Angewandte
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Ph-BPE and nBu3SiH was accomplished with all of the
bifunctional catalysts 4a, 4b, and 5 (entries 8–10) and merely
trace amounts of acetophenone were observed.
With three promising candidates as racemization catalysts
in hand, we next investigated their activity in the aforemen-
tioned racemic background reactions (Scheme 2). Impor-
tantly, none of the pincer complexes catalyzed the racemiza-
tion of the silyl ether (S)-6a in basic medium (Scheme 2, top).
Additionally, both the Ru- and the Os-CNN complexes 4a
and 4b did not promote the dehydrogenative coupling of 1-
phenylethanol (2a) and nBu3SiH (Scheme 2, bottom). Con-
versely, when Ru-PNP complex 5 was employed as the
precatalyst, substantial amounts of silyl ether 6a were formed
within 14 h, therefore thwarting its suitability for our
endeavor. Without any obvious differences in catalytic
activity between 4a and 4b, we continued with Ru-CNN
complex 4a as the designated racemization precatalyst.
We then tried the DKR of racemic 1-phenylethanol (2a)
in the presence of a copper catalyst, employing ruthenium
complex 4a and our established resolving system consisting of
CuCl, NaOtBu, (R,R)-Ph-BPE, and nBu3SiH in toluene. With
1.1 equiv of the hydrosilane, quantitative conversion of the
alcohol was achieved after stirring for 14 h at room temper-
ature, and the corresponding silyl ether (S)-6a was isolated
with 86% ee. It is worth noting that the reaction also
proceeded smoothly in polar protic solvents such as tert-amyl
alcohol with unchanged reactivity and enantioselectivity (see
the Supporting Information for details). However, reducing
the reaction temperature to 08C did not improve the
enantiomeric excess although good reactivity was maintained.
With suitable conditions established, we explored the
scope of this DKR (Scheme 3). Various sterically and
electronically modified benzylic alcohols 2a–z were tested,
and the corresponding silyl ethers (S)-6a–y were generally
obtained in almost quantitative yields with good to high levels
of enantioselection. Both electron-donating (Me and OMe)
and electron-withdrawing substituents (aryl, heteroaryl, hal-
ogens, CF3, and CO2tBu) were well tolerated but a trend
regarding the reactivity or selectivity was not apparent. A
detailed survey of the aryl groupꢁs steric effects revealed that
monosubstitution of the parent 1-phenylethanol (2a) with
Figure 1. Candidates for catalytic alcohol racemization.
Table 1: Identification of a suitable racemization catalyst.[a]
Entry
Catalyst
Variation
ee [%][c]
1
2
3
1
1
1
1
w/o (R,R)-Ph-BPE, nBu3SiH
0
32
93
95
93
94
90
0
w/o nBu3SiH
w/o (R,R)-Ph-BPE
none
4
5
6
7
8
9
10
3a
3b
3c
4a
4b
5
none
none
none
none
none
none
0
0
[a] All reactions were performed on a 0.1 mmol scale. [b] Estimated by in
situ 1H NMR analysis; ratio based on the integration of baseline-
separated resonance signals. [c] Determined by HPLC analysis on
a chiral stationary phase.
nBu3SiH were added (entries 2–4).[12] A similar result was
obtained with the structurally related, more robust ruthenium
complex 3a where one carbonyl ligand is replaced by
triphenylphosphine (entry 5).[3b] The electronic modification
of this complex using electron-poorer and -richer phosphines
as in 3b and 3c had no effect (entries 6 and 7). We then turned
our attention towards bifunctional catalysts bearing a metal-
bound amino group that have been employed in transfer
hydrogenation with great success.[13] In contrast to the half-
sandwich complexes 1 and 3a–c operating through inner-
sphere mechanisms,[14] these catalysts likely operate through
outer-sphere mechanisms involving hydrogen-bonding net-
works between the substrate and the NH2 functionality as
a crucial feature.[15] In recent years, transition-metal pincer
complexes have emerged as highly active and robust catalysts,
with [M(CNN)(dppb)Cl] 4a (M = Ru)and 4b (M = Os) as
well as [Ru(PNP)(CO)HCl] 5 achieving particularly high
turnover frequencies.[16] Also, ruthenium complex 5 was
reported to facilitate the catalytic hydrogenolysis of chlor-
osilanes to produce hydrosilanes,[17] thereby making catalyst
poisoning under our setup unlikely. Indeed, fast racemization
of (S)-1-phenylethanol [(S)-2a] in the presence of both (R,R)-
Scheme 2. Interrogation of potential background reactions. All reac-
tions were performed on a 0.2 mmol scale. Enantiomeric excesses
were determined by HPLC analysis on a chiral stationary phase after
cleavage of the silyl ether. Conversions were determined by GLC
analysis using tetracosane as an internal standard.
2
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Angew. Chem. Int. Ed. 2020, 59, 1 – 6
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