Asymmetric Autocatalysis and the Origin of Homochirality - ACS

Chapter 3

Asymmetric Autocatalysis and the Origin of Homochirality Downloaded by UNIV OF FLORIDA on November 11, 2017 | http://pubs.acs.org Publication Date (Web): November 2, 2017 | doi: 10.1021/bk-2017-1258.ch003

Kenso Soai* and Arimasa Matsumoto Department of Applied Chemistry, Tokyo University of Science, Kagurazaka, Shinjuku-ku, Tokyo, 162-8601 Japan *E-mail: [email protected]

Asymmetric autocatalysis is a reaction in which chiral product acts as chiral catalyst for its own production. Pyrimdin-5-yl-iso-propylcarbinol (pyrimidyl alkanol) was found to be a highly efficient asymmetric autocatalyst in the enantioselective addition of diisopropyl zinc to pyrimidine-5-carbaldehyde to produce more of itself (Soai reaction). The process is the automultiplicaion, i.e., catalytic self-replication, of a chiral compound. Pyrimidyl alkanol with extremely low enantiomeric excess (ee) automultiplies and is amplified during three consecutive asymmetric autocatalyses to reach >99.5% ee. By using asymmetric autocatalysis, the origin of homochirality has been examined. Circularly polarized light, inorganic chiral crystals such as quartz, enantiotopic surface of achiral inorganic chiral such as gypsum, chiral crystals of achiral organic compounds, chiral compounds of isotope (hydrogen, carbon, oxygen, nitrogen) substitution, acting as chiral initiators in asymmetric autocatalysis, can be correlated to the highly enantioenriched products. Spontaneous absolute asymmetric synthesis was achieved without the intervention of any chiral factor.

1. Introduction Living organisms on Earth are composed of highly enantioenriched compounds such as L-amino acids and D-sugars (Figure 1). Enzymes composed of random mixtures of D- and L-amino acids would not operate, and neither © 2017 American Chemical Society Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

Downloaded by UNIV OF FLORIDA on November 11, 2017 | http://pubs.acs.org Publication Date (Web): November 2, 2017 | doi: 10.1021/bk-2017-1258.ch003

would DNA and RNA composed of D- and L-(deoxy)ribose. The origin of chiral homogeneity of biomolecules, often called as biological homochirality, has been a research subject of broad interest, because the biological homochirality is considered to be essential for the origin and evolution of life. Several theories have been proposed for the origins of the chirality of organic compounds (1–7). However, the enantiomeric excesses of organic compounds induced by the proposed mechanisms have been very low (99.5% ee as an asymmetric autocatalyst, (S)-2-alkynyl-5-pyrimidyl alkanol 4 with >99.5% ee, was obtained in a yield of >99% (Scheme 3) (27). The product 4 in one round was used as an asymmetric autocatalyst for the following round. Even after the tenth round, the yield of newly formed 4 was >99% and the ee was >99.5%. Thus, 2-alkynylpyrimidyl alkanol 4 was found to be a practically perfect asymmetric autocatalyst.

Scheme 3. Practically perfect asymmetric autocatalysis.

3. Amplification of Enantiomeric Excess in Asymmetric Autocatalysis An asymmetric non-autocatalytic reaction with amplification of ee was first reported by Kagan (28). In asymmetric autocatalysis, if the amplification of ee is observed, the reaction would become a more powerful method for amplifying a very low ee to a very high ee by applying consecutive asymmetric autocatalysis. Indeed, we found that the ee of pyrimidyl alkanol increased during the asymmetric autocatalysis (26). We used the product of one round as the asymmetric autocatalyst for the following round. 30 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


When (S)-pyrimidyl alkanol 2 with only 2% ee was employed as the initial asymmetric autocatalyst, the reaction afforded (S)-2 with an increased ee of 10% (Scheme 4) (26). The (S)-2 with 10% ee was then used as the asymmetric autocatalyst for the following consecutive asymmetric autocatalysis. The ee was further amplified step-by-step to 57, 81, and 88% ee. The overall process is the asymmetric autocatalysis of (S)-2 starting from a low 2% ee and with significant amplification of ee to 88% ee. This is the first asymmetric autocatalysis with amplification of ee. Regarding the amplification of ee, in sharp contrast to non-autocatalytic amplification reactions, One of the striking features of the amplification of ee in the present asymmetric autocatalysis is that there is no need for any chiral auxiliary other than the asymmetric autocatalyst itself.

