Heterolytic cleavage of hydrogen molecule by rhodium thiolate complexes that catalyze chemoselective hydrogenation of imines under ambient conditions

Article abstract of DOI:10.1021/ja905835u

(Chemical Equation Presented) The bis(thiolate)Rh(III) complex having a tris(3,5-dimethylpyrazolyl)borate (Tp^Me2) coligand [Tp ^Me2Rh(SPh)₂(MeCN)] reacts reversibly with H₂ to form the hydridothiolato complex [Tp^Me2RhH(SPh)(MeCN)] and PhSH. The benzenedithiolate analogue [Tp^Me2Rh(o-S₂C ₆H₄) (MeCN)], which can also heterolytically activate H₂, catalyzes hydrogenation of imines under ambient temperature and pressure with high chemoselectivity.

Full text of DOI:10.1021/ja905835u

Published on Web 09/29/2009
Heterolytic Cleavage of Hydrogen Molecule by Rhodium Thiolate Complexes
That Catalyze Chemoselective Hydrogenation of Imines under Ambient
Conditions
Yoshiyuki Misumi, Hidetake Seino,* and Yasushi Mizobe*
Institute of Industrial Science, The UniVersity of Tokyo, Komaba, Meguro-ku, Tokyo 153-8505, Japan
Received July 14, 2009; E-mail: ymizobe@iis.u-tokyo.ac.jp; seino@iis.u-tokyo.ac.jp
Heterolytic activation of dihydrogen is a key step in catalytic
hydrogenation of polar bonds and hydrogen metabolism mediated
by hydrogenases. Many transition-metal complexes, especially
electron-poor ones, are known to be capable of cleaving H2
generally in the manner that H+ is split off from the highly acidic
H2 ligand to leave H- on the metal center.1 In this conversion, the
proton is accepted by an external Lewis base or more efficiently
by an internal ancillary ligand, as is typically observed in
metal-ligand bifunctional catalysts represented by organoamide
complexes.2 Anionic S-donor ligands, RS- and S2-, are also
expected to become internal proton acceptors, and some mecha-
nisms proposed for the function of [NiFe]-hydrogenases postulate
proton transfer from the coordinated H2 to the Ni-bound terminal
S(Cys) moiety.3 However, such reactions proceeding on thiolate
complexes are uncommon.4-6 There are only a few cases that
clearly confirm the formation of M(H)-S(H)R from M-SR with
As reversible heterolysis of H2 molecule at the Rh-S bond in 1
was disclosed, catalytic hydrogenation was examined to probe the
reactivity of the resulting hydrogen atoms. As shown in Table 1, 1
was found to be effective for the hydrogenation of styrene and
N-benzylideneaniline under 1 atm H2 with low to moderate activity
at 20-50 °C (entries 1-4). Although higher activity was expected
toward the more polarized CdO bond, benzaldehyde and acetophe-
none were not hydrogenated under these conditions. Isolated 2
showed slightly higher activity than 1 toward styrene, implying
that the active species in CdC reduction is close to 2 (entry 5).
However, conversion of N-benzylideneaniline was much deterio-
rated when 2 alone was used, and therefore, not only the Rh-H
but also the S-H hydrogen are essential for hydrogenating the
CdN bond (entry 6). The activity of the diiodo complex
[TpMe2RhI2(MeCN)] (3) toward this substrate was also poor (entry
7), but the Se analogue of 1, [TpMe2Rh(SePh)2(MeCN)] (4),7c was
found to be much more active than 1, even at 20 °C (entry 8).
Whereas 3 remained almost intact under a H2 atmosphere at 50
°C, 4 formed the hydrido complex analogously to 1. These results
indicate that catalytic activity for the hydrogenation of CdN bonds
sharply depends on the ability to heterolyze H2 molecules and that
this is strongly affected by ligating elements.
5
H2 and scarce applications to catalysis.6
We have been engaging in studies involving activation of small
molecules by transition-metal thiolate complexes.7 We herein report
that the bis(thiolate)Rh(III) complex having a tris(3,5-dimethylpyra-
zolyl)borate (TpMe2) coligand, [TpMe2Rh(SPh)2(MeCN)] (1),7b
reacts reversibly with H2 to form the hydridothiolato complex
