Tuning Catalytic Activity in the Hydrogenation of Unactivated Olefins with Transition-Metal Oxos as the Lewis Base Component of Frustrated Lewis Pairs

The
steric and electronic demands of the catalytic olefin hydrogenation
of tert-butylethylene with oxorhenium/Lewis acid
FLPs were evaluated. The sterics of the ligand were altered by installing
bulkier isopropyl groups in the 2,6-positions of the diamidopyridine
(DAP) ligand. Lewis acid/base adducts were not isolated for complexes
with this ligand; however, species incorporating isopropyl groups
were still active in catalytic hydrogenation. Modifications were also
made to the Lewis acid, and catalytic reactions were performed with
Piers’ borane, HB­(C6F5)2,
and the aluminum analogue Al­(C6F5)3. The rate of catalytic hydrogenation was shown to strongly correlate
with the size of the alkyl, aryl, or hydride ligand. This was confirmed
by a linear Taft plot with the steric sensitivity factor δ =
−0.57, which suggests that reaction rates are faster with sterically
larger X substituents. These data were used to develop a catalyst
((MesDAP)­Re­(O)­(Ph)/HB­(C6F5)2) that
achieved a TON of 840 for the hydrogenation of tert-butylethylene at mild temperatures (100 °C) and pressures (50
psi of H2). Tuning of the oxorhenium catalysts also resulted
in the hydrogenation of tert-butylethylene at room
temperature.