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Condensed Matter Chemistry in Asymmetric Catalysis and Synthesis

Ru Jiang , Chenxu Liu , Ping Yang , Shuli You

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (7) : 1537 -1547.

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Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (7) : 1537-1547 DOI: 10.7536/PC220221
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Condensed Matter Chemistry in Asymmetric Catalysis and Synthesis
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Abstract

Condensed matter chemistry, as a science of studying condensed matter in chemical reactions, has attracted extensive attention recently. In this review, condensed matter chemistry related to asymmetric catalysis and synthesis are briefly summarized, and different condensed matter phenomena in asymmetric catalytic reactions were classified. Besides, relative works were discussed in detail to illustrate the effects of the multi-level structures and composition of condensed matter on the catalytic activity, enantio- and regioselective control. It is hoped that the system of condensed matter science would be well developed by attracting more attention and bringing more thoughts to the essence of reaction from the perspective of condensed matter chemistry.

Contents

1 Introduction

2 Chirality and asymmetric catalysis

2.1 Importance of chirality

2.2 Asymmetric catalysis

3 Condensed matter chemistry in asymmetric catalysis and synthesis

3.1 Condensation of catalysts:nonlinear effect

3.2 Condensation of catalysts and solvents

3.3 Condensation of catalysts and additives

3.4 Condensation of catalysts and substrates:phase-transfer catalysis

3.5 Condensation of enantiomers:SDE

4 Conclusion and outlook

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Keywords

condensed state / asymmetric catalysis / enantioselectivity / nonlinear effect / solvent effect / phase-transfer catalysis / self-disproportionation of enantiomers

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Ru Jiang, Chenxu Liu, Ping Yang, Shuli You. Condensed Matter Chemistry in Asymmetric Catalysis and Synthesis. Progress in Chemistry, 2022, 34(7): 1537-1547 DOI:10.7536/PC220221

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1 Introduction

Recently, the concept of "condensed matter chemistry" proposed by academician Xu Ruren of Jilin University has attracted wide attention from chemists in different fields[1,2]. Atoms, ions and molecules, which are regarded as the main body of reaction in traditional chemistry, often form condensed substances with different structural levels in actual reactions, and the real composition, structure, properties and functions of these substances are the key factors affecting the reaction[3]. For traditional organic chemistry, the reaction system is relatively simple and basically carried out in solution, so it is generally regarded as an ideal intermolecular reaction. In recent years, with the vigorous development of organic chemistry, the reaction system has gradually become complex and diverse, and more and more researchers have found that the active species in many reactions is no longer a simple molecule, but a more complex condensed matter with multi-level structure. Therefore, it is not enough to study these complex organic chemical reactions only at the molecular level. However, there are few systematic studies on condensed matter chemistry in organic reactions. In this paper, some special chemical phenomena caused by molecular level aggregation in chiral catalysis and synthesis are listed and discussed from the point of view of condensed matter chemistry.

2 Chirality and chiral catalysis

2.1 The importance of chiral

The word Chirality comes from the Greek word "Cheir". The left hand and the right hand are mirror images of each other, but they can't coincide completely. Therefore, an object is called chiral if it does not coincide with its mirror image[4]. Chirality is a basic property of nature-from small molecules to macromolecules to macroscopic substances, such as natural amino acids, proteins, genetic material DNA, shell threads, nebulae and so on, chiral substances constitute a colorful world. In addition, we refer to this substance, which has two different forms like "left-handed and right-handed", as an enantiomer (enantiomer)[4]. Interestingly, some chiral substances exist in nature or life as almost a single isomer, such as amino acids, nucleosides, sugars, etc., or show completely different properties between the two isomers, such as (S) -asparagine, which has a bitter taste, while its enantiomer has a sweet taste. However, in the early days, people did not fully understand chirality, ignoring that the interaction between enantiomers and their receptors might produce different results, which also led to some tragedies. For example, in the application of chiral drugs, the "thalidomide" incident in Europe in the 1960s. Since then, the preparation of single mirror isomers (enantiopure compounds) has become one of the keys to drug development.

