%0 Journal Article
%T Transition Metal Ions as Efficient Catalysts for Facile Ortho-Formylation of Phenols under Vilsmeier¨CHaack Conditions
%A F. Aneesa
%A K. C. Rajanna
%A Y. Arun Kumar
%A M. Arifuddin
%J Organic Chemistry International
%D 2012
%I Hindawi Publishing Corporation
%R 10.1155/2012/289023
%X Aromatic compounds (phenols), when treated with Vilsmeier Haack (V-H) reagent in the presence of transition metal ions such as Cu(II), Ni(II), Co(II), Cd(II), and Zn(II) under reflux conditions, afforded corresponding ortho-formyl derivatives in good yields. Under normal conditions the metal-ion-free V-H reactions are too sluggish and resulted in poor yields. This protocol provides highly regioselective formylation under a mild and efficient condition with simple workup. 1. Introduction One of the most fascinating developments seen in organic synthesis during the recent years is the application of transition metals for ortho functionalization of aromatic compound [1]. Ever since the application of transition metals in organic reactions is introduced, a number of chemical reactions have been reported to functionalize nonreactive aromatic C¨CH bond ortho to hydroxyl and other functional groups [2]. Formylation of aromatic compounds is a classical reaction in inorganic chemistry and numerous methods are available [3]. The formyl substituted phenols are important compounds in organic chemistry [4], for instance substituted salicyladehydes (O-hydroxybenzaldehydes) are important intermediates in organic synthesis for the preparation of a variety of oxygen containing heterocyclic compounds and as a source for silane ligands [5], and only few of them are commercially available even today. Even though the synthesis of salicyladehydes from corresponding phenols has been carried out by various methods, most of them afforded only low yields, leading to deformylation or lack in regioselectivity [6]. The Vilsmeier-Haack reaction is a mild method for the introduction of a formyl group to various aromatic and heteroaromatic compounds [7, 8]. It is also utilized in the synthesis of a large number of heterocyclic compounds [9¨C11]. Some interesting cyclization reactions under Vilsmeier conditions have been reported recently [12¨C18]. It is seen that despite of good formylating ability of V-H reagent only sporadic reports are found on ortho-formylation of phenols using V-H reaction. One patent is reported in 1927 for synthesis of vanillin by formylation of Guaiacol with n-methyl formanilide-phosphoryl chloride complex [19, 20]. It is very well known that V-H reagent will produce aryl formats from phenols. In continuation of our ongoing work on V-H reactions under various modified conditions we have reported a certain V-H reaction in micellar media [21] and under ultrasonic conditions [22]. 2. Results and Discussion Recently we have found that metal ions are good promoters
%U http://www.hindawi.com/journals/oci/2012/289023/