Efficient Z-Selective Semihydrogenation of Internal Alkynes Catalyzed by Cationic Iron(II) Hydride Complexes

Article abstract of DOI:10.1021/jacs.9b09907

The bench-stable cationic bis(σ-B-H) aminoborane complex [Fe(PNP^NMe-iPr)(H)(η²-H₂B = NMe₂)]⁺ (2) efficiently catalyzes the semihydrogenation of internal alkynes, 1,3-diynes and 1,3-enynes. Moreover, selective incorporation of deuterium was achieved in the case of 1,3-diynes and 1,3-enynes. The catalytic reaction takes place under mild conditions (25 °C, 4-5 bar H₂ or D₂) in 1 h, and alkenes were obtained with high Z-selectivity for a broad scope of substrates. Mechanistic insight into the catalytic reaction, explaining also the stereo- and chemoselectivity, is provided by means of DFT calculations. Intermediates featuring a bisdihydrogen moiety [Fe(PNP^NMe-iPr)(η²-H₂)₂]⁺ are found to play a key role. Experimental support for such species was unequivocally provided by the fact that [Fe(PNP^NMe-iPr)(H)(η²-H₂)₂]⁺ (3) exhibited the same catalytic activity as 2. The novel cationic bisdihydrogen complex 3 was obtained by protonolysis of [Fe(PNP^NMe-iPr)(H)(η²-AlH₄)]₂ (1) with an excess of nonafluoro-tert-butyl alcohol.

Full text of DOI:10.1021/jacs.9b09907

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Article
Efficient Z-Selective Semi-Hydrogenation of Internal
Alkynes Catalyzed by Cationic Iron(II) Hydride Complexes
Nikolaus Gorgas, Julian Brünig, Berthold Stöger, Stefan Vanicek, Mats Tilset, Luis F. Veiros, and Karl Kirchner
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b09907 • Publication Date (Web): 07 Oct 2019
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Efficient Z-Selective Semi-Hydrogenation of Internal Alkynes
Catalyzed by Cationic Iron(II) Hydride Complexes
Nikolaus Gorgas,† Julian Brünig,† Berthold Stöger,‡ Stefan Vanicek,§ Mats Tilset,§ Luis F. Veiros,# and Karl
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Kirchner*,†
†Institute of Applied Synthetic Chemistry and ‡X-Ray Center, Vienna University of Technology, Getreidemarkt 9, A-1060 Vienna,
AUSTRIA; §Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, NORWAY; #Centro de Química
Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais No. 1, 1049-001 Lisboa, PORTUGAL
ABSTRACT: The bench-stable cationic bis(σ-B-H) aminoborane complex [Fe(PNPNMe-iPr)(H)(η2-H2B=NMe2)]+ (2)
efficiently catalyzes the semi-hydrogenation of internal alkynes, 1,3-diynes and 1,3-enynes. Moreover, selective
incorporation of deuterium was achieved in the case of 1,3-diynes and 1,3-enynes. The catalytic reaction takes place
under mild conditions (25oC, 4-5 bar H2 or D2) in 1 h and alkenes were obtained with high Z-selectivity for a broad
scope of substrates. Mechanistic insight into the catalytic reaction, explaining also the stereo- and chemoselectivity, is
provided by means of DFT calculations. Intermediates featuring a bis-dihydrogen moiety [Fe(PNPNMe-iPr)(η2-H2)2]+ are
found to play a key role. Experimental support for such species was unequivocally provided by the fact that [Fe(PNPNMe
-
iPr)(H)(η2-H2)2]+ (3) exhibited the same catalytic activity than 2. The novel cationic bis-dihydrogen complex 3 was
obtained by protonolysis of [Fe(PNPNMe-iPr)(H)(η2-AlH4)]2 (1) with an excess of nonafluoro-tert-butyl alcohol.
and high abundance, it is their intrinsic properties that
may provide new opportunities paving the way to
INTRODUCTION
unprecedented reactivities and selectivities in catalytic
transformations.10 Iron may be considered as
a
The semi-hydrogenation of alkynes to alkenes is an
important transformation for the industrial manufacture
of bulk and fine chemicals.1 Since the reduction of C≡C
triple bonds to alkenes may potentially lead to the
formation of (E)- or (Z)-alkenes as well as saturated
hydrocarbons, it remains a challenging task to control
the chemo- and stereoselectivity of this reaction.2 For
example, (Z)-olefins are traditionally obtained by
heterogenous hydrogenation with the Lindlar catalyst
being the most prominent example.3 However, these
systems require a careful control of the reaction
conditions in order to avoid isomerization or
overreduction of the products.