2.2 Chiral catalysis

At present, the main methods to obtain enantiopure compounds are chiral resolution, substrate induction and chiral catalysis[4]. Among them, chiral catalysis (asymmetric catalysis), including enzymatic catalysis and chemical catalysis, can efficiently and selectively convert prochiral or achiral substrates into chiral target products, making the reaction highly efficient and effective. It is worth mentioning that chemical catalysis dominated by chiral organometallic catalysts and small organic molecule catalysts has made rapid progress in recent decades, which has greatly promoted the application of asymmetric catalytic reactions in academic research and industrial production. In recognition and affirmation of the role of asymmetric catalysis in promoting the development of human society, the 2001 Nobel Prize in Chemistry was awarded to Dr. Knowles, Professor Noyori and Professor Sharpless for their outstanding contributions to asymmetric hydrogenation, asymmetric epoxidation and dihydroxylation.Professor Arnold, the pioneer of directed evolution of enzymes, shared the Nobel Prize in Chemistry in 2018, and the Nobel Prize in Chemistry in 2021 was awarded to Professor List and Professor MacMillan in the field of small organic molecule catalysis.

3 Condensed matter chemistry in chiral catalysis and synthesis

In asymmetric catalytic reactions, chiral control (also known as enantioselective control) has always been a difficulty and challenge that synthetic organic chemists are interested in and committed to breaking through. In theory, the stereoselectivity control of asymmetric catalytic reactions is mainly determined by the catalyst. However, in the actual reaction, some condensed substances formed by catalysts, reaction solvents, additives and even chiral products themselves will greatly affect the reaction results. Therefore, in this paper, these condensed matter chemistries are briefly classified and introduced according to different aggregation modes or aggregation States, including aggregation between catalysts, aggregation between catalysts and substrates, aggregation between catalysts and additives, aggregation between catalyst and solvent, and aggregation between enantiomers, respectively.

3.1 Aggregation of Catalysts and Catalysts: Nonlinear Effects

In asymmetric synthesis, there should be a theoretical linear correlation between the ee value (enantiomeric excess) of the product and the ee value of the catalyst (as shown by line B in fig. 1) without interference from other factors. However, in many practical reactions, due to the complexity of the catalytic species in the reaction mechanism or the interaction between them, such as aggregation, there are Nonlinear effects between the ee value of the product and the ee value of the chiral ligand or chiral catalyst (as shown by curves A and C in Figure 1)[5,6]. In essence, this nonlinear relationship is due to the existence of two different aggregation modes between enantiomers of chiral compounds, such as self-aggregation of compounds with the same configuration, also known as Homochiral associates. The aggregation between compounds of opposite configuration is also called Heterochiral associates. In general, the two species differ in activity. If the nonlinear relationship is like curve A, it is called positive nonlinear effect [ (+) -NLE], indicating that the activity of homochiral aggregated species is higher than that of heterochiral species in the reaction. This means that even if a ligand or catalyst with low enantiomeric purity is used, the reaction can smoothly produce products with high enantiomeric purity, so this effect is also called asymmetric amplification effect. If the nonlinear relationship is like curve C, it is called negative nonlinear effect [ (-) -NLE], indicating that the activity of heterochiral aggregated species is higher than that of homochiral species in the reaction. Such reactions require ligands or catalysts of high enantiopurity, otherwise the enantioselective control of the reaction will be significantly affected, so this effect is also called asymmetric reduction effect. In general, the study of nonlinear effects can not only provide guidance for the enantiopurity selection of reaction catalysts, but also provide relevant information on the mechanism of partial reactions. Next, this paper will introduce and analyze the application of nonlinear effects in asymmetric catalysis and synthesis through some classical examples.

