2- Butene-1,4-diones hydrogenation

Other PK variations include microwave conditions, solid-phase synthesis, and the fixation of atmospheric nitrogen as the nitrogen source (27—>28). Hexamethyldisilazane (HMDS) is also an excellent ammonia equivalent in the PK synthesis. For example, 2,5-hexanedione and HMDS on alumina gives 2,5-dimethylpyrrole in 81% yield at room temperature. Ammonium formate can be used as a nitrogen source in the PK synthesis of pyrroles from l,4-diaryl-2-butene-l,4-diones under Pd-catalyzed transfer hydrogenation conditions. 

Similar reaction conditions as those by Bose were used for a range of other applications, for example, the synthesis of heterocycles. A combination of a microwave-assisted Paal-Knorr reaction15 with a transfer hydrogenation takes place in the preparation of 2,5-di- and 1,2,5-trisubstituted pyrroles from -l,4-diaryl-2-butene-l,4-diones in a one-pot operation. Hydrogenation was achieved with ammonium formates and 10% Pd/C as catalyst in PEG-200. Yields of up to 92% were obtained within 0.5-2 min (Scheme 4.2)16. 

During the reaction, trialkyltin chloride is produced as the by-product. Tributyltin chloride is known to react with the palladium complex to produce the hydropalladium species via /3-hydrogen elimination. If the reaction takes a long time, 2-buten-l,4-dione moiety is reduced (Scheme... 

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