The open d-shell enforces the active space in 3d metal catalysis: Highly enantioselective chromium(ii) pincer catalysed hydrosilylation of ketones

Article abstract of DOI:10.1039/c8cc05172k

Bis(oxazolinyldimethylmethyl)pyrrol (PdmBox) stereodirecting ligands provided the key to the chromium(ii)-catalysed highly enantioselective hydrosilylation of ketones. A rare square planar, chiral chromium(ii) alkyl complex was found to serve as a potent precatalyst for the reduction of a broad range of aryl alkyl and dialkyl ketone derivatives. The stereoelectronic preference of the open d⁴ shell of chromium(ii) firmly locks the molecular catalyst in a square planar geometry giving rise to two blocked quadrants of the coordination sphere. This earth-abundant base metal catalytic platform produces the corresponding chiral alcohols in excellent isolated yields with up to 98 %ee under mild reaction conditions (-40 °C to rt) and at low catalyst loadings (as low as 0.5 mol%).

Full text of DOI:10.1039/c8cc05172k

View Article Online
View Journal
ChemComm
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: C. Schiwek, V.
Vasilenko, H. Wadepohl and L. H. Gade, Chem. Commun., 2018, DOI: 10.1039/C8CC05172K.
This is an Accepted Manuscript, which has been through the
Volume 52 Number
1 4 January 2016 Pages 1–216
Royal Society of Chemistry peer review process and has been
accepted for publication.
ChemComm
Accepted Manuscripts are published online shortly after
Chemical Communications
www.rsc.org/chemcomm
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1359-7345
COMMUNICATION
Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
=
3
rsc.li/chemcomm

Please do not adjust margins
ChemComm
Page 1 of 5
View Article Online
DOI: 10.1039/C8CC05172K
Journal Name
COMMUNICATION
The Open d-Shell Enforces the Active Space in 3d Metal Catalysis:
Highly Enantioselective Chromium(II) Pincer Catalysed
Hydrosilylation of Ketones
Received 00th January 20xx,
Accepted 00th January 20xx
Christian H. Schiwek,a,† Vladislav Vasilenko,a,† Hubert Wadepohla and Lutz H. Gade*,a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Bis(oxazolinyldimethylmethyl)pyrrol (PdmBox) stereodirecting developed the non-conjugated but more flexible
ligands provided the key to the chromium(II)-catalysed highly bis(oxazolinyldimethyl-methyl)pyrrole ("PdmBox") ligand.8
enantioselective hydrosilylation of ketones. A rare square planar, Initial attempts of transferring chiral information by 3d
chiral chromium(II) alkyl complex was found to serve as a potent metal(II) complexes using this re-designed ligand family gave
precatalyst for the reduction of a broad range of aryl alkyl and rise to modest enantioselectivities.9
A careful structural
dialkyl ketone derivatives. The stereoelectronic preference of the investigation revealed that this outcome could be attributed to
open d4 shell of chromium(II) firmly locks the molecular catalyst in the preferential formation of tetrahedral MII complexes (for M
a square planar geometry giving rise to two blocked quadrants of = Mn, Fe and Co), featuring an arrangement of the oxazoline
the coordination sphere. This earth-abundant base metal catalytic substituents which is unfavourable for efficient enantio-
platform produces the corresponding chiral alcohols in excellent discrimination. We envisioned that the desired square-planar
isolated yields with up to 98 %ee under mild reaction conditions coordination, which provides
a
geometrically suitably
(−40 °C to rt) and at low catalyst loadings (as low as 0.5 mol%).
structured active space for chirality transfer (Figure 1), could
be enforced stereoelectronically by the open d-shell of the
metal centre. This would be expected for the (high spin) d4 as
well as d8 configuration of the divalent group 6 and 10 metal
ions.
Applications of chromium salts or well-defined chromium
complexes in homogeneous catalysis1 include inter alia the
asymmetric Nozaki-Hiyama-Kishi-reaction,1b,d,2 poly-3 and
oligomerizations4 of olefins or cross-coupling reactions.5
However, chromium-catalyzed reductions of C=O bonds have
not been investigated in a systematic fashion so far. In
particular, there are no reports on enantioselective versions of
the latter catalysed by chromium complexes.
(a)
(b)
We have recently explored a range of 3d metal-catalysed
stereoselective transformations and investigated the
associated mechanistic pathways.6 Our studies have shown
that application of conjugated, rigid and planar N,N,N pincer-
type ligands in combination with 3d metals in their formal MII
oxidation state furnished catalytic systems of unprecedented
activity and selectivity.2c,6c,e,7 In these cases, the rigidity of the
stereodirecting ligand gave rise to a structurally well-defined
coordination sphere which was thought to deliver the
observed high enantioselectivity.
