Ligand self-sorting and nonlinear effects in dinuclear asymmetric hydrogenation: Complexity in catalysis

Article abstract of DOI:10.1002/chem.201301966

Self-sorting species: Nonlinear effects are observed in asymmetric hydrogenation reactions if self-sorted dinuclear complexes are generated. The nature of the substrate also influences this effect, because it affects the solubility of racemic self-sorted homochiral dinuclear complexes (see scheme). Copyright

Full text of DOI:10.1002/chem.201301966

DOI: 10.1002/chem.201301966
Ligand Self-Sorting and Nonlinear Effects in Dinuclear Asymmetric
Hydrogenation: Complexity in Catalysis
Frꢀdꢀric G. Terrade,[a] Martin Lutz,[b] and Joost N. H. Reek*[a]
Nature has been a source of inspiration for scientists as
billion years of evolution have resulted in magnificent exam-
ples of how processes can be controlled efficiently. In the
field of supramolecular catalysis, enzymes have been the
major source of inspiration. As such, many synthetic systems
have been prepared to mimic certain aspects of enzymes,
with a strong focus on connecting catalytically active sites to
cavities or binding sites having affinity for the substrate.[1,2]
Although such approaches have resulted in interesting new
tools to control selectivity,[3–5] we are not nearly close to the
abilities of Nature to control chemical transformations. One
of the major differences between Nature and synthetic ap-
proaches is that in biological systems, chemical processes
take place in a complex out-of-equilibrium environment.[6]
In a cell, many chemical transformations occur simultane-
ously or in controlled sequence, involving an impressive
number of different components. An important challenge
for biologists is to understand how this organized complexity
leads to emerging properties. As a result, new fields such as
systems biology[7] including non-equilibrium thermodynam-
ics[8] have been developed. Synthetic chemists traditionally
aim for systems that are as clean and pure as possible,[9] and
it is only recently that complexity in chemical systems re-
ceived attention.[10–12] Despite the interesting perspectives,
complexity in homogeneous catalysis has not been a re-
search focus in itself, although many aspects related to com-
plexity have been reported. Product inhibition and catalyst
poisoning are relevant to complex chemistry as they imply
processes with feedback loops. Dynamic catalyst libraries
and combinatorial catalysis require selection procedures,[6,13]
and nonlinear effect (NLE) in asymmetric catalysis[14–16] can
lead to emerging properties. With this in mind we decided
to study in detail NLE using dinuclear hydrogenation cata-
lysts C that have four chiral ligands. Application of non-
enantiopure ligands can lead to formation of heterochiral
complexes (with more than two chiral ligands) with different
properties and these complexes can theoretically be more
active and selective than their homochiral analogues.[16] Cat-
alytically active complexes with four chiral ligands are rare,
and for hydrogenation complex C is, to the best of our
knowledge, the only example. The use of a racemic ligand
could lead to the formation of ten stereoisomers of C
(Figure 1).
Figure 1. The [Rh2(L)4] complex C and a schematic representation of the
10 potential complexes that can form from a racemic mixture (pink rep-
resents the ligand in R configuration and blue the S). For L1, R=4-n-bu-
tylbenzene, for L2, R=CF3 and for L3, R=CH3.
[a] F. G. Terrade, Prof. Dr. J. N. H. Reek
Van’t Hoff Institute for Molecular Sciences
University of Amsterdam
Herein we report the self-sorting of chiral ligands at these
tetraligated complexes, but not for some of the precursor
complexes, the in situ enantiopurification of the system
caused by the relative insolubility of the racemate of homo-
chiral complexes. As a result, in asymmetric hydrogenation
using non-pure ligands (S/R=70:30) the product can be
formed in high enantiopurity (90% ee). The outcome, how-
ever, is strongly dependent on starting conditions (concen-
Science Park 904, 1098 XH, Amsterdam (The Netherlands)
Fax : (+31)205255604
E-mail: j.n.h.reek@uva.nl
[b] Dr. M. Lutz
Crystal and Structural Chemistry
Bijvoet Center for Biomolecular Research
Utrecht University, Padualaan 8, 3584 CH Utrecht (The Netherlands)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301966.
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COMMUNICATION
tration, incubation, substrate) as intermediate complexes to-
wards the formation of C are not self-sorted and yet active
in hydrogenation.
only homocomplexes [Rh2((R)-L)
AHCTUNGTRENNUNG
2ACHTUNGTRENNUN(G nbd)2] and [Rh2((S)-L)2-
give a different signal in the NMR spectrum. To confirm
this self-sorting behaviour, the experiment was repeated
using pseudoenantiomers (S)-L1 and (R)-L2, mirror-image
ligands that have a different R group on the sulphonamide.
