C O M M U N I C A T I O N S
IMes-Ru-Cp* angle, which is 4.2° larger than the ICy-Ru-Cp*
angle, on average, along the reaction pathway. Further support for this
hypothesis emerges from the analysis of the key ꢀ-agostic intermediate
G (Figure 3). In addition to the larger NHC-Ru-Cp* angle, the higher
steric stress of the system bearing the IMes ligand is evidenced by the
shorter Ru-O distance and by the quite longer Ru-H agostic distance.
Indeed, in the case of ICy the Cꢀ-H distance is remarkably long, and
the overall complex resembles the π-coordinated ketone species H.
The major source of steric stress in the case of the IMes complex
involves the interaction between the Me group on the Cꢀ atom, which
is a short distance from one of the Meortho groups of the nearby mesityl
ring, and forces a remarkable rotation of this ring around the N-Cipso
bond (the Ccarbene-N-Cipso-Cortho dihedral angle is 115.5°). This
rotation is limited by steric interaction between the other Meortho group
and Me groups of the Cp* ligand. The increased steric stress in these
Comparison of the overall racemization pathway indicates that,
in the case of ICy, the magnitude of all energy barriers supports a
ruthenium-mediated racemization reaction occurring at room tem-
perature. Conversely, in the case of IMes, several intermediates
and transition states are significantly higher in energy, an energy
profile which is in qualitative agreement with the reduced racem-
ization activity of the Ru-IMes complex.
In the context of the development of new catalysts enabling
alcohol racemization, we previously reported the use of highly
efficient 16-electron ruthenium complexes. Here, we support a
proposed reaction mechanism by spectroscopic detection of inter-
mediates that are analyzed by computational methods. These results
clearly highlight a mechanism involving ruthenium hydride inter-
mediates, without any direct ligand participation. Furthermore, the
reaction mechanism proposed consists of a sequence of reaction
steps among the most typical in organometallic chemistry: ꢀ-H
elimination, π to σ isomerization of a coordinated ketone, and
ketone insertion into the Ru-H bond.
NHC complexes can be quantified by the total percent buried volume,
26,29
%VBur
,
occupied by the NHC and the Cp* ligands in the IMes
complex (73.6%) vs the smaller value of 70.3% for the ICy complex.29a
We believe these results will prove helpful in the understanding
of the mechanism of other racemization catalysts and should guide
the development of new catalysts for racemization reactions.
Acknowledgment. The ERC (Advanced Investigator Award
FUNCAT to SPN), EPSCR National Mass Spectrometry Service
Centre (for HRMS analyses), the HPC team of Enea (www.enea.it)
for using the ENEA-GRID, and the HPC facilities CRESCO
A.P. thanks the Spanish MICINN for a Ramo´n y Cajal contract.
S.P.N. is a Royal Society-Wolfson Research Merit Award holder.
Supporting Information Available: NMR data supporting the
existence of hydride intermediates and full computational material. This
Figure 3. DFT structure of the ꢀ-agostic intermediates. Distances in Å,
angles in deg. (a and b) NHC ) ICy and IMes, respectively.
The reaction moves to completion by release/exchange of the
racemized phenylethanolate ligand with a free alcohol molecule,
through proton transfer (Figure 4). This step begins with the
approach of a free alcohol molecule to the coordinated alkoxide F′
via a H-bond interaction, as in intermediate J′, which is 0.9 above
and 1.9 kcal ·mol-1 below F′ for IMes and ICy, respectively. This
indicates a clear steric stress in the H-bonded intermediate J′. The
proton transfer reaction proceeds through transition state J′-J with
a barrier of 10.3 kcal ·mol-1 for IMes, while it is remarkably lower
for ICy, only 1.8 kcal ·mol-1. The system collapses into the
H-bonded complex J, from which the racemized alcohol is released.
Finally, it is noteworthy that entropic effects, not included in the
present calculations, would disfavor phenylethanol H-bonding to
F′, thus increasing the barrier for alcohol exchange.
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Figure 4. Schematic reaction pathway for release of the racemized alcohol,
energies in kcal ·mol-1
.
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