An efficient method for chemoselective nitration of phenols under mild and heterogeneous conditions
چکیده
The nitration of aromatic compounds may be achieved with many nitrating reagents and is a very useful way in organic synthesis 1 . Also, nitro compounds find use in many industrial applications2.3 and para isomers are commonly the more commercially desirable products,e.g., p-nitrotoluene vs o-nitrotoluene or p-nitrophenol vs o-nitrophenol and production of 2-nitroestrone versus 4-nitroestrone is important from pharmaceutical point of view4 . Nitration of the phenol as a special case has been studied by various nitrating agents under different conditions4.1 5. Very recently, Rodrigues et al. reported that phenol can be nitrated by silica gel-supported acetyl nitrate with excellent ortho regioselectivity l6. Although a survey of the l iterature indicates the lack of entire selectivity of para nitration of phenol but almost selective ortho nitration of phenol was performed with N204 and pyridine derivatives carrying transferable nitro group or silica gel supported acetyl nitrate6. 16. It is clear that, on the basis of the above 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 17. We have also demonstrated that in situ formation of HN03 is a major factor for an effective nitration of phenols with metal nitrates (containing covalent nitrato groups ) 1 7a. Our goal, in undertaking this line of work, was two-fold: a) to overcome the limitations and drawbacks of the reported methods such as : tedious work-up,9. 1 1 strongly acidic media (Ho_8),4b oxidation ability of the reagents and safety problems (storage, handling, using and also presence of toxic transition metal cations such as Cr+3, Hg+2, Cu+2 , . . . within molecular structure of the reagents) 1 8.19, (b) moreover, constraining a reaction to the surface of solid habitually allows to use milder conditions and increases its reactivity2o. 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 purposes21 . Therefore, we decided to seek a heterogeneous system for selective nitration of phenol, and we have investigated a number of different reaction conditions based upon the in situ generation of HN02 by relatively strong solid inorganic acidic salts [NaHS04.H20, Mg(HS04h. pka-2] and sodium nitrite. We wish to report here in one-pot heterogeneous procedure for nitration of phenols.