RNH C w Rnhcrnh Rnhcme

Scheme 4.21 Carbamation of amines with DMC in the presence of CO2.

and aromatic amines to give the corresponding carbamates quantitatively at 90°C. In these conditions high yields of carbamates are obtained in a few minutes (Table 4.7). Since aliphatic amines are harder nucleophiles than aromatic ones, they react faster with the carbonyl of DMC (entries 3-7). No N,N-dimethyl derivates were observed.

This pronounced double selectivity has been explained in terms of Pearson's HSAB theory.28 According to this procedure, N-methylcarbamates have been prepared from primary aliphatic and aromatic amines, either at reflux temperature of

TABLE 4.6 Reaction of Amines and DMC in the Absence of a Base

Entry

Amine T (oC)

Time (h) MNM (%)

DNM (%)

Carbamate (%)

1

Benzylamine 90

6

12

6

4

2

Aniline 200

24

15

37

9

Conditions: Amine/DMC molar ratio: 1/40. MNM = mono-

-N-methylation; DNM =

N,N-dimethylation.

TABLE 4.7 Reaction of Amines and DMC in the Presence of a Base

Entry

Amine

Time (min)

Base

Carbamate (%)

1

p-Anisidinea

25

C4H9OK

77

2

p-Chloroanilineb

20

C4H9OK

86.5

3

Benzylaminec

1

C4H9OK

100

4

n-Decylamined

3

CH3ONa

100

5

Hexane-1,6-diamine

10

CH3ONa

100

6

n-Octylamine

5

CH3ONa

100

7

Phenethylamine

5

CH3ONa

100

Conditions: Reflux temperature. 90°C; Amine/Base/DMC molar ratio: 1/1.2/40. aAfter18h, 73% of N-methyl carbamate. b After 18h, 87% of N-methyl carbamate. c After 0.5 h, 70% of N-methyl carbamate. dAfter 6 h, 78% of N-methyl carbamate.

Conditions: Reflux temperature. 90°C; Amine/Base/DMC molar ratio: 1/1.2/40. aAfter18h, 73% of N-methyl carbamate. b After 18h, 87% of N-methyl carbamate. c After 0.5 h, 70% of N-methyl carbamate. dAfter 6 h, 78% of N-methyl carbamate.

DMC (90°C) or at 230°C in autoclave. The reaction can be carried out in one step or through the isolation of the carbamate and the subsequent methylation reaction with DMC.

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