Absolute Rate Constants for the Addition of Cyanomethyl (·CH2CN) and (tert-Butoxy)carbonylmethyl (·CH2CO2C(CH3)3) radicals to alkenes in solution

Intermolecular Addition of Carbon-Centered Radicals to Ynamides. A Regio- and Stereoselective Route to Persubstituted α-Iodo-enamides

Rather surprisingly, C-C bond formation through "intermolecular" radical addition to internal ynamides has never been reported. Actually, ynamides are excellent acceptors for "electrophilic" carbon-centered radicals. These processes enable the introduction of functionalized alkyl chains at Cβ, groups that have not yet been introduced via the addition of organometallics. Radical carboiodination affords persubstituted α-iodo-enamides in moderate to high yield. The addition is totally stereoselective. Theoretical support to the mechanism and the scope and limitation of the reaction are discussed.

Photoredox meets gold Lewis acid catalysis in the alkylative semipinacol rearrangement: a photocatalyst with a dark side

The alkylative semipinacol rearrangement of a variety of TMS protected α-styrenyl substituted cyclic alcohols with unactivated bromoalkanes that merge photoredox and Au(i)/Au(iii) catalysis has been achieved.

Organic Covalent Patterning of Nanostructured Graphene with Selectivity at the Atomic Level

Organic covalent functionalization of graphene with long-range periodicity is highly desirable-it is anticipated to provide control over its electronic, optical, or magnetic properties-and remarkably challenging. In this work we describe a method for the covalent modification of graphene with strict spatial periodicity at the nanometer scale. The periodic landscape is provided by a single monolayer of graphene grown on Ru(0001) that presents a moiré pattern due to the mismatch between the carbon and ruthenium hexagonal lattices. The moiré contains periodically arranged areas where the graphene-ruthenium interaction is enhanced and shows higher chemical reactivity. This phenomenon is demonstrated by the attachment of cyanomethyl radicals (CH2CN(•)) produced by homolytic breaking of acetonitrile (CH3CN), which is shown to present a nearly complete selectivity (>98%) binding covalently to graphene on specific atomic sites. This method can be extended to other organic nitriles, paving the way for the attachment of functional molecules.

Electrophilicity and Nucleophilicity Index for Radicals

Radicals can be regarded as electrophilic/nucleophilic, depending on their tendency to attack sites of relatively higher/lower electron density. In this paper, an electrophilicity scale, global as well as local, and a nucleophilicity scale for 35 radicals is reported. The global electrophilicity scale correlates well with the nucleophilicity scale, suggesting that these concepts are inversely related.

Respective Contributions of Polar vs Enthalpy Effects in the Addition/Fragmentation of Mercaptobenzoxazole-Derived Thiyl Radicals and Analogues to Double Bonds

The formation and the reactivity of three selected sulfur-centered radicals formed from mercaptobenzoxazole, mercaptobenzimidazole, and mercaptobenzothiazole toward four double bonds (methyl acrylate, acrylonitrile, vinyl ether, and vinyl acetate) are investigated. The reversibility of the addition/fragmentation reaction in these widely used photoinitiating systems of radical polymerization was studied, for the first time, through the measurement of the corresponding rate constants by time-resolved laser spectroscopy. The combination of these results with quantum mechanical calculations clearly evidences that, contrary to previous studies on other aryl thiyl radicals, the addition rate constants (ka) are governed here by the polar effects associated with the very high electrophilic character of these radicals. However, interestingly, the back-fragmentation reaction (k-a) is mainly influenced by the enthalpy effects as supported by the relationship between the rate constants and the addition reaction enthalpy DeltaHR. The addition and fragmentation rate constants calculated from the transition state theory (TST) are in satisfactory agreement with the experimental ones. Therefore, molecular orbital (MO) calculations offered new opportunities for a better understanding of the sulfur-centered radical reactivity.

