Mechanistic insights into carbamate formation from CO2 and amines: the role of guanidine–CO2 adducts

Capture of CO2 by amines is an attractive synthetic strategy for the formation of carbamates. Such reactions can be mediated by superbases, such as 1,1,3,3-tetramethylguanidine (TMG), with previous implications that zwitterionic superbase–CO2 adducts are able to actively transfer the carboxylate gro...

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Veröffentlicht in:Catalysis science & technology 2021-01, Vol.11 (20), p.6877-6886
Hauptverfasser: Mannisto, Jere K, Pavlovic, Ljiljana, Tiainen, Tony, Nieger, Martin, Sahari, Aleksi, Hopmann, Kathrin H, Repo, Timo
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Sprache:eng
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Zusammenfassung:Capture of CO2 by amines is an attractive synthetic strategy for the formation of carbamates. Such reactions can be mediated by superbases, such as 1,1,3,3-tetramethylguanidine (TMG), with previous implications that zwitterionic superbase–CO2 adducts are able to actively transfer the carboxylate group to various substrates. Here we report a detailed investigation of zwitterionic TMG–CO2, including isolation, NMR behavior, reactivity, and mechanistic consequences in carboxylation of aniline-derivatives. Our computational and experimental mechanistic analysis shows that the reversible TMG–CO2 zwitterion is not a direct carboxylation agent. Instead, CO2 dissociates from TMG–CO2 before a concerted carboxylation occurs, where the role of the TMG is to deprotonate the amine as it is attacking a free CO2. This insight is significant, as it opens a rational way to design new synthesis strategies. As shown here, nucleophiles otherwise inert towards CO2 can be carboxylated, even without a CO2 atmosphere, using TMG–CO2 as a stoichiometric source of CO2. We also show that natural abundance 15N NMR is sensitive for zwitterion formation, complementing variable-temperature NMR studies.
ISSN:2044-4753
2044-4761
2044-4761
DOI:10.1039/d1cy01433a