3.1.1 Asymmetric addition of organozinc reagents to aldehydes

In 1986, Noyori et al. Reported the highly enantioselective addition of dialkylzinc to aldehydes catalyzed by β-dialkylamino alcohols[7]. They found that when the ratio of β-dialkyl amino alcohol to zinc reagent was different, the species formed in the reaction were different, which significantly affected the yield of the reaction. For example, a series of different alkylzinc reagents can undergo asymmetric addition to aldehydes and produce the corresponding chiral secondary alcohols when 2 mol% (−) -DAIB is used to catalyze the reaction (Scheme 1). However, the alkylation reaction could not take place when one equivalent of (-) -DAIB was used. This is because when one equivalent of (-) -DAIB is added, the dialkylzinc reagent will first form a 1/1 species (-) -Ⅰ with (-)-DAIB, which can only activate the carbonyl group of benzaldehyde and can not react with it, so excessive dialkylzinc can only be added.(-) -I can be further converted into the dinuclear dialkylzinc species (-) -II, which is the alkylation active species (Scheme 2), and then the addition reaction occurs to produce the alkylation product in excellent yield and enantioselectivity.
In addition to the change of reactivity caused by the different ratio of β-dialkyl amino alcohol to zinc reagent, there is a significant positive nonlinear effect [ (+) -NLE] with the change of enantiopurity of β-dialkyl amino alcohol. For example, when 8 mol% of (-) -DAIB with an enantiomeric purity of only 15% ee was used as the catalyst in the reaction, the product with up to 95% ee could still be obtained, which was not very different from the result of the standard reaction (Scheme 3)[8]. After studying the mechanism, Noyori et al. Found that the mixture of enantiopure DAIB and 1 equiv of dialkylzinc reagent can form a single homochiral dimeric binuclear species Ⅲ, but Ⅲ is unstable and easy to decompose into the corresponding monomer Ⅰ to participate in the catalytic reaction. Interestingly, when the homochiral dimeric species (-) (-) -Ⅲ and (+) (+) -Ⅲ formed by the two enantiomers were mixed, the enantiomeric monomers (S) and (R) -Ⅰ formed by in situ decomposition could heterochirally aggregate with each other to form a stable heterochiral dimeric species (-) (+) -Ⅲ, but (-. In other words, DAIB with low enantiopurity will preferentially form the catalytically inactive heterochiral dimeric species (-) (+) -III with alkylzinc reagents, and the remaining single excess enantiomer can form the unstable homochiral dimeric species (-) (-) -III or (+) (+) -III, which can further decompose to participate in the catalytic reaction. Therefore, even if a catalyst with low enantiopurity is used in the reaction, good reaction results can still be obtained only by increasing the amount of catalyst.

3.1.2 Amino acid catalyzed asymmetric aldol reaction

As an important small molecule catalyst, proline plays an important role in asymmetric aldol reaction, Mannich reaction and amino acylation. In 2004, Blackmond's group found that there was an obvious asymmetric amplification effect in the repeated proline-catalyzed aldehyde α-aminoxylation reaction (Figure 4A), which was different from the strict linear correlation between the ee value of the product and the catalyst in the asymmetric adol reaction previously reported by List's group, which aroused Blackmond's group's interest in the mechanism of amino acid catalysis[11][12]. In 2006, they found that in the proline-catalyzed asymmetric aldol reaction (Scheme 4B), there are actually two different relationships between the ee value of proline and the ee value of the aldol product[13]. When the concentration of proline is low, it can be completely dissolved in the reaction system, and the ee value of the reaction product is linearly related to the ee value of the catalyst (fig. 2a, square); On the contrary, if a high concentration of proline is used as the catalyst, it is not completely dissolved and is in solid-liquid equilibrium, and there is a nonlinear relationship between the ee value of the reaction product and the ee value of the catalyst (fig. 2a, dots). In order to explore the cause of this nonlinear relationship, they measured the relationship between the ee value of proline in the solution phase (the concentration of proline is basically maintained between 0.02 and 0.03 mol/L) and its overall ee value in the high concentration proline system.That is, when the ee value of the proline population changes from almost racemic to enantiopure, the ee value of proline in the solution phase remains basically unchanged (about 50% ee) (fig. 2b), which corresponds to the enantioselectivity control result of the reaction. This is due to the low solubility of proline in the mixed solution of DMSO/H2O. When the concentration is high and the ee value is low, the heterochiral dimerization is dominant, and the proline precipitates as racemic crystals, resulting in the enantiopurity of proline in the solution being higher than that of the whole. When the concentration is high and the ee value is high, the homochiral dimer is dominant, and enantiopure crystals are mainly precipitated, so the enantiopurity of proline in the solution is lower than that in the whole. In addition to proline, other amino acids as catalysts also have significant nonlinear effects under heterogeneous conditions.
This study not only reveals the nature of nonlinear effects in amino acid-catalyzed reactions, but also provides guidance for how to achieve chiral amplification in amino acid-catalyzed reactions.It also leads to further thinking about the origin of chirality in nature, that is, starting from trace or low ee chiral initiators, the initial small enantiomeric imbalance can be amplified through chiral amplification[13⇓~15].

