Pivotal Role of the Basic Character of Organic and Salt Catalysts in C−N Bond Forming Reactions of Amines with CO₂

Abstract

Organocatalysts promote a range of C−N bond forming reactions of amines with CO2. Herein, we review these reactions and attempt to identify the unifying features of the catalysts that allows them to promote a multitude of seemingly unrelated reactions. Analysis of the literature shows that these reactions predominantly proceed by carbamate salt formation in the form [BaseH][RR′NCOO]. The anion of the carbamate salt acts as a nucleophile in hydrosilane reductions of CO2, internal cyclization reactions or after dehydration as an electrophile in the synthesis of urea derivatives. The reactions are enhanced by polar aprotic solvents and can be either promoted or hindered by H‐bonding interactions. The predominant role of all types of organic and salt catalysts (including ionic liquids, ILs) is the stabilization of the carbamate salt, mostly by acting as a base. Catalytic enhancement depends on the combination of the amine, the base strength, the solvent, steric factors, ion pairing and H‐bonding. A linear relationship between the base strength and the reaction yield has been demonstrated with IL catalysts in the synthesis of formamides and quinazoline‐2,4‐diones. The role of organocatalysts in the reactions indicates that all bases of sufficient strength should be able to catalyze the reactions. However, a physical limit to the extent of a purely base catalyzed reaction mechanism should exist, which needs to be identified, understood and overcome by synergistic or alternative methods.

Interrupted aza-Wittig reactions using iminophosphoranes to synthesize 11C–carbonyls

“…These effects are likely due to the increased availability of soluble CO2 in the form of activated complexes. 22,23 We assessed the compatibility of the iminophosphorane-CO2-fixation method with a diverse scope of nucleophiles (Scheme 2a). Under our developed conditions, carbamates derived from benzyl, isopropyl, and 4-methoxyphenethyl alcohol were isolated in 84-94% yields (4)(5)(6).…”

Recent Advances in the Chemistry of Metal Carbamates

“…Thus, a range of nucleophilic reagents, including neutral N -heterocyclic carbenes [ 25 , 26 ], are known to react with CO 2 even under mild conditions, and some chemistry at transition metal centers is provided by the possibility of CO 2 insertion into the bond between a metal atom and a suitable anionic ligand, e.g., alkyl, allyl, alkoxide and hydride [ 27 , 28 , 29 , 30 , 31 , 32 ]. In this context, amines are key reactants towards carbon dioxide, and indeed carbon dioxide/amine systems have been intensively investigated in the field of capture/storage [ 33 , 34 , 35 ] and exploited for the incorporation of the CO 2 moiety within organic structures [ 36 , 37 , 38 , 39 ]. Furthermore, CO 2 is also prone to insertion reactions into a variety of metal-amide bonds, generating a carbamato ligand; however, metal complexes containing carbamato ligands are easily available through diverse synthetic routes not requiring the use of pressurized CO 2 .…”

Interrupted Aza-Wittig Reactions Using Iminophosphoranes to Synthesize 11C-Carbonyls

“…These effects are likely due to the increased availability of soluble CO2 in the form of activated complexes. 22,23 We assessed the compatibility of the iminophosphorane-CO2-fixation method with a diverse scope of nucleophiles (Scheme 2a). Under our developed conditions, carbamates derived from benzyl, isopropyl, and 4-methoxyphenethyl alcohol were isolated in 84-94% yields (4-6).…”