Tuning the\nPyridone Scaffold within a Rhodium-NHC\nPlatform for <i>gem</i>-Specific Alkyne Dimerization via\na Ligand-Assisted Proton Shuttle Mechanism

A series of mononuclear square-planar Rh{κ<sup>2</sup><i>N,O</i>-BHetA}(η<sup>2</sup>-coe)(NHC)\n(BHetA = Bis-Heteroatomic\nAcidato) complexes have been prepared. Modifications of the pyridonato\nBHetA-type ligand architecture include 4-Me, 5-Me, 6-Me, 3-Br, 4-Br,\n4-OMe, and 5-NO<sub>2</sub> substitutions as well as pyrimidonato,\nsuccinimidato, and 2-piperidonato catalysts. Two structural isomers\nhave been observed for the complexes, depending on the stereoelectronic\nproperties of the ligand. The structure–activity relationship\nhas been studied for <i>gem</i>-specific alkyne dimerization\nvia a cooperative ligand-assisted proton shuttle mechanism. Density\nfunctional theory calculations have revealed a mechanistic pathway\ninvolving the hemilabile coordination of the BHetA ligand, CMD deprotonation,\nπ-alkyne protonation, and reductive elimination. The increase\nin oxygen basicity imparted by the substituent in the pyridonato ligand\nis key, the 4-methyl derivative being the most active catalyst. However,\na favored iminol–amide tautomerization precludes an increase\nin catalytic activity for the more basic saturated piperidonato catalyst.