Simple Alkaline-Earth Metal Catalysts for Effective Alkene Hydrogenation

Article abstract of DOI:10.1002/anie.201810026

Alkaline earth metal amides (AeN′′₂: Ae=Ca, Sr, Ba, N′′=N(SiMe₃)₂) catalyze alkene hydrogenation (80–120 °C, 1–6 bar H₂, 1–10 mol % cat.), with the activity increasing with metal size. Various activated C=C bonds (styrene, p-MeO-styrene, α-Me-styrene, Ph₂C=CH₂, trans-stilbene, cyclohexadiene, 1-Ph-cyclohexene), semi-activated C=C bonds (Me₃SiCH=CH₂, norbornadiene), or non-activated (isolated) C=C bonds (norbornene, 4-vinylcyclohexene, 1-hexene) could be reduced. The results show that neutral Ca or Ba catalysts are active in the challenging hydrogenation of isolated double bonds. For activated alkenes (e.g. styrene), polymerization is fully suppressed due to fast protonation of the highly reactive benzyl intermediate by N′′H (formed in the catalyst initiation). Using cyclohexadiene as the H source, the first Ae metal catalyzed H-transfer hydrogenation is reported. DFT calculations on styrene hydrogenation using CaN′′₂ show that styrene oligomerization competes with styrene hydrogenation. Calculations also show that protonation of the benzylcalcium intermediate with N′′H is a low-energy escape route, thus avoiding oligomerization.

Full text of DOI:10.1002/anie.201810026

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Simple Alkaline-Earth Metal Catalysts for Effective Alkene
Hydrogenation
Authors: Heiko Bauer, Mercedes Alonso, Christian Fischer, Bastian
Rösch, Holger Elsen, and Sjoerd Harder
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201810026
Angew. Chem. 10.1002/ange.201810026
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http://dx.doi.org/10.1002/ange.201810026

10.1002/anie.201810026
Angewandte Chemie International Edition
Simple Alkaline-Earth Metal Catalysts
for Effective Alkene Hydrogenation
Heiko Bauer,[a] Mercedes Alonso*, [b] Christian Fischer, [a] Bastian Rösch, [a] Holger Elsen,[a]
Sjoerd Harder*[a]
Dedicated to Professor Alexander C. Filippou on the occasion of his 60th birthday.
[a]
[b]
Dr. H. Bauer, C. Fischer, B. Rösch, H. Elsen, Prof. Dr. S. Harder*
Chair of Inorganic and Organometallic Chemistry, Universität Erlangen-Nürnberg, Egerlandstrasse 1,
91058 Erlangen, Germany
Fax: (+49) 9131-8527387
E-mail: sjoerd.harder@fau.de
Dr. M. Alonso
Algemene Chemie, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Elsene, Belgium
E-mail: malonsog@vub.be
Supporting information for this article is available on the WWW under http//:www.angewandte.chemie.org or
from the author.
Keywords: Alkaline Earth Metal – Alkene – Hydrogenation – DFT calculations
Table of Contents
Keep it simple: The widely used, easily accessible, alkaline earth metal amides M(NR2)2 (M = Ca, Sr,
Ba, R = SiMe3) are highly active alkene hydrogenation catalysts. Alkene oligomerization is fully
suppressed by trapping the reactive intermediate with R2NH. A first example of group 2 metal
hydrogen transfer catalysis is given.
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10.1002/anie.201810026
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2
Already in 1874, von Wilde demonstrated the catalytic reduction of ethylene by Pt/H2,[1] a ground-
breaking discovery that laid the foundations for Sabatier’s 1912 Nobel prize on alkene
hydrogenation.[2] Despite its early roots, alkene hydrogenation is by no means old hat[3] and arguably
is the chemical transformation that find most applications in industry. Although the majority of
catalysts is heterogeneous, homogeneous hydrogenation catalysts contributed to the early successes
of organometallic chemistry. Merits of the classical Wilkinson catalyst lie in its great selectivity that
originates from a clear reactivity sequence for C=C double reduction (Scheme 1).[4] Although the most
efficient catalysts are based on precious platinum group metals,[3] there is a growing awareness of
the toxicity and limited availability of such metals.[5] Current research efforts focus on using earth-
abundant first-row transition metals,[6] early main group metals[7] or even on metal-free routes.[8,9]
Slaugh reported alkene hydrogenation mediated by main group metal hydrides under extreme
conditions (150-225 °C, 150-250 bar H2, 25 mol% cat.).[10] We introduced the first transition metal-
free alkene hydrogenation under relatively mild conditions (20-60 °C, 20 bar H2, 2.5-5 mol% cat.)[11]
which broke the dogma that efficient alkene activation needs partially filled d-orbitals for -
backbonding.
