Organic Letters
Letter
bench-stable PhPNS ligand L-1. Mn-1 can be readily
synthesized by the treatment of L-1 with 1 equiv of
Mn(CO)5Br as a metal precursor in toluene at 100 °C for
16 h. The bright-yellow complex was isolated in 86% yield and
was characterized by NMR, IR, and mass spectrometry
(Scheme 1).
The use of manganese complexes, Mn-2, Mn-3, and Mn-4,
provided less satisfactory results (Table 1, entries 2−4). To
our delight, catalyst Mn-5 showed better reactivity, resulting in
60% conversion and a selectivity of 98:2, whereas dibenzyl was
not detected (Table 1, entry 5). The application of Mn-6
resulted in a lower reactivity albeit a similar selectivity when
compared with Mn-1 (Table 1, entry 6). A control experiment
showed that the reaction does not take place without the
catalyst (Table 1, entry 7). Interestingly, the use of K2CO3 or
Cs2CO3 for the activation of the catalyst was not successful
(Table 1, entries 8 and 9). The use of polar-aprotic THF as a
solvent resulted in 11% conversion of 1a (Table 1, entry 10),
whereas no reaction occurred if polar-protic MeOH was used
(Table 1, entry 11). Decreasing the hydrogen pressure to 20
bar helped to reduce the formation of undesired over-
hydrogenation products (Table 1, entry 12). Performing the
reaction at 50 °C led to the same result (Table 1, entry 13);
nevertheless, 60 °C appeared to be more suitable for a
substrate scope preparation. Additionally, no impact was
observed when a drop of mercury was added to the reaction
mixture, which suggests the homogeneous nature of the
catalyst under these reaction conditions (Table 1, entry 14).21
With the optimized reaction conditions in hand, we started
to explore the substrate scope for the selective semi-
hydrogenation of alkynes using our new PhPNS−Mn catalyst
(Table 2). A range of substrates bearing different electronic
and steric properties were well tolerated and provided the
corresponding (Z)-alkenes in good yields with excellent
chemoselectivity. It should be noted that the substrates bearing
electron-withdrawing substituents were significantly more
reactive than the ones bearing electron-donating groups.
Additionally, the hydrogenation of 1i, bearing ester function-
ality, proceeded chemoselectively toward alkyne hydrogena-
tion, and the ester group remained intact. Importantly, alkynes
that contain heterocycles (1o−q, 1x, 1y) could also be applied
and provided excellent reactivity and selectivity. Remarkably,
no proto-dehalogenation of C−Cl and C−Br bond took place
when 1-chloro-4-(phenylethynyl)benzene (1l) and 1-bromo-3-
(phenylethy-nyl)benzene (1n) were applied as substrates.
Moreover, our protocol was suitable for the application of
triisopropyl(phenylethynyl)silane 1r in the hydrogenation
reaction. Because of the higher steric hindrance of the
substrate, the reaction required 5 mol % of Mn-1, a slightly
higher hydrogen pressure of 30 bar, and 24 h of the reaction
time, resulting in a 76% yield of (Z)-triisopropyl-(styryl)silane
as a single isomer. Furthermore, the reduction of aryl-alkyl
alkynes, including protected propargylic alcohols 1s−y, led to
the formation of the corresponding (Z)-allylic alcohols,
demonstrating the wide scope of substrates. Additionally, a
gram-scale synthesis of (Z)-stilbene could also be achieved
using only 0.5 mol % of Mn-1, leading to the formation of a
99% yield of the desired product (Scheme 2), implying that the
described protocol could be suitable for the industrial
production of (Z)-alkenes.
Scheme 1. Synthesis of Mn-1
Our newly synthesized catalyst Mn-1 was subsequently
investigated in the hydrogenation of diphenylacetylene. Our
initial attempts proceeded by applying 1 mol % of Mn-1 and
2.5 mol % of KOtBu in toluene at 60 °C under 30 bar of H2 for
16 h. To our delight, diphenylacetylene was fully consumed,
producing the desired (Z)-stilbene in 88% GC yield as well as
1% of (E)-stilbene and 11% of dibenzyl as a result of
overhydrogenation (Table 1, entry 1).
a
Table 1. Optimization of the Reaction Condition
b
b
entry
[Mn]
Mn-1
Mn-2
Mn-3
Mn-4
Mn-5
Mn-6
base
conv. (%)
ratio 2a/3a/4a
1
2
3
4
5
6
7
8
9
10
11
12
13
14
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
K2CO3
Cs2CO3
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
>99
nr
nr
05
60
48
nr
nr
17
11
nr
>99
>99
>99
88:01:11
nd
nd
82:18:00
98:02:00
95:05:00
nd
Mn-1
Mn-1
Mn-1
Mn-1
Mn-1
Mn-1
Mn-1+Hg
nd
90:10:00
51:49:00
nd
96:01:03
96:01:03
93:01:06
c
d
To prove whether the described reaction proceeds via
metal−ligand cooperativity, we attempted to synthesize the
corresponding manganese N-Me derivative of Mn-1 because it
would establish whether the proton transfer from the ligand N-
atom would occur. Because the formation of the Mn-1(N-Me)
catalyst was not successful after several attempts, using
different solvents and temperatures, we prepared the
corresponding N-Me manganese complex for Mn-(6), which
also showed reactivity in the hydrogenation. As expected, the
e
e f
,
e g
,
a
Reaction conditions: 1a (1 mmol), [Mn] (1 mol %), and base (2.5
mol %) in 2 mL of toluene at 60 °C under 30 bar of H2 for 16 h.
b
Determined by the GC analysis using m-xylene as an internal
c
d
e
standard. Reaction in THF. Reaction in methanol. 20 bar of H2.
f
g
50 °C. One drop of mercury was added.
B
Org. Lett. XXXX, XXX, XXX−XXX