“…Alternatively, radical 361 undergoes a concerted fragmentation to give the allyl radical 363 directly (Scheme 45). 152 This latter process is somewhat akin to that advanced by Giese for the rapid elimination of acetate from radical 275 via transition state 278 (section III.A). Time-resolved laser flash photolytic studies were conducted in a variety of solvents and led to the determination of Arrhenius parameters and rate constants for the formation of the allyl radical 363 (Table 8).…”
ContentsI. Introduction 3273 II. Rearrangements 3275 A. β-(Acyloxy)alkyl Rearrangement 3275 1. Esters 3275 2. Lactones 3280 3. Thiocarbonyl Esters 3280 B. β-(Phosphatoxy)alkyl Rearrangement 3282 C. β-(Nitroxy)alkyl and β-(Sulfonatoxy)alkyl Rearrangements 3285 D. (Acyloxyalkyl)silyl Radical Rearrangement 3285 III. Fragmentation Reactions 3286 A. β-(Acyloxy)alkyl Radicals and Their Thiocarbonyl Analogues 3286 B. β-(Phosphatoxy)alkyl Radicals 3288 1. Anaerobic Conditions 3288 2. Aerobic Conditions 3293 C. β-(Sulfonatoxy)alkyl and Related Radicals 3294 IV. Mechanism 3294 A. β-(Ester)alkyl Radicals Susceptible to Neither Rearrangement Nor Fragmentation 3294 B. Fragmentation Reactions 3295 C. Rearrangement Reactions 3295 1. Noncage Dissociative Pathways 3296 2. Stepwise Pathways via Five-Membered Cyclic Intermediate Radicals 3296 3. Use of Isotopic and Stereochemical Labeling as Probes of Mechanism 3296 4. Quantitative Structure Activity Relationships 3298 5. Computational Studies and ESR Considerations 3299 V. Overview of β-(Ester)alkyl Radical Reactions 3303 VI. Related Reactions Involving Radical C−O Bond Shift and Cleavage 3305 A. Rearrangement and Fragmentation of β-Hydroxyalkyl Radicals 3305 B. Schenck or Allylperoxy Radical Rearrangement 3307 C. β-(Vinyloxy)alkyl to 4-Ketobutyl Rearrangement 3308 VII. Acknowledgments 3309 VIII. References 3309 Scheme 6
“…Alternatively, radical 361 undergoes a concerted fragmentation to give the allyl radical 363 directly (Scheme 45). 152 This latter process is somewhat akin to that advanced by Giese for the rapid elimination of acetate from radical 275 via transition state 278 (section III.A). Time-resolved laser flash photolytic studies were conducted in a variety of solvents and led to the determination of Arrhenius parameters and rate constants for the formation of the allyl radical 363 (Table 8).…”
ContentsI. Introduction 3273 II. Rearrangements 3275 A. β-(Acyloxy)alkyl Rearrangement 3275 1. Esters 3275 2. Lactones 3280 3. Thiocarbonyl Esters 3280 B. β-(Phosphatoxy)alkyl Rearrangement 3282 C. β-(Nitroxy)alkyl and β-(Sulfonatoxy)alkyl Rearrangements 3285 D. (Acyloxyalkyl)silyl Radical Rearrangement 3285 III. Fragmentation Reactions 3286 A. β-(Acyloxy)alkyl Radicals and Their Thiocarbonyl Analogues 3286 B. β-(Phosphatoxy)alkyl Radicals 3288 1. Anaerobic Conditions 3288 2. Aerobic Conditions 3293 C. β-(Sulfonatoxy)alkyl and Related Radicals 3294 IV. Mechanism 3294 A. β-(Ester)alkyl Radicals Susceptible to Neither Rearrangement Nor Fragmentation 3294 B. Fragmentation Reactions 3295 C. Rearrangement Reactions 3295 1. Noncage Dissociative Pathways 3296 2. Stepwise Pathways via Five-Membered Cyclic Intermediate Radicals 3296 3. Use of Isotopic and Stereochemical Labeling as Probes of Mechanism 3296 4. Quantitative Structure Activity Relationships 3298 5. Computational Studies and ESR Considerations 3299 V. Overview of β-(Ester)alkyl Radical Reactions 3303 VI. Related Reactions Involving Radical C−O Bond Shift and Cleavage 3305 A. Rearrangement and Fragmentation of β-Hydroxyalkyl Radicals 3305 B. Schenck or Allylperoxy Radical Rearrangement 3307 C. β-(Vinyloxy)alkyl to 4-Ketobutyl Rearrangement 3308 VII. Acknowledgments 3309 VIII. References 3309 Scheme 6
“…Our groups have reported kinetic , and labeling , studies of β-phosphatoxyalkyl radical reactions that, we believed, were most consistent with concerted reactions. More recently, however, we reported detailed studies of a competing migration and elimination reaction of a β-phosphatoxyalkyl radical that are not consistent with our original conclusions; specifically, the results indicated that both products arose from a common intermediate, an ion pair produced by heterolysis, even in low polarity solvents .…”
mentioning
confidence: 80%
