In Situ Mass Spectrometric and Kinetic Investigations of Soai's Asymmetric Autocatalysis

Article abstract of DOI:10.1002/chem.202003260

Chemical reactions that lead to a spontaneous symmetry breaking or amplification of the enantiomeric excess are of fundamental interest in explaining the formation of a homochiral world. An outstanding example is Soai's asymmetric autocatalysis, in which small enantiomeric excesses of the added product alcohol are amplified in the reaction of diisopropylzinc and pyrimidine-5-carbaldehydes. The exact mechanism is still in dispute due to complex reaction equilibria and elusive intermediates. In situ high-resolution mass spectrometric measurements, detailed kinetic analyses and doping with in situ reacting reaction mixtures show the transient formation of hemiacetal complexes, which can establish an autocatalytic cycle. We propose a mechanism that explains the autocatalytic amplification involving these hemiacetal complexes. Comprehensive kinetic experiments and modelling of the hemiacetal formation and the Soai reaction allow the precise prediction of the reaction progress, the enantiomeric excess as well as the enantiomeric excess dependent time shift in the induction period. Experimental structural data give insights into the privileged properties of the pyrimidyl units and the formation of diastereomeric structures leading to an efficient amplification of even minimal enantiomeric excesses, respectively.

Full text of DOI:10.1002/chem.202003260

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Accepted Article
Accepted Article
Title: In situ Mass Spectrometric and Kinetic Investigations of Soai's
Asymmetric Autocatalysis
Authors: Oliver Trapp, Saskia Lamour, Frank Maier, Alexander Siegle,
Kerstin Zawatzky, and Bernd F. Straub
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202003260
Link to VoR: https://doi.org/10.1002/chem.202003260
01/2020

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
10.1002/chem.202003260
Chemistry - A European Journal
Research Article
In situ Mass Spectrometric and Kinetic Investigations of Soai's Asymmetric
Autocatalysis
Oliver Trapp,*[a,b] Saskia Lamour,[a,b] Frank Maier,[a] Alexander F. Siegle,[a] Kerstin Zawatzky,[a] and
Bernd F. Straub[c]
Abstract: Chemical reactions that lead to a spontaneous
symmetry breaking or amplification of the enantiomeric excess
are of fundamental interest in explaining the formation of a
homochiral world. An outstanding example is Soai’s asymmetric
autocatalysis, in which small enantiomeric excesses of the added
product alcohol are amplified in the reaction of diisopropylzinc and
pyrimidine-5-carbaldehydes. The exact mechanism is still in
dispute due to complex reaction equilibria and elusive
intermediates. In situ high-resolution mass spectrometric
measurements, detailed kinetic analyses and doping with in situ
reacting reaction mixtures, show the transient formation of
rarely observed. Mechanistic explanations for such reactions with
positive nonlinear effects were discussed by Kagan[ 5 ] and
Noyori.[6,7] considering reversible monomer-dimer associations.
Frank[8] postulated a theoretical model leading to a spontaneous
asymmetric synthesis. If dimers can be formed from their
monomeric building blocks, e.g. by intermolecular interactions,
they are of the same configuration (homochiral) or of opposite
configuration (heterochiral). Since these homochiral and
heterochiral dimers are diastereomeric to each other, they have
different intrinsic properties, which are reflected in their solubility,
rate of formation and other physical properties. The formation of
hemiacetal complexes, which can establish an autocatalytic cycle. heterochiral dimers from an enantiomerically enriched mixture
We propose a mechanism that explains the autocatalytic
amplification involving these hemiacetal complexes.
can increase the enantiomeric excess of the free monomeric
major enantiomer.[9]
Comprehensive kinetic experiments and modelling of the
hemiacetal formation and the Soai reaction allow the precise
prediction of the reaction progress, the enantiomeric excess as
well as the enantiomeric excess dependent time shift in the
induction period. Experimental structural data give insights into
the privileged properties of the pyrimidyl units and the formation
of diastereomeric structures leading to an efficient amplification of
even minimal enantiomeric excesses, respectively.
