[a] 1 H NMR yields of enamine product 2a vs. mesitylene as internal standard in DMSO-d6. Isolated yield given in parentheses. tert-Butyl tetramethylguanidine (BTMG) gave the highest yield, affording 88% of product. This improvement is likely attributed to its strong basicity and the enhanced solubility of the Nickel source in DME. Nevertheless, combining BTMG with an Ir-based photocatalyst ultimately resulted in quantitative formation of the desired product (Entry 8). The product was isolated in 76% yield, with the loss attributed to the high volatility of the product. Although the less expensive 4CzIPN could be an alternative, challenges associated with column chromatography necessitated the use of easier to separate Ir((CF3)ppy)2(dtbbpy)PF6. Control experiments (Table , Entries 9-13) revealed that no product was formed in the absence of light, the Ir photocatalyst, or base showcasing that direct excitation is not feasible and initial deprotonation is essential. However, omitting the Nickel catalysts gave a low yield of 27% (Entry 12), which is likely due to the direct photo-oxidation of the alcohol. This observation is consistent with the visible-lightdriven photocatalytic oxidation of benzyl alcohol enabled using QDs, as well as our previous work on the meal-free oxidation of secondary alcohol to ketones. Moreover, the formation of 47% product in the absence of TphBr may be attributed to the low half-wave-potential E1/2 of the starting materials, which could act as sacrificial additive instead of the aryl bromide. With the optimized conditions in hand, we explored the scope of the reaction with a variety of α-hydroxy acids, including tertiary and quaternary substrates (Scheme 1). Simple Mandelic acids bearing both electron-donating and -withdrawing functional groups were applicable to our methodology respectively, affording products 2b to 2i in excellent yields. Remarkably, substrate 2e, which contains a free hydroxyl group, was tolerated albeit affording only a moderate yield for Vanillin. In contrast, substrate 2f, which contains an aryl bromide, competes in the oxidative addition step, resulting in a significantly reduced yield of the desired aldehyde. As anticipated, 4-arylated Mandelic acids were well tolerated, and 2g to 2i gave good to excellent isolated yields. Furthermore, quaternary α-cyclohexyl or α-cyclopentyl Mandelic acid and benzilic acid, exhibited quantitative yields of product 3a-3c, likely due to the formation of more stabilized tertiary radical intermediates. A scale up of the reaction for substrate 3b on a 1.5 mmol scale produced a comparable outcome, affording an 88% yield and 230 mg of the desired product. Based on the results above, we expected that carboxy fluorenol 4 would undergo decarboxylative oxidation. However, no oxidation product was observed, and the reaction stopped at the formation of fluorenyl alcohol in 90% yield (Scheme 2A). Overall, we propose that following the initial decarboxylation by the photocatalyst, α-hydroxy radicals are efficiently intercepted by the Ni(II) catalysts, obtained from oxidative addition into the bromothiophene. The subsequent dehydrogenation of the hydroxy groups either occurs either from a Nickel β-hydride elimination, a base-assisted oxidation process, or halide radicalmediated hydrogen atom transfer step. The turn-over of the Nickel catalyst likely occurs from reductive elimination of the dehalogenated thiophene, as confirmed by GCMS, and subsequent reduction to Ni(0) by the reduced photocatalyst. Indication for such a radical mechanism was provided by TEMPO trapping experiments, which yielded a diminished 32% of product 2a with 5 equiv (Scheme 2B). Nonetheless, TEMPO can also act as redox mediator in alcohol oxidation reactions. A TEMPO-adduct 2a' was confirmed by GC-HRMS, with an NMR yield of 27%. An initial decarboxylation event is crucially required for achieving high dehydrogenation reactivity (Scheme 2C). Only traces of benzyl alcohol were converted to benzaldehyde (2b), which is also known as a common sidereaction under photochemical control.