Alkene Transfer Hydrogenation with Alkaline-Earth Metal Catalysts

Article abstract of DOI:10.1002/anie.201813910

The alkene transfer hydrogenation (TH) of a variety of alkenes has been achieved with simple AeN′′₂ catalysts [Ae=Ca, Sr, Ba; N′′=N(SiMe₃)₂] using 1,4-cyclohexadiene (1,4-CHD) as a H source. Reaction of 1,4-CHD with AeN′′₂ gave benzene, N′′H, and the metal hydride species N′′AeH (or aggregates thereof), which is a catalyst for alkene hydrogenation. BaN′′₂ is by far the most active catalyst. Hydrogenation of activated C=C bonds (e.g. styrene) proceeded at room temperature without polymer formation. Unactivated (isolated) C=C bonds (e.g. 1-hexene) needed a higher temperature (120 °C) but proceeded without double-bond isomerization. The ligands fully control the course of the catalytic reaction, which can be: 1) alkene TH, 2) 1,4-CHD dehydrogenation, or 3) alkene polymerization. DFT calculations support formation of a metal hydride species by deprotonation of 1,4-CHD followed by H transfer. Convenient access to larger quantities of BaN′′₂, its high activity and selectivity, and the many advantages of TH make this a simple but attractive procedure for alkene hydrogenation.

Full text of DOI:10.1002/anie.201813910

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Accepted Article
Title: Alkene Transfer Hydrogenation with Alkaline Earth Metal
Catalysts
Authors: Sjoerd Harder, Heiko Bauer, Katharina Thum, Mercedes
Alonso, and Christian Fischer
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201813910
Angew. Chem. 10.1002/ange.201813910
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http://dx.doi.org/10.1002/ange.201813910

10.1002/anie.201813910
Angewandte Chemie International Edition
Alkene Transfer Hydrogenation with Alkaline Earth Metal Catalysts
Heiko Bauer,[a] Katharina Thum,[a] Mercedes Alonso*, [b] Christian Fischer,[a] Sjoerd Harder*[a]
[a]
Dr. H. Bauer, K. Thum, C. Fischer, 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
[b]
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 – Transfer Hydrogenation – DFT calculations
Table of Contents
The hydrogen shuffle: Transfer of hydrogens from 1,4-cyclohexadiene to a variety of alkenes is
catalyzed by simple alkaline earth metal amides. The proposed mechanism for this convenient highly
selective transfer hydrogenation is supported by DFT calculations.
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The catalytic reduction of unsaturated bonds is an integral part in the industrial production of bulk
products as well as fine chemicals.[1] Whereas hydrogenation with readily available H2 is a highly
atom-efficient reaction, transfer hydrogenation (TH) is rapidly gaining popularity.[2] The latter process
is defined by reduction using an alternative hydrogen source that preferentially is readily available,
inexpensive and easy to handle and recycle. Avoiding use of pressurized H2 eliminates potential
safety hazards and circumvents the need for elaborate experimental set-ups and high-pressure
reactors.
Although we are currently experiencing “the golden age of TH”,[2] first observations for such
reactivity already date back more than a century. In 1903, Knoevenagel described the concurrent
dehydrogenation/hydrogenation of a substituted 1,4-cyclohexadiene (Scheme 1).[3] The Meerwein-
Ponndorf-Verley ketone reduction is another early example of TH reactivity using a main group metal
Al catalyst and iPrOH as the hydrogen donor.[4] Spurred on by Henbest’s Ir catalysts,[5] the first well-
defined Ru catalysts for TH have been introduced by Noyori[6] and Shvo.[7] These bifunctional
catalysts transfer protic and hydridic H’s from iPrOH to a ketone. The turn of the century witnessed
major contributions in metal-free catalyzed TH. List and Rueping reported the enantioselective TH of
ketones using catalytic quantities of a Brønsted acidic chiral phosphoric acid and the Hantzsch ester
for hydrogen delivery.[8] Stephan and co-workers pioneered Lewis acid catalyzed imine
hydrogenation using iPr2NH as H2 substitute,[9] a process that was later extended by Oestreich et al.
