Helicenes as Chirality-Inducing Groups in Transition-Metal Catalysis: The First Helically Chiral Olefin Metathesis Catalyst

Article abstract of DOI:10.1002/chem.201802786

Helical chirality is a novel enantioselectivity-inducing property in transition-metal-catalyzed transformations. The principle is illustrated herein for the example of asymmetric olefin metathesis. This work reports the synthesis of the first helically chiral Ru-NHC alkylidene complex from an aminohelicene-derived imidazolium salt, which was ligated to the first generation Hoveyda–Grubbs catalyst. Kinetic data were acquired for benchmark test reactions and compared to an achiral catalyst. The helically chiral Ru-catalyst was evaluated in asymmetric ring-closing metathesis (RCM) and ring-opening metathesis–cross-metathesis (ROM/CM) reactions, which proceeded with promising levels of enantioselectivity. Extensive NMR-spectroscopic investigations and a DFT geometry optimization were performed. These results led to a topographic steric map and calculation of percent-buried-volume values for each quadrant around the metal center.

Full text of DOI:10.1002/chem.201802786

A Journal of
Accepted Article
Title: Helicenes as chirality inducing groups in transition metal
catalysis: the first helically chiral olefin metathesis catalyst
Authors: Manfred Karras, Michał Dąbrowski, Radek Pohl, Jiří Rybáček,
Jaroslav Vacek, Lucie Bednárová, Karol Grela, Ivo Starý,
Irena G. Stará, and Bernd Schmidt
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.201802786
Link to VoR: http://dx.doi.org/10.1002/chem.201802786
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Helicenes as chirality inducing groups in transition metal
catalysis: the first helically chiral olefin metathesis catalyst
Manfred Karras,[a,b] Michał Dąbrowski, Radek Pohl, Jiří Rybáček, Jaroslav Vacek, Lucie
[c]
[b]
[b]
[b]
[
b]
[c]
[b]
[b]
[a]
Bednárová, Karol Grela, Ivo Starý,* Irena G. Stará* and Bernd Schmidt*
Abstract: Helical chirality is a novel enantioselectivity inducing
element in transition metal catalyzed transformations. The principle is
illustrated herein for the example of asymmetric olefin metathesis. We
describe the synthesis of the first helically chiral Ru-NHC alkylidene
complex from an aminohelicene-derived imidazolium salt, which was
ligated to the first generation Hoveyda-Grubbs catalyst. Kinetic data
were aquired for benchmark test reactions and compared to an achiral
catalyst. The helically chiral Ru-catalyst was evaluated in asymmetric
RCM and ROM/CM reactions, which proceeded with promising levels
of enantioselectivity. Extensive NMR-spectroscopic investigations
and a DFT geometry optimization were performed. These results led
to a topographic steric map and calculation of percent-buried-volume
values for each quadrant around the metal centre.
for instance, been demonstrated that the reactivity of Ru-
metathesis catalysts with unsaturated NHC ligands can be
improved by increasing the -donor capacity of the ligand through
introduction of donor substituents in the backbone.[
Unfortunately, enantioselectivities are normally low for this ligand
design if the chiral N-substituent can freely rotate around the C-
N-bond,[6] whereas higher levels of enantioselectivity are
5]
observed with multicyclic,[ bidentate
7]
[8]
or cyclometalated
NHC’s.[9] On the downside, opportunities for steric and electronic
modulation through backbone substituents are restricted for these
ligand systems. The alternative approach to chiral NHC-ligands
requires one or two centres of chirality in the backbone. One of
the earliest examples for this strategy is the first chiral Ru-
metathesis catalyst 5a.[ This approach has later been used for
other chiral metathesis catalysts with unsymmetrical NHC
ligands.[11-12] In catalysts such as 5a the stereoinduction relies on
a so-called “gearing”-effect:[13-15] the N-aryl substituents rotate out
of plane to avoid steric interaction with the substituents in the
backbone. In the process, one of the two enantiotopic
coordination hemispheres becomes more crowded than the other.
