On the NH effect in ruthenium-catalysed hydrogenation of ketones: Rational design of phosphine-amino-alcohol ligands for asymmetric hydrogenation of ketones

Article abstract of DOI:10.1002/chem.201000790

Chemical equation presented Redesign and reduce: The graphic shows some Ru complexes of tridentate ligands that do not contain NH substituents but can catalyse ketone reduction with zero-order dependence on ketone. In tridentate systems, a secondary amine, not a primary amine, promotes hydrogen activation. Leading on from these observations, the Ru complex of a phosphine-amino-alcohol shown above has been found to be a good hydrogenation catalyst.

Full text of DOI:10.1002/chem.201000790

DOI: 10.1002/chem.201000790
On the NH Effect in Ruthenium-Catalysed Hydrogenation of Ketones:
Rational Design of Phosphine-Amino-Alcohol Ligands for Asymmetric
Hydrogenation of Ketones
Scott D. Phillips, Josꢀ A. Fuentes, and Matthew L. Clarke*[a]
The asymmetric hydrogenation of simple ketones, such as
acetophenones, has become an important synthetic method
as a result of the development of [RuCl2(diphosphine)-
co-ordination environment that is more accessible for bulky
substrates relative to the Noyori catalysts.[4]
Rather than synthesising a very large library of new
P^N^NH2 catalysts, we felt that an investigation of catalyst–
structure activity relationships might throw up some leads
for new catalysts, along with shedding light on some mecha-
nistic issues, since it had not been possible to isolate the re-
action intermediates in our previous studies. In this commu-
nication, we report some surprising findings from our kinetic
experiments and the introduction of phosphino-amino-alco-
hol ligands for Ru-catalysed hydrogenation.
ACHTUNGTRENNUNG
(diamine)] catalysts by Noyori and co-workers.[1] A large
range of catalysts of this general type have now been pre-
pared.[2] Their reactivity is quite in contrast to simple M-
ACHTUNGTRENNUNG(diphos)Xn salts that are barely active as catalysts for ke-
tones that cannot chelate to the metal centre. The enhanced
reactivity for reduction of simple ketones is proposed to be
À
due to the “bifunctional mechanism” in which the ketone
hydrogen bonds to the primary amine terminus of the
ligand, activating it to attack by Ru–hydride and controlling
stereoselectivity.[1c] There are reports that suggest these cata-
lysts are not effective for certain ketones, such as bulky ke-
tones (low reactivity), sterically similar aryl–aryl ketones
(low selectivity), some ketones with strongly co-ordinating
substituents (low reactivity), and alkyl–alkyl ketones (low
selectivity) as well as a range of individual substrates that
do not undergo asymmetric reduction readily. The discovery
of structurally distinct, new classes of ketone hydrogenation
catalyst[3] that deviate from the [RuCl2(diphosphine)(di-pri-
mary-amine)] blueprint therefore might present one of the
best opportunities to solve these problems; the importance
of chiral secondary alcohols requires that methodology
exists for every possible type of substrate in order to see
this technology widely exploited in industry and more gener-
ally in synthesis. We have shown that Ru catalysts derived
from P^N^NH2 ligands can hydrogenate some poorly reac-
tive ketones with good enantioselectivity. The initial design
of the catalyst envisaged hydrogenation of polar bonds fa-
cilitated by the primary amine terminus of the ligand in a
The new achiral ligands, 1–3 can be prepared in one step
from commercial starting materials and then converted into
complexes of type [RuCl 2A
N
CHTUNGTREN(NUGN dmso)] by micro-
wave-assisted complexation with [RuCl CTHUNGRTEN(NUNG dmso)4] in THF at
1208C (Scheme 1). Complexes 4–7 can be obtained pure by
chromatography or recrystallisation and are easily handled
in air.
We carried out kinetic studies on the hydrogenation of
a,a-dimethylpropiophenone, a substrate that was initially re-
[a] S. D. Phillips, Dr. J. A. Fuentes, Dr. M. L. Clarke
School of Chemistry, University of St. Andrews
EaStCHEM, St. Andrews, Fife, KY16 9ST (UK)
Fax : (+44)1334-463808
E-mail: mc28@st-andrews.ac.uk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000790.
