Under nitrogen atmosphere, treatment of 4-phenyl-1-butyne 1a with benzyl 2-diaxo-2-phenylacetate 2a in the presence of Co(OAc)2 and N-oxazolinylphenyl 8-aminoquinoline (OPAQ) L1 as cocatalysts and phenyl silane and H2O as hydrogen donor, delivered the desired product 3a in 83% yield. Control experiments showed that all of the reagents were necessary for the reaction to occur.

The practicality of this new alkyne functionalization methodology was examined with gram scale synthesis of 3a. Further transformations of the semi-hydrogenation/hydrohydrazidation products into hydrazine hydrochloride 5, pyrazole 6, 1,3-disubstituted indazole 7 and even the protected primary amines 8a-8b, and an antihistamine drug promethazine 9a was also synthesized by the simple three-step operation.

Scheme 4. Further transformations of the obtained semi-hydrogenation/hydrohydrazidation products.

To gain insights into the reaction mechanism, several control experiments were conducted, as well as deuterium labelling and radical clock experiments. The preliminary data suggested a cobal-hydride species being the catalytic reactive species, which is responsible both for the semi-hydrogenation of alkynes and for the hydrohydrazidation of the in situ generated alkenes.

Scheme 5. Mechanstic studies and a plausible reaction mechanism.