Ethanol as hydrogen donor: Highly efficient transfer hydrogenations with rhodium(I) amides

Article abstract of DOI:10.1002/anie.200704685

Catalysts take to the bottle: Rhodium amides with a saw-horse structure serve as very efficient catalysts for the transfer hydrogenation of ketones and activated olefins using ethanol as hydrogen donor. Under mild conditions, the corresponding alcohols and ethyl acetate are formed with high efficiency, with a turnover frequency above 500 000 h^-1. (Chemical Equation Presented).

Full text of DOI:10.1002/anie.200704685

Angewandte
Chemie
DOI: 10.1002/anie.200704685
Homogeneous Catalysis
Ethanol as Hydrogen Donor: Highly Efficient Transfer
Hydrogenations with Rhodium(I) Amides**
Theo Zweifel, Jean-Val›re Naubron, Torsten Büttner, Timo Ott, and Hansjörg Grützmacher*
Homogeneously catalyzed transfer hydrogenation has
become a powerful tool in synthetic chemistry, and a wide
range of unsaturated substrates can be employed in this
to a reported synthesis protocol (ArF = 3,5-(CF3)2C6H3, OT f=
CF3SO3ꢀ).[7] The structures of 1a and 1c were determined by
X-ray diffraction (Figure 1).[9] The cations in both complex
reaction.[1] Impressive activities (turnover frequencies TOF >
1 ” 106 hꢀ1
)
and selectivites have been reached. Rutheniu-
[2]
m(II) arene complexes and rhodium(III) cyclopentadienyl
complexes in combination with 2-propanol or formic acid/
triethylamine mixtures as hydrogen donors are among the
most popular catalytic systems.[3] Ethanol is a renewable
resource and has spurred considerable interest as an alter-
native to fossil fuels and as a potential feedstock for the
chemical industry.[4] Although reduced organometallic com-
plexes are often prepared by reacting a complex with the
metal in a higher oxidation state with ethanol (for example,
RhIII!RhI or RuIII!RuII), ethanol has not been investigated
systematically as a hydrogen source in transfer hydrogena-
tion.[5] This may be due to the fact that ethanol frequently
poisons the catalyst by forming stable and inactive carbonyl
complexes[6] and that under basic conditions, aldol condensa-
tion products are easily formed with acetaldehyde.
Figure 1. A) Ortep plot (ellipsoids set at 30% probability) of the
structure of 1. The BArF anion and carbon-bound hydrogen atoms are
4
omitted for clarity. Selected bond lengths [Š] and angles [8]: Rh1-N1
2.155(2), Rh1-P1 2.279(1), Rh1-ct2 2.079(8), Rh1-C5 2.193(3), Rh1-C4
We reported that the d8 RhI diolefin amide [Rh(trop2N)-
(PPh3)] (2a) is an active catalyst for ketone and imine
=
=
2.191(2), Rh1-C19 2.195(4), Rh1-C20 2.189(4), C4 C5 1.406(4), C19
C20 1.389(4); N1-Rh1-P1 173.1(2), ct1-Rh1-ct2 144.7(4). B) Ortep plot
(ellipsoids set at 30% probability) of the structure of 2. Carbon-bound
hydrogen atoms are omitted for clarity. Selected bond lengths [Š] and
angles [8]: Rh1-N1 2.147(1), Rh1-P1 2.203(1), Rh1-ct 1 2.074(4), Rh1-
ct2 2.133(5), Rh1-C4 2.180(2), Rh1-C5 2.201(2), Rh1-C19 2.241(1),
hydrogenation
with
H2
(trop2N = bis(5-H-dibenzo-
[a,d]cyclohepten-5-yl)amide).[7] We report herein that such
RhI amide complexes are very efficient catalysts for the
reaction in Equation (1).
=
=
Rh1-C20 2.247(2), C4 C5 1.412(2), C19 C20 1.396(2); N1-Rh1-P1
170.1(4), ct1-Rh1-ct2 145.5(5).