particularly promising candidate in this respect.11 For
example, much progress could be achieved in the field of
iron catalyzed hydrogenations of olefins.12 Concerning
the selective reduction of alkynes, some iron complexes
could successfully be applied that operate via
hydrofunctionalization
or
transfer
hydrogenation
procedures.13 However, only two examples are currently
known that can promote this transformation using
hydrogen gas as the reductant (Scheme 1).8a,14
Scheme 1. Iron Catalysts for the Semi-Hydrogenation
of Alkynes
a)
+
P'
Although significant progress could be achieved in
the field of heterogeneous catalysis and homogeneous
transfer hydrogenations, the number of molecular
defined catalysts that operate under an atmosphere of
H2 is surprisingly low.4-6 Concerning the stereoselectivity
of the reaction, recent examples were shown to deliver
(E)-alkenes which, except in some cases,7 are formed due
to secondary isomerization processes.8 Consequently, the
development of new and selective homogeneous semi-
hydrogenation catalysts that are able to produce (Z)-
alkenes without isomerization or overreduction would
still be of great advantage.9
H
H
Bianchini 1989
H
P
H2 (1 bar)
r.t.
Fe
P'
H
H
P' = PPh2
R
P'
H
R
H
restricted to terminal alkynes
Me
N
b)
H
H
R
P'
H
H2 (4-10 bar)
90°C
N
R
Milstein 2013
Fe
N
R
H
B
P'
P' = PPh2
R
H
Me
H
(E)-selective due to isomerization
H
H
this work:
+
H
+
Me
H
Me
H
N
P'
R
N
P'
H
H
H
H2 (5 bar)
r.t.
N
Fe
H
H
B
N
Fe
or
R
Within this context, homogeneous catalysts based on
first row transition metals attracted particular interest.
Apart from obvious advantages such as their low price
N
P'
R
R
N P'
Me
Me
N
H H
Me
Me
high (Z)-selectivity / no isomerization
2
P' = PiPr2
3
P' = PiPr2
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Page 2 of 9
In 1989, Bianchini et al. discovered that the non-
double bond of the enyne substrates remained
unaffected.
This circumstance was further examined by
1
2
3
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8
classical polyhydride [Fe(PP3)(H)(2-H2)]+ is capable of
catalyzing the semi-hydrogenation of terminal alkynes.14
More recently, the group of Milstein reported on a novel
acridine based pincer type complex that bears an imino
borohydride co-ligand.8a This complex was found to
reduce internal alkynes selectively to the respective (E)-
olefins. The reaction requires several hours and rather
high reaction temperatures (90 °C) to achieve high yields
and selectivities. Additional studies revealed that the
hydrogenation initially affords (Z)-alkenes which,
however, are isomerized to the respective (E)-olefins by
the same catalyst.
conducting the hydrogenation in presence of D2
(Scheme 2). The reaction of 1,3-enyne A16b with D2
catalyzed by 2 resulted in complete and selective
deuterium incorporation at the former alkyne carbons,
whereas an isotope scrambling into the existing olefin C–
H positions was not observed. On the other hand, the
respective 1,3-diyne A16a gave the fully deuterated
butadiene product under the same reaction conditions.
Thus, the novel iron hydride complex represents an
interesting catalytic system that allows for the selective
hydrogen isotope labelling of butadiene derivatives.
Based on the experimental observations, it is likely
that the reaction proceeds via a classical insertion
mechanism. Due to the labile nature of the dihydrogen
and bis(σ-B-H) aminoborane ligands, the pre-catalyst
gets activated by the addition of the substrate in the
presence of dihydrogen. In contrast, dissociation of the
aminoborane ligand in 2 under an atmosphere of
dihydrogen to form 3 was not be observed.15
The hydrogenation of A1 was conducted at
different hydrogen pressures as well as substrate
concentrations and the initial turnover frequencies were
determined after a reaction time of five minutes. A
significant increase of the initial TOFs could be observed
at higher hydrogen pressures (see supporting
information, Table S1). At 20 bar H2, after 5 min the
product was obtained in >99% yield corresponding to a
TOF of 2400 h-1. In contrast, essentially no acceleration
took place when the substrate concentration
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We describe here the application of the well-defined
bench-stable cationic aminoborane complex [Fe(PNPNMe
-
iPr)(H)(η2-H2B=NMe2)]+ (2) described recently15 as highly
efficient pre-catalysts for the semi-hydrogenation of
internal alkynes, 1,3-diynes and 1,3-enynes with
molecular hydrogen under mild conditions. We take
advantage of the fact that the aminoborane ligand,
which is coordinated to the metal center via two weak σ-
B-H bonds in 2-fashion, is substitutionally labile and
upon dissociation readily provides two vacant
coordination sites to bind dihydrogen and alkynes.
