Amine-Borane Dehydrogenation and Transfer Hydrogenation Catalyzed by α-Diimine Cobaltates

Article abstract of DOI:10.1002/chem.201804811

Anionic α-diimine cobalt complexes, such as [K(thf)_1.5{(^DippBIAN)Co(η⁴-cod)}] (1; Dipp=2,6-diisopropylphenyl, cod=1,5-cyclooctadiene), catalyze the dehydrogenation of several amine-boranes. Based on the excellent catalytic properties, an especially effective transfer hydrogenation protocol for challenging olefins, imines, and N-heteroarenes was developed. NH₃BH₃ was used as a dihydrogen surrogate, which transferred up to two equivalents of H₂ per NH₃BH₃. Detailed spectroscopic and mechanistic studies are presented, which document the rate determination by acidic protons in the amine-borane.

Full text of DOI:10.1002/chem.201804811

A Journal of
Accepted Article
Title: Amine-Borane Dehydrogenation and Transfer Hydrogenation
Catalyzed by α-Diimine Cobaltates
Authors: Thomas M. Maier, Sebastian Sandl, Ilya G. Shenderovich,
Axel Jacobi von Wangelin, and Robert Wolf
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Amine-Borane Dehydrogenation and Transfer Hydrogenation
Catalyzed by α-Diimine Cobaltates
Thomas M. Maier,[a]† Sebastian Sandl,[b] Ilya G. Shenderovich,[c] Axel Jacobi von Wangelin,*[b] Jan J.
Weigand,[d] and Robert Wolf*[a]
Abstract: Anionic -diimine cobalt complexes such as
[K(thf)1.5{(DippBIAN)Co(4-cod)}] (1, cod = 1,5-cyclooctadiene) catalyze
the dehydrogenation of several amine-boranes. Based on the
excellent catalytic properties, an especially effective transfer
hydrogenation protocol for challenging olefins, imines, and
N-heteroarenes has been developed. NH3BH3 was used as
dihydrogen surrogate, which transferred up to two equiv. H2 per
NH3BH3. Detailed spectroscopic and mechanistic studies are
presented, which document the rate determination by acidic protons
in the amine-borane.
advanced with the abundant and cheaper late 3d metals, despite
the recent progress with Ti, Mn, Fe, Co, and Ni catalysts.[7]‒[12]
While
a
number of iron catalysts for amine-borane
dehydrogenations have been studied recently,[8] effective cobalt
[9]-[11]
catalysts are scarce.
To our knowledge, only three well-
defined molecular cobalt catalysts have been reported to date
(Figure 1). Peters and co-workers reported bis(phosphino)boryl
(PBP) cobalt catalysts (Figure 1) for the dehydrogenation of
dimethylamine-borane (DMAB)[9] and applications to the transfer
hydrogenation of styrene. Waterman and co-workers reported
that the cyclopentadienylcobalt complexes CpRCo(CO)2I (R= H,
Me, Figure 1) catalyze ammonia borane (AB) dehydrogenation at
elevated temperatures (65°C).[10] The authors performed catalytic
transfer hydrogenations with styrenes, alkynes, and olefins with
an excess (8 equiv.) of AB at 65°C within 6 h. Tripodal
polyphosphine cobalt(I) hydrides (Figure 1) recently reported by
Shubina and co-workers exhibited similar activity in the AB-
dehydrogenation.[11] A mechanism was proposed based on DFT
calculations.
