Ruthenium-catalyzed transfer hydrogenation of amino- and amido-substituted acetophenones

Article abstract of DOI:10.1002/ejoc.201301020

The ruthenium-catalyzed transfer hydrogenation of electron-rich amino-substituted acetophenones is reported. Variation of the reductant, ligands, base, and solvent allowed reaction optimization. A key discovery was the use of 1,4-butanediol as an irreversible reducing agent, which significantly improved the conversion. A range of amino- and amido-substituted aryl ketones were explored, and they all gave the corresponding alcohols in good yield, which demonstrates the wider applicability of this process. The ruthenium-catalyzed reduction of electron-rich amino-substituted acetophenones with 1,4-butanediol as an irreversible reducing agent is reported. Optimization of the conditions and variation of the amino substituent are explored as is the use of amido- and sulfonamidoacetophenones with varying results. Copyright

Full text of DOI:10.1002/ejoc.201301020

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201301020
Ruthenium-Catalyzed Transfer Hydrogenation of Amino- and
Amido-Substituted Acetophenones
Andrew J. A. Watson*[a] and Antony J. Fairbanks[a]
Keywords: Ruthenium / Homogeneous catalysis / Hydrogen transfer / Reduction
The ruthenium-catalyzed transfer hydrogenation of electron-
rich amino-substituted acetophenones is reported. Variation
of the reductant, ligands, base, and solvent allowed reaction
optimization. A key discovery was the use of 1,4-butanediol
as an irreversible reducing agent, which significantly im-
proved the conversion. A range of amino- and amido-substi-
tuted aryl ketones were explored, and they all gave the cor-
responding alcohols in good yield, which demonstrates the
wider applicability of this process.
amino-substituted acetophenones was particularly diffi-
cult.[12] In fact, a survey of the existing literature revealed
only a single report of the ruthenium-catalyzed transfer
hydrogenation of amido-substituted acetophenones;[13]
moreover, there were only two examples of the reduction
present in that study. Similarly, there exist only a limited
number of examples of the direct hydrogenation of this type
of substrate class by using ruthenium catalysts.[14]
Introduction
Transfer hydrogenation is a long-established process that
dates back to the metal-catalyzed reactions originally dis-
covered by Meerwein,[1] Ponndorf,[2] and Verley.[3] Al-
though organocatalytic transfer hydrogenation, for exam-
ple, by using diimide[4] or Hantzsch esters[5] as reductants,
has become a field of considerable interest, metal-catalyzed
processes are still very significant. Some areas of current
research activity include the development of transition-
metal catalysts for the asymmetric reduction of prochiral
ketones,[6] imines,[7] and heterocycles.[8] Of the many transi-
tion metals that have been used to mediate transfer hydro-
genation,[9] ruthenium complexes have demonstrated high
catalytic activity (Scheme 1).
Results and Discussion
The lack of a precedent in the literature for the reduction
of amido- and amino-substituted aryl ketones initiated an
investigation into the transfer hydrogenation of these sub-
strates. Commercially available electron-rich amino-substi-
tuted acetophenone 1 was chosen for reaction development.
It was reasoned that if reduction of electron-rich amino-
substituted aryl ketones could be achieved, then reduction
of the less electron-rich amido-substituted acetophenones
should also be possible under similar conditions.
Previously, a trace amount of transfer hydrogenation had
been observed under the reaction conditions reported by
Williams and co-workers[12] for which a “borrowing hydro-
gen”[15] approach was applied to the synthesis of these
alcohols, and therefore, a similar, but simplified, system was
chosen as the starting point. Using the same model sub-
strate (1), catalyst {[Ru(PPh3)3(CO)(H)2]}, and solvent
(DMSO), a screen of typical reducing agents was under-
taken (Scheme 2). The use of formic acid as the stoichio-
metric reductant was a promising starting point, as alcohol
2 was produced with 45% conversion, although a large ex-
cess amount of the reductant was required (Scheme 2). The
use of 2-propanol also resulted in reduction, but with a sig-
nificantly reduced conversion (25%). A more recent ap-
proach to the reduction of ketones by transfer hydrogena-
tion involved the use of 1,4-butanediol,[16,17] which upon
Scheme 1. Related work.
The use of ruthenium complexes to catalyze the transfer
hydrogenation of acetophenones is well established.[10]
However, challenges still exist in this field, such as the ex-
tension of the basic process to allow the reduction of elec-
tron-rich aromatic ketones.[11] For example, during a recent
study it was noted that the transfer hydrogenation of
[a] Department of Chemistry, University of Canterbury,
20 Kirkwood Avenue, Christchurch 8041, New Zealand
E-mail: a.watson@canterbury.ac.nz
http://www.chem.canterbury.ac.nz/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301020.
