J. CHEM. RESEARCH (S), 1998 823
and Mn(TNP)OAc occurred in the process, reactions with
Mn(TMP)OAc and Mn(TDCPP)OAc were stopped with
some catalyst still remaining. In the course of measuring the
turnover numbers, the aqueous phase was renewed whenever
the reaction stopped.
By correlating the intensity of the Soret bands of various
Mn±porphyrins with their concentrations in the oxidation
of cyclooctane, according to the conditions described in
Table 1, consumption percentages of Mn(TPP)OAc of
81%, Mn(TNP)OAc of 56%, Mn(TMP)OAc of 24% and
Mn(TDCPP)OAc of 17% were obtained at the end of the
reactions.
Mn-Porphyrin
Mn(TPP)OAc
Mn(TNP)OAc
Mn(TMP)OAc
Mn(TDCPP)OAc
Ar
Phenyl
1-Naphthyl
2,4,6-Trimethylphenyl
2,6-Dichlorophenyl
Experimental
Results of oxygenation of the arylalkanes and cyclo-
alkanes, and also consideration of the smaller number of
In
a
standard reaction procedure, to
a
solution of
Mn(porphyrin)OAc (0.006 mmol) in dichloromethane (2 ml) the
phase-transfer catalyst (0.024 or 0.24 mmol) imidazole (0.06 mmol)
and then substrate (0.5 mmol) were successively added. To the
resulting solution mixture, a solution of NaIO4 (1 mmol) in H2O
(10 ml) was added and the two phases were stirred vigorously for
the appropriate time at room temperature. Formation of products
and consumption of substrates were monitored by GLC.
Products were characterised by comparison of their spectroscopic
(IR, 1H NMR) properties with those of authentic samples. The
purity determination of the substrates and reactions monitoring
were accomplished by GLC.
±
±
benzylic C H bonds relative to those of secondary C H
bonds, clearly demonstrate that the latter bonds are less
susceptible to oxidation than the former ones. Low con-
version competitive oxidation of cyclooctane and tetralin
with an Mn(TPP)OAc:imidazole:cyclooctane:tetralin:oxidant
ratio of 1:10:83.33:83.33:166.66 led to a conversion of 18%
of tetralin and 4% of cyclooctane to their oxidised forms
in 30 min. These results are consistent with the greater
±
reactivity of the benzylic C H bonds [0(18/4) Â(16/4)
18-fold] relative to those of cyclooctane, under our exper-
imental conditions.
Support of this work by Research Council of Shiraz
University is greatly appreciated.
Mn(TNP)OAc is a better catalyst than Mn(TPP)OAc
for oxygenation of all the substrates and particularly
for cycloalkanes and ethylbenzene. The sterically hindered
Mn(TMP)OAc complex acts very similarly to Mn(TNP)OAc
in the oxidation of cycloalkanes and ethylbenzene, whereas
in the oxygenation of indane and tetralin Mn(TMP)OAc
shows a relatively poor activity. This may be related to the
steric eects of the ortho-methyl substituents of the phenyl
groups of the TMP ligand.
The high stability of the halogenated Mn(TDCPP)OAc,
under the oxidising conditions, and also the relatively slower
oxidation reactions that are induced by this complex, led
us to increase both the reaction time (24 vs. 3.5 h) and
concentration of the oxidation in the CH2Cl2 phase by
enhancing the PTC/Mn±porphyrin molar ratio (40 vs. 10),
for this catalyst. Under these conditions high conversion of
cycloalkanes (45±87%), and arylalkanes (80±87%) to their
oxidised forms is observed.
Received, 1st June 1998; Accepted, 2nd September 1998
Paper E/8/04080J
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Treatment of the green CH2Cl2 solution of Mn(TPP)OAc
with NaIO4 under the catalytic reaction conditions in
the absence of a substrate produced a new broad band at
398 nm at room temperature. Addition of tetralin to the
solution caused a gradual loss of intensity of the Soret
absorbance at 398 nm with
a concomitant increase in
absorbance at 478 nm. The band at 398 nm, although not
very close to that expected for a classical high-valent
oxo-manganese complex (Soret band at 420±425 nm)5
is attributed to the presence of an active high-valent
manganese oxidising species. The possible complexation of
periodate and other anionic ligands, existing in the reaction
mixture, to Mn(TPP) is expected to give Soret bands at
>478 nm.6
Maximum turnover numbers for various catalysts
(assuming two oxidations for generation of cyclooctanone)
with
a molar ratio of 1:10:167:1250 for Mn-catalyst:
imidazole:oxidant:cyclooctane are Mn(TPP)OAc (100 in
7 h), Mn(TNP)OAc (130 in 15 h), Mn(TMP)OAc (350 in
36 h) and Mn(TDCPP)OAc (475 in 72 h). It should be
noted that while complete destruction of Mn(TPP)OAc