Mechanistic insights into carbamate formation from CO and amines: the role of guanidine-CO adducts
Capture of CO 2 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-CO 2 adducts are able to actively transfer the carboxylate g...
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Veröffentlicht in: | Catalysis science & technology 2021-10, Vol.11 (2), p.6877-6886 |
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Zusammenfassung: | Capture of CO
2
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-CO
2
adducts are able to actively transfer the carboxylate group to various substrates. Here we report a detailed investigation of zwitterionic TMG-CO
2
, including isolation, NMR behavior, reactivity, and mechanistic consequences in carboxylation of aniline-derivatives. Our computational and experimental mechanistic analysis shows that the reversible TMG-CO
2
zwitterion is not a direct carboxylation agent. Instead, CO
2
dissociates from TMG-CO
2
before a concerted carboxylation occurs, where the role of the TMG is to deprotonate the amine as it is attacking a free CO
2
. This insight is significant, as it opens a rational way to design new synthesis strategies. As shown here, nucleophiles otherwise inert towards CO
2
can be carboxylated, even without a CO
2
atmosphere, using TMG-CO
2
as a stoichiometric source of CO
2
. We also show that natural abundance
15
N NMR is sensitive for zwitterion formation, complementing variable-temperature NMR studies.
This work explores the reactivity of a reversible superbase-CO
2
zwitterion, which can be used as a stoichiometric source of CO
2
. |
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ISSN: | 2044-4753 2044-4761 |
DOI: | 10.1039/d1cy01433a |