All of the examples discussed above describe reactions that exhibit sigmoidal kinetic profiles, but none meets the criteria set by the Frank model for an asymmetric autocatalytic reaction leading to the emergence of homochirality. In further pursuit of Frank's challenge, a number of groups turned to reactions involving alkyl metal reagents. These molecules can form complex higher-order chiral structures that may introduce nonlinear effects into the reaction network. Such nonlinear effects may constitute an inhibition mechanism resulting in asymmetric amplification that is the second key feature of the emergence of homochirality in Frank autocatalysis.