Chemistry Reference
In-Depth Information
forgotten until 1969 [1298]. Mannich reagents 1765, such as methyleneammonium
salts, react with nucleophiles by aminomethylation.
H
H
Cl
O
O
O
H
Cl
Cl
H
H
Cl
N
R
N
R
N
R
H
Cl
Cl
H
R
H
R
Cl
R
Cl
Cl
N
Cl
N
N
H
R
Cl
R
Cl
R
1765
1766
1767
Dichloromethyleneammonium salts 1767 having a tertiary nitrogen (chlorinated
Vilsmeier salts) are relatively easy to obtain, some even being commercially avail-
able, while those with secondary and primary nitrogen can be prepared at least in
situ from dichloromethyleneamine and cyanogen chloride [1298]. These salts are
generally strongly electrophilic reagents, and may be compared with methylam-
monium and chloromethyleneammonium salts (Mannich reagents and Vilsmeier-
Haack or Arnold reagents). Recent applications of dimethylphosgeneiminium
chloride (''Viehe's salt'') have been as a dehydrating agent for the preparation of
carbodiimides from ureas (see Section 5.4.3.1) [1254], and its use in the direct
conversion of tetrahydro-2-pyranyl (THP) protected alcohols into the correspond-
ing halides in the presence of a tetraalkylammonium halide (see also Section 4.4.1
on the chlorination of alcohols) [997].
As
NR
2
B
PO
2
Cl
2
C
] and Arnold [1766] reagents give the corresponding imonium com-
pounds and particularly, by their hydrolysis, aldehydes. They have found extensive
use as formylating, halogenating, and dehydroxylating reagents [1299].
The following section highlights some selected recent applications of the use of
phosgene equivalents in the preparation of Vilsmeier-type chlorinated derivatives
of amides and ureas. Thionyl chloride, carbonyl bromides, phosphorus oxy-
chloride, phosgene, triphosgene, oxalyl chloride, and p-toluenesulfonyl chloride are
all e
cient oxophilic promoters capable of generating Vilsmeier-type chloro imi-
nium ion intermediate 1770 by reaction with formamides, particularly dimethyl-
formamide, and ureas.
chloromethyleneammonium salts,
the Vilsmeier-Haack
[ClCH
y
COCl
Cl
O
O
(CCl
3
O)
2
CO
R
2
R
2
Cl
R
2
Cl
R
1
R
1
R
1
N
N
N
CH
2
Cl
2
, 0°C
- CO
2
R
3
R
3
R
3
1768
1770
1769
