Synthesis of Carbamates from Amines and Dialkyl Carbonates: Influence of Leaving and Entering Groups

Abstract

A number of carbamates were synthesised through a halogen-free process by reacting amines with symmetrical and unsymmetrical carbonates. The results obtained showed a specific trend of preferred leaving groups (in the dialkyl carbonates) depending on whether a catalyst or a base was used. On the other hand, investigations conducted on the preferred entering groups (amines) for the synthesis of carbamates showed the same trend regardless of whether a catalyst or a base was used. Finally, in accordance with the results obtained, it was possible to synthesise sterically hindered carbamates in high yield by transesterification of methyl carbamate with a sterically hindered alcohol.

Key words

green chemistry - amine - dialkyl carbonate - carbamate - protecting groups

References and Notes

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10

General procedure for the reaction of unsymmetrical carbonates with (2-phenylethyl)amine (Table  [¹] and Table  [²] ): In a 25 mL round-bottom flask, the amine (9.30 mmol) and the carbonate (18.50 mmol) were added, followed by either zinc acetate (0.46 mmol) or potassium tert-butoxide (2.30 mmol). The reaction mixture was heated to 60 ˚C with continuous agitation. Samples were taken at regular time intervals and analysed by ^¹H NMR spectroscopy (see Supporting Information).

11

General procedure for the reaction of symmetrical carbonates with (2-phenylethyl)amine (Table  [³] and Table  [4] ). In a 25 mL round-bottom flask, the amine (9.30 mmol), DMC (18.50 mmol) and the carbonate (18.50 mmol) were added, followed by either zinc acetate (0.46 mmol) or potassium tert-butoxide (2.30 mmol). The reaction mixture was heated to 60 ˚C with continuous agitation. Samples were taken at regular time intervals and analysed by ^¹H NMR spectroscopy (see Supporting Information).

12

General procedure for the reaction of amines with dimethyl carbonate (Table  [5] and Table  [6] ). In a 25 mL round-bottom flask, phenyl ethyl amine (4.65 mmol) the selected amine (4.65 mmol) and DMC (18.50 mmol) were added, followed by either zinc acetate (0.460 mmol) or potassium tert-butoxide (1.60 mmol). The solution was heated to 60 ˚C with continuous agitation. Samples were taken at regular time intervals and analysed by ^¹H NMR spectroscopy (see Supporting Information).

14

General procedure for the transesterification of a urethane with an alcohol Table  [7] : In a 25 mL round-bottom flask, the urethane (9.30 mmol) and the alcohol (93.00 mmol) were added, followed by potassium tert-butoxide (11.60 mmol). The contents were heated to 60 ˚C with continuous agitation. Samples were taken at regular time intervals and analysed by ^¹H NMR spectroscopy.

15

General procedure for the transesterification of methyl carbamates with alcohols (Table  [7] ). In a 25 mL round-bottom flask, the selected methyl carbamate (9.30 mmol) and the selected alcohols (93.00 mmol) were added, followed by potassium tert-butoxide (11.60 mmol). The solution was heated to the relevant temperature with continuous agitation. Samples were taken at regular time intervals and analysed by ^¹H NMR spectroscopy (see Supporting Information).