Molybdenum(0)-Catalyzed Tandem Cyclization Reactions of Aminoalkynes with Diketones for Multiply Substituted Azaheterocycles.

An efficient new molybdenum(0)-catalyzed tandem cyclization reaction to give structurally diverse azaheterocycles is presented here. The reaction between a terminal 1,4-aminoalkyne and acetylacetone in the presence of 5 mol % of [Mo(CO)4(COD)] as a catalyst in CH3CN was optimized to yield its multiply substituted indoline derivative 3a in an excellent yield. The analogous reaction of a terminal 1,5-aminoalkyne with acetylacetone yielded its multiply substituted tetrahydroquinoline 7 in a moderate yield. Various derivatives of indolines (18) and tetrahydroquinolines (5) were isolated. In both types of reactions, regioselective products (72-95% for indolines) were obtained using unsymmetrical 1,3-diketones and their structures were confirmed by the X-ray diffraction method. Products 3a and 7a were also prepared using water as a solvent. The HRMS (ESI+) spectra provide evidence for the enamine, methyl ketone, and diol in their catalytic reaction mixtures. A plausible mechanism involving the intramolecular hydroamination, a nucleophilic attack of an enamine on a 1,3-diketone leading to a diol formation followed by the elimination of water is proposed based on several control reactions, including reactions with the isolated iminium ion salt.