RESULTS AND DISCUSSION
The aminoborane complex [Fe(PNPNMe-iPr)(H)(η2-
H2B=NMe2)]+ (2) was tested as pre-catalyst for the
hydrogenation of a variety of different alkyne substrates
in order to examine the general applicability and
functional group tolerance of this novel system. Apart
from 1-phenylpropyne, also dialkyl and diphenyl
substrates could be reduced to the respective (Z)-alkenes
(Table 1, A1-A3) without isomerization of the products.
Neither an increase of the catalyst loading nor higher
pressures led to a further reduction of the alkene.
Terminal alkynes, however, are further hydrogenated to
yield the saturated alkanes. Overreduction could be
prevented by the introduction of trimethylsilyl (TMS)
moieties as protecting groups which were tolerated
throughout without cleavage of the C–Si bond. Even
those substrates were quantitatively reduced affording
the respective alkenes with excellent (Z)-selectivity
(except for the TMS-protected phenylacetylene A4 for
which minor amounts of the trans-isomer were found in
the isolated product). The reaction proceeds well also in
case of diynes (A13, A14) which could successfully by
converted to the corresponding (Z,Z)-dienes.
Table 1. Semi-hydrogenation of Alkynes Catalyzed by
2.
Encouraged by these results, we extended the
substrate scope to 1,3-diynes (A16a) as well as 1,3-
enynes (A16b-A20). In any case, the respective (Z,Z)-
butadienes could be obtained with excellent
stereoselectivity (Table 2). Functional groups such as
esters or amine groups are tolerated whereas the C=C
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Journal of the American Chemical Society
H
2 (5 bar)
Table 2. Hydrogenation of 1,3-Diynes and 1,3-Enynes
Catalyzed by Complex 2.
H
R
H
R
2
(1.0 mol%)
1
R
R
CH2Cl2, r.t.
1h
2
R
3
4
5
6
H
H
R
H
2 (5 bar)
H
H
R
R
2
(1.0 mol%)
or
H
H
or
H
H
CH2Cl2, r.t.
1h
R
R
H
H
R
R
TMS
7
A1a
A2a
A3a
A4a
8
9
>99% (86%)
Z/E > 99
>99% (83%)
Z/E > 99
>99% (82%)
Z/E > 99
>99% (89%)
Z/E = 89/11
10
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A16aa
A16ba
A17b
F3C
>99% (91%)
Z,Z/E,E > 99
>99% (93%)
Z,Z/E,E > 99
>99% (89%)
Z,Z/E,E > 99
TMS
TMS
TMS
TMS
F
tBu
A5a
A6a
A7a
A8a
H2N
>99% (83%)
Z/E > 99
>99% (91%)
Z/E > 99
>99% (87%)
Z/E > 99
>99% (85%)
Z/E > 99
O
tBu
MeO
O
O
tBu
OMe
TMS
TMS
TMS
S
HO
TMS
O
NH2
A18b
A19b
A20a
A9a
A10a
A11b
A12c
>99% (89%)
Z,Z/E,E > 99
>99% (84%)
Z,Z/E,E > 99
>99% (87%)
Z,Z/E,E > 99
>99% (94%)
Z/E > 99
>99% (92%)
Z/E > 99
>99% (90%)
Z/E > 99
>99% (87%)
Z/E > 99
Conditions: (a) alkyne (1.0 mmol), 2 (1.0 mol%), CH2Cl2 (1 mL), r.t., 1
h, conversion and selectivity determined by 1H NMR, isolated yields
given in parenthesis. (b) 10 bar H2.
