Photochemical Dehalogenation of Aryl Halides: Importance of Halogen Bonding

Abstract

Upon UVA irradiation, aryl halides can undergo dehalogenation in presence of bases and methanol as a hydrogen donor. This catalyst-free photochemical dehalogenation is furnished through a facile radical-chain reaction under mild conditions. The chain reaction follows UVA irradiation of the reaction mixture in a transition metal-free environment. Mechanistic studies support a chain mechanism in which initiation involves absorption by a methoxide-bromoarene complex facilitated by halogen bonding interactions. The methoxide-bromine interaction leads to a weakened Br-C bond that is prone to facile cleavage during the initiation and propagation steps.

Coupling photocatalytic water oxidation with reductive transformations of organic molecules

“…In addition, some of EWG-substituted bromobenzenes underwent the transformation to provide moderate to high yields of coupling products in the case of reducing the reaction time to 10 h or decreasing the incident light intensity to 0.10 W/cm 2 (see 2b , 2c , 2 f, and 2 g ). These results reveal that the presence of the EDG would decrease the substrate reactivity, which has been observed in literatures where homolytic cleavage of phenyl C-Br bonds begins with a single electron transfer (SeT) from the promoter to the substrate 53 .

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“…Strangely, aryl iodides gave the targeted product in lower yields with lower conversions although we and other chemists provided evidence that aryl iodides are more reactive than aryl bromides (see Fig. 6a ) 39 , 40 , 53 . We inferred that the resulting I − from the coupling of iodobenzenes would block the present reaction by poisoning our photocatalyst.…”

Coupling photocatalytic overall water splitting with hydrogenation of organic molecules: a strategy for using water as a hydrogen source and an electron donor to enable hydrogenation

“…In theory, such an electronic effect agrees with the SeT mechanism because an increase in the electron density of the aromatic ring would accelerate the SeT from the catalyst to the substrate by improving the electron-accepting capacity of the substrate. 67–69 Moreover, such an electronic effect has been authenticated in previous literature as SeT-mediated cleavage of aryl C–Br bonds. 67–69 In contrast, oxidative metal addition-mediated reactions have a different electronic effect: electron-deficient aryl halides have lower reactivity.…”

“…67–69 Moreover, such an electronic effect has been authenticated in previous literature as SeT-mediated cleavage of aryl C–Br bonds. 67–69 In contrast, oxidative metal addition-mediated reactions have a different electronic effect: electron-deficient aryl halides have lower reactivity. 71,72 In summary, the observed electronic effect under our conditions is consistent with SeT rather than the oxidative metal addition mechanism, which confirms the reliability of our conclusion that the present reactions proceed via the SeT mechanism.…”

A photoresponsive palladium complex of an azopyridyl-triazole ligand: light-controlled solubility drives catalytic activity in the Suzuki coupling reaction

“…Such photoinduced dehalogenations are precedented in the literature and likely proceed via the formation of an aryl radical that abstracts one hydrogen from the solvent. [31][32][33] Due to the larger conversions in response to increased light intensity along with decreased selectivity toward the production of the coupled product, we explored how shorter irradiation times would affect the conversion and the product ratio. For this, aer the addition of the cis-rich 5-Pd solution, the reaction mixture was irradiated for 30 min (30 W), and then the light was switched off for the rest of the reaction time.…”