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Doi:10.1016/j.catcom.2003.11.003Aerial oxidation of substituted aromatic hydrocarbons catalyzed Kala Nair, Dhanashri P. Sawant, G.V. Shanbhag, S.B. Halligudi * Inorganic Chemistry and Catalysis Division, National Chemical Laboratory, Pune 411 008, India Received 31 July 2003; received in revised form 1 November 2003; accepted 1 November 2003 Aerial oxidation of substituted aryl aromatic hydrocarbons were carried out using Co/Mn/BrÀ catalyst system in water-dioxane medium in the range 14–56 bar air and temperature 383–423 K. The combination of optimum catalyst concentration of saltsCo(OAc)2, Mn(OAc)2 and NaBr (1:3:10 molar ratio) in water-dioxane (1:2 mole ratio) is found catalyze aerial oxidation ofsubstituted aryl aromatic to give corresponding oxygenated products. Under the optimized conditions, p-cymene gave p-isopropylbenzaldehyde (33.1%), p-isopropylbenzyl alcohol (54.7%) and p-isopropyl benzoic acid (3.3%), respectively, at p-cymene conversion40.2%. Similarly, oxidation of p-methoxy toluene gave p-methoxy benzaldehyde (87.4%), benzyl alcohol (5.5%), and p-methoxybenzoic acid (6%), while oxidation of p-tert-butyl toluene yielded p-tert-butyl benzaldehyde (87%), p-tert-butyl benzyl alcohol (5.7%)and p-tert-butyl benzoic acid (6.1%), at conversions 16.4% and 36.1%, respectively. It is found that Co/Mn/BrÀcatalyst system inwater-dioxane medium is eﬀective in the aerial oxidation of substituted aromatic hydrocarbons to get corresponding alcohol andaldehydes in greater yields.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Aryl aromatic hydrocarbons; Oxidation; Co/Mn/BrÀ catalyst; Water-dioxane operating system. In addition to these, there is a sig-niﬁcant loss in acetic acid (5–10 wt% of TA) getting Oxidation reactions occupy a prominent place in oxidized to CO and CO2, which requires the lowering both the sciences of catalysis and catalysis-based mod- the reaction temperature without reducing reaction ern chemical industry [1,2]. They have vastly contributed to the development of the modern society, for their Oxidation of p-cymene is important, because, of its products are incorporated into an amazingly large pro- oxidation products p-isopropyl benzylalcohol and p- portion of the materials and commodities in daily use. In isopropyl benzaldehyde is used in perfumes and has chemical industry more than 60% of products are ob- export potential. The latter is also used as a ﬂavoring tained by catalytic oxidation routes. Terephthalic acid agent for food materials and p-isopropyl benzoic acid is (TA) is commercially manufactured by the dioxygen used as a pharmaceutical intermediate. Oxidation of p- oxidation of p-xylene using Co/Mn/BrÀ catalyst system cymene by molecular oxygen to p-isopropylbenzoic acid in acetic acid medium [3–5]. The issues to be addressed or its intermediate alcohol and aldehyde catalyzed by in liquid phase catalytic oxidation of aromatic hydro- Co and Mn complexes in acetic acid medium with ac- carbons are to ﬁnd eﬃcient catalyst systems, free from etaldehyde or methyl ethyl ketone as radical initiators bromide promoter and replacement of acetic acid sol- [6,7], cerium (IV) ammonium nitrate in acetic acid to vent, both these factors cause corrosion problems in give nitro and acetate substituted derivatives  andoxides of Cr, Mn and Se have been reported . Simi-larly, liquid and vapor phase oxidation of p-methoxy- toluene and p-tert-butyltoluene, wherein conventional Corresponding author. Tel.: +91-20-5893300; fax: +91-20-5893761.