Scheme 4. Asymmetric autocatalysis with amplification of ee. After examining the effect of the substituent at 2 position of pyrimidyl alkanol, 2-alkynylpyrimidyl alkanol 4 was finally found to exhibit the most efficient amplification of ee. Thus, starting from alkanol 4 with ee as low as ca. 0.00005% ee, three consecutive asymmetric autocatalyses amplified ee to 57, 99, and >99.5% ee (Figure 2) (29). The initially slightly major (S)-enantiomer of 4 automultiplied by a factor of ca. 630,000 during these three consecutive asymmetric autocatalyses, whereas the initially slightly minor (R)-enantiomer of 4 automultiplied by a factor of less than 1,000. As described, we found highly efficient asymmetric autocatalysts with significant amplification of ee.

4. The Origin of Chirality Examined by Using Asymmetric Autocatalysis The origins of chirality proposed so far can induce chiral compounds with very low enantiomeric excess. A chiral compound with low ee may act as a chiral initiator for the asymmetric autocatalysis of pyrimidyl alkanol (30). The subsequent amplification of ee during the asymmetric autocatalysis affords highly enantioenriched pyrimidyl alkanol with the corresponding absolute configuration to that of the chiral initiator (31). Thus, asymmetric autocatalysis can act as a link between the low ee induced by the origin of chirality and compounds with high ee observed in nature (Scheme 5). 31 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 2. Automultiplication of (S)- and (R)-4 in consecutive asymmetric autocatalysis.

Scheme 5. Schematic correlation of the origin of chirality to chiral compound with high enantiomeric excess. 32 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

4.1. Circularly Polarized Light


4.1.1. Chiral Compounds Induced with Circularly Polarized Light Initiate Asymmetric Autocatalysis Circularly polarized light (CPL) has long been considered one of the physical chiral factors of the origin of chirality (4, 6, 32). Asymmetric photolysis of racemic leucine by right-handed CPL (r-CPL, 213 nm) induces only 2% ee in residual L-leucine (33). Hexahelicene with low ee (99.5% ee (Scheme 7) (37). On the contrary, upon the irradiation of righthanded (r)-CPL, (R)-4 with >99.5% ee was formed. The rationalization regarding the relationship between the handedness of CPL and alkanol 4 can be made as follows: (R)- and (S)-pyrimidyl alkanols 4 exhibit positive and negative circular dichroism (CD) spectra at 313 nm, respectively. This means that (R)-4 absorbs l-CPL preferentially. Then the direct irradiation of l-CPL to racemic alkanol 4 33 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


would induce the asymmetric photodegradation of (R)-4 preferentially and leave the (S)-4 with low ee. Thus, even if the ee of the remaining (S)-4 was extremely low after the asymmetric photodegradation by CPL, the remaining compound 4 itself would serve as an asymmetric autocatalyst in the asymmetric autocatalysis with significant amplification of ee.

Scheme 7. A route to obtain a near enantiopure compound by CPL irradiation in conjunction with asymmetric autocatalysis.

4.2. Chiral Inorganic Material 4.2.1. Chiral Inorganic Crystals as the Origins of Chirality in Conjunction with Asymmetric Autocatalysis Quartz is a chiral inorganic crystal and exhibits either a dextrorotatory (d) or a levorotatory (l) enantiomorph. The rotation of plane polarized light, i.e., optical activity, was found with quartz. Thus, quartz has been considered for many years the origin of chirality in nature; however, it is unclear whether quartz induces the apparent enantiomeric imbalance in organic compounds. Thus, it is a challenge to use quartz as a chiral initiator in the asymmetric autocatalysis of pyrimidyl alkanol. When 2-alkynylpyrimidine-5-carbaldehyde 3 was reacted with i-Pr2Zn in the presence of d-quartz powder as a chiral initiator, (S)-pyrimidyl alkanol 4 with 97% ee was formed in 95% yield (Scheme 8) (38). In sharp contrast, in the presence of l-quartz powder, (R)-4 with 97% ee was formed in 97% yield. These results clearly show that the chirality of quartz controls the absolute configurations of the obtained pyrimidyl alkanol 4. Thus, the first experimental realization was achieved that a chiral inorganic crystal as the origin of chirality is correlated to that of a chiral organic compound with high ee.

