Metal–ligand cooperative proton transfer as an efficient trigger for rhodium-NHC-pyridonato catalyzed gem-specific alkyne dimerization

The mononuclear square-planar Rh{κ2-X,N-(Xpy)}(η2-coe)(IPr) (X = O, NH, NMe, S) complexes have been synthesized from the dinuclear precursor [Rh(μ-Cl)(IPr)(η2-coe)]2 and the corresponding 2-heteroatom-pyridinate salts. The Rh-NHC-pyridinato derivatives are highly efficient catalysts for gem-specific alkyne dimerization. Particularly, the chelating N,O-pyridonato complex displays turnover frequency levels of up 17 000 h–1 at room temperature. Mechanistic investigations and density functional theory calculations suggest a pyridonato-based metal–ligand cooperative proton transfer as responsible for the enhancement of catalytic activity. The initial deprotonation of a Rh-π-alkyne complex by the oxo-functionality of a κ1-N-pyridonato moiety has been established to be the rate-limiting step, whereas the preferential protonation of the terminal position of a π-coordinated alkyne accounts for the exclusive observation of head-to-tail enynes. The catalytic cycle is closed by a very fast alkenyl–alkynyl reductive elimination.
Financial support from the Spanish Ministerio de Ciencia e Innovación (MICINN/FEDER) under the Projects PID2019-103965GB-I00 and PGC2018-099383-B-I00, and the Diputación General de Aragón (FEDER 2014-2020 “Building Europe from Aragón”, group E42_20R) are gratefully acknowledged. A.D.G. thanks the Spanish Ministerio de Economía y Competitividad (MINECO) for the postdoctoral grant Juan de la Cierva - Incorporación 2015 (IJCI-2015-27029). A.U. thankfully acknowledges the Spanish MECD for a FPU fellowship (FPU 2017/05417). The authors would like to acknowledge the use of Servicio General de Apoyo a la Investigación-SAI, Universidad de Zaragoza and the computational resources provided by the Institute for Biocomputation and the Physics of Complex Systems (BIFI)–Universidad de Zaragoza.