Evaluation of Fe and Ru Pincer-Type Complexes as Catalysts for the Racemization of Secondary Benzylic Alcohols

Article abstract of DOI:10.1002/chem.201601793

Fe and Ru pincer-type catalysts are used for the racemization of benzylic alcohols. Racemization with the Fe catalyst was achieved within 30 minutes under mild reaction conditions, with a catalyst loading as low as 2 mol %. This reaction constitutes the first example of an iron-catalyzed racemization of an alcohol. The efficiency for racemization of the Fe catalyst and its Ru analogue was evaluated for a wide range of sec-benzylic alcohols. The commercially available Ru complex proved to be highly robust and even tolerated the presence of water in the reaction mixture.

Full text of DOI:10.1002/chem.201601793

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Accepted Article
Title: Evaluation of Fe and Ru Pincer-Type Complexes as Catalysts for the Racemi-
zation of Secondary Benzylic Alcohols
¨
Authors: Jan-E. Backvall; Christoph Bornschein; Karl Gustafson; Oscar Verho; Matt-
hias Beller
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Chemistry - A European Journal
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COMMUNICATION
Evaluation of Fe and Ru Pincer-Type Complexes as Catalysts for
the Racemization of Secondary Benzylic Alcohols
Christoph Bornschein,+[a,b] Karl P. J. Gustafson,+[a] Oscar Verho,[a] Matthias Beller[b] and Jan-E.
Bäckvall*[a]
Abstract: Herein, two Fe and Ru pincer-type catalysts are used
for the racemization of benzylic alcohols. Racemization with the
Fe catalyst was achieved within 30 minutes under mild reaction
conditions, with a catalyst loading as low as 2 mol%. This
reaction constitutes the first example of an iron-catalyzed
racemization of an alcohol. The efficiency of the Fe catalyst and
its Ru analogue was evaluated on a wide range of sec-benzylic
alcohols. The commercially available Ru complex proved to be
highly robust and even tolerated the presence of water in the
reaction mixture.
active and robust catalytic systems for the racemization of
secondary alcohols in very short reaction times.[17] The Bäckvall
group showed that racemization of secondary alcohols by
pentaphenylcyclopentenylruthenium catalyst 2 (0.5 mol%) could
be achieved in 10 minutes at ambient conditions,[6d,6e] which to
date is one of the most versatile racemization catalysts for
secondary alcohols. Later, Park and co-workers reported on a
dimeric cyclopentenylruthenium complex 3 (2 mol%) that could
be activated with light to obtain the desired racemization under
mild conditions (30 °C, air) within 10 minutes reaction time.[18]
A
similar catalyst to 2 was created by the groups of Kanerva and
Leino.[19] They developed the pentabenzyl analogue of 2, a
complementary catalyst working especially well on electron-rich
alcohols, however, requiring slightly higher catalyst loadings.
One example of a recently developed racemization catalyst is
the Ru/indenyl complex 4 reported by the Nolan group.[6a, 20] By
applying this complex to the racemization of (S)-1-phenylethanol,
a complete racemization was achieved at room temperature (1
mol% catalyst) within 20 minutes. Furthermore, the group of
Baratta has reported on a new type of racemization catalyst,
composed of a pincer type Ru or Os catalyst working as
racemization catalyst for secondary alcohols.[21]
The synthesis of enantiomerically pure compounds
constitutes a fundamental part of modern organic chemistry.
Asymmetric synthesis is widely applied in the preparation of
pharmaceuticals,[1] electronic and optical devices,[2] polymer
components[3] and biological probes.[4]
The resolution of racemates is still the most commonly used
method for obtaining enantiomerically pure compounds on an
industrial scale.[1c] A drawback of all resolution protocols is that
only a maximum theoretical yield of 50% of the desired
enantiomer can be obtained. To allow for quantitative yield, the
undesired enantiomer has to be interconverted into the desired
one e.g. via racemization.
A coupling of resolution and
racemization leads to a so-called dynamic kinetic resolution
(DKR), where an in situ racemization catalyst keeps the starting
material racemic.[5]
One of the most popular methods to racemize secondary
alcohols involves reversible hydrogen transfer mediated by a
metal catalyst that can be e.g. Ru-,[5a,6] Rh-[7] or Ir-[8] based.[9] [10]
Racemization of alcohols can also be accomplished via a dehy-
dration-hydration mechanism catalyzed by acidic zeolites[11] or
resins.[12] There are also a number of substrate-specific catalysts,
such as a vanadium catalyst that racemizes allylic alcohols[13] or
a palladium catalyst that is mainly useful for allyllic esters.[14]
The development of metalloenzymnatic DKR began in the
end of the 1990s[5a,15] and one of the racemizatiomn catalysts
used was the Ru dimeric Shvo complex 1.[16] Following this work,
various Ru-cyclopentadienyl type transfer hydrogenation
catalysts were developed that allow for an efficient racemization
at ambient temperature.[17]
Figure 1. Powerful Ru-based racemization catalyst (a) and PNP-pincer-based
de- and hydrogenation catalysts (b).
Recent efforts in optimizing such protocols have led to highly
Despite the extensive research and the recent progress in
the field of alcohol racemization, there is still a need of finding
new and broadly applicable racemization catalysts that are cost-
effective, easily accessible and composed of earth abundant
elements. In line with these criteria, we decided to study the two
PNP-pincer type catalysts 5 and 6, which should be capable of
mediating the racemization of alcohols given their
dehydrogenation/hydrogenation properties. For instance, Ru-
Macho® 6 is a commercially available and fairly inexpensive
catalyst, which was previously used for the hydrogenation of
ester derivatives.[22] The Fe-pincer catalyst 5 was recently
utilized by the Beller group for the hydrogenation of carboxylic
acid derivatives[23] and for the dehydrogenation of methanol.[24]
[a]
[b]
[+]
Department of Organic Chemistry, Arrhenius Laboratory, Stockholm
University, SE-10691 Stockholm, Sweden
Fax: +46-8-154908; Phone: +46-8-674 7178
E-mail: jeb@organ.su.se
Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-
Einstein-Strasse 29a, 18059 Rostock, Germany
Fax: 0049 381 1281 51113; Tel: 0049 381 1281 113
E-mail: matthias.beller@catalysis.de
These two authors contributed equally to this work
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))

