UPDATES
Table 1. Optimization of Reaction Conditions.[a]
In the last few years, we have demonstrated that
nanoporous gold (AuNPore) having a unique three-
dimensional (3D) network nanopore structure was an
active and selective heterogeneous nanocatalyst in
various organic transformations.[8,9] The high surface
area compared to bulk metals, distinct electronic
property arising from hyperboloid-like ligaments, non-
toxicity, high thermal and mechanical rigidity, simple
recovery, and high reusability of AuNPore make it
more green and sustainable in the field of heteroge-
neous catalysis. Moreover, the bulk metallic feature of
AuNPore could prevent the aggregation-induced cata-
lytic deactivation and AuNPore without any supports
may help to understand the intrinsic catalytic activity
more easily by elimination of the support-effect.
During the investigation of the catalytic activity for the
dissociation of various chemical bonds in the last
decade, we have disclosed that the SiÀ H bond is
readily activated by AuNPore, reacting with water to
produce silanols and H2,[9a] which was successfully
extended to the selective reduction of various unsatu-
rated chemical bonds.[9b–f] Herein, we report that the
unsupported AuNPore is a highly active heterogeneous
catalyst for the selective reduction of tertiary amides to
Entry
SiÀ H
Yield [%][b]
1
PhMe2SiH
Et3SiH
(Me2SiH)2O
(i-Pr)3SiH
Ph2SiH2
99 (93)
2
34
45
0
67
0
0
0
0
3[c]
4
5[c]
6[d]
7[e]
8[f]
9[g]
PhSiH3
PhMe2SiH
PhMe2SiH
PhMe2SiH
[a] Reactions were carried out with 1a (0.5 mmol) and SiÀ H
(1.5 mmol) in the presence of 2 mol% of AuNPore catalyst at
°
80 C for 6 h.
[b] 1H NMR determined using CH2Br2 as an internal standard.
[c] 1.5 equiv. of (Me2SiH)2O was used.
[d] 1.5 equiv. of PhSiH3 was used.
[e] Without using catalyst.
[f] Au30Ag70 alloy was used instead of AuNPore.
tertiary amines with hydrosilanes as reductants under [g] PdNPore was used instead of AuNPore.
mild conditions (Scheme 1). AuNPore exhibited a
remarkably high reusability, which was used for twelve
times without any loss of catalytic activity.
2a in 93% isolated yield (entry 1). Further investiga-
The AuNPore catalyst was fabricated according to tion of other hydrosilanes revealed that the steric
our previously reported method by corrosive deal- hindrance of substituents on silicon drastically influ-
loying of Au30Ag70 alloy in 70 wt% nitric acid at enced the reaction efficiency. For example, the use of
ambient temperature for 18 h.[9b] The scanning micro- triethylsilane
and
1,1,3,3-tetramethyldisiloaxne
scope (SEM) image in Figure 1a clearly indicate the (TMDS) gave moderate yields of 2a, whereas triiso-
formation of 3D open pore network structure con- propylsilane was used to be totally ineffective (en-
structed by hyperboloid-like ligaments. The average tries 2–4). Diphenylsilane (1.5 equiv.) also can be used
diameter of Au ligaments was estimated to be around as reductant, giving 2a in good yield (entry 5).
30 nm.
However, the use of phenylsilane resulted in no
In light of our previously observed hydrosilane- reaction and the amide 1a was recovery quantitatively
effect on the AuNPore-catalyzed semihydrogenation of (entry 6). It was noted that the reduction without
alkynes,[9b] we initially investigated various hydro- catalyst or with the AuNPore precursor, AuAg alloy as
silanes in the reduction of N-phenyl-2-pyrrolidone (1a) a catalyst did not proceed at all (entries 7 and 8).
to N-phenylpyrrolidine (2a) using 2 mol% of AuNPore Moreover, nanoporous palladium (PdNPore) prepared
in EtOAc at 80 C (Table 1). The reaction with from a PdAl alloy[10] was used to be totally inactive
°
PhMe2SiH as a reductant afforded the desired product (entry 9), indicating the distinct catalytic performance
of AuNPore on the reduction of amides with hydro-
silanes. In addition, among various solvents tested
instead of EtOAc, toluene was also effective, produc-
ing 2a in 94% yield, but other solvents such as THF
(68%) and CH3CN (24%) were less effective.
The catalytic performance of AuNPore was further
studied for the reduction of various tertiary amides to
the corresponding tertiary amines using PhMe2SiH as a
°
reductant in EtOAc at 80 C (Table 2). The reductions
of 5-, 6-, and 8-membered cyclic amides having
benzyl, phenyl, and methyl substituents on the nitrogen
atom proceeded effectively, affording the correspond-
Figure 1. SEM Images of AuNPore: (a) before reaction, (b)
after twelfth reuse.
ing N-substituted pyrrolidine (2b), piperidine (2c),
Adv. Synth. Catal. 2019, 361, 4817–4824
4818
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