Radical and Ionic Mechanisms in Rearrangements of o-Tolyl Aryl Ethers and Amines Initiated by the Grubbs–Stoltz Reagent, Et3SiH/KOtBu

Rearrangements of o-tolyl aryl ethers, amines, and sulfides with the Grubbs–Stoltz reagent (Et3SiH + KOtBu) were recently announced, in which the ethers were converted to o-hydroxydiarylmethanes, while the (o-tol)(Ar)NH amines were transformed into dihydroacridines. Radical mechanisms were proposed, based on prior evidence for triethylsilyl radicals in this reagent system. A detailed computational investigation of the rearrangements of the aryl tolyl ethers now instead supports an anionic Truce–Smiles rearrangement, where the initial benzyl anion can be formed by either of two pathways: (i) direct deprotonation of the tolyl methyl group under basic conditions or (ii) electron transfer to an initially formed benzyl radical. By contrast, the rearrangements of o-tolyl aryl amines depend on the nature of the amine. Secondary amines undergo deprotonation of the N-H followed by a radical rearrangement, to form dihydroacridines, while tertiary amines form both dihydroacridines and diarylmethanes through radical and/or anionic pathways. Overall, this study highlights the competition between the reactive intermediates formed by the Et3SiH/KOtBu system.

Photoredox/palladium co-catalyzed propargylic benzylation with internal propargylic carbonates

The developed photo/palladium dual catalytic system provided a novel route to internal propargylic benzylation products. A radical coupling mechanism between the propargylic radical and benzyl radical was proposed.

A Reinvestigation of the Deceptively Simple Reaction of Toluene with ●OH, and the Fate of the Benzyl Radical. I. a Combined Thermodynamic and Kinetic Study on the Competition Between ●OH Addition and Hydrogen Abstraction Reactions.

This work reports density functional and composite model chemistry calculations performed on the reactions of toluene with the hydroxyl radical. Both experimentally observed H-abstraction from the methyl group and possible additions to the phenyl ring were investigated. Reaction enthalpies and heights of the barriers suggest that H-abstraction is more favorable than ●OH addition to the ring. The calculated reaction rates at room temperature and the radical-intermediate product fractions support this view. This is somehow contradictory with the fact that, under most experimental conditions, cresols are observed in a larger concentration than benzaldehyde. Since the accepted mechanism for benzaldehyde formation involves H-abstraction, a contradiction arises that begs for an explanation. In this first part of our work we give the evidences that support the preference of hydrogen abstraction over ●OH addition and suggest an alternative mechanism which shows that cresols can actually arise also from the former reaction and not only from the latter.

A Reinvestigation of the Deceptively Simple Reaction of Toluene with ●OH, and the Fate of the Benzyl Radical. I. a Combined Thermodynamic and Kinetic Study on the Competition Between ●OH Addition and Hydrogen Abstraction Reactions.

This work reports density functional and composite model chemistry calculations performed on the reactions of toluene with the hydroxyl radical. Both experimentally observed H-abstraction from the methyl group and possible additions to the phenyl ring were investigated. Reaction enthalpies and heights of the barriers suggest that H-abstraction is more favorable than ●OH addition to the ring. The calculated reaction rates at room temperature and the radical-intermediate product fractions support this view. This is somehow contradictory with the fact that, under most experimental conditions, cresols are observed in a larger concentration than benzaldehyde. Since the accepted mechanism for benzaldehyde formation involves H-abstraction, a contradiction arises that begs for an explanation. In this first part of our work we give the evidences that support the preference of hydrogen abstraction over ●OH addition and suggest an alternative mechanism which shows that cresols can actually arise also from the former reaction and not only from the latter.

