REGIOSELECTIVITY OF COMPLEXATIONS OF SUBSTITUTED PHENYL BENZOATES AND PHENYL PHENYLACETATES WITH Cr(CO)6

Article abstract of DOI:10.1135/cccc19942095

Selectivity of complexation of substituted phenyl benzoates is very low.In most cases, comparable yields of both regioisomeric complexes are isolated.Exception is 4-chlorophenyl ester, where benzoic acid moiety is complexed nearly exclusively.Very high regioselectivity of complexation was observed with substituted phenyl phenylacetates.The substituent of substituted phenols has not any effect on the complexation, and only phenylacetic acid moiety is complexed.This observation supports the recently proposed mechanism of the catalytic activity of the esters at arene complexation.

Full text of DOI:10.1135/cccc19942095

Short Communication
2095
REGIOSELECTIVITY OF COMPLEXATIONS OF SUBSTITUTED PHENYL
BENZOATES AND PHENYL PHENYLACETATES WITH Cr(CO)6
Pavol HRNCIAR, Peter CERNAK, Vladimir GAJDA and Stefan TOMA
Department of Organic Chemistry,
Comenius University, 842 15 Bratislava, The Slovak Republic
Received March 15, 1994
Accepted May 10, 1994
Selectivity of complexation of substituted phenyl benzoates is very low. In most cases, comparable
yields of both regioisomeric complexes are isolated. Exception is 4-chlorophenyl ester, where ben-
zoic acid moiety is complexed nearly exclusively. Very high regioselectivity of complexation was
observed with substituted phenyl phenylacetates. The substituent of substituted phenols has not any
effect on the complexation, and only phenylacetic acid moiety is complexed. This observation sup-
ports the recently proposed mechanism of the catalytic activity of the esters at arene complexation.
In our previous work1 a regioselectivity of complexation of substituted 3-benzylidene-
phthalide has been studied. It was found that the complexation is directed by the substi-
tuent, which was characterized by the molecular electrostatic potential in the middle of
both aromatic moieties present in the free ligand. It was of interest therefore to check if
similar regioselectivity would be observed in other cases, too. In the present work sub-
stituted phenyl esters of benzoic and phenylacetic acids were chosen as model com-
pounds for such a study.
The starting phenyl esters of benzoic as well as phenylacetic acids were prepared by
standard method starting from acid chlorides and phenols2. The complexation of the
esters was carried out with Cr(CO)6 in boiling decalin, as described earlier3. The course
of the complexation of phenyl benzoates is depicted in Scheme 1, and the results are
given in Table I.
The structure assignment of complexation products of phenyl benzoates was straight-
forward from their 1H NMR spectra as it was easy to see whether the chemical shifts of
benzoic acid or phenol residue were shifted to the higher field due to the complexation
with Cr(CO)3 (see Experimental). The colour of the complexes or that of their solutions
was very indicative, too. The complexes in which benzoic acid residue was complexed
gave red solutions while those having complexed phenol moiety gave yellow solutions.
This was expressed also in their UV-VIS spectra (the λ values are given in Scheme 1).
To get an additional proof to this assignment, (η6-phenyl)tricarbonylchromium ben-
zoate was prepared via acylation of (η6-phenol)tricarbonylchromium with benzoyl
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