“…The rate constants for reactions of 1 increase smoothly as the solvent polarity increases from THF to aqueous acetonitrile solutions, similar to the kinetic behavior observed in other β-ester radical reactions. For example, the 15-fold acceleration in reaction of 1 upon proceeding from THF to acetonitrile is comparable to the acceleration in reactions of radicals 6 − 8 in these solvents. ,, The obvious difference between radicals 6 − 8 and 1 is that the reactions of 6 − 8 do not give different products in the different solvents; 6 and 7 react by diphenyl phosphate and trifluoroacetate migrations, respectively, and radical 8 reacts in both solvents by a combination of diphenyl phosphate migration and diphenylphosphoric acid elimination reactions. ,, The log A terms in the Arrhenius functions (the entropic terms) for reactions of radical 1 in THF and acetonitrile are similar to one another as well as to those for reactions of radicals 6 − 8 in the same solvents. ,, …”
[formula: see text] The 2-(diethylphosphatoxy)-2-(p-methoxyphenyl)-1,1-dimethylethyl radical (1) reacted to give the benzylic radical product from phosphate migration or a radical cation (or a mixture of the two) as a function of solvent. Smooth acceleration in rates of reactions of 1 in solvents of increasing polarity and consistent entropies of activation indicate that radical 1 reacts by common mechanism irrespective of the final products formed, specifically by initial heterolysis to a radical cation-phosphate anion pair.
“…Our respective groups have addressed the mechanisms of β-phosphatoxyalkyl radical reactions from experimental and computational perspectives. Stereochemical labeling studies implicated concerted migration reactions. , Laser flash photolysis (LFP) kinetic studies of β-phosphatoxyalkyl radicals coupled with product analyses found that phosphate migrations, an elimination and a nucleophilic substitution reaction proceeded without diffusively free radical cation intermediates, and the kinetic results were considered to be most consistent with concerted processes. , Low energy pathways for concerted [1,2]- and [3,2]-phosphate shifts, for concerted syn-[1,3] eliminations, and for nucleophilic β-displacement of phosphate were found computationally …”
β-Phosphatoxyalkyl radical reactions were studied experimentally and computationally. The 1,1-dibenzyl-2-(diphenylphosphatoxy)-2-phenylethyl radical (1) reacted to give the migration product 2-benzyl-2-(diphenylphosphatoxy)-1,3-diphenylpropyl radical (2) and the elimination product 2-benzyl-1,3-diphenylallyl
radical (3) in a variety of solvents. A modest kinetic solvent effect for reactions of 1 was found. Variable
temperature studies in THF and acetonitrile gave Arrhenius functions with similar log A terms; the entropies
of activation are ∼−5 eu. A deuterated analogue of radical 1 reacted in THF and acetonitrile with rate constants
indistinguishable from those of 1, but the ratio of products 2:3 increased for the deuterated radical requiring
kinetic isotope effects (KIEs) in reactions following the rate-limiting step. In aqueous acetonitrile solutions,
the β,β-dibenzylstyrene radical cation (4) was detected as a short-lived intermediate, and the rate constants for
formation of 3 and 4 indicated that both species derived from a common intermediate. The 1,1-dimethyl-2-(dimethylphosphatoxy)ethyl radical (C1) was studied computationally. Transition states for concerted phosphate
migrations and phosphoric acid elimination were found with energies in the order [1,2]-migration < [1,3]-elimination < [3,2]-migration; each transition state was more polarized than radical C1. A transition state for
homolytic fragmentation of C1 to give 2-methylpropene and the dimethylphosphatoxyl radical could not be
found, but the reaction from the ensemble of these two entities to give the 2-methylallyl radical and phosphoric
acid was followed computationally. KIEs and solvent dielectric effects were computed for each concerted
reaction of C1. The results indicate that radical 1 reacts in all solvents studied by a common pathway involving
initial heterolysis. The first-formed contact pair of radical cation 4 and diphenyl phosphate anion collapses to
products 2 and 3 and, as a minor process, evolves to diffusively free radical cation 4 in aqueous acetonitrile
solutions. A model for heterolytic fragmentation of β-ester radicals involving contact and solvent-separated
ion pairs is presented.
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