a Cyclohexane was included as a 1 H NMR standard. In the reaction with 1,4cyclohexadiene (20 equiv) Bu4P + BF4 -was also included as a Please do not adjust margins Please do not adjust margins A reaction mixture of 1a and excess TEMPO, a common trap for radicals, was used to monitor evolution of methyl radical over time. Reaction progress of this mixture upon irradiation with blue LEDs was monitored using 1 H and 31 P NMR spectroscopy. Consumption of the starting material occurred cleanly along with formation of TEMPO-CH3 as the sole observable organic product in all cases (Figure ). No reactivity was observed in the absence of light when irradiation was intermittently ceased, demonstrating that the system is well-behaved and inconsistent with a sustained radical chain process. Additional experiments were performed to test the possibility of thermal reaction caused by heat from the LEDs, despite the use of cooling fans in these reactions. A mixture of 1a and TEMPO remained unchanged in the dark at RT after 5 h as observed by 1 H and 31 P NMR. Even heating at 60 °C for 1.5 h gave no significant spectroscopic changes (Scheme 4, top). In contrast, another sample of 1a and TEMPO kept at RT while irradiating for 1.5 h yielded a comparable amount of TEMPO-CH3 as the previous reactions (Scheme 4, bottom). These results support the requirement for visible light to form methyl radicals from 1a and the absence of a significant thermal background reaction. Considering the strong Pd(II)-CH3 bond was effectively cleaved, it should be expected other (weaker) Pd(II)-C(sp 3 ) bonds would also be susceptible to homolysis under blue light irradiation. To test this idea, a new T-shaped complex (4) bearing a neosilyl group was prepared. Complex 4 did indeed react with TEMPO upon blue LED irradiation and generated the corresponding radical trapping product TEMPO-CH2SiMe3 in nearly quantitative yield as determined by 1 H NMR (Scheme 5). Formation of an unidentified Pd product 5a (vide infra) in high yield was also observed by 31 P NMR. respectively. The yield of TEMPO-CH3 was also high in all cases (70-89%). Importantly, the 31 P resonance of product 5b does not match the reported shift for [( t Bu3P)Pd I (µ-Br)]2 6 (δP 86.3 ppm) indicating a unique Pd product formed under these conditions. Spiking the crude solution of 5b with an authentic sample of 6 led only to an increase in the intensity of the 31 P signal for 5b and no detectable 6, suggesting the Pd(I) species may be an intermediate on the path to formation of the unknown 5b.