Scheme 1. Reactivity of olefinic C=C bonds towards
hydrogenation with the Wilkinson catalyst
(Ph3P)3RhCl decreases with substitution. Endocyclic
bonds show a higher reactivity.
Benzyl Ca, Sr and K complexes catalyzed hydrogenation of various alkenes (Scheme 2). Stoichiometric
reactions with a model catalyst and full characterization of all reaction intermediates (Scheme 2b),
suggested that catalysis likely proceeds through the slow, rate-determining, formation of a metal
hydride intermediate. Single reduction of cyclohexadiene or myrcene suggested that the method
could be limited to activated (conjugated) alkenes like styrenes or butadienes. Preference for the
reduction of conjugated bonds was explained by the stability of the intermediate: benzylic or allylic
carbanions are more stable than alkyl carbanions. Since styrenes are also very sensitive towards
polymerization, in most cases large quantities of oligomers were obtained as an undesired side-
product (2-20%). This could to some extent be suppressed by increasing the H2 pressure to 100 bar.
More recently, Okuda and coworkers introduced a set of cationic Ca hydride catalysts for alkene
hydrogenation (Scheme 2c).[12,13] Stabilized by a neutral aza-crownether ligand, Me4TACD, they could
be pictured to consist of the fragments (Me4TACD)CaH2 and/or (Me4TACD)CaH+. Their Ph4Bˉ salts are
insoluble in aromatic solvents, however, in THF they were found to be highly efficient in alkene
hydrogenation converting activated alkenes like styrene already at room temperature and 1 bar H2
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10.1002/anie.201810026
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3
pressure. More importantly, also non-conjugated alkenes like 1-hexene could be converted albeit at
higher temperatures (60 °C) and with long reaction times (24-36 h). This significant ability to react
with isolated double bonds was attributed to their positive charge thought to be critical in imparting
sufficient electrophilicity to the electropositive calcium center.[13] Despite their high reactivity, di-
substituted unactivated double bonds were not
converted. Moreover, activated substrates like
styrene were prone to oligomerization as a side
reaction.
Scheme 2. (a) First early main group metal catalyst for
alkene hydrogenation under mild conditions.[11] (b)
Proposed catalytic cycle according to stoichiometric
conversions. (c) Cationic Ca hydride complexes with
the Me4TACD ligand.[13]
Notwithstanding these breakthroughs, group 2
metal alkene hydrogenation catalysis is far from
being up to par with transition metal applications.
Preventing oligomerization, functional group
tolerance, activity improvement and hydrogen
transfer protocols are important issues. Herein
we present a set of simple well-known catalyst
precursors, Ae[N(SiMe3)2]2 (AeN’’2; Ae = Ca, Sr,
Ba), tackling all of these goals. These common
reagents can be conveniently prepared at a large
scale.[14,15] Although their reaction with H2 to form
the active hydride N’’AeH and N’’H is unpredicted
[16]
(the pKa of 49 for H2 is much higher than the
25.8 value[17] for N’’H), we could recently show
that these elementary reagents are converted to
metal
hydride
catalysts
for
imine
hydrogenation.[18] This rather unexpected amide-
to-hydride conversion is supported by the very
recently reported reversible activation of H2 by
similar alkali metal reagents.[19]
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10.1002/anie.201810026
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A proposed mechanism for alkene hydrogenation catalyzed by simple AeN’’2 reagents is shown in
Scheme 3. As shown earlier,[18] precatalysts AeN’’2 are indeed able to deprotonate H2, the driving
force being aggregation of N’’AeH to larger undefined clusters. Exemplary some of these clusters
could previously be trapped and structurally characterized (Scheme 3).[20,21] Their high reactivity is
the key to alkene conversion. Precatalyst CaN’’2 could fully convert styrene to ethylbenzene (Table 1,
entries 1-5). Since increasing the temperature had a larger accelerating effect than increasing the
pressure, catalysis could also conveniently be run at an ambient H2 pressure of only 1 bar (entry 5)
but room temperature conditions gave hardly conversion (entry 3). Whereas MgN’’2 is essentially
inactive (entry 6), the heavier Ae amides show very high activity (entries 7-8). The most reactive
BaN’’2 even allowed for full conversion at room temperature (entry 9) and, most important, enabled
low catalyst loadings down to 1 mol% (entries 10-11). Alkali metal amides are hardly active (entries
12-14) suggesting that the 2+ charge of Ae is important for Lewis acidic substrate activation. The
latter is supported by the complete inactivity of the system in the solvent THF. Most remarkably,
polymerization sensitive styrene is converted to ethylbenzene without oligomeric side-products. We
attribute this finding to the presence of the relatively acidic free amine N’’H (pKa = 25.8), which reacts
much faster with the benzylmetal intermediate than H2 (pKa ≈ 49) thus keeping its concentration low
and preventing styrene oligomerization (Scheme 3). Only the longer runs with 1 mol% catalyst
loading (entries 10-11) gave some oligostyrene on account of thermal polymerization. Addition of 9
mol% N’’H, however, accelerated the trapping reaction and inhibited oligomer formation (Figure S6).
