Molecules aren’t Tinkertoys. Chemists can’t just pluck atoms out of a box and connect them however they want. Rather, they rely on an inventory of reactions, accumulated over generations of research, for manipulating molecular structures. Building a new molecule means solving an intricate puzzle of which reactions to perform in which order.

Those reactions can be temperamental. They can depend sensitively on solvents, temperatures, and other parameters. The reactants don’t always find each other, and they don’t always react as planned. In a complex environment, they often react with the wrong thing entirely to form unwanted by-products. In a multistep synthesis of a complicated molecule, the inefficiencies quickly compound, and chemists often need an enormous amount of starting material to make even a tiny amount of product.

A few special reactions buck the trend. Their reactants seek out and react only with each other, with nearly 100% efficiency, regardless of...

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