Chemistry - A European Journal
10.1002/chem.201601793
COMMUNICATION
(Table 1, entries 1, 4 and vs. 6 and 8-11). In the absence of
base in MTBE no racemization took place (entry 12).[26]
The influence of the relative amount of base, using 2 mol%
of catalyst, that was required for sufficient activation was also
studied (Table 1, entries 1, 5, 6 and 12). Two equivalents of
base proved to be the most successful combination for
achieving an efficient racemization (Table 1, entries 1 and 4).
Table 1. Racemization of (S)-1-Phenylethanol (7a) using Fe-Pincer catalyst
5.[a],[b]
Catalyst /
mol%
Base /
(mol%)
ee-value
(rac)-7a[c]
Rel. amount
of 8a[c]
Entry
1
2
2
1
t-BuOK (4)
t-BuOK (2)
t-BuOK (1)
t-BuOK (5)
t-BuOK (2)
t-BuOK (10)
t-BuOK (4)
NaH (20)
2%
82%
99%
0%
5%
4%
0%
5%
0%
5%
0%
0%
6%
0%
0%
0%
Table 2. Racemization of (S)-1-Phenylethanol (7a) using Ru-Macho® catalyst
6.[a],[b]
3
0.5
2.5
2
4[d]
5
Entry
Catalyst /
mol%
Base /
(mol%)
ee-value
(rac)-7a[c]
Rel. amount
of 8a[c]
99%
7%
6
2
1
2
1
1
-
99%
0%
0%
2%
2%
0%
4%
3%
3%
2%
5%
0%
7[e]
8
2
99%
99%
84%
99%
99%
99%
t-BuOK (2)
NaH (5)
2
3
0.5
0.5
0.25
0.5
0.5
0.5
0.5
0.1
5%
9
2
NEt3 (100)
Cs2CO3 (100)
Na2CO3 (100)
-
4
Na2CO3 (100)
t-BuOK (0.5)
t-BuOK (1)
t-BuOK (0.5)
t-BuOK (1)
t-BuOK (1)
t-BuOK (0.2)
99%
0%
10
11
12
2
5[d]
6 [e]
7
2
0%
2
7%
8[f]
9[g]
10
2%
[a] General reaction conditions: 0.25 mmol (S)-7a, 1.25–6.25 µmol 5, 0–0.25
mmol base, 1 mL MTBE, 50 °C, 30 min. [b] for complete optimization table see
Supporting Information. [c] ee-values and yields of side product 8a were
determined by chiral GC. [d] 15 min reaction time. [e] 1 equiv. water was
added to the reaction mixture.
0%
99%
[a] General reaction conditions: 0.5 mmol (S)-7a, 0.5–5.0 µmol 6, 0–0.5 mmol
base, 2 mL MTBE, 50 °C, 30 min. [b] for complete optimization table see
Supporting Information. [c] ee-values and yields of side product 8a were
determined by chiral GC [d] 3 h reaction time [e] 10 min reaction time. [f]. 1
equiv. water was added to the reaction mixture. [g] reaction under air
The evaluation of catalysts 5 and 6 commenced with the
racemization of 1-(S)-phenylethanol 7a. First, the performance
of the Fe-PNP-pincer complex 5 to racemize 7a was evaluated
in a set of different solvents, employing two equivalents of t-
BuOK as base (Table S1, Supporting Information). Among the
many solvents tested, only ethers (THF and MTBE) led to
detectable racemization of 7a (Table S1, entries 2, 3, 12 and 15-
17). No racemization of 7a was observed in other more strongly
coordinating solvents, such as dimethyl formamide and 2-
propanol[25] or in non-coordinating solvents, such as toluene,
cyclohexane and pentane (Table S1, entries 18-22). MTBE was
chosen as the solvent for further experiments with 2 mol% of 5
and 2 equivalents of t-BuOK (4 mol%) (Table 1, entry 1). These
In contrast to iron catalyst 5 the ruthenium catalyst 6 was
found to exhibit a broader tolerance to the reaction conditions as
complete racemization was observed in several solvents, such
as toluene, THF and MTBE (Table S2), with either t-BuOK or
NaH as bases (Table 2, entries 2 and 3). Even significant
racemization was observed in 2-propanol as solvent (Table S2,
entry 18). Gratifyingly, a catalyst loading as low as 0.25 mol%
was sufficient for the racemization of 7a within 3 h at 50 °C
(Table 2, entry 5). By increasing the catalyst loading to 0.5 mol%,
complete racemization was obtained within 10-15 min (Table 2,
entry 6). An excess of base was required for maintaining high
racemization activity, and consequently a 1:1 ratio between
catalyst and base was insufficient for obtaining full racemization
with in 30 min (Table 2, entry 7). Interestingly, further trials with
addition of one equivalent of water, or even performing the
reaction under air did not inhibit the activity of this catalyst
(Table 2, entries 8 and 9). Additionally, the long-term stability
was established by adding a second portion of the starting
material after 1 h, showing a complete racemization after an
additional hour.
conditions allowed for
a low amount of the undesired
dehydrogenation product 8a within the observed reaction time.
The optimization showed that the amount of catalyst required is
at least 2 mol% while with 1 and 0.5 mol% of 5 inefficient or no
racemization was observed (entries 2 and 3). A slightly higher
catalyst loading (2.5 mol%) led to a faster reaction and the
reaction now gave full racemization within 15 min (Table 1, entry
4). The addition of 1 equiv. of water completely shut down the
racemization (entry 5).
Next, the effect of different bases was studied and it was
found that only t-BuOK promoted the desired racemization