In this experiment also, only homochiral complexes
The METAMORPhos family, a new class of sulfonamide–
phosphorus ligand was recently disclosed.[17–19] Their coordi-
nation to rhodium gives rise to unique complexes consisting
of two metal centres and four METAMORPhos ligands
[Rh2L4] (C). Two deprotonated ligands form a bridge be-
tween the two metals and two ligands coordinate in a P–O
chelating fashion. These complexes display unrivalled high
selectivity in asymmetric hydrogenation of benchmark sub-
strates and challenging substrates such as cyclic enamides.[19]
Studies on the coordination chemistry of enantiopure li-
gands revealed that C is formed from the precursor [Rh-
[Rh2((S)-L1)2ACHTUNGTRENNUG(nbd)2] and [Rh2((R)-L2)2ACHTUNGTERN(NUGN nbd)2] can be seen
in the 31P NMR spectra (Figure 2). Control experiments
show that the substituents on the sulphur atom are not re-
sponsible for this chiral self-recognition.[19,21]
ACHTUNGTRENNUNG(nbd)2]BF4 (nbd=norbornadiene) in three steps (Scheme 1).
One equivalent of ligand (L1, L2 or L3) reacts with the met-
Figure 2. 31P NMR spectrum of [Rh2L
b) (R)-L2 and c) a mixture of (S)-L1 and (R)-L2.
2ACHTUNGTNER(NUNG nbd)2] synthesized from a) (S)-L1,
The reaction of [RhACTHNUTRGNEUNG(nbd)2]BF4 and (rac)-L2 also resulted
in the formation of self-sorted homochiral complexes, as
showed by the 31P NMR spectrum (see the Supporting Infor-
mation) and further confirmed by X-ray analysis (Figure 3).
Scheme 1. Coordination of the METAMORPhos ligands to rhodium in
three steps to form the final tetraligated dinuclear complex C.
The homocomplexes A, [Rh2((R)-L2)2ACHTUNGTREN(UNGN nbd)2] and [Rh2((S)-
L2)2A(nbd)2], crystalize as a racemate of homochiral com-
CTHUNGTRENNUNG
allic precursor to yield A, [Rh2L
(nbd)2], a bimetallic com-
plexes. Notably, the ligands have the same coordination
mode and the complex has the same conformation as previ-
ously established by NMR studies and DFT calculation.[19]
plex where two METAMORPhos ligands form a bridge be-
tween the metals, and each rhodium atom is chelated by an
nbd ligand. This dimeric complex reacts with two additional
ligands (one per rhodium atom) to give the momomeric
complex B, [RhL2ACHTUNGTRENNUNG(nbd)]. The monomeric nature of this
complex is evident from the mass spectrum and the doublet
in the 31P NMR spectrum, indicating a symmetric complex
with one rhodium atom and two ligands. The structure of
this complex is similar to the precatalyst synthesized from
monodentate phosphorus ligands, such as the well-studied
MONOPhos.[20] The final dimeric complex [Rh2L4] quantita-
tively forms when 5 bars of H2 is applied to this monomer,
which effectively removes the nbd ligands.
We explored the coordination of racemic ligands under
the same conditions for the formation of the three com-
plexes A–C. When a racemic mixture of ligand is used,
three diastereomers of A can be formed: [Rh2((R)-L)2-
ACHTUNGTRENNUNG(nbd)2], [Rh2((S)-L)2ACHTNURTEGGN(NUN nbd)2] and [Rh2((R)-L)((S)-L)ACHTNGUERTN(NUGN nbd)2].
Surprisingly, the 31P NMR spectrum of A is identical for
complexes based on (R)-L1 and (rac)-L1, suggesting that
Figure 3. 31P NMR spectrum of [Rh2L4] synthesized from a) (R)-L2, b)
(S)-L3 and c) a mixture of (R)-L2 and (S)-L3.
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J. N. H. Reek et al.
In contrast to what is observed for complex A, if (rac)-L1
(or (rac)-L2) is used to form complex B, the 31P NMR spec-
trum shows two doublets in a 1:1 ratio indicating the forma-
tion of two diastereomeric complexes. One of the doublets
corresponds to the previously observed homochiral complex
as the boat-shaped conformation, previously proposed on
the basis of DFT calculations. A noticeable difference be-
tween the calculated and the crystalized complex is that the
P–O coordinated ligands are deprotonated by triethylamine
in the solid state.
