COBALT(II)CHLORIDE CATALYSED ACYLATION OF ANISOLE

Article abstract of DOI:10.1016/S0040-4039(00)93460-9

Cobalt(II)chloride catalysed acylation of anisole with variety of acid chlorides proceeds via an acyl radical to give p-acylated anisoles in good yields.

Full text of DOI:10.1016/S0040-4039(00)93460-9

Tetrahe.xkon Letters, Vol.32, No.38, pp 5179-5182, 1991
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C O B A L T ( I I ) C H L O R I D E C A T A L Y S E D A C Y L A T I O N OF A N I S O L E
Javed Iqbal
, M.A. Khan and Naresh K. Nayyar
Dept. of Chemistry, Indian Institute of Technology, Kanpur-208016, India
Abstract
:
Cobalt(ll)chloride
variety of acid
to give p-acylated anisoles in good yields.
catalysed
acylation
of
anisole
with
chlorides proceeds via an acyl radical
The acylation of aromatic ring with acid halides in presence
of aluminium trichloride (The Friedel-Crafts reaction) is an extremely
useful organic transformation in the domain of electrophilic aromatic
substitution reactions. These acylations can be promoted by other Lewis
acids, however, the scope of this reaction is severely limited owing
to the requirement of the Lewis acid in stoichiometric amounts.
acylation of activated arenes like anisole may be promoted 2 by Bronsted
acid in moderate yields. In contrast, copper(I) promoted acylation 3
The
of anisole with selenol esters may be achieved in good yields. However,
the use of selenium compounds in these acylations severely limits the
scope of these reactions.
In view of the above limitations,
catalyst during these acylations seemed an attractive alternative
therefore, we now show that cobalt(II)chloride is an efficient
the use
of
a
and
catalyst for the acylation of anisole with various acyl chlorides.
Typically,
acid
chloride
(16 mmol)
and
aromatic
compound
(8 mmol) is added to
a
solution of anhydrous cobalt(II)chloride (5
mol %) in dry acetonitrile (30 ml) and the resulting mixture is heated
at 80°C for 10-15 hours. The acetonitrile is removed under vacuum and
the residue taken into ether (80 ml) and washed successively with satura-
ted solution of sodium bicarbonate and water. Drying (MgSO4) and evapo-
ration of the organic layer yielded
chromatography gave the acylated products
(Schemel).
pounds
a
liquid which on flash column
2
and in good yields
3
These reactions work well with electron rich aromatic com-
The acylation of anisole gave mainly the para
like anisole.
acylated compounds (2a-d) which are accompanied by
( ~ 5 % ) of the corresponding ortho isomers. Benzene, toluene and thio-
anisole do not give good yield of the acylated products under these
conditions. These acylations are always accompanied by 5-10% of the
derived from the corresponding acid chloride. Small amounts
and the coupled product were obtained in all the reactions.
a
very small amount
a
biacyl
of ester
3
4
5179

5180
OMe
OCOR
O.Me i , ; ~ 'I
I01 I01
OMe
+
R"ll'cl
~
+
+
~
O M ~
6
(a)
R = M e
6 8 " / ,
1 0 %
10%
5 - 1 0 %
(b}
(c}
R = C 6 H 5
R = C ~ H 7
72%
10%
12%
15"/,
-
-
7 8 " / ,
-
(d)
R - - C H 3 - C H = C H - -
71 "/,
( S c h e m e
1 )
The
upon
equivalents of acetyl chloride always results in the formation of the
coupled product 4a in high yields. However, the use of equivalents
of acetyl chloride leads to the formation of 2a in good yields. The
acylation of p-methyl anisole with acetyl chloride m a i n l y g i v e s rise
reaction of anisole with acetyl chloride is critically dependent
the quantity of the latter during the acylations. The use of
2
5
to the coupled product and the corresponding e s t e r as the m a j o r product
(Scheme 2). Any attempt to encourage the acylation of p-methylanisole
met with very little success.
Mechanistically,
these reactions
seem
to
be
very
interesting
as we observe the formation of biacyl, esters
3
and the coupled products
4. The formation of these products can be explained only if we assume
the formation of the radical species during these reactions. The forma-
tion
of the radical species in presence of
a
paramagnetic cobalt(II)
complex is not surprising as we have already s h o w n earlier that the
biacyls are obtained 4 as byproducts during cobalt(II)chloride catalysed
acylative cleavage of ethers with acid chlorides. Thus, the mechanism
of these reactions can be understood if we invoke an initial transfer
of electron from cobalt(If) complex (A) to an acyl halide to give an
acyl radical (B) and cobalt(III) species (C) (Scheme 3). The cobalt(IZI)
complex (C) w~ll oxidize the anisole to give the corresponding radical
c a t i o n (D) and cobalt(II) complex (A). The acyl radical (B) will react
with complex (A) to y i e l d an acyl cobalt complex (E). The radical cation
(D)
may react with the acyl cobalt complex (E)
in an SH2 m a n n e r to

5181
OMe
OM
OCOCH3
o
ColIcl2
H3C
52"/.
5
45%
6
(Scheme 2]
y i e l d the para acylated products
2
and the complex (A) will be regene-
rated to complete the cycle. The oxidation of ethers by cobalt(III)
complex have already been proposed in our earlier studies 4. The oxida-
tion of anisole by cobalt(III) complex will be very facile as it leads
to
the formation of
a
radical cation w h i c h can be stabilised by the
aromatic ring.
The high para selectivity in these reactions indicates
that these acylations are taking place via an acyl cobalt complex (E) 5
w h i c h may not react with the ortho position due to its bulkiness. This
OMe
0
0
7
OMe
OMe
(A)
Scheme 3 )

5182
observation
is
supported
by
product
and
radical cation (F) followed by an O-acylation of the resulting phenoxy
radical. On the other hand the coupling of (F) will yield products
or (Scheme 4). These reactions are remarkably suppressed by addition
of 2,
the
fact
that
p-methylanisole
does
not
give any
ortho
acylated
under
these conditions
(Scheme 2).
The formation of esters
3
6
can be explained by demethylation of
4
5
6-di-t-butyl-4-methylphenol
(BHT) 6, and this observation further
Co
R
R
(F) 1_MeC| O"
o
o
3 o r
6
---
R
R
( S c h e m e ~)
supports the presence of radical species during these acylations.
In
conclusion
the present study indicate
that
the
acylation
of anisole with acyl chloride can be catalysed by cobalt(II)chloride
via
a non-ionic process.
REFERENCES AND N O T E S
Friedel-Crafts and related
1933, 504, 267 (b) Dorofeenko,
I.
2.
G.A.
Olah:
reactions.
Interscience,
New York, 1964, vol. 3, P-3.
(a) Unger, F., Liebigs Ann. Chem.
G.N., Zh. Obshch. Khim. 1961, 31, 994 (c) Effenberger, F., Epple,
G., Angew, Chem. Internat. Edit., 1972, 11, 300.
3.
4.
5.
Kozikowski, A.P.; Ames, A.; J. Am. Chem. So.,
Iqbal, J.; Srivastava, R.R.; Tetrahedron, 1991, 47, 3155.
For acylcobalt complex see Francalance, F.; Garduo, A.;
L.; Fiorani, T.; Foa, M., J. Organomet. Chem. 1983, 243, 87.
1980,
102, 860.
:
Abis,
6 .
Murata, S.; Tamatani, A.; Suzuki, K.; Miure, M.; Nomura, M., Chem.
Lett., 1990, 757.
(Received in UK 29 May 1991)