Figure 1. Illustration of the metal-induced stereo-electronic shaping of the active space
in tetrahedral (a) vs. square-planar (b) PdmBox (3d) metal complexes. Arrows highlight
the sterically favoured substrate binding sites.
The exceptional inertness of square-planar PdmBox d8-
nickel(II) complexes in the hydrosilylation of ketones led us to
investigate their presumably isostructural PdmBox d4-
chromium(II) congeners and, based on previous experience
with 3d metal reduction catalysis, chose the chromium(II)
alkyls as potential pre-catalysts.
The conjugated π-electron system of the rigid chiral pincers
rendered these ligands redox non-innocent which in some
cases gave rise to competitive one-electron radical
7g
To overcome this limitation we recently
chemistry.6f,
The chromium(II) complexes of this type were readily
accessible following a standard two-step protocol: Initial
lithiation of the protioligand followed by addition of
b 10
(tmeda)CrCl2 gave chlorido complexes 1a, . Subsequent
alkylation of these compounds with a dialkyl magnesium
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 2 of 5
View Article Online
DOI: 10.1039/C8CC05172K
COMMUNICATION
Journal Name
source MgR2(THF)2 (R = CH2SiMe3) produced precatalysts 2a
,
b
The solid state structural data is complemented by the
in moderate overall yields (Scheme 1).
magnetic susceptibilities observed in solution (ꢀeff = 4.75 ꢀb
for 1a
determined by Evans’ method.13 Such values reflect the
, ꢀeff = 4.70 ꢀb for 1b and ꢀeff = 4.73 ꢀb for 2a)
N
N
N
1. LiHMDS
2. (tmeda)CrCl2
Mg(CH2SiMe3)2(THF)2
toluene, rt, 12 h
H
presence of a paramagnetic d4-high spin complex, being in the
O
O
O
O
O
O
N
Cr
N
N
Cr
N
N
N
THF, rt, 12 h
same range as the theoretical spin-only value (ꢀso = 4.90 ꢀb
)
∗
∗
∗
∗
∗
∗
Cl
and matching other reported chromium(II) complexes.4d, 12a
SiMe3
R
R
R
R
R
R
1a (R = Ph, 62 %)
1b (R = iPr, 64 %)
2a (R = Ph, 70 %)
2b (R = iPr, 73 %)
PdmBox
(R = Ph, iPr)
Table 1. Optimization of the reaction conditions for the chromium(II) catalysed
hydrosilylation of ketones.
Scheme 1. Synthesis of PdmBox chromium(II) chlorido (1a,b) and alkyl (2a,b)
complexes.
R(PdmBox)Cr(CH 2SiMe3) (5.0 mol%), silane (2 eq.)
O
OH
solvent, -40°C to rt, 2 h
then K2CO3/MeOH
In addition to a characterization by EA, MS, NMR and
determination of the magnetic moment in solution (vide infra),
we were able to obtain single crystals suitable for X-ray
3a
4a
#
1c
R a
Ph
Ph
Ph
iPr
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Solvent
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
Silane
Conv. [%]b
>99
>99
>99
>99
>99
>99
58
ee [%]b
87
diffraction for both classes of compounds (1b
The coordination geometry around the chromium centre is
best described by slightly distorted square-planar
, 2b, Figure 2).
(EtO)2MeSiH
(EtO)2MeSiH
(EtO)2MeSiH
(EtO)2MeSiH
(EtO)3SiH
2
95
3d
4
95
a
75
arrangement with calculated geometry factors of τ4 = 0.14 (for
1b) und τ4 = 0.09 (for 2b) (τ4 = 0 describes an ideal square-
planar coordination).11 This square-planar geometry is the
expected behaviour for d4-chromium(II) complexes and is
facilitated by a twist/torsion of the pyrrole-backbone due to
the inherent flexibility of the ligand. A close structural
resemblance of both molecular structures is observed,
indicating that co-ligand exchange only has a minor impact on
the structural parameters of the complex. All bonds were in
the expected range for chromium(II) complexes with
tridentate pincer ligands.3c, 4c, 12 Both compounds feature the
typical short M-N bond length for the anionic nitrogen as
opposed to the neutral oxazoline N donors. In addition, a
noticeable trans-influence of the alkyl ligand is detected.