3.1.3 Enantioselective allylic substitution catalyzed by iridium metal

In transition metal-catalyzed asymmetric reactions, if the catalytic active species is a mono-ligand-coordinated metal complex, the ee value of the product is generally linear with the ee value of the ligand. If the catalytic active species is a metal complex coordinated by multiple chiral ligands (MLn), the ee value of the product and the ee value of the ligand generally show a nonlinear relationship, in which case the nonlinear effect is mainly due to the difference in catalytic activity between the complex formed by homochiral ligands and the complex formed by heterochiral ligands[5]. For example, when the catalytic activity of M[(R)-L]2 or M[(S)-L]2 is significantly higher than that of M [ (R) -L] [ (S) -L], the reaction presents a positive nonlinear effect [ (+) -NLE]; On the contrary, if the catalytic activity of M [ (R) -L] [ (S) -L] is higher than that of M[(R)-L]2 or M[(S)-L]2, the reaction shows a negative nonlinear effect [ (-) -NLE]. Based on this principle, the study of nonlinear effect is also an important means to obtain the information of ligand composition in active catalysts.
You Shuli's group recently prepared different types of iridium catalysts in situ by changing the feeding ratio of iridium precursor to chiral phosphorus-olefin ligand (Carreira ligand) in the asymmetric allylic substitution reaction catalyzed by metal iridium, and realized the kinetic resolution of branched allylic substrates and the Z-form retention reaction of linear allylic substrates, respectively (Scheme 5 and 6)[16,17]. In the mechanism study, they explored the relationship between the ee value of the product and the ee value of the ligand under different catalytic systems. When the feed ratio of the iridium precursor [Ir(cod)Cl]2 to the Carreira ligand (S) -L1 is 1/4, the reaction shows a positive nonlinear effect, and the catalytic active species is the double-ligand iridium complex K1 (Scheme 5)[18]. When the feed ratio of iridium precursor to Carreira ligand is 1/2, there is a strict linear relationship between the ee value of the product and the ee value of the ligand, and the catalytic active species K2 of the reaction contains only one ligand coordination (Scheme 6).
It is worth mentioning that the two iridium complexes with different coordination modes also have significant differences in catalytic activity and selectivity. Specifically, the reaction with the two-ligand-coordinated iridium complex K1 as the catalytic active species generally uses the terminal branched allyl precursor as the substrate, and the reaction has excellent branched selectivity[19,20]. Through the analysis of the single crystal structure of complex K1, Carreira's group found that the bond between the iridium center and the carbon atom C3 on the allyl fragment is significantly longer than that between the iridium center and C1[18]. They speculate that this is mainly because of the strong anti-site effect of the phosphorus atom, which weakens the Ir-C3 bond, so that the site is more vulnerable to nucleophile attack (Scheme 7). In contrast, in the reactions catalyzed by the mono-ligand-coordinated iridium complex K2, both the terminal branched or straight-chain allylic precursors can be well used, and these reactions have excellent straight-chain selectivity[21⇓~23]. However, the source of the regioselectivity of the K2-catalyzed reaction has not yet been elucidated.
In asymmetric catalytic reactions, especially for homogeneous catalytic systems, the catalytically active species are difficult to prepare and characterize, but the enantiopurity information of the product and the catalyst (or ligand) is readily available.The information on the ligand composition in the active catalyst can be quickly obtained by a simple nonlinear effect study, which can provide some guidance for the reaction mechanism study.

