Organometallics
expected due to the nonuniversal nature of the functional. In
addition, the discrepancy between experimentally and
theoretically predicted % ee might be affected by the additional
mechanisms of the generation of chirality caused by energeti-
cally accessible conformers of the catalyst−substrate complex,
the possible involvement of Ru -pathway, and specific
R
solvation of propan-2-ol. These pathways, however, seem to
17
play a minor role as discussed elsewhere. Optimized M06-
2
1
X-D3 geometries for the transition states are shown in Figure
.
By inspecting optimized geometries, which are first-order
saddle-points on the potential energy surface, multiple close
contacts defined as a separation that is less than the sum of the
van der Waals radii of the respective atoms could be identified
6
in the region of (tethered) η -arene ligand and the region of
3
6
SO moiety. These contacts include not only known CH−π,
2
37
38
lone pair (lp)−π, and C−H···H−C interactions, but also
39
apparently a novel lp···H−C noncovalent interaction (d
H···O
40
=
2.36 Å, cf. 2.70 Å for the sum of van der Waals radii ). In
addition, we also note several short C−H···F proximities of
.64−2.71 Å observed for transition states leading to R-
product of 2b (Figure 1). To visualize noncovalent interactions
2
41
present in these geometries, noncovalent interaction (NCI)
plots, which are based on the M06-2X-D3 electron density and
its derivative analysis, were adapted (Figure 2). In all cases
Figure 4. Calculated M06-2X-D3/def2-TZVP/SMD(pronan-2-ol)
electrostatic potential (ESP) surfaces (ρ = 0.001 au) of benzene,
hexafluorobenzene, acetophenone, 2′,3′,4′,5′,6′-pentafluoroacetophe-
none, and two conformers of 1-cyclohexylethanone.
(
i.e., eight transition states), the (green) isosurfaces confirm
the presence of delocalized weak noncovalent interactions in
6
the region of (tethered) η -arene ligand and the region of SO
2
moiety of the catalyst. The topological features of electron
density in these regions were further analyzed with the well-
established quantum theory of atoms in molecule (QTAIM)
analysis (Figure 3). The presence of noncovalent interactions
is confirmed by the presence of bond critical points (BCPs) of
molecular electron density. Furthermore, cage critical points
The π cloud of benzene (shown for comparison) and to
some extent acetophenone (1a) creates a negative region of
ESP, called the heap, above and below the molecular plane
42
leading to a negative sign of quadrupole moment tensor Q (z-
zz
46
direction is normal to the molecular plane) (see the SI). In
contrast, the similar region (“hole”) of ESP is positive for
hexafluorobenzene (shown for comparison) and 2′,3′,4′,5′,6′-
pentafluoroacetophenone (1b), leading to a positive sign of
(
CCPs) are observed in the regions of OSO/arene in all
three studied cases of 1a and 1b and are characteristic for lp−π
interactions.
4
3
quadrupole moment tensor Q . One therefore should expect
zz
What causes the reverse of the sense of enantioselection
when going from 1a to 1b with catalyst (R,R)-I? The analysis
presented above indicates that transition states leading to R-
product are comparably somewhat equally stabilized via CH−π
interactions (Figure 1, two top left structures). However, a
large difference is observed for transition states leading to S-
product in the region of lp−π interactions (Figure 1, two top
right structures). Although any chemical bond is a dynamic
that the aromatic ring of 1a will repel the negative oxygen atom
of the SO moiety of the catalyst, whereas the one of 1b will
2
attract it. To conclude, lp−π interaction in the region of the
SO moiety of the catalyst seems to be the major driving force,
2
which causes the reverse of the sense of enantioselection when
going from 1a to 1b with catalyst (R,R)-I.
What makes a further dramatic improvement of the % ee for
b when going from catalyst (R,R)-I to (R,R)-II? The
1
44
equilibrium between attractive and repulsive forces, there is
apparently more attraction between the lone pair of (SO)O
oxygen of the catalyst and π-electron density of 2′,3′,4′,5′,6′-
pentafluoroacetophenone (1b) vs π-electron density of
acetophenone (1a). This is evidenced by a much shorter
centroid (CNT)···O bond distance present in the case of 1b
with respect to 1a (Δ = −0.29 Å; Figure 1), as well as greater
values of electron densities (ρ) for bond critical points in the
transition states leading to the major S-product seem to be
stabilized by lp−π interaction on an equal footing (“identical”
CNT···O bond distance of ∼2.94 Å as well as a ρ of ∼0.70 ×
−2
10
au). In contrast, there seems to be more destabilization
present for the diastereomeric transition state leading to a
minor R-product with (R,R)-II (Figure 1, second structure
from bottom right), thus kinetically blocking its accumulation.
The origin of this destabilization is the “tethered” arm, which
increases steric bulkiness. As a result, the aromatic ring of 1b
−
2
corresponding transition-state structures (Δ ∼ 0.16 × 10 au
on average; Figure 3). More attraction implies more exergonic
stabilization of the corresponding transition state, e.g., a
kinetical deblockage to accumulate the S-enantiomer through
lowering the position of the first-order saddle-point on the
potential energy surface. The purely electrostatic component of
these lp−π interactions can be further understood by
2
3
experiences forced rotation around the C(sp )−C(sp ) bond.
Even though the resultant structure is stabilized by C−H···X
(X = C, F) interactions, the overall destabilization plays a
major role. Therefore, the reason why (R,R)-II gives 2b with
much improved % ee than (R,R)-I relies on the kinetical
blockage of the pathway leading to a minor R-product through
significant destabilization of the corresponding diastereomeric
45
examining electrostatic potential (ESP) maps of 1a and 1b,
6
as shown in Figure 4.
transition state in the region of (tethered) η -arene ligand.
1
406
Organometallics 2021, 40, 1402−1410