Introduction
Transition metal-catalyzed dehydrogenations of amine-boranes
have attracted great attention as a potentially versatile method of
hydrogen storage and B-N materials synthesis.[1]‒[3] Amine-
boranes can serve as solid hydrogen surrogates in transfer
hydrogenations.[4] Various dehydrogenation and transfer
hydrogenation protocols have been developed with precious
metal catalysts, and the underlying mechanisms have been
thoroughly studied.[5] By contrast, dehydrogenations are far less
[a]
[b]
M.Sc. T. M. Maier, Prof. Dr. R. Wolf
University of Regensburg
Institute of Inorganic Chemistry
93040 Regensburg, Germany
E-mail: robert.wolf@chemie.uni-regensburg.de
M. Sc. S. Sandl, Prof. Dr. A. Jacobi von Wangelin
University of Regensburg
Institute of Organic Chemistry
93040 Regensburg, Germany
new address:
University of Hamburg
Dept. of Chemistry
20146 Hamburg, Germany
E-mail: axel.jacobi@chemie.uni-hamburg.de
[c]
[d]
Dr. I. G. Shenderovich
University of Regensburg
Institute of Organic Chemistry
93040 Regensburg, Germany
Prof. Dr. J. J. Weigand
Technical University Dresden
Chair of Inorganic Molecular Chemistry
01062 Dresden, Germany
Figure 1. Homogeneous cobalt catalysts for amine-borane dehydrogenation
(Dipp = 2,6-diisopropylphenyl, Mes = 2,4,6-trimethylphenyl, cod = 1,5-cyclo-
octadiene).
Supporting information and ORCID(s) for the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/XXXXXXX.
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The paucity of cobalt-based amine-borane dehydrogenation and
transfer hydrogenation catalysts[9]-[11] prompted us to investigate
reaction proceeds through B. In case of C, both dihydrogen
molecules are eliminated in the first step and a cycloaddition gives
the terminal product D. We cannot rule out that B was also
converted into D by the loss of one molecule H2. The side product
HB(NMe)2 was reported for many catalytic DMAB hydrogenations
in the literature.[5],[8],[9] Related mechanisms for DMAB
dehydrogenation have also been described for dehydrogenation
catalysts based on zirconium and iridium.[22] Monitoring the
time-dependent H2 formation (see the SI) revealed an induction
period of approx. 1 min (SI, Figure S13). A comparison of the
initial rates indicates that the catalytic dehydrogenation activity of
2 is five times higher than that of 1 (SI, Figure S13).
the
efficacy
of
complexes
containing
redox-active
bis(iminoacenaphthene)diimine (BIAN) ligands.[13] This ligand
class was deemed particularly suitable because it offers a
convenient synthesis from commercial precursors (>60 g scales),
redox-activity, modular structure, and
a persistent ligand
backbone.[13] BIANs have mainly been exploited in noble metal
catalysis so far,[14] while applications to 3d metal catalysis have
only been reported very sporadically; systematic investigations
are still in their infancy.[15]
Results and Discussion
Key discoveries and model reactions. We previously
investigated the catalytic properties of low-valent ferrate and
cobaltate anions in the hydrogenation of olefins, ketones, and
imines.[16]
The
pre-catalysts
[K([18]crown-6)(thf)2][M(η4-
anthracene)2] (M = Fe, Co)[17] and [K(thf)x][Co(η4-cod)2][18],[19] (cod
= 1,5-cyclooctadiene) enabled the hydrogenation of disubstituted
alkenes, ketones and imines. Poor activities were observed for
the
hydrogenation
of
tri-substituted
alkenes
and
dehydrogenations of amine-boranes.[19] We therefore set out to
manipulate the stereoelectronic properties of the catalysts by
incorporation of redox-active bis(imino)acenaphthene ligands.
The synthesis of [K(thf)1.5{(DippBIAN)Co(η4-cod)] (1) (Dipp =
2,6-diisopropylphenyl; cod = 1,5-cyclooctadiene BIAN = bisaryl-
(imino)acenaphthene, Figure 1) was recently reported.[20] 1 and
the closely related mesityl-derivative [K(thf){(MesBIAN)Co(η4-cod)]
(2) (Mes = 2,4,6-trimethylphenyl, see the SI for details) were
readily accessible in high yields from a straightforward ligand
exchange reaction of [K(thf)x][Co(η4-cod)2] with ArBIAN (Ar = Dipp
or Mes). The redox-active BIAN moiety in 1 and 2 may facilitate
metal-centered redox processes by its ability to accommodate
two electrons, while cod can serve as a placeholder for vacant
coordination sites. We commenced our studies with
dimethylamine-borane (NHMe2BH3, DMAB, Scheme 1 and
Figure 2) as model substrate and monitored its consumption by
11B-NMR spectroscopy. With 5 mol% catalyst loading of 1 at 25°C,
DMAB was completely consumed within 34 h. The formation of
two main products, tetramethyl-1,3-diaza-2,4-diboretane (74%)
and N,N’-dimethylaminoborane (22%), and one minor
Figure 2. Time-dependent 11B-NMR spectra (160.4 MHz, 300K, C6D6-capillary)
of the DMAB-dehydrogenation with catalyst 1 (0.2 mmol DMAB in 2.5 mL THF).