3.2 Aggregation of Catalyst and Solvent: Solvent Effect

Solvent is an important component in organic chemical reactions, which can change the activation energy of reacting molecules through long-range interactions such as electrostatic forces or dispersion forces.It can also change the properties of the reaction molecules by forming short-range interactions such as coordination bonds and hydrogen bonds with the reaction molecules, thus affecting the equilibrium and rate of the reaction, which is also called solvent effect[24].
In catalytic asymmetric reactions, aggregation between solvent molecules and catalyst can also lead to changes in the structural level of the catalyst. For example, solvent molecules act as ligands to coordinate with the metal center, resulting in changes in the spatial environment of the catalyst and the electrical properties of the metal center, thus affecting the activity or selectivity of the catalyst. In 2004, Tang Yong's group studied the copper-catalyzed enantioselective Friedel-Crafts reaction of indole and benzylidene malonate, and found that in the catalytic system derived from copper source Cu(OTf)2 and C2-symmetric chiral tridentate ligand TOX,When the solvent was changed from chlorinated or aromatic solvents to solvents containing oxygen atoms (such as alcohols, ethers, acetone), the enantioselectivity control of the reaction was reversed, that is, the configuration of the product changed from S to R (Table 1)[25,26]. In addition, they also found that alcohol solvents with good coordination can greatly improve the reaction activity, and the enantioselectivity control of the reaction increases with the increase of the steric hindrance of the alcohol molecule (Table 1, entries 3 ~ 6). For example, when isopropanol is used as the solvent, the reaction takes only 2 H to complete. In order to explain the solvent effect in the reaction, they combined the experimental results with previous related studies to propose two stereocontrol models — when the solvent can coordinate with the copper ion center, such as isopropanol, the stereocontrol model of the reaction is shown in Figure 3A[27]. In this model, the bulky X oxazoline substituent (OX) in the side arm shields the Si face of the benzylidene malonate substrate, so the nucleophile can only attack from the Re face to give the S configuration product. In a non-coordinating solvent environment, the C2-symmetric TOX ligand shields the Re-face of the benzylidene malonate substrate by switching the side arm group, as shown in Model 3B, and the nucleophile attacks from the Si-face to give the R-configuration product.

3.3 Aggregation of Catalyst and Additive: Additive Effect

In addition to the solvent, additives can sometimes aggregate with the catalytically active species due to weak interactions, affecting the transition state of the reaction, resulting in the inversion of the enantioselectivity control of the reaction.
By adding molecular sieves or silica gel as additives, Oh's group achieved the immobilization of copper-brucine derivative complexes and formed multifunctional heterogeneous chiral catalysts in situ[28,29]. Using an immobilized heterogeneous catalyst versus an unimmobilized homogeneous catalyst, they smoothly realized the orthogonal enantioselective indole Friedel-Crafts alkylation reaction (Scheme 8) and proposed a stereochemical control model under both catalytic systems (Fig. 4). The difference between the two catalytic systems is that there are hydrogen bonding interactions between the catalyst, substrate and additive in the heterogeneous catalytic system, which leads to the opposite configuration of the dominant transition state in the two catalytic systems.