using 1,4-cyclohexadienes as a reducing agent.[10] The Stephan group reduced activated alkenes with
a R3SiH/HNR2 combination using a Frustrated Lewis Pair (FLP) catalyst.[11] Very recently, the Melen
and Morrill groups reported TH of silylenol ethers using a FLP catalyst [12] and the groups of Paradies
and Hoge introduced a Lewis acidic phosphine catalyst for TH of 1,1,-diphenylethylene.[13]
Interestingly, high energy “H2 substitutes” like NH3BH3 or in situ generated HN=NH have been shown
to reduce activated alkenes without using a catalyst.[14,15]
While early main group metal catalysis is a rapidly developing field,[16] TH with these metals is
currently restricted to transferring hydrogen from iPrOH to ketones with simple bulk catalysts like
NaOH or KOH.[17] Apart from this isolated example, the conversion of imines or alkenes hitherto has
not been reported. During our investigation on alkene hydrogenation with H2 using simple alkaline
earth (Ae) metal amide catalysts,[18] the substrate 1,3-cyclohexadiene (1,3-CHD) showed dual
reactivity and was not only hydrogenated to cyclohexene but also dehydrogenated to benzene. This
striking observation implied that this diene may function as a H source offering an alternative route
to formation of the metal hydride catalyst. Indeed, a mixture of 1,3-CHD and styrene could without
H2 be converted to ethylbenzene and benzene.[18] Although concomitant reduction of 1,3-CHD also
gave considerable quantities of cyclohexene, this reaction suggested the feasibility of Ae metal
catalyzed alkene TH. Ketone TH, and to a lesser extent imine TH, are well-studied procedures but
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there are hardly catalysts for the more challenging TH of the significantly less polar C=C bond.[13,19]
Herein we report first comprehensive investigations towards alkene reduction by TH using easily
accessible AeN’’2 catalysts (N’’ = N(SiMe3)2, Scheme 1).
Scheme 1. Various concepts for transfer hydrogenation.
The first task was the selection of a hydrogen source that would be a convenient alternative to H2 for
the efficient generation of a metal hydride complex. Since 1,3-CHD contains conjugated double
bonds, it not only delivers H but is also easily reduced to cyclohexene. The latter process does not
take place using 1,4-CHD with isolated (unactivated) double bonds. The reaction of 1,4-CHD to
1
benzene with stoichiometric quantities of a metal reagent was monitored by H NMR in C6D6
(Scheme 2). For CaN’’2 full conversion was obtained at 60 °C within 15 hours. In line with the
increasing reactivity of group 2 metal reagents with metal size, BaN’’2 was found to be considerably
more reactive (99%, 1.5 h). As the THF adduct CaN’’2∙(THF)2 did not react with 1,4-CHD, the presence
of THF is detrimental to conversion. Based on this observation, we propose a mechanism in which
1,4-CHD forms an adduct with the metal reagent (Scheme 2). This activates 1,4-CHD for
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intramolecular deprotonation to give a highly unstable cyclohexadienyl anion. Such Meisenheimer
anions can be stabilized by strongly electron withdrawing substituents (e.g. NO2), but generally
eliminate a hydride to give an aromatic product.[20,21] The dibenzyl calcium reagents
(DMAT)2Ca∙(THF)2 and (4-tBu-benzyl)2Ca clearly underperformed (DMAT = 2-Me2N--Me3Si-benzyl).
Conversion of 1,4-CHD to benzene could also be achieved by KH in THF (60 °C, >99%, 2.5 h) but not
with nBuLi in C6D6 (60 °C, 5%, 48 h) whereas the superbase mixture nBuLi/KOtBu reacted fast (20 °C,
[22]
>99%, 0.25 h). The recently introduced highly reactive ethylcalcium complex [(DIPPBDI)CaEt]2
reacted already at room temperature (22%, 1.5 h) but also gave ligand exchange to homoleptic
species (Figure S11-12); DIPPBDI = CH[C(Me)N-DIPP]2, DIPP =2,6-diisopropylphenyl. Although we could
not intercept the labile cyclohexadienyl intermediate, the formation of known[22] [(DIPPBDI)CaH]2
confirms that the reaction with 1,4-CHD is an alternative route to metal hydride species.
Scheme 2. Stoichiometric conversion of 1,4-CHD
to benzene and the metal hydride catalyst
(monitored by 1H NMR in C6D6).
Using 1,4-CHD as a H source, the catalytic TH of styrene by catalyst LCaR was explored (Scheme 3; R =
reactive group, L = spectator ligand). It was found that the nature of the catalyst strongly determines
the course of the reaction. The simple and easily accessible Ca amide catalyst CaN’’2 gave selective
conversion to ethylbenzene (Table 1, Entry 1). In agreement with our earlier observations, the
catalyst CaN’’2 has the unique property to inhibit styrene polymerization (<1% oligomers).[18] The key
to this selectivity is the presence of the relatively acidic amine N’’H (pKa = 25.8)[23] formed during
catalyst initiation. This amine quenches the reactive benzylcalcium intermediate thus preventing
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styrene polymerization (Scheme 3). Earlier DFT calculations confirm that reaction of N’’CaCH(Me)Ph
with N’’H is a low energy escape route (G‡ = +10.2 kcal/mol) which is clearly preferred over styrene
polymerization (G‡ = +14.5 kcal/mol).[18]
Scheme 3. Transfer hydrogenation of styrene using 1,4-cyclohexadiene as a H source and catalyst LCaR.