Addition of Carbon-Centered Radicals to Double Bonds: Influence of the Alkene Structure

The radical addition reaction to the double bond is well-recognized in organic chemistry as a powerful tool for C-C bond formation. The reactivity of three selected carbon centered radicals (aminoalkyl, methyl, and cyanomethyl) toward five double bonds, also representative of widespread monomers (vinyl ether, vinyl acetate, acrylonitrile, methyl acrylate, and ethylene), was examined in detail by using molecular orbital calculations. The observed reactivity is strongly influenced by the reaction exothermicity demonstrating that the energy barrier is governed in large part by the enthalpy term. The polar effect, as computed by molecular orbital calculations from the transition state structures, can drastically enhance the reactivity. A clear separation and quantification of the relative role of the polar and enthalpy effects in the different radical/double bond systems are obtained and the observed trend of reactivity is discussed. In addition to the effect of the charge-transfer configurations on the barrier, a large influence on the transition state geometry was evidenced.

Role of charge-transfer configurations on the addition reaction of aminoalkyl radicals onto acrylate double bonds

The major factors affecting the reactivity of the important class of aminoalkyl radicals toward the addition reaction onto acrylate double bonds were studied. The rate constants of interaction, directly measured by using laser-induced photoacoustic calorimetry, range from 2.5 x 10(7) to 4.5 x 10(7) M(-)(1) s(-)(1). The reactivity of these radicals was compared to that of alkyl radicals. Calculations show that the reaction enthalpy is not the driving factor. Molecular orbital calculations of the transition states clearly demonstrated the large participation of charge-transfer configurations relative to nonpolar configurations, leading to a decrease of the barrier height. Moreover a quite good correlation is found when plotting the charge-transfer character in the transition state against the ionization potential. The roles of the different factors governing the reactivity of these species were elucidated.

Factors Controlling the Addition of Carbon-Centered Radicals to Alkenes-An Experimental and Theoretical Perspective

The successful exploitation of syntheses involving the generation of new carbon-carbon bonds by radical reactions rests on some prior knowledge of the rate constants for the addition of carbon-centered radicals to alkenes and other unsaturated molecules, and of the factors controlling them. Two former classical reviews in Angewandte Chemie by Tedder (1982) and by Giese (1983) provided mechanistic insight and led to various qualitative rules on the complex interplay of enthalpic, polar, and steric effects. In the meantime, the field has experienced very rapid progress: many more experimental absolute rate constants have become available, and there have been major advances in the efficiency and reliability of quantum-chemical methods for the accurate calculation of transition structures, reaction barriers, and reaction enthalpies. Herein we review this progress, recommend suitable experimental and theoretical procedures, and display representative data series for radical additions to alkenes. On this basis, and guided by the pictorial tool of the state-correlation diagram for radical additions, we then offer a new and more stringent quantification of the controlling factors. Our analysis leads to a partial revision of the previous qualitative rules, and it more clearly exhibits the interplay of the reaction enthalpy effects, polar charge-transfer contributions, and steric substituent effects on the reaction energy barrier. The various contributions are cast into the form of new, simple, and physically meaningful but non-linear, predictive equations for the preestimation of rate constants. These equations prove successful in several tests but call for additional theoretical and experimental foundation. The kinetics of related reactions such as polymer propagation, copolymerization, and the addition of radicals to alkynes and aromatic compounds is shown to follow the same principles.

Entropy control of the cross-reaction between carbon-centered and nitroxide radicals

Absolute rate constants for the cross-coupling reaction of several carbon-centered radicals with various nitroxides and their temperature dependence have been determined in liquids by kinetic absorption spectroscopy. The rate constants range from <2 x 10(5) M(-)(1) s(-)(1) to 2.3 x 10(9) M(-)(1) s(-)(1) and depend strongly on the structure of the nitroxide and the carbon-centered radical. Grossly, they decrease with increasing rate constant of the cleavage of the corresponding alkoxyamine. In many cases, the temperature dependence shows a non-Arrhenius behavior. A model assuming a short-lived intermediate that is hindered to form the coupling product by an unfavorable activation entropy leads to a satisfactory analytic description. However, the behavior is more likely due to a barrierless single-step reaction with a low exothermicity where the free energy of activation is dominated by a large negative entropy term.