3.4 Aggregation of Catalyst and Substrate: Phase Transfer Catalysis

Phase transfer catalysis (PTC), as an important organic small molecule catalysis, has become an important organic synthesis method in industry and laboratory research because of its relatively low catalyst dosage, mild reaction conditions, easy operation and large-scale[30,31]. In a phase transfer catalytic reaction, the catalyst (Q+R-, fig. 5) can accelerate or cause the reaction of substances in two immiscible solvents (liquid-liquid two-phase system or solid-liquid two-phase system).For the chiral cation phase transfer catalytic reaction, the specific reaction mechanism is shown in Fig. 6: firstly, the inorganic base (MOH) and the substrate undergo a deprotonation process at the interface between the organic phase and the aqueous phase to form a M+R-(Step I); Asymmetric phase transfer catalysts often contain many highly lipophilic substituents, which are difficult to move freely into the aqueous phase, so the Q*+X- of M+R- and chiral phase transfer catalysts will exchange ions at the interface between the two phases, resulting in lipophilic species Q*+R-(Step II). This species can enter the organic phase and has high reactivity, that is, the asymmetric nucleophilic substitution reaction with the electrophilic species R ′ X occurs to produce the optically active target product R ′ -R, and the chiral phase-transfer catalyst R 'X is regenerated to complete the catalytic cycle (Step III)[32]. For such reactions, the key to success is the rapid ion exchange between the chiral phase-transfer catalyst Q*+X- and the M+R-, that is, the chiral fragment of the catalyst and the effective reactive fragment of the substrate aggregate to form an active species Q*+R-, which not only has high reactivity, but also can achieve chiral induction of the reaction.
The first example of phase transfer catalytic reaction was reported by Starks' group in 1971[33]. They successfully realized the cyanation of alkyl chlorides by using (nBu)3P+C16H33Br- as a phase transfer catalyst (Scheme 9). The first asymmetric phase transfer catalytic reaction was reported by Merck in 1984. Dolling et al. Successfully realized the α-asymmetric methylation of carbonyl compounds by using cinchonine-derived quaternary ammonium salt (PTC 1) as a chiral phase transfer catalyst and chloroform as a methyl source (Figure 10)[34]. Through systematic studies, they speculated that a Tight ion pair intermediate was formed between the cation and anion in the catalytically active species through hydrogen bonding and π-π stacking, which could effectively induce the chirality of the product (Scheme 10).
In addition to the phase transfer catalyst derived from Chiral cations, Toste's group reported the first chiral anion phase transfer (Chiral anionic phase transfer, CAPT) catalytic reaction in 2011, which successfully realized the enantioselective olefin electrophilic fluorination and successfully constructed a series of spiro compounds (Fig. 11)[35]. The specific reaction process is as follows (Figure 8): because the fluorinated reagent Selectfluor is almost insoluble in non-polar solvents (such as fluorobenzene), it can not directly react with the substrate dissolved in organic solvents.However, it can undergo ion exchange with soluble chiral phosphate at the two-phase (solid-liquid phase) interface to form a tight chiral ion pair, thus bringing the cation with fluorination function in the fluorination reagent into the organic phase. The chiral ion pair further reacts with the olefin substrate in the organic phase to generate the target compound, and the catalyst is regenerated to realize the asymmetric phase transfer catalysis between the solid phase and the organic phase.