1
BH3-containing compound (quartet at -9.5 ppm, JB-H = 134 Hz)
was observed. The less bulky pre-catalyst 2 was far less selective
as illustrated by the observation of significant quantities of
N,Nʹ-dimethylaminoborane (19%) and unknown BH3-containing
species (17%). We therefore employed pre-catalyst 1 for further
dehydrogenation studies.
A kinetic analysis by 11B-NMR spectroscopy showed that the
reaction likely proceeded through
involving the linear intermediate B (Me2N-BH2-NMe2-BH3) and the
unsaturated intermediate (Me2N=BH2) (Scheme 1). As
a stepwise mechanism
C
proposed by Schneider and co-workers for dehydrogenations
catalyzed by a Ru-amido pincer complex and by Weller and
co-workers with a cationic Rh-phosphine complex,[21] the loss of
two molecules dihydrogen operates over two steps when the
Scheme 1. Dehydrogenation of dimethylamine-borane (NMe2HBH3 = DMAB)
(top); proposed mechanism based on observed intermediates (bottom).
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The dehydrogenation of ammonia borane (AB) is of particular
interest due its high hydrogen content of 19.6 wt%.[1] The
cyclopentadienyl carbonyl cobalt and tripodal phosphine cobalt
complexes reported by the groups of Waterman and Shubina,[8b,c]
respectively, are the only previously reported molecular cobalt
catalysts for the AB-dehydrogenation (Figure 1).[9] Hence, we
sought to compare the properties of pre-catalyst 1 with these
benchmark systems that both operate at elevated temperature
(65°C). When 1 (5 mol%) was added to a solution of AB in THF,
the evolution of H2 commenced immediately. This indicates a
rapid onset of catalytic dehydrogenation already at ambient
temperature. The characterization of reaction intermediates
(Scheme 3) was performed by 11B-NMR spectroscopy. The
starting material AB was completely consumed after 24 h.
Borazine (30 ppm) and polyborazine (26 ppm) were identified as
the two main soluble products. However, it is noteworthy that a
white precipitate formed during the reaction in THF. This solid was
studied by magic angle spinning (MAS) 11B NMR spectroscopy
with proton decoupling and cross-polarization (11B-CPMAS-NMR)
as well as without cross-polarization from protons and with proton
coupling. The 11B MAS NMR spectrum of this material showed
two signals at 2 ppm and -19 ppm (Figure 4). Proton decoupling
(11B{1H} MAS NMR) reduced the linewidth of the -19 ppm
resonance while it did not affect the signal at -2 ppm. The intensity
of the former signal was strongly enhanced in the 11B{1H} CPMAS
spectrum. Thus, this signal should be assigned to a boron atom
bonded to hydrogen(s). In contrast, the signal at -2 ppm may be
assigned to a boron atom bearing no H atoms. We believe that
this solid is polyaminoborane for which similar solid-state NMR
data, particularly similar chemical shifts, were reported by
Schneider and co-workers.[8]
Scheme 2.
N-methylamine-borane (b).