Enthalpies of formation of the benzyloxyl, benzylperoxyl, hydroxyphenyl radicals and related species on the potential energy surface for the reaction of toluene with the hydroxyl radical

<p></p><p>The reaction of toluene (T) with OH^● produces addition products as well as the benzyl radical (TR). TR can react with OH^● or O₂ to produce oxygenated species, for many of which there is no experimental information available. We present here theoretically determined heats of formation (HFs) of 17 such species using the non-isodesmic reactions on the potential energy surface (PES) of TR+O₂ and T+OH^●+O₂. For those species the experimental HFs of which are known, we obtained a good correlation between experimental and theoretical values at the G4 (r²=0.999) and M06/cc-pVQZ (r²=0.997) levels, thus showing the goodness of the methods used. Previously unknown HFs of other radicals (benzyloxyl, spiro [1,2-dioxetane benzyl], hydroxyphenyl, and benzylperoxyl) and closed shell species (salicylic alcohol, benzo[b]oxetane and p-hydroxy cyclohexa-2,5-dienone) were later determined using those methods.<b></b></p><br><p></p>

Enthalpies of formation of the benzyloxyl, benzylperoxyl, hydroxyphenyl radicals and related species on the potential energy surface for the reaction of toluene with the hydroxyl radical

<p></p><p>The reaction of toluene (T) with OH^● produces addition products as well as the benzyl radical (TR). TR can react with OH^● or O₂ to produce oxygenated species, for many of which there is no experimental information available. We present here theoretically determined heats of formation (HFs) of 17 such species using the non-isodesmic reactions on the potential energy surface (PES) of TR+O₂ and T+OH^●+O₂. For those species the experimental HFs of which are known, we obtained a good correlation between experimental and theoretical values at the G4 (r²=0.999) and M06/cc-pVQZ (r²=0.997) levels, thus showing the goodness of the methods used. Previously unknown HFs of other radicals (benzyloxyl, spiro [1,2-dioxetane benzyl], hydroxyphenyl, and benzylperoxyl) and closed shell species (salicylic alcohol, benzo[b]oxetane and p-hydroxy cyclohexa-2,5-dienone) were later determined using those methods.<b></b></p><br><p></p>

[Co(TPP)]-Catalyzed Formation of Substituted Piperidines

Radical cyclization via cobalt(III)–carbene radical intermediates is a powerful method for the synthesis of (hetero)cycles. Building on the recently reported synthesis of N-heterocyclic pyrrolidines catalyzed by Co(II) porphyrins, we herein report the [Co(TPP)]-catalyzed formation of desirable six membered N heterocyclic piperidines, directly from linear aldehydes. Piperidines were obtained in overall high yields, with linear alkenes being formed as side products in small amounts. A DFT study was performed to gain a deeper mechanistic understanding of the cobalt(II)-porphyrin-catalyzed formation of pyrrolidines, piperidines and linear alkenes. The calculations show that the alkenes are unlikely to be formed through 1,2-HAT. Instead, the calculations are consistent with a pathway involving benzyl radical formation followed by radical rebound ring-closure to form the piperidines. Competitive 1,5-HAT from the beta-position to the benzyl radical explains the formation of linear alkenes.<br>

[Co(TPP)]-Catalyzed Formation of Substituted Piperidines

Radical cyclization via cobalt(III)–carbene radical intermediates is a powerful method for the synthesis of (hetero)cycles. Building on the recently reported synthesis of N-heterocyclic pyrrolidines catalyzed by Co(II) porphyrins, we herein report the [Co(TPP)]-catalyzed formation of desirable six membered N heterocyclic piperidines, directly from linear aldehydes. Piperidines were obtained in overall high yields, with linear alkenes being formed as side products in small amounts. A DFT study was performed to gain a deeper mechanistic understanding of the cobalt(II)-porphyrin-catalyzed formation of pyrrolidines, piperidines and linear alkenes. The calculations show that the alkenes are unlikely to be formed through 1,2-HAT. Instead, the calculations are consistent with a pathway involving benzyl radical formation followed by radical rebound ring-closure to form the piperidines. Competitive 1,5-HAT from the beta-position to the benzyl radical explains the formation of linear alkenes.<br>