Transition metal complexes with P,N-ligands and silylenes : synthesis and catalytic studies

We were interested to extend our library of P,N-ligands, and to investigate the influence of a
five-memebered ring-chelate, since most of the previously tested ligands form six-membered
ring-chelates. In consequence, twelve new phosphinomethyl-oxazolines and their
corresponding iridium-complexes were prepared and tested in the enantioselective
hydrogenation of unfunctionalized and functionalized olefins. The tetrasubstituted olefin 2-
(4’-methoxyphenyl)-3-methyl-2-butene was reduced with higher enantioselectivity than
reported for other iridium catalysts.
Another objective was to examine the effect of a strong π-accepting and planar phosphorusmoiety.
Three chiral phosphininoxazolines were prepared by dehydration of a water-adduct,
wherein the phosphorus atom is present as a secondary phosphine oxide. The corresponding
cationic iridium-complexes were synthesized. One of these complexes was also tested in the
iridium-catalyzed hydrogenation and transfer-hydrogenation showing good conversion, but
low to moderate enantioselectivity.
We were interested in the scope of iridium-PHOX complexes in other catalytic reactions.
Initial studies towards the application of this system in asymmetric catalytic Pauson-Khand
reaction were completed regarding pressure influence, reproducability and the influence of the
counteranion on the enantioselectivity of the reaction. 3-Phenylsubstituted enynes undergo
cycloaddition with good to very good yields and good enantioselectivities. Steric hindrance at
the alkene moiety leads to a strong drop in yield. Among the tested counteranions, small and
weakly-coordinating anions, such as triflate, tetrafluoroborate, hexafluorophosphate and
hexafluoroantimonate proved to give the highest enantioselectivities.
Inspired by the recent success of NHCs in the iridium catalyzed hydrogenation, we envisioned
the synthesis of iridium- and rhodium-complexes containing N-heterocyclic silylenes. Two
cationic rhodium(I)-silylene complexes 187 and 188 were prepared from [Rh(cod)2]BArF 185
and 1,3-di-tert-butyl-2,3-dihydro-1H-1,3,2-diazasilol-2-ylidene 171 and its saturated analogue
172, respectively (Figure 1). Only catalytically inactive complexes were formed. The
corresponding iridium-complexes were not accessible by the same method.