Selecting Double Bond Positions with a Single Cation-Responsive Iridium Olefin Isomerization Catalyst

The catalytic transposition of double
bonds holds promise as an
ideal route to alkenes of value as fragrances, commodity chemicals,
and pharmaceuticals; yet, selective access to specific isomers is
a challenge, normally requiring independent development of different
catalysts for different products. In this work, a single cation-responsive
iridium catalyst selectively produces either of two different internal
alkene isomers. In the absence of salts, a single positional isomerization
of 1-butene derivatives furnishes 2-alkenes with exceptional regioselectivity
and stereoselectivity. The same catalyst, in the presence of Na+, mediates two positional isomerizations to produce 3-alkenes.
The synthesis of new iridium pincer-crown ether catalysts based on
an aza-18-crown-6 ether proved instrumental in achieving cation-controlled
selectivity. Experimental and computational studies guided the development
of a mechanistic model that explains the observed selectivity for
various functionalized 1-butenes, providing insight into strategies
for catalyst development based on noncovalent modifications.