Benzyl radical addition reaction through the homolytic cleavage of a benzylic C–H bond

Ueda, Masafumi; Kondoh, Eiko; Ito, Yuta; Shono, Hiroko; Kakiuchi, Maiko; Ichii, Yuki; Kimura, Takahiro; Miyoshi, Tomomitsu; Naito, Takeaki; Miyata, Okiko

doi:10.1039/c0ob01148g

Cited by 30 publications

(15 citation statements)

References 58 publications

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“…To validate our proposal, we studied the Giese-type radical conjugate addition 22 to dimethyl fumarate 3a (Fig. 2b) [23][24][25][26] . The experiments were conducted in acetonitrile (CH3CN) using commercially available γ-terpinene as a cheaper and more stable surrogate of 1,4-CHD.…”

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confidence: 86%

Photochemical generation of radicals from alkyl electrophiles using a nucleophilic organic catalyst

et al. 2018

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Chemists extensively use free radical reactivity for applications in organic synthesis, materials science, and life science. Traditionally, generating radicals requires strategies that exploit the bond dissociation energy or the redox properties of the precursors. Here, we disclose a photochemical catalytic approach that harnesses different physical properties of the substrate to form carbon radicals. We use a nucleophilic dithiocarbamate anion catalyst, adorned with a well-tailored chromophoric unit, to activate alkyl electrophiles via an SN2 pathway. The resulting photon-absorbing intermediate affords radicals upon homolytic cleavage induced by visible light. This catalytic SN2-based strategy, which exploits a fundamental mechanistic process of ionic chemistry, grants access to open-shell intermediates from a variety of substrates that would be incompatible with or inert to classical radical-generating strategies. We also describe how the method's mild reaction conditions and high functional group tolerance could be advantageous for developing C-C bond-forming reactions, for streamlining the preparation of a marketed drug, for the late-stage elaboration of biorelevant compounds, and for enantioselective radical catalysis.Free radical chemistry offers powerful ways of making molecules that are often complementary to classical methods proceeding via ionic pathways 1 . This explains why radical processes have found application in material science, drug discovery, agrochemistry, and other disciplines 2 . Advances within the field have been spurred by the identification of effective radical-generating strategies. One traditional method relies on initiators 3 . Initiators are high-energy compounds (e.g. diazo compounds or peroxides), which, upon homolysis induced by heat or high-energy light, generate a reactive radical that can abstract a hydrogen or halogen atom from a substrate S (Fig. 1a, left panel). This approach ultimately relies on the bond dissociation energy (BDE) of S to form the target openshell intermediate I. Another classical route to radicals exploits the tendency of a substrate S to engage in redox processes, which can be triggered by stoichiometric oxidants/reductants 3 or by electrochemical means 4 (Fig. 1a, right panel). Radical ions emerging from the single-electron transfer (SET) event can then fragment to yield the target radical I. These classical strategies are powerful. However, they require relatively harsh conditions, including hazardous and toxic reagents, high temperatures, and/or UV-light irradiation, overall limiting the selectivity and the functional group tolerance of the ensuing radical process.A crucial step towards milder reaction conditions and, consequently, more selective reactions has been the use of substrates bearing thio functions and acting as both radical precursors and reactants (Fig. 1b, left). Methods introduced by Barton 5-7 and further popularized by Zard with xanthate transfer chemistry 8,9 greatly expanded the synthetic potential of radicals, but still relied on pu...

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confidence: 86%

Photochemical generation of radicals from alkyl electrophiles using a nucleophilic organic catalyst

et al. 2018

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“…Watson and co‐workers reported that Cu(I) catalysts initiate radical reactions of nitroalkanes with benzyl bromides 10b. However, to the best of our knowledge, there are no examples of the direct substitution of benzyl alcohols instead of benzyl halides via benzyl radicals 12…”

Section: Introductionmentioning

confidence: 99%

Fast Pyrolysis of Wood for Biofuels: Spatiotemporally Resolved Diffuse Reflectance In situ Spectroscopy of Particles

et al. 2014

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Fast pyrolysis of woody biomass is a promising process capable of producing renewable transportation fuels to replace gasoline, diesel, and chemicals currently derived from nonrenewable sources. However, biomass pyrolysis is not yet economically viable and requires significant optimization before it can contribute to the existing oil-based transportation system. One method of optimization uses detailed kinetic models for predicting the products of biomass fast pyrolysis, which serve as the basis for the design of pyrolysis reactors capable of producing the highest value products. The goal of this work is to improve upon current pyrolysis models, usually derived from experiments with low heating rates and temperatures, by developing models that account for both transport and pyrolysis decomposition kinetics at high heating rates and high temperatures (>400 °C). A new experimental technique is proposed herein: spatiotemporally resolved diffuse reflectance in situ spectroscopy of particles (STR-DRiSP), which is capable of measuring biomass composition during fast pyrolysis with high spatial (10 μm) and temporal (1 ms) resolution. Compositional data were compared with a comprehensive 2D single-particle model, which incorporated a multistep, semiglobal reaction mechanism, prescribed particle shrinkage, and thermophysical properties that varied with temperature, composition, and orientation. The STR-DRiSP technique can be used to determine the transport-limited kinetic parameters of biomass decomposition for a wide variety of biomass feedstocks.

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“…Benzylation of olefins starting directly from alkylbenzenes has only a few precedents in the literature. As an example, Naito and co‐workers reported the radical benzylation of electron‐poor olefins by means of the Et 3 B/O 2 system at 180 °C 15. Quite common is the use of peroxides16,17 for the generation of the radical intermediate, generally in the presence of metal catalysts 17.…”

Section: Resultsmentioning

confidence: 99%

Of Flexing and Vibrating

2013

ChemPhysChem

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The direct benzylation of electrophilic alkenes with alkylbenzenes has been achieved by decatungstate photocatalysis in a high ionic strength medium (5:1 acetonitrile/water mixed solvents, 0.5 M lithium perchlorate). The reaction has been extended to ring‐substituted (Cl, F, CN) toluenes and further alkylbenzenes. Intramolecular selectivity (p‐cymene, 4‐methylanisole) and deuteration effects support the atom transfer character of the process that, however, requires a sufficient stabilization of the intermediate polar exciplex to occur. The reactivity of the decatungstate anion is compared with that of aromatic ketones, the benchmark of photochemical hydrogen/electron transfer processes.

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Benzyl radical addition reaction through the homolytic cleavage of a benzylic C–H bond

Cited by 30 publications

References 58 publications

Photochemical generation of radicals from alkyl electrophiles using a nucleophilic organic catalyst

Photochemical generation of radicals from alkyl electrophiles using a nucleophilic organic catalyst

Fast Pyrolysis of Wood for Biofuels: Spatiotemporally Resolved Diffuse Reflectance In situ Spectroscopy of Particles

Of Flexing and Vibrating

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