2 R2C¼O þ 2 EtOH ! 2 R2HCꢀOH þ MeCOOEt,
ð1Þ
In this reaction, ethanol serves as hydrogen donor and is
converted to acetic acid ethyl ester (ethyl acetate).[8] The
reaction in Equation (1) is practically irreversible and for
many substrates exothermic by about 10 kcalmolꢀ1. Conse-
quently, it should be possible to perform the transfer hydro-
genation (TH) in neat ethanol at high substrate concentra-
tions.
salts adopt saw-horse structures with N-Rh-P angles of about
1728 and ct-Rh-ct angles of about 1458 (where ct is the center
=
of the coordinated C C bond). There are no close contacts
between the cations and anions.[10] The NH functional groups
in the [Rh(trop2NH)(PR3)]+ cations in 1a–c are sufficiently
acidic[11] to be quantitatively deprotonated by KOtBu or
Li[N(SiMe3)2] to give the neutral amides [Rh(trop2N)(PR3)]
(Scheme 1; 2a: R = Ph, 2c: R = OPh). These react with two
equivalents of methanol or ethanol in stoichiometric reactions
to give quantitatively the hydrides [RhH(trop2NH)(PR3)]
(3a,c) and formic methyl ester (HCOOMe) or ethyl acetate
(MeCOOEt). The structures of a related amide 2 and amino
hydride 3 are known from our previous investigations (with
PR3 = PPh2tol),[7] and a comparison between 1, 2, and 3 shows
that there is little change in the corresponding bond lengths
and angles (see the Supporting Information).
The complexes [Rh(trop2NH)(PPh3)]+ OTfꢀ (1a), [Rh-
ꢀ
(trop2NH)(PPh3)]+ BArF
(1b), and [Rh(trop2NH){P-
4
(OPh)3}]+ OTfꢀ (1c) were prepared in high yield according
[*] T. Zweifel, Dr. J.-V. Naubron, Dr. T. Büttner, T. Ott,
Prof. Dr. H. Grützmacher
Department of Chemistryand Applied Biology
ETH-Hönggerberg, 8093 Zürich (Switzerland)
Fax: (+41)446-331-418
The rhodium amide complexes 2a,c are direct catalysts in
the reactions described below. But because of their sensitivity,
it is more convenient to use the easily storable amino
complexes 1a–c as catalyst precursors in combination with a
small amount of base (KOtBu or a suspension of potassium
E-mail: gruetzmacher@inorg.chem.ethz.ch
[**] This work was supported bythe Swiss National Science Foundation
(SNF) and the ETH Zürich.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3245

Communications
Scheme 1. Synthesis of amino olefin complexes 1a–c, the correspond-
ing amido complexes 2a,c and their reaction with methanol or ethanol
to give the amino hydride complexes 3a,c.
carbonate). No catalytic turnover was observed with 1a–c in
Scheme 2. Simplified catalytic transfer hydrogenation cycle by which
substrates 4–8 are quantitativelyconverted into the corresponding
alcohols with catalysts 2a,c.
the absence of base. The anion (OTfꢀ, BArF ꢀ) of the
4
precatalyst has no influence on the catalyst activity. Methanol
is not an efficient hydrogen donor in catalytic TH, and only a
few catalytic cycles were observed. However, the efficiency
with ethanol is excellent (Table 1, entries 1–3).[1,2] 2-Propanol
can be used as hydrogen donor but is less efficient and
requires more dilute conditions to obtain comparable con-
versions (Table 1, entries 11, 12).
reaction proceeds with TOF50 = 500000 hꢀ1 at room temper-
ature. The further results listed in Table 1 show that the
catalysts 2a,c tolerate a variety of functional groups and are
not deactivated by nitrogen donors (Table 1, entry 4). Nota-
bly, with the triphenylphosphite complex 1c as catalyst
precursor, ortho-bromoacetophenone (Table 1, entry 5) and
the nitroacetophenones o/m/p-6 (Table 1, entries 6–8) are
converted with high activity under mild conditions (408C,
1 mol% K2CO3).[12] No reduction of the nitro moiety was
observed, and no aldol-type condensations were detected,
Table 1: Transfer hydrogenations with complexes 1a–c as catalyst
precursors or amide 2a as catalyst. In all cases, greater than 98%
conversion was achieved.
EntrySubstrate
S/C
TOF
[hꢀ1
]
50
1
2
3
4
5
6
7
8
9
acetone[a]
100000
100000
100000
100000
5000
500000
750000
600000
300000
5000
ꢀ
despite the high C H acidity. The high efficiency with which
cyclohexanone[a]
electron-poor olefins such as acrylic acid methyl ester (7) or
itaconic acid dimethyl ester (8) are cleanly converted at
substrate/catalyst (S/C) ratios of 10000 under base-free
conditions is also remarkable (Table 1, entries 9 and 10).
