The selective reduction of monosubstituted alkenes was similarly achieved by relying on e-HAT (Conditions C). Canonical hydrogenation with H2 over Pd/C can accomplish this kind of reduction; however, the chemoselectivity of that method can be poor when competing for reductively labile functionalities are present. Accordingly, the e-HAT was tested with substrates that can be challenging with such well-established reduction protocols. Thus, olefins containing amine-diol functionalities (45), trisubstituted alkenes (46, 47), Cbz-protected amines (48), benzyl-protected carboxylic acids (48), and thioanisole moieties (49), which can all be problematic under typical hydrogenation conditions, were smoothly reduced. Although classical HAT chemistry has been studied for over 30 years, precise control of chemoselectivity when multiple olefins are present in a substrate has remained underdeveloped due to the complexity of tuning the hydride donor, oxidant, and catalyst. The modularity of e-HAT can potentially address such a challenge to achieve unique and useful selectivity with polyunsaturated systems and was thus pursued using the three distinct e-HAT conditions (A-C) disclosed above. Compound 50, which contains two different mono-and disubstituted terminal alkenes, was chosen as a case study (Figure ). By applying Conditions B (Co(salen)-1/HFIP), the exocyclic alkene was selectively isomerized to form the thermodynamically favoured trisubstituted alkene product 51 in 59% yield. In contrast, exposing compound 50 to Conditions A (CoBr2/4,4'-MeO-bpy/Et3NHBF4) led to the formation of disubstituted alkene 52 in 92% yield with an E:Z isomeric ratio of 3:1. To date, such selectivity has not been reported using canonical HAT reactivity. Similarly, evaluation of the e-HAT alkene reduction displayed exquisite selectivity towards terminal monosubstituted alkenes. Triene 53 was subjected to Conditions C which selectively reduced the desired alkene -over the 1,1-disubstituted and endocyclic olefins -in 95% isolated yield (Figure ). Likewise, diene 55 was also examined under the same conditions and exhibited similar chemoselectivity towards the monosubstituted olefin in the presence of an internal disubstituted alkene to furnish 56 in 81% yield. As there are numerous alkene reduction methods available, the results of e-HAT could be placed into context through a direct comparison with the prior art. Using H2 with Pd/C, dimide, PET (Co-Salen with Ru catalyst and light) , and Co/H + -based systems resulted in an inseparable mixture of reduced or isomerized products (see SI).