E-mail address: Halligudi).
catalysts consisting Co, Mn and NaBr salts in acetic acid 1566-7367/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2003.11.003 K. Nair et al. / Catalysis Communications 5 (2004) 9–13 medium and vanadium pentoxide based catalysts, for sponding aldehydes, alcohols and carboxylic acids and the the vapor phase oxidation of p-methoxy toluene to p- results along with the reaction conditions are presented in methoxy benzaldehyde have also been reported [10–17].
Table 2. The oxidation reactions and their products ob- Mechanisms of oxidations using Co/Mn/BrÀ catalyst tained are shown in Schemes 1–3, respectively.
systems in acetic acid medium, are well established in the The oxidation of p-cymene was carried with diﬀerent literature, however, the oxidation of substituted aryl mole ratios of catalyst salts to know their eﬀect of on the aromatic hydrocarbons to get corresponding alcohol conversion of p-cymene and product selectivities. From and aldehydes in higher yields is still important from the results (entry nos. 1–3, Table 1), it is found that Co:Mn:BrÀ (1:3:10, mole ratio) gave marginally higher Therefore, we are reporting here our results on the conversion (39.1%) of p-cymene at 42 bar air and 423 K oxidation of substituted aryl aromatic hydrocarbons in after 3 h. However, the selectivities for p-isopropyl water-dioxane medium using conventional Co/Mn/BrÀ benzyl alcohol were higher than p-isopropyl benzalde- catalyst system and optimized reaction conditions for hyde and p-isopropyl benzoic acid. Hence, Co:Mn:BrÀ higher substrate conversions and product yields.
(1:3:10, mole ratio) was used for studying the eﬀect oftemperature, air pressure and time on stream on theconversion and product distribution in the oxidation of Cobalt acetate, manganese acetate, sodium bromide and 1,4-dioxane (AR grade) were procured fromM/s. Loba Chemie, Mumbai. Double distilled water and The inﬂuence of temperature on the oxidation of p- air from the cylinder was used as a source of molecular cymene was studied by varying the temperature from oxygen in the oxidation reaction. p-Cymene, p-methoxy 383 to 443 K, with Co:Mn:BrÀ (1:3:10, mole ratio), 42 toluene and p-tert-butyl toluene supplied by M/s. Malti bar air and 2 h. From the results (entry nos. 4A–4C, Chemicals, Baroda were distilled and purity checked Table 1) it is seen that maximum conversion of p-cym- from GC prior to their use in the oxidation experiments.
ene (49.5%) was observed at 443 K. Corresponding al- Oxidation experiments were conducted in a 300 ml cohol and aldehyde selectivities were found to be 54.1% pressure reactor (M/s Parr Instruments Co., USA).
and 39.1%, respectively. However, at lower temperature In a typical run, known amounts of substrate, metal conversions was comparatively less, while selectivities salts Co(OAc)2 Á 4H2O, Mn(OAc)2 Á 4H2O, NaBr, water, for p-isopropylbenzyl alcohol were higher. The selec- 1,4-dioxane was mixed in the reactor. The reactor was tivities for p-isopropyl benzaldehyde increased with in- pressurized with air from the cylinder and the bomb was crease in temperature, because, of consecutive reactions heated to attain the required temperature. At this stage, steps involved in the oxidation of p-cymene. The side the agitation was started to initialize the reaction and products were higher particularly at lower temperatures sample was withdrawn through sample valve. The re- and for a reasonable conversion minimum temperature action was continued for a period of 2–3 h and stopped.
should be around 403 K for the oxidation of p-cymene.