Dehydrogenation
of
diisopropylamine-borane
(a)
and
Furthermore, we studied catalytic dehydrogenations with the
sterically more demanding diisopropylamine-borane and the
primary N-methylamine-borane (Scheme 2). These reactions
have rarely been reported under base metal catalysis.[8],[23]
Diisopropylamine-borane exclusively afforded the iminoborane
iPr2N=BH2 after 72 h in THF, which exhibited the characteristic
triplet at 34.8 ppm in the 11B NMR spectrum (Figure 3, top). The
formation of oligomeric [MeHN-BH2]n (n = 6-11) from
N-methylamine-borane was corroborated by ESI-MS and
11B-NMR spectroscopy. The ESI-MS spectra showed peaks at
m/z 186.3 to 443.6 at intervals of 43.1 (corresponding to the
monomeric unit H2B-NMeH, see Figures S19-S21). 11B-NMR
spectra recorded in THF displayed a broad triplet at -4.8 ppm with
the typical line broadening of 1JBH = 106 Hz. Significantly broader
peaks are expected for a polymer (Figure 3, bottom).[24]
Figure 3. 11B{1H}-NMR (128.4 MHz, 300K, C6D6) of dehydrogenation products
of diisopropylamine-borane- (top) and N-methylamine-borane (bottom) in THF.
Figure 4. 11B-NMR spectra at 300 K of polyaminoborane; MAS at 6 kHz; relax
= relaxation.
Mechanistic studies of dehydrogenation. AB dehydrogenation
was directly monitored by H2 evolution at 1-12.5 mol% catalyst
loading (Figure 5). An induction period was apparent at low
catalyst concentrations, indicating that 1 might act as a pre-
catalyst that is converted to the active catalyst species under
reaction conditions. The formation of 0.5 equivalents H2 per AB
Scheme 3. Dehydrogenation of AB catalyzed by 1 (5 mol%).
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was observed within the first 2 min with 5 mol% (10 mM) catalyst.
Subsequently, the reaction became much slower, indicating
catalyst deactivation and possibly a change in the reaction
mechanism. A plot of the initial rates vs. catalyst concentrations
(SI, Figure S16) showed a 2nd order rate in catalyst. A linear
relationship between reaction rate and substrate concentration
from 50 – 200 mM was established from dehydrogenations with
different AB concentrations and constant catalyst concentration
(SI, Figure S18). Higher substrate concentrations afforded no
significant enhancement of the initial rate constant. Based on
these data, the following rate law can be formulated:
species.[25] The analysis of changes of catalyst activity by the
presence of selective catalyst poisons is an instructive tool for the
distinction between homotopic and heterotopic catalysis
pathways.[19],[26] Mercury (675 equiv. per [Co]) and P(OMe)3
(0.2 equiv. per catalyst) barely had an influence on the overall
reaction rate (5 mol% catalyst, see Figure 7). Both additives are
known to selectively poison heterogeneous catalysts.[19],[25],[26]
A
complementary experiment was performed with the strong
π-ligand dibenzo[a,e]cyclooctatetraene (dct), which selectively
deactivates soluble metal complexes in low oxidation states and
therefore is a powerful poison of homogeneous catalysts.[19],[25]-[27]
Addition of AB to a solution of the catalyst (5 mol% 1) and dct
(2 equiv. per [Co]) significantly slowed down the reaction. The
inhibition was not complete as dct underwent partial
hydrogenation to E/Z-dibenzocyclooctene and dibenzo-
cyclooctane (GC-MS). These poisoning studies support the
notion of a homotopic reaction mechanism.
d(H2)/dt = k·[catalyst 1]2·[NH3BH3]
(1)
Figure 5. Dehydrogenation of AB catalyzed with different catalyst loading of 1.
Reaction conditions: 0.2 mmol (mM) AB in THF (1 mL) at 25°C.
Figure 6. Observation of kinetic isotope effects in the dehydrogenation of
ammonia borane. Reaction conditions: 5 mol% 1, 0.2 mmol AB, THF (1 mL),
25°C.
Further mechanistic evidence was gathered from GC-MS
investigations of the reaction mixtures, which documented the
formation of cyclooctene and cyclooctane arising from (partial)
hydrogenation of the 1,5-cyclooctadiene ligand in 1. No H2
formation was observed in control experiments with NMe3BH3 and
NH3BEt3. A crossover experiment with a substrate mixture of
NMe3BH3 and NH3BEt3 did not result in any H2 formation.