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Ruthenium-Catalyzed Transfer Hydrogenation
oxidation cyclized to the lactol, and this was then further Table 1. Optimization of conditions.[a]
oxidized to give γ-butyrolactone and 2 equiv. of hydro-
gen.[18] In a similar manner, the use of 1,4-butanediol
proved to be advantageous in this study, as a much im-
proved conversion (63%) was obtained.
Entry
Catalyst
Solvent[b]
Additive[c] Conv. [%][d]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
[Ru(PPh3)3(CO)(H)2]
[RuCl2(PPh3)3]
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
73
39
16
60
65
57
56
40
26
9
41
32
15
32
46
81
58
71
79
74
[[Ru(p-cymene)Cl2]2]
[Ru(PPh3)3(CO)(H)2]
[Ru(PPh3)3(CO)(H)2]
[Ru(PPh3)3(CO)(H)2]
[Ru(PPh3)3(CO)(H)2]
[Ru(PPh3)3(CO)(H)2]
[Ru(PPh3)3(CO)(H)2]
[Ru(PPh3)3(CO)(H)2]
[Ru(PPh3)3(CO)(H)2]
[Ru(PPh3)3(CO)(H)2]
[Ru(PPh3)3(CO)(H)2]
[Ru(PPh3)3(CO)(H)2]
[Ru(PPh3)3(CO)(H)2]
[Ru(PPh3)3(CO)(H)2]
[Ru(PPh3)3(CO)(H)2]
[Ru(PPh3)3(CO)(H)2]
[Ru(PPh3)3(CO)(H)2]
[Ru(PPh3)3(CO)(H)2]
NaOH
Na2CO3
KOH
K2CO3
KOtBu
dppm
dppe
dppp
dppb
dpppent
xantphos
DPEphos
–
Scheme 2. Initial study.
These initial results indicated that the use of reversible
hydrogen transfer agents such as 2-propanol led to poor
conversion. Resonance stabilization of the ketone through
conjugation with the ring nitrogen atom, which is presum-
ably the underlying reason why catalytic reduction of these
ketones is so difficult, should also mean that for a reversible
process the position of the equilibrium lies firmly to the
side of the ketone. However, the use of an irreversible hy-
drogen donor had allowed the reaction to be driven to a
higher conversion, and so this avenue of investigation was
pursued further.
DMSO
toluene
heptane
1,4-dioxane
tert-amyl
alcohol
DME
Bu2O
H2O
DME
DME
DME
DME
DME
–
–
–
–
21
22
23
24
25
26
27
28
[Ru(PPh3)3(CO)(H)2]
[Ru(PPh3)3(CO)(H)2]
[Ru(PPh3)3(CO)(H)2]
[Ru(PPh3)3(CO)(H)2]
[Ru(PPh3)3(CO)(H)2]
[Ru(PPh3)3(CO)(H)2]
[Ru(PPh3)3(CO)(H)2]
[Ru(PPh3)3(CO)(H)2]
–
–
–
–
–
85
75
0
85[e]
74[f]
60[e,g]
16[e]
66[e]
1,4-Butanediol was used as the reaction solvent as well as
the hydrogen source by Williams and co-workers to reduce
ketones in similar ruthenium-catalyzed systems.[19] Further-
more, the fact that it is readily available from renewable
sources by the fermentation of sugars[20] makes it a very
attractive hydrogen source. It can also be easily removed by
an aqueous wash, or through chromatography.
–
dppp
Na2CO3
[a] Conditions: Ketone (1 mmol), 1,4-butanediol (5 equiv.), catalyst
(5 mol-% in Ru), 115 °C, 24 h. [b] 1 mL, DME = 1,2-dimethoxye-
thane. [c] 5 mol-% in the case of phosphines, 10 mol-% for bases.
dppm = 1,1-bis(diphenylphosphino)methane, dppe = 1,2-bis(di-
phenylphosphino)ethane, dppp = 1,3-bis(diphenylphosphino)pro-
pane, dppb = 1,4-bis(diphenylphosphino)butane, dpppent = 1,5-
bis(diphenylphosphino)pentane, xantphos = 4,5-bis(diphenylphos-
A comparison of efficacy of selected ruthenium precata-
lysts under the chosen conditions highlighted the choice of
[Ru(PPh3)3(CO)(H)2] (Table 1, Entry 1) in preference to
either [RuCl2(PPh3)3][21] (Table 1, Entry 2) or [Ru(p-cym-
phino)-9,9-dimethylxanthene, DPEphos
= bis[2-(diphenylphos-
1
[22]
phino)phenyl] ether. [d] Determined by analysis of 2 by H NMR
spectroscopy. [e] 2 equiv. of diol. [f] 1 equiv. of diol. [g] 3 mol-% cat-
alyst loading.
