As chemists have come to better understand the correlation between catalyst design and selectivity, a theme has emerged in which researchers seek to make different products from the same reactants simply by altering the catalyst. In a new example, Frank Glorius and coworkers at the University of Münster, in Germany, switched the reactivity of enals with hydrazones in cycloaddition reactions to make 1,2-diazepines or pyrazoles, depending on the choice of N-heterocyclic carbene organocatalyst (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja510737n). The team discovered that a bulkier mesityl-substituted catalyst leads to [4+3] annulation, whereas a less bulkier methoxyphenyl-substituted catalyst leads to [4+1] annulation (shown). “The electronic and steric properties of the N-heterocyclic carbene organocatalyst play a vital role in controlling the reaction pathway, allowing selective access to diverse 1,2-diazepine and pyrazole derivatives from identical substrates,” the researchers note. These compounds are important heterocycles present in a range of natural products and bioactive synthetic compounds.