Organometallics
Note
reactivity of the Ru center is sensitive to the ligand
environment.
With these novel complexes, we first studied their catalytic
activities for the well-known transformation of alcohols to
esters via a dehydrogenative homocoupling reaction. An initial
test was carried using 1-hexanol as the model substrate with 0.1
mol % catalyst loading, as shown in Table 1. It was found that
high conversion and yields (entries 16 and 17). This
methodology offers a straight strategy for the selective
oxidation of benzylamines to the corresponding imines under
the base- and oxidant-free conditions.
8
Since the Milstein catalyst is capable of catalyzing the
4a
dehydrogenative coupling of amines and alcohols to amides,
complexes 1−3 were also studied using 1-hexanol and
benzylamine (eq 1). While it has been demonstrated that a
Table 1. Esterification of Primary Alcohol with Ruthenium
a
Complexes
number of ruthenium/ligand combinations show great activity
9
in this type of dehydrogenative amide generation, it was rather
surprising that our ruthenium complexes only afforded low
yields for the amide products (<10%), with the formation of a
mixture of imines and ester, presumably due to the enhanced
reactivity of complexes 1−3 toward the amine. Further
evidence was observed when the reactions were stopped at
low conversion of imine (e.g., 50%). The GC/MS and MS
fragmentation pattern analysis indicated that the homocoupled
imine was generated preferentially (in a ∼6:2:1 ratio, eq 1),
with only a negligible amount of amide formed, suggesting that
more benzylamine was dehydrogenated than the alcohol under
the same conditions. Accordingly, we propose that our Ru
complex with an imine arm is able to dehydrogenate the
primary amine to imine, which then reacts with another 1 equiv
of the primary amine to form the homocoupled imine product
with elimination of one molecule of ammonia (Scheme 2). 1-
a
b
1
Under argon. Determined by GC and H NMR.
3
the PN P complex 1 was active for the reaction (entry 1) and
8
0% conversion was achieved within 24 h. The efficiency was
N
C
increased to 98% when P N N complex 2 was employed
N
C
(
(
entry 2). P N N complex 3 displayed moderate activity
60%) for the same reaction (entry 3), although the analogous
H releases the hydrogen molecule to regenerate catalyst 1.
3b
2
Milstein complex 4 has shown superior activity. Complex 2
was chosen for further examination and showed high activities
toward other linear aliphatic alcohols (butyl alcohol 99%, entry
Both H and NH gases were detected from the reaction.
2
3
CONCLUSION
■
4
; pentanol 97%, entry 5). Benzyl alcohol can also be converted
In summary, we have developed a protocol for the dehydrogen-
ative homocoupling of primary alcohols to esters and coupling
of amines to imines catalyzed by a class of novel pincer
ruthenium complexes containing an imine arm. We have
demonstrated that replacing the CH with an N group in the
phosphine arm significantly enhances the reactivities of these
Ru complexes toward amines. Selective oxidation of benzyl-
amines to imines was accomplished using aniline derivatives as
the substrate and solvent. This simple, direct, and base-free
system should find useful applications in imine production
processes.
to the corresponding ester effectively (entry 6).
Encouraged by the results in the dehydrogenative ester
formation, the activities for the homocoupling of amines were
investigated and Milstein catalyst 4 was also examined for
comparison (Table 2). Using benzylamine as the model
substrate, PN P complex 1 showed excellent catalytic activity
for this reaction (entry 1) while P N N complex 2 was only
slightly less active (entry 2). P N N complex 3 also displayed
moderate activity under the same conditions (entry 3).
Intriguingly, Milstein’s catalyst 4 exhibited lower reactivity in
the reaction than catalysts 1−3 (entry 4). These observations
strongly suggest that introducing the imine arm into the pincer
complex indeed enhances the reactivity of the Ru complex
toward amines. A series of substituted benzylamine derivatives
were also tested. The presence of electron-donating sub-
stituents on the phenyl ring offered slightly lower yields of the
imine product as a result of further dehydrogenation of the
amine to give nitrile products (entries 5−7). The formation of
nitrile could be reduced when the reactions were conducted
under neat conditions (entries 8−11). Carrying out the
reactions using aniline as the solvent (e.g., aniline) enabled
the cross-coupling product N-arylimines to be cleanly produced
3
N
C
N
C
EXPERIMENTAL SECTION
■
General Procedure for the Esterification of Primary Alcohol.
In a typical experiment, a mixture containing alcohol (5 mmol), and a
suitable amount of ruthenium complex in a Schlenk flask equipped
with a cooling finger were heated at the stated temperature under an
argon atmosphere. Thereafter, 0.1 mL of the reaction mixture was
sampled and immediately diluted with 0.5−1.0 mL of i-PrOH for GC
analysis. After the reaction was complete, the reaction mixture was
purified by silica gel column chromatography to afford the ester
product, which was identified by comparison with an authentic sample
1
by GC-MS and H NMR analysis.
General Procedure for the Oxidation of Primary Amine to
Imine. In a typical experiment, a mixture containing a suitable amount
of amine and ruthenium complex in toluene, as a neat mixture, or in
aniline in a Schlenk flask equipped with a cooling finger were heated at
the stated temperature under an argon atmosphere. Thereafter, 0.1 mL
of the reaction mixture was sampled and immediately diluted with
(
entries 12−17). These results suggested that the formation of
nitrile in the case of 4-methyl- or 4-methoxybenzylamine was
greatly minimized with increased amine concentrations. Aniline
containing electron-withdrawing substituents such as F and
electron-donating substituents such as MeO could also reach
5
209
dx.doi.org/10.1021/om300422v | Organometallics 2012, 31, 5208−5211
He, Li-Peng
