992
R. D. Patil, Y. Sasson
An oxidation of ruthenium chloride by aq. NaOCl in situ
generates ruthenium tetroxide (RuO4) which has good
solubility in DCE reaction solvent and act as true liquid
phase oxidant during the present reactions [21–23]. To our
delight; we found that using RuCl3-NaOCl system the
reaction rate of various activated as well as non-activated
naphthalenes oxidation has greatly improved under phase
transfer catalytic conditions. To the best of our knowledge,
RuCl3-NaOCl reaction system for the liquid phase partial
oxidation of naphthalene to phthalic acid under phase
transfer conditions has been not reported.
Keywords Phase transfer catalysis Á Naphthalene’s Á
Phthalic acids Á Ruthenium catalyst Á Oxidation
1 Introduction
The partial oxidation of naphthalenes to the corresponding
phthalic acids has been an effective and useful reaction for
the production of aromatic dicarboxylic acids. The prod-
ucts phthalic acids have numerous practical applications in
various industries such as polymers, adhesives and glues,
electronics, synthetic perfumes etc. [1, 2]. Phthalic acids
can be synthesized through the oxidation of their starting
precursors such as naphthalenes, xylenes, benzaldehydes
[3–5]. However the catalytic oxidation of naphthalene
directly to phthalic anhydride and subsequent hydrolysis of
anhydride into the phthalic acids has been remains the most
common route for phthalic acid synthesis [3, 6]. There are
several literature reports for the oxidation of naphthalenes
using CeO2/Al2O3, M (Ru, Pt, Pd, Co etc.)/Al2O3, V2O5,
[RuIV(2,6-Cl2tpp)Cl2] complex, RuO4 etc. has reported [7–
14]. Apart from this; metal salts (MOAc)/photoirradiation
and superoxide (KO2) were utilized for the naphthalenes
oxidation [15–17].
For the present study; we have chosen 1-chloronaph-
thalene as model substrate, ruthenium chloride (RuCl3-
3H2O) as catalyst, aq. NaOCl (house bleach) as reagent and
experimental results obtained are presented in Table 1.
Based on our earlier studies [22, 23]; we have selected 1,2-
dichloroethane (DCE) is reaction solvent and ruthenium to
PTC ratio was maintained at 1:5 mol% for the present
study. Initially the reaction was performed using
1-chloronaphthalene, stoichiometric amount of aq. NaOCl
(molar ratio of NaOCl/substrate = 8), 1 mol% RuCl3-
3H2O and 5 mol% TBAB at room temperature. After
completion of the reaction at 1 h, 1-chloronaphthalene
afforded 54 % isolated yield with 61 % selectivity for
desired 3-chloro-phthalic acid 2 (Table 1, entry 1). The
increase of molar ratio of NaOCl/substrate from 8 to 12
increases the reaction yield up to 75 % along with small
increase in the selectivity for product 2 (Table 1, entry 2).
A reduction of the ruthenium salt from 1 mol% to
0.5 mol% increases the reaction yield up to 84 % (Table 1,
entry 3). Under similar reaction conditions; increase of
NaOCl/substrate molar ratio from 12 to 16 decreases
reaction yield (Table 1, entry 4) up to 62 %. An effort to
decrease ruthenium salt from 0.5 to 0.2 mol%, causes
decrease in the reaction yield (Table 1, entry 5). Next to
optimize reaction temperature; reactions were carried out at
variable temperatures (from 0 to 55 °C, Table 1, entries
6–8). As obtained results indicated that the best yield of
product 2 could be obtained at room temperature (Table 1,
entry 3). In the case of reaction carried out using didecyl
dimethyl ammmonium bromide (DDAB) instead of TBAB
as PTC; relatively lower yield and selectivity of product 2
was observed (Table 1, entry 9). Reaction without PTC
(TBAB) given only 68 % yield (Table 1, entry 10).
Reaction did not proceed in absence of either ruthenium
catalyst or aq. NaOCl (Table 1, entries 11–13). After
details optimization study it was found that 0.5 mol% of
ruthenium catalyst, 2.5 mol% PTC (TBAB), NaOCl/sub-
strate in a molar ratio of 12, and reaction at room tem-
perature were optimized parameters to achieve best yield
and selectivity of product 2 under given conditions
(Table 1, entry 3).
An aqueous sodium hypochlorite (aq. NaOCl) is well
known and useful oxidant in organic synthesis for an
oxidation of various organic compounds [18]. An oxida-
tion of naphthalenes carried out using aq. NaOCl at
60–70 °C during the period of 4–6 h [19, 20], however
this method has limitations to oxidize strongly electron
withdrawing substituted naphthalene’s [19, 20]. The
method utilizing aq. NaOCl in conjunction with ruthe-
nium salt (RuO2) has reported for oxidation of naph-
thalenes [21]. This RuO2-NaOCl system has advantages
over utilizing only aq. NaOCl (without ruthenium salt)
such as; reaction proceed at room temperature (instead at
60–70 °C) and efficient to oxidize various activated as
well as non-activated naphthalenes [19–21]. However
RuO2-NaOCl system has slow reaction rate towards
strongly electron withdrawing substituted naphthalenes
and took longer reaction time of some days in case of
nitronaphthalenes (4–7 days) [21].
Our research group was introduced phase transfer
catalysis in such related oxidation reactions [22, 23]. It has
been found that when RuO4 combined with suitable phase
transfer catalyst, its activity (RuO4) substantially changed
[24, 25]. Therefore we hypothesize that reaction rate of
naphthalenes (particularly non-activated naphthalenes)
oxidation using RuO2-NaOCl [21] system could be sub-
stantially enhanced using phase transfer catalyst for both
electron donating as well as electron withdrawing
substituents.
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