Scheme 1. Synthesis of P^N^N ligands and their ruthenium complexes.
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COMMUNICATION
ported to be nearly unreactive with [RuCl
(binap)
E
an induction period and contrasting rate profile with the tri-
methylated catalyst is good evidence that the catalyst is not
converted into another species (i.e., demethylation), al-
though this remains a very unlikely possibility until a cata-
lytic intermediate is isolated. The fastest reactions are limit-
ed, most likely by several factors including an event involv-
ing the ketone substrate (most likely either ketone binding
or nucleophilic attack of hydride), whereas the slower cata-
lyst is slow due to other steps in the catalytic cycle, most
likely hydrogen activation, being retarded to a greater
degree when the NH is not present.
(BINAP = 2,2’-bis(diphenylphosphino)-1,1’-binaphthyl) cat-
alysts (6% yield),[1c] but can be reduced readily with catalyst
4 at room temperature, or more conveniently for kinetic
studies at 708C. A catalyst that can reduce this ketone in
high yield therefore has significant competence in ketone
hydrogenation. We were intrigued to find in batch experi-
ments that all the catalysts reduced a,a-dimethylpropiophe-
none in high yield after 16 h reaction time. Reactions car-
ried out with real-time gas-uptake and subsequent process-
ing of the data in graphical form[5] enable a number of in-
sights to be gained. For brevity, the kinetics graphs are
placed in the Supporting Information. Plotting a graph of
rate against time clearly shows a significant induction time;
maximum turnover frequencies and average initial turnover
frequencies during the first 35 min for all four catalysts are
shown in Table 1, along with notes on the rate profile.
There is a drop in activity and an increase in induction
time in going from catalyst 4 to the achiral P^NH^NH2 cat-
alyst, 5. This indicates that the shape of the catalyst, and
perhaps the geometry at the Ru centre has a very significant
influence on the catalyst productivity. Surprisingly, exchang-
ing the NH2 groups for an NMe2 group as in 6 gives a cata-
lyst with quite similar activity, casting significant doubt on
the primary amine-assisted transition state that would have
seemed likely with these catalysts. There is a significant
drop in activity in switching to the P^NMe^NMe2 catalyst,
7, although this catalyst does slowly reduce a,a-dimethylpro-
piophenone through to completion in about 12 h. The fact
that even trimethylated, 7 does function as a catalyst for this
low reactivity substrate suggests that an inner-sphere mecha-
nism can operate, albeit rather slowly. Both the P^NH^NH2
catalysts 4 and 5 catalyse reduction with pseudo first-order
dependence in ketone concentration. The P^NH^NMe2 cat-
alyst, 6 clearly undergoes a relatively sudden drop in activity
during reaction, although is of broadly similar rate as the
catalyst 5. Most interestingly, the P^NMe^NMe2 catalyst re-
duces this low reactivity ketone with no dependence on
ketone concentration. This was also checked in individual
batch experiments at different concentrations. The lack of
With these results in hand, we were intrigued if similar re-
activity patterns would be present in Noyori catalysts; the
details of these hydrogenations has generated intense inter-
est,[6,7] because of their significant importance. However, de-
spite the quality and depth of the studies reported, the rela-
tive importance of the NH effect to each step in the catalyt-
ic cycle has been rather problematic to study experimentally
since the hydrogenation is fast, and the NH substituents un-
dergo exchange in the protic solvents which are required for
effective catalysis. Noyori reported early on that at least one
primary NH2 group is necessary for effective reaction.[1d]
However, some ketone reduction catalysts that do not have
NH ligands have been reported. We carried out hydrogena-
tions with the “non-NH” catalyst [RuCl2ACTHNUTRGNEUNG
(binap)Py2][8] at dif-
ferent concentrations of acetophenone. These results reveal
that the reactions are also pseudo-zero order in ketone, in
contrast to the pseudo first order kinetics that have been de-
scribed for [RuCl2ACTHNUGTRENNUG(binap)ACHUTNGTREN(NNGU dpen)] (dpen = 1,2-diphenylethy-
lenediamine) under similar conditions (Scheme 2).[6] Thus
ketone binding or reduction is not rate-determining in the
slower hydrogenations catalysed by the non-NH containing
catalyst [RuCl2ACHTNUTRGENNUG(binap)Py2] either.