At the end of a run, reactor was cooled to room tem-perature and the reaction mixture was transferred to a beaker. The reaction mixture withdrawn in the begin-ning and at the end of the reaction were analyzed by gas The oxidation of p-cymene was carried out to know chromatography (Shimadzu, GC-14B) with a 6 ft 10% the eﬀect of air (O2) pressure ranging between 14 and 56 SE-30 S.S.packed column, 1/8 in O.D and FID (detec- bar at 423 K. From the results (entry nos. 5A–5D, Table tor). From GC analysis, the conversion of substrate 1), it is seen that the conversion of p-cymene increased and selectivities for products were obtained. The iden- from 20.6% to 60.2% with increase in pressure. The se- tities of the oxidation products were made by GC–MS lectivity for p-isopropylbenzyl alcohol steadily increased and p-isopropyl benzaldehyde decreased with increase inpressure. Higher pressure seems to stabilize a primaryoxidation product p-isopropyl benzyl alcohol leading to Oxidation of p-cymene catalyzed by Co/Mn/BrÀ catalyst system with air gave p-isopropylbenzyl alcohol,p-isopropyl benzaldehyde and p-isopropyl benzoic acid To ﬁnd the eﬀect of BrÀ concentration in the aerial and the results along with reaction conditions are oxidation of p-cymene, the mole ratio of Co:Mn:Br is presented in Table 1. Similarly, aerial oxidation of p- varied in such a way that the total moles (moles added) tert-butyl toluene and p-methoxy toluene gave corre- varied was in the range 3.5–14 mmol. The variation in K. Nair et al. / Catalysis Communications 5 (2004) 9–13 Table 1Catalytic data of oxidation of p-cymenea a Reaction conditions: medium p-cymene (33.58 mmol) + NaBr (10 mmol) + H2O (311 mmol) + 1,4-dioxane (568 mmol), time ¼ 3 h, air pres- sure ¼ 42 bar, temperature ¼ 423 K.
b The sources of Co and Mn are Co(OAc)2 Á 4H2O and Mn(OAc)2 Á 4H2O, respectively.Values in the parenthesis refer to the number of mmol of catalyst. In runs 1–5D, 10 mmol of NaBr was used.
c Turnover frequency (TOF) ¼ mole of p-cymene converted per mole of metal salt per hour.
d A ¼ p-isopropylbenzyl alcohol, B ¼ p-isopropyl benzaldehyde, C ¼ p-isopropyl benzoic acid.
e In runs 4A–4C, p-cymene ¼ 17.16 mmol, time ¼ 2 h, temperature ¼ 383, 403, 423 K, respectively.
f In runs 5A–5D, p-cymene ¼ 17.16 mmol, time ¼ 2 h, air pressure ¼ 14, 28, 42, 56 bar, respectively.
g In runs 6, 7, 8, p-cymene ¼ 17.16 mmol, time ¼ 2 h, air pressure ¼ 42 bar, temperature ¼ 423 K.
h In runs 9A–9C, p-cymene ¼ 33.58 mmol, air pressure ¼ 56 bar, temperature ¼ 423 K, time ¼ 2 h, 4 h, 6 h, respectively, (each time reactor was cooled, gas discharged and fresh air charged to continue the run).
Table 2Catalytic data of oxidation of diﬀerent substrates Conditions: medium substrate (33.58 mmol), Co (1 mmol) + Mn (3 mmol) + NaBr (10 mmol) + H2O (311 mmol) + 1,4-Dioxane (568 mmol), time ¼ 2 h, air pressure ¼ 56 bar, temperature ¼ 423 K. The sources of Co and Mn are Co (OAc)2 Á 4H2O and Mn (OAc)2 Á 4H2O, respectively.
a p-isopropyl benzaldehyde.
b p-isopropylbenzyl alcohol.
c p-isopropyl benzoic acid.
d p-tert-butyl benzaldehyde.
e p-tert- butyl benzyl alcohol.
f p-tert-butyl benzoic acid.
g p-methoxy benzaldehyde.
h p-methoxy benzyl alcohol.
i p-methoxy benzoic acid.
j Turnover frequency (TOF) ¼ mole of substrate converted per mole of metal salt per hour.
total moles of catalyst components, actually amounts to Table 1 (entry nos. 6–8), that the conversion of p-cym- the variation in total catalyst concentration in the re- ene decreased with decrease in catalyst concentration.
action mixture. It is seen from the results presented in At all catalyst concentrations, the selectivities for K. Nair et al. / Catalysis Communications 5 (2004) 9–13 p-tert-butyl toluene was conducted and the results ob- tained are presented in Table 2. It is seen that, p-cymeneand p-methoxy toluene could be oxidized easily than p-tert-butyl toluene. In comparison with p-cymene, theoxidation of p-methoxy toluene and p-tert-butyl tolueneis found to be more selective to corresponding aldehydes Scheme 1. Oxidation of p-cymene. A, p-isopropylbenzyl alcohol; B, p-isopropyl benzaldehyde; C, p-isopropyl benzoic acid.
