Li et al.
JOCArticle
complexes,8 we were pleased to find that the chemoselectivity
could be completely switched from CdO to CdC bonds
through further polarization of the olefins, and high enantio-
selectivity (up to 89% ee) was obtained in the asymmetric
transfer hydrogenation of prochiral R,R-dicyanoolefins.9
Chemoselective reduction of R,β-unsaturated carbonyl com-
pounds has been widely studied, and the selective reduction of the
carbonyl group of R,β-unsaturated ketones to allylic alcohols has
been achieved with relative ease.10 In contrast, the catalytic trans-
fer hydrogenation of the alkenic double bonds of conjugated
enones, especially using environmentally benign hydride source
and solvent, is limited.11 The conjugate reduction of enones has
been performed by use of ruthenium,12 rhodium,13 and iridium14
complexes as catalyst in transfer hydrogenation manner, and the
ligands adopted included phosphine,12,13a-f,14a,b carbene,13h,i
Phebox (Phebox=bis(2-oxazolinyl)phenyl),13j and bipyridine.14c
To the best of our knowledge, the conjugate reduction catalyzed
by the corresponding metals containing amine ligands10b-f has
not been investigated. Herein, we would like to report the highly
chemoselective transfer hydrogenation of R,β-unsaturated ke-
tones catalyzed by Rh-diamine complex employing HCO2Na as
hydride source. It was notable that the chemoselectivity could
dramatically switch from CdO to CdC bonds when the reaction
was conducted in aqueous media.
TABLE 1. Catalytic Effect of Metal Precursors and N-Sulfonylated
Diamine Ligands
selectivity (%)b
(2a:3a):4a
entry
1a
metal
ligand
time (h)
conv (%)b
5a
5a
5a
5a
5b
5b
5c
5c
5a
5c
5b
5b
5b
5b
5b
6a
6b
6c
6c
6c
6c
6c
6c
6c
6c
6c
6a
6b
6d
6e
12
12
12
85
12
85
12
85
0.5
0.5
0.5
0.5
0.5
0.5
0.5
48
33
40
92
17
21
17
27
95
91
98
91
98
36
9
19 (16:3):81
34 (31:3):66
56 (50:6):44
64 (40:24):36
87 (84:3):13
87 (85:2):13
73 (71:2):27
87 (84:3):13
63 (3:60):37
95 (83:12):5
98 (42:56):2
79 (45:34):21
92 (11:81):8
96 (95:1):4
2a
3a
4a
5a
6a
7a
8a
9c
10c
11c
12c
13c
14c
15c
94 (93:1):6
aThe reaction was performed with 0.5 mmol of 1a and 0.2 mL of
TEAF in 1 mL of DCM under argon atmosphere at 28 °C. bDetermined
by GC. cThe reaction was performed with 0.5 mmol of 1a and 7.5 equiv
of HCO2Na in 1 mL of degassed water under argon atmosphere at 60 °C
monosulfonylated diamine complexes in organic solvent and
aqueous phase. Initial studies were carried out in DCM at 28 °C
using the azeotrope of formic acid and triethylamine (5:2)
(TEAF) as hydrogen source. The precatalyst was generated
by in situ treatment monosulfonylated diamine with metal
precursor in degassed MeOH for 1 h in the presence of 2 equiv
of Et3N (triethylamine) when refluxing. Similar to the previous
example catalyzed by monosulfonylated diamine-Ru(II) com-
plex,15 the model substrate 1a was reduced via 1,4-reduction
and 1,2-reduction pathways at the same time and afforded the
1,4-adduct 2a (4-phenylbutan-2-one), 1,2-adduct 4a ((E)-4-
phenylbut-3-en-2-ol), and alcohol 3a (4-phenylbutan-2-ol) with
both CdC and CdO double bonds reduced (Table 1, entries
1-8). The monosulfonylated diamine ligand TsEN (TsEN =
N-(p-toluenesulfonyl)-1,2-ethylenediamine) had an obvious
effect on the chemoselectivity, furnishing the desired 1,4-ad-
ducts 2a and 3a in 56% selectivity and 40% total conversion
after 12 h (entry 3). The chemoselectivity could slightly increase
after prolonged time, and the ratio of 1,4-reduction reached up
to 64% (entry 4). In order to further improve the chemoselec-
tivity, we investigated other metal precursor (Figure 1),
[RhCl2Cp*]2 (Cp* = C5Me5) and [IrCl2Cp*]2, and found that
better selectivity could be obtained in the presence of [RhCl2-
Cp*]2 with TsEN in DCM (entries 5-8). To our surprise, the
conversion could not be improved for a prolonged period of
time (85 h) (entries 5 vs 6 and entries 7 vs 8).
Results and Discussion
Table 1 shows the representative results for the transfer
hydrogenation of benzylideneacetone 1a catalyzed by various
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