ene)Cl2]2
(Table 1, Entry 3), both of which were pre-
viously used for transfer hydrogenation. This higher activity
can be attributed to the active hydride species that is already
preformed.[21c] This was more apparent upon adding a
range of bases to the reaction (Table 1, Entries 4–8), which (Table 1, Entries 19–22) generally did not result in an im-
resulted in lower conversions (40–65%). Unexpectedly, the provement in the conversion, except in the case of DME
addition of a variety of diphosphine ligands to the reaction (85%). This could be due to the solvent acting as a ligand
mixture did not improve the conversion (Table 1, Entries 9– in the reaction. The use of water (Table 1, Entry 23) led to
15), a result that was rather surprising given the previous no conversion, and only starting material was recovered,
improvements in transfer hydrogenation that were observed which suggests that the catalyst was not stable under these
upon combining [Ru(PPh3)3(CO)(H)2] with either conditions. It was concluded that DME was the optimal
DPEphos[16] or xantphos.[23]
solvent for the reaction owing to a combination of the
A further improvement in conversion was achieved upon slightly improved conversion and its ease of removal at the
using DMSO as the cosolvent (Table 1, Entry 16), in a pro- end of the reaction, particularly if compared to DMSO.
cess similar to the initially trialed conditions (Scheme 2). A
With DME as the solvent, the amount of 1,4-butanediol
range of other solvents were also screened (Table 1, En- could be reduced to 2 equiv. (Table 1, Entry 24) without af-
tries 17–23). The use of heptane (Table 1, Entry 18) resulted fecting the conversion. However, the use of less than
in an improved conversion relative to that obtained with 2 equiv. (Table 1, entry 25) had a detrimental effect on the
toluene (71 vs. 58%). The use of oxygen-containing solvents reaction conversion. This result contrasts somewhat with
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6785

A. J. A. Watson, A. J. Fairbanks
SHORT COMMUNICATION
the stoichiometry of the reaction, as each molecule of 1,4- Table 2. Reaction scope.[a]
butanediol should be capable of reducing two molecules of
ketone, and so only half an equivalent should be required.
Final tuning reactions showed that lower conversions
were obtained if a lower catalyst loading was used (Table 1,
Entry 26) and that the same conversion was obtained after
both 24 and 48 h, which was indicative of catalyst deactiva-
tion during the reaction. Re-introduction of base or phos-
phine (Table 1, Entries 27 and 28) also hindered the reac-
tion, as seen previously.
Having established a set of conditions that gave the prod-
uct in good conversion, a range of amino ketones were scre-
ened to evaluate the reaction scope (Table 2). Initial studies
on a range of tertiary amines showed a large variation of
reaction efficiency with ring size in the case of the cyclic
compounds. The amine containing a seven-membered ring
(Table 2, Entry 4) returned the lowest yield (47%), whereas
the corresponding six-membered ring material (Table 2, En-
try 3) gave the highest yield (74%). The five-membered ring
compound (Table 2, Entry 2) reacted similarly to that con-
taining a dimethylamino group (Table 2, Entry 1). The in-
troduction of further heteroatoms into the ring (Table 2,
Entries 5–7) had little effect on the yield of the reaction, all
of which were higher than 75%; this indicated the tolerance
of the process to remote functionalization and its potential
application to pharmaceutically active compounds.
To further test the reaction scope, the use of amido and
sulfonamido substrates were investigated. Reduction of a
Boc-protected aromatic amine (Table 2, Entry 8) was pre-
viously reported,[13] and although the yield obtained here
was higher (59 vs. 43% reported[13]), the direct comparison
of these yields is somewhat unfair, as in this case the reac-
tion was run for eight times longer at a much higher tem-
perature. Interestingly, the other example reported in that
paper,[13] the N-acetamide (Table 2, Entry 9), also gave a
poor yield of product (31 vs. 28% reported[13]). In contrast,
the N-benzamide (Table 2, Entry 10) gave the correspond-
ing product in a significantly higher yield (74%). This was
surprising as one may expect that the reactivity of the
ketones in the acetamide and benzamide should be very
similar.