5
83
6
nBuSiH3
PMHSe
84
7
84
8f
PhSiH3
>99
0
70
9f
Ph2SiH2
nd
nd
90
10f
11
12
13
14
15
16
17
Me2PhSiH
(EtO)2MeSiH
(EtO)2MeSiH
(EtO)2MeSiH
(EtO)2MeSiH
(EtO)2MeSiH
(EtO)2MeSiH
(EtO)2MeSiH
0
n-pentane
n-hexane
Et2O
>99
>99
>99
>99
>99
0
90
90
THF
86
tmeda
DCM
80
nd
nd
MeCN
0
a (S)-enantiomer for R = iPr, (R)-enantiomer for R = Ph. b Determined by chiral
HPLC. c Reaction was perfomed at rt. d Reaction was perfomed with 0.5 mol%
catalyst loading. e PMHS = Poly(methylhydrosiloxane). f Work-up with NaOH
in iPrOH. All reactions were performed at 0.12 mmol scale.
An initial assessment of the activity of this system with
acetophenone 3a as the benchmark substrate and
(EtO)2MeSiH as the silane showed full conversion in less than
15 min at rt and gave a promising enantioselectivity of 87 %ee
([Cr] = 5 mol% of 2a). The fast reaction rate led us to
investigate this transformation at reduced temperatures
(Table 1). Employing the phenyl derivative of the PdmBox
ligand (complex 2a), we were able to improve the selectivity
for this reaction to 95 %ee while warming from -40 °C to rt
over a period of two hours. Despite its similar activity, the
isopropyl substituted derivative 2b produced a significantly
reduced selectivity and no further improvements could be
achieved by a variation of the silane (Table 1, entries 2 and
3-8). While excellent conversions and a somewhat diminished
enantiomeric excess were observed for the more reactive
silanes, no product formation was detected for Ph2SiH2 and
Me2PhSiH. These findings could suggest that the silane is
Chart 2. Molecular structures of chlorido complex 1b and alkyl precatalyst 2b were
determined by X-ray diffraction. Displacement ellipsoids have been set at 30%
probability level, hydrogen atoms have been omitted for clarity. Selected bond lengths
[Å] and angles [°]: 1b: Cr-Cl 2.3305(6), Cr-N(1) 2.0735(18), Cr-N(2) 2.0144(17), Cr-N(3)
2.079(2); N(1)-Cr-Cl 95.43(5), N(1)-Cr-N(3) 169.87(8), N(2)-Cr-N(3) 86.47(7), N(2)-Cr- involved in the rate-determining step of the catalysis or that
N(1) 86.79(7), N(2)-Cr-Cl 170.70(6), N(3)-Cr-Cl 92.43(6); torsion angle N(1)-Cr-N(2)-C(5) -
catalyst activation is dependent on a suitable silane source
20.4(2). 2b: Cr-N(1) 2.081(6), Cr-N(2) 2.066(6), Cr-N(3) 2.084(6), Cr-C(23) 1.843(8), N(1)-
(vide infra).
Cr-C(23) 95.4(3), N(1)-Cr-N(3) 168.9(2), N(2)-Cr-N(3) 84.5(2), N(2)-Cr-N(1) 84.4(2), N(2)-
A solvent screening revealed that although (aromatic) low
Cr-C(23) 177.1(3), N(3)-Cr-C(23) 95.7(3); torsion angle N(1)-Cr-N(2)-C(5) -37.44(6).
polarity solvents (toluene, n-pentane, n-hexane) give rise to
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 3 of 5
View Article Online
DOI: 10.1039/C8CC05172K
Journal Name
COMMUNICATION
slightly higher enantioselectivities, coordinating and polar determining kinetics. The activity of the catalyst in the
solvents such as tetrahydrofuran or even tmeda can in hydrosilylation of acetophenone 3a remained unaffected by
principle afford the product in good selectivity and excellent presence of the radical traps triphenylmethane and
yield (Table 1, entries 10-14). This behaviour contrasts with 9,10-dihydroanthracene, suggesting heterolytic rather than
that of many other ketone hydroelementation catalysts which homolytic bond activation and absence of radical
generally prefer low polarity and weakly coordinating intermediates. Similarly, hydrosilylation of acetophenone 3a
solvents.28,31 No conversion was observed in dichloromethane using deuterated (EtO)2MeSiD gave a high degree of D
and acetonitrile, presumably due to rapid precatalyst incorporation at the C1 carbon after work-up with
decomposition in these solvents (Table 1, entries 15 and 16). K2CO3/MeOH and no deuteration of the methyl group
Notably, the activity of the chromium catalyst allowed a (Scheme 3, top). This manifests the role of the silane as the
reduction of the catalyst loading down to 0.5 mol% (at 3 mmol sole hydrogen atom source.
scale) without impairing the enantioselectivity.