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Hrnciar, Cernak, Gajda, Toma:
1
chloride (Eq. (A)), and H NMR as well as UV-VIS spectra of the products of both
routes were compared.
(A)
SCHEME 1
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Short Communication
2097
TABLE I
Products of complexation of substituted phenyl benzoates (Scheme 1)
Calculated/Found
% C
M.p.,°C
Yield, %
Formula
(M.w.)
Compound
% H
IIa
99.5 – 102
33
C16H10CrO5
(334.3)
57.49
57.53
3.01
3.01
IIIa
IVa
IIb
129.5 – 131
18
C
C
C
C
C
C
C
C
16H10CrO5
(334.3)
57.49
57.44
3.01
2.90
>150 dec.
13
19H10CrO8
(470.3)
48.52
48.75
2.14
2.16
87 – 89
9
17H12CrO5
(348.3)
58.62
59.10
3.47
3.54
IIIb
IIc
125 – 128
28
17H12CrO5
(348.3)
58.62
58.20
3.47
3.34
106 – 107
12.5
17H12CrO5
(348.3)
58.62
59.02
3.47
3.82
IIIc
IId
132 – 135
32
17H12CrO5
(348.3)
58.62
57.16
3.47
3.23
89 – 91
7
17H12CrO5
(348.3)
58.62
58.14
3.47
3.23
IIId
IIe
128 – 131
19
17H12CrO5
(348.3)
58.62
59.04
3.47
3.56
124 – 125.5
4.7
C18H14CrO5
(362.3)
59.72
60.03
3.89
3.52
IIIe
IIf
70 – 72
31.6
C
18H14CrO5
(362.3)
59.72
59.05
3.89
3.20
93.5 – 95
28
C
18H14CrO5
(362.3)
59.72
59.30
3.89
3.87
IIIf
IVf
IIg
119.5 – 121.5
40.2
C18H14CrO5
(362.3)
59.72
59.10
3.89
3.28
>149 dec.
5
C
21H14Cr2O8
(498.3)
52.20
52.10
2.83
2.91
116.5 – 117.5
13.6
C16H9ClCrO5
(368.7)
52.12
51.87
2.46
2.44
IIIh
135 – 136.5
23
C
16H9ClCrO5
(368.7)
52.12
51.83
2.46
2.45
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Hrnciar, Cernak, Gajda, Toma:
From the results given in Table I it follows that there is practically no regioselectiv-
ity of complexation of different phenyl benzoates. Only in the case of chloro sub-
stituted derivatives, the benzene ring not bearing chlorine was attacked predominantly
at complexation of Ig (R2 = H, R1 = Cl), and a product of the reductive dehalogenation
(IIId) was isolated, too. In these cases there are practically two electron-withdrawing
groups (Cl−, −OCOR or Cl−, −COOR) on the same ring. This observation is in accord
with the results of 3-benzylidenephthalide complexation1.
It is of interest to mention that the main product of the complexation of unsubstituted
phenyl benzoate is the product of benzoic acid moiety complexation, in spite of the fact
that molecular electrostatic potential calculated by AM1 method4,5 is more negative on
the phenol moiety (−1.76 eV) than on the benzoic acid one (−1.511 eV). The complex-
ation of benzoic acid moiety was observed even when the phenol moiety was sub-
stituted by one or two electron-donating methyl groups. This points to some
autocatalytic or directing effect of the ester functionality (C=O bond).
The course of the complexation of substituted phenylacetates V is depicted in Eq. (B),
and the results are given in Table II.
(B)
The structure assignment of the complexation products of phenyl phenylacetates was
not so straightforward especially for the unsubstituted and some methyl substituted
derivatives, as both aromatic moieties form just a broad singlet or multiplet in NMR
spectra of the free ligand. For the above mentioned reason, (η6-phenyl)tricarbonylchro-
mium phenylacetate was prepared by an independent route (Eq. (C)).
(C)
The chemical shift of the −CH2− group of this compound was found at 3.79 ppm,
while the same chemical shift of the free ligand IVa was found at 3.83 ppm, and the
product of complexation of Va has the chemical shift of the −CH2− group at 3.61 ppm.
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A very characteristic feature of the 1H NMR spectra of the complexation product of Va
is a broad singlet of C6H5Cr(CO)3 group at 5.37 ppm. Inspection of the NMR spectra of
all complexation products (see Table III) reveals that the chemical shift of −CH2−
group is located around 3.60 ppm, and all of them have a broad singlet for
C6H5Cr(CO)3 around 5.37 ppm. That means that in all cases the complexation took
place on the phenylacetic acid moiety, in spite of the fact that calculated molecular
electrostatic potential is nearly equal at C6H5CH2− (−1.927 eV) and C6H5O− (−1.881 eV)
moiety.
These results document that the ester CO groups have a directing effect upon the
complexation. It seems reasonable to propose that the first step of complexation is
formation of the >C=O → Cr(CO)5 intermediate, and the Cr(CO)3 group is then
“ushered” underneath the appropriate benzene ring. This is in accord with the proposed
catalytic effect of the esters at the complexations of arenes with Cr(CO)6 (ref.6).
TABLE II
Products of complexation of substituted phenyl phenylacetates (Eq. (B))
Calculated/Found
M.p.,°C
Yield, %
Formula
(M.w.)
Compound
% C
% H
VIa
VIb
VIc
VId
VIe
VIf
87 – 88.5
C17H12CrO5
(348.3)
58.62
58.48
3.47
3.38
47
107 – 108
C18H14CrO5
(362.3)
59.67
60.03
3.89
3.37
30
112 – 114
C18H14CrO5
(362.3)
59.67
59.28
3.89
3.49
32
111
24
C18H14CrO5
(362.3)
59.67
59.34
3.89
3.52
86.5 – 88
C19H16CrO5
(376.3)
60.64
59.95
4.28
4.14
48
105.5 – 107.5
C19H14CrO6
(390.3)
58.46
58.83
3.61
3.56
45
VIg
VIIa
103 – 105.5
C17H11ClCrO5
(382.7)
53.40
54.10
2.89
3.20
14.6
96 – 97.5
C17H12ClCrO5
(348.3)
58.62
58.67
3.47
3.42
14.5
a Equation (C).
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