10]
Asymmetric syntheses through transition metal catalyzed
reactions have evolved from academic curiosities into industrially
relevant processes for the production of pharmaceuticals, crop
protecting agents and other fine chemicals. Asymmetric (or
enantioselective) catalysis requires the presence of a chiral ligand
at the metal, which induces either directly or indirectly a
dissymmetry around the catalytically active centre. For
a
successful enantioinduction the chiral information needs to be
embedded in a steering ligand, which is tightly bound to the metal
throughout the entire catalytic cycle. In this regard, N-heterocyclic
carbenes (NHC’s) are particularly suitable due to their high -
donor capacity and strong M-NHC bond.[1-2] In most cases
described so far NHC-ligands have been chirally modified by
introducing centres of chirality, either in the N-substituent or in the
NHC-backbone (Figure 1).[3-4] The former approach offers not
only the advantage of a convenient ligand synthesis from
homochiral primary amines, but also the opportunity for tuning the
electronic ligand properties through backbone substituents. It has,
Figure 1. Implementation of chirality in monodentate and monocyclic NHC’s.
[
[
a]
b]
M. Karras, Prof. B. Schmidt
Institut fuer Chemie
Universität Potsdam
We wondered whether enantiocontrol in asymmetric transition
metal catalysis is possible even for the most straightforward NHC-
ligand design (monocyclic, monodentate, chiral information only
in the N-substituent) if chirality elements other than point chirality
are used. A literature search revealed only two examples that
strictly fall into this category. Andrus and coworkers described a
Rh-NHC complex with planary chiral N-para-cyclophane
substituents for enantioselective conjugate addition of boronic
acids,[16] and Sankararaman and coworkers developed a Pd-NHC
Karl-Liebknecht-Str. 24-25, D-14476 Potsdam, Germany
E-mail: bernd.schmidt@uni-potsdam.de
M. Karras, R. Pohl, J. Rybáček, J. Vacek, L. Bednárová, Dr. I. Starý,
Dr. I. G. Stará
Institute of Organic Chemistry and Biochemistry
Czech Academy of Sciences
Flemingovo n. 2, 166 10 Prague 6, Czech Republic
E-mail: stary@uochb.cas.cz; stara@uochb.cas.cz
M. Dąbrowski, Prof. K. Grela
Faculty of Chemistry, Biological and Chemical Research Centre
University of Warsaw
[
c]
complex substituted with
a
[
chiral para-cyclophane for
17]
Żwirki i Wigury 101, 02-089 Warsaw, Poland
enantioselective hydrogenation.
Other transition metal NHC-
catalysts with “non-classical” elements of chirality (i. e. planar or
axial chirality) have either bidentate[18-20] NHC ligands or carry
additional centres of chirality.[19, 21]
Supporting information for this article is given via a link at the end of
the document.
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COMMUNICATION
[
36-
Helical chirality has so far been unexplored in asymmetric
transition metal catalysis as a stereoinformation inducing element.
Recent efforts by some of us have greatly facilitated the synthesis
preferences arising from a rotation around the Ru-NHC bond.
3
7]
As a substituent for the second nitrogen we chose an N-mesityl
group, because we expected this substituent to be sufficiently
bulky to stabilize the catalytically active 14-electron species
arising from dissociation of the temporary ligand, but not too
sterically demanding to inhibit any catalytic activity. For the
of enantiopure helicenes[22-23] via
a diastereoselective Ni-
catalyzed alkyne cyclotrimerization with point-to-helical chirality
transfer.[24] This strategy has in the meantime been successfully
applied to the synthesis of enantiopure aminohelicenes,[25-26]
which we in turn expect to open the door to NHC ligands with
helically chiral N-substituents and transition metal catalysts
derived thereof. Literature precedence for transition metal-NHC
complexes with N-helicenyl substituents is scarce,[27-28] and these
have not been investigated in asymmetric catalysis. In addition,
few other helicene substituted imidazolium salts have been
synthesized via different routes, but not ligated to transition
metals.[29-30]
As a proof of concept we investigated the feasibility of asymmetric
olefin metathesis reactions,[14, 31-35] catalyzed by a helically chiral
metathesis initiator which was designed according to the general
principles outlined above and summarized in Figure 1. Our
synthesis of such a catalyst (M)-6 starts from enantiopure 2-
amino[6]helicene ((M)-1).[26] To avoid steric overcrowding of the
NHC ligand, which might result in problems upon ligation and low
catalytic activity,[11] we aimed at the synthesis of an
unsymmetrical NHC with just one N-helicenyl substituent.
