A problem with transition state analogue selection approach is that catalytic activity is not a screening criterion. TSAs do not necessarily represent real transition states and so a catalyst obtained from screening could just be the best receptor for a TSA but is not necessarily the best catalyst. To circumvent this problem, catalytic activity needs to be measured directly and also quickly. To develop a high-throughput screen, substrates could be designed to change color or release a fluorescent product upon reaction. For example, Crabtree and coworkers utilized this method in screening for a hydrosylation catalysts for alkene and imine. Unfortunately the prerequisite for such substrates narrow down the range of reactions for study.