[TpMe2RhH(SPh)(MeCN)] (2) and PhSH, as shown in Scheme 1.
On the basis of this heterolysis of H2, a hydrogenation catalyst that
operates under ambient temperature and pressure with high
chemoselectivity toward imines has been developed.
Table 1. Hydrogenation of PhCHdZ Catalyzed by
[TpMe2RhX1X2(MeCN)]a
entry
catalyst (X1; X2)
Z
temp (°C)
time (h)
yield (%)b
1
2
3
4
5
6
7
8
1 (SPh; SPh)
1 (SPh; SPh)
1 (SPh; SPh)
1 (SPh; SPh)
2 (H; SPh)
2 (H; SPh)
3 (I; I)
4 (SePh; SePh)
5 (o-S2C6H4)
5 (o-S2C6H4)
CH2
CH2
NPh
NPh
CH2
NPh
NPh
NPh
NPh
NPh
20
50
20
50
50
50
50
20
20
20
20
10
20
10
10
10
10
6
16
72
21
63
86
18
27
99
98
48
Scheme 1
9
1
2
10c
a Conditions: substrate (1.00 mmol), catalyst (0.01 mmol), THF
(5 mL), H2 (1 atm). b Yields of ethylbenzene and N-benzylaniline were
determined by GLC analyses. The sum of the yields of product and
remaining substrate was no less than 98% in each reaction. c Conducted
in benzene (5 mL).
Treatment of a 10 mM solution of 1 in C6D6 with 1 atm H2 at
20 °C for 2 h gave an equilibrium mixture containing 1, 2, and
PhSH in a 1:10:10 ratio. When this mixture was set under a N2
1
atmosphere, 1 was slowly regenerated. The H NMR spectrum of
2 showed a doublet at δ -13.80 (JRhH ) 11.6 Hz) due to the hydrido
ligand and 10 singlets corresponding to the three inequivalent
pyrazolyl groups (two methyls and one ring proton for each) and
MeCN. Although we previously found 2 in the reaction of
[TpMe2Rh(C8H14)(MeCN)] with 1 equiv of PhSH, its isolation was
hampered by simultaneous production of 1 (1/2 ≈ 1:2).7e Here,
addition of hexane to an equilibrium mixture prepared from 1 under
a H2 atmosphere gave yellow crystals of pure 2, with which the
structure was fully determined by X-ray crystallography (see the
Supporting Information).
To improve the catalytic efficiency, the benzenedithiolato
complex [TpMe2Rh(o-S2C6H4)(MeCN)] (5) was newly synthesized
according to Scheme 2. The active intermediate generated from 5
may contain both RhH and SH moieties within the same molecule.
As expected, 5 achieved much more rapid hydrogenation of
N-benzylideneaniline at 20 °C than the other complexes mentioned
above (Table 1, entry 9). Moreover, it was found that reduction of
the CdC bond is suppressed and inertness toward the CdO bond
is still preserved, as shown in Table 2. Thus, the CdN bonds in
various aldimines are efficiently hydrogenated under ambient
9
14636 J. AM. CHEM. SOC. 2009, 131, 14636–14637
10.1021/ja905835u CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
temperature and pressure with coexisting CdC or CdO functions
unaffected, with the exception that partial reduction occurred for
the CdC bond conjugated with the CdN group (entry 3). On behalf
of inertness to CdO bonds, reductive amination could be performed
by mixing aldehyde and primary amine directly under hydrogenation
conditions (entry 5). Conversion of a ternary iminium salt into a
tertiary ammonium salt also took place quantitatively (entry 6).
of hydride and proton to the CdO bond,6 as is widely accepted for
bifunctional molecular catalysts.2
It is still unclear whether deprotonation from the H2 adducts of
Rh thiolate complexes occurs directly at the stage of η2-H2 or after
formation of Rh(H)-S(H) species. However, their catalytic func-
tions may have some relevance to [Fe]-hydrogenases, which
generate a proton and transfer a hydride to an organic molecule at
a monoiron site bound to a cysteine residue.12,13
Scheme 2
Acknowledgment. This work was supported by Grants-in-Aid
for Scientific Research [18065005 on Priority Area “Chemistry of
Concerto Catalysis” and 21350033 (B)] from the Ministry of
Education, Culture, Sports, Science, and Technology, Japan.
Supporting Information Available: Experimental details and X-ray
analysis data for 2 (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
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