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Hrnciar, Cernak, Gajda, Toma:
TABLE III
1H NMR spectra of new compounds
Compound
No. 1H NMR spectrum (CDCl3; δ, ppm; J, Hz)
Phenyl (η6-benzoate)tricarbonyl-
chromium
IIa
5.00 t, 2 H (m-C6H5Cr(CO)3); 5.55 t, 1 H
(p-C6H5Cr(CO)3); 6.25 d, 2 H (o-C6H5Cr(CO)3);
7.37 m, 5 H (Ph)
(η6-Phenyl)tricarbonylchromium
benzoate
IIIa
5.00 t, 2 H (m-C6H5Cr(CO)3); 5.42 m, 3 H
(o,p-C6H5Cr(CO)3); 7.5 m, 3 H (o,p-C6H5Cr(CO)3);
8.12 bd, 2 H (m-Ph)
(η6-Phenyl)tricarbonylchromium
(η6-benzoate)tricarbonylchromium
p-Tolyl (η6-methylbenzoate)-
IVa
IIb
5.00 t, 1 H; 5.40 m, 7 H; 6.17 d, 2 H
2.37 s, 3 H (CH3); 5.42 t, 2 H (m-C6H5Cr(CO)3);
5.55 t, 1 H (p-C6H5Cr(CO)3); 6.22 d,
tricarbonylchromium
2 H (o-C6H5Cr(CO)3); 7.07 d, 2 H, J(A,B) = 6 (C6H4);
7.20 d, 2 H, J(A,B) = 6(C6H4)
(η6-p-Tolyl)tricarbonylchromium
benzoate
IIIb
IIc
2.14 s, 3 H (CH3); 5.35 d, 2 H (C6H4Cr(CO)3); 5.55 d,
2 H (C6H4Cr(CO)3);
7.57 m, 3 H (p-Ph); 8.05 d, 2 H (o-Ph)
p-Tolyl (η6-benzoate)-
tricarbonylchromium
2.18 s, 3 H (CH3); 5.07 t, 1 H (C6H4Cr(CO)3); 5.28 d,
2 H (C6H4Cr(CO)3); 5.50 d, 2 H (C6H4Cr(CO)3);
7.82 m, 3 H (Ph); 8.40 d, 2 H (Ph)
(η6-o-Tolyl)tricarbonylchromium
benzoate
IIIc
2.26 s, 3 H (CH3); 5.25 t, 2 H (m-C6H4Cr(CO)3);
5.62 t, 1 H (p-C6H4Cr(CO)3); 6.25 d,
2 H (o-C6H4Cr(CO)3); 7.21 m, 4 H (C6H4)
2.29 s, 3 H (CH3); 4.89 d, 1 H, J(A,B) = 6 (C6H4);
5.94 bs, 2 H (C6H4Cr(CO)3); 5.77 t, 1 H
(C6H4Cr(CO)3); 7.57 m, 3 H (o,p-Ph); 8.15 d,
2 H (m-Ph)
m-Tolyl (η6-benzoate)-
tricarbonylchromium
IId
(η6-m-Tolyl)tricarbonylchromium
benzoate
IIId
2.38 s, 3 H (CH3); 5.32 t, 2 H (m-C6H4Cr(CO)3);
5.58 t, 1 H (p-C6H4Cr(CO)3); 6.24 d,
2 H (o-C6H4Cr(CO)3); 7.12 m, 4 H (C6H4)
3,4-Dimethylphenyl (η6-benzoate)- IIe
tricarbonylchromium
2.11 s, 3 H (CH3); 2.28 s, 3 H (CH3); 5.47 m,
4 H (m-C6H4Cr(CO)3); 7.58 m, 3 H (m,p-Ph);
8.13 d, 2 H (o-Ph)
(η6-3,4-Dimethylphenyl)tricarbonyl- IIIe
chromium benzoate
2.27 s, 6 H (CH3); 5.31 t, 2 H (m-C6H4Cr(CO)3);
5.59 t, 1 H (p-C6H4Cr(CO)3); 6.23 d, 2 H (o-
C6H4Cr(CO)3); 6.94 d, 2 H (C6H3); 7.11 s, 1 H (C6H3)
2,6-Dimethylphenyl (η6-benzoate)- IIf
tricarbonylchromium
2.12 s, 3 H (CH3); 2.22 s, 3 H (CH3); 4.95 d, 1 H
(C6H3Cr(CO)3); 5.47 t, 2 H (C6H4Cr(CO)3); 7.62 m,
3 H (Ph); 8.20 d, 2 H (Ph)
(η6-2,6-Dimethylphenyl)tricarbonyl- IIIf
chromium benzoate
2.20 s, 3 H (CH3); 2.32 s, 3 H (CH3); 5.27 t, 2 H
(m-C6H5Cr(CO)3); 5.65 t, 1 H (p-C6H5Cr(CO)3);
6.30 d, 2 H (o-C6H5Cr(CO)3); 7.00 t, 3 H (C6H3)
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Short Communication
2101
TABLE III
(Continued)
Compound
No. 1H NMR spectrum (CDCl3; δ, ppm; J, Hz)
(η6-2,6-Dimethylphenyl)tricarbonyl- IVf
chromium (η6-benzoate)-
tricarbonylchromium
2.15 s, 3 H (CH3); 2.20 s, 3 H (CH3); 4.95 d, 1 H
(C6H3Cr(CO)3); 5.30 t, 2 H (C6H3Cr(CO)3); 5.45 t,
2 H (m-C6H5Cr(CO)3); 5.70 t, 1 H
(p-C6H5Cr(CO)3); 6.25 d, 2 H (o-C6H5Cr(CO)3)
4-Chlorophenyl (η6-benzoate)-
tricarbonylchromium
IIg
5.50 d, 2 H, J(A,B) = 2 ((C6H4ClCr(CO)3); 5.65 d,
3 H, J(A,B) = 3 (C6H4ClCr(CO)3); 7.57 m, 3 H (Ph);
8.12 d, 2 H (Ph)
(η6-4-Chlorophenyl)tricarbonyl-
chromium benzoate
IIIg
IIIh
