An efficient method for chemoselective nitration of phenols under mild and heterogeneous conditions

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1 ndian Journal of Chemistry Vol. 0, Decemer 00, pp. 9-9 An efficient method for chemoselective nitration of phenols under mild and heterogeneous conditions Mohammad Ali Zolfigol*, Elahe Madrakian & Ezat Ghaemi Department of Chemistry, College of Science, University of Bu-Ali Sina, zip code 6 7, Hamadan,.R. ran, Fax: (9) 70 Received Feruary 000; accepted (revised) July 00 Nitrophenols can e otained via nitrosation-oxidation of phenols with Mg(HS0h or NaHS0.H0, NaN0 and wet Si0 at room temperature in moderate to high yields. n situ generation of HN0 and radical cation mechanis_m via nitrous acid catalyzed (NAC) pathway appear to e applicale to phenol nitration using these reagents. The nitration of aromatic compounds may e achieved with many nitrating reagents and is a very useful way in organic synthesis. Also, nitro compounds find use in many industrial applications. and para isomers are commonly the more commercially desirale products,e.g., p-nitrotoluene vs o-nitrotoluene or p-nitrophenol vs o-nitrophenol and production of -nitroestrone versus -nitroestrone is important from pharmaceutical point of view. Nitration of the phenol as a special case has een studied y various nitrating agents under different conditions.. Very recently, Rodrigues et al. reported that phenol can e nitrated y silica gel-supported acetyl nitrate with excellent ortho regioselectivity l6. Although a survey of the literature indicates the lack of entire selectivity of para nitration of phenol ut almost selective ortho nitration of phenol was performed with N 0 and pyridine derivatives carrying transferale nitro group or silica gelsupported acetyl nitrate t is clear that, on the asis of the aove facts there is a need for more regioselective control of nitration of aromatic compounds. Recently, in this connection we have reported the applications and mechanism of some hydrated metal nitrates and their dinitrogen tetroxide complex analogues for nitration of phenols under different conditions 7. We have also demonstrated that in situ formation of HN0 is a major factor for an effective nitration of phenols with metal nitrates (containing covalent nitrato groups ) 7 a. Our goal, in undertaking this line of work, was two-fold: a) to overcome the limitations and drawacks of the reported methods such as : tedious work-up,9. strongly acidic media (Ho_), oxidation aility of the reagents and safety prolems (storage, handling, using and also presence of toxic transition metal cations such as Cr, Hg, Cu,... within molecular structure of the reagents).9, () moreover, constraining a reaction to the surface of solid haitually allows to use milder conditions and increases its reactivity o. Very recently, we among many others have demonstrated that heterogeneous reagent systems have many advantages such as simple experimental procedures, mild reaction conditions and minimization of chemical wastes as compared to their liquid phase counterparts. We have reported simple procedures for in situ generation of 'nitrosonium ion (NO) under mild and heterogeneous conditions and also applications of it for different purposes. Therefore, we decided to seek a heterogeneous system for selective nitration of phenol, and we have investigated a numer of different reaction conditions ased upon the in situ generation of HN0 y relatively strong solid inorganic acidic salts [NaHS0.H 0, Mg(HS0h. pka-] and sodium nitrite. We wish to report here in one-pot heterogeneous procedure for nitration of phenols. Results and Discussion During the course of our studies on the utilization of NO in functional groups transformations, we thought that phenol must e converted to the p-nitrosophenol selectively y Mg (HS0h () or NaHS0.H 0 () [( eq), one equivalent of or were needed for oxidation step], NaN0 [(), ( eq)]

2 9 NDAN J CHEM., SEC B, DECEMBER 00 and wet Si0 (0% wlw) in CHCl as usale solvent via ill situ generation of HN0. Since, phenol nitrosation is rapid and yields almost entirely the para isomer which can e readily converted to p-nitrophenol via mild oxidation with HN0, H0/NaWO/ etc. Therefore, we decided to produce p-nitrophenol via nitrosation-oxidation strategy - in one pot reaction under mild and heterogeneous condition y addition of NaN0 (or other ionic metal nitrates) to the reaction mixture ( as a source for in situ generation of HN0 in the presence of or and wet Si0 ) after performing the nitrosation reaction (, path a). Whereas, in contrast to the reported procedures under aqueous media - we oserved that nitrosation and oxidation were occurring simultaneously without addition of any oxidant to the reaction mixture for the oxidation of nitrosophenol. Hence, in this reaction mono nitration has occurred and o-nitrophenol and p-nitrophenol were otained (, Tale, path ). Different kinds of -sustituted phenols were also sujected to nitration reaction in the presence of inorganic acidic salts e. g. Mg(HS0h () or NaHS0.H0 (), NaN0 (), and wet Si0 (0% wlw) in dichloromethane ( ). The nitration reactions were performed under mild and completely heterogeneous conditions at room temperature in moderate to excellent yields (, Tale ). The present nitration reactions can e readily carried out y placing the nitrating agents, phenols ( or ) and the usale solvent in a reaction vessel and efficiently stirring the resultant heterogeneous mixture at room temperature. The mono nitrophenols can e otained y simple filtration and evaporation of the solvent. This alternative method provides nitrated phenols directly, in short reaction times and good yields. n fact, a comination of sodium nitrite and solid acids can act as solid HN0 which can e readily weighed, handled and used for different purposes in the presence of moist Si0. n order to show the chemoselectivity of this method a competitive reaction was performed etween phenol and anisole. t was oserved that exclusive phenol nitration proceeded whereas anisole remained intact in the reaction mixtures after hr ( ). This method is also very mild as the hydrolysis product of -cyanophenol was not oserved ( a N'N0. NO rno, N0 N0 l or V, Entries 9 and 0). Selective mononitration of,' dihydroxydiphenyl was also achieved y controlling stoichiometry of reagents (Tale, Entries and 6). Although the reaction occurs without wet Si0 ut, the reaction time period is very long and completed after several days. Therefore we think that the presence of wet Si0 will act as a media and will provide a heterogeneous effective surface area for in situ generation of HN0. t also will make easy work up. Meanwhile, nitration was not occurred in the asence of inorganic acidic salts (Tale, Entry 7). This new system e.g. a comination of inorganic acidic salts or and sodium nitrite is similar to N0 (N0 :::> NO N0 ) 7 as a nitrosating agent via in situ generation of HN0 (Eqn ) and NO (Eqn ). Therefore on the asis of our oservations, the previously reported results concerning the applications of N0,.6 metal nitrate dinitrogen tetroxide complexes [M(N0)m.nN0], 7. oxidation of HN0 with oxygen and production of N0 (Eqn _6),7 very recent reported mechanism for nitration of phenols. 9 and the products which are otained, the following nitrous acid catalyzed mechanism (NAC) may e proposed ( V). n conclusion, the cheapness and the availaility of the reagents, easy and clean work-up, and high yields make this method attractive for the large-scale operations. This simple procedure is highly selective and contamination y oxidation side-products is avoided. Moreover, the new element here is that the reaction is heterogeneous. This could e worthwhile in an industrial settingo.