Substrate scope was investigated for precatalysts CaN’’2 and BaN’’2 using comparable reaction
conditions (6 bar, 120 °C, 10 mol% cat.). Functional group tolerance was evaluated using para-
methoxy or chloro styrene (entries 15-18). Whereas ether substituents do not pose problems the
aromatic C-Cl group led to catalyst decomposition by a nucleophilic aromatic substitution of Cl for
N’’, a known reactivity.[22] Di-substituted conjugated alkenes like 1,1-diphenylethylene, trans-stilbene
or -Me-styrene react somewhat slower than styrene but gave very fast clean conversion with the
highly reactive BaN’’2 catalyst (entries 19-24). 1,3-Cyclohexadiene, in which the double bonds are
activated by conjugation, could be converted mainly to cyclohexene, leaving the remaining (isolated)
double bond unaffected (entries 25-26). Interestingly, up to 23% benzene could be detected as a
side-product. This formal dehydrogenation reaction can only be explained by deprotonation followed
by ß-H elimination (Scheme 3, top right). This reactivity is supported by reaction of cyclohexadiene
with BaN’’2 without H2 pressure giving equal quantities of benzene and cyclohexene (Figure S25).
Dual dehydrogenation/hydrogenation of cyclohexadiene implies that BaN’’2 could be used as a
transfer hydrogenation catalyst. Indeed, a mixture of cyclohexadiene (used in here as a H source) and
BaN’’2 (10 mol%) fully converted styrene into ethylbenzene (80 °C, 1.5 h, Figure S26), demonstrating
first hydrogen transfer catalysis for this metal class.
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10.1002/anie.201810026
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Scheme 3. Postulated mechanism for alkene hydrogenation with AeN’’2 catalysts including trapping,
oligomerization and H transfer pathways. The inset shows an isolated example of a larger aggregate (Ae = Ca or
Sr and L = PMDTA).[20]
The outstanding reactivity of the Ba catalyst motivated us to investigate its performance for
hydrogenation of isolated double bonds. The alkene Me3SiCH=CH2 was smoothly hydrogenated
(entries 27-28). The latter, however, should be regarded semi-activated by the silyl substituent which
substantially polarizes the C=C bond and stabilizes the intermediate anion Me3Si(Me)CHˉ by negative
hyperconjugation. Norbornadiene could be fully transformed into mixtures of norbornene,
nortricyclene and norbornane (entries 29-30). Similar, its double bonds are semi-activated by
homoconjugation.[23] Such a through-space interaction is demonstrated by formation of nortricyclene
through an intramolecular nucleophilic addition followed by protonation. Norbornene was cleanly
converted to norbornane (entries 31-33) but its double bond is semi-activated by ring strain. 1-
Hexene, an alkene that is truly unactivated, was fully converted by BaN’’2, however, a substantial
quantity of 2-hexene was observed (entry 35). Independent experiments showed that this internal
alkene cannot be further hydrogenated. 1-Hexene isomerization was also observed for cationic Ca
hydride catalysts.[13] This side-reaction could proceed through “wrong” addition followed by ß-H
elimination (Eq. 1) or through a deprotonation/protonation protocol (Eq. 2). As the free amine N’’H is
likely involved in this last step, we investigated 1-hexene hydrogenation with DMAT2Ca∙(THF)2. Thus,
avoiding N’’H formation, isomerization-free full conversion to hexane could be achieved (entry 36).
