Asymmetric autocatalysis and its implications for the origin of homochirality

Abstract

An autocatalytic reaction in which the reaction product serves as a catalyst to produce more of itself and to suppress production of its enantiomer serves as a mechanistic model for the evolution of homochirality. The Soai reaction provided experimental confirmation of this concept, nearly 50 years after it was first proposed. This Perspective offers a rationalization of the Soai autocatalytic reaction; accounting for enantiomeric excess and rate observations, that is both simple as well as gratifying in its implications for the chemical origin of life.

Scheme 1.

The Soai autocatalytic reaction.

Fig. 1.

Simple asymmetric autocatalytic reactions (unlike the Soai reaction) necessarily show an erosion of ee over time. Plotted is the final product ee as a function of turnover number in asymmetric autocatalytic reactions for two hypothetical of autocatalysts at different ee and different product enantioselectivity: a catalyst with an initial ee_cat,0 = 0.99 exhibiting an enantioselectivity of 0.99 (solid blue line), and a catalyst with an initial ee_cat,0 = 0.88 exhibiting an enantioselectivity of 0.88 (shaded magenta line).

Scheme 2.

Models for including mutual antagonism in autocatalytic systems. (A) Specific mutual antagonism: enantiomeric R and S catalysts form a reservoir of inactive heterochiral dimers. If the initial ratio of S:R enantiomers is not 1:1, a greater fraction of the minor enantiomer is extracted into the dimer reservoir, which has total S:total R ratio equal to 1:1. (B) Unspecific mutual antagonism: enantiomeric R and S catalysts form a reservoir of inactive homochiral and heterochiral dimers in statistical proportions.

Fig. 2.

Experimental reaction rate as a function of fraction conversion of the aldehyde 1b in the Soai autocatalytic reaction shown in Scheme 1 employing enantiopure and racemic catalyst. The experimental rate for the racemic catalyst has been multiplied by a factor of ≈2 (1.93).

Fig. 3.

Schematic depicting how the catalyst concentration increases for enantiopure and racemic catalysts as the autocatalytic reaction of Scheme 1 proceeds, as predicted by the experimentally measured reaction rates shown in Fig. 2.

Scheme 3.

ml₂ model for autocatalysis. The monomeric R and S enantiomers form homochiral (RR and SS) and heterochiral (SR) dimers that themselves serve as the active catalysts in the autocatalytic reaction.

Scheme 4.

Proposed models for the Soai reaction based on trimeric (A) and tetrameric (B) transition states, where the order refers to the number of prochiral aldehyde plus ex-aldehyde (alcohol/alkoxide or nascent alkoxide species) that are brought together (although not simultaneously) in the reaction event.