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conditions in almost 86% yield (entry 8). Reaction of benzyl
benzoate with dihydrogen resulted in almost quantitative
yield of benzyl alcohol after 7 h (entry 5); Reaction at room
temperature led to quantitative formation of the trans
dihydride ruthenium complex 4, although no alcohol was
formed. The aromatic methyl benzoate is quantitatively
hydrogenated to benzyl alcohol and methanol (entry 6) in a
relatively short time (4 h). As expected, the activated ester
dimethyl terephthalate is converted into the corresponding
dialcohol, 1,4-dimethanolbenzene, in high yield (97%) under
our conditions (Table 1, entry 10). In a previous report,
dimethyl phthalate was hydrogenated to phthalide using a
ruthenium hydride complex in a yield of 11.5% after 144 h at
1808C under a hydrogen pressure of 128 atm.[5a] The bulky
ester, tert-butyl acetate, was not effectively hydrogenated
(10.5% conversion after 24 h; Table 1, entry 9), probably
because of steric reasons.
Figure 1. ORTEP diagram of a molecule of complex 1 with the thermal
ellipsoids set at 50% probability. Selected bond lengths [] and
angles [8]: Ru1–C20 1.844(7), Ru1–N1 2.163(5), Ru1–P1 2.350(2), Ru1–
P2 2.291(2), Ru1–H1r 1.48(11), C1–C2 1.450(9), P1–C1 1.803(6), P2–
C7 1.843(7), C6–C7 1.552(9); C20-Ru1-N1 171.4(2), P1-Ru1-P2
153.1(1), H1r-Ru1-P1 72(4), H1r-Ru-P2 85(4).
A possible cycle for catalysis of ester hydrogenation by 2 is
presented in Scheme 2. Initially, dihydrogen addition to
complex 2 results in aromatization, to form the coordinatively
saturated, trans dihydride complexes 4, as experimentally
observed. Dissociation of the amine “arm” can provide a site
for ester coordination to the ruthenium center, to give the
intermediate A. A hydride ligand subsequently transfers to
Upon treatment of complex 1 with dihydrogen, the trans
dihydride complex 3 is quantitatively formed (see Scheme 1).
This complex can also be prepared by treatment of the
corresponding hydridochloride complex with NaHBEt3.[9]
Complex 3 slowly looses H2 at room temperature to
regenerate complex 1. The PNN complex 2 was recently
reported by us,[8] although crystals suitable for a single-crystal
X-ray diffraction study could not be obtained. This complex
shows analogous reactivity with dihydrogen, and it catalyzes
the dehydrogenation of primary alcohols to esters.[8,11]
Complexes 1 and 2 were studied as catalysts for the
hydrogenation of esters. Upon treatment of ethyl benzoate
with dihydrogen (5.3 atm) at 1158C for 16 h with a catalytic
amount of 3 (1 mol%), 7.5% conversion of the ester to yield
7% of benzyl alcohol and 7.5% of ethanol were obtained.
Performing the reaction at 1408C resulted in 12% conversion
to these alcohols. A trace amount of the transesterification
product benzyl benzoate was also formed (Table 1, entries 1
and 2).
ꢀ
the carbonyl group of the ester, followed by O H elimination
of a hemiacetal and regeneration of complex 2. The hemi-
acetal is in equilibrium with the aldehyde, which is readily
hydrogenated following a similar catalytic cycle via inter-
mediate B to form the corresponding alcohol.[12] The much
lower catalytic activity of complex 1 is attributable to the
difficulty in opening the chelate ring to provide a coordina-
tion site for ester coordination, to generate an intermediate
analogous to A, possibly indicating the importance of ligand
hemilability for ester hydrogenation. This mechanism differs
significantly from the ones outlined for ruthenium-catalyzed
ketone hydrogenation, in which binding of the ketone to the
metal is not required and hydrogenation takes place by a
concerted hydride/proton transfer.[13]
Remarkably, using complex 2 (1 mol%) as catalyst under
the same conditions (1158C) for 4 h resulted in 99.2%
conversion of ethyl benzoate into benzyl alcohol and ethanol
(Table 1, entry 3). Hydrogenation of hexyl hexanoate resulted
in formation of 1-hexanol in 82% yield after 5 h (entry 4).
Ethyl acetate was hydrogenated to ethanol under similar
In conclusion, non-activated aromatic and aliphatic esters
can be effectively hydrogenated to the corresponding alcohols
under relatively mild, neutral conditions using a (PNN)ru-
thenium hydride complex as catalyst. This reaction involves
an unusual aromatization/dearomatization sequence. The
analogous PNP complex is much less active, suggesting that
Table 1: Hydrogenation of esters catalyzed by 1 and 2.[a]
Entry
Ester
cat.
t [h]
T [8C]
Conversion [%]
Products (yield [%])[b]
1
2
3
4
5
6
7
8
9
ethyl benzoate
ethyl benzoate
ethyl benzoate
hexyl hexanoate
benzyl benzoate
methyl benzoate
ethyl butyrate
ethyl acetate
tert-butyl acetate
dimethyl terephthalate
1
1
2
2
2
2
2
2
2
2
16
16
4
5
7
4
4
12
24
5
115
140
115
115
115
115
115
115
115
115
7.5
12
benzyl alcohol (7); ethanol (7.5)[c]
benzyl alcohol (11.5); ethanol (12)[c]
benzyl alcohol (96); ethanol (99)[c]
1-hexanol (82.2)
99.2
82.2
98.5
100
100
86
benzyl alcohol (98)
benzyl alcohol (97); methanol (100)[c]
1-butanol (98); ethanol (98.6)
ethanol (85.6)
10.5
100
ethanol (10.5); tert-butanol (10.5)
1,4-dimethanolbenzene (97); methanol (100)
10
[a] Reaction conditions: ester (2 mmol); catalyst (0.02 mmol); H2 (5.3 atm); dioxane (2 mL). [b] Percentage of maximum possible amount of each of
the product alcohols; determined by GC. [c] A trace of benzyl benzoate was also formed: 0.3% (entries 1 and 2); 1% (entries 3 and 6).
1114
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 1113 –1115