Autocatalysis and in particular self-amplifying chemical processes
are of great interest as they provide an explanation for the efficient
replication of molecules with intrinsic error correction in general,[1]
and the appearance of mirror image molecules with the same
handedness, namely homochirality. Such processes are of
fundamental importance in symmetry breaking[2] related to the
emergence of life.[3] In an asymmetric reaction or catalysis, it is
usually expected that the enantiomeric excess of the reagent or
catalyst used will be transferred linearly to the product formation.
Positive nonlinear effects,[4] which means that the use of only
enantiomerically enriched reagents or catalysts lead to a
significant increase in the enantiomeric excess in the product, are
Scheme 1. Soai‘s asymmetric autocatalytic reaction. a) Conversion of
pyrimidine-5-carbaldehyde 4 with diisopropylzinc (iPr2Zn) in the presence of a
catalytic amount of pyrimidyl alcohol 1. b) Formation of homochiral (R,R)-3/
(S,S)-3 and heterochiral (R,S)-3 dimers of the isopropylzinc pyrimidyl
alkoxides 2. *The dimers can exchange their monomeric moieties without
formation of the monomers 2.[32] R = (H3C)3C-CC-.
[a]
Prof. Dr. O. Trapp, Dr. S. Lamour, Dr. F. Maier, Dr. A.F. Siegle,
Dr. K. Zawatzky
Department of Chemistry
Ludwig-Maximilians-University Munich
Butenandtstr. 5-13
81377 Munich (Germany)
E-mail: oliver.trapp@cup.uni-muenchen.de
In 1995, Soai[ 10 , 11 ] reported an extremely remarkable
[b]
[c]
Max-Planck-Institute for Astronomy
Königstuhl 17
69117 Heidelberg (Germany)
reaction. When pyrimidine-5-carbaldehyde
4
reacts with
diisopropylzinc (iPr2Zn) in the presence of a catalytic amount of
pyrimidyl alcohol 1 with a low ee, asymmetric autocatalytic
amplification of the enantiomeric excess gives the pyrimidine
alcohol 1 with high ee as the final product (Scheme 1a).
Autocatalysis and amplification are also observed in pyridyl-3-
carbaldehydes,[12] however the 2-alkynyl substituted pyrimidine
analogues are superior in amplification of the ee. Even a trace
imbalance of chiral molecules[13] such as an extremely low ee of
Prof. Dr. B.F. Straub
Organisch-Chemisches Institut
Ruprecht-Karls-Universität Heidelberg
Im Neuenheimer Feld 270
69120 Heidelberg (Germany)
Supporting information for this article is given via a link at the end of
the document
This article is protected by copyright. All rights reserved.

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
10.1002/chem.202003260
Chemistry - A European Journal
Research Article
the initial catalyst of only ~510-5 %[14] or other chiral triggers like
1H/2H,[15] 12C/13C,[16] 14N/15N[17] and 16O/18O[18] isotopically labelled
cryptochiral compounds,[19] cryptochiral compounds,[20] circularly
In this context, another highly remarkable discovery was
reported by Hawbaker and Blackmond, where hydroxy ethers
interfere with the Soai reaction and even inhibit the reaction.[19]
More recently, Denmark and co-workers[ 40 ] performed
investigations of the Soai reaction with focus on the role of the
nitrogen atoms in the pyrimidine/ pyridyl moiety, the structure of
the Zn-alkoxides in solution by NMR spectroscopic studies and
in-situ IR kinetic studies using pyridyl-3-carbaldehyde as
surrogate, (‘Trojan-Horse’ substrate). An alternative mechanism
is proposed, considering a ‘cube escape’ model. Such cube-type
structures were also proposed by Noyori and co-workers[6a] in the
enantioselective addition of dialkylzincs to aldehydes promoted
21
]
22 ]
polarized light,[
(enantiomorph) crystals[
and other
compounds[23] are able to induce enantioselectivities, that lead to
an amplification greater than 99.5% ee in a few cycles. A highly
interesting feature of the reaction is, that spontaneous symmetry
breaking with stochastic distribution of the final (R)-1 or (S)-1
product is possible, even when no chiral additive is employed.[24]
Numerous findings and reports contributed to the
mechanistic understanding of the Soai reaction.[2] Still, open
questions remain, especially regarding (a) the origin of
enantioselectivity, (b) kinetic aspects of the reaction, i.e. the
reliable prediction of the chiral amplification, and (c) the privileged
structure of the pyrimidyl-5-carbaldehydes and corresponding
alcohols. While the last point (c) can be explained by experimental
findings of similar reactions with the possible coordination of the
nitrogen containing pyridyl or pyrimidyl rings and the associated
activation of the alkyl zinc compounds as well as the formation of
by
chiral
-amino
alcohols,
i.e.