Unsymmetrical NHC’s can be used to modulate steric congestion
around the metal centre and influence conformational
[38]
synthesis of the required imidazolium salt Fürstner’s method
was used. In this method, oxazolium salts such as 3 are reacted
with amines, in our case 2-amino[6]helicene (M)-1. Imidazolium
salt (M)-4 was obtained in fair overall yield and its identity
confirmed by MALDI-MS and NMR spectroscopy. It was
converted to the carbene by deprotonation with K-amylate and
subsequently treated with 5b.[39] In the presence of the phosphine
scavenger CuCl[40-41] the helically chiral Ru-alkylidene (M)-6 was
obtained. Like other NHC-Ru alkylidene complexes with a
hemilabile ortho-isopropoxybenzylidene ligand,[
40,
42]
complex
(M)-6 is a dark green solid. The assigned structure was confirmed
by MALDI-MS and a full NMR-spectroscopical characterization.
Indicative for a successful ligation of the NHC ligand are the
chemical shift values for the -proton of the Ru-alkylidene unit (
= 16.64 ppm for (M)-6,  = 17.45 ppm for 5b ) and for the
methine proton of the isopropoxy substituent ( = 4.90 ppm for
[40]
[40]
(M)-6,  = 5.28 ppm for 5b ). Similar shifts to lower -values have
also been observed for 5c and were attributed to the increased -
donor capacity of NHC ligands (Scheme 1).[
40]
Scheme 1. Synthesis of a helically chiral Ru-NHC-metathesis catalyst (M)-6. Selected NOE-interactions are indicated by red arrows.
In the next step the catalytic activities of (M)-6 and 5c were
investigated by NMR spectroscopic experiments.
representative example is the time-conversion curve for the RCM
diethyl diallylmalonate and the ring closing enyne metathesis of
A
diphenylpropargyl allyl ether have similar kinetic profiles (see
supporting information). The reduced activity of (M)-6 points at a
preferred orientation of the N-helicenyl substituent to the
catalytically active centre. We also investigated the cross
metathesis of allylbenzene and (E)-1,4-diacetoxy-2-butene to
check whether the sterically demanding helicenyl substituent
affects E/Z-selectivity, but similar yields and selectivities towards
the E-product were observed with both catalysts.
of 7 (eq. 1).
Although the helicenyl substituent in (M)-6 leads to a notable
deceleration of the initiation rate of the metathesis reaction
compared to 5c, a conversion of 97% for the RCM of 7 is still
achieved after 40 minutes with 1 mol % (Figure 2). The RCM of
As a test substrate for enantioselective RCM the allyl ether 9 was
chosen (Scheme 2a), because this prochiral triene has previously
been used to evaluate other chiral metathesis catalysts,[
11, 34, 43]
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including 5a.[
10]
Under optimized conditions (for details see
supported by NOE 5 of backbone proton H4 with the opposite
“upper” terminus of the helicene. These conclusions are
confirmed by the results of the DFT-geometry optimization
(Figure 3a).
supporting information) (R)-10 was obtained in 56% ee with
quantitative conversion of the starting material. Grubbs and
coworkers reported for the same test reaction under similar
conditions an ee of 35% at 82% conversion with catalyst 5a.[10]
Catalyst (M)-6 was also tested in the ROM-CM-reaction of meso-
11 and styrene. With 1 mol % of catalyst the expected all-cis-
configured cyclopentane 12 was obtained with high E-selectivity
and 40% ee at 86% conversion (Scheme 2b). Although
substantially higher enantioselectivities have been observed for
these and similar substrates with chiral Schrock-type Mo-
alkylidenes,[44] in particular the “chiral-at-Mo” catalysts,[45] the
enantioinduction observed with (M)-6 proves the concept outlined
above.
