Enantiomer surface chemistry: conglomerateversusracemate formation on surfaces

Abstract

Research on surface chirality is motivated by the need to develop functional chiral surfaces for enantiospecific applications. While molecular chirality in 3D has been the subject of study for almost two centuries, many aspects of 2D chiral surface chemistry have yet to be addressed. In 3D, racemic mixtures of chiral molecules tend to aggregate into racemate (molecularly heterochiral) crystals much more frequently than conglomerate (molecularly homochiral) crystals. Whether chiral adsorbates on surfaces preferentially aggregate into heterochiral rather than homochiral domains (2D crystals or clusters) is not known. In this review, we have made the first attempt to answer the following question based on available data: in 2D racemic mixtures adsorbed on surfaces, is there a clear preference for homochiral or heterochiral aggregation? The current hypothesis is that homochiral packing is preferred on surfaces; in contrast to 3D where heterochiral packing is more common. In this review, we present a simple hierarchical scheme to categorize the chirality of adsorbate-surface systems. We then review the body of work using scanning tunneling microscopy predominantly to study aggregation of racemic adsorbates. Our analysis of the existing literature suggests that there is no clear evidence of any preference for either homochiral or heterochiral aggregation at the molecular level by chiral and prochiral adsorbates on surfaces.

Chirality in amino acids beyond the Cα configuration

“…They analyzed 1308 arbitrarily selected chiral organic compounds and found that ~90% of racemic mixtures crystallize as racemates and 5% to 10% of neutral, chiral organic molecules crystallize as conglomerates, whereas the probability of conglomerate crystallization of ionic and zwitterionic organic molecules is 2 to 3 times higher. These results found their way into reviews and textbooks of stereochemistry . A recent detailed review repeated the result of Jacques et al, mentioning that the formation of conglomerates increased at low crystallization temperatures …”

“…These results found their way into reviews and textbooks of stereochemistry. [10][11][12][13] A recent detailed review repeated the result of Jacques et al, 14 mentioning that the formation of conglomerates increased at low crystallization temperatures. 15,16 In our analysis of 1179 amino acids, we checked whether they crystallized as conglomerates or racemates (column 10 en/rac in Tables S1-S19, Supporting Information).…”

“…Thus, dramatically less structures with +ψ conformation are available for analysis. This shows up in the ratio of numbers for dr(θ) +ψ (total) and especially for dr(θ) <av and dr(θ) <av (columns [10][11][12]. Ratios of only few structures are not significant.…”

Heterochiral or Homochiral Self‐Assembly: Mercury(II), Cadmium(II), and Spontaneous Resolution of Silver(I) Complexes Derived from a Racemic Bis(pyridyl) Ligand

“…Chirality is an important property and commonly used in biological functions and material sciences. Chiral ligands have attracted an extensively attention in chiral separation, asymmetric synthesis and catalysis, as well as supramolecular architectures due to their ability to generate an asymmetric environment . In principle, chiral ligands are employed to achieve enantio‐ or diastereoselective assembly, but a more complex and less explored situation arises when racemic ligands are used .…”

“…Chiral ligands have attracted an extensively attention in chiral separation, asymmetric synthesis and catalysis, as well as supramolecular architectures due to their ability to generate an asymmetric environment. [1][2][3][4][5][6][7][8] In principle, chiral ligands are employed to achieve enantioor diastereoselective assembly, but a more complex and less explored situation arises when racemic ligands are used. [9][10][11][12][13] In such systems, reactions with racemic ligands either result in homochiral complexes through a ligand self-recognition (also known as narcissistic self-sorting) process [14][15][16] or heterochiral complexes through a ligand self-discrimination (also called social self-sorting) process.…”