3.5 Aggregation of Enantiomers: Enantiomeric Self-disproportionation

From the above introduction, it can be seen that the aggregation between the enantiomers of the catalyst can cause the change of the activity or enantiopurity of the catalytic species, thus affecting its chemical behavior in asymmetric catalysis and synthesis. In addition to catalysts, the enantiomers of chiral products in chemical reactions can also aggregate and show significant differences in physical properties in an achiral environment. Based on this phenomenon, the enantiopurity of non-racemic products can also be improved by some conventional physical operations, such as recrystallization, column chromatography, distillation, sublimation and other physical means. Academically, under the condition of not introducing chiral factors, the non-racemate is spontaneously aggregated into an enantiomeric mixture with a higher ee value and an enantiomeric mixture with a lower ee value by conventional physical means such as recrystallization, column chromatography, sublimation or distillation,Self-disproportionation of enantiomers (SDE) is a phenomenon in which enantiomers are enriched and chiral compounds with high optical purity are obtained[36⇓~38].
First of all, the basic principle of enantiomeric self-disproportionation to achieve enantiomeric enrichment in non-racemic compounds is introduced[37]. For racemic compounds, the aggregates produced by homochiral aggregation are still enantiomers, while heterochiral aggregation can only produce racemic dimers, so the enantiomers of racemic compounds cannot be separated without additional chiral factors (Fig. 9b); For non-racemic compounds, when the enantiomers are homochiral, they will form two different aggregates, which can be separated by physical means due to the difference in physical properties. When the enantiomers aggregate heterochirally, in addition to forming racemic dimers, there is an excess of a single configurational enantiomer, which also belongs to different aggregates and can be separated by differences in physical properties (Fig. 9c). For example, in the amino acid-catalyzed Aldol reaction in Section 3.1.2, the ee value of the amino acid in the solution phase is different from that in the solid phase, which is essentially enantiomeric self-disproportionation. Next, this paper will introduce the self-disproportionation of enantiomers under different physical actions through some examples.
In 2006, Soloshonok's group found that there was obvious enantiomeric self-disproportionation of β-amino esters during column chromatography[36]. In the experiment, they used β-amino esters with different enantiomeric purities as starting materials or different ratios of developing agents for column chromatography of β-amino esters, and then collected the eluates from different time periods with test tubes and analyzed the β-amino esters by HP LC (Table 2). Interestingly, they found that the enantiopurity of the β-amino ester in different tubes was different, with the first eluted β-amino ester having a lower enantiopurity and the second eluted β-amino ester having a higher enantiopurity (fig. 10).The degree of difference varies with the type of raw material, the initial enantiomeric purity of the raw material, and the ratio of the developing agent, and the enantiomeric purity of the β-amino ester in the eluent can be up to optical purity (Table 2). Coincidentally, the self-disproportionation of enantiomers was also observed in the separation of 1,2-dihydroisoquinoline derivatives by column chromatography[39]. Contrary to what was observed by Soloshonok et al., they found that the product eluted first was very enantiopure, while the product eluted last was almost racemic (Fig. 10). Through the structural analysis of the racemate and the single crystal formed by the single enantiomer of 1,2-dihydroisoquinoline derivative, they found that there was a significant π-π stacking interaction between the R and S enantiomers in the crystal formed by the racemate (Fig. 11), while there was no such interaction in the single crystal formed from the single enantiomer. This indicates that the racemate formed by heterochiral aggregation between the enantiomers of 1,2-dihydroisoquinoline derivatives has the characteristics of more thermodynamic stability and poor solubility.
Similarly, in achiral liquid preparative chromatography, some chiral compounds will also undergo enantiomeric self-disproportionation on the chromatographic column, resulting in a group of overlapping peaks on the spectrum, and the enantiopurity of components with different retention times is different[40,41].
Because of differences in structure and intermolecular forces, the boiling point or sublimation temperature of the crystal formed by the racemate and the single enantiomer is different, in addition to differences in density and solubility. Taking advantage of this, the enantiomeric self-disproportionation of some non-racemic compounds can also be achieved by distillation or sublimation. For example, in 1989, Koppenhoefer and Trettin, while distilling the chiral compound trifluoroacetamido ester with an enantiomeric purity of 91% ee, found that the difference in enantioselectivity of the product in the remaining mother liquor and the collected fraction could reach about 10% ee (fig. 12)[42]. In 2007, when Soloshonok's group purified the trifluoromethyl-substituted α-hydroxy acid, it was found that the enantiomeric purity of the sample remaining at the bottom to be sublimated gradually increased with time, and it could reach optical purity after 56.5 H (Fig. 13A)[43]. Through the comparative study of racemic and enantiopure α-hydroxy acid crystals, they found that the racemic single crystals formed by heterochiral aggregation had fewer hydrogen bonds and obvious repulsion between fluorine atoms, which may be the main factors leading to low melting point, instability and easy sublimation of racemic crystals. Inspired by Soloshonok's work, Escolano's group also realized the enantiomeric self-disproportionation of cyclic amino esters through sublimation experiments[44]. They found that the sublimed solid enantioselectively had only 27.2% ee, while the solid to be sublimed left at the bottom had an enantioselective purity of > 99.9% ee (fig. 13b).

4 Conclusion and prospect

In this paper, the condensed matter chemistry phenomena in chiral catalysis and synthesis were analyzed, including the effects of the specific composition and hierarchical structure of the condensed matter formed by the aggregation of various reaction elements on the catalytic activity, enantioselectivity and regioselectivity control of the reaction. Due to the limitation of few examples and corresponding discussions on condensed matter chemistry in chiral synthesis, and even less systematic research, this paper is not a complete summary, but an analysis of individual phenomena based on the author's understanding, trying to explain them with condensed matter chemistry. We firmly believe that the results of many organic reactions should be closely related to condensed matter chemistry, but the importance of condensed matter chemistry in chiral synthesis and organic chemistry as a whole has not been given due attention. With the improvement of monitoring methods, especially in situ monitoring technology, combined with the enhancement of theoretical computing power, it is possible to systematically study condensed matter chemistry in organic reactions. It is of great help to the in-depth study of condensed matter science, the understanding of the nature of organic chemical reactions, and the regulation of reactivity and selectivity.

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Funding

National Natural Science Foundation of China(91856201)

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