Reversal of the sense of enantioselectivity between 1- and 2-aza[6]helicenes used as chiral inducers of asymmetric autocatalysis

Catalytic asymmetric construction of helicenes via transformation of biaryls

This review showcases a systematic overview of the available tools for the catalytic asymmetric transformation of biaryl substrates toward the construction of challenging enantioenriched helicenes and the conceptual aspects associated with each type of transformation. Depending on the properties of the biaryl and the nature of the process, several methodologies have been developed, including olefin metathesis, hydroarylation of alkynes, C-X (X = C, O, N) coupling, and C-H functionalization. Pioneering studies and an array of representative reactions are discussed to underscore the potential of these synthetic protocols.

London Disperse Interactions Assist Chiral Induction in the Soai Autoamplifying Reaction Provoked by 1- and 2-Aza[6]helicenes

In this paper, DFT computations revealed the mechanisms of the asymmetric catalytic reactions of diisopropylzinc with pyrimidylaldehyde catalyzed by 1- and 2-aza[6]helicenes, which make them effective inductors of the autocatalytic chiral amplification Soai reaction. The generation of chirality takes place through the formation of adducts of aldehyde and helicenes stabilized via non-covalent disperse interactions strictly defining the orientation of the aldehyde molecule in the corresponding transition state.

Synthesis of Aza[n]helicenes (n = 4-7) via Photocyclodehydrochlorination of 1-Chloro-N-aryl-2-naphthamides

A series of aza[n]helicenes (n = 4-7) was synthesized using a photocyclodehydrochlorination of 1-chloro-N-aryl-2-naphthamides as a general synthetic procedure introducing the nitrogen atom to the third ring of the helicene framework. The effect of the nitrogen presence in the helicene skeleton on the physicochemical properties of the prepared aza[n]helicenes was studied and compared to those of the parent carbo-analogues. The insertion of a nitrogen atom into the outer edge of the helicene molecule has a severe impact on certain physicochemical properties such as optical rotation, electrostatic potentials, and intermolecular interactions. On the other hand, some other properties such as UV/vis, fluorescence, and phosphorescence spectra remained almost unaffected when compared to the parent carbohelicenes. A nitrogen atom can be also used for further derivatization, which can lead to further modification of helicene properties, as manifested here in the fluorescence changes induced by protonation.

Absolute Asymmetric Catalysis, from Concept to Experiment: A Narrative

The generally accepted hypothesis to explain the origin of biological homochirality (that is to say, the fact that proteinogenic amino acids are left-handed, and carbohydrates right-handed, in all living beings) is to assume, in the course of prebiotic chemical evolution, the appearance of an initial enantiomeric excess in a set of chiral molecular entities by spontaneous mirror-symmetry breaking (SMSB), together with suitable amplification and replication mechanisms that overcome the thermodynamic drive to racemization. However, the achievement of SMSB in chemical reactions taking place in solution requires highly specific reaction networks showing nonlinear dynamics based on enantioselective autocatalysis, and examples of its experimental realization are very rare. On the other hand, emergence of net supramolecular chirality by SMSB in the self-assembly of achiral molecules has been seen to occur in several instances, and the chirality sign of the resulting supramolecular system can be controlled by the action of macroscopic chiral forces. These considerations led us to propose a new mechanism for the generation of net chirality in molecular systems, in which the SMSB takes place in the formation of chiral supramolecular dissipative structures from achiral monomers, leading to asymmetric imbalances in their composition that are subsequently transferred to a standard enantioselective catalytic reaction, dodging in this way the highly limiting requirement of finding suitable reactions in solution that show enantio­selective autocatalysis. We propose the name ‘absolute asymmetric catalysis’ for this approach, in which an achiral monomer is converted into a nonracemic chiral aggregate that is generated with SMSB and that is catalytically active.

Our aim in this Account is to present a step-by-step narrative of the conceptual and experimental development of this hitherto unregarded, but prebiotically plausible, mechanism for the emergence of net chirality in molecular reactions.

