Chelating bis-carbene rhodium(III) complexes in transfer hydrogenation of ketones and imines

Article abstract of DOI:10.1039/b109491b

Chelating rhodium(III) carbene complexes are accessible via a simple synthesis and are catalytically active for hydrogen transfer from alcohols to ketones and imines.

Full text of DOI:10.1039/b109491b

Chelating bis-carbene rhodium(III) complexes in transfer hydrogenation
of ketones and imines†
Martin Albrecht,a Robert H. Crabtree,*a Jose Matab and Eduardo Peris*b
a Yale Chemistry Dept., 225 Prospect St., New Haven, CT 06520, USA. E-mail: robert.crabtree@yale.edu
b Departamento de Química Inorgánica y Orgánica, Universitat Jaume I, E-12080, Castellón, Spain.
E-mail: eperis@qio.uji.es
Received (in Purdue, CA, USA) 17th October 2001, Accepted 13th November 2001
First published as an Advance Article on the web 6th December 2001
Chelating rhodium(III) carbene complexes are accessible via
a simple synthesis and are catalytically active for hydrogen
transfer from alcohols to ketones and imines.
over 16 h in the presence of NaOAc and KI to give 2a–c
(46–78% yield).‡ In an alternative route, 1a–c react with
[Rh(OAc)2]2 in refluxing EtCN over 10 h to give 2a–c in ca.
50% yield. The air stable red complexes were purified by
column chromatography. Mechanisms for these syntheses are
not understood and, in view of the redox processes necessarily
involved, they may well be complex.
Homogeneous organometallic catalysis has long depended on
phosphine ligands, PR3.1 Only recently have N-heterocyclic
carbenes (NHCs) offered the promise of an alternative ligand
environment for organometallic catalysis.2 The precursor
imidazolium salts are usually easier to synthesize than are PR3,
but have proved harder to install on the metal—often requiring
n-BuLi treatment for prior generation of the free carbene.2
To be useful as spectator ligands, carbenes must survive the
conditions of the catalytic reaction. Once bound to a metal,
monodentate carbenes can be reactive, however, undergoing
reductive elimination with alkyl groups in some cases.3 We4
and others2,5,6 have therefore been developing chelate carbenes
that should better resist degradation under catalytic conditions
by benefiting from the chelate effect and stereoelectronic
barriers to undesired degradation pathways. Palladium com-
plexes of NHCs have been most studied;2–4 Rh, also very
catalytically active,1 is rarely studied with polydentate C,C
donors.6
We now report direct metallation of the imidazolium
precursors to give [Rh(bis-carbene)I2(OAc)] (2a–c) and their
application to catalytic alkene isomerization and transfer
hydrogenation of ketones and imines.
N-Alkyl imidazoles readily react with CH2I2 in toluene at 120
°C to give the precursor iodide salts 1a,b in high yields (Scheme
1).7 The o-phenylene-bis-imidazolium precursor 1c has also
been prepared by alkylation of o-phenylene diimidazole.8 We
now find that 1a–c react with [(cod)RhCl]2 in refluxing MeCN
Evidence for a C,C-bidentate chelating bonding mode of the
carbene ligand in 2 comes from NMR spectroscopy, which
1
shows that the imidazole rings are symmetry-related. The H
NMR spectrum of 2b, for example, reveals one set of signals for
the isopropyl groups, and a sharp singlet for the methylene
protons around d 6.2. In the 13C{1H} NMR spectrum, the
carbene signal is observed as a doublet at d 154 with coupling
constants that are diagnostic for Rh binding (1JCRh 43 Hz).
One derivative formed crystals suitable for X-ray diffrac-
tion.§ Fig 1 shows the structure of 2c. The carbene is chelating
with a ruffled conformation and a bite angle (C1–Rh1–C11) of
92.2(4)°. The Rh–C distances, 1.992(9) and 2.000(10) Å, are
normal for Rh–C s bonds and imply a symmetric ligand
coordination mode. The high trans effect of the carbenes is
evident in the rather long Rh–OAc distances (Rh–O 2.166(6),
2.181(6) Å) compared to those in the parent carboxylate
complexes [Rh(OAc)2(L)]2 (2.01–2.06 Å).9
Complex 2b (0.1 mol%) catalyses the hydrogenation of CNO
and CNN groups (eqn. 1) via hydrogen transfer from i-PrOH/
KOH at 82 °C.10
(1)
RCHNE + Me2CHOH ? Me2CO + RCH2–EH
Aryl and alkyl ketones are converted to the corresponding
alcohols in good yields (Table 1), though benzophenone, entry
5, required longer reaction times. Pyridine nitrogens do not
poison the catalyst: reduction of acetylpyridine occurs readily
Scheme 1 Synthetic routes: i, [(cod)RhCl]2, NaOAc, KI, EtCN; ii,
[Rh(OAc)2]2, MeCN.
Fig. 1 A view of 2c (50% probability level, hydrogen atoms omitted for
clarity). Pertinent bond lengths (Å) and angles (°): Rh1–C1 1.992(9), Rh1–
C11 2.000 (10), Rh1–I1 2.6744(12), Rh1–I2 2.6602(12), Rh1–O1 2.181(6),
Rh1–O2 2.166(6); C1–Rh1–C11 92.2(4), C1–Rh1–O1 162.8(3), C11–Rh1–
O1 104.7(3), O1–Rh1–O2 60.5(3), I1–Rh1–I2 174.92(4), C1–Rh1–I1
89.6(3), C1–Rh1–I2 95.2(3), C11–Rh1–I1 88.2(3), C11–Rh1–I2 93.2(3).
† Electronic supplementary information (ESI) available: spectroscopic data