Less-activated or electron-rich olefins such as styrene or 3,4-
dihydro-2H-pyrane are not hydrogenated.
acetophenone[a]
2-acetylpyridine (4)[b]
2-bromoacetophenone (5)[b]
2-nitroacetophenone (o-6)[b]
3-nitroacetophenone (m-6)[b]
4-nitroacetophenone (p-6)[b]
acrylic acid methyl ester (7)[c]
itaconic acid dimethyl ester (8)[c]
cyclohexanone[d]
5000
5000
5000
10000
20000
10000
10000
100000
10000
25000
300000
90000
150000
100000
Addition of
a large excess of triphenylphosphine
10
11
12
(100 equiv relative to 2a) had no influence on the catalystꢀs
activity. This finding supports our assumption that 2a is the
catalyst and not a species formed from 2a by PPh3 dissoci-
ation. The TH of acetophenone in [D5]ethanol resulted in
complete deuteration of the 1-position in the product 1-
phenylethanol (Scheme 3). When itaconic acid dimethyl ester
7 was transfer hydrogenated with [D5]ethanol, deuterium was
incorporated exclusively in the b-position of methylsuccinic
acid dimethyl ester. Furthermore, the amide 2a cleanly
dehydrogenates propionic acid methyl ester to give acrylic
acid methyl ester 8 and the hydride 3a.[14] These findings
suggest a Noyori-type mechanism for the transfer hydro-
acetophenone[d]
[a] 1a or 1b, 1 mol% KOtBu, substrate 2m in EtOH, room temperature.
[b] 1c, 1 mol% K2CO3, Substrate 2m in EtOH, 408C. [c] 2a, substrate 2m
in EtOH, room temperature. [d] 1a, 1 mol% KOtBu, substrate 0.5m in
iPrOH.
The performance of 2a,c is impressively demonstrated
when acetone, the byproduct in classical transfer hydro-
genation with 2-propanol as hydrogen donor, is quantitatively
=
converted to 2-propanol in the reaction Me2C O + 2EtOH!
[13e]
ꢀ
=
Me2CH OH + MeCO(OEt) (Table 1, entry 1; see also genation of activated C C double bonds.
Scheme 2). The computed reaction enthalpy for this reaction
The observation that the formation of ethyl acetate is
efficiently promoted by the isolated amide 2a in the absence
is DHr = ꢀ14 kcalmolꢀ1. Under the given conditions, this
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of any additional external base prompted us to investigate this
process by DFTcalculations (B3PW91/BS211B3PW91/BS1;
for computational details see the Supporting Information).
The mechanism is divided into two parts, which are displayed
in Figure 2. Step 1 shows the reaction of the model complex
[Rh(cht2N)(PH3)] (2’) with ethanol, leading to adducts A, B,
or C (cht = cycloheptatrienyl). Adduct A, in which ethanol is
merely H-bonded to the Rh amide nitrogen atom, is slightly
more stable than B and C, in which the oxygen center also
interacts with the Rh atom. Adducts A and B are intercon-
verted by inversion at the oxygen center and are in rapid
equilibrium. Adduct C, best described as an ethoxide com-
plex, is almost isoenergetic to B, and the activation barrier
Ea(B,C) via TS1 is very low (2.9 kcalmolꢀ1). Adduct A lies on
the reaction coordinate that leads to the formation of the
Scheme 3. Selective deuterium incorporation into acteophenone and
itaconic acid dimethyl ester and dehydrogenation of propionic acid
methyl ester promoted by 2a.
ꢀ
primary oxidation product acetaldehyde. We find that the O
H bond of the coordinated ethanol molecule is cleaved first
via TS2a, leading to intermediate D; subsequently, the a-CH
Figure 2. Formation of acetaldehyde (step 1) and ethyl acetate (step 2a or 2b) catalyzed by rhodium amide [Rh(cht2N)(PH3)] (2’) according to DFT
calculations.
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3247

Communications
obtained from CHCl3/n-hexane. M.p.: 2058C (decomp). 1H NMR
bond is broken via TS2b. However, TS2a is lower in energy
3
2
(500.1 MHz, CDCl3): d = 3.61 (dd, JPH = 5.7 Hz, JRhH = 2.2 Hz, 1H,
NH), 4.91 (ddd, 3JHH = 8.2 Hz, 3JPH = 2.5 Hz, 2JRhH = 0.2 Hz, 2H,
CHolefin), 5.25 (dd, 3JRhH = 1.4 Hz, 4JPH = 7.3 Hz, 2H, CHbenzyl),
6.40 ppm (ddd, 3JHH = 8.9 Hz, 2JRhH = 3.7 Hz, 3JPH = 2.8 Hz, 2H,
CHolefin). 13C NMR (101.6 MHz, CDCl3): d = 81.7 (d, 1JRhC = 7.3 Hz,
2C, CHolefin), 91.4 ppm (d, 1JRhC = 12.5 Hz, 2C, CHolefin). 31P NMR
than the intermediate D, and at this point we simply note that
the potential surface is very flat in this region of the reaction.
