.
Angewandte
Communications
DOI: 10.1002/anie.201303500
Synthetic Methods
Metal-Free HB(C6F5)2-Catalyzed Hydrogenation of Unfunctionalized
Olefins and Mechanism Study of Borane-Mediated s-Bond
Metathesis**
Yuwen Wang, Weiqiang Chen, Zhenpin Lu, Zhen Hua Li,* and Huadong Wang*
Homogeneous hydrogenation of olefins is of great impor-
tance for both laboratory and industry synthesis.[1] While most
hydrogenation reactions are catalyzed by transition-metal
complexes, the development of metal-free hydrogenation
catalysts has recently attracted much attention because of
their potential benefits of low cost and toxicity.[2–4] Early
efforts towards the metal-free catalytic hydrogenation of
olefins or polyarenes employed boranes or diboranes, such as
iBu3B,[2b,c] Et2BBEt2, or Pr2BBPr2
as catalysts. These
[2e,f]
reactions require harsh reaction conditions (reaction temper-
ature at 170–2258C and H2 pressure at 100 bar), which often
led to unwanted pyrolytic chain cleavage. The recent emerg-
ing chemistry of frustrated Lewis pairs provides a new
strategy for metal-free hydrogenation which allows hydro-
genation to be carried out under mild reaction conditions.[3,4]
Various frustrated Lewis pair systems have been designed to
effectively hydrogenate unsaturated complexes, such as
imines,[4a–g] enamines,[4h] silylenol ethers,[4i] N-heterocyclic
compounds,[4j] ynones,[4k] polycyclic aromatic hydrocarbons,[4l]
amine-substituted benzenes,[4m] electron-rich alkenes,[4n] and
electron-poor alkenes.[4o,p] However, despite such progress,
metal-free hydrogenation of unfunctionalized, purely alkyl-
substituted olefins under mild reaction conditions still
remains an unmet challenge.
Scheme 1. Working hypothesis for the hydroborane-catalyzed hydro-
genation.
employed as catalysts to hydrogenate unfunctionalized ole-
fins. The working hypothesis is illustrated in Scheme 1. First,
hydroboration of olefins affords alkyl-substituted organo-
À
borane complexes. Subsequently, under H2, the C(alkyl) B
bond of these complexes can undergo hydrogenolysis to yield
the hydrogenation product and regenerate the catalyst. We
speculated that such hydrogenolysis could occur under
comparably mild reaction conditions when highly Lewis-
acidic organoboranes are applied.
Recently, several groups have reported that highly Lewis-
acidic borane complexes can react with H2 without Lewis
bases under mild reaction conditions.[5–7] For example, Piers
et al. reported that antiaromatic pentaarylboroles react with
H2 at ambient temperature.[5] Our group as well as Nikonov
et al. observed that highly Lewis-acidic hydroborane species
such as HBArF2 (ArF = 2,4,6-tris(trifluoromethyl)phenyl)[6] or
Cyclohexene was chosen as the standard substrate as its
hydrogenation product cyclohexane can be easily identified
1
by H NMR spectroscopy. Initially we sought to use HBArF
2
as the hydrogenation catalyst. However, upon mixing HBArF
2
and cyclohexene in toluene, no hydroboration took place
even at 1108C, possibly because of the large steric bulkiness
[7]
HB(C6F5)2 can activate D2 to afford the corresponding
around the boron center of HBArF .[8] We then examined
2
deuteroboranes at 608C. These discoveries prompted us to
examine if highly Lewis-acidic hydroboranes can be
HB(C6F5)2 as the catalyst for the hydrogenation reaction, as
HB(C6F5)2 is known to be a very reactive hydroboration
reagent for olefins.[9] When cyclohexene was mixed with
20 mol% of HB(C6F5)2 in C6D6 under 6 bar H2 at 1108C, the
hydrogenation product cyclohexane was obtained in 53%
yield after 72 hours. After the reaction temperature was
increased to 1408C, cyclohexene was quantitatively converted
into cyclohexane within 72 hours. For comparison, the less
Lewis-acidic 9-borabicyclo[3.3.1]nonane (9-BBN) was also
tested as the catalyst, and yielded no hydrogenation product
after 72 hours at 1408C.
Using HB(C6F5)2 as the catalyst, we performed the
catalytic hydrogenation of a variety of purely alkyl-substi-
tuted olefins (Table 1). For most mono-, di-, or trisubstituted
olefins, complete hydrogenation can be achieved after
72 hours. For some olefins (entries 2, 6, and 8), a longer
[*] Y. Wang,[+] W. Chen,[+] Z. Lu, Prof. Z. H. Li, Prof. H. Wang
Shanghai Key Laboratory of Molecular Catalysis and
Innovative Material, Department of Chemistry, Fudan University
Shanghai, 200433 (China)
E-mail: huadongwang@fudan.edu.cn
[+] These authors contributed equally to this work.
[**] Financial support from the Shanghai Science and Technology
Committee (10ZR1404200, Shanghai Rising-Star Program), Shang-
hai Leading Academic Discipline Project (B108), the National
Nature Science Foundation of China (21102018, 21273042), and the
National Basic Research Program of China (2011CB808505) is
gratefully acknowledged.
Supporting information for this article is available on the WWW
7496
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 7496 –7499