An Annelated Mesoionic Carbene (MIC) Based Ru(II) Catalyst for Chemo- and Stereoselective Semihydrogenation of Internal and Terminal Alkynes

The catalytic utility
of [RuL1(CO)2I2] (1),
containing an annelated π-conjugated imidazo-naphthyridine-based
mesoionic carbene (MIC) ligand (L1), is evaluated for E-selective alkyne semihydrogenation. The precatalyst 1, in combination with 2 equiv of AgBArF, semihydrogenates
a broad range of internal alkynes with molecular hydrogen (5 bar)
in water. (E)-Alkenes are accessed in high yields,
and a number of reducible functional groups are tolerated. A chelate
MIC ligand and two cis carbonyls provide a well-defined
platform at the Ru center for hydrogenation and isomerization. The
loss of two iodides and the presence of two carbonyls render the Ru
center electron deficient and thus the formation of metal vinylidenes
with terminal alkynes is avoided. This is leveraged for the semihydrogenation
of terminal alkynes by the same catalytic system in isopropyl alcohol.
Reaction profile, isomerization, kinetic, and DFT studies reveal initial
alkyne hydrogenation to a (Z)-alkene, which further
isomerizes to an (E)-alkene via metal-catalyzed Z → E isomerization.