Consistently, no dehydrogenation products were observed by
11B-NMR. These results proved that the presence of H-N and H-B
entities within one molecule are required to enable
dehydrogenation of amine-boranes. In an effort to gain more
insight into the operating reaction mechanism, we performed
dehydrogenations of the deuterated species ND3BH3, NH3BD3,
and ND3BD3. Experiments with 5 mol% catalyst 1 and ND3BH3
revealed
a
kinetic isotope effect (KIE) k(NH3BH3)
/
k(ND3BH3) = 1.6 (2° KIE), while with NH3BD3 a negligible KIE
k(NH3BH3) / k(NH3BD3) of 0.9 was observed. This is strongly
indicative of a participation of a protic H-N in the rate determining
step. Fully deuterated ammonia borane (ND3BD3) showed a
strong KIE k(NH3BH3) / k(ND3BD3) of 2.0 (Figure 6).
Complementing the kinetic studies, we conducted poisoning
experiments in order to study the nature of the catalytically active
Figure 7. Poisoning experiments in the dehydrogenation of AB. Reaction
conditions: 5 mol% 1, 0.2 mmol AB, THF (1 mL), 25°C.
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The insight gained by these studies can be summarized in a
tentative mechanistic scheme (Scheme 3). CataIysis is initiated
by the (partial) hydrogenation the cyclooctadiene ligand. This
results in an induction period observed in the reaction-time
profiles at low catalyst concentrations. The poisoning studies
indicate that a homogeneous (molecular) catalyst is operative,
while the 2nd order rate law with respect to cobalt suggests that
the rate-determining step involves two cobalt atoms. While the
exact structure of the active species still remains obscure
presently, it should be noted that numerous transition metal
hydrides catalyze amine-borane dehydrogenation,[1] and there is
extended to imines and quinoline derivatives (Figure 9).
Hydrogenations of quinolines are of particular interest due to the
formation of 1,2,3,4-tetrahydroquinolines, which constitute key
motifs of several bioactive compounds.[30],[31]
literature precedent for dinuclear cobalt hydride complexes.[33]
A
dinuclear cobalt hydride species thus might be a plausible on-
cycle intermediate. The basic nature of the hydride ligands might
explain why N-H transfer appears to be rate determining in this
case.
Scheme 3. Summary of the mechanistic information gained for amine-borane
dehydrogenation and transfer transfer hydrogenation (X = CR2, NHRʹʹ).
Figure 8. Transfer hydrogenation of alkenes with 1 (5 mol%). Standard
conditions: alkene and AB (each 0.2 mmol), THF (1 mL); yields were
determined by quantitative GC vs. internal n-pentadecane. [a] 40 h.
Scope of transfer hydrogenations. Next, we expanded the
catalytic applications of 1 and 2 to transfer hydrogenations of C=C
and C=N bonds using AB as formal hydrogen donor. Only a few
molecular cobalt catalysts are known to be competent in transfer
hydrogenations of olefins and imines (Figure 1).[9] We performed
initial studies with the combination of NH3BH3 and
α-methylstyrene (SI, Table S1). Pre-catalysts 1 and 2 gave similar
results. Optimizations with 1 showed best activities and full
conversion at 5 mol% catalyst loading and equimolar
concentrations of alkene and AB (0.2 mol L-1 in THF, see SI:
Table S1). Allylbenzene, linear α-olefins, and 4-octene were
successfully hydrogenated under these conditions (Figure 8).
Complete hydrogenation of 1,1-diphenylethylene proceeded
within 40 h at ambient temperature. The reaction conditions were
compatible with ethers, esters, amines, CF3, F, and free alcohols
(Figure 8). Minor dehydrohalogenation (3%) was observed for
Very few heterogeneous catalysts for the transfer hydrogenation
of quinolines and related N-heterocycles were described by Beller
and co-workers,[28] while molecular catalysts are also scarce.[30]
Using catalyst 1, various quinolines was hydrogenated to
1,2,3,4-tetra-hydroquinolines at room temperature within 16 h.