We were interested to investigate if the presence of NH
groups had any effect on the hydrogenation of alkenes. We
carried out hydrogenation of styrene, a substrate that can be
assumed to be reduced exclusively by inner-sphere process-
es. In our original studies, we found that catalyst 4 does
show significant chemoselectivity for reduction of C=O over
C=C bonds, but it is less chemoselective than Noyori cata-
lysts unless conditions are care-
fully controlled. Thus at 708C,
styrene is hydrogenated within
12 h with all the catalysts with
similar rates for each catalyst
and no significant induction
period. These results argue for
the NH effect (and the sensitiv-
ity to ligand shape) being spe-
cific for ketone hydrogenation;
our view is that the most likely
possibility is that alkene hydro-
genations share very little of
the ketone hydrogenation cata-
lytic cycle, with the cycle oper-
ating being relatively insensitive
to ligand structure.
Table 1. Summary of kinetic data for the hydrogenation of a,a-dimethylpropiophenone using Ru catalysts 4–
7.
Catalyst[a]
Maximum
TOF[b]
Average
Notes on kinetics[c]
initial TOF[b]
4
5
6
7
655
653
403
188
350
106
115
35
induction period followed by pseudo-first-order kinetics
longer induction period followed by pseudo-first-order kinetics
Induction period, then fast reaction with sudden decrease in rate
constant rate throughout; zero order in ketone substrate
[a] Reaction carried out as described in the Scheme and in the Supporting Information, at constant pressure of
70 bar with hydrogen feed monitored. [b] Average initial TOF in mol product per mol catalyst per h measured
from first 35 min of the reaction. Maximum TOF in mol product per mol catalyst per h is the rate reached
after the induction period take from a plot of rate against time.
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M. L. Clarke et al.
À
Scheme 2. Hydrogenation of acetophenone using the non-N H Noyori
catalyst, [RuCl2A(binap)(pyridine)2]; productivity is independent of ketone
N
ACHTUNGTRENNUNG
concentration. IPA = isopropyl alcohol.
À
The NH ketone hydrogen bond in the Noyori mechanism
almost certainly defines a well-defined transition state that
enables a high level of enantioselectivity, but in terms of
productivity, itꢁs more significant role is in enhancing hydro-
gen activation. It is likely there are many highly reactive
ketone-hydrogenation catalysts that do not use NH ligands
waiting to be discovered, with an efficient mechanism for
hydrogen activation being the more important feature that
needs to be addressed in designing a new catalyst. Some
recent studies have suggested that the original proposal that
heterolytic splitting of hydrogen takes place between an
amido NH(À) ligand and Ru needs to be modified, since
the amido complex seems to be protonated readily.[6] How-
ever, when a final detailed picture of the mechanism is de-
cided, it should reconcile the importance of the NH ligand
in hydrogen activation as well as ketone binding. Our work-
ing hypothesis for catalyst 4 is that the secondary amine
part of the ligand provides a weak interaction that places
(but does not strongly activate) one face of the ketone in
the correct orientation for the hydride attack. This could be
followed by either deprotonation of the NH ligand to deliv-
er product or proceed by the “re-entry mechanism” postu-
lated by Bergens and co-workers[7u] (early part of transition
Scheme 3. Synthesis of a phosphine-amino-alcohol catalyst, and new ke-
tones reduced in Table 2.
Table 2. Comparison of some hydrogenations of deactivated ketone sub-
strates using complexes 4 and 11 as catalyst.
