.
Angewandte
Communications
DOI: 10.1002/anie.201400099
Hydrogen Transfer
A Concerted Transfer Hydrogenolysis: 1,3,2-Diazaphospholene-
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Catalyzed Hydrogenation of N N Bond with Ammonia–Borane**
Che Chang Chong, Hajime Hirao,* and Rei Kinjo*
Abstract: 1,3,2-diazaphospholenes catalyze metal-free transfer
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hydrogenation of a N N double bond using ammonia–borane
under mild reaction conditions, thus allowing access to various
hydrazine derivatives. Kinetic and computational studies
revealed that the rate-determining step involves simultaneous
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breakage of the B H and N H bonds of ammonia–borane.
The reaction is therefore viewed as a concerted type of
hydrogenolysis.
A
mmonia–borane (AB; H3NBH3), an air-stable and non-
flammable white solid, has a high gravimetric hydrogen
density (19.6 wt%) and small molecular weight
a
(30.87 gmolꢀ1). In the past decade, not only the challenge of
using AB as a H2 storage material[1,2] but also the develop-
ment of synthetic methodologies utilizing AB directly as
a source of hydrogen has received considerable attention.[3]
For the latter, transfer hydrogenation using a source in place
of H2 gas is considered to be greatly advantageous over the
conventional methods for hydrogenation of unsaturated
compounds in organic synthesis. Therefore, a deep mecha-
nistic comprehension of elementary hydrogen-transfer steps
involved therein is essential to establishing excellent transfer-
hydrogenation systems.[4,5] Recently, Berke and co-workers
reported the metal-free stoichiometic hydrogenation of
organic molecules such as polar olefins, aldehydes, ketones,
and imines, using AB.[6] Importantly, the hydrogen transfer
from AB to imines was proven to follow a concerted
mechanism involving a six-membered transition state (Sche-
me 1a).[6d,7] An analogous mechanism was also predicted for
the process of Meerwein–Pondorf–Verley reduction.[8] Sim-
ilarly, Manners et al. demonstrated the hydrogen transfer
between amine-boranes and aminoboranes also occurs bimo-
lecularly in a single-step process.[9] A concerted pathway for
a homopolar H2 exchange process between ethane and
ethylene was proposed to involve a very high barrier based
on density functional theory (DFT) calculations.[10] These
results indicate only hydrogen acceptors with polar p bonds,
Scheme 1. Concerted elementary processes for transfer hydrogenation
(a) and transfer hydrogenolysis (b; X, Y, X’, and Y’: fragments of main
group elements). c) I(PIII)/II(PV) redox catalysis for transfer hydrogena-
tion of azobenzene with ammonia–borane.
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such as C N or B N, may lower the activation barrier to
enable the facile, direct, double hydrogen transfer. To the best
of our knowledge, hydrogenolysis, which involves a concerted
hydrogen transfer to the single bond of a hydrogen acceptor
has never been reported (Scheme 1b).
Recent breakthroughs in using p-block elements as
mimics for transition metals demonstrate that various main-
group compounds featuring both strong electron-donor and
electron-acceptor sites can activate small molecules.[11] There
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are a few reports on the activation of B H and N H bonds of
AB by main-group compounds; however, their applications in
catalysis still remain highly challenging.[12,13] Radosevich and
co-workers showed that a catalytic amount of the phosphorus
compounds I promotes a transfer hydrogenation of azoben-
zene using an excess amount of AB (Scheme 1c).[14] Remark-
ably, the highly strained T-shape geometry of I allows the
phosphorus center to possess both donating and accepting
orbitals which interact effectively with protic and hydridic
hydrogens from AB. As a result, the reaction of I and AB
gave the dihydridophosphorane II, which was shown to be the
resting state of the catalyst. Note that the unique I-
(PIII)$II(PV) redox mechanism is critical to building the
catalytic cycle. All these pioneering studies encouraged us to
[*] C.-C. Chong, Prof. Dr. H. Hirao, Prof. Dr. R. Kinjo
Division of Chemistry and Biological Chemistry, School of Physical
and Mathematical Sciences, Nanyang Technological University
Nanyang Link 21,Singapore 637371 (Singapore)
E-mail: hirao@ntu.edu.sg
[**] We are grateful to Nanyang Technological University and the
Singapore Ministry of Education (MOE2013-T2-1-005) for financial
support, and Dr. Rakesh Ganguly and Dr. Yongxin Li (Nanyang
Technological University) for assistance in X-ray crystallographic
analysis.
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investigate whether even a simple X’ Y’ single bond can
activate AB when the bond is strongly polarized (cX’ ! cY’).
Thus, a negatively polarized Y’ may act as a donor to form
Supporting information for this article is available on the WWW
3342
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 3342 –3346