TMS
TMS
TMS
A13a
A14a
A15a
>99% (84%)
Z,Z/E,E > 99
>99% (91%)
Z,Z/E,E > 99
>99% (90%)
Z/E > 99
Scheme 2. Selective Deuterium Incorporation
through Hydrogenation of 1,3-Diynes and 1,3-Enynes
by 2 with D2
H
H
H
H
H
H
D
2 (4 bar)
H
H
H
D
D
2
(2.0 mol%)
overreduction in case of terminal alkynes
CH2Cl2, r.t.
1h
D
D
Conditions: (a) alkyne (1.0 mmol), 2 (1.0 mol%), CH2Cl2 (1 mL), 25 oC,
1 h, conversion and selectivity determined by 1H NMR, isolated
yields given in parenthesis. (b) 3 (2.0 mol%), 2h reaction time. (c) 7
(1.5 mol%), MeOH /CH2Cl2 (1:9, 1 mL), 1h.
A16
-d4
H
H
D
2 (4 bar)
H
H
2
(2.0 mol%)
CH2Cl2, r.t.
1h
D
D
A17
-d2
was raised. These results indicate that the rate
determining step of the mechanism should be loss of the
aminoborane ligand and formation of the active species
being in line with the a very small energy barrier (11
kcal/mol) calculated for the catalytic reaction (vide infra).
A series of DFT calculations were carried out in
order to gain more detailed insight into the catalytic
A more detailed picture is provided by the free
energy profile in Scheme 4 employing 1-phenylpropyne
as the substrate. Insertion of the alkyne into the iron
hydride bond proceeds easily with an activation barrier
of 1 kcal/mol in an exergonic step (ΔG = –11 kcal/mol).
This occurs with a H-shift between two adjacent
coordination positions and results in intermediate 1-B
with trans dihydrogen and vinyl ligands. Coordination of
a H2 molecule to the resulting iron vinyl complex affords
intermediate 1-D in which there are two dihydrogen
ligands parallel to the P-Fe-P axis. 1-D formation
overcomes a barrier of only 3 kcal/mol and that step is
exergonic with ΔG = –5 kcal/mol. From 1-D, a second H-
transfer with concomitant re-orientation of the H2 ligand
produces 1-E, where the recently formed olefin is loosely
coordinated as a C–H σ-complex. Requiring 11 kcal/mol,
this step represents the highest barrier in the catalytic
cycle. The overall free energy balance of the cycle is
favorable with ΔG = –22 kcal/mol and closing the cycle
with liberation of the olefin and addition of a new alkyne
molecule regenerates the initial species (1-A) in a slightly
exergonic process (ΔG = –3 kcal/mol).
cycle as well as the origin of the observed selectivity.16
A
simplified catalytic cycle for the semi-hydrogenation of
internal alkynes is depicted in Scheme 3. The cycle starts
with an alkyne dihydrogen complex of the type
[Fe(PNPNMe-iPr)(H)(η2-H2)(η2-MeCCPh)]+ as the active
species. The η2-coordinated alkyne inserts into the iron
hydride bond resulting in a dihydrogen vinyl species (I).
After coordination of a second equivalent of H2 (II), this
intermediate split the H-H bond under release of the
alkene product and recovery of the initial hydride
dihydrogen complex (III).
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Regarding the chemoselectivity of the reaction, we
Page 4 of 9
transfer between the two adjacent coordination sites
and, thus, both the H2 as well as the alkyl ligands occupy
two apical positions trans to each other, in 1-H. This step
corresponds to a formal olefin insertion in a Fe–H bond
and parallels the first step of the entire path (alkyne
insertion, over 1-TSAB). This is an endergonic process
with ΔG = 6 kcal/mol and the associated barrier (13
kcal/mol) is 2 kcal/mol higher than the one found for
1
2
3
4
5
6
7
8
also considered a second hydrogenation step to the
respective alkane in our calculations (right side of the
profile in Scheme 4). Here, the loosely bonded olefin in
intermediate 1-E rearranges to η2-coordinated (in 1-G).
This process occurs in two steps. First there is H-
exchange, bringing the H2 ligand back to its original
position, i.e., opposite to the Npy-atom, in intermediate
1-F. Then, a reorientation of the olefin brings it to the η2-
coordination mode present in intermediate 1-G. These
steps have negligible barriers (≤ 1 kcal/mol) and 1-G is 7
alkyne
insertion,
indicating
that
the
second
9
hydrogenation is less favorable than the first one, in
agreement with the experimental results. However, the
small difference between the calculated barriers may
suggest the possibility of competitive processes. After H2
coordination (from 1-H to 1-I) a final H-transfer from the
dihydrogen ligand to the terminal alkyl C-atom produces
the final intermediate, 1-J, with an alkane bonded as a
C–H σ-complex. This is a facile process with a barrier of 6
kcal/mol and, also, clearly favorable with ΔG = –27
kcal/mol.
