The amount of labeling in both 1 and 2 increases even after isomerization to 2 reaches the maximum yield. When the enal product 2 is subjected to the standard isomerization conditions under D2, 9% of 1 is formed with partial D-incorporation. Recovered 2 is labelled at both the methyl and alkenyl positions. The extent of labeling suggests a rate of isomerization that is bracketed by the rate of Co-DAT catalyst formation by reaction with D2 and "Co-D" (the exact structure of the reactive hydride species generated from Co(dmg) catalysts is not resolved) is generated reversibly. If the active Co-DAT catalyst was generated only in an initial reaction with D2, product and starting material would be labelled in ~2.5% (i.e. the catalyst loading), while if the "Co-D" was in rapid equilibrium with Co ½ D2, the product and substrate would converge and become completely labelled at the alkenyl and allylic positions. Thus, the lower extent of deuteration of the internal double bond reflects its less preferential reaction with "Co-D".