Transfer hydrogenation is only observed for LCaR = CaN’’2 (N’’ = N(SiMe3)2). Catalyst (DIPPBDI)CaN’’ gave
exclusive dehydrogenation whereas the catalyst (DMAT)2Ca∙(THF)2 only led to polystyrene.
In contrast to Entry 1, catalyst (DIPPBDI)CaN’’[24] converted 1,4-CHD into H2 and benzene and only
traces of ethylbenzene were observed (Entry 2). This implies that in situ formed (DIPPBDI)CaH prefers
deprotonation of N’’H over styrene addition (Scheme 3). Indeed, reaction of (DIPPBDI)CaH with a
mixture of N’’H and styrene only gave H2 and (DIPPBDI)CaN’’ leaving styrene unconverted (Figure S10).
On the other hand, in situ formed N’’CaH (or aggregates thereof)[25] apparently prefers styrene
addition over N’’H deprotonation but in case styrene is absent also slow dehydrogenation of 1,4-CHD
into benzene and H2 was observed: 5 mol% CaN’’2 in C6D6, 60 °C, 99% conversion after 4 days.
Using the dibenzylcalcium catalysts (DMAT)2Ca∙(THF)2 or THF-free (4-tBu-benzyl)2Ca changed the
course of the catalytic reaction again and gave only styrene polymerization (Entries 3-4). This is due
to the absence of a Brønsted acidic proton source like N’’H and implies that the benzylcalcium
intermediate reacts much faster with styrene than with 1,4-CHD. The latter reaction would close the
TH cycle but is not observed (Scheme 3).
This first set of experiments demonstrates that the nature of the Ca catalyst is crucial to success in
alkene TH catalysis. The presence of the amide ligand N’’ is essential and bulky spectator ligands
should be avoided (Scheme 3). Replacing Ca for the group 1 metals Li or K was not successful (Entries
5-6), however, using the heavier group 2 metals Sr and Ba lowered the reaction time substantially
(Entries 7-8). The catalyst BaN’’2 gave even at room temperature full conversion (Entry 9). Using only
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one equivalent of 1,4-CHD gave also nearly quantitative conversion but clearly extended the reaction
time (Entry 10). Traces of H2 were only formed in the final stage, clearly indicating strong preference
for TH over dehydrogenation. In contrast to the detrimental influence of THF in Ca-catalyzed styrene
TH, THF ligands do not affect the activity of the Ba catalyst (Entry 11). This can be explained by the
considerably weaker Ba∙∙∙THF bond. There are cases in which Ba∙∙∙arene coordination, an interaction
of a large soft metal cation with a soft aromatic -system, is competitive with THF coordination.[26]
Therefore, TH of para-methoxystyrene by BaN’’2 posed no problems (Entry 12) but para-
chlorostyrene suffered from partial Cl/N’’ exchange (entry 13). This aromatic nucleophilic
substitution is possibly facilitated by Ba∙∙∙arene bonding. Higher substituted styrene substrates like
1,1-diphenylethylene, trans-stilbene or -Me-styrene could be reduced quantitatively within 1-2
hours (Entries 14-16). Even the trisubstituted C=C bond in 1-Ph-cyclohexene was fully hydrogenated
but only after raising the temperature to 120 °C (Entry 17). At this higher temperature, part of the
reducing agent is lost due to H2 evolution. To compensate for this undesired side-reaction, 5
equivalents of 1,4-CHD have been used.
In addition, a series of substrates with non-conjugated (isolated) double bonds could be converted.
The C=C bond in Me3SiCH=CH2 was fully reduced at the lower temperature of 60 °C but should be
regarded semi-activated by the strongly polarizing Me3Si-substituent (Entry 18). Semi-activated
norbornadiene was fully hydrogenated to norbornane and nortricyclononane (arising from
intramolecular nucleophilic addition) at the higher temperature of 120 °C (Entry 19). The fact that
norbornene could not be detected is in agreement with efficient reduction of norbornene to
norbornane (Entry 20). The cyclic double bond in cyclohexene is somewhat less activated and could
only be reduced in traces (Entry 21) whereas 3-vinyl-cylohexene gave partial reduction of the
exocyclic terminal double bond (Entry 22). This is in agreement with the problematic hydrogenation
of non-conjugated internal alkenes like 2-hexene, however, the truly unactivated C=C bond in 1-
hexene was fully converted to hexane (Entry 23). In contrast to 1-hexene reduction with BaN’’2 and
H2,[18] TH is selective and no isomerization to inert internal alkenes like 2- or 3-hexene could be
observed.
