Ir/Thioether–Carbene, −Phosphinite, and −Phosphite Complexes for Asymmetric Hydrogenation. A Case for Comparison

We
studied for the first time the potential of novel and simple
Ir/thioether-NHC complexes in the asymmetric hydrogenation of unfunctionalized
olefins and cyclic β-enamides. For comparison, we prepared and
applied the analogues thioether–phosphinite/phosphite complexes.
We found that the efficiency of the new Ir/thioether-NHC catalyst
precursors varies with the type of olefin. Thus, while the Ir/thioether-NHC
catalyst precursors provided lower catalytic performance than their
related Ir/thioether-P complexes in the hydrogenation of olefins lacking
a coordinating group, the catalysts had similar good performance for
the reduction of functionalized olefins (e.g., tri- and disubstituted
enol phosphonate derivatives). Catalytic results together with the
studies of the reactivity toward H2 indicated that the
thioether-carbene design favors the formation of inactive trinuclear
species, which are responsible for the low activities obtained with
these carbene-type catalysts. Nevertheless, this catalyst deactivation
can be avoided by using functionalized olefins such as enol phosphonates.
We also report the discovery of simple-to-synthesize Ir/thioether-P
catalysts containing a simple backbone that gave high enantioselectivities
for some trisubstituted olefins, some challenging 1,1′-disubstituted
olefins, and cyclic β-enamides.