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C. Kucukturkmen et al. / Catalysis Communications 74 (2016) 122–125
increased to reflux, useful levels of enantioselectivity could be obtained.
Although the reactions proceeded more slowly, the enantioselectivity
observed was both reproducible and reliable. Various bases such as
i
t
t
NaOH, KOH, NaO Pr, NaO Bu, KO Bu and different substrate/catalyst
ratio have been explored with the aim of optimizing the reaction con-
ditions. To date, the best conversions and enantioselectivities were
attained when the reactions were performed with 1000:1–1000:3
Scheme 3. Application of 4b–d for the catalytic asymmetric transfer hydrogenation of
acetophenone.
i
substrate-complex ratio in the presence of 1 M solution of NaO Pr in
2-propanol.
around the Ru(1) atom arises from the P(1)–Ru(1)–Cl(2) axis with the
angle of 175.11(8) which is not exactly perpendicular to the N(1)/
N(3)/Cl(1)/P(1)/Ru(1) coordination plane. The average bond lengths
of the configuration around the Ru(1) atom are 2.4529(18),
With optimized conditions in hand, acetophenone and its deriva-
o
tives were examined in the ATH reaction (Table 1). When the reaction
was performed in the presence of L-alanine-based ruthenium-catalyst
4b, enantioselectivity from moderate to good (71–86%) and excellent
conversions (86–99%) were obtained except 4-methoxyacetophenone
(59% ee). It is noteworthy to state that similar results in the ATH
reaction were achieved with L-t-leucine-based ruthenium-catalyst 4d
and 64–91% ees and 77–99% conversions were obtained and also
the lowest enantiomeric excess was observed in the ATH of 4-
methoxyacetophenone (59% ee) as well. On the contrary, L-valine-
based ruthenium catalyst 4c showed low activity (9–63% ee) and also
extended the reaction time and lower ees. High enantioselectivity was
achieved for ATH of 2-methoxyacetophenone with all ruthenium cata-
lysts 4b–d.
Substrate scope was expanded using catalyst 4d because of given
the best ee rates. Further 12 different ketones were examined in ATH
reaction and in most cases high conversions and good enantiose-
lectivities were achieved (Table 2). Ortho and para substituted
chloroacetophenones (entries 1 and 2) gave slightly better conversions
and ees than bromo counterparts (Table 1, entries 5 and 7). In the case
of ethyl phenyl ketone had a high activity (entry 5), while bulkier
isopropyl phenyl ketone showed a low activity (entry 6). It has been
succeeded by excellent conversion and high enantioselectivity with
hetereoaromatic 2-acetylpyridine (entry 8), however 2-acetylthio-
phene relatively yielded moderate selectivity (entry 7). Although very
good conversions for ATH of aliphatic ketones were obtained, unfortu-
nately the enantioselectities remained very low (entries 10–12).
2
.2805(17) and 2.1741(53) Å for the Ru–Cl, Ru–P and Ru–N bonds, re-
spectively, which are also comparable with the values in the literature
7,35]. In the complex, a C–H∙∙∙Cl inter-molecular hydrogen bonding in-
teraction is observed [C(32)∙∙∙Cl(2); 3.595(8) Å, C(32)–H(32)∙∙∙Cl(2);
[
o
1
52.00(5) ]. The crystal packing is stabilized by N–H∙∙∙Cl, C–H∙∙∙N and
C–H∙∙∙Cl intra-, and C–H∙∙∙Cl inter-molecular hydrogen bonds. Addition-
ally, C–H∙∙∙π interactions are available in the unit cell.
Along with L-alanine 5b, commercially available L-Valine 5c and L-t-
Leucine 5d which are more sterically hindered group on chiral center
were used to synthesize ruthenium complexes performing the same
procedure. While complex 4c synthesized from L-Valine 5c was obtain-
ed as a single isomer after rinsing with ethyl ether and petroleum ether,
a single isomer from complex 4d could not be obtained by purification.
Therefore, a mixture of isomers in the ratio of 5:1 was used in the sub-
sequent ATH reactions.
The chiral ruthenium complexes 4b–d were then used in the asym-
metric transfer hydrogen reactions as catalysts (Scheme 3). Initially,
the ATH was examined using ruthenium complexes in the presence of
i
i
NaO Pr as a base and PrOH as a hydrogen source and solvent at reflux
temperature. Two different methods were applied according to differ-
ences during the addition of reagents. In the first method, the solution
i
of acetophenone in the PrOH was heated at reflux temperature and
then the complex and base were added into the reaction mixture.
Although rapid conversions were achieved, the enantiomeric excesses
remained very low using this protocol. Moreover, repeated trials using
this method led to highly variable enantiomeric excess measure-
ments. On the other hand, when all of the reagents were added into
3. Conclusion
In summary, a series of new chiral quinazoline-based ruthenium
complexes have been synthesized and characterized. In order to
i
the PrOH at ambient temperature and the reaction was gradually
Fig. 2. Molecular structure of the title compound 4b with the atom-numbering. The thermal ellipsoids are drawn at 30% probability level.