The equimolar stoichiometry of quinolines and AB underlines the
high efficacy of catalyst 1 as 2 equiv. H2 per AB are being
transferred. Quinoxaline containing two C=N bonds was fully
hydrogenated.
Mechanistic studies of transfer hydrogenation. We
investigated the reaction-time profile of the hydrogenation of
α-methyl-styrene (blue curve in Figure 10). The reaction onset is
very fast (50% conversion after 3 min) and very similar to the
dehydrogenation of AB (Figure 5). The reaction between
α-methyl-styrene and AB under an atmosphere of 1 bar D2
showed no deuterium incorporation after 5 min (GC-MS, see SI:
Figure S31). This indicates a direct (i.e. intramolecular) hydrogen
4-chloro-α-methylstyrene.
Alkyl
cinnamates
underwent
competitive carbonyl hydrogenation to give 3-phenyl-1-propanol.
Challenging trisubstituted olefins such as 1-phenylcyclopentene,
1-phenylcyclohexene, and 1,1',2-triphenylethylene as well as
arene moieties remained untouched even at elevated
temperatures and with an excess of AB. Hydrogenation of such
unsaturated functions could be realized by applying external H2
pressure (vide infra). The scope of transfer hydrogenations was
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Hydrogenation of alkenes. The inefficacy of the transfer
hydrogenation protocol for sterically hindered and some
functionalized substrates prompted us to develop
a
hydrogenation protocol that would combine the rapid catalyst
activation mechanism by catalytic amounts of AB with a
hydrogenation
mechanism
in
the
presence
of
(super)stoichiometric amounts of H2 gas (Table 1).[34],[35],[36] Pre-
catalyst 2 proved slightly more active than 1 in the hydrogenation
of the model substrate 1,1’,2-triphenylethylene. With 3 mol% of 2,
the hydrogenation proceeded cleanly at 20 bar H2 and 60°C in the
presence of several amine-boranes as catalyst activators. Amines
.
and BH3 THF were unreactive; NMe2HBH3 fared much poorer
than AB.
Table 1. Screening of different additives in the hydrogenation of
1,1’,2-triphenylethylene with catalyst 2.
Figure 9. Transfer hydrogenation of imines. Standard conditions: substrate
(0.2 mmol), THF (1 mL); yields were determined by quantitative GC vs. internal
n-pentadecane. Conditions for isolated (isol.) substrates: substrate (0.4 mmol),
THF (2 mL).
Additive
w/o
Yield (conversion) in [%]
transfer from AB to the alkene (see Scheme 3 above) which is
orders of magnitude faster than the reduction of the alkene by D2.
0 (0)
92
NMe2HBH3
NH3BH3
Furthermore, this observation argues against
a stepwise
mechanism involving H2 formation from AB followed by cobalt-
catalyzed hydrogenation of the alkene. Deuterated cumenes
(appr. 15-20%, mostly cumene-d1, little cumene-d2-7) were only
observed after long reaction times (16 h, GC-MS, see SI: Figures
S30 and S32).[32] Catalyst poisoning studies with dct suggested
that the reaction follows a homotopic mechanism. The reaction
was immediately inhibited after dct addition at 50% conversion
(1.0 equiv. per [Co], Figure 10). The partial hydrogenation of the
catalyst poison dct to a mixture of dibenzocyclooctene and
dibenzocyclooctane resulted in the recovery of low catalyst
activity after a few minutes (GC-MS).
> 99
[b]
NH3BH3
> 99
NEt3
0 (13)
1 (14)
0 (12)
2 (52)[c]
Pyridine
Piperidine
BH3·(THF)
[a] Standard conditions: 2 (3 mol%), substrate (0.2 mmol) in THF (1 mL).
Yields of hydrogenated products were determined by quantitative GC vs.
internal n-pentadecane. [b] catalyst 1 instead of 2. [c] possibly due to
hydroboration of triphenylethylene.
