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N-Heterocyclic carbene-carbodiimide (NHC-CDI) adducts are versatile compounds that can be used as ligands and (pre)catalysts, but their systematic structure–property relationships are underexplored. Herein, we investigated how structural electronic variations on both the NHC and CDI affect the inherent kinetic and thermodynamic properties of the adducts. Using in situ carbene trapping and variable-temperature NMR spectroscopy, we measured the rates of dissociation and the equilibrium constants and then used Eyring and van’t Hoff analyses to calculate ΔG‡ and ΔG, respectively. Linear free-energy relationships indicate that changing the para position of the CDI substituents yields a similar effect to changing the NHC core. These CDI structural modifications affected the adducts’ thermodynamics (ΔG) more than the kinetics (ΔG‡) and were found to be influenced more by inductive, rather than resonance, factors. Preliminary results suggest a steric threshold beyond which steric effects dominate electronic effects in governing the strength of the adduct bond. This systematic investigation provides valuable insight into the design of NHC-CDIs for current and future applications.
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Switchable Organocatalysis from N ‐heterocyclic Carbene‐Carbodiimide Adducts with Tunable Release Temperature**
Abstract 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., differentpara‐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.
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- Award ID(s):
- 2154633
- PAR ID:
- 10470727
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Angewandte Chemie International Edition
- Volume:
- 62
- Issue:
- 48
- ISSN:
- 1433-7851
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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