(-)-3-exo-(dimethyl-
amino)isoborneol (DAIB) and has been discussed by Brown and
co-workers in the context of the Soai reaction as potential
tetrameric structure of the Zn-alkoxides.[25]
Furthermore, the Soai reaction shows some peculiarities
that are well documented, but still unexplained such as (a) an
unusual inverse temperature dependence on the reaction kinetics,
supramolecular structures,[6a] points a) and b) are not that obvious. i.e. the maximum reaction rate increases significantly as the
The elucidation of the mechanism is highly challenging due to
complex reaction equilibria and elusive intermediates.[25-29] It is
well established that isopropylzinc pyrimidyl alkoxides 2 can form
dimers, tetramers[30,31] and oligomeric compounds.[2] The dimers
can be either homochiral ((R,R)-3 or (S,S)-3) or heterochiral
((R,S)-3) (Scheme 1b).[ 32 ] It is important to note, that the
interconversion of the dimers 3 can proceed by direct exchange
of the monomeric moieties without formation of the monomers.[32]
The implication of this equilibrium is, that the equilibrium constant
for the heterochiral dimer formation Khetero is twice the equilibrium
constant for the homochiral dimer formation Khetero/Khomo = 2.[33]
Thus, nonlinear effect can be well explained, because an
imbalance of the enantiomers leads to amplification as soon as
more stable heterochiral dimers (R,S)-3 have formed. Blackmond
and Brown developed a model considering dimers 3 as
catalytically active species based on reaction progress analysis
by calorimetric measurements and NMR spectroscopy. These
dimers, tetramers[30,31] and oligomers[34] were characterized by
comprehensive NMR spectroscopic measurements and single-
crystal X-ray diffraction analysis,[ 35 ] and these findings are
supported by quantum chemical computations.[36] Kinetic studies
corroborate these results with pronounced effects of the additive
concentration[37] and ee leading to an induction period and a
sigmoidal kinetic profile typical for autocatalytic processes.[2]
Schiaffino performed quantum chemical calculations of the kinetic
constants at the M05-2X/6-31G(d) level of theory and investigated
the effect of the aza group in the pyrimidine moiety to activate the
zinc reagent in the tetrameric complex.[38]
reaction temperature is decreased,[ 41 ] and (b) a prolonged
induction period, which are not yet rationalized by properties of
potential catalyst structures or corresponding kinetic models.
Here we seek to investigate the open mysteries of Soai’s
asymmetric autocatalysis.
First, we performed reaction kinetic investigations using
multiplexing HPLC[42] in the flow-injection mode[43] (Chiralpak IB,
mobile phase n-hexane/THF 55:45, 1.2 mL/min), which provides
temporal resolution of the stereoisomers formed (Figure 1 in the
Supplementary Information). Reaction progress was observed
injection of sample pulses with a time interval of 2.1 min from the
reaction mixture onto the chiral separation column (Figure 1a and
b). We systematically varied the concentrations of the reactants
and additives of the Soai reaction (2-(tert-butylacetylene-1-
yl)pyrimidyl-5-carbaldehyde 4: 10.6
– 41 mM, ((R)-2-(tert-
butylacetylene-1-yl)pyrimidyl-5-(iso-butan-1-ol) (R)-1 (ee
>
99.9%): 0.266 – 4 mM; iPr2Zn: 30 – 130 mM (Figure 1b and
Figures 6-23 in the Supplementary Information).
