Phosgenation Reactions - Phosgenations - page 215

ther drawback of this methodology is that alkylation of the amine affords un-
wanted by-products. This side reaction can be avoided by performing the alkylation
on alkylammonium N-alkylcarbamates; these are easily obtained from primary
amines and CO 2 in the presence of 18-crown-6, which can be quantitatively recov-
ered at the end of the reaction [598].
An advantage that increases the industrial interest in this methodology is the
possibility of reducing the production of chloride wastes. In fact, alkylating agents
other than alkyl chlorides can also be employed. The entire process occurs with
yields close to 100% [599].
CO 2 has been used as a cyclocarbamating agent in a limited number of cases,
described only in patents. The reaction of 2-(methylamino)ethanol with CO 2 gave
solely 1,4-dimethylpiperazine in aqueous solution without any catalyst [600], while
1-methylamino-2-propanol reacted with CO 2 to give 3,5-dimethyl-2-oxazolidinone
in 58% yield under similar conditions [601].
It has been reported, on the other hand, that by using triphenylstibine oxide as
catalyst, even 2-(methylamino)ethanol reacted with CO 2 to give 3-methyl-2-oxazo-
lidinone in 48% yield [602]. Unfortunately, this catalyst failed to promote the reac-
tion of 2-aminoethanols lacking an N-substituent, giving only small amounts of 2-
oxazolidinones. The addition of some dehydrating reagents, such as phosphorus
compounds [603] or carbodiimides [604], was found to promote the reaction, al-
though this strategy inevitably led to increased costs and by-product formation.
This was also the case in relation to the utilization of aziridines, the dehydrated
form of 1,2-amino alcohols, in their cycloaddition reactions with CO 2 to form 2-
oxazolidinones [605].
It has been found that NMP is an excellent solvent and that n-Bu 2 SnO can be
used as a catalyst for the dehydrative condensation of 1,2-aminoethanols 813 with
CO 2 to give 2-oxazolidinones 814 [606].

R 3

R 2 R 3

n- Bu 2 SnO, CO 2 O

O

solvent

R 1 HN OH N

R 2

R 1

813 814

R 1 = Me, R 2 = R 3 = H, NMP, 94%
R 1 = Et, R 2 = R 3 = H, NMP, 76%
R 1 = R 2 = R 3 = H, NMP, 53%
R 1 = R 2 = H, R 3 = Me, NMP, 73%
R 1 = H, R 2 = R 3 = Me, NMP, 85%

It may be noteworthy that this catalyst is commercially available in the form of a
powder, and is easy to handle because of its stability in air.
A proposed reaction mechanism involves CO 2 insertion to form a cyclic tin car-
bamate 817, and subsequent intramolecular nucleophilic attack of an alkoxy group
on a carbonyl carbon atom leading to elimination of 2-oxazolidinone 818 and re-
generation of the starting tin oxide 819.