3 ZOLFGOL et al. : AN EFFCENT METHOD FOR CHEMOS ELECTVE NTRATON OF PHENOLS l or a c d e F Cl Br CN Ph N f g h CH) OCH) COCH) CO j k CH Ph NHOAc -HOC6H- Tale -Mononitration of phenols to their corresponding nitro derivatives with a comination Mg(HS0h () or NaHS0.H0 (), Entry Sustrate Sa Sa c c d d e e Sf Sf g g h h i i j j k k NaN0 () and wet Si0 (0% w/w) in dichloromethane at room temperature. m.p. (0C) Yields Time Product Reag.lSust. (mrnole)" Reported Found % (min) 0 0 6a 6a 6 6 6c 6c 6d 6d 6e 6e 6f 6f 6g 6g 6h 6h 6i 6i 6j 6j 6k 6k 6 6, (h) l NO ReactionC <: 7-7<:. 9 9 ".. ". " <: <: 66) 66. ". ",. " a \'. et Si0 : sustrate ( ) (0. g : mmole). solated Yields. C Reaction was not occurred in the asence of inorganic acidic salts. Experimental Section General. Chemicals were purchased from Fluka, Merck and Aldrich chemical companies. Yields refer to isolated pure products. The nitration products were -:haracterized y comparison of their spectral (JR, H_ NMR, C_NMR), TLC and physical data with the authentic samples, 7. Mononitration of phenol () with NaHS0.H0 (), NaN0 and wet Si0 : A typical procedure. A suspension of compound 0. g, 0.0 mole),

4 NDAN J CHEM., SEC B, DECEMBER 00 9 HJ c9j l or $-N0 N0 l or 0r 6 00% 0% J, 7 en _ or &N0 CN 00% 0% V H N0' HN0 H0 HN0 HN0) NO HN0 H0 NO N0 N0) N0 N0N0 O N0 NO [ or S -- (j ' (eq ) (eq ) (eq ) (eq ) l -N c$j c$j -- &N0 CO (eq ) (eq 6) J' or 6 V (.76 g, 0.0 mole), NaN0 (. g, 0.0 mole) and wet Si0 (0% w/w, g) in CHCh (0 ml) was stirred magnetically at room temperature. The reaction was completed after 0 minutes and then filtered. The residue was washed with CHCh (x l O ml). Anhydrous NaS0 ( g ) was added to the filtrate. After minutes the resulting mixture was also filtered. Dichloromethane was removed y water ath (-0 0C) and simple distillation. Residue otained was -nitrophenol ( insolule in /pantane), l. g ( %), mp 0- DC (lit7 mp DC). The n-pantane was evaporated y water ath (-0 DC), -nitrophenol, 0.9 g, %, mp - DC (lit7 mp DC) was also otained (Tale, ). Mononitration of -cyanophenol (d) with NaHS0.H0 (), NaN0 and wet Si0 : A typical procedure. A suspension of compound d (0. g, mmole), (0.76 g mmole), wet S i0 (0% w/w, 0. g) and NaN0 (0. g, mmole) in dichloromethane ( ml) was stirred at room temperature for hr (the progress of the reaction was monitored y TLC) and then filtered. Anhydrous NaS0 ( g) was added to the filtrate. After min the resulting mixture was also filtered. Dichloromethane was removed y simple distillation on water ath (-0 DC). The yield was 0.99 g (9%) of crystalline pale yellow solid (6d), mp 0- C (Lit.c mp C). H-NMR (FT-90 MHz, CDCh, TMS) : 0 7. (d, H), 7.77 (dd, H),. (d, H), 0.7 (, H). H-NMR spectra were identical with reference spectrac. Acknowledgement The authors gratefully acknowledge partial support of this work y the Research Affair of Bu-Ali Sina University, Hamadan,.R ran. References Gu S, Jing Wu J & Liang Y, Synth Commun, 7, 997, 79. Smith K, Musson A & DeBoos G A, Chem Commun, 996, 69. Waller F G, Barrett A G M, Braddock D C & Ramprasad D, Chem Commun, 997, 6 and references cited therein: (a) Delaude L. Laszlo P & Smith K, Acc Chem Res , 607. () Laszlo P, Acc Chem Res, 9, 96.. (c) Comelis A, Laszlo P & Pennetreau P, Bull Soc Chim Be/g. 9, 9, 96. Zeegers P J, J Chem Ed. 70, 99, 06. H,

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