C O M M U N I C A T I O N S
benzoate.14 Formation of the ester became very slow after 6 h,
perhaps because of retardation of the reaction by the high ester
concentration. However, almost quantitative formation of benzyl
benzoate was obtained (entry 7).
Although further studies are required, a novel mechanism
involving aromatization/dearomatization and amine arm hemilability
seems plausible. Upon reaction of 3 with the alcohol, an aromatic,
coordinatively saturated alkoxy hydride complex may be generated.
Amine “arm” opening would enable the â-H elimination process,
followed by aldehyde elimination to give complex 4. Dihydrogen
loss from 4 regenerates 3, as described above. Mechanistic studies
are now underway.
In conclusion, new Ru(II) hydride complexes based on electron-
rich PNP and PNN ligands catalyze alcohol dehydrogenation to
esters. Catalyst design has resulted in the novel complex 3 which
is an outstanding catalyst for the acceptorless dehydrogenation of
primary alcohols to esters under mild, neutral conditions, providing
an environmentally benign method for the direct synthesis of esters
from alcohols.
To further improve the reaction, we aimed at totally eliminating
the need for a base. A possible role of the base is deprotonation of
2 to the corresponding Ru(0) complex. Exploring this possibility,
2 was treated with 1 equiv of KOBut at -32 °C. Interestingly,
deprotonation of the benzylic phosphine “arm”, rather then the
hydride ligand, took place, resulting in the brown-red Ru(II)
complex 314 in 89% yield. 31P{1H} NMR of 3 shows a singlet at
94.7 ppm, representing an upfield shift of 14 ppm relative to
complex 2. The hydride ligand gives rise to a doublet at -26.45
1
ppm (JPH ) 25.5 Hz) in H NMR. A one-proton singlet at 3.66
1
ppm in H NMR and a doublet at 65.25 (JPC ) 50.3 Hz) in 13C-
{1H} NMR indicate formation of an anionic PNN system. The CO
ligand absorbs at 1899 cm-1 in the IR spectrum.
Acknowledgment. Support of this project by the German-Israeli
Project Cooperation (DIP-G.7.1) and by the Israel Science Founda-
tion is gratefully acknowledged. J.Z. thanks the Aron Zandman
Foundation for a Postdoctoral Fellowship. D.M. is the Israel Matz
Professor of Organic Chemistry.
Transition-metal complexes with an anionic PNP ligand
(C5H3N(CHPPh2)(CH2PPh2)) were reported.15
In an unusual observation, reaction of complex 3 with excess
dihydrogen resulted in aromatization, yielding the trans-dihydride
complex 4, which was fully characterized.14 31P{1H} NMR of 4
shows a singlet at 124.9 ppm, downfield shifted by 30 ppm relative
to complex 3. The two magnetically equivalent hydride ligands give
Supporting Information Available: Experimental procedures and
characterization of the PNN ligand and complexes 1-3, procedure for
catalytic reactions. X-ray data for complex 2 in CIF format. This
1
rise to a doublet at -4.06 ppm (JPH ) 17.0 Hz) in H NMR. A
doublet (2H) at 3.12 ppm (JPH ) 8.5 Hz) and a singlet (2H) at
3.83 ppm for the two benzylic methylene groups in 1H NMR,
respectively, indicate the presence of a regular aromatic PNN
system. Significantly, 4 slowly loses H2 at room temperature to
regenerate complex 3 (Scheme 1).
References
(1) Larock, R. C. ComprehensiVe Organic Transformations; VCH: New York,
1989; p 966 and references therein.
(2) Ishihara, K.; Ohara, S.; Yamamoto, H. Science 2000, 290, 1140. (b) Corma,
A.; Nemeth, L. T.; Renz, M.; Valencia, S. Nature 2001, 412, 423. (c)
Hoydonckx, H. E.; De Vos, D. E.; Chavan, S.; Jacobs, P. A. Top. Catalysis
2004, 27, 83.
(3) Kraus, M. In Handbook of Heterogeneous Catalysis; Ertl, G., Kno¨zinger,
H., Weitkamp, J., Eds.; VCH Verlagsgesellschaft mbH: Weinheim, 1997;
Vol. 5, p 2159.
(4) Murahashi, S.-I.; Naota, T.; Ito, K.; Maeda, Y.; Taki, H. J. Org. Chem.
1987, 52, 4319.