[Rh(L)
(nbd)], whereas the other indicates the formation of
Before evaluating nonlinear effects in the hydrogenation
of the benchmark substrates dimethylitaconate (4) and
methyl-2-acetamidoacrylate (5), we first determined the sol-
ubility of the racemate of homochiral complexes based on
ligand L2 in pure dichloromethane and in the presence of
typical amounts of the substrates (substrate/Rh ratio of 25
and initial Rh concentration of 25 mm), as this plays a role
in the anticipated nonlinear effects. In the absence of sub-
strate, the solubility of the racemic self-sorted complex is
0.64 mm. Interestingly, the presence of substrate 4 reduces
this solubility to 0.41 mm whereas 5 increases it to 1.58 mm,
substrate-induced properties that could influence the cata-
lytic outcome of the reaction. We first hydrogenated 4 using
complexes based on ligands L2 with an enantiopurity of the
ligand varying between 0 and 100%. At high catalyst con-
centration ([Rh]=25 mm), when the complexes are pre-
formed prior to substrate addition, precipitation of the race-
mate occurs, leaving in solution homochiral complexes of
the ligand that is in excess. Subsequent addition of substrate
4 enhances this enantiopurification of the reaction mixture
by lowering the solubility of the racemate even further. This
explicit reservoir effect[14,16] leads to a very strong positive
nonlinear effect. Lowering of the enantiomeric excess of the
ligand from 100 to 40%, leads to a drop of only 7% for the
ee value of the product (Figure 5). Importantly, the same ex-
periments but without incubation that allow this self-sorting
process and precipitation to occur, results in the opposite
effect. Instead of a positive nonlinear effect, a strong nega-
tive nonlinear effect is observed. This suggests that under
these conditions, catalysis happens before self-sorting and
subsequent precipitation has completed. The kinetic com-
plexes formed before the metal–ligand system reaches a
thermodynamic equilibrium are responsible for most of the
conversion.
a meso heterochiral complex. X-ray diffraction confirmed
the monomeric nature of the complex. Just as observed for
complex A, B crystalizes as a racemate of homochiral com-
plexes (crystals of the heterochiral complex were not ob-
tained).
When two equivalents of (rac)-L1 were reacted with [Rh-
ACHTUNGTRENNUNG(nbd)2]BF4 under H2 pressure to form complex C, the
31P NMR spectrum of the solution was identical to the very
characteristic AA’BB’XX’ pattern observed for the homo-
chiral complex C prepared from (R)-L1 (see the Supporting
Information). This suggests that C forms with a high fidelity
chiral self-sorting: only [Rh2((R)-L1)4] and [Rh2((S)-L1)4]
are present in solution. To confirm these findings, we used
pseudoenantiomers consisting of (S)-L1 and (R)-L2. This
strategy allowed us to distinguish the pseudodiastereomers
by mass spectroscopy and gave a better separation of the
31P NMR signals. Mostly homochiral complexes were
formed, but now the self-sorting was not perfect: minor side
products could be distinguished by spectroscopic techniques
(see the Supporting Information). Control experiments
show that the substituent on the sulphur atom is not respon-
sible for self-sorting. Using a mixture of (S)-L1 and (S)-L2
to form complex C leads to a multitude of signals in
31P NMR consistent with a statistical mixture of complexes
(see the Supporting Information). The use of the structurally
more similar ligands (R)-L2 and (S)-L3 showed strong self-
sorting behaviour, as only homochiral complexes could be
seen in the 31P NMR spectrum (Figure 3). In line with this,
in the MS spectra, only homocomplexes could be identified.
The reaction of two equivalents of (rac)-L2 with [Rh-
ACHTUNGTRENNUNG(nbd)2]BF4 under H2 pressure yielded a compound that is
very poorly soluble in dichloromethane. Analysis by NMR
spectroscopy was prohibited by the low solubility, but X-ray
structure determinations on isolated crystals demonstrated
that the solid state consisted of the racemate of the homo-
chiral complexes C (see Figure 4). Like for complex A,
these crystal structures confirmed the coordination mode of
the ligands, previously established by NMR studies, as well
Similar hydrogenation experiments of 4 were done under
more diluted conditions ([Rh]=2 mm), again with and with-
out complex preformation prior to substrate addition. The
same results are obtained in both these experiments, as ex-
pected, as under these dilute conditions no precipitation
Figure 4. Crystal structures of A, B and C synthesized from (rac)-L2. Hydrogen atoms, solvent and, in the case of C, the triethylammonium counterion
are omitted for clarity (see ref. [22]).