34 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Scheme 8. Asymmetric autocatalysis triggered by quartz.

Cinnabar (HgS) and Retgersite (NiSO4•6H2O) are other examples of chiral inorganic crystal. These chiral inorganic crystals were also found to trigger the asymmetric autoctalysis (39, 40). In addition to the chiral inorganic crystals, chiral inorganic nano-structure also trigger the asymmetric autocatalysis, and helical silica and helical mesoporous silica can direct the enantiomeric outcome of the asymmetric autocatalysis (Scheme 9) (41, 42).

Scheme 9. Asymmetric autocatalysis triggered by helical silica gel.


4.2.2. Enantiotopic Face of Achiral Inorganic Crystal


Surface structures also have chirality and the crystal surfaces sometimes exhibit chirality even the whole crystal structure itself is achiral. Gypsum (CaSO4•2H2O) is an achiral inorganic mineral, but is known to have enantiotopic faces. The asymmetric autocatalysis performed on the enantiotopic surface of gypsum afforded enantio-enriched chiral compounds. The addition reaction of diisopropyl zinc vapor to the adsorbed aldehyde 3 gives the enantioenriched alkanol 4 with the corresponding absolute structure to the adsorbed crystal surface of the gypsum (Scheme 10). This result is the first example of the enantioselectivity control of the reaction product by the surface chirality of achiral inorganic crystal (43).

Scheme 10. Asymmetric autocatalysis on the enantiotopic surface of achiral Gypsum.

4.3. Asymmetric Autocatalysis Triggered by Chiral Organic Crystals Composed of Achiral Organic Compounds Some of the achiral organic compounds crystallize in chiral forms. It was unclear whether chiral crystals of achiral compounds can act as a chiral inducer in enantioselective reactions. We found an enantioselective reaction using these chiral crystals as chiral inducers of asymmetric autocatalysis. Cytosine is an essentially flat achiral molecule and a nucleobase of cytidine and deoxycytidine. Cytosine is known to form a chiral crystal 5 (space group: P212121) when it is crystallized from methanol. It was found that achiral cytosine when crystallized from methanol with stirring and without adding any seed crystal affords powder-like crystals that exhibit either a plus or minus Cotton effect in solid-state CD spectra at ca. 310 nm in Nujol mulls (44). The distribution of the formation of [CD(+)310Nujol]-5 and [CD(–)310Nujol]-5 was stochastic. Next, the chiral crystals 5 were used as chiral triggers for asymmetric autocatalysis (Scheme 11) (44). When pyrimidine-5-carbaldehyde 3 and i-Pr2Zn were reacted in the presence of [CD(+)310Nujol]-5, enantioenriched (R)-pyrimidyl alkanol 4 was obtained after the subsequent asymmetric autocatalysis. On the 36 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


other hand, [CD(–)310Nujol]-5 afforded (S)-4. These results clearly show that the cytosine crystal acts as the origin of chirality in asymmetric autocatalysis.

Scheme 11. Asymmetric autocatalysis triggered by chiral crystal of achiral cytosine.

Cytosine crystallizes from water as an achiral monohydrate (space group: P21/ c). It was found that the enantioselective formation of a chiral crystal of cytosine takes place through the dehydration of crystallization water by heating or in vacuo from the enantiotopic faces (Figure 3) (45, 46).

Figure 3. Chirality generation by dehydration of crystallization water.

In addition to cytosine, various achiral organic compound which crystallized in chiral structure act as chiral initiator of asymmetric autocatalysis (Figure 4) (47–53). 37 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 4. Enantiomorphs of achiral organic compounds, which can act as chiral initiators of asymmetric autocatalysis. 4.4. Asymmetric Autocatalysis Triggered by Isotope Chirality 4.4.1. Chiral H/D Isotopomers Isotope substitution sometimes make a chirality in achiral compounds (54). Glycine and α-methylalanine are known to be achiral amino acids. However, deuterium substitution of one of the hydrogen atoms of the methylene group of glycine and one methyl group of α-methylalanine makes these compounds chiral: glycine-α-d 17 and α-methyl-d3-alanine 18 (Scheme 12).

