Room Temperature Iron-Catalyzed Transfer Hydrogenation and Regioselective Deuteration of Carbon–Carbon Double Bonds

An
iron catalyst has been developed for the transfer hydrogenation
of carbon–carbon multiple bonds. Using a well-defined
β-diketiminate iron­(II) precatalyst, a sacrificial amine and
a borane, even simple, unactivated alkenes such as 1-hexene undergo
hydrogenation within 1 h at room temperature. Tuning the reagent stoichiometry
allows for semi- and complete hydrogenation of terminal alkynes. It
is also possible to hydrogenate aminoalkenes and aminoalkynes without
poisoning the catalyst through competitive amine ligation. Furthermore,
by exploiting the separate protic and hydridic nature of the reagents,
it is possible to regioselectively prepare monoisotopically labeled
products. DFT calculations define a mechanism for the transfer hydrogenation
of propene with nBuNH2 and
HBpin that involves the initial formation of an iron­(II)-hydride active
species, 1,2-insertion of propene, and rate-limiting protonolysis
of the resultant alkyl by the amine N–H bond. This mechanism
is fully consistent with the selective deuteration studies, although
the calculations also highlight alkene hydroboration and amine–borane
dehydrocoupling as competitive processes. This was resolved by reassessing
the nature of the active transfer hydrogenation agent: experimentally,
a gel is observed in catalysis, and calculations suggest this can
be formulated as an oligomeric species comprising H-bonded amine–borane
adducts. Gel formation serves to reduce the effective concentrations
of free HBpin and nBuNH2 and
so disfavors both hydroboration and dehydrocoupling while allowing
alkene migratory insertion (and hence transfer hydrogenation) to dominate.