(5) (a) Blum, Y.; Shvo, Y. J. Organomet. Chem. 1984, 263, 93. (b) Blum,
Y.; Shvo, Y. Isr. J. Chem. 1984, 24, 144. (c) Meijer, R. H.; Ligthart, G.
B. W. L.; Meuldijk, J.; Vekemans, J. A. J. M.; Hulshof, L. A.; Mills, A.
M.; Kooijman, H.; Spek, A. L. Tetrahedron 2004, 60, 1065.
(6) (a) Charman, H. B. J. Chem. Soc., B 1970, 584 (b) Morton, D.; Cole-
Hamilton, D. J. Chem. Commun. 1988, 1154.
(7) (a) Dobson, A.; Robinson, S. D. Inorg. Chem. 1977, 16, 137. (b) Jung,
C. W.; Garrou, P. E. Organometallics 1982, 1, 658. (c) Ligthart, G. B.
W. L.; Meijer, R. H.; Donners, M. P.; Meuldijk, J. J.; Ekemans, V. J. A.
J. M.; Hulshof, L. A. Tetrahedron Lett. 2003, 44, 1507.
(8) Shinoda, S.; Kojima, T.; Saito, Y. J. Mol. Catal. 1983, 18, 99.
(9) Matsubara, T.; Saito, Y. J. Mol. Catal. 1994, 92, 1.
(10) Lin, Y.; Ma, D.; Lu, X. Tetrahedron Lett. 1987, 28, 3115.
Complex 3 is the best homogeneous catalyst for acceptorless
dehydrogenative esterification of alcohols. When used as catalyst
without any base, ester yields of over 90% (TON > 900) were
obtained from the alcohols in relatively short reaction times (Table
1, entries 8-11). This reaction provides a convenient method for
the synthesis of esters because of its high efficiency, simplicity,
and facile isolation of the desired products. Murahashi et al reported
that heating Ru(H)2(PPh3)4 with 1-butanol at 180 °C in toluene
(sealed tube) resulted in 40 turnovers of butyl butyrate after 24 h.4
For direct comparison with our system, 0.1 mol % Ru(H)2(PPh3)4
was refluxed in 1-butanol at 117 °C for 72 h under argon
atmosphere, resulting only in 2% butyraldehyde and no formation
of ester, while the reaction catalyzed by 3 gave 90% of the ester
after 5 h under the same conditions. Heating the Shvo catalyst η4-
Ph4C4CO)Ru(CO)3 with 1000 equiv of benzyl alcohol in toluene
at 115 °C for 24 h resulted in 2% benzaldehyde; no ester was
observed, although this complex catalyzes ester formation at higher
temperatures.11
Dehydrogenation of primary alcohols to esters may, in principle,
proceed by dehydrogenation to the aldehyde followed by (a)
hemiacetal formation from the aldehyde and alcohol followed by
its dehydrogenation to the ester4,16 or (b) a Tischenko-type
disproportionation involving the aldehyde.17 When complex 3 was
heated with 100 equiv of benzaldehyde in toluene at 115 °C for 12
h, no benzyl benzonate was formed. On the other hand, heating
complex 3 with 100 equiv each of benzaldehyde and benzyl alcohol
in toluene at 115 °C for 12 h resulted in formation of benzyl
benzonate in 100% yield, indicating that the hemiacetal pathway
is likely to be operative.
(11) Blum, Y.; Shvo, Y. J. Organomet. Chem. 1985, 282, C7; yields and
reaction times were not reported.
(12) Homogeneous catalytic dehydrogenative lactonization of diols: (a) Lin,
Y.; Zhu, X.; Zhou, Y. J. Organomet. Chem. 1992, 429, 269; attempted
use of primary alcohols resulted in no catalysis. (b) Zhao, J.; Hartwig, J.
F. Organometallics 2005, 24, 2441.
(13) Zhang, J.; Gandelman, M.; Shimon, L. J. W.; Rozenberg, H.; Milstein,
D. Organometallics 2004, 23, 4026.
(14) See Supporting Information.
(15) Sacco, M.; Vasapollo, G.; Nobile, C. F.; Piergiovanni, A. J. Organomet.
Chem. 1988, 356, 397. The aromatic phosphor-ylide resonance form also
contributes to this structure.
(16) Menashe, N.; Shvo, Y. Organometallics 1991, 10, 3885.
(17) Ito, T.; Horino, H.; Koshiro, Y.; Yamamoto, A. Bull. Chem. Soc. Jpn.
1982, 55, 504
JA052862B
9
J. AM. CHEM. SOC. VOL. 127, NO. 31, 2005 10841