Kinetic analysis of these data gives a reaction order of 1.9
in [4], an order of 1 in [1], and an order of 0 in [iPr2Zn] (Figures
24-26 in the Supplementary Information), confirming previous
studies.[41]
.
푑[1]
푑푡
= 푘 [4]1.9[1]1[iPr2Zn]0
(Eq. 1)
Surprisingly, when we performed the HPLC separation of the
reaction mixture using isopropanol instead of THF in the mobile
phase we observed the enantiomers of another compound
connected by plateau formation with 4. We identified these peaks
as the isopropyl hemiacetals (R)-5iPr or (S)-5 iPr (Figure 2 a), i.e.
in the case of chiral alcohols diastereomeric hemiacetals are
expected (vide infra).
Remarkably, these hemiacetals 5iPr are subject to a
dynamic interconversion, which we investigated by temperature-
dependent enantioselective dynamic HPLC (DHPLC)[44,45] (Figure
2 b and Figures 28-35 in the Supplementary Information).
The thermodynamic parameters of the formation of the
hemiacetal 5iPr were determined by linear regression of the
thermodynamic Gibbs free energies G(T), obtained from the
In 2012 Brown, Blackmond and co-workers[39] reported the
identification of a transient hemiacetal intermediate in the Soai
reaction of 2-(adamantylacetylene-1-yl)pyrimidine-5-carbalde-
hyde and 2-(adamantylacetylene-1-yl)pyrimidyl alcohol by 1H
NMR spectroscopic kinetic studies. Gridnev and Vorobiev
investigated by quantum chemical DFT calculations and kinetic
analysis potential acetal intermediates. They concluded that the
acetals are off-loop species because they are not precursor of the
reaction product.[27]
This article is protected by copyright. All rights reserved.

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
10.1002/chem.202003260
Chemistry - A European Journal
Research Article
equilibrium constants K, vs. the temperatures T (correlation
coefficient r = 0.9949) to be G0 = 3 kJ/ mol, H0 = -15.6 kJ/ mol
and S0 = -62.5 J/(Kmol) (Figure 36 in the Supplementary
Information).
function as a transient chiral ligand to activate the dialkylzinc
reagent, very similar to the -dialkylaminoalcohols in Noyori’s
DAIB catalysis[6a] or Blackmond’s hydroxy ethers.[19] The in-situ
formation of a transient catalyst by reaction or interaction of
molecules participating in the reaction is
a fundamental
mechanism leading immediately to autocatalysis and
amplification. Similar mechanisms are well known in substrate
activated enzyme catalysis to regulate biochemical reaction
46
]
networks and in artificial systems.[
Furthermore, the
thermodynamic data indicate that the formation of the hemiacetal
is favored at lower temperature and if it is involved in the
autocatalytic cycle, it correlates with the observation that the Soai
reaction is accelerated at lower temperature. Moreover, the
measured reaction kinetics of the formation of the hemiacetals
agrees with the observed induction period of the Soai reaction.
1H NMR spectroscopic studies in CD3OD reveal that the
electron-deficient pyrimidine moiety of 4 and the unsubstituted
pyrimidyl-5-carbaldehyde 4H promote the formation of deuterated
methyl hemiacetals (characteristic hemiacetal proton at  = 5.7
ppm; cf. Figures 39 and 42 in the Supplementary Information) in
95% yield at room temperature, while for comparison
benzaldehyde yields only 9% (Table 4 in the Supplementary
Information). Temperature dependent measurements confirm the
trend that the hemiacetal formation is favored at lower
temperature (Figures 40 and 41 in the Supplementary
Information). This formation of hemiacetals is also observed in
toluene, the solvent used in the Soai reaction: Reaction of 4 and
4H with rac-2-methyl-1-phenylpropan-1-ol in [D8]toluene give
diastereomeric hemiacetals in 11% and 9% yield, respectively.