D
OH
2a [Cr] (1.0 mol%), (EtO)2MeSiD (2 eq.)
With the optimized reaction conditions in hand, we
explored the substrate scope of the chromium(II)-catalysed
ketone reduction (Scheme 2): A broad range of o- and p-
substituted aryl methyl ketones was tolerated, furnishing the
chiral alcohols in good to excellent yields and
Ph
4a-d1
toluene, rt, 2 h, then K2CO3/MeOH
O
>96% D integration
Ph
2a [Cr] (1.0 mol%)
(EtO)2MeSiH (1 eq.) : (EtO)2MeSiD (1 eq.)
3a
H/D OH
Ph
enantioselectivities up to 98 %ee (entries 4a-i). In the case of
toluene, rt, 2 h,
then K2CO3/MeOH
4a
very electron-poor aryl groups, a reduced yield and selectivity
was detected, finally leading to no conversion in the extreme
case of the C6F5 aryl substituent (cf. entry 4l). Naphthones and
aryl alkyl ketones with a linear alkyl chain were converted
readily with good to excellent enantiodiscrimination, as were
kH/kD = 1.66
Scheme 3. Deuterium labelling study of the chromium(II)-catalysed reduction of 3a
(top) and determination of the kinetic isotope effect for this reaction by a competitive
hydrosilylation experiment (bottom).
cyclic derivatives (entries 4m-s, up to 96 %ee) as well as
The 1H/2H kinetic isotope effect (KIE) of this transformation
heterocyclic substrates, highlighting the potential of the
presented method. In addition, the protocol proved useful
even in the case of dialkyl ketones, provided that the
substituents featured sufficiently dissimilar steric bulk (entries
was determined in
a competitive experiment employing
equimolar amounts of hydrosilane and deuterosilane in the
chromium(II)-catalysed hydrosilylation (Scheme 3, bottom).
From the fraction of the deuterated product, we calculated a
4v vs. 4w,x).
significant KIE of kH/kD = 1.66, indicating that E-H/D bond
scission is a major contribution to the rate-limiting step of the
3a [Cr]
(1.0 mol%), (EtO)2MeSiH (2 eq.)
toluene, -40°C to rt, 2 h
O
OH
R'
catalytic pathway in this particular derivative (vide infra).
then K2CO3/MeOH
R
R'
R
3
4
OH
OH
OH
OH
4d
OH
MeO
Ph
4a
4b
4c
4e
89 %, 95 %ee
87 %, 98 %ee
89 %, 94 %ee
91 %, 98 %ee
92 %, 90 %ee
OH
OH
OH
OH
OH
Br
MeO2C
MeO
F
Cl
4i
4f
4g
4h
4j
92 %, 86 %ee
91 %, 90 %ee
87 %, 90 %ee
88 %, 89 %ee
31 %, 50 %ee
OH
OH
OH
OH
OH
F5
MeO
4k
4l
4m
88 %, 96 %ee
OH
4n
4o
90 %, 92 %ee
no conv.
86 %, 85 %ee
88 %, 90 %ee
OH
Figure 3. Hammett correlation for the chromium-catalysed hydrosilylation of various
OH
OH
OH
MeO
O
p-substituted acetophenone derivatives (R
= H, F, Br, Cl, Me, OMe). Substituent
C6H13
constant values were taken from reference 14.
4p
4q
4r
4s
4t
93 %, 93 %ee
91 %, 87 %ee
89 %, 94 %ee
67 %, 90 %ee
90 %, 86 %ee
OH
OH
OH
OH
Finally, we evaluated the electronic properties of the rate-
limiting step by means of a Hammett analysis. Relative
reaction rates of six p-substituted acetophenone derivates (R =
H, F, Cl, Br, Me, OMe) were determined in a competitive
experiment by 13C NMR spectroscopy.15 Interestingly, the
Hammett correlation produced a non-linear curve which can
be subdivided into two linear segments. For substrates with
electron-donating groups the reaction parameter was found to
be positive (ρ = +0.99 ± 0.04), which is generally interpreted in
S
4u
4v
4w
4x
90 %, 90 %ee
84 %, 30 %ee
81 %, 95 %ee
62 %, 90 %ee
Scheme 2. Substrate scope of the chromium(II)-catalysed hydrosilylation of ketones.