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Scheme 3. Simplified Catalytic Cycle for the Semi-
hydrogenation of Internal Alkynes Catalyzed by 2.
H
+
Me
N
PR2
N
Fe
H
H
B
N PR2
Me
Me
N
2
Me
The hydrogenation of phenylacetylene was also
considered by means of DFT calculations and the energy
profile obtained for the corresponding reaction is
depicted in Scheme 5. The mechanism is entirely
equivalent to the one discussed above for the
hydrogenation of 1-phenylpropyne. Thus, it also
comprises two stepwise H-atom additions to CC
unsaturated bonds. The first can be viewed as an
acetylene insertion into a hydride bond and results in a
vinyl complex (2-B). Then, there is H2 coordination
followed by another H-transfer producing the olefin
(styrene, in this case) and regenerating an hydride ligand
(intermediate 2-E). A repetition of this sequence leads to
the hydrogenation of styrene and the formation of the
alkane (ethylbenzene) in the final intermediate, 2-K.
The most important difference between the two
paths is the relative value of the highest barrier for each
H2 addition. In the case of the internal alkyne (1-
phenylpropyne, Scheme 4) the barrier for the second
addition is the higher one, justifying the observed semi-
hydrogenation of the substrate. However, in the case of
the terminal alkyne (phenylacetylene, Scheme 5) the
opposite occurs. That is,
- H2B=NMe2 + PhC CMe + H2
H
H
H
+
Me
N
PR2
Me
H
N
Fe
H
N PR2
Me
Me
Me
I
III
H
H
+
+
Me
Me
H
H
N
PR2
H
N
PR2
H
N
Fe
N
Fe
H
N PR2
Me
N PR2
H
Me
Me
Me
II
H
H
kcal/mol more stable than 1-E, reflecting the additional
stability associated with stronger coordination of the
olefin. From 1-G, there is H-transfer to the inner C-atom
of the C=C double bond with formation of an alkyl
complex (1-H). The process occurs with concomitant H-
Scheme 4. Free Energy Profile for the Hydrogenation of 1-Phenylpropyne. Free Energies (kcal/mol) refer to
Intermediate 1-A.
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+
Me
H
H
N
PR2
+
H
H
Me
1
N
Fe
H
N
PR2
H
+
N
PR2
2
3
4
5
6
7
8
9
Me
N
Fe
H
H
Me
N
PR2
Me
H
N
PR2
Me
H
N
Fe
Me
H
H
1
N
PR2
Me
0
H
Me
1-TSAB
+
H
H
+
H
+
1-A
Me
Me
H
H
H
N
PR2
N
PR2
-5
1-TSDE
Me
H
N
Fe
N
PR2
N
Fe
H
+ H2
-8
+
H
H
H
Me
N
Fe
N
PR2
H
N
PR2
H
H
N
PR2
Me
-11
H
H
Me
-11
1-C
1-TSCD
N
PR2
H
11
Me
H
N
Fe
Me
Me
Me
1-B
+
+
H
H
Me
N
PR2
H
H
-16
-16
1-D
H
N
PR2
Me
+
-17
1-TSIJ
Me
H
Me
H
H
H
N
Fe
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11
12
13
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N
PR2
1-TSGH
Me
N
PR2
H
H
+ H2
H
N
PR2
N
Fe
N
Fe
+
-22
-22
H
H
H
Me
H
Me
H
-23
-23
1-I
N
PR2
Me
13
N
PR2
Me
N
PR2
1-TSEF
1-E
H
Me
Me
-28
1-TSFG
+
Me
H
1-H
H
N
Fe
H
+
H
H
Me
H
+
N
PR2
Me
Me
+
Me
-29
-29
1-G
N
PR2
H
H
Me
N
PR2
N
PR2
H
N
Fe
H
1-F
H
H
N
Fe
N
Fe
H
N
PR2
H
H
H
+
Me
Me
N
PR2
H
Me
N
PR2
Me
H
H
H
Me
N
PR2
N
PR2
Me
H
H
H
Me
Me
N
Fe
N
Fe
H
H
H
H
Me
N
PR2
N PR2
G = -3 kcal/mol
H
Me
Me
H
Me
H
Me
+
H
Fe
H
Me
H
N
PR2
H
Me
-50
2-J
N
H
H
N
PR2
Me
H
H
Me
Scheme 5. Free Energy Profile for the Hydrogenation of Phenylacetylene. Free Energies (kcal/mol) refer to
Intermediate 2-A.