Entry Catalyst T [8C] t [h] Ketone Conversion [%][a] ee
1
2
4
50
35
50
50
50
50
50
50
3
16
3
16
16
16
16
65
8
8
8
12
12
13
13
13
>99
>99
>99
>99
>99 {93}
0
77 (S)
79 (S)
80 (S)
94
97
n.d
11
11
4
11
4
3[b]
4[c]
5[d]
6
7
8
11
11
15
65 {46}
n.d
64 (S)
[a] Reactions carried out using 0.5 mol% of the (R,R) catalyst with base/
catalyst ratio of 2:1 for catalyst 4, and 5:1 for catalyst 11 in isopropyl al-
cohol as solvent and using initial pressure of 50 bar hydrogen unless
stated otherwise. Conversion determines against methyl naphthalene in-
ternal standard, with products isolated in selected experiments; {yields
after chromatography}. [b] 1 mol% catalyst used. [c] (R,R)+(S,S)/meso=
6:1. [d] (R,R)+(S,S)/meso=7:1.
À
À
state is outer-sphere with concerted C H and Ru O bond
formation). All of the NH-containing catalysts presumably
benefit from the ability of the NH ligand to promote some
part of the hydrogen activation process. This could utilise
the interplay between amino/amido bonding or the reduced
steric demand of NH substituents. In any case, we felt these
observations gave us a lead to investigate catalysts that do
not contain a primary amine terminus, and herein we also
report a new type of catalyst based on P^NH^OH ligands.
We have found that an analogous phosphine-amino-alco-
perimental variability, they do seem to be reproducible; we
have probably run the reduction of 8 with catalyst 4 over
100 times and never got over 77% ee). There are, however,
some differences between catalysts 4 and 11. Whereas, the
original catalyst is somewhat unusual in operating well using
a Ru/base co-catalyst ratio of just 2, catalyst 11 requires a
Ru/base ratio of 5 to work well. Another difference is that
the P^NH^OH catalyst is exclusively a pressure-hydrogena-
tion catalyst and does not promote transfer hydrogenation
with reasonable rates. In general, catalyst 11 is slightly less
reactive catalyst than 4, but in some cases shows significant-
ly increased reactivity; thus reduction of ketone 13, deacti-
vated by bulk and the heterocycle completely inhibits hydro-
genation with 4, but can be reduced albeit with quite moder-
ate results with catalyst 11.
hol ligand 10 (Scheme 3) on treatment with [RuCl
gave a similar type of complex of type [RuCl
(P^NH^OH)] (11). There were no problems connected to
(dmso)4],
CTHUNGTRENNUNG
ACHTUNGTRENNUNG
OH deprotonation or lability despite the weaker binding
and more reactive OH function.
Our structure–activity studies suggested that swapping the
primary amine terminus for an alcohol should still deliver
active catalysts for the reduction of a deactivated ketone,
and a validation of our model would be complex 11 giving
the same sense of enantioselection with similar ee. Pleasing-
ly, complex 11 does reduce ketones readily and seems to
give either very similar or slightly higher enantioselectivity
when compared to the original P^NH^NH2 catalyst
(Table 2) (although the differences are only just outside ex-
In conclusion, the NH rather than the NH2 substituent
within tridentate P^NH^NH2 ligands is of significant impor-
tance to the catalytic cycle. Kinetic studies suggest that a
step in the catalytic cycle that does not involve the substrate,
most likely hydrogen activation, is most significantly en-
hanced by an NH group in both our catalysts and the
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Chem. Eur. J. 2010, 16, 8002 – 8005

Asymmetric Hydrogenation of Ketones
COMMUNICATION
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amino-alcohol ligands might be useful in Ru-catalysed
ketone hydrogenation. The significance of these results is
primarily in opening up this class of complex as a potential
candidate for effective hydrogenation catalysis. Our search
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substrate with high enantioselectivity and reactivity will now
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from the plethora of enantiopure amino alcohols.
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Keywords: chirality · hydrogenation · kinetics · phosphines ·
tridentate ligands
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Received: March 29, 2010
Published online: June 25, 2010
Chem. Eur. J. 2010, 16, 8002 – 8005
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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8005