The key catalytic step in this process involves how the catalytic cycles are closed. The atom-transfer pre-catalyst, either , which concurrently regenerates the ground state photocatalyst A-iii and the active group transfer species, namely BPA•Cu(II)-Cl2 and [(bpmen)Fe(III)Cl2]Cl, both of which have been isolated and characterized through X-ray diffraction of a single crystal. Our previous studies on the mechanism of azido-arylation reaction indicated the generation of active azide transfer catalyst, namely [BQA•Cu(II)(N3)2], relies on a bridging azide ligand to shuttle an electron from the BQA•Cu(I)-species to the [Cu(II)(dpp)2] 2+ complex, which also returns ground-state photocatayst . However, such inner-sphere electron-transfer (ISET) was not viable in our studies on the aryl-chlorination of vinylboronic esters, wherein [(bpmen)Fe(II)Cl2] and [Ru(II)(bpy)3] 2+ are used to facilitate the reaction; the coordinatively saturated nature of the Ru(III)-bpy complex prevents the formation of a bridging chloride ligand . Hence, in the current catalytic systems, understanding the mechanism through which redox balancing is leveraged between the oxidized photocatalyst, the two discrete chlorine-transfer pre-catalysts and the reaction anion is crucial for the wider applications of this type of reaction. First, we explored under what conditions the closing of the photocatalytic cycle could be attained under the two different reaction regimes. In the case of the Cu/Cu system favored for electron deficient alkene aryl-chlorination, cyclic voltammetry measurements suggested that BPA•Cu(I) species should be sufficiently reactive to reduce [Cu(II)(dpp)2] 2+ and regenerate the ground state photocatalyst A-iii. However, treatment of [Cu(II)(dpp)2](ClO4)2 (prepared independently to represent the oxidized photocatalyst, E1/2 [Cu(II)/Cu(I)] = +0.36 V vs Fc + /Fc in MeCN) with Cu(I)-atomtransfer pre-catalyst species [BPA•Cu(I)] (E1/2 [Cu(II)/Cu(I)] = +0.36 V vs Fc + /Fc in MeCN) did not result in the formation of the ground state Cu-photocatalyst (A-iii), strongly suggesting that thermodynamic redox balancing was not sufficient to close the photocatalytic cycle (entry i, Figure ) and was consistent with our observations for the related alkene azido-arylation reaction with the same catalyst pair. As expected, however, addition of 20 equivalents of KCl to the same reaction resulted in the formation of an 80% yield of the ground state Cu-photocatalyst (A-iii) (entry ii, Figure ) and supports a mechanism involving an ISET pathway: a single electron is shuttled from the BPA•Cu(I) species to [Cu(II)(dpp)2] 2+ via a bridging chloride ligand. Meanwhile, a separate control experiment with oxidized photocatalyst surrogate [Cu(II)(dpp)2](ClO4)2 and excess KCl led to partial displacement of dpp ligand from the Cu(II)-center (~12% free dpp observed by 1 H NMR) with no formation of [Cu(I)(dpp)2] + observed (entry iii, Figure ), which suggests the chloride anion alone is not a competent reductant for regeneration of the ground state photocatalyst. In the case of the Cu/Fe catalyst system, treatment of the same oxidized Cu-photocatalyst with the Fe(II)-atom-transfer pre-catalyst, [(bpmen)Fe(II)Cl2] (E1/2 [LFe(III)/LFe(II)] = -0.08 V vs Fc + /Fc in MeCN) resulted, directly, in a 75% yield of the ground state Cuphotocatalyst (A-iii), suggesting that thermodynamic redox balancing of the catalytic cycles was operational (entry iv, Figure ). Interestingly, addition of 20 equivalents of KCl to this reaction resulted in almost quantitative formation of the ground state Cu photocatalyst A-iii (entry v, Figure ). While the oxidation potentials of the metal species involved are clearly capable of balancing the catalytic cycles through a thermodynamic pathway, the enhanced yield of A-iii observed in the presence of excess KCl suggests that an ISET pathway may also be operational, and that their impact on reaction outcome cannot be distinguished. An important take home message from these studies suggests that, while the requirements of mediating the redox state of the two catalytic cycles in the presence of the BPA•Cu(I) group transfer pre-catalyst may preclude nucleophiles of non-bridging ligation mode, a broader range of nucleophiles could be transferred with the Fegroup transfer system since the closure of both photocatalytic and catalytic group-transfer cycles could be simply driven by the complementary redox potentials of the catalytic species. Together, these features should broaden the scope of future reactions. Next, we sought to gain more insight into the nature of the species driving the group transfer for the carbon-chlorine bond formation. Looking first at the Cu/Cu system, a chloride-bridged dimeric Cu(II)-complex [Cu(II)(BPA)(μ(1,1)-Cl)Cl]2 (4) was prepared and isolated in its crystalline form to represent the active chlorine-transfer species, BPA•Cu(II)-Cl2. Exposure of 4 to azo-bis-isobutyronitrile (AIBN) under thermal condition in MeCN-d3 led to the formation of 2-chloro-2-methylpropanenitrile (5) (Figure ), an expected product from the chlorination of 2-cyanopropyl radical (int-V), strongly implicating the involvement of 4 in chlorine-transfer. Meanwhile, chlorine-transfer to int-IIB should reduce 4 to a putative Cu(I)-BPA species, which is supported by UV-Vis spectroscopy. Comparison of a 'post reaction' mixture of thermal chlorinetransfer experiment and a Cu(I)-complex independently prepared from [Cu(I)(CH3CN)4](PF6) and BPA (peaks at λ= 472 nm) shows a clear overlap in the spectral trace. Further UV-Vis measurements of Cu(I)-complexes obtained from [Cu(I)(CH3CN)4](PF6) and BPA with or without excess KCl (20 equivalent) displayed identical absorption properties, indicating the predominance of a cationic [(BPA)Cu(I)] + chlorine-transfer pre-catalyst C. A similar set of experiments were also performed with [(bpmen)Fe(III)Cl2]Cl (6) in our previous mechanistic studies towards the aryl-chlorination of vinyl boronic esters, which confirmed the role of bpmen•Fe(III) species as an active chlorine atom-transfer catalyst 14 . In conclusion to our mechanistic studies, under the optimal reaction conditions for each dual catalysis pair (Cu/Fe and Cu/Cu), the mechanism (outlined in Figure ) is proposed to start with visiblelight excitation of the Cu(I) photocatalyst, which is then oxidatively quenched by the diphenyliodonium triflate 1a to form the aryl-radical int-I and an oxidized [Cu(II)(dpp)2] 2+ species. The aryl radical can undergo addition to C-C double bond to form the homobenzylic radical int-II. Following the formation of the active Fe(III) (Figure ) or Cu(II)(Figure ) chlorine-transfer species, transfer of a chlorine atom to the homobenzylic radical int-II forms the desired aryl-chlorination product.