Recently, some chemists found that many reactions proceed efficiently in the solid surfaces. Indeed, in many cases, surfaces of solids have properties that are not duplicated in the solution or gas phase, entirely new chemistry may occur. Even in the absence of new chemistry, a surface reaction may be more desirable than a solution counterpart, because the reaction is more convenient to run, or a high yield of product is attained. For these reasons, synthetic surface organic chemistry is a rapidly growing field of study. Experiments using these solid phase catalysts generally have the following features; (i) it is often easy to isolate the products and to separate the catalyst; (ii) comparing the reaction conditions with those of related homogeneous reactions, they are so mild that a high yield of specific products and suppression of by-product formation are expected; (iii) selectivity and activity of the catalysts are often comparable to those of enzymes. Several classes of solids have commonly been used for surface organic chemistry including aluminas, silica gels, and clays. Basic alumina, the material used commonly for column chromatography, is certainly one of the most interesting of these solids because it has surface properties that suggest a very rich organic chemistry may occur there. This report describes the efficient application of basic alumina and PCl5 in synthesis of nitriles directly from aldehydes. Nitriles are of particular interest in preparative organic chemistry due to their rich chemistry. They serve as useful precursors for the synthesis of amines, carboxylic acids, amides, ketones, and hetrocyclic compounds such as tetrazoles, thiazoles, oxazoles, 2-oxazolines and 1,2diarylimidazoles. It has also been well documented that the cyano group itself is present in HIV protease inhibitors, 5lipoxygenase inhibitors, and many other bioactive significant molecules. They are usually prepared by nucleophilic substitution with the cyanide anion or by regenerating the cyano group via oxidation, rearrangement, or elimination. The conversion of aldehydes into nitriles is a useful transformation and a topic of current interest to organic chemists. As a result, a number of reagents have been emerged for this purpose, such as triethylamine sulfurdioxide, sulphuryl chloride fluoride, montmorillonite KSF, formaldehyde, etc. However some of these methods suffer from disadvantages such as, preparation of triethylamine sulf uryldioxide and sulphuryl chloride fluoride is inconvenient (−70 C), dehydration with KSF, zeolite, and envirocat EPZG requires high temperature or long reaction times. Therefore, we reasoned that use of an immobilized system, via the application of solid phase reagents, could lead to a more efficient and cleaner route to these important materials. Here, we decided to apply an inexpensive and environmentally friendly catalyst, basic alumina, for the preparation of nitriles from aldehydes in one pot without solvents (Scheme 1). Alumina / PCl5 was shown to have a remarkably high activity for the conversion of alkyl, aryl and heterocyclic aldehydes into nitriles in high yields, without any of the environmental disadvantages of using toxic solvents. In a typical experiment, aldehyde, alumina, hydroxylamine hydrochloride and phosphorus pentachloride (PCl5) were mixed thoroughly. The mixture was heated in an oil bath at 120 C without use of any solvents for the appropriate time (Table 1). The products obtained were analyzed by IR, and NMR spectroscopy, and by direct comparison with authentic samples. The mechanism of the reaction could be briefly proposed
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Jan 8, 2009
Vanchiappan Aravindan + 1
Open Access