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Table 1. Optimization of the reaction conditions.[a]
Entry
Solvent
Silane
t [h]
Yield [%][b]
[c]
1
2
3
4
5
6
7
8[e]
9[f]
10[g]
11
THF
THF
THF
THF
CH2Cl2
MeOH
toluene
THF
THF
THF
Et3SiH
Cl3SiH
Ph2SiH2
24
24
7
3.5
5
4
12
1
–
–
[d]
89
96
90
93
76
99
85
PhMe2SiH
PhMe2SiH
PhMe2SiH
PhMe2SiH
PhMe2SiH
PhMe2SiH
PhMe2SiH
16
24
24
[h]
–
–
[i]
[c]
THF
–
[a] Conditions: Benzaldehyde (0.1 mmol), aniline (0.1 mmol), silane
(0.15 mmol), AuCNTs (50 mL of a 4 mm suspension in THF, 0.2 mol%), sol-
vent (1 mL), RT. [b] Yield of isolated product. [c] No reaction. [d] The prod-
ucts were formed as a complex mixture. [e] The reaction was performed
with 0.4 mol% of the catalyst. [f] The reaction performed with
0.004 mol% of the catalyst. [g] The reaction performed without a catalyst.
[h] None of the desired product was obtained; only the imine was isolat-
ed. [i] The reaction was performed without the silane.
Figure 2. TEM image of the AuCNT catalyst; inset shows a 300% magnifica-
tion of the area indicated by the black rectangle.
3 nm (see the Supporting Information, Figure S14). The AuCNT
catalyst was suspended in water and the gold content in the
aqueous suspension of the nanohybrid was determined to be
4 mm by inductively coupled plasma mass spectrometry
(ICP-MS).
were to be calculated by only taking into account the surface
gold atoms (those in contact with the medium, as calculated
by using the method reported by Djꢂga-Mariadassou and co-
workers)[18] and not the actual Au loading (total gold content),
more-flattering TOF and TON values would be obtained
(TON=70833, TOF=4426 hꢀ1). Notably, no reaction was ob-
served in the absence of either the AuCNT catalyst (Table 1,
entry 10) or the silane (Table 1, entry 11).
Our studies started with the one-pot condensation of ben-
zaldehyde (1 equiv) with aniline (1 equiv) in the presence of
a silane hydride source (1.5 equiv) and a catalytic amount of
AuCNT (0.2 mol%). To avoid hydrolysis of the transient imine
(which is formed in situ), water was depleted from the catalyst
by repeated precipitation and resuspension in dry THF. Initially,
two silanes (i.e., Et3SiH and Cl3SiH; Table 1, entries 1 and 2)
were initially screened, albeit without success as no conversion
was detected. Then, diphenylsilane was tested as the hydride
source (Table 1, entry 3), which promoted the expected reduc-
tive amination reaction in 89% yield after 7 h. These promising
results prompted us to screen a few more conditions and fur-
ther improvement was achieved by using PhMe2SiH, which
permitted access to amine 3a in 96% yield after just 3.5 h
(Table 1, entry 4). Next, we investigated the influence of differ-
ent solvent systems and found that, under similar conditions,
CH2Cl2, MeOH, and toluene (Table 1, entries 5, 6, and 7) were
less effective, because the quantitative conversion of the sub-
strates required prolonged reaction times. Thus, PhMe2SiH and
THF were selected to further investigate the direct reductive
amination process.
With the optimized reaction conditions in hand, we exam-
ined the scope of our catalytic AuCNTs/PhMe2SiH system in the
direct reductive amination of aldehydes. We started by study-
ing the effect of substituents on the aromatic ring of the ben-
zaldehyde substrate. Whereas aldehydes that contained para-
halogenated aromatic rings (Table 2, entries 2 and 3) provided
the corresponding secondary amines (3b and 3c) in almost-
quantitative yields (97–98%) within 1.5–2 h, the introduction
of an electron-withdrawing NO2 group (Table 2, entry 4) led to
a slightly lower yield (91%) and required an extended reaction
time (5 h). In the case of aldehydes that contained electron-do-
nating groups (Table 2, entries 5 and 6), the reactions also pro-
ceeded smoothly; meta-tolualdehyde led to amine 3e in 95%
yield within 1.5 h and para-methoxybenzaldehyde provided
amine 3 f in 96% yield after 3 h. Bulkier 2-naphthaldehyde
(Table 2, entry 7) was also converted into the desired secon-
dary amine (3g) in high yield (88%) within 3 h. Under our opti-
mized conditions, the conversion of 2-thiophenecarboxalde-
hyde into the corresponding amine (3h) was easily performed
(93%, 4.5 h; Table 2, entry 8), as well as that of cinnamalde-
hyde to yield compound 3i (86%, 4 h; Table 2, entry 9). The re-
action of aliphatic phenyl acetaldehyde with para-toluidine
(Table 2, entry 10) was more sluggish and required more time
Upon increasing the catalyst loading to 0.4 mol%, amine 3a
was formed in quantitative yield and the reaction time was
shortened to 1 h (Table 1, entry 8). The reductive amination
was still operative with only 0.004 mol% of the AuCNTs
(Table 1, entry 9) and, under these conditions, remarkable
values of the turnover number (TON=21250) and turnover fre-
quency (TOF=1328 hꢀ1) were calculated. If the kinetic values
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