+
Me
H
H
N
PR2
N
Fe
H
N
PR2
Me
H
H
+
+
H
Me
H
Me
15
2-TSAB
H
N
PR2
N
PR2
H
N
Fe
N
Fe
H
H
N
PR2
H
N
PR2
H
H
H
Me
Me
15
H
H
+
H
+
Me
H
+
H
H
H
Me
Me
1
+ H2
0
N
PR2
N
PR2
N
PR2
H
N
Fe
H
N
Fe
2-B
N
Fe
-4
H
H
-3
2-TSCD
2-A
H
+
Me
-5
2-TSDE
N
PR2
H
H
N
PR2
+
N
PR2
H
N
PR2
2-C
Me
H
H
H
Me
H
H
H
H
H
Me
H
+
Me
Me
PR2
N
Fe
N
H
H
H
N
PR2
H
N
Fe
H
H
N
PR2
H
H
N
Fe
Me
-14
2-D
-14
H
-14
2-E
-15
-16
2-G
-15
2-TSGH
N
PR2
+
Me
N
H
H
N
PR2
Me
H
H
2-TSEF
+
PR2
2-TSFG
Me
H
H
-20
+ H2
H
H
N
Fe
-22
2-H
Me
H
H
12
H
H
+
+
Me
2-TSIJ
Me
N
PR2
N
PR2
H
-25
2-I
Me
N
PR2
Me
N
PR2
H
N
Fe
H
H
-27
H
Me
H
H
N
Fe
H
+
N
Fe
H
N
PR2
N
PR2
H
2-F
H
N
PR2
H
Me
+
N
PR2
H
H
H
H
H
N
Fe
H
Me
+
Me
Me
N
PR2
-34
H
H
H
H
H
H
H
N
PR2
H
N
PR2
N
Fe
H
Me
H
2-J
H
N
Fe
N
PR2
H
H
H
H
H
+
H
Me
Me
N
N
PR2
H
G = -18 kcal/mol
Me
H
N
PR2
H
Fe
H
H
N
PR2
H
Me
H
H
H
H
H
The observed chemoselectivity can also be related
to the stability of the η2-olefin complexes in each case
(1-G and 2-F). In fact, those are the initial species in the
second H2 addition, from olefin to alkane. Considering
the competition between H2 and the olefin for the six
the barrier for olefin hydrogenation (12 kcal/mol, 2-TSFH)
is lower than the one calculated for the hydrogenation of
the alkyne (15 kcal/mol, 2-TSAB). Both processes
correspond to the first H-transfer, from the H2 ligand to
the corresponding CC unsaturation. The results above
indicate that in the case of phenylacetylene the reaction
is expected to go all the way until the saturated product.
coordination
position
(Figure
S1,
supporting
information), it becomes clear that the olefin complex is
more stable in the case of the terminal olefin (styrene, in
2-F), while the opposite happens in the case of the
internal olefin (1-phenylpropene, in 1-G). Therefore, the
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Journal of the American Chemical Society
Page 6 of 9
existence of the initial species for the second
activation energy of just 5.0 kcal/mol being in line with
the experimentally observed fluxional behaviour (Scheme
7).
1
2
3
4
5
6
7
8
hydrogenation process is favorable in the case of
substrates with a terminal C=C double bond, contrarily
to what occurs for internal ones.
