after
(initial turnover frequency about 4 h–1). After 8 h, the MeO- longer reaction times the alkyl-substituted substrate was
substituted ethynylbenzene (entry 3) reaches slightly lower converted considerably less and reached only 63% after 24 h
conversion
(entry 2). A similar reaction profile was noted when the
substrate contained a trimethylsilyl rather than an n-
butyl group (entry 3).
Table 3 Semi-hydrogenation of terminal alkynes catalysed by 1b.a
1b, 7 mol%,
Table 4 Semi-hydrogenation of internal alkynes catalysed by 1b.a
Me(EtO)2SiH,
1b, 7 mol%,
R
R
1,2-DCE, 60 ο
C
R’
Me(EtO)2SiH,
R’
+
entry
R
conversion /%b
R
R’
R
1,2-DCE, 60 ο
C
R
2 h
41
39
39
42
39
38
25
8 h
95
91
83
94
98
91
57
24 h
100
100
93
1
2
3
4
5
6
7
Ph
entry
R
R’
conversion /%b
4-BrPh
4-MeOPh
4-CH3Ph
4-CF3Ph
4-NCPh
n-Hex
2h
8h
75
31
51
38
35
97
89
24h
88
Z/E ratio
96:4
1
2
Ph
Ph
Ph
n-Bu
SiMe3
Me
19
14
17
21
13
87
77
26
14
-
99
63
>99:1
95:5
100
100c
76
3
Ph
61
4
n-Bu
65
n.d.
5
n-Bu
n-Bu
n-Pr
Et
41
n.d.
98:2c
>99:1c,d
79:21
n.d.
a
General conditions: Alkyne (0.2 mmol), methyldiethoxysilane (0.4 mmol),
6
CH2OH
(CH2)2OH
CH2OBn
CH2NHBn
CH2Br
100
89
complex 1b (14 μmol), C6Me6 (20 μmol; NMR internal standard) or hexadecane
7
b
(0.2 mmol; GC standard), 1,2-DCE (1.25 mL), 60 °C. Conversion determined
spectroscopically by 1H NMR or GC analysis.
8
n-Pr
Et
59(88:12)
89
21e
25e
c
Over-reduction to 4-
9
20
19
ethylbenzonitrile after 24 h observed (28%).
10
Et
n.d.
a
General conditions: Alkyne (0.2 mmol), methyldiethoxysilane (0.4 mmol),
(83%) than phenylacetylenes with other substituents (91–
98%), and does not reach full conversion after 24 h. Slight
deactivation has tentatively been attributed to ether
coordination to the iron centre (cf lower conversion after 8 h
in THF vs DCE). Again, the semi-hydrogenated functionalised
styrene was the only detectable product for all
phenylacetylene derivatives screened here. An exception is 4-
ethynylbenzonitrile, which is partially over-reduced to 4-
cyanoethylbenzene after prolonged reaction times.6e,10c,18
After 24 h, the alkene/alkane distribution of products was
about 3:1. If the catalytic reaction was stopped at 8 h (91%
conversion), no alkane compound is present and 4-
cyanostyrene was the exclusive product. Over-reduction may
be promoted by the largely positive Hammett parameter of
the 4-CN substituent (σp = +0.66),19 which induces a higher
polarisation of the vinyl moiety. The enhanced propensity for
further reduction of the cyano-styrene product therefore
points to a nucleophilic hydride attack as the initial step of
hydrogenation. In comparison to phenylacetylenes, 1-octyne
was reacted much slower and after 8 h, conversion was only
57% (Table 3, entry 7) and imcomplete even 24 h In addition,
the aliphatic substrate was not converted selectivity to 1-
octene. While the terminal olefin was the major product
complex 1b (14 μmol), C6Me6 (20 μmol; NMR internal standard) or hexadecane
b
(0.2 mmol; GC standard), 1,2-DCE (1.25 mL), 60 °C. Conversion and Z/E ratio
determined spectroscopically by GC (entries 1,5) or 1H NMR spectroscopy (all
c
e
other entries). Product distribution determined after addition of Me4NF.
Substantial amounts of side products formed.
Interestingly, the –SiMe3 group is preserved during the
reduction, a result that disfavours a mechanistic pathway
involving first alkyne hydrosilylation and subsequent
desilylation,. Likewise, the catalytic activity of 1b towards
dialkylated acetylenes is lower than phenylacetylene semi-
hydrogenation. For example, 2-heptyne is reacting at similar
rates to 1-phenyl-1-hexyne (entry 4) reaching 65% conversions
after 24 h, while 5-decyne, was converted only to 41% after 24
h (entry 5). However, the catalytic activity during the first 8 h
was essentially identical to that measured for phenyl-hexyne
(cf. entry 2), suggesting some catalyst deactivation by the 5-
decene product.
