Cobalt- and Iron-Catalyzed Isomerization–Hydroboration of Branched Alkenes: Terminal Hydroboration with Pinacolborane and 1,3,2-Diazaborolanes

The synthesis and
characterization of a series of structurally
varied N-phosphinoamidinate-ligated cobalt complexes
is described, along with the successful application of these and a
related iron complex as precatalysts in the isomerization–hydroboration
of terminal, geminal, and internal alkenes. These reactions proceed
under mild conditions (23–65 °C), at relatively low base-metal
loadings (1–5 mol %), typically without cosolvent, and with
high terminal hydroboration selectivity across a broad spectrum of
branched alkenes. With some of the alkene substrates examined, the N-phosphinoamidinate-ligated precatalysts employed herein
are shown to provide alternative terminal selectivity versus other
previously reported precatalyst classes for such transformations.
Reports of terminal-selective metal-catalyzed alkene isomerization–hydroboration
disclosed thus far in the literature employ pinacolborane (HBPin);
while effective in the system herein, we also report the first examples
of such transformations employing either 1,3-dimethyl-1,3-diaza-2-boracyclopentane
or benzo-1,3,2-diazaborolane. The application of these 1,3,2-diazaborolanes
in place of HBPin in some instances enables novel terminal selectivity
in the isomerization–hydroboration of branched alkenes.