Chemistry Reference
In-Depth Information
times, high temperatures, or inconvenient reaction conditions for their execution,
and are often accompanied by painstaking chromatographic separations to remove
spent reagents from the desired products.
R
1
R
1
1.
(COCl)
2
2. Hunig's base
Cl
N
Cl
N
Ar
Ar
Br
2
R
R
Cl
Br
R
CHO
N
R
1
Cl
or TFA
N
R
1
N
R
1
1780
1781
1782
R
1
O
R
= H (62%), 4-F (62%), 4-Br (71%),
4-Cl (65%), 2-Cl(20-40%),
3-Cl (11%), 4-Me (58%),
4-OMe (66%), 4-NO
2
(34%)
N
H
2
O
Ar
Cl
Br
R
O
R
N
N
R
1
R
1
1784
1783
An interesting protocol for converting amides and lactams to their correspond-
ing thio derivatives 1786, through in situ thionation of intermediate chloro imi-
nium ions 1770 with hexamethyldisilathiane TMS
2
S, has been reported [1312].
Cl
TMS
TMS
S
S
TMS
2
S
R
2
Cl
R
2
R
1
R
2
R
1
N
Cl
N
- 2 TMSCl
R
1
N
R
3
R
3
Cl
R
3
1770
1785
1786
The procedure seems to be sensitive to the degree of substitution at nitrogen and
relatively insensitive to the size of the alkyl groups on the nitrogen or a to the car-
bonyl. Oxalyl chloride and phosphorus oxychloride were both found to be quite ef-
fective in mediating iminium ion 1770 formation (at
78
C); however, it should
be noted that oxalyl chloride was only effective in instances where no protons were
a to the carbonyl. Triphosgene also provided a convenient method for monitoring
the formation of these Vilsmeier intermediates as the evolution of CO
2
occurred
with concomitant formation of the iminium ion 1770 (at 0
C). The methodology
was found to give results comparable to those obtained with Lawesson's reagent.
Lawesson's reagent
S
S
MeO
P
P
OMe
S
S
General procedure. Thioamides 1786 or thiolactams with oxalyl chloride [1312]:
A so-
lution of the amide or lactam (ca. 1 mmol) in dry dichloromethane (2 mL) was
