Chemistry Reference
In-Depth Information
with aluminum chloride gave 2,2
0
-dihydroxy-4,4
0
-dimethoxybenzophenone 1262,
identical to that obtained from 1,3-dimethoxybenzene 1260 and phosgene in the
presence of aluminum chloride.
OMe
OMe
O
OMe
(COCl)
2
No catalyst
MeO
MeO
OMe
1260
1261
OH
O
OH
AlCl
3
MeO
OMe
1262
The AlCl
3
-mediated chlorocarbonylation of arenes with oxalyl chloride has been
used in the preparation of symmetrical and unsymmetrical diaryl ketones 1266
[945, 946].
O
O
R
1
1265
(COCl)
2
Cl
R
1
AlCl
3
R
R
R
1263
1264
1266
Apparently, success in this reaction is strongly curtailed by the distinct tendency of
oxalyl chloride to form [COCl
2
] or [COCl
.
], which subsequently substitutes hydro-
gen atoms (aliphatic or aromatic) by chlorocarbonylation [947]. Chlorocarbony-
lation is especially favored under free radical conditions and in chlorinated sol-
vents, such as trichloroethene or dichloromethane. Neubart and Fishel reported
the reaction of alkylbenzenes with oxalyl chloride and AlCl
3
under these condi-
tions, resulting in 4-alkylbenzoyl chlorides in yields of up to 80%, depending on
the length of the alkyl substituent [948].
A recent application of enzyme catalysis (oxalyl chloride, 50 U aminoacylase E.C.
3.5.1.14, hexane, 4 days, 20
C) for the synthesis of unsymmetrical diaryl ketones
has been reported [949].
The best results were obtained when the less activated substrate was used in the
initial acylation and the more activated arene was used as the subsequent acceptor.
With less activated substrates, such as chloro- and bromobenzenes, the chlorocar-
bonylation required a longer time and higher temperatures. Yields of 54-77% were
obtained (Table 4.43) using 1.0 equiv. of each arene, rather than the several fold
excess of one or the other often previously employed [950]. Moreover, the con-
ditions of the second Friedel-Crafts reaction are su
ciently mild that even an o-