The maximal calculated barrier on the way from A to the
acetaldehyde adduct E is given by EZPE(TS2b)ꢀEZPE(A) =
7.5 kcalmolꢀ1.
1
(162.0 MHz, CDCl3): d = 40.3 ppm (d, JRhP = 143.5 Hz). 103Rh NMR
Dissociation of acetaldehyde from E to give the amino
hydride F is slightly endothermic. Overall, the dehydrogen-
ation of ethanol by the rhodium amide follows the well-
established mechanism of metal–ligand bifunctional cataly-
sis.[5a,13] Possible routes for the formation of ethyl acetate are
shown in steps 2a and 2b (Figure 2). In step 2a, a concerted
reaction is shown, which starts with the ethanol adduct A, to
which acetaldehyde (from step 1) is added. In a single step via
the transition state TS3, simultaneous nucleophilic attack of
the acetaldehyde carbonyl group by the oxygen atom of the
coordinated ethanol molecule and concerted transfer of the
OH and CH hydrogen atoms gives the rhodium amino
hydride F and ethyl acetate. The calculated activation barrier
for this process is low (8.2 kcalmolꢀ1). A second route is
shown in step 2b. The ethoxide complex C reacts with
acetaldeyhde to give the adduct G which immediately
rearranges via TS4 (which is slightly lower in energy than G
indicating again a flat potential surface in this region; see
discussion for D and TS2a above) to give the hemiacetal
complex H. The latter may easily rearrange into the reactive
conformation J. A concerted hydrogen transfer from the OH
and a-CH groups via the very low-lying transition state TS5
results in the exothermic formation of ethyl acetate and the
rhodium amino hydride complex F. The latter transfers
hydrogen to the substrate to give the hydrogenated product
under regeneration of catalyst 2’ (see the Supporting
Information for the reaction profile calculated for acetone
as substrate).
In summary, the rhodium amides 2a,c with a saw-horse
structure are highly efficient catalysts for the transfer hydro-
genation of ketones and activated olefins using ethanol as
hydrogen donor, which is irreversibly converted to ethyl
acetate. The reactions can be performed at high substrate
concentrations in neat ethanol at room temperature.
Although we do not exclude that the hemiacetal MeHC(OH)-
(OEt) is formed classically in a non-metal-assisted reaction
(and enters the catalytic cycle via H or J; see step 2b in
Figure 2), results from DFTcalculations show that its
formation may be also a metal-catalyzed reaction. Note that
according to the calculations only very low activation barriers
(less than 10 kcalmolꢀ1) are encountered along the reaction
path, which explains the high catalytic activity.
(12.6 MHz, CDCl3): d = 1053.1 ppm (d, 1JRhP = 144 Hz).
1c: [RhCl(trop2NH){P(OPh)3}] (150 mg, 0.18 mmol; see the
Supporting Information and reference [7]) and AgOTf (47 mg,
1.83 mmol, 1.03 equiv) were dissolved in CH2Cl2 (5 mL), and the
resulting suspension was stirred for 12 h and subsequently filtered
over a plug of celite. CH2Cl2 was removed under reduced pressure,
and the resulting red solid was recrystallized from acetone/n-hexane
and dried under vacuum. Yield: 162 mg, 0.169 mmol, 95%. Crystals
suitable for X-ray diffraction were obtained by layering a solution of
the complex in CH2Cl2 with n-hexane. M.p.: 2338C (decomp).
2
1H NMR (300.1 MHz, CDCl3): d = 5.09 (dd, 3JPH = 7.3 Hz, JRhH
=
1.0 Hz, 1H, NH), 5.32 (dd, 4JPH = 13.0 Hz, 2JRhH = 0.8 Hz, 2H,
CHbenzyl), 5.55 (dd, 3JHH = 8.6 Hz, 3JPH = 1.2 Hz, 2H, CHolefin),
6.63 ppm (ddd, 3JHH = 8.6 Hz, 2JRhH = 3.8 Hz, 3JPH = 2.9 Hz, 2H,
CHolefin). 13C NMR (75.5 MHz, CDCl3): d = 75.1 (br. s 2C, CHolefin),
79.8 ppm (d, 1JRhC = 11.7 Hz, 2C, CHolefin). 31P NMR (121.5 MHz,
1
CDCl3): d = 105.7 ppm (d, JRhP = 227.0 Hz). 103Rh NMR (12.6 MHz,
1
CDCl3): d = 1131.4 ppm (d, JRhP = 227.0 Hz).