Although different reaction conditions make it difficult to compare the performance of this TH
method with classical alkene hydrogenation using group 2 metal catalysts, conversion rates are
certainly on a par with BaN’’2 catalyzed alkene hydrogenation using H2 as a reductant.[18] Okuda’s
cationic Ca hydride catalysts were found to be substantially more active in alkene/H2 reduction than
neutral Ca catalysts[27] but alkene TH with BaN’’2 and 1,4-CHD is in most cases significantly faster. The
highly reactive THF-free Ca hydride complex [(DIPPBDI)CaH]2 recently reported by the Hill group,[22]
hydrogenates a large range of alkene substrates (including 1-hexene) already at room temperature
but full conversion generally took up to 3 weeks.[28] In the latter case, catalyst performance is clearly
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limited by thermal stability: slight increase in temperature led to ligand scrambling and loss of the
catalyst by (CaH2)∞ precipitation. In contrast, BaN’’2 is an extremely robust catalyst that can
withstand operation temperatures up to 120 °C. The exact nature of the catalytic species is unclear
but likely larger aggregates N’’xBayHz are formed.[25b]
The transfer of hydrogen from 1,4-CHD to styrene catalyzed by CaN’’2 was investigated by DFT
methods at the M06/6-311++G(d,p)//M06/6-31++G(d,p) level of theory.[29] G values at 60 °C were
corrected for benzene solvent effects.[30] The energy profile given in Scheme 4 starts with a series of
reactions that initiate the formation of the Ca hydride catalyst N’’CaH (CAT, route I). The hydrogen
source 1,4-CHD is deprotonated by CaN’’2 to an intermediate with a cyclohexadienyl anion (I4) which
could be described as a labile hydride-Meisenheimer complex. The substantial activation energy
needed for its formation (I2*, +22.9 kcal/mol) explains the need for higher temperatures. At these
temperatures, the subsequent hydride elimination through transition state I5* (+22.3 kcal/mol) is
competitive. The activation energies for N’’CaH catalyst formation from CaN’’2 and 1,4-CHD (via I2*
and I5*) are higher than the energy barrier for the reaction CaN’’2 + H2 → N’’CaH + N’’H (+18.7
kcal/mol).[18] On the other hand, the latter reaction which formally is the deprotonation of H2 (pKa H2
= 49)[31] by a weak base (pKa N’’H = 25.8)[23] was calculated to be clearly endergonic (+11.9
kcal/mol)[18] whereas generation of N’’CaH from 1,4-CHD is slightly exergonic: CaN’’2 + 1,4-CHD →
N’’CaH + N’’H + benzene (-1.4 kcal/mol). This is due to energy release from the conversion of the
unconjugated diene 1,4-CHD to the highly stable aromatic benzene.
The subsequent hydrogenation pathway (H) starts from N’’CaH (CAT). Styrene insertion and
subsequent quenching of the benzylcalcium intermediate with N’’H are clearly the lower energy
processes along the reaction profile (H5*). The activation energy for styrene polymerization (P2*) is
4.3 kcal/mol higher and this process may only be feasible in case N’’H is not present (Entries 3-4). The
total energy release for transfer of hydrogen from 1,4-CHD to styrene is 34.7 kcal mol which is
considerably more exergonic than styrene hydrogenation by H2, releasing 21.4 kcal/mol. The
difference of 13.3 kcal/mol is the aromatization energy for converting 1,4-CHD into benzene and H2.
As part of our theoretical studies, we investigated an alternative route (A) that circumvents
formation of the metal hydride catalyst (CAT). However, transferring the hydride directly from the
cyclohexadienyl anion to styrene is substantially higher in energy (A2*, 28.2 kcal/mol) and most likely
not feasible.
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10.1002/anie.201813910
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Scheme 4. Energy profile for the transfer hydrogenation of styrene with 1,4-CHD and CaN’’2; N’’ = N(SiMe3)2 at
the M06/6-311++G(d,p)//M06/6-31++G(d,p) level (* indicates transition state). G values (60 °C, kcal/mol)
were corrected for benzene solvent effects. The catalytic cycle is initiated by formation of N’’CaH (I in grey),
followed by styrene hydrogenation (H in green). A non-competitive alternative pathway (A) and styrene
polymerization (P) are shown in red and blue, respectively. Insets show selected transition states (H’s partially
shown, distances in Å).
