In 2014, the Shenvi and Herzon groups independently published olefin hydrogenation by manganese-and cobalt-catalyzed mHAT respectively, trapping the transient alkyl radical with a second HAT step. 8b, e, f mHAT has some advantages over typical transition metalcatalyzed hydrogenations of olefins, which proceed by additions of H2. First, unactivated olefins react well in mHAT methods, enabled by the high reactivity (low bond dissociation energy) of the weak metal-hydride bond. The hydride source and reductant in mHAT is a silane or borohydride, 8a, 8b, 10 enabling selective monoreduction of a polyalkene via stoichiometry control, which would be more challenging using a traditional hydrogenation in which the reductant is 1 atm of H2 or solvent alcohol. Finally, the radical mechanism offers complementary selectivity relative to a typical transition-metal catalyzed mechanism, with improved chemoselectivity and preservation of sensitive functional groups. One problem with these single-catalyst mHAT systems (Scheme 1b) is that they require at least stoichiometric loading of an oxidant, such as tert-butyl hydroperoxide (TBHP), which is necessary to turn over the M 2+ /M 3+ cycle. While enabling, the inclusion of this oxidant reduces the atom economy of the reaction and can introduce some additional safety considerations. In 2020, West and coworkers addressed this problem with the cooperative hydrogen atom transfer (cHAT) method for the chemoselective hydrogenation of unactivated olefins (Scheme 1c). In cHAT, an Fe-based catalyst is complemented by a catalytic amount of a thiol that rapidly traps the alkyl radical to give the net hydrogenation product. The catalysts can be regenerated while avoiding the use of a stoichiometric oxidant. The cHAT system provides a facile method for olefin hydrogenation with up to 93% yield.