N-Heterocyclic carbenes (NHCs) are powerful organocatalysts, but practical applications often require in situ generation from stable precursors that “mask” the NHC reactivity via reversible binding. Previously established “masks” are often simple small molecules, such that the NHC structure is used to control both catalytic activity and activation temperature, leading to undesirable tradeoffs. Herein, we show that NHC-carbodiimide (CDI) adducts can be masked precursors for switchable organocatalysis and that the CDI substituents can control the reaction profile without changing the NHC structure. Large electronic variations on the CDI (e.g., alkyl versus aryl) drastically change the catalytically active temperature, whereas smaller perturbations (e.g., different para-substituted phenyls) tune the catalyst release within a narrower window. This control was demonstrated for three classic NHC-catalyzed reactions, each influencing the NHC-CDI equilibrium in different ways. Our results introduce a new paradigm for controlling NHC organocatalysis as well as present practical considerations for designing appropriate masks for various reactions.
Supporting Information for Switchable organocatalysis from N-heterocyclic carbene-carbodiimide adducts with tunable release temperature
Additional and tabulated data. Explanation of calculated values. Experimental details, including instrumentation, methods, reagent sources, synthetic procedures, characterization of all new compounds.
Crystallographic data for 1,3-bis(2,4,6-trimethylphenyl)-1,3-dihydro-2H-imidazolium-2-[(N-phenyl-N’-cyclohexyl)carbodiimide] (1a)
The .cif file for the single crystal X-ray crystallography data of 1,3-bis(2,4,6-trimethylphenyl)-1,3-dihydro-2H-imidazolium-2-[(N-phenyl-N’-cyclohexyl)carbodiimide] (1a). These data have been deposited at CCDC number 2202030.