Generation of Methyl Radicals by Decomposition of Bibenzyl Compounds Containing α-Methoxy Substituents

Hartzell, G. E.; Huyser, Earl S.

doi:10.1021/jo01034a051

Cited by 22 publications

(10 citation statements)

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“…If addition of the resultant tertiary radical were slow to rebound into a nickel catalyst, we posited that unimolecular β-scission could occur to generate a methyl radical and dimethyl carbonate (Δ G = −25.1 kcal/mol and Δ G ‡ = 11.4 kcal/mol) . Coupling the generation of high energy alkyl radicals with the formation of a stable carbonyl byproduct via β-scission has been well-studied, but only recently emerged as a strategy in transition-metal-catalyzed cross-coupling. Reported examples of β-scission from carbon-centered radicals in cross-coupling are limited to xanthate esters that must be presynthesized and afford only stabilized radical species, thus precluding access to methyl radical .…”

Section: Introductionmentioning

confidence: 99%

Nickel/Photoredox-Catalyzed Methylation of (Hetero)aryl Chlorides Using Trimethyl Orthoformate as a Methyl Radical Source

et al. 2020

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Methylation of organohalides represents a valuable transformation, but typically requires harsh reaction conditions or reagents. We report a radical approach for the methylation of (hetero)aryl chlorides using nickel/photoredox catalysis wherein trimethyl orthoformate, a common laboratory solvent, serves as a methyl source. This method permits methylation of (hetero)aryl chlorides and acyl chlorides at an early and late stage with broad functional group compatibility. Mechanistic investigations indicate that trimethyl orthoformate serves as a source of methyl radical via β-scission from a tertiary radical generated upon chlorine-mediated hydrogen atom transfer.

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Section: Introductionmentioning

confidence: 99%

Nickel/Photoredox-Catalyzed Methylation of (Hetero)aryl Chlorides Using Trimethyl Orthoformate as a Methyl Radical Source

et al. 2020

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“…Thus in the common acrylic or methacrylic esters, such as methyl, ethyl, or n-butyl acrylate or methacrylate, benzophenone has only a very weak initiator action, if any. On the other hand, if benzophenone is exposed to light in a mixture of Wissenschaftliches Hauptlaboratoriumder Bayer AG 41 5 Krefeld-Uerdingen (Germany) these esters with a solvent containing ether groups (diethyl ether, tetrahydrofuran, polyethylene glycol, or methyl glycol acetate), polymerization begins after a very short timeC3]. Benzophenone has an equally good initiator action even without solvents in esters of acrylic acid with an alcohol containing ether groups, such as diethylene glycol diacrylate.…”

Section: Chain Initiation By Hydrogen Abstractionmentioning

confidence: 99%

Aromatic Keto Compounds as Initiators in Photopolymerizations

Heine

Rosenkranz

Rudolph

1972

Angew. Chem. Int. Ed. Engl.