1 Introduction: The Origin of Biological Homochirality and Spontaneous Mirror-Symmetry Breaking

2 Experimental Chemical Models for Spontaneous Mirror-Symmetry Breaking: The Soai Reaction and Beyond

3 Spontaneous Mirror-Symmetry Breaking in Supramolecular Chemistry: Plenty of Room at the Top

4 Absolute Asymmetric Catalysis: An Alternative Mechanism for the Emergence of Net Chirality in Molecular Systems

5 Experimental Realization of Top-Down Chirality Transfer to the

Molecular Level

6 Conclusions and Outlook

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

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.

Simultaneous Control of Central and Helical Chiralities: Expedient Helicoselective Synthesis of Dioxa[6]helicenes

An expedient synthesis of a new family of configurationally stable dioxa[6]helicenes was established using a sequential helicoselective organocatalyzed heteroannulation/eliminative aromatization via enantioenriched fused 2-nitro dihydrofurans featuring both central and helical chiralities. Starting from simple achiral precursors, a broad range of these previously unknown chiral heterocyclic scaffolds were obtained with good efficiency, and their aromatization proceeded with very high enantiopurity retention in most cases.

Synthesis, Structure, and Complexation of an S-Shaped Double Azahelicene with Inner-Edge Nitrogen Atoms

An S-shaped double azahelicene (1) was synthesized in excellent yield by a palladium-catalyzed double dehydrogenative C-H coupling reaction. The stereochemistry of 1 was confirmed to be dl by single-crystal X-ray diffraction analysis. Selective formation of dl-1 was attributed to the isomerization of the kinetically controlled product (meso-1) into the more thermodynamically stable dl-1 under the applied reaction conditions. dl-1 can coordinate to palladium(II) in a bidentate trans-chelating fashion, which was confirmed by X-ray absorption fine structure (XAFS) as well as by X-ray photoelectron spectroscopy (XPS), diffuse reflectance (DR) UV/Vis, and far-infrared (FIR) absorption spectroscopy. Theoretical calculations of palladium complex 16 revealed a weak attractive interaction between palladium and carbon atoms on the central dimethoxynaphthalene core, which could facilitate a disproportionation between a trans-chelating (dl-1)⋅PdCl₂ complex and PdCl₂ to form 16.

Kinetic relationship in parallel autocatalytic amplifications of pyridyl alkanol and chiral trigger pyrimidyl alkanol

Experimental and kinetic analysis of a chemical system combines autocatalytic amplification of 2-alkynyl-5-pyrimidyl alkanol 2 and 6-alkynyl-3-pyridyl akanol 4 in which 2 acts as a chiral trigger and 4 being the subsequent autocatalyst. Starting from a very low initial ee, both alkanols are produced with high enantiopurity in one single cycle. This provides insight into a dual nonlinear amplification of chirality observed with amplifying trigger 2 and accelerated amplification of autocatalyst 4. These kinetic studies reveal a five-fold magnitude superior amplification rates of 4 associated with trigger's enantiopurity at the outset.

Enantioselective Control of Both Helical and Axial Stereogenic Elements though an Organocatalytic Approach

A highly diastereo- and enantio-selective method for the asymmetric synthesis of molecules containing helicenes and stereogenic axes was developed based on organocatalysis. Various compounds bearing both helical and axial stereogenic elements were obtained in excellent enantioselectivities. The mechanism study revealed that the reaction proceeded through two stages: 1) The first cyclization produces a reaction intermediate containing a stereogenic axis. 2) The dynamic kinetic resolution of helix reaction intermediate following with cyclization generates a helix and another stereogenic axis.