The reactions were performed on a 0.52 mmol scale and all values refer to the isolated
products. Selectivities have been determined by chiral HPLC or GC.
A series of control experiments was carried out in order to
identify the nature of the elementary steps and the rate-
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 4 of 5
View Article Online
DOI: 10.1039/C8CC05172K
COMMUNICATION
Journal Name
terms of a build up a negative charge in the active complex of chromium in enantioselective catalysis besides the NHK
the rate-limiting step. In contrast, substrates with electron- reaction. Further investigations towards the extension of this
withdrawing groups gave rise to a negative reaction parameter catalyst to other substrates and additional studies of the
(ρ = -0.66 ± 0.05), which can be attributed to build-up of a reaction mechanism are currently underway in our
positive charge in the active complex of the rate-limiting step. laboratories.
The non-linear Hammett correlation with a maximum rate for
We acknowledge the award of a pre-doctoral fellowship to
a derivative with a small Hammett substituent constant is V. V. from the Landesgraduiertenförderung (LGF Funding
typical of a single mechanism with a change in the rate-limiting Program of the state of Baden-Württemberg) and funding by
step depending on the electronic properties of the substrate. the Deutsche Forschungsgemeinschaft (Ga 488/9-2). We thank
Thus, the positive reaction parameter of the electron-rich Clemens Blasius and Dr. Jan Wenz for their advice and support.
derivatives is in accordance with a rate-determining insertion
step, whereas the negative reaction parameter of the electron-
Conflicts of Interest
poor derivatives agrees well with a rate-limiting metathesis
process. This is consistent with a deactivation of electron-rich
ketones compounds towards hydride insertion and an
activation of the corresponding alkoxides with respect to a
metathesis step. On the other hand, insertion is expected to
be fast for electron-poor ketones, with the metathesis step
being decelerated in this case. The hypothesis of a change in
the rate-limiting step is also supported by KIE analysis in the
two distinct kinetic regimes: compared to acetophenone a
somewhat larger KIE is observed for the para-bromo derivative
There are no conflicts to declare.
References
1. a) T. Agapie, Coord. Chem. Rev., 2011, 255, 861-880; b) G. C.
Hargaden, P. J. Guiry, Adv. Syn. Catal., 2007, 349, 2407-2424; c)
K. H. Theopold, Eur. J. Inorg. Chem., 1998, 1998, 15-24; d) G.
Zhang, Q. Tian, Synthesis, 2016, 48, 4038-4049.
2. a) M. Bandini, et al., Angew. Chem. Int. Ed., 1999, 38, 3357-
3359; b) A. Berkessel, et al., Angew. Chem. Int. Ed., 2003, 42,
1032-1035; c) Q. H. Deng, H. Wadepohl, L. H. Gade, Chem. Eur.
J., 2011, 17, 14922-14928; d) A. Fürstner, N. Shi, J. Am. Chem.
Soc., 1996, 118, 12349-12357; e) A. Gil, F. Albericio, M. Alvarez,
Chem. Rev., 2017, 117, 8420-8446; f) G. C. Hargaden, P. J. Guiry,
Stereoselective Synthesis of Drugs and Natural Products, 2013,
1-22; g) M. Inoue, T. Suzuki, M. Nakada, J. Am. Chem. Soc.,
2003, 125, 1140-1141; h) J. J. Miller, M. S. Sigman, J. Am. Chem.
Soc., 2007, 129, 2752-2753; i) K. Sugimoto, S. Aoyagi, C.
Kibayashi, J. Org. Chem., 1997, 62, 2322-2323.
(kH/kD
=
ketone featuring a para-methoxy substituent (kH/kD
1.78) as opposed to a significantly reduced KIE for a
1.18).
=
The large KIE for σ-bond metathesis is in line with our previous
work6b,g and we envision that the smaller values for electron-
rich ketones can be attributed to
a critical associative
formation of a chromium-hydride ketone complex (vide infra)
in the rate-determining step.
3. a) A. Döhring, et al., Organometallics, 2000, 19, 388-402; b) M.
Enders, et al., J. Organometallic Chem., 2003, 687, 125-130; c) Z.
Z. Liu, et al., Organometallics, 2011, 30, 749-756; d) M.
Ronellenfitsch, et al., Macromol., 2016, 50, 35-43; e) B. J.
Thomas, K. H. Theopold, J. Am. Chem. Soc., 1988, 110, 5902-
5903.
