The elements of a green catalyst

Chemists agree that a sustainable future requires reducing reliance on precious metals such as palladium and rhodium. Many of them hold up earth-abundant first-row transition metals as green alternatives to the precious metals used to catalyze cross couplings and other important organic reactions needed to synthesize drug molecules. First-row metals are less costly and less carbon intensive to refine, and they can do a lot of useful chemistry. But making catalytic reactions more sustainable requires thinking about more than just swapping out precious-metal catalysts for earth-abundant ones. On a summer day about 20 years ago, Paul Chirik returned to his laboratory after a conference with a question on his mind: Why do chemists use expensive rhodium to do hydrogenation reactions? Why don't they use something more available, such as iron? Chemists are rightfully proud of the transition-metal catalysts they developed to enable certain organic transformations. For example, rhodium catalyzes asymmetric