hydroformylation product 1. While an array of protocols for the methylenic isomerization of ketones, esters, amides, and styrenes have been reported, this type of isomerization with aldehyde substrates was unknown (Fig ). This is presumably due to challenges associated with conducting metal hydride mediated isomerizations in the presence of a reducible aldehyde unit and the lack of strong thermodynamic driving force to favour the product. Our computational estimates suggest the energy difference between enal 1 and 2 is only ~1 kJ/mol in MeCN (see the SI for details). Given the utility of enals as synthetic intermediates, the discovery of a general, efficient process to isomerize easily synthesized methylenic enals into more highly substituted products would be valuable. Here we show that Co(dmg)2-type catalysts promote such enal isomerizations under H2 to give trisubstituted products in yields >80%, at low catalyst loadings (0.1 mol%), and with minimal alkene or aldehyde reduction. Comprehensive reaction screening and mechanistic studies showed the classical isomerization catalyst RhCl3 was a reasonable candidate to promote the desired enal isomerization. In buffered ethanol/dioxane catalyzed the isomerization of enal 1 to target 2 in ~50% yield was observed (Fig ). Further improvements could not be achieved because the enals were found to be in equilibrium and were unstable under the optimized conditions. Reactions under more forcing conditions led to enal hydrogenation, while reactions conducted with a 1:1 mixture of 1 and 2 quickly showed ratios of 1:2 = 33:67. These studies show the challenges associated with developing high yielding isomerizations of sensitive enals like 1 (see the SI for additional screening data). It was not until a hydrogen atom transfer (HAT) approach using Co(dmg)2-type catalysts , was taken that trisubstituted enal 2 could be obtained in good yield (Fig 2b). Reactions with Co(dmgBF2)2(H2O)2 proceed quickly to ~90% conversion and 80% yield of 2 under 75 psi H2. Alkene hydrogenation is slow and saturated aldehyde 3 is only observed in >10% well after the isomerization is complete. Reactions could be conducted at 0.1 mol% of Co(II)-catalyst and still gave good yields (83%).