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The present article deals with the primary photochemical reactions of carbonyl compounds commonly used as photoinitiators. It is shown that radical formation results mainly from three processes, i. e. hydrogen abstraction, P-cleavage, and a-cleavage. The difference in the suitability of the radicals for chain initiation is discussed. IntroductionPhotopolymerizations are initiated by radical formation through the action of light on light-sensitive compounds (photoinitiators) added to the vinyl monomers. Compounds of many classes can be used for this initiation [']; the most important of these are carbonyl compounds with UV absorptions in the range between 300 and 400 nm. The known compounds of this type that are suitable for practical use and the reactions of these compounds that lead to the initiation of a polymerization chain form the subject of the present article. Chain Initiation by Hydrogen AbstractionThe photoreduction of benzophenone ( 1 ) to benzopinacol ( 3 ) is probably one of the best-known light-induced reactions. There have been numerous attempts to make useof the ability of benzophenone to form free radicals by hydrogen abstraction in the excited triplet state for the initiation of vinyl polymerizations[21. However, these attempts have led to conflicting results. Thus in the common acrylic or methacrylic esters, such as methyl, ethyl, or n-butyl acrylate or methacrylate, benzophenone has only a very weak initiator action, if any. On the other hand, if benzophenone is exposed to light in a mixture of these esters with a solvent containing ether groups (diethyl ether, tetrahydrofuran, polyethylene glycol, or methyl glycol acetate), polymerization begins after a very short timeC3]. Benzophenone has an equally good initiator action even without solvents in esters of acrylic acid with an alcohol containing ether groups, such as diethylene glycol diacrylate. Very slow polymerization is observed, on the other hand, when benzophenone and alkyl acrylates are exposed to light in isopropanol.This observation is surprising at first, since kinetic measurements indicate that hydrogen abstraction occurs at almost equal rates on exposure of benzophenone to light in ethers and in isopropan~l[~I, and the quantum yield of the photoreduction in isopropanol is almost 1. According to investigations by E l a d 5 ] , however, the free radicals formed from ethers have a strong tendency to add to double bonds. Though a corresponding addition of semipinacol radicals has occasionally also been observed ['], ether radicals add preferentially when present together with semipinacol radicals[71.The difference in the initiator action of benzophenone in the presence of isopropanol and ethers corresponds to the variation in the ease of addition of the free radicals formed by hydrogen abstraction from the solvent. The role of the semibenzopinacol radicals (2) invariably formed in photoinitiation with benzophenone, which, according to Braun[*], also initiate polymerizations by hydrogen transfer, appears to be of minor importance accord...

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“…A viable alternative is the methanolysis of alcohols which has been reported over a variety of Brønsted and Lewis acidic compounds such as HCl, 2 H 2 SO 4 , 3 p-MeC 6 H 4 SO 3 H, 4 CF 3 SO 3 H, 5 CAN [(NH 4 ) 2 Ce-(NO 3 ) 6 ], 6 FeX 3 (X = Cl, NO 3 ), 7 RE(OTf) 3 (RE = Yb, Sc), 8 and NaHSO 4 /SiO 2 . 9 These methods, however, are limited to secondary and tertiary substrates, [2][3][4][5][6][7] or to primary benzyl alcohols only if activated by OH and OR para-substituents. 8,9 Safer and selective methylation protocols can be conceived with the non-toxic dimethyl carbonate (MeOCO 2 Me, DMC).…”

Section: Introductionmentioning

confidence: 99%

The methylation of benzyl-type alcohols with dimethyl carbonate in the presence of Y- and X-faujasites: selective synthesis of methyl ethers

2008

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At 165-200 uC, in the presence of sodium-exchanged faujasites (NaX or NaY) as catalysts, the reaction of dimethyl carbonate with benzyl-, o-and p-methoxybenzyl-, p-hydroxybenzyl-, diphenylmethyl-, and triphenylmethyl-alcohols (1a, 2a,b, 3a, 4a, and 4c, respectively), produces the corresponding methyl ethers in up to 98% yields. A peculiar chemoselectivity is observed for hydroxybenzyl alcohols (compounds 3a and 3b, para-and ortho-isomers) whose etherification takes place without affecting the OH aromatic groups. Acid-base interactions of alcohols and DMC over the faujasite surface offer a plausible explanation for the catalytic effect of zeolites NaY and NaX, as well as for the trend of reactivity shown by the different alcohols (primary . secondary . tertiary). However, in the case of substrates with mobile protons in the b-position (i.e. 1-phenylethanol and 1,1-diphenylethanol), the dehydration reaction to olefins is the major, if not the exclusive, process.

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Generation of Methyl Radicals by Decomposition of Bibenzyl Compounds Containing α-Methoxy Substituents

Cited by 22 publications

References 0 publications

Nickel/Photoredox-Catalyzed Methylation of (Hetero)aryl Chlorides Using Trimethyl Orthoformate as a Methyl Radical Source

Nickel/Photoredox-Catalyzed Methylation of (Hetero)aryl Chlorides Using Trimethyl Orthoformate as a Methyl Radical Source

Aromatic Keto Compounds as Initiators in Photopolymerizations

The methylation of benzyl-type alcohols with dimethyl carbonate in the presence of Y- and X-faujasites: selective synthesis of methyl ethers

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