Catalytic semi-hydrogenation is accelerated by hydroxyl-
functionalized substrates. For example, 2-hexyn-1-ol is semi-
hydrogenated in 87% within 2 h (entry 6; cf ca 40% conversion
of phenylacetylenes in the same time span, Table 3). Similarly,
the regioisomer 3-hexyn-1-ol is converted to 77% (entry 7).
Presumably the hydroxyl group coordinates to the iron centre,
which facilitates Si–H bond cleaveage across the Fe–O bond. In
detected by GC analysis,
a considerable number of
unidentified side-products appeared at higher retention times.
However, there was no evidence of over-reduction to octane.
The optimised conditions determined for terminal alkynes
were applied for the catalytic semi-hydrogenation of internal
alkynes using catalyst 1b (Table 4). In general, the conversion
agreement with such
a model, the semi-hydrogenation
product of hydroxyl-containing substrates features a silylated
alcohol group, which was deprotected by treatment with
Me4NF in CH3CN. When a benzyl group was installed as a
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(b) D. Teschner, J. Borsodi, A. Wootsch, Z. Révay, M.
protecting group onto 2-hexyn-1-ol, the rate of conversion of
the resulting alkyne was significantly decreased compared to
the deprotected alcohol (entry 8). Amine and halide groups are
not compatible with the iron catalyst and lead to poor
conversions after 24 h and no detectable quantities of olefin
product (entries 9,10).
Hävecker, A. Knop-Gericke, S. D. Jackson and R. Schlögl,
Science, 2008, 320, 86-89. (c) T. Yusuke, H. Norifumi, H.
Takayoshi, S. Shogo, M. Takato, M. Tomoo, J. Koichiro and K.
Kiyotomi, Chem. Lett., 2011, 40, 405-407; (d) M. W. Tew, H.
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Brown, P. T. Bishop and S. C. E. Tsang, Nat. Commun., 2014,
,
Z-selectivity of the iron NHC catalyst is very high for phenyl
substituted internal acetylenes with >95% cis-olefin formation
(entries 1-3), and exclusive formation of the Z-isomer in the
reduction of 1-phenyl-1-hexyne. The selectivity is lower for the
benzylether-containing substrate (entry 8) with a 4:1 bias
towards the cis-olefin. The initial selectivity was higher (7:1
after 4 h and at 59% conversion), suggesting a gradual drift in
catalyst conformation that may be product-induced, and
concomitant loss of selectivity. In contrast, the same substrate
with a free hydroxyl group affords the cis-product in excellent
Z/E ratio (98:2, entry 6) and the isomeric 3-hexyn-1-ol is
reduced exclusively to the cis-alkene.20
5
, 5787.
4
5
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In summary, we have employed 1,2,3-triazolylidene and
IMes iron piano-stool complexes for the catalytic semi-
hydrogenation of alkynes using silanes as reducing agents.
Monodentate complexes are more active than the NHC-
pyridine chelate. In contrast to catalytic hydrogenation using
platinum group transitions metals, the reduction is selective to
alkynes, and alkenes are typically not converted, thus
preventing any over-reduction to saturated alkanes. This
distinct reactivity demonstrates the complementarity of iron
as a catalyst compared to precious metals. The reduction of
internal alkynes revealed excellent Z-selectivity using this
protocol, offering an attractive and cheap methodology to
access this synthetically valuable functional group. Further
work is on-going to reduce the catalyst loading and to
efficiently convert aliphatic alkynes by enhancing the reactivity
of the iron centre through appropriate ligand tuning.
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9
We thank the European Research Council (ERC CoG 615653),
the Swiss National Science Foundation (200021_162868 and
206021_170755), and the Irish Research Council (fellowship to
C.J.) for financial support.
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Conflicts of interest
There are no conflicts to declare.
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Notes and references
‡ Upon extended reaction times (>10 h), the spectroscopic yields
tend to decrease slowly, which has been attributed to the
inherent instability of the styrene products towards
polymerization in the absence of a stabilizer (see also ESI, Table
S1).
1623.
13 G. Wienhofer, F. A. Westerhaus, R. V. Jagadeesh, K. Junge, H.
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20 Due to overlap in the 1H NMR spectrum, cis/trans ratios
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Piano-stool NHC iron complexes catalyze the selective semi-
hydrogenation of alkynes without any over-reduction and with
high selectivity towards Z-alkynes when starting from
disubstituted alkynes.
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