Complex 3 was also applied as pre-catalyst for the
hydrogenation of alkynes. Experiments were conducted
In order to obtain support for the mechanistic
studies from an experimental point of view, in particular
the existence of bis-dihydrogen intermediates, we
prepared the novel cationic bis-dihydrogen complex
[Fe(PNPNMe-iPr)(H)(η2-H2)2]+ (3). This complex was readily
obtained by reacting [Fe(PNPNMe-iPr)(H)(η2-AlH4)]2 (1)15
with an excess of nonafluoro-tert-butyl alcohol (Scheme
6) in THF at room temperature. The coordinated [AlH4]-
anion is protonated by the acidic alcohol thereby
liberating H2 with concomitant formation of a mixture of
several poorly coordinating counterions of the types
[Al(OC(CF3)3)4-nHn]- as detected by 19F{1H} NMR
spectroscopy and ESI-MS.16
o
in C6D6 under an H2 pressure of 5 bar at 25 C using 1-
phenylpropyne as test substrate. By employing 1 mol%
of in situ prepared 3 the alkyne could be quantitatively
reduced to the corresponding alkyne within 30 min. The
product was formed with >99% Z-selectivity and no
hydrogenation to the respective alkane could be
observed even when the reaction time was extended to
several hours. Accordingly, complexes 2 and 3 are
obviously synthons for the active catalyst which reacts
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
with alkynes in the presence of H2 to form [Fe(PNPNMe
-
iPr)(H)(η2-H2)(η2-RCCR’)]+ thereby initiating the catalytic
cycle as shown in Schemes 3-5. It has to be noted that
the neutral non-classical iron(II) polyhydride complex
[Fe(PNP)(H)2(η2-H2)] did not catalyzed the hydrogenation
of alkynes, but the dimerization of alkynes to give 1,3-
eneynes with high Z-selectivity. 20
Scheme 6. Preparation of a Cationic Non-Classical
Fe(II) Polyhydride Complex via Protonolysis of 1a
Scheme 7. Free Energy Profile (kcal/mol) Calculated
for the Interconversion of cis- and trans-Isomers of 3.
1
Inset: H NMR Spectrum of 3 (Hydride Region, 250
MHz, THF-d8, 20oC).
+
H
H
Me
N
PR2
H
+
H
N
Fe
H
H
Me
Me
N
PR2
N
PR2
N
PR2
H
THF, r.t.
H
H
Me
N
Fe
H
N
Fe
CF3
5.0
TSBC
F3C
F3C
N PR2
N PR2
H
Al
H
H
H
Me
O
Me
H
1
H
(10 equiv.)
3
2.0
2
-14.6 -14.7 -14.8
+
H
H
Me
1.0
1.0
N
PR2
a
Selected bond distances (Å) and angles (o): Fe1-N1
+
H
Me
TSAB
N
Fe
H
H
N
PR2
H
+
H
1.9867(9), Fe1-P2 2.1443(3), Fe1-P1 2.1461(3), Fe1-Al1
2.3507(4), Fe1-H1 1.46(2), Fe1-HAL1 1.48(2), Fe1-HAL2
1.55(2), Al1-HAL1 1.82(2), Al1-HAL2 1.73(2), Al1-HAL3
1.65(2), Al1-HAL4 1.54(2), P2-Fe1-P1 157.22(1).
0.0
Me
H
N
PR2
H
N
Fe
H
Me
H
N
PR2
H
N
PR2
A
N
Fe
C
H
Me
H
N
PR2
B
H
Me
1
The H NMR spectrum of 3 in THF-d8 features a
CONCLUSION
broadened triplet resonance at -14.68 ppm that
integrates to five hydrogen atoms, while a singlet at
187.3 ppm could be observed in the 31P{1H} NMR
spectrum. Owing to fast exchange between classical and
non-classical hydrides, it was not possible to determine
separate proton resonances for the individual hydride
ligands. Even at -100 °C only a slight broadening of the
The bench-stable cationic bis(σ-B-H) aminoborane
complex [Fe(PNPNMe-iPr)(H)(η2-H2B=NMe2)]+ (2) turned
out to be an efficient pre-catalyst for the semi-
hydrogenation of internal alkynes, 1,3-diynes and 1,3-
enynes. With 1,3-diynes and 1,3-enynes deuterium could
be selectively incorporated in the presence of D2. This
was exemplarily shown with 1,4-diphenylbuta-1,3-diyne
and (Z)-but-1-en-3-yne-1,4-diyldibenzene were the
1
hydride signal was observed. H NMR spectra recorded
at variable temperatures revealed an extremely short
relaxation time T1(min) of 12 ms (-65 °C, 500 MHz)17 which
isotopomeres
d2)dibenzene
(Z,Z-buta-1,3-diene-1,4-diyl-1,2-
(Z,Z-buta-1,3-diene-1,4-diyl-
is
characteristic
of
coordinated
dihydrogen
and
molecules.18,19
d4)dibenzene, respectively, were obtained. The catalytic
reaction takes place under mild conditions (1 h, 25oC, 4-5