for the rhodium(III) complexes. See http://www.rsc.org/suppdata/cc/
b109491b/
32
CHEM. COMMUN., 2002, 32–33
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Table 1 Catalytic transfer hydrogenation using the rhodium(III) catalystsa
Entry
Catalyst (mol %)
Substrate
Yield (%)
TON
1
2b
3
2b (0.1)
2b (0.1)
2b (0.1)
2b (0.5)
2b (0.1)
2b (0.1)
2b (0.1)
2b (0.005)
2b (0.1)
2b (0.1)
2b (0.1)
2c (0.1)
2c (0.1)
2c (0.1)
None
Acetophenone
Acetophenone
2-Acetylpyridine
4-Acetylpyridine
Benzophenone
> 98
17
> 98
> 98
1000
170
1000
200
890
0
1000
19000
1000
1000
1000
1000
950
4
5
6b
7
8
9
10
11
12
13
14c
15
72 (89 after 18 h)
0
> 98
> 98
> 98
> 98
85 ( > 98 after 18 h)
54 ( > 98 after 36 h)
78 (95 after 18 h)
80 ( > 98 after 18 h)
< 10
Benzophenone
Cyclohexanone
Cyclohexanone
Hexan-3-one
Benzylidene aniline
Benzylidene methylimine
Benzylidene methylimine
Benzophenone
Benzophenone
Acetophenone
1000
—
a 2 mmol substrate, 10 mL 0.1 M KOH in i-PrOH, reflux temperature for 10 h, unless stated otherwise; yields determined by 1H NMR or GC; TON = mol
product/mol catalyst. b At 25 °C. c In 2-BuOH as solvent, reflux temperature.
(entries 3 and 4). Cyclic and acyclic aliphatic ketones have been
hydrogenated, including ketones with substituents bulkier than
Notes and references
‡ Typical procedure for the synthesis of the bis-carbene rhodium(III
)
methyl groups (entries 7 and 9). Generally, 2b hydrogenates
aliphatic substrates faster than aromatic ones. For example,
formation of cyclohexanol from cyclohexanone is complete
within 6 h, whereas hydrogenation of benzophenone requires
more than 12 h. Attempts to carry out transfer hydrogenation at
room temperature had only limited success (entries 2 and 6):
acetophenone was converted slowly while benzophenone was
unreactive. No aldol condensation byproducts from the acetone
were observed. Incubation of the catalyst with KOH/i-PrOH at
82 °C prior to substrate addition did not improve the catalytic
performance,11 so activation of the catalyst system is fast.
Control reactions without Rh gave no significant transfer
hydrogenation.
Interestingly, 2b also catalyzes the reduction of imines to the
corresponding amines (entries 10 and 11). These N-substituted
benzylidene imines react slower than ketones.
The more rigid catalyst system 2c exhibited similar catalytic
activity to 2b. The reduction of benzylidene methylimine and
benzophenone proceeded slightly faster (entries 12 and 13). A
20 °C increase of the reaction temperature by changing the
solvent from i-PrOH to 2-BuOH did not improve catalyst
performance significantly (entry 14).
Transfer hydrogenation of alkene CNC double bonds failed
with our catalysts. Reactions with monosubstituted, terminal
olefins (allylbenzene) or disubstituted alkenes (cis or trans,
cyclooctadiene or trans-b-methylstyrene) gave no detectable
amounts of hydrogenation products under the reaction condi-
tions used for ketone and imine reduction. Attempted reductions
of conjugated enones as in 1-phenylbuten-3-one gave a mixture
of polymeric products that were poorly soluble in hexane.
Clearly, our catalyst systems are not applicable to this type of
substrate. Mechanistic studies are in progress but no inter-
mediates have proved isolable so far.
In summary, chelating bis-carbenes can be readily installed
on rhodium(III) under mild conditions. The products give robust
and air stable catalysts for hydrogen transfer.
We gratefully acknowledge financial support from the Swiss
National Foundation (M. A.), the NSF and DOE (R. H. C.), the
DGESIC (E.P., PB98-1044) and the Generalitat Valenciana for
a fellowship (J. M.).
complexes: a mixture of methylene-bis(N-isopropylimidazolium) diiodide
1b (488 mg, 1.0 mmol), NaOAc (328 mg, 4.0 mmol), KI (332 mg, 2.0
mmol) and [RhCl(cod)]2 (243 mg, 0.5 mmol) was stirred in EtCN (20 mL)
at reflux for 16 h. After cooling, volatiles were removed under reduced
pressure and the residue purified by gradient column chromatography.
Elution with CH2Cl2 gave a [RhX(cod)]2 fraction (X
= Cl, I) and
subsequent elution with CH2Cl2–acetone (3+1) gave 2b as an orange solid
(473 mg, 74%). Analytically pure material was obtained by crystallization
from CH2Cl2–heptane. Anal. Calc. for C15H23I2N4O2Rh (648.08): C 27.80,
H 3.58, N 8.65. Found: C 27.58, H 3.59, N 8.37%.
§ Crystal data for 2c: orange prisms (0.13 3 0.09 3 0.05 mm) Mw
=
=
738.20, monoclinic, space group P21/c (no. 14), a
= 8.654(3), b
17.372(6), c = 18.001(6) Å, b = 103.260(8)°, V = 2634.2(16) Å3, Z = 4,
Dc = 1.861 g cm23, m = 3.016 mm, Mo–Ka radiation (l = 0.71073 Å),
11164 reflections measured, 3212 unique, observed reflections (I
>
3.00s(I)) 280 parameters and converged with unweighted and weighted
agreement factors of R = 0.0424, Rw = 0.0940, S = 1.103.CCDC reference
number 172875. See http://www.rsc.org/suppdata/cc/b1/b109491b/ for
crystallographic data in CIF or other electronic format.
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