4. a) A. Carter, et al., Chem. Commun., 2002, 858-859; b) R.
Emrich, O. Heinemann, P. W. Jolly, et al., Organometallics, 1997,
16, 1511-1513; c) Y. Shaikh, et al., Organometallics, 2012, 31,
7427-7433; d) T. Simler, P. Braunstein, A. A. Danopoulos,
Organometallics, 2016, 35, 4044-4049.
5. a) X. Cong, H. Tang , X. Zeng, J. Am. Chem. Soc., 2015, 137,
14367-14372; b) O. M. Kuzmina, P. Knochel, Org. Lett., 2014, 16,
5208-5211; c) A. K. Steib, et al., J. Am. Chem. Soc., 2013, 135,
15346-15349; d) A. K. Steib, et al., Chem. Eur. J., 2015, 21, 1961-
1965.
6. a) T. Bleith, et al., Angew. Chem. Int. Ed., 2016, 55, 7852-7856;
b) T. Bleith, L. H. Gade, J. Am. Chem. Soc., 2016, 138, 4972-4983;
c) Q. H. Deng, H. Wadepohl, L. H. Gade, J. Am. Chem. Soc., 2012,
134, 2946-2949; d) C. A. Rettenmeier, et al., Inorg. Chem., 2016,
55, 8214-8224; e) V. Vasilenko, et al., Angew. Chem. Int. Ed.,
2017, 56, 8393-8397; f) J. Wenz, et al., Chem. Commun., 2016,
52, 202-205; g) V. Vasilenko, C. K. Blasius, L. H. Gade, J. Am.
Chem. Soc., DOI: 10.1021/jacs.8b05340.
7. a) T. Bleith, H. Wadepohl, L. H. Gade, J. Am. Chem. Soc., 2015,
137, 2456-2459; b) Q. H. Deng, et al., J. Am. Chem. Soc., 2013,
135, 5356-5359; c) Q. H. Deng, et al., Chem. Eur. J., 2014, 20, 93-
97; d) Q. H. Deng, H. Wadepohl, L. H. Gade, J. Am. Chem. Soc.,
Scheme 4. Mechanistic proposal for the chromium(II)-catalyzed hydrosilylation of
ketones.
Based on these initial mechanistic experiments presented
herein and our previously reported results in the related
iron(II)- and manganese(II)-catalyzed hydrosilylations, we
propose a catalytic cycle consisting of three critical steps
(Scheme 4):6b,e,7a After activation of the chromium(II) alkyl
precatalyst and formation of a hypothetical Cr(II)-H species,
ketone coordination (I) and insertion into the Cr-H bond (II)
occurs. Subsequent σ-bond metathesis regenerates the
hydride and releases the product (III). This proposed direct
hydride transfer pathway is consistent with labelling- and
Hammett correlation studies as well as the absence of radical
intermediates.
In conclusion, we have designed and isolated the first
chromium(II)-based precatalyst for the highly enantioselective
hydrosilylation of a broad range of ketones containing various
functional groups. This work emphasizes the application of
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 5 of 5
View Article Online
DOI: 10.1039/C8CC05172K
Journal Name
COMMUNICATION
2012, 134, 10769-10772; e) B. K. Langlotz, H. Wadepohl, L. H.
Gade, Angew. Chem. Int. Ed., 2008, 47, 4760-4764; f) C. Mazet,
L. H. Gade, Chem. Eur. J., 2003, 9, 1759-1767; g) C. Rettenmeier,
H. Wadepohl, L. H. Gade, Chem. Eur. J., 2014, 20, 9657-9665.
8. J. Wenz, et al., Inorg. Chem., 2017, 56, 3631-3643.
9. J. Wenz, et al., Eur. J. Inorg. Chem., 2017, 2017, 5545-5556.
10. S. K. Hao, et al., Organometallics, 1994, 13, 1326-1335.
11. A. Okuniewski, et al., Polyhedron, 2015, 90, 47-57.
12. a) K. P. McGowan, K. A. Abboud, A. S. Veige, Organometallics,
2011, 30, 4949-4957; b) A. Alzamly, S. Gambarotta, I. Korobkov,
Organometallics, 2014, 33, 1602-1607.
13. D. F. Evans, J. Chem. Soc., 1959, 0, 2003-2005.
14. C. Hansch, A. Leo, R. W. Taft, Chem. Rev., 1991, 91, 165-195.
15. H. M. Yau, A. K. Croft, J. B. Harper, Chem. Commun., 2012, 48,
8937-8939.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins