Highly regioselective radical alkylation of 3-substituted pyrroles

Guadarrama‐Morales, Oscar; Méndez, Francisco; Miranda, Luis D.

doi:10.1016/j.tetlet.2007.04.146

Cited by 42 publications

(19 citation statements)

References 23 publications

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“…As trapping reagents that avoid the formation of several cross‐coupling products, we selected mesitylacetate MesAc and N ‐methylpyrrole‐2‐carboxylate NMPCA. With the former, all ring positions except meta are occupied; with the latter, it is well known that the addition of radicals to pyrroles almost exclusively occurs at the ring position adjacent to the nitrogen atom . Proof of principle for the viability of our approach is obtained from the NMR spectra in SI‐5.3.…”

Section: Resultsmentioning

confidence: 99%

“…meta are occupied;with the latter,i tiswell known that the addition of radicals to pyrroles almoste xclusively occurs at the ring position adjacent to the nitrogen atom. [38][39][40][41][42][43] Proof of principle fort he viability of our approachi so btained from the NMR spectra in SI-5.3. In the case of MesAc, the deceleration of the traget reactionb yt he overcrowdedt rapping reagent only allows am oderate yield of the coupling product, with benzoate remaining the main product;h owever, the sterically less demanding NMPCA turns the table and approximately reverses the product distribution (Scheme3).…”

Section: Carbon-carbon Bond Formationsmentioning

confidence: 99%

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Micellized Tris(bipyridine)ruthenium Catalysts Affording Preparative Amounts of Hydrated Electrons with a Green Light‐Emitting Diode

Naumann

Lehmann

Goez

2018

Chemistry A European J

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We have explored alkyl substitution of the ligands as a means to improve the performance of the title complexes in photoredox catalytic systems that produce synthetically useable amounts of hydrated electrons through photon pooling. Despite generating a super-reductant, these electron sources only consume the bioavailable ascorbate and are driven by a green light-emitting diode (LED). The substitutions influence the catalyst activity through the interplay of the quenching parameters, the recombination rate of the reduced catalyst OER and the ascorbyl radical across the micelle-water interface, and the quantum yield of OER photoionization. Laser flash photolysis yields comprehensive information on all these processes and allows quantitative predictions of the activity observed in LED kinetics, but the latter method provides the only access to the catalyst stability under illumination on the timescale of the syntheses. The homoleptic complex with dimethylbipyridine ligands emerges as the optimum that combines an activity twice as high with an undiminished stability in relation to the parent compound. With this complex, we have effected dehalogenations of alkyl and aryl chlorides and fluorides, hydrogenations of carbon-carbon double bonds, and self- as well as cross-coupling reactions. All the substrates employed are impervious to ordinary photoredox catalysts but present no problems to the hydrated electron as a super-reductant. A particularly attractive application is selective deuteration with high isotopic purity, which is achieved simply by using heavy water as the solvent.

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Section: Resultsmentioning

confidence: 99%

Section: Carbon-carbon Bond Formationsmentioning

confidence: 99%

Micellized Tris(bipyridine)ruthenium Catalysts Affording Preparative Amounts of Hydrated Electrons with a Green Light‐Emitting Diode

Naumann

Lehmann

Goez

2018

Chemistry A European J

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“…Monitoring the yields of dechlorination and cross-coupling in parallela llows factoring out all the complex issues influencing the formation of the s radicals, such as catalyst stability and parasitic e À aq scavenging, because ac hloride ion is only liberated when a s radicali sb orn.H ence, the ratio of each pair of yields in Scheme 2, which for convenience has also been in-cluded in Table S2 of SI-4.2, specifies the fraction of the s radicals intercepted by the trapping reagent, with the only proviso that all end products arising through the adduct have been captured and summed. Thec ross-coupling product 6 represents the only case in point;t he pyrrole and indole derivates, which constitute the majority of trapping reagents in Scheme 2, are known to couple only in the ring position adjacent to the nitrogen; [26][27][28][29][30][31] and their substitution pattern allows no isomers, as is the case for mesityl acetate. The yield ratios so obtained exhibit ac lear dichotomy:t hey are near unity for uncharged trappingc omponents (products 1, 2, 8, 9,a nd 11) and half as large for the negatively charged ones.…”

Section: Laboratory-scale Synthesesmentioning

confidence: 99%

First Micelle‐Free Photoredox Catalytic Access to Hydrated Electrons for Syntheses and Remediations with a Visible LED or even Sunlight

Naumann

Goez

2018

Chemistry A European J

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Hydrated electrons are super-reductants, yet can be generated with visible light when two photons are pooled, most efficiently through storing the energy of the first photon in a radical pair formed by the reduction of an excited catalyst by a sacrificial donor. All previous such systems for producing synthetically useable amounts of hydrated electrons with an LED in the visible range had to resort to compartmentalization by SDS micelles to curb the performance-limiting recombination of the pair. To overcome micelle-imposed constraints on sustainability and applications, we have instead attached carboxylate groups to a ruthenium (tris)bipyridyl catalyst such that its pentaanionic radical strongly repels the dianionic radical of the bioavailable donor urate. We have explored the influence of the Coulombic interactions on the electron generation by a time-resolved study from microseconds to hours, including for comparison the unsubstituted complex, which forms a monocationic radical, with and without SDS micelles. The new homogeneous electron source is best with regard to stability and total electron output; it has a broader synthetic scope because it does not entail micellar shielding of less hydrophilic compounds, which particularly facilitates cross-couplings; and it tolerates supramolecular containers as carriers of water-insoluble substrates or products. As an application in the field, we demonstrate the solar remediation of a recalcitrant chloro-organic bulk chemical.

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“…Intermolecular radical alkylation of some 3-substituted pyrroles with xanthates mediated by dilauroyl peroxide occurs at C2 in moderate to good yields with high regioselectivity, as shown by preparation of the product 476 from 3-phenylpyrrole 477 (Scheme 4.150) [740].…”

Section: Addendummentioning

confidence: 99%