Journal
ADVANCED SYNTHESIS & CATALYSIS
Volume 358, Issue 24, Pages 3983-3991Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/adsc.201600629
Keywords
amination; C-H activation; radical stability; remote functionalization
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Funding
- Deutsche Forschungsgemeinschaft [SFB 749, C6]
- COST action [CM1201]
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The stability of N-centered radicals and radical cations of potential relevance in C-H amidation reactions has been quantified using highly accurate theoretical methods. Combination with available C-H bond energies for substrate fragments allows for the prediction of reaction enthalpies in 1,5-hydrogen atom transfer (HAT) steps frequently encountered in reactions such as the Hofmann-Loffler-Freytag (HLF) reaction. Protonation of N-radicals is found to be essential in classical HLF reactions for thermochemically feasible HAT steps. The stability of neutral N-radicals depends strongly on the type of N-substituent. Among the electron-withdrawing substituents, the trifluoroacetyl (TFA) group is the least and the toluenesulfonyl (tosyl) group the most stabilizing. This implies that TFA-aminyl radicals have the broadest and tosyl-aminyl radicals the smallest window of synthetic applicability. In how far the intramolecular C-H amidation reactions compete with hydrogen abstraction from common organic solvents can be judged based on a comparison of reaction thermodynamics.
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