The carbamation of primary amines with DMC is a known process, though only catalyzed reactions proceed with good yields and selectivity (Scheme 4.20).39
Our group has very recently reported that, when temperatures are 130-140°C, the reaction of Scheme 4.21 can proceed efficiently (yields up to 83% in methyl carbamates), without any catalyst in the presence of scCO2 at 90 bar.40 ScCO2 plays a double role in this reaction: (1) it acts as a catalyst in the formation of the active nucleophiles (Scheme 4.21), and (2) at a pressure over 90 bar, it inhibits the formation of N-methylated carbamates [RN(Me)CO2Me], which are possible by-products.
Scheme 4.20 Carbamation of amines with DMC. Catalyst: strong bases, Pb(II), alumina.
The mechanism of the reaction between amines with DMC has been investigated further.41 While in the absence of bases they give methylation and carboxy-methylation reactions without selectivity (BAl2 and BAc2 mechanisms, respectively) (Table 4.6), in the presence of bases the BAc2 mechanism prevails. (Table 4.7) The carbamate already formed reacts further with DMC via BAl2 mechanism to give the corresponding N-methyl derivative. These strong bases (potassium tert-butylate or sodium methoxide) catalyze the reaction of aliphatic
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