Chemistry - A European Journal
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COMMUNICATION
To establish the scope of Fe-complex 5, it was evaluated for
the racemization of a range of secondary alcohols (Figure 2).
Various 1-phenylethanol derivatives with different substituents
underwent efficient racemization with catalyst 5. With p-methoxy
and p-trifluoromethyl substituted aryls (7b and 7f) a hydrogen
atmosphere had to be applied to obtain full racemization. The 4-
Br derivative 7c, required four times longer reaction time
compared to the other benzylic alcohols. Notably, the allylic
alcohol 7j was obtained fully racemic together with 21% of the
unsaturated ketone. However, none of the usually observed
isomerization side product was formed when 5 was used as
catalyst. Furthermore, water- and oxygen-free reaction
conditions were required for a satisfactory activity.
bromo-substituted substrates constitute an exception and
needed prolonged reaction times. Nonetheless, 12 different
ortho-, meta- and para-substituted 1-arylethanol derivatives
worked excellently under the applied conditions. In the case of
the 1-(para-cyanophenyl)ethanol (7o) an undesired catalyst-
substrate interaction mediated by the nitrile group might be
responsible for the incomplete racemization. It is worthy to note,
that reducible nitrile- and nitro-functionalities stay intact
throughout the experiments (GC/MS). Unfortunately, as for the
Fe-analogue, the Ru-complex did not exhibit any racemization
activity towards aliphatic alcohols.
Figure 2. Substrate scope for the racemization of secondary alcohols.
Reaction conditions (unless otherwise noted): secondary alcohol 7 (0.25
mmol), Fe-pincer (0.00625 mmol), t-BuOK (0.0125 mmol), tert-butyl methyl
ether (MTBE, 1 mL), 50 °C, 30 min; enantiomeric excess (ee) was determined
Figure 3. Substrate scope for the racemization of secondary alcohols.
Reaction conditions (unless otherwise noted):: secondary alcohol
7 (0.5
mmol), Ru-Macho (0.0025 mmol), t-BuOK (0.005 mmol), MTBE (2 mL), 50 °C,
15 min. ee-value was determined either with chiral GC/MS or HPLC. [a] (S)-7k
[b]
either with chiral GC/MS or HPLC. [a] 1 atm H2-atmosphere during reaction;
[e]
90% ee; [b] (S)-7m 60% ee; [c] 30 min reaction time; [d] 120 min reaction time.
120 min reaction time. [c] (S)-7m 90% ee;.
reaction in THF for 18h [f] (S)-7t 65% ee. [g] 60 min reaction time.
The developed protocol for the Fe-catalyzed racemization
was found to be sensitive to steric effects and substrates
bearing a larger α-alkyl substituent (7k) or ortho-substitution (7p)
were not tolerated. Also, the Fe-catalyst did not racemize
aliphatic alcohols as demonstrated by the failed racemization of
substrate 7r.
Next, the optimized reaction conditions for the commercially
available Ru-Macho® complex 6 were applied for the racemi-
zation of a variety of secondary alcohols (Figure 3). Interestingly,
the Ru catalyst 6 displayed a broader substrate scope compared
to its Fe analogue. It was found that an increased steric demand
at the -position had no noticeable influence on the performance
of catalyst 6 (7k, 7l and 7m). Also, as in the case of the Fe
catalyst 5, no general trend in reactivity was observed for
electron-withdrawing or -donating aryl substituents. Again,
By comparing the performance of the two pincer catalysts, it
was found that the Ru-Macho® catalyst is more stable and
generally give lower amounts of the undesired ketone 8 than its
Fe analogue. Attempts to use the racemization catalysts in a
DKR were unsuccessful. When these catalysts were
incorporated into a DKR protocol, no racemization was observed
after the addition of various acyl donors. A strong interaction of
the acyl donor with the active catalyst intermediate is likely to be
responsible for this deactivation.
In conclusion, we have investigated the utility of two new
pincer-type catalysts in the racemization of secondary alcohols.
These two catalysts showed great activity in the racemization of
benzyl alcohols. For the first time an iron-based catalyst was
demonstrated to efficiently racemize alcohols. With this protocol