However, an analogous trend was observed with the
sulfonamides; the methyl sulfonamide (Table 2, Entry 11)
only gave a poor yield of the product (35%) in a similar
manner to the acetamide, whereas the tosyl sulfonamide
(Table 2, Entry 12) performed more than twice as well
(76%). At this point, it is unclear why this is the case, as
analyses of the crude reaction mixtures by NMR spec-
troscopy revealed that the only other compound present
was the starting material, which precludes yield reduction
by side reactions.
Cyclic imides such as succinimide and phthalimide were
not tolerated; the ring-opened amido esters were obtained
instead of any reduced products (Scheme 3).
To conclude this study, three other compounds were ex-
amined as substrates (Scheme 4). Moving the morpholino
substituent to the ortho position as in 4, which may be ex-
pected to increase the steric bulk around the carbonyl
[a] Conditions: Ketone (3 mmol), 1,4-butanediol (2 equiv.),
[Ru(PPh3)3(CO)(H)2] (5 mol-%), DME (3 mL), 115 °C, 24 h.
[b] Boc = tert-butoxycarbonyl, Ac = acetyl, Bz = benzoyl, Ms =
methanesulfonyl, Ts = para-toluenesulfonyl. [c] Yield of isolated
product. [d] Conversion was determined by 1H NMR spectroscopy.
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Ruthenium-Catalyzed Transfer Hydrogenation
was then concentrated, and the crude product was purified by flash
column chromatography to afford the corresponding alcohol in
good yield.
Standard Procedure for the Reduction of Amidoacetophenones: 1,4-
Butanediol (0.53 mL, 6 mmol) and 1,2-dimethoxyethane (3 mL)
were added to an oven-dried, nitrogen-purged Young’s tube con-
taining [Ru(PPh3)3(CO)(H)2] (138 mg, 0.15 mmol) and the amido
ketone (3 mmol). The reaction tuve was then purged with nitrogen
for 10 min before it was heated to 115 °C for 24 h. On completion,
the reaction mixture was cooled to room temperature before con-
centrating under vacuum. The crude product was purified by flash
column chromatography before recrystallizing from CH2Cl2/hexane
to afford the corresponding alcohol in good yield.
Scheme 3. Ring opening of imides.
group, actually had very little effect on the reaction yield
(88%). Finally, both ethyl- and phenyl-substituted ketones
5 and 6 were found to be good substrates for the reaction
(73 and 68%, respectively), which indicates application of
the process beyond acetophenones.
Supporting Information (see footnote on the first page of this arti-
cle): Complete experimental details, characterization data, and
1
copies of the H and 13C NMR spectra.
Acknowledgments
We would like to thank the University of Canterbury for providing
funding for A. J. A. W. We would also like to thank Dr. Marie
Squire for her help with running the MS and NMR services.
Scheme 4. Substrate scope.
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This study represents the first systematic examination of
ruthenium-catalyzed transfer hydrogenation of amino- and
amido-substituted aromatic ketones. It was found that 1,4-
butanediol was the best hydrogen source under the reaction
conditions. Furthermore, the yields were generally good for
a challenging reduction reaction, and a range of amines,
amides, and substituted ketones were tolerated. It is there-
fore concluded that this method provides a robust catalytic
alternative to traditional reducing agents. Future areas for
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Experimental Section
Standard Procedure for the Formation of Amino Ketones: 4-Fluoro-
acetophenone (3.6 mL, 30 mmol) was dissolved in DMSO (40 mL)
under N2, and the amine (66 mmol, 2.2 equiv.) was added before
the mixture was heated to 115 °C for 16 h. The mixture was then
cooled to room temperature before it was poured into H2O
(100 mL) and mixed with brine (100 mL). The mixture was ex-
tracted with Et2O (3ϫ 100 mL), and the combined organic layers
were then dried (MgSO4), filtered, and concentrated under vacuum.
The crude material was then purified by column chromatography
(petroleum ether/EtOAc) to afford the corresponding ketone in
good yield.
Standard Procedure for the Reduction of Aminoacetophenones: 1,4-
Butanediol (0.53 mL, 6 mmol) and 1,2-dimethoxyethane (3 mL)
were added to an oven-dried, nitrogen-purged Young’s tube con-
taining [Ru(PPh3)3(CO)(H)2] (138 mg, 0.15 mmol) and the amino
ketone (3 mmol). The reaction tube was then purged with nitrogen
for 10 min before diluting with CH2Cl2 (50 mL). The mixture was
then extracted with aqueous NaOH (1 m, 20 mL). The organic layer
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A. J. A. Watson, A. J. Fairbanks
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Received: July 10, 2013
Published Online: September 23, 2013
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Eur. J. Org. Chem. 2013, 6784–6788