Role of Asymmetric Autocatalysis in the Elucidation of Origins of Homochirality of Organic Compounds

Pyrimidyl alkanol and related compounds were found to be asymmetric autocatalysts in the enantioselective addition of diisopropylzinc to pyrimidine-5-carbaldehyde and related aldehydes. In the asymmetric autocatalysis with amplification of enantiomeric excess (ee), the very low ee (ca. 0.00005%) of 2-alkynyl-5-pyrimidyl alkanol was significantly amplified to >99.5% ee with an increase in the amount. By using asymmetric autocatalysis with amplification of ee, several origins of homochirality have been examined. Circularly polarized light, chiral quartz, and chiral crystals formed from achiral organic compounds such as glycine and carbon (13C/12C), nitrogen (15N/14N), oxygen (18O/16O), and hydrogen (D/H) chiral isotopomers were found to act as the origin of chirality in asymmetric autocatalysis. And the spontaneous absolute asymmetric synthesis was also realized without the intervention of any chiral factor.

Gas phase synthesis of [4]-helicene

A synthetic route to racemic helicenes via a vinylacetylene mediated gas phase chemistry involving elementary reactions with aryl radicals is presented. In contrast to traditional synthetic routes involving solution chemistry and ionic reaction intermediates, the gas phase synthesis involves a targeted ring annulation involving free radical intermediates. Exploiting the simplest helicene as a benchmark, we show that the gas phase reaction of the 4-phenanthrenyl radical ([C₁₄H₉]^•) with vinylacetylene (C₄H₄) yields [4]-helicene (C₁₈H₁₂) along with atomic hydrogen via a low-barrier mechanism through a resonance-stabilized free radical intermediate (C₁₈H₁₃). This pathway may represent a versatile mechanism to build up even more complex polycyclic aromatic hydrocarbons such as [5]- and [6]-helicene via stepwise ring annulation through bimolecular gas phase reactions in circumstellar envelopes of carbon-rich stars, whereas secondary reactions involving hydrogen atom assisted isomerization of thermodynamically less stable isomers of [4]-helicene might be important in combustion flames as well.

Helicenes represent key building blocks leading eventually to carbonaceous nanostructures. Here, exploiting [4]-helicene as a benchmark, the authors present a synthetic route to racemic helicenes via a vinylacetylene mediated gas phase chemistry with aryl radicals involving ring annulation.

The influence of nitrogen position on charge carrier mobility in enantiopure aza[6]helicene crystals

A computational study exploring the influence of the nitrogen position on charge carrier mobility in enantiopure aza[6]helicene crystals

Asymmetric autocatalysis. Chiral symmetry breaking and the origins of homochirality of organic molecules

Biological homochirality, such as that of l-amino acids, has been a puzzle with regards to the chemical origin of life. Asymmetric autocatalysis is a reaction in which a chiral product acts as an asymmetric catalyst to produce more of itself in the same absolute configuration. 5-Pyrimidyl alkanol was found to act as an asymmetric autocatalyst in the enantioselective addition of diisopropylzinc to pyrimidine-5-carbaldehyde. Asymmetric autocatalysis of 2-alkynyl-5-pyrimidyl alkanol with an extremely low enantiomeric excess of ca. 0.00005% exhibited significant asymmetric amplification to afford the same pyrimidyl alkanol with >99.5% enantiomeric excess and with an increase in the quantity of the same compound. We have employed asymmetric autocatalysis to examine the origin of homochirality. Asymmetric autocatalysis triggered by circularly polarized light, chiral minerals such as quartz, chiral organic crystals composed of achiral compounds gave highly enantioenriched pyrimidyl alkanol with absolute configurations corresponding with those of the chiral triggers. Absolute asymmetric synthesis without the intervention of any chiral factor was achieved. Chiral isotopomers acted as chiral triggers of asymmetric autocatalysis.

Stereodivergent Catalysis

This review covers diastereo- and enantiodivergent catalyzed reactions in acyclic and cyclic systems using metal complexes or organocatalysts. Among them, nucleophilic addition to carbon-carbon and carbon-nitrogen double bonds, α-functionalization of carbonyl compounds, allylic substitutions, and ring opening of oxiranes and aziridines are considered. The diastereodivergent synthesis of alkenes from alkynes is also included. Finally, stereodivergent intramolecular and intermolecular cycloadditions and other cyclizations are also reported.