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10.1002/anie.201810026
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Our most active catalysts were unable to hydrogenate 1,1-diethylethylene and hydrogenation of 4-
vinylcyclohexene led to reduction of the terminal alkene but not of the endocyclic C=C bond (entry
37-39). Although the scope of these catalysts could not be extended to unactivated di-substituted
alkenes, the activated trisubsitituted alkene Ph-cyclohexene could be fully converted (entry 40).
The proposed catalytic cycle for hydrogenation of styrene by CaN’’2 was calculated by DFT methods
at the M06/6-311++G(d,p)//M06/6-31++G(d,p) level of theory.[24] G values (80 °C, 6 bar) were
corrected for benzene solvent effects.[25] The energy profile (Scheme 4) consists of four parts:
initiation (I), hydrogenation (H), dimerization (D) and trapping (T).
Initiation (I) of the cycle starts with precatalyst CaN’’2, exchanging an amide ligand for a hydride
through transition state I1* (+18.7 kcal/mol). As predicted by the large pKa differences between H2
(49) and N’’H (25.8), this step is clearly endothermic (I2, 10.8 kcal/mol). Release of the bulky amine
N’’H to give the model catalyst (CAT) is slightly endothermic. A second amide/hydride exchange to
give CaH2 is even more endothermic (+27.3 kcal/mol)[18] and was not further considered. Note that
although this initiation reaction is endothermic, further aggregation to larger cluster (e.g. shown in
Scheme 2) will be exothermic, balancing energy loss. For simplicity, the mechanism has been
modeled with the simplest Ca hydride catalyst N’’CaH (CAT).
Hydrogenation (H) follows the normal route in which styrene coordinates and inserts in the Ca-H
bond (H1→H2*→H3). The benzylcalcium intermediate H3 reacts with H2 via a -bond metathesis
(H3→H4*→H5) to give ethylbenzene and the catalyst (CAT). Relative to the model catalyst CAT,
styrene hydrogenation is exergonic by 21.4 kcal/mol. The highest point along the energy profile is the
transition state for insertion (H2*, +24.5 kcal/mol). However, the activation energy for this step
(CAT→H2*, +12.6 kcal/mol) is lower than that for the -bond metathesis step (H3→H4*, +16.9
kcal/mol). The structure of transition state H4* is reminiscent of H2 activation by a FLP reagent (see
Scheme 4). The benzyl anion in H4* is bound with its Ph ring to Ca2+. The free carbanion center and
Ca2+ span a Lewis base/acid pair that activates the H-H bond.
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10.1002/anie.201810026
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Dimerization (D), the first step of oligomerization, starts with benzylcalcium intermediate H3 that
after styrene coordination gives insertion (H3→D1→D2*) with an activation energy of 14.5 kcal/mol,
i.e. somewhat lower than that for hydrogenation (H4*). It is also thermodynamically favored over
hydrogenation: formation of D3 is 8.6 kcal/mol more endergonic than hydrogenation (H5). The
stability of D3 may be explained by its unusual structure in which Ca is sandwiched between two
aromatic rings (see Scheme 4).
Trapping (T) of the benzylcalcium intermediate H3 proceeds by endergonic N’’H coordination to form
T1 followed by protonolysis of the benzyl anion. Transition T2* (+9.8 kcal/mol) is significantly lower
than those for either hydrogenation H4* (+16.5 kcal/mol) or dimerization D2* (+14.1 kcal/mol). This
explains that even at low N’’H concentration, styrene oligomerization is prevented by trapping of the
intermediate.
Scheme 4. Energy profile for the initiation (I), hydrogenation (H), dimerization (D) and trapping rection (T) at
the M06/6-311++G(d,p)//M06/6-31++G(d,p) level.[24] G values at 80 °C and 6 bar in kcal/mol were
corrected for benzene solvent effects.[25]
We demonstrated that: (i) widely used and conveniently accessible precursors in Ae chemistry,
AeN’’2, catalyze alkene hydrogenation tolerating ether functional groups, (ii) these catalysts fully
suppress alkene oligomerization by trapping of the highly reactive intermediate with N’’H, (iii)
especially BaN’’2 is a highly active catalyst with a broad scope, (iv) reduction of isolated, non-
activated, alkenes like 1-hexene is not limited to cationic catalysts, and (v) BaN’’2 is an efficient
hydrogen transfer catalyst. This study not only reveals the hidden potential of common Ae reagents
in catalysis but is especially useful for further development of this rapidly growing field.