Va
5.30 t, 2 H (m-C6H5Cr(CO)3); 5.62 t, 1 H
(p-C6H5Cr(CO)3); 7.10 d, 2 H, J(A,B) = 8 (C6H4Cl);
7.40 d, 2 H, J(A,B) = 8 (C6H4Cl)
(η6-Phenyl)tricarbonylchromium
4-chlorobenzoate
5.00 bt, 1 H (p-C6H5Cr(CO)3); 5.50 m,
4 H (C6H5Cr(CO)3); 7.53 d, 2 H (C6H4Cl); 8.03 d,
2 H (C6H4Cl)
Phenyl (η6-phenylacetate)-
3.61 s, 2 H (CH2); 5.37 m, 5 H (C6H5Cr(CO)3);
tricarbonylchromium
7.07 dd, 2 H (o-Ph); 7.32 m, 3 H (Ph)
EXPERIMENTAL
All the starting materials and solvents were purified by standard procedures. 1H NMR spectra were
measured in deuteriochloroform solution on Tesla BS 587 instrument at 80 MHz, using tetramethyl-
silane as an internal standard (δ, ppm; J, Hz). UV-VIS spectra were measured by Hewlett–Packard
diode array spectrophotometer 8452A in methanolic solution. Phenyl benzoates and phenyl phenylace-
tates were prepared by standard procedures2.
General Procedure for Complexation of Phenyl Benzoates and Phenylacetates
The reaction mixture which consisted of the appropriate ester and Cr(CO)6 in the molar ratio 3 : 1 in
decalin was purged for 20 min with argon, then it was evacuated and again argon was passed through
it. This procedure was repeated 2 – 3 times. After that, the reaction mixture was refluxed (bath tem-
perature ≈ 260 °C) until the evolution of CO or decomposition of the complex started. All experi-
ments were carried out with 0.5 g (2 mmol) of Cr(CO)6. The reaction mixture was cooled to 20 °C,
filtered through Kieselguhr, and then chromatographed under N2 on SiO2 column using 2-methylpen-
tane as the eluent for elution of decalin and then a mixture of 2-methylpentane–ethyl acetate (5 : 1
to 4 : 1, v/v) for elution of the reaction products. The results are given in Tables I and II.
(η6-Phenyl)tricarbonylchromium benzoate (IIIa). The complexation of phenol (0.27 g, 2.8 mmol)
with Cr(CO)6 (0.5 g, 2.2 mmol) was carried out as described above. After the complexation was over,
the reaction mixture was cooled to 20 °C, and K2CO3 (0.5 g, 36 mmol) and benzoyl chloride (0.8 g,
5.7 mmol) were added under stirring. The mixture was stirred for 20 min at 45 °C, cooled to 20 °C
and chromatographed. Chromatography afforded 0.16 g (24%) of (η6-phenyl)tricarbonylchromium
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

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Hrnciar, Cernak, Gajda, Toma:
benzoate. Crystallization from mixture of 2-methylpentane–ethyl acetate (9 : 1, v/v) yielded yellow
crystals melting at 129 – 130.5 °C.
(η6-Phenyl)tricarbonylchromium phenylacetate (VII). The experiment was carried out as described
for the synthesis of IIIa, except that 0.5 g of K2CO3 and 0.6 g (0.4 mmol) of phenylacetyl chloride
were added to the (η6-phenol)tricarbonylchromium solution. Chromatography yielded 0.10 g (14.5%)
of (η6-phenyl)tricarbonylchromium phenylacetate (VII) as yellow crystals from 2-methylpentane–
ethyl acetate (9 : 1, v/v) melting at 96 – 97.5 °C.
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1965.
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(1985).
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