that this aqua complex might facilitate the H-atom transfer
from water to late transition metals usually employed as
hydrogenation catalysts (2) to give metal-dihydride species
such as 3.7 These species could subsequently bring about
alkene (and alkyne) hydrogenation as depicted in Scheme
1. In this way, formation of the expected hydrogenation
Table 1. H2O/Cp2TiCl-Based Hydrogenation of Alkenes and
Alkynes Catalyzed by Late Transition Metals
Scheme 1. Anticipated Hydrogen Transfer from
TiIII-Coordinated H2O to Alkenes Catalyzed by Late Transition
Metals
products might additionally serve to prove the occurrence
of H-atom transfers from TiIII-aqua complexes to other
transition metals, a phenomenon that has not been reported
to date.
To check our hypothesis we treated a series of alkenes
and alkynes with Cp2TiCl (2.5 equiv),5 H2O (10 equiv), and
a substoichiometric quantity (0.1 equiv) of different hydro-
genation catalysts, including Pd/C, Pd/alumina, Pd(dba)2,8
Rh/alumina and Wilkinson, and Lindlar catalysts. All these
experiments gave us the expected hydrogenation products
(Table 1),9 supporting the anticipated hydrogen-transfer
process depicted in Scheme 1.
a Besides 14 we obtained 15% decanal, presumably deriving from Pd-
induced alkene isomerization.7b
(5) Bis(cyclopentadienyl)titanium(III) chloride, generated in situ by
stirring commercial Cp2TiCl2 with Mn dust in THF, exists as an equilibrium
mixture of the monomer Cp2TiCl and the dinuclear species (Cp2TiCl)2;
see: (a) Enemærke, R. J.; Larsen, J.; Skrydstrup, T.; Daasbjerg, K. J. Am.
Chem. Soc. 2004, 126, 7853-7864. (b) Daasbjerg, K.; Svith, H.; Grimme,
S.; Gerenkamp, M.; Mu¨ck-Lichtenfeld, C.; Gansa¨uer, A.; Barchuck, A.;
Keller, F. Angew. Chem., Int. Ed. 2006, 45, 2041-2044. (c) Gansa¨uer, A.;
Barchuk, A.; Keller, F.; Schmitt, M.; Grimme, S.; Gerenkamp, M.; Mu¨ck-
Lichtenfeld, C.; Dassbjerg, K.; Svith, H. J. Am. Chem. Soc. 2007, 129,
1359-1371. For clarity’s sake we represent this complex as Cp2TiCl. For
recent reports on the synthetic applications of this single-electron-transfer
reagent, see: (d) Barrero, A. F.; Oltra, J. E.; Cuerva, J. M.; Rosales, A. J.
Org. Chem. 2002, 67, 2566-2571. (e) Barrero, A. F.; Rosales, A.; Cuerva,
J. M.; Oltra, J. E. Org. Lett. 2003, 5, 1935-1938. (f) Rosales, A.; Oller-
Lo´pez, J. L.; Justicia, J.; Gansa¨uer, A.; Oltra, J. E.; Cuerva, J. M. Chem.
Commun. 2004, 2628-2629. (g) Justicia, J.; Rosales, A.; Bun˜uel, E.; Oller-
Lo´pez, J. L.; Valdivia, M.; Ha¨ıdour, A.; Oltra, J. E.; Barrero, A. F.; Ca´rdenas,
D. J.; Cuerva, J. M. Chem. Eur. J. 2004, 10, 1778-1788. (h) Justicia, J.;
Oltra, J. E.; Cuerva, J. M. J. Org. Chem. 2004, 69, 5803-5806. (i) Justicia,
J.; Oller-Lo´pez, J. L.; Campan˜a, A. G.; Oltra, J. E.; Cuerva, J. M.; Bun˜uel,
E.; Ca´rdenas, D. J. J. Am. Chem. Soc. 2005, 127, 14911-14921. (j) Justicia,
J.; Oltra, J. E.; Cuerva, J. M. J. Org. Chem. 2005, 70, 8265-8272. (k)
Este´vez, R. E.; Oller-Lo´pez, J. L.; Robles, R.; Melgarejo, C. R.; Gansa¨uer,
A.; Cuerva, J. M.; Oltra, J. E. Org. Lett. 2006, 8, 5433-5436. For recent
reviews see: (l) Gansa¨uer, A.; Lauterbach, T.; Narayan, S. Angew. Chem.,
Int. Ed. 2003, 42, 5556-5573. (m) Cuerva, J. M.; Justicia, J.; Oller-Lo´pez,
J. L.; Oltra, J. E. Top. Curr. Chem. 2006, 264, 63-91.
Furthermore, when we treated ethyl cinnamate (9) with
D2O instead of H2O we obtained the double-deuterated
derivative 21 with a 75% incorporation of deuterium (Scheme
2), thus confirming that the incoming hydrogen atoms came
Scheme 2. Synthesis of Double-Deuterated Derivative 21
from water. The results obtained with the Wilkinson catalyst
also open up the possibility of using well-established rhodium
chiral catalysts for enantioselective H2O/Cp2TiCl-based al-
kene hydrogenations.10
The cis configuration of reduction products 19 and 20
coincided with that expected for conventional alkyne hy-
drogenation with the Lindlar catalyst.1,7b These results
provided additional support for our hypothesis that the
(6) Cuerva, J. M.; Campan˜a, A. G.; Justicia, J.; Rosales, A.; Oller-Lo´pez,
J. L.; Robles, R.; Ca´rdenas, D.; Bun˜uel, E.; Oltra, J. E. Angew. Chem., Int.
Ed. 2006, 45, 5522-5526.
(7) For homogeneous hydrogenation catalysts see: (a) Takaya, H.;
Noyori, R. In ComprehensiVe Organic Synthesis; Trost, B. M., Fleming,
I., Eds.; Pergamon: Oxford, UK, 1991; Vol. 8, pp 443-469. For
heterogeneous catalysts, see: (b) Siegel, S. In ComprehensiVe Organic
Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon: Oxford, UK, 1991;
Vol. 8, pp 417-442.
(9) For detailed experimental procedures see the Supporting Information.
(10) For a Nobel Lecture including outstanding rhodium chiral catalysts
see: Noyori, R. Angew. Chem., Int. Ed. 2002, 41, 2008-2022.
(8) Takahashi, Y.; Ito, T.; Sakai, S.; Ishii, Y. J. Chem. Soc., Chem.
Commun. 1970, 1065-1066.
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