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Dinuclear Asymmetric Hydrogenation
COMMUNICATION
without complex preformation were significantly different,
showing that for this substrate, hydrogenation is significantly
faster than the formation of the self-sorted complexes (see
the Supporting Information). Like observed for the hydro-
genation of 4 (at [Rh]=25 mm), the kinetically formed
active complexes give different selectivity than the thermo-
dynamic self-sorted complexes. The exact nature of those
complexes remains elusive, as they convert very fast into the
stable dimer and no direct observation could be made. In
analogy with well-known systems, we expect solvated bis-li-
gated monomers to be active intermediates.[20] As they are
formed from the non-self-sorted intermediate B, they are
not self-sorted either, and the weakly coordinated solvent
molecules make them very reactive towards H2 and sub-
strates.
Figure 5. NLE curves for the hydrogenation of 4 with 2 as a ligand.
In summary, we studied in detail a catalytic system for
asymmetric hydrogenation based on METAMORPhos li-
gands and rhodium as a complex chemical system. These
studies revealed self-sorting of ligands at dinuclear com-
plexes leading to homochiral complexes, behaviour that is
not observed for the mononuclear complexes that form
during the incubation phase of a hydrogenation experiment.
We confirmed the nature of dimeric and mononuclear com-
plexes with their X-ray structure. The racemate of the self-
sorted homochiral complex [Rh2(L2)4] synthesized from the
racemic ligand were found to be very insoluble. This proper-
ty leads to a very strong positive nonlinear effect in asym-
metric hydrogenation, with a high ee value obtained for the
product while the ligand is present in low enantiopurity
(40%). This is, however, only observed when the substrate
is introduced after an incubation time in which the self-
sorted complexes are formed. Addition of the substrates at
the beginning of the experiment gives completely different
effects showing that intermediate complexes, which form
before the thermodynamic equilibrium of the ligand–metal
system is reached, are catalytically active. The complexity of
the system was further demonstrated by showing a feedback
loop: the substrate influences the solubility of the racemate
of homochiral complexes, a characteristic that is important
for the nonlinear effects observed. The substrate thus influ-
ences the extent of nonlinear effect. This is the first example
of self sorted ligation at dinuclear complexes that are active
in asymmetric hydrogenation. Detailed information as pre-
sented here is relevant for process optimization and devel-
opment of more complex catalytic systems that may play an
important role in future processes.
spontaneously occurs after incubation. The small deviation
from linearity for the curve of ee value of product as a func-
tion of ee value of ligand may be due to imperfect self-sort-
ing (as also observed the NMR experiment using (S)-L1 and
(R)-L2). We attempted to record the 31P NMR spectrum of
the reaction mixture (with a racemic ligand) under these
catalysis conditions to see these heterocomplexes arising
from incomplete self-sorting. Unfortunately, even after an
overnight acquisition, the signal-to-noise ratio was too small
and only the signals for the homochiral complexes could be
observed. In the NMR tube, crystals of the self-sorted race-
mate had appeared during the NMR experiment, (their
nature was confirmed by X-ray crystal structure determina-
tion). Apparently, the amount of non-self-sorted complexes
is very small, yet they have an influence on the outcome of
the reaction.
Similar studies on the nonlinear behaviour of the com-
plexes were performed using substrate 5. For [Rh]=25 mm,
the results were comparable as those observed for 4. How-
ever, the nonlinear effect observed after incubation is less
pronounced (Figure 6), likely owing to the substrate-induced
higher solubility of the racemate of self-sorted complexes.
This shows that a feedback loop operates, in which the sub-
strate controls, by solubility of complexes, the outcome of its
own hydrogenation. For [Rh]=2 mm, the results with and
Acknowledgements
We acknowledge the NRSCC for financial support and Dr. A. M.
Kluwer, Dr. R. J. Detz, Dr. J. I. v.d. Vlugt and Prof. Dr. B. de Bruin for
fruitful discussions.
Keywords: asymmetric
catalysis
·
complexity
·
Figure 6. NLE curves for the hydrogenation of 5 with 2 as a ligand.
hydrogenation · nonlinear effects · self-sorting
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Received: May 22, 2013
Published online: July 10, 2013
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