Chemistry - A European Journal
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COMMUNICATION
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Typical procedure for the Fe-catalyzed racemization of secondary
alcohols. Fe-pincer catalyst 5 (2.5 mg, 0.00625 mmol) was dissolved in
dry and degassed MTBE 1 mL. After achieving the desired reaction
temperature of 50 °C, t-BuOK (1.4 mg, 0.0125 mmol) was added to the
yellow solution, which caused an immediate color change to deep red.
Shortly after base addition, the secondary alcohol 7 (0.25 mmol) was
added to the reaction mixture under an argon atmosphere. After 30
minutes reaction time, an aliquot was withdrawn and analyzed by HPLC
and/or GC.
Typical procedure for the Ru-catalyzed racemization of secondary
alcohols Ru-Macho®-catalyst 6 (1.5 mg, 0.0025 mmol) was dissolved in
dry and degassed MTBE 2 mL. After achieving the desired reaction
temperature of 50 °C under argon, t-BuOK (0.6 mg, 0.005 mmol) was
added to the reaction mixture, followed by the addition of secondary
alcohol 7 (0.5 mmol). After 15 minutes reaction time an aliquot was
withdrawn and analyzed by HPLC and/or GC.
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
Acknowledgements
[23]
This research was supported by the Swedish Research Council
(621-2013-4653), the European Research Council (ERC Adg
247014), the Berzelii Center EXSELENT, and the Knut and Alice
Wallenberg Foundation. The support by the DAAD (German
Academic Exchange Service) for a three-month exchange (C.
B.) is highly appreciated.
[24]
[25]
Keywords: iron • ruthenium • racemization • secondary alcohols
• pincer-type catalysis
[26]
Noteworthy, in THF in the absence of base a complete loss of optical
purity was achieved, however, significant amounts of dehydrogena-
tion product, acetophenone 8a, were formed (Table S1, entry 17).
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Chemistry - A European Journal
10.1002/chem.201601793
COMMUNICATION
COMMUNICATION
An efficient procedure for the
racemization of secondary benzylic
alcohols is presented. Using pincer-
type complexes, the first Fe-catalyzed
C. Bornschein, K. P. J. Gustafson, O.
Verho, M. Beller, J.-E. Bäckvall*
Page No. – Page No.
protocol
was
developed.
The
Evaluation of Fe and Ru Pincer-Type
Complexes as Catalysts for the
Racemization of Secondary Benzylic
Alcohols
efficiency of the Fe catalyst and its Ru
analogue was evaluated for a wide
range of sec-benzylic alcohols. It was
found that the commercially available
Ru complex tolerates water and air in
the
reaction
mixture
during
racemization.