We demonstrated first successful catalytic transfer hydrogenation from 1,4-CHD, a green waste
product from plant oil refinery,[32] to a variety of alkenes using group 2 metal catalysts. The choice of
catalyst fully controls the course of the reaction. Amide ligands are crucial and bulky spectator
ligands should be avoided for successful TH. Especially BaN’’2 was shown to be a highly effective and
selective TH catalyst, quantitatively reducing activated double bonds (e.g. styrene) without
oligomeric side-products and hydrogenating truly unactivated alkenes (e.g. 1-hexene) without double
bond isomerisation. Convenient access to larger quantities of BaN’’2,[33] which is widely used as
precursor in Ba chemistry, combined with the advantages inherent to TH (easy experimental set-up
avoiding high pressure) make Ba catalyzed alkene TH a highly attractive procedure. The mechanism is
different from the generally practised transition metal or acid catalyzed TH. Group 2 metal TH
catalysis should therefore be considered a novel area of research, clearly demonstrating the rapidly
expanding versatility of early main group metal catalysts.
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Table 1. Catalytic alkene transfer hydrogenation.[a]
Entry
Catalyst
mol%
Substrate
Eq. 1,4-CHD T [°C]
t [h]
Product
%
1
2
3
4
5
6
7
8
9
CaN”2
5
5
5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1
60
60
60
60
60
60
60
60
20
20
60
44
24
20
20
24
24
6
0.75
8
21
0.75
99
traces[b]
0[c]
(
DIPPBDI)CaN”
(DMAT)2Ca∙(THF)2
(p-tBu-benzyl)2Ca
LiN’’
5
0[c]
10
10
5
5
5
5
5
30[b]
traces
99
99
99
KN’’
SrN’’2
BaN”2
BaN”2
10
11
BaN”2
99
99
BaN”2∙(THF)2
1.5
12
13
BaN”2
BaN”2
5
5
1.5
1.5
60
60
1
99
14
traces
14
15
16
BaN”2
BaN”2
BaN”2
5
5
5
1.5
1.5
1.5
60
60
60
1.5
1.5
99
99
99
0.75
17
18
BaN”2
BaN”2
10
5
5
120
60
3
99[d]
99
1.5
4.25
19
20
BaN”2
BaN”2
10
10
5
5
120
120
7
5
75/25[d,e]
99[d]
21
BaN”2
10
5
120
24
Traces[d]
22
23
BaN”2
BaN”2
10
10
5
5
120
120
24
16
50[d]
99[d]
[a] [Substrate] = 0.33 M in C6D6. Reaction times for essentially full conversion (99 %, determined by 1H NMR)
have been optimized in 0.25 h steps. For slower reactions the conversion after 24 h is given. GC-MS was also
used for product identification. [b] 1,4-CHD is dehydrogenated to benzene and H2; also isomerisation to 1,3-
CHD observed. [c] Full conversion to polystyrene. [d] At the higher reaction temperature toluene-d8 was used
as solvent. [e] Conversion determined by GC-MS measurement.
This article is protected by copyright. All rights reserved.

10.1002/anie.201813910
Angewandte Chemie International Edition
11
Acknowledgments S.H. kindly acknowledges 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, selected NMR spectra and details for the DFT
investigations including XYZ coordinates for all optimized minima and transition states.
Abstract:
The challenging alkene transfer hydrogenation (TH) of a variety of alkenes has been achieved with
simple AeN’’2 catalysts (Ae = Ca, Sr, Ba; N’’ = N(SiMe3)2) using 1,4-cyclohexadiene (1,4-CHD) as a H
source. Reaction of 1,4-CHD with AeN’’2 gave benzene, N’’H and the metal hydride species N’’AeH (or
aggregates thereof) which is a catalyst for alkene hydrogenation. Complex BaN’’2 is by far the most
active catalyst. Hydrogenation of activated C=C bonds (e.g. styrene) proceeded already at room
temperature without polymer formation. Unactivated (isolated) C=C bonds (e.g. 1-hexene) needed a
higher temperature (120 °C) but proceeded without double bond isomerization. The choice of the
catalyst is critical to successful TH. The ligands fully control the course of the catalytic reaction which
can be: (i) alkene TH, (ii) 1,4-CHD dehydrogenation or (iii) alkene polymerization. DFT calculations
support formation of a metal hydride species by deprotonation of 1,4-CHD followed by H‾ transfer.
Convenient access to larger quantities of BaN’’2, its high activity and selectivity and the many
advantages of TH make this a simple but attractive procedure for alkene hydrogenation.
This article is protected by copyright. All rights reserved.