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Table 1. Catalytic alkene hydrogenation.[a]
H2
(bar)
T
(°C)
80
120
25
80
80
120
80
80
t
Entry
Catalyst
mol%
Substrate
Product
%Conv.
(h)
1.5
0.5
24
1
24
24
0.5
0.5
15
24
24
24
24
24
1.5
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
CaN"2
CaN"2
CaN"2
CaN"2
CaN"2
MgN"2
SrN"2
BaN"2
BaN"2
CaN"2
BaN"2
LiN"
10
10
10
10
10
10
10
10
10
1
6
6
6
12
1
6
6
6
6
6
6
6
6
6
6
99
99
14
99
97
traces
99
99
99
74
99
traces
20
68
99
25
120
120
120
120
120
120
1
10
10
10
10
NaN"
KN"
CaN"2
16
BaN"2
10
6
120
0.25
99
17
18
CaN"2
BaN"2
10
10
6
6
120
120
24
24
15
19
19
CaN"2
10
6
120
24
57
20
21
22
23
24
BaN"2
CaN"2
BaN"2
CaN"2
BaN"2
10
10
10
10
10
6
6
6
6
6
120
120
120
120
120
0.75
24
0.5
24
0.25
99
10
99
64
99
25
26
CaN"2
BaN"2
10
10
6
6
120
120
24
24
85/0/15[c]
76/1/23[c]
27
28
29
CaN"2
BaN"2
CaN"2
10
10
10
6
6
6
120
120
120
12
0.5
24
99
99
3/40/52/5[b]
30
BaN"2
10
6
120
24
0/29/39/32[b]
31
32
CaN"2
BaN"2
10
10
10
10
10
10
10
10
10
6
6
6
6
6
6
6
6
6
120
120
120
120
120
120
120
120
120
24
10
10
24
24
20
24
24
24
55
99
99
33 (DMAT)2Ca∙(THF)2
34
35
36 (DMAT)2Ca∙(THF)2
37
38
CaN"2
BaN"2
70/4/26[b]
1/58/42[b]
0/0/99
15
CaN"2
BaN"2
73
95
39 (DMAT)2Ca∙(THF)2
40 BaN"2
10
6
120
20
99
1
[a] [Alkene] = 0.5 M in C6D6. Reaction times for >99% conversion (determined by H NMR) have been optimized
in 15 minutes steps. For slower reactions the conversion (%) after 24 hours is given. GC-MS was also used for
product identification. [b] Conversion obtained by GC-MS measurement. [c] Toluene-d8 used as a solvent in
order to detect benzene.
This article is protected by copyright. All rights reserved.

10.1002/anie.201810026
Angewandte Chemie International Edition
10
Acknowledgments S.H. kindly acknowledge the Deutsche Forschungsgemeinschaft (DFG) for funding
this project (HA 3218/9-1). M.A. thanks the FWO for a postdoctoral fellowship (12F4416N) and the
VUB for financial support. Computational resources and services were provided by the Shared ICT
Services Centre funded by the Vrije Universiteit Brussel, the Flemish Supercomputer Center (VSC)
and FWO.
Supporting Information Experimental procedures and details for the DFT investigations including XYZ
coordinates for all optimized minima and transition states.
Abstract:
Alkaline earth metal amides (AeN’’2: Ae = Ca, Sr, Ba, N’’ = N(SiMe3)2) catalyze alkene hydrogenation
(80-120°C, 1-6 bar H2, 1-10 mol% cat.), the activity increasing with metal size. Various activated C=C
bonds (styrene, p-MeO-styrene,-Me-styrene, Ph2C=CH2, trans-stilbene, cyclohexadiene, 1-Ph-
cyclohexene), semi-activated C=C bonds (Me3SiCH=CH2, nornornadiene) or non-activated (isolated)
C=C bonds (norbornene, 4-vinylcyclohexene, 1-hexene) could be reduced. The results show that
neutral Ca or Ba catalysts are active in the challenging hydrogenation of isolated double bonds. For
activated alkenes (e.g. styrene) polymerization is fully suppressed due to fast protonation of the
highly reactive benzyl intermediate by N’’H (formed in catalyst initiation). Using cyclohexadiene as
the H source, the first Ae metal catalyzed hydrogen transfer hydrogenation is reported. DFT
calculations on styrene hydrogenation using CaN’’2 show that styrene oligomerization competes with
styrene hydrogenation. Calculations also show that protonation of the benzylcalcium intermediate
with N’’H is a low-energy escape route, thus avoiding oligomerization.
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