Chemistry Reference
In-Depth Information
For a description and mechanistic aspects regarding CDC, see Sections 4.5.1.1 and
4.5.2.7.
_
CDC
1 eq
S
Cl
Cl
R
R
R´
N
N
H
H
N
N
R´
2 eq Et
3
N
CH
2
Cl
2
rt, 4-21 h
+
40-88 %
1699
1740
CDC
General procedure. Carbodiimides 1699 from thioureas [1137]: To a solution of a thi-
ourea 1740 (1 equiv.) and CDC (1 equiv.) in dichloromethane, triethylamine (2
equiv.) was added dropwise at room temperature. The reaction mixture was stirred
at room temperature for 4-21 h, then poured into water, and extracted with di-
chloromethane. The combined organic phases were successively washed with 5%
aq. HCl, saturated aq. NaHCO
3
, and water, dried (MgSO
4
), and concentrated to
dryness. The residue was purified by short column chromatography (SiO
2
) to give
the carbodiimide 1699 (yields 40-88%).
4.5.3.4
Oxidative Addition Reactions of Primary Amines with Isocyanides
Formally, carbodiimides may be generated by the addition of a (nucleophilic) pri-
mary amine to an isocyanide (with an electron deficiency; see constitutional for-
mula 1515b and considerations on isocyanides in the introduction to Section 4.5.2)
under release of a reduction equivalent (two electrons) and two protons. The re-
quired oxidation equivalent can be provided most easily with an oxygen atom.
Dioxygen
Reaction of isocyanides with primary amines in the presence of selenium and
DBU, followed by the introduction of molecular oxygen in refluxing THF, affords
unsymmetrical and symmetrical carbodiimides in isolated yields of 58-87% [1282]
(see also Section 4.5.3.5, Table 4.48). The oxidation reaction is mediated by sele-
nium with the assistance of base DBU.
Typical procedure. N-tert-Butyl-N
0
-n-butylcarbodiimide 1747 [1282]: A mixture of tert-
butyl isocyanide 1745 (2 mmol), n-butylamine 1746 (2 mmol), selenium (2 mmol),
and DBU (8 mmol) was stirred for 1 h in refluxing THF (5 mL). Then, molecular
oxygen was introduced by means of a peristaltic pump at 20 mL min
1
for 5 h
while maintaining reflux. After deposited selenium had been filtered off, the fil-
trate was diluted with Et
2
O (50 mL), washed with brine (3
50 mL), dried over
MgSO
4
, and concentrated in vacuo to give essentially pure 1747 in 82% yield.
1.
Se
, DBU
THF
reflux, 1 h
+
H
2
O
3
N
N
+
NC
H
2
N
2.
O
2
reflux, 5 h
1746
82 %
1747
1745