Scheme 12. Chiral hydrogen isotopomers of glycine and α-methylalanine trigger asymmetric autocatalysis. 38 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

The chiral glycine-α-d 6 and α-methyl-d3-alanine 7 resulting from isotope substitution were found to act as chiral initiators in asymmetric autocatalysis to afford pyrimidyl alkanol 4 with high ee (Scheme 12) (55). In the presence of (S)-6, (S)-5-pyrimidyl alkanol 4 was formed with high ee. On the other hand, (R)-4 was formed in the presence of (R)-6 instead of (S)-6. It was also found that (R)- or (S)-α-methyl-d3-alanine 7 act as the chiral initiator of asymmetric autocatalysis. Thus, (R)-7 afforded (S)-7, while (S)-7 afforded (R)-4. These results are the first examples of a highly enantioselective reaction induced by chirally deuterated amino acids.


4.4.2. Chiral Carbon Isotope (13C/12C) Chirality Many achiral organic molecules may become chiral by carbon isotope substitution (Figure 5). However, because the chirality originates from the very small difference between the carbon isotopes (13C/12C), it has been experimentally difficult to discriminate carbon isotope chirality. It has been a question whether chiral carbon chirality can induce chirality in some reactions. We found that carbon isotope (13C/12C) chiral compounds trigger asymmetric autocatalysis (56).

Figure 5. Carbon isotope (12C/13C) substitution generates chirality.

The carbon isotopically chiral compound methyl-13C-methylphenyl methanol 8 arising from 13C substitution of the methyl group was used as a chiral trigger. The preparation of alkanol 8 is shown in Scheme 13. In the presence of (R)-alkanol 8, when i-Pr2Zn and pyrimidine-5-carbaldehyde 3 were reacted, (S)-pyrimidyl alkanol 4 was formed with high ee (Scheme 14). On the contrary, (S)-8 afforded (R)-4. Chiral alcohols 9 and 10 resulting from 13C substitution act as chiral triggers of asymmetric autocatalysis to afford pyrimidyl alkanols 4 with high ee, and which have the corresponding absolute configurations of the isotopically substituted carbon chirality of 9 and 10. 39 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Scheme 13. Asymmetric synthesis of 13C-labeled dimethylphenylmethanol 8.

Scheme 14. Asymmetric autocatalysis triggered by chiral carbon isotopomer.

4.4.3. Asymmetric Autocatalysis Triggered by Oxygen (16O/18O) and Nitrogen (14N/15N) Isotope Chirality Similar to the carbon, other atoms in typical organic compounds such as oxygen and nitrogen have stable isotopes. Recognition of chirality only from isotopes becomes much more difficult when the isotope atoms become heavier due to the small relative mass difference. 40 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Asymmetric autocatalysis can recognize these isotope chiralities by oxygen (18O/16O) and nitrogen (15N/14N) (Scheme 15).

Scheme 15. Asymmetric autocatalysis triggered by chiral oxygen and nitrogen isotopomers. (1R,2S)-1,2-Diphenylethandiol-1,2-diol 11 is meso diol. However, introduction of one oxygen isotope 18O results in the isotopically chiral [18O](R)-11 and [18O](S)-11. Similarly, the achiral triol glycerin 12 becomes oxygen isotopically chiral glycerin by substitution of oxygene at 1- or 3- position by oxygen isotope 18O. These oxygen isotopically chiral compounds act as chiral initiators of asymmetric autocatalysis and the [18O](R)-11 afforded (S)-pyrimidyl alkanol 4 and the [18O](S)-11 gave the opposite enantiomer (57, 58). Nitrogen isotope substitution generates an isotope chirality in achiral diamine. Tetraethyl-3,4-butandiamine is meso diamine but the 15N substitution on one nitrogen reduces its symmetry, and isotopically chiral diamine [15N](S)-13 and [15N](R)-13 were obtained (59). These nitrogen isotopically chiral compounds also act as a chiral initiator in asymmetric autocatalysis. As mentioned, these results are the first examples of asymmetric induction by carbon, oxygen and nitrogen isotopically chiral compounds.

