Butynes Hydrogenation with Parahydrogen over Immobilized Iridium Catalyst

The Ir–P@SiO₂ catalyst was prepared by immobilizing a dimeric Ir complex via interaction with –PPh₂ groups of 2-diphenylphosphinoethyl-functionalized silica gel. The catalyst was tested in hydrogenations of butynes (1-butyne and 2-butyne) with parahydrogen. The experimentally observed ¹H NMR spectra of hyperpolarized products are well-fitted with theoretical PASADENA spectra simulated with the use of spin density matrix formalism. The analysis of ¹H NMR spectral lines of the products reveals that the catalyst is stereoselective in such reactions and the syn-addition of p-H₂ takes place. Moreover, we demonstrate that the lineshape of hyperpolarized NMR signals of cis-2-butene is indicative of the mechanism of its formation. It was found that 2-butene produced during 1-butyne hydrogenation is formed from hyperpolarized 1-butene via isomerization. The maximum observed ¹H polarization levels for 1-butene were 5.2 ± 0.5%.