Catalyses: Protocol 1: A solution of 1c in ethanol (1 mgmLꢀ1
,
1.04 mm) was added to a Schlenk tube containing a 2m solution of the
substrate in ethanol. For the solid substrates 3-nitroacetophenone and
4-nitroacetophenone,
a 1m solution in THF/ethanol (1:1) was
prepared. The solution was degassed by three freeze-pump-thaw
cycles, and 1 mol% solid K2CO3 was added under argon. The
suspension was warmed to 408C and the reaction monitored by
NMR spectroscopy. Protocol 2: Compound 2a in THF (1 mgmLꢀ1
,
1.1 mm) was added to a 2m solution of the substrate in ethanol. The
reaction was monitored by GC and NMR spectroscopy. TOF values
were determined after 50% conversion.
Received: October 10, 2007
Revised: November 8, 2007
Published online: March 17, 2008
Keywords: densityfunctional calculations · ethanol ·
.
homogeneous catalysis · rhodium · transfer hydrogenation
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Floch and co-workers (cyclohexanone: TOF = 1.33 ” 106 hꢀ1
;
acetophenone 1.2 ” 106 hꢀ1 at 908C and S/C = 20 ” 106 with a
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Experimental Section
A description of all experiments and detailed listing of spectroscopic
data is given in the Supporting Information. All experiments were
performed under argon.
1b: Compound 1a (103 mg, 0.113 mmol) and NaBArF (100 mg,
4
0.11 mmol) were dissolved in CH2Cl2 (10 mL). The solution was
stirred for 2 h. The formed NaOTf was removed by filtration over
celite. CH2Cl2 was removed under reduced pressure; the product was
washed with pentane and dried under vacuum. Yield: 165 mg,
0.10 mmol, 90%. Crystals suitable for X-ray diffraction could be
3248
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Angewandte
Chemie
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independent reflections, R1 = 0.0337 for 11225 reflections with
I > 2s(I) and wR2 = 0.0755, 595 parameters. CCDC-631186 (1a)
and 631185 (1b) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
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derivative [Rh(trop2NMe)(PPh3)]OTf: P. Maire, A. Sreekanth,
T. Büttner, J. Harmer, I. Gromov, H. Rüegger, F. Breher, A.
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[11] For related complexes, we determined pKa values below 19 in
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2005, 117, 6483; Angew. Chem. Int. Ed. 2005, 44, 6325; ; b) T.
Büttner, F. Breher, H. Grützmacher, Chem. Commun. 2004,
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symmetrical esters RCO(OR) has been reported recently: a) J.
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[9] Crystal data: 1a (C80H50BF24NPRh): Mr = 1625.90; crystal size
3
¯
0.14 ” 0.32 ” 0.28 mm , triclinic, space group P1, a = 13.0678(14),
b = 15.4796(29),
c = 18.7612(35) Š,
a = 78.540(16),
b =
=
77.412(12), g = 82.493(12)8, V= 3614.86 Š3, Z = 2, 2qmax
56.568, 17903 independent reflections, R1 = 0.0538 for 11119
reflections with I > 2s(I) and wR2 = 0.1619, 973 parameters. 1b
(C52H44F3NO7PRhS): Mr = 1017.80, crystal size 0.42 ” 0.32 ”
[14] For the reaction 2’ + H2!3’, DHr = ꢀ32.6 kcalmolꢀ1 was com-
puted (0 K, gas phase, DFTat B3PW91/BS211B3PW91/BS1
level); the reported hydrogenation enthalpy for acrylic acid is
0.20 mm3, triclinic, space group P1, a = 11.0060(15), b =
given as ꢀ30.3 kcalmolꢀ1
.
¯
11.9617(15), c = 20.1302(51) Š, a = 79.873(15), b = 73.504(18),
g = 64.311(13)8, V= 2285.64 Š3, Z = 2, 2qmax = 63.628, 14050
[15] Gaussian03, M. J. Frisch et al., Gaussian, Inc., Wallingford CT,
2004, full reference given in the Supporting Information.
Angew. Chem. Int. Ed. 2008, 47, 3245 –3249
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 3249