More important, the diastereomeric ratio of 1:2.6 for the
unsubstituted pyrimidine hemiacetal improves to 1:5.2 for the 2-
(tert-butylacetylene-1-yl substituted pyrimidine hemiacetal
(HSQC spectra depicted in in Figures 43 and 44 in the
Supplementary Information). In this context, it is important to note
that the formation of stereolabile hemiacetals offers the
conceptual mechanism of minor enantiomer recycling[47] leading
to the amplification of the major enantiomer/ diastereoisomer. In
the next step, we investigated the Soai reaction by in-situ high-
resolution mass spectrometric experiments. In these experiments
there is no separation column involved to avoid quenching of the
reaction intermediates. We monitored the course of the Soai
reaction under inert conditions (anhydrous toluene, argon
atmosphere) by feeding the reacting reaction mixture
continuously (10 µL/ min) but pulsed (time interval of 30 s) via a 5
µL sample loop of a 6-port valve (anhydrous toluene as eluent,
flow rate 200 µL/ min) into a high-resolution Orbitrap mass
spectrometer (Figure 45 in the Supplementary Information) using
atmospheric pressure chemical ionization (APCI) under mild
ionization conditions (T = 150°C, N2).
Figure 1. a) Reaction progress analysis by flow-injection analysis using
enantioselective HPLC to separate the reactand 4 and product enantiomers
(R)-1 (green) and (S)-1 (red). Reaction conditions: 26.0 mM 2-(tert-
butylacetylene-1-yl)pyrimidyl-5-carbaldehyde 4, 1.3 mM (R)-2-(tert-
butylacetylene-1-yl) pyrimidyl-5-(iso-butan-1-ol) (R)-1 (ee > 99.9%) and 40 mM
iPr2Zn in toluene (grey) at 20°C. Separation conditions: Chiralpak IB column
(25 cm, I.D. 4.6 mm, particle size 5 µm), n-hexane/THF 55:45 (v/v), 1.2
mL/min. b) Concentration of (R)-1, (S)-1 and 4 vs. time t.
Figure 2. a) Hemiacetal formation of 4 in presence of i-propanol. b)
Temperature-dependent enantioselective DHPLC measurements of the
formation of the hemiacetal 5iPr with isopropanol (Chiralpak IC-3 (15 cm, I.D.
4.6 mm, particle size 3 µm), n-hexane/isopropanol 60:40 (v/v), 1.0 mL/min. c)
Eyring plot for the determination of the activation parameters H╪ and S╪ of
the hemiacetal formation (red data points) and the hemiacetal decomposition
(blue data points) obtained from the DHPLC experiment. The upper and lower
curves represent the error bands (21 data points each) of the linear regression
with a level of confidence of 95%.
The activation enthalpies Hǂ for the hemiacetal 5iPr
formation and decomposition were obtained via the slope and the
activation entropies Sǂ via the intercept of the Eyring plots
(ln(k/T) vs. 1/T) (Figure 2c). Deviations of the activation
parameters Hǂ and Sǂ have been calculated by error band
analysis of the linear regression with a level of confidence of 95%.
The activation parameters of the hemiacetal 5iPr formation are
Hǂ = 26.3  0.2 kJ/mol and Sǂ = -195  34 J/(Kmol) (r = 0.9990,
To identify all intermediates and transient intermediates
during the Soai reaction, all pulsed injections were summed up
over the complete reaction progress and the mass range between
m/z 180 and 800 in a first step (Figure 3a; Figures 46-49 in the
Supplementary Information). This mass spectrum shows the
complexity of intermediates formed in the Soai reaction. We
identified (cf. Figure 3b) the alcohol 1a and its fragment 1b, the
monomeric Zn-alkoxides 2a and 2b, the dimeric Zn-alkoxides 3a
and 3b (different charge states), pyrimidine-5-carbaldehyde 4, the
residual deviation sy
= 0.0306) and the hemiacetal 5iPr
decomposition are Hǂ = 47.7  0.2 kJ/mol and Sǂ = -112  1
J/(Kmol) (r = 0.9994, sy = 0.0630). The formation of hemiacetal
5iPr from 4 is endergonic, however this is a highly dynamic process
and interestingly, the kinetic parameters k1(293 K) = 4.110-3
(mols)-1 and k-1(293 K) = 1.310-2 s-1). Hemiacetals, formed from
the pyrimidine-5-carbaldehyde and its corresponding alcohol can
This article is protected by copyright. All rights reserved.