The general conditions were applied to a series of trisubstituted
olefins (Figure 11). It is noteworthy that, unlike the transfer
hydrogenation protocol, no dehalogenation was observed for
4-halo-α-methylstyrenes (X = Cl, Br) under these hydrogenation
conditions. Naphthalene and pinene were hydrogenated at
elevated temperature.
Conclusions
We have shown that for the first time that highly reduced cobalt
anions such as [K(thf)1.5{
DippBIAN)Co(η4-cod)}] (1) can be used as
active catalysts for the dehydrogenation of ammonia borane (AB)
and related amine-boranes under mild conditions. The activitiy of
1 surpasses that of other molecular cobalt catalysts by
Figure 10. Catalyst poisoning in the transfer hydrogenation with 1.
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Acknowledgements
We thank the mass spectrometry department of the University of
Regensburg for ESI-MS analysis, Matthew J. Margeson
(University of Regensburg) for experimental assistance, and
Christoph Ziegler, Philipp Büschelberger, and Dr. Daniel J. Scott
(University of Regensburg) for valuable comments on the
manuscript. Financial support by the Fonds der Chemischen
Industrie
(fellowship
to
T.M.M.),
the
Deutsche
Forschungsgemeinschaft (JA 1107/6-1, WO 1496/6-1), and the
European Research Council (CoG 683150) is gratefully
acknowledged.
Keywords: cobalt • catalysis • amine-boranes • transfer
hydrogenation • dehydrogenation
Figure 11. Substrate scope of alkene hydrogenations involving AB-mediated
catalyst activation. Bonds in blue indicate sites of π-bond hydrogenation.
Standard conditions: 0.2 mmol alkene, THF (1 mL). Yields determined by
quantitative GC-FID vs. internal n-pentadecane; conversions given in
parentheses if <90%.
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Waterman[10] and Shubina[11] (Figure 1), which require elevated
temperatures for an effective dehydrogenation reaction.
Pre-catalyst 1 displayed a similar activity as Peter’s PBP pincer
complex[9] for the dehydrogenation of DMAB. A mixture of
polyaminoborane, borazine, polyborazine was obtained using
catalyst 1, indicating that >1 equiv. H2 was released from AB.
Reaction monitoring and poisoning experiments strongly indicate
the operation of a homotopic catalyst. Transfer hydrogenation of
olefins, imines, and quinolines have attracted increased attention
only recently.[28],[29] Catalyst 1 is also able to catalyze such
transformations effectively, which involved the transfer of up to 2
equiv. H2 from AB. Mechanistic studies documented that the rate-
determining step likely involves proton transfer from the amine-
borane, while the rate law suggested that more than one Co atom
may be involved. Poisoning experiments again supported a
homogeneous mechanism. [K(thf){MesBIAN)Co(η4-cod)}] (2)
exhibited similarly good catalytic activity in the transfer
hydrogenation reaction between AB and alkenes/imines. A
related protocol was used for the hydrogenation of challenging
trisubstituted olefins which involved catalyst activation by AB and
subsequent hydrogenation under 10 bar H2. This initial study
demonstrates the significant potential of highly reduced -diimine
cobaltate for (de)hydrogenation reactions for the first time. The
results have direct ramifications for the development of related
reductive transformations and H2 storage processes under base
metal catalysis, which are the subject on on-going investigation in
our laboratories.
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See the supporting information for experimental details.
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Entry for the Table of Contents
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A new class of cobalt catalysts with
redox-active bisiminoacenaphthene
diimine (BIAN) ligands efficiently
catalyzes transfer hydrogenation and
dehydrogenation reactions of amine-
boranes. Alkenes, imines and
quinolines were effectively reduced
with ammonia borane (AB) as a
dihydrogen surrogate. Moreover, AB
can be used as an initiator for the
hydrogenation of highly challenging
trisubstituted olefins.
Thomas M. Maier, Sebastian Sandl,
Ilya G. Shenderovich, Axel Jacobi von
Wangelin*, and Robert Wolf*
Page No. – Page No.
Amine-borane Dehydrogenation and
Transfer Hydrogenation Catalyzed
by α-Diimine Cobaltates
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