Using diphenylsilane (Ph 2 SiH 2 ) several sources of nickel were screened (Table ). Of these, NiBr 2 (dme) gave the best results (entry 6). Surprisingly, the nickel(0) source Ni(cod)2 (entry 7) led to only 15% of the aldehyde (by NMR). The nature of the counterion associated with the initial nickel salt also seemed crucial, as switching from bromide to chloride (i.e., NiCl2(dme)) gave inferior results. Insofar as other ligands are concerned, dtbbpy proved to be the most effective on Ni in catalyzing these reductions. This observation may be reflective of the ease in the reductive elimination step of the catalytic cycle due to increased electron density as well as the bulkiness imparted by the t-butyl groups. Nickel-based catalysts are known to potentially lose activity (i.e., are poisoned) resulting from metal chelation by the presence of heteroatoms in the starting materials, or products/by-products formed. Under these aqueous conditions, a similar observation was made due to chelation of Ni by the 2-mercaptopyridine released from DPDTC. Addition of zinc chloride was very effective as the thiol scavenger, leading to the complete reduction of the acid to the aldehyde in high yield (by NMR). While zinc bromide gave similar results, other thiol scavengers like copper thiocarboxylate (CuTC), CuMeSal, and N-ethylmaleimide (see ESI) resulted in no product being formed. To neutralize the HCl released by ZnCl2, several bases were examined. 2,6-Lutidine and 2,4,6-collidine gave similar results, whereas triethylamine and Hunig's base gave inferior results. 2,4,6-Collidine, therefore, was chosen over lutidine because it afforded better emulsification properties of the aqueous reaction mixture. Inorganic bases were not considered because of possible precipitation with 2-mercaptopyridine, resulting in inadequate stirring of the reaction mixture. Lowering the catalyst loading to 5% Ni(II) still provided sufficient reactivity (see ESI), although this was substrate-dependent. Control experiments confirmed that both the nickel catalyst as well as zinc dust were essential for the reaction to occur. When the reaction was carried out in the absence of base, the yield dropped. In terms of temperature, at 60 o C a slightly lower yield was observed due to hydrolysis of the thioester. Based on these studies the optimized conditions were determined to be: reactions are run at 40 o C at 0.5 M in 2 wt % TPGS-750-M/H2O, using 5-10 mol % NiBr2(dme) as the pre-catalyst, 1.2 equiv of zinc chloride as a thiol scavenger, and 2.5 equiv of the base and reducing agent.