transition state involving a protonated ruthenacycle since the
protonation of ruthenacyclopentatriene intermediates and sub-
sequent nucleophilic attack of conjugate bases onto the remain-
ing carbene carbon have been proposed in a series of studies by
Dixneuf’s group.13 The protonation of a model ruthenacycle
methoxo complex with HCl was estimated to occur with an
activation barrier of DGz = 19.6 kcal molꢁ1, which is smaller
than that of TS-II. Although the protonation is endothermic
(DG = 5.4 kcal molꢁ1), a further lower activation barrier of DGz
= 16.0 kcal molꢁ1 was calculated for the subsequent hydride-
transfer process via TS-IV (Fig. S9, ESIw). As a result, the
protonation/hydride transfer mechanism is favorably supported
by these DFT calculations. In addition, the subsequent isomer-
ization of the resultant ruthenacyclopentene into a Z4-diene
complex is estimated to be facile and thermodynamically favor-
able owing to a small activation barrier of DGz = 2.2 kcal molꢁ1
and a large exothermicity of 31.2 kcal molꢁ1 (Fig. S9, ESIw).
Therefore, the step of the highest barrier in the overall process is
the reversible protonation (B $ C), and this is in good accor-
dance with the significant rate decrease observed when using
CH3OD in place of CH3OH. In addition, a smaller yet signifi-
cant barrier was estimated for the hydride-transfer step, and this
result was experimentally corroborated by the observation of
KIE for the competitive reaction between CH3OH and CD3OH.
This research was supported by the MEXT, Grant-in-Aid for
Scientific Research (B) (20350045), JSPS Research Fellowship
for Young Scientists (KY, 10438), and the Global COE program.
Scheme 4 Plausible catalytic cycle and transition states with
calculated activation barriers DGz(298 K).
but there was a negligible difference in the rates between
CD3OH and CH3OH. In addition, a small kinetic isotope
effect of kH/kD = 1.9 was observed when 5c0 was treated with
a
1 : 1 mixture of CH3OH and CD3OH in refluxing
THF (ESIw).
Scheme 4 outlines a plausible catalytic cycle of the
Cp*RuCl-catalyzed reductive cyclization of the 1,6-diynes.
On the basis of the observation of the transfer hydrogenation
of 1 (Scheme 1), it is reasonable to propose that the corres-
ponding ruthenacycle with a general formula A is a key
intermediate. In a polar solvent, A must be in equilibrium
with methoxo analogue B, which undergoes reversible proto-
nation with concomitantly produced HCl. Subsequently,
hydride abstraction from the methoxo ligand by an electrophilic
carbene center occurs via transition state D, giving rise to
ruthenacyclopentene E and one molecule of formaldehyde.
The isomerization of E must occur in a disrotatory fashion to
furnish more thermodynamically stable Z4-1,3-diene complex F.
The intermediacy of F is again supported by the isolation of 3
(Scheme 1). Finally, the displacement of the resultant
1,3-diene ligand with a 1,6-diyne substrate is followed by
oxidative cyclization to reproduce A.
Notes and references
1 For reviews, see: (a) I. Ojima, M. Tzamarioudaki, Z. Li and
R. J. Donovan, Chem. Rev., 1996, 96, 635; (b) J. M. Takacs,
S. C. Bolto and Y.-C. Myoung, Curr. Org. Chem., 1998, 2, 233;
(c) S. M. A. Sohel and R.-S. Liu, Chem. Soc. Rev., 2009, 38, 2269.
2 B. M. Trost and D. C. Lee, J. Am. Chem. Soc., 1988, 110, 7255.
3 (a) H.-Y. Jang and M. J. Krische, J. Am. Chem. Soc., 2004, 126, 7875;
(b) I. G. Jung, J. Seo, S. I. Lee, S. Y. Choi and Y. K. Chung,
Organometallics, 2006, 25, 4240; (c) K. T. Sylvester and P. J. Chirik,
J. Am. Chem. Soc., 2009, 131, 8772.
4 Selected reviews for transfer hydrogenations from alcohols, see:
(a) J. S. M. Samec, J.-E. Backvall, P. G. Andersson and P. Brandt,
¨
Chem. Soc. Rev., 2006, 35, 237; (b) T. Ikariya and A. J. Blacker,
Acc. Chem. Res., 2007, 40, 1300; (c) M. H. S. A. Hamid,
P. A. Slatford and J. M. J. Williams, Adv. Synth. Catal., 2007,
349, 1555; (d) J. F. Bower, I. S. Kim, R. L. Patman and
M. J. Krische, Angew. Chem., Int. Ed., 2009, 48, 34.
5 For a review on Cp*Ru-catalyzed reactions, see: B. M. Trost,
M. U. Frederiksen and M. T. Rudd, Angew. Chem., Int. Ed., 2005,
44, 6630.
6 Y. Yamamoto, K. Yamashita and H. Nishiyama, Chem. Commun.,
2011, 47, 1556.
7 M. I. Bruce, G. A. Koutsantonis and E. R. T. Tiekink,
J. Organomet. Chem., 1991, 420, 271.
8 (a) B. M. Trost and M. T. Rudd, J. Am. Chem. Soc., 2003,
125, 11516; (b) M. Zhang, H.-F. Jiang, H. Neumann, M. Beller
and P. H. Dixneuf, Angew. Chem., Int. Ed., 2009, 48, 1681.
9 R. Barrios-Francisco and J. J. Garcıa, Inorg. Chem., 2009, 48, 386.
´
10 K. Tani, K. Ueda, K. Arimitsu, T. Yamagata and Y. Kataoka,
J. Organomet. Chem., 1998, 560, 253.
11 (a) B. L. Lucht, S. S. H. Mao and T. D. Tilley, J. Am. Chem. Soc.,
1998, 120, 4354; (b) W. M. F. Fabian and J. M. Kauffman,
J. Lumin., 1999, 85, 137.
12 P. Hauwert, G. Maestri, J. W. Sprengers, M. Catellani and
C. J. Elsevier, Angew. Chem., Int. Ed., 2008, 47, 3223.
The proposed catalytic cycle involves an unprecedented
elementary step where B evolves into E through transition state
D. Although metallacyclic mechanisms were proposed in the
previous examples,3a,c this is the first experimental evidence for
hydrogen transfer from MeOH to a metallacyclopentatriene.
Moreover, this novel hydride transfer mechanism is well corro-
borated by a preliminary theoretical study on model systems. We
could not locate any transition state for normal b-hydride
elimination of a model methoxo complex (TS-I, Scheme 4). On
the other hand, a transition state for hydride transfer from the
methoxo ligand onto one of the carbene carbons could be located
(TS-II). However, this process is estimated to occur with an
activation energy of DGz = 20.3 kcal molꢁ1, and is thermo-
dynamically unfavorable with an endothermicity of DG =
4.7 kcal molꢁ1 (Fig. S8 in ESIw). We, hence, sought an alternative
13 (a) J. Le Paih, S. De
´
rien and P. H. Dixneuf, Chem. Commun., 1999,
rien, P. H. Dixneuf, E. Clot
1437; (b) J. Le Paih, F. Monnier, S. De
´
and O. Eisenstein, J. Am. Chem. Soc., 2003, 125, 11964; (c) M. Zhang,
H. Jiang and P. H. Dixneuf, Adv. Synth. Catal., 2009, 351, 1488.
c
11554 Chem. Commun., 2011, 47, 11552–11554
This journal is The Royal Society of Chemistry 2011