SGBD paralelo

The ability of photoredox catalysts to initiate organic transformations has been employed by utilizing the reductive quenching cycle; however, competing side reactions (i.e. hydrogen atom abstraction and enamine coupling)48 from reactive intermediates in the intermolecular processes hinder the completion of the desired transformation. Alternatively, the oxidative quenching pathway has been explored to suppress this issue and to develop highly useful synthetic methods. In 2012, the Yoon group employed an oxidative quenching cycle to engage electron-rich bis(styrenes) in [2+2] cycloadditions via visible light photocatalysis (54-92% yield).21 Methyl viologen (MV2+) was selected to oxidatively quench Ru(bpy)32+* to generate the

strongly oxidizing Ru(bpy)33+ species. Subsequent oxidation of the electron-rich styrene 27

provides its radical cation 28 along with regenerating the ground state photocatalyst Ru(bpy)32+.

The radical cation then undergoes a [2+2] cycloaddition followed by reduction to furnish the cis- substituted cyclobutane adduct 29 in excellent yield. A limitation of the scope is the requirement

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of at least one of the styrenes to bear an electron-donating substituent (typically a methoxy group) at the para or ortho position. Notably, the efficiency of the Ru(bpy)32+/MV2+ system to

promote the radical cation mediated cycloaddition was highlighted on a gram scale, completed in 2.5 h while providing identical yields to the smaller-scale experiments.

Previously, the Akita group used the atom transfer radical addition (ATRA) method to develop a photoredox-catalyzed trifluoromethylative difunctionalization of olefins using electrophilic CF3 reagents such as Umemoto’s reagent (sulfonium salt) and Togni’s reagent

(hypervalent iodine species).49 Their success inspired them to expand the method to aminative difunctionalization of olefins. An application of utilizing the oxidative quenching cycle was disclosed by the group for the intermolecular aminotrifluoromethylation of alkenes catalyzed by Ru(bpy)3(PF6)2 (Scheme 1.8, Eq.1).50 This photocatalytic method enabled rapid access to CF3-

containing derivatives, which are structurally important in bioactive compounds. Driven by visible light, highly efficient and regioselective functionalization of C=C bonds were

accomplished to yield a range of β-trifluoromethylamines. The substrate scope included terminal alkenes, specifically styrene derivatives, and internal alkenes. They propose that the

photoexcited Ru(bpy)32+* is oxidatively quenched by Umemoto’s reagent 30, thus generating the

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trifluoromethyl radical (•CF3). Interception of the β-trifluoromethylated carbocation 31 and

hydrolysis via Ritter—type amination affords the aminotrifluoromethylated adduct 32. Recently, the authors employed a similar strategy for the intermolecular aminohydroxylation of olefins shown in Scheme 1.8, Eq. 2.51 This route is an alternative to the Os-catalyzed system developed by Sharpless that uses toxic Os species.52 Synthesis of vicinal aminoalcohol derivatives 35 was realized by the regiospecific aminohydroxylation using a photocatalytic system. The key reagent, N-protected 1-aminopyridinium salt 33, serves both as an electron acceptor and an amidyl radical precursor in the presence of the strong photoexcited state reductant fac-Ir(ppy)3.

Most recently, the MacMillan group extended their development of engaging photoredox catalysis in organic transformations, particularly sp3 carbon-fluorine bond formation, by

unveiling a direct conversion of aliphatic carboxylic acids to the corresponding alkyl fluorides.53 Although important advances have been made in the development of sp3 C-F formation, it

remains challenging to develop methods possessing attractive features of high regioselectivity, operation simplicity, bond strength independence, and accessibility to inexpensive starting

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material. Their previous findings of a photon-induced decarboxylation strategy54 was a blueprint to their investigated decarboxylative fluorination of sp3 carboxylic acids, in which

Ir(dF(CF3)ppy)2(dtbbpy)PF6 was chosen as the optimal photocatalyst with blue LEDs as the light

source. Selecfluor was selected as the electrophilic source of fluorine. A proposed pathway detailed an oxidative quenching of photoexcited Ir(III)* [Ir(IV)/ Ir(III)* = -0.89V) by N-F bond of Selectfluor 38 (+ 0.33 V) via SET process to provide the strongly oxidizing Ir(IV) agent (Scheme 1.9). Subsequent oxidation of the aliphatic carboxylic acid 36, then decarboxylation generates the sp3-alkyl radical 41, while concomitantly reducing Ir(IV) to Ir(III) thus completing the catalytic cycle. The formed radical 41 can abstract a fluorine atom from Selectfluor 38 to afford the desired fluoroalkane 37, while producing the corresponding Selectfluor radical cation 39. The radical cation 39 can then serve as an appropriate electron acceptor to generate the strong oxidant Ir(IV) from Ir(III)* in subsequent photoredox cycles. The redox-neutral decarboxylative fluorination protocol successfully tolerated a wide range of substituted

carboxylic acids, including primary, secondary, and tertiary alkyl carboxylic acid to furnish alkyl fluorides in excellent yields (70-99%).

Lastly, another example of utilizing the oxidative quenching cycle was presented by the Stephenson group in a fac-

Ir(ppy)3 catalyzed radical

reductive dehalogenations of unactivated alkyl, alkenyl, and aryl iodides (Scheme 1.10).55 Unlike their previous reports of

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carbon-bromide bonds via the reductive quenching cycle, this method is driven by the strong reducing power of photoexcited state fac-Ir(ppy)3 (Ir(IV)/Ir(III)* = -1.73 V). Harnessing Ir(III)’s

capability of cleaving carbon-iodide bonds in alkyl, alkenyl, and aryl iodides is favorable due to their high negative reduction potentials that are measured between -1.59 V and -2.24 V. The resulting carbon-centred radical may undergo intramolecular cyclization and/ or hydrogen atom abstraction from tributylamine in combination with Hantzsch ester or formate. The reductive protocol exhibited excellent functional group compatibility and easy scale-up to provide good to excellent yields under mild conditions.

The recent contributions by the pioneering groups have demonstrated the broad utility of transition metal photoredox catalysis in organic synthesis by highlighting the various new modes of reactivity. The discussed synthetic applications of visible light photoredox catalysis

emphasize the unique dual nature of ruthenium and iridium complexes by employing either a reductive or oxidative quenching cycle. More importantly, the unique photophysical properties of photocatalysts and its ability to modulate permit challenging organic transformations to be accomplished. The relevance of photoredox catalysis in organic synthesis continues to grow and show great promises.