bar H2 or D2) and all alkenes were obtained with high Z-
selectivity for a broad scope of substrates. Mechanistic
The experimental observations are further
supported by DFT calculations. The cis- and trans-
isomers of the bis(dihydrogen) complex differ merely by
2.0 kcal/mol and their interconversion requires an
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Journal of the American Chemical Society
REFERENCES
insight into the catalytic reaction is provided by means of
DFT calculations. The stereo- and chemoselectivity was
fully explained in agreement with the experimental
observations. Intermediates featuring a bis-dihydrogen
moiety [Fe(PNPNMe-iPr)(η2-H2)2]+ are found to play a key
role. Experimental support for such species was provided
by the fact that [Fe(PNPNMe-iPr)(H)(η2-H2)2]+ (3) exhibited
the same catalytic activity than 2. The novel cationic bis-
dihydrogen complex 3 was obtained by protonolysis of
[Fe(PNPNMe-iPr)(H)(η2-AlH4)]2 (1) with an excess of
nonafluoro-tert-butyl alcohol. Thus, complexes 2 and 3
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presence of H2 to form [Fe(PNPNMe-iPr)(H)(η2-H2)(η2-
RCCR’)]+ thereby initiating the catalytic cycle.
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ASSOCIATED CONTENT
Supporting Information
The supporting information is available free of charge on
the ACS Publications website at DOI: xxxxx
Synthetic procedures, NMR spectra of all compounds, and
crystallographic data (PDF)
X-ray crystallographic data for 1 (CCDC entry 1951434) (CIF)
Optimized cartesian coordinates for DFT-optimized
structures (XYZ)
AUTHOR INFORMATION
Corresponding Author
* E-mail (K. Kirchner): karl.kirchner@tuwien.ac.at, Tel: (+43) 1
58801 163611. Fax: (+43) 1 58801 16399.
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ORCID
Nikolaus Gorgas: 0000-0003-2919-1042
Berthold Stöger: 0000-0002-0087-474X
Stefan Vanicek: 0000-0002-8539-7961
Mats Tilset: 0000-0001-8766-6910
Luis F. Veiros: 0000-0001-5841-3519
Karl Kirchner: 0000-0003-0872-6159
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
(7) (a) Radkowski, K.; Sundararaju, B.; Fürstner, A.
A
NG and KK gratefully acknowledge the Financial support by
the Austrian Science Fund (FWF) (Project No. P29584-N28).
NG thanks the COST-CHAOS action for granting a STSM to
the University of Oslo. SV thanks the Austrian Science Fund
(FWF) for an Erwin Schrödinger Postdoctoral Fellowship
(Project No. J4158). LFV thanks Fundação para a Ciência e
Tecnologia, UID/QUI/00100/2013. The Research Council of
Norway supported this work through the Norwegian NMR
Platform, NNP (226244/F50). Senior Engineer Dirk Petersen
(University of Oslo NMR center) is thanked for variable
temperature NMR assistance. This paper is dedicated to
Professor Marko D. Mihovilovic on the occasion of his 50th
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(14) (a) Bianchini, C.; Meli, A.; Peruzzini, M.; Vizza, F.; Zanobini,
F.; Frediani, P. A Homogeneous Iron(II) System Capable of
Selectivity Catalyzing the Reduction of Terminal Alkynes to
Alkenes and Buta-1,3-Dienes. Organometallics 1989, 8, 2080–
2082. (b) Bianchini, C.; Meli, A.; Peruzzini, M.; Frediani, P.;
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Hydrogenation of 1-Alkynes to Alkenes Catalyzed by an Iron(II)
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Table of Contents (TOC)
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- fast semi-hydrogenation
R
Fe-cat.
H
R
H
R
- highly (Z)-selective
- bench-stable catalyst precursor
- selective deuterium incorporation
H
2 (5 bar), r.t.
R
7
8
9
catalyst design:
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
+
H
H
H
+
Me
Me
Me
N
PR2
N
PR2
N
PR2
H
H
H
H
N
Fe
H
N
Fe
N
Fe
H
H
B
N PR2
N PR2
H
N PR2
Me
Me
Me
Me
H
N
active
stable
active
unstable
Me
not active
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