Thus, the speciﬁc activity (TOF) for p-cymene con- version is enhanced by the presence of Co, Mn andNaBr concentrations in water-dioxane medium. The conversion of p-cymene increased at higher temperatureand air pressure. High yields of aldehydes and alcohols could be obtained if reactions were carried out at a low temperature under conditions of oxygen starvation(pressure requirement). On the other hand if the reaction Scheme 2. Oxidation of p-tert-butyl toluene. G, p-methoxy benzyl al- temperatures and oxygen concentration (air pressure) cohol; H, p-methoxy benzaldehyde; I, p-methoxy benzoic acid.
are at higher levels, the intermediate product aldehydequickly oxidizes to carboxylic acid.
A decrease in catalyst mole ratio from 14 to 3.5 de- creased the conversion of p-cymene. From the observa-tions made, it is concluded that the optimum conditions for maximum conversion of p-cymene (>93%) and highproduct selectivity for alcohol and aldehyde is to carryout the reaction at 423 K and 56 bar air for 6 h, with Co/Mn/BrÀ catalyst mole ratio 1:3:10 in water-dioxane Scheme 3. Oxidation of p-methoxy. D, p-tert-butyl benzyl alcohol; E, medium. Similarly, p-methoxy toluene and p-tert-butyl p-tert-butyl benzaldehyde; F, p-tert-butyl benzoic acid.
toluene are oxidized more selectively to correspond-ing aldehydes under the optimized reaction conditions.
p-isopropylbenzyl alcohol and p-isopropyl benzaldehyde Therefore, we conclude that the aerial oxidation of were higher with smaller amounts of p-isopropyl benzoic substituted aryl hydrocarbons could be eﬀectively carried acid. Side products were higher at higher conversion of out with the conventional catalyst system under liquid phase conditions in water-dioxane medium, which avoidsthe loss of acetic acid and minimize corrosion problems In order to obtain maximum conversion of p-cymene, the oxidation reaction was carried out for the total pe- riod of 6 h. After every 2 h, the reactor was cooled andcharged with fresh air. From the results presented in  A. Bielanski, J. Haber, Oxygen in Catalysis, Marcel Dekker, New Table 1 (entry nos. 9A–9C), that a maximum conversion  H.H. Kung, Transition Metal Oxides: Surface Chemistry and of p-cymene (93.8%) was obtained at the end of 6 h with Catalysis, Elsevier, Amsterdam, 1989.
p-isopropylbenzyl alcohol and p-isopropyl benzaldehyde  W. Partenheimer, Catal. Today 23 (1995) 69.
selectivities 54.2% and 14.4%, respectively. Entry no. 9B  S.A.H. Zaidi, Appl. Catal. 27 (1986) 99.
(Table 1) at p-cymene conversion (74.8%), the highest  S.A. Chavan, S.B. Halligudi, D. Srinivas, P. Ratnasamy, J. Mol.
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Under the optimized conditions of p-cymene oxida- Mitsui, T. Yoshihisa, T. Masatoshi, Bull. Chem. Soc. Jpn. 61 (3) tion, the aerial oxidation of p-methoxy toluene and K. Nair et al. / Catalysis Communications 5 (2004) 9–13  N. Shimizu, N. Saito, U. Michio, Stud. Surf. Sci. Catal. 44 (1989)  D.H.R. Barton, A.E. Martell, D.T. Sawyer (Eds.), The Activation of Dioxygen and Homogeneous Catalytic Oxidation, Plenum  A.F. Kristi, US Patent US4740614.
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