Figure 2. Catalytic activity of (M)-6 and 5c in the RCM of 7.
Figure 3. Ball-and-stick-representation of DFT-geometry optimized structure (a)
and topographic steric map of (M)-6 (b).
Based on the DFT-optimized structure of (M)-6 a topographic
steric map was prepared and the “percent buried volume” (%Vbur
value was calculated, using the SambVca 2.0 web tool (Figure
b).[46] Topographic maps are a tool to visualize the distribution of
)
3
steric hindrance around a metal centre. The coordination sphere
is viewed as a projection along the z-axis onto the x-y-plane,
which means that in our case the complex is viewed from the
bottom. The steric extension along the z-axis is visualized by iso-
contour curves, with red colour indicating high steric demand, and
blue colour indicating low steric demand. “Percent buried volume”
was introduced by Nolan, Cavallo and coworkers as a parameter
to quantify the bulkiness of NHC ligands. It is defined as the
Scheme 2. Enantioselective RCM (a) and ROM-CM catalyzed by (M)-6.
To gain further insight into its preferred conformation, a ROESY-
experiment and a structure optimization using DFT-methods were
performed for (M)-6. Selected NOE’s are indicated by red arrows
in the structure shown in Scheme 1: from NOE 1 it can be
concluded that the N-mesityl substituent resides over the
benzylidene unit and that free rotation around the Ru-NHC bond
is not possible. NOE’s 2 and 3 indicate spatial proximity of one
ortho- and its vicinal meta-position and the isopropoxy group of
the alkylidene unit. The other ortho-position of the helicenyl
substituent is close to H4 of the NHC backbone (NOE 4). These
three NOE’s indicate that the helicene rotates out of plane around
the N-C-bond and that it is locked in this conformation. This is
relative amount of the total volume of a sphere around the metal
that is occupied by the ligand.[47-48] For instance, for complex 5c
a %Vbur value of 33.7 was reported,[48-49] and for a derivative of its
unsaturated IMes-analogue
a
%Vbur value of 31.9 was
calculated.[48, 50] For our helically chiral complex (M)-6 the
overall %Vbur value was determined to 33.6. In the case of
unsymmetrical NHC-ligands which are not freely rotatable around
the M-NHC-bond an overall %Vbur value is only of limited
relevance. Therefore the individual values for each four quadrants
were determined. While the %Vbur values of northwest (NW),
southwest (SW) and northeast (NE) quadrant vary only slightly
(
from 30.4 to 32.0), the south-east (SE) quadrant is sterically
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COMMUNICATION
significantly more congested than the other three, with
a %VburMax value of 40.5. In this regard (M)-6 differs from other
unsymmetrical but achiral Ru-NHC alkylidene complexes that
have recently been analyzed in a similar fashion: for these
complexes similar %Vbur values were obtained for adjacent
eastern and western quadrants, respectively.[37]
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In summary, we have devised a synthesis of the first olefin
metathesis catalyst with a helically chiral NHC-ligand. NMR-
spectroscopic investigations and DFT-calculations suggest that
the complex is conformationally rigid and that the N-helicenyl
substituent adopts a preferred conformation in which it rotates out
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Helical chirality is a novel
enantioselectivity inducing element for
transition metal catalyzed
transformations. The concept is
exemplified for an enantioselective
olefin metathesis catalyst.
Manfred Karras, Michał Dąbrowski,
Radek Pohl, Jiří Rybáček, Jaroslav
Vacek, Lucie Bednárová, Karol Grela,
Ivo Starý, Irena G. Stará and Bernd
Schmidt
Page No. – Page No.
The first helically chiral olefin
metathesis catalyst
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