5. Absolute Asymmetric Synthesis based on Statistical Fluctuation and Amplification Spontaneous absolute asymmetric synthesis, i.e., the formation of enantioenriched compounds from achiral reagents without the intervention of any chiral factor, has been proposed as the origin of chirality (1, 60). However, in the usual reaction, the enantiomeric excesses of the products are far below the detection level, i.e., racemate. As described, asymmetric autocatalysis can amplify enantiomeric excess from extremely low to very high. We expected that when i-Pr2Zn was treated with pyrimidine-5-carbaldehyde without adding any chiral substance, initial statistically generated slight enantiomeric excess in the formation of (the zinc 41 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


alkoxide of) pyrimidyl alkanol, and that the amplification of enantiomeric excess by subsequent asymmetric autocatalysis would afford the pyrimidyl alkanol with detectable enantiomeric excess (Figure 6).

Figure 6. Absolute asymmetric synthesis of pyrimidyl alkanol without the intervention of chiral factor in conjunction with asymmetric autocatalysis. Indeed, when 2-alkynylpyrimidine-5-carbaldehyde 3 was reacted with i-Pr2Zn, and the subsequent asymmetric autocatalysis with amplification of ee gave (S)- or (R)-pyrimidyl alkanol 4 with enantiomeric excess above the detection level (61, 62). The absolute configurations of the product pyrimidyl alkanol 4 exhibit an approximate stochastic distribution of S- and R-enantiomers. In the presence of achiral silica gel (63) or achiral amine (64), it was also found that (S)and (R)-4 were formed with approximate stochastic distributions in the reaction of aldehyde 3 with i-Pr2Zn in toluene (Figure 7).

Figure 7. Histograms of the absolute configuration and ee of pyrimidyl alkanols. 42 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

These results fulfill one of the conditions necessary for spontaneous absolute asymmetric synthesis. Thus, absolute asymmetric synthesis between pyrimidine5-carbaldehyde and diisopropylzinc was achieved in conjunction with asymmetric autocatalysis.


6. Reaction Models and Crystal Structures of Asymmetric Autocatalysis Several groups have published papers on the reaction model of the present asymmetric autocatalysis (65–84). The enantiomeric amplification in the usual asymmetric catalysis is sometimes explained by the formation of dimer. However, in the mechanism of the amplification of enantiomeric excess from very low ee, an additional mechanism such as more aggregation may be involved. Actually, kinetic analysis of pyrimidyl alkanol suggested that the reaction is mainly of the first order in the zinc alkoxide of pyrimidyl alkanol, i.e., catalyst and second order in aldehyde and show an interesting temperature dependence. NMR studies also support the existance of higher order structures. The X-ray analysis of the single crystals of asymmetric autocatalyst, i.e., alkylzinc alkoxide of pyrimidyl alkanol, gave the following information: isopropylzinc alkoxide of enantiopure pyrimidyl alkanol with excess amount of i-Pr2Zn form tetrameric structure with two 4-membered rings of Zn-O-Zn-O, one 12-membered ring, and with the coordination of 6 molecule of i-Pr2Zn per one tetramer. When the amount of i-Pr2Zn is small, oligomeric crystal is formed. The results indicate that various aggregation status should exist in the reaction system and this accessibility to the higher aggregation state may play an important role in this high enantiomeric excess amplification (85, 86).

7. Conclusions Pyrimidyl alkanol was discovered as asymmetric autocatalyst with amplification of enantiomeric excess in the reaction between pyrimidine-5carbaldehyde and i-Pr2Zn. The process is a catalytic self-replication of a chiral molecule. The reaction exhibits significant amplification of enantiomeric excess, from extremely low ee to >99.5% ee. Thus, we proved that there is a chemical reaction in which the initial low ee can become >99.5% ee by asymmetric autocatalysis with amplification of ee. We apply asymmetric autocatalysis with amplification of ee for the correlation between the origin of chirality and chiral organic compounds with high enantiomeric excesses. Various chiral compounds and chiral factors trigger asymmetric autocatalysis. Irradiation of CPL to the racemate of pyrimidyl alkanol and the subsequent asymmetric autocatalysis affords a highly enantioenriched compound. Chiral inorganic crystals such as d- and l-quartz and cinnabar act as chiral triggers to afford highly enantioenriched compounds. Spontaneous absolute asymmetric synthesis was achieved for the first time in the reaction of pyrimidine-5-carbaldehyde and i-Pr2Zn in conjunction with asymmetric autocatalysis without the intervention of chiral factor. 43 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

Acknowledgments The authors are grateful to the coworkers whose names appear in the papers. Financial support from Japan Society for the Promotion of Science (JSPS) is gratefully acknowledged.

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