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Finally, the general applicability of the methodology was
evaluated through the hydrogenation of a series of aromatic
and aliphatic nitriles (Table 3). For the majority of substrates,
good yields of the corresponding primary amines were ob-
tained at 508C by using 15 bar of H2. Electron-releasing
(Table 3, entries 2–6) as well as electron-withdrawing (Table 3,
entries 7–9) substituted benzonitriles were successfully hydro-
genated with the [Ru(cod)(methylallyl)2]/L2 system, although
higher catalyst loadings were needed (2 mol%) in the latter
case. Interestingly, ester and amido functionalities were also
tolerated (Table 3, entries 10 and 11) and the corresponding
amines were isolated in high yields. However, slightly higher
temperatures (708C for methyl 4-cyanobenzoate and 1008C for
N-(4-cyanophenyl)acetamide) and 2 mol% catalyst loading
were required for these examples. Furthermore, thiophene-2-
carbonitrile could also be hydrogenated at mild conditions by
using 2 mol% catalyst (Table 3, entry 12).
Then, a series of aliphatic nitriles was tested by using the
[Ru(cod)(methylallyl)2]/L2 system (Table 3, entries 13–20) at
508C, with 15 bar of H2, and 0.5 mol% catalyst loading. Under
these conditions, aliphatic nitriles bearing a short alkyl chain
afforded the corresponding amines in high yields (Table 3, en-
tries 13–15). In contrast, substrates with longer alkyl chains
(Table 3, entries 16 and 17) or branched (Table 3, entry 18),
needed either higher catalyst loadings (1 mol%) or tempera-
tures (708C) to be successfully hydrogenated, which is proba-
bly due to the major steric hindrance of these substrates. Final-
ly, cyclic nitriles (Table 3, entries 19 and 20) were converted
into the corresponding primary amines at mild conditions, too.
In comparison with the already described Ru base-free sys-
tems for the hydrogenation of nitriles, most of which are
based on pincer complexes operating in a temperature range
between 90 and 1358C,[11a,b,d] the [Ru(cod)(methylallyl)2]/L2
system presents the advantage of being able to hydrogenate
a broad range of substrates at 508C.
Figure 3. Yield versus time kinetic profiles using [Ru(cod)(methylallyl)2]/L2
1 mol%, at 508C, 15 bar H2, 6 h for: a) benzonitrile 3 hydrogenation to
benzyl amine 4; b) 1-octanenitrile 5 hydrogenation to octyl amine 6; c) con-
comitant benzonitrile 3 and 1-octanenitrile 5 hydrogenation. Insets for a and
b correspond to the initial rates plots in which [TOF]0 is calculated as
[r0 (mol%)À1] where r0 is the slope of the linear equation: yield
(%)=r0 time (h) defined at initial reaction times. Figure S1 (in the Support-
ing Information) shows the initial rates for all the experiments.
fined at initial reaction times) for the aromatic nitrile is approxi-
mately 3.5 times higher than the value for the aliphatic one. In-
terestingly, in the experiment performed with both nitriles (Fig-
ure 3c), although the hydrogenation rates for both com-
pounds decreased, the ratio between the [TOF]0 values corre-
sponding to each substrate is very similar, being four times
higher for the aromatic nitrile with respect to the aliphatic
one. These results clearly demonstrate that the studied catalyt-
ic system is more active for the hydrogenation of aromatic ni-
triles than for aliphatic ones.
Conclusions
We have described a novel catalytic system based on the com-
bination of [Ru(cod)(methylallyl)2] and ligand L2 for the selec-
tive hydrogenation of nitriles to primary amines. This new pro-
tocol presents the advantage of not needing any basic additive
to achieve good selectivities to the primary amine. Further-
more, a variety of aromatic and aliphatic nitriles can be hydro-
genated at mild conditions (508C, 15 bar H2).
With the aim of gaining mechanistic clues about the catalyt-
ic system, a yield versus time profile of the hydrogenation of
1-octanenitrile was performed after preactivation of the [Ru(-
cod)(methylallyl)2]/L2 combination with H2 (Figure S2a in the
Supporting Information). Under these conditions, the induction
period was significantly mitigated, indicating that a Ru–hydrido
complex could be an active species in the catalytic cycle. More-
over, the prevention of the induction period was also observed
when the hydrogenation of 1-octanenitrile was performed in
the presence of 10 mol% KOtBu (Figure S3b in the Supporting
Information).
Experimental Section
General procedure for nitrile hydrogenation with [Ru(cod)-
(methylallyl)2]/L2
A 4 mL glass vial containing a stirring bar was charged with
[Ru(cod)(methylallyl)2] (0.8 mg, 0.0025 mmol, 0.5 mol%) and L2
(1.5 mg, 0.0025 mmol, 0.5 mol%). The vial, sealed with a septum
equipped with a syringe needle, was evacuated and subsequently
flushed with argon three times. Dry isopropanol (2 mL) and the
corresponding nitrile (0.5 mmol) were added under argon. The vial
was set in an alloy plate and introduced into a 300 mL autoclave
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