Radical Addition Rates to Alkenes by Time-Resolved CIDNP: 2-Hydroxy-2-propyl Radicals

Addition Rate Constants of Phosphorus- and Carbon-Centered Radicals to Double Bond of Monomers as Studied by a Pulsed Electron Paramagnetic Resonance Method

Addition reaction of a radical to the double bond of a monomer, which is important at early stage of photopolymerization, has been studied by time-resolved (TR-) and pulsed electron paramagnetic resonance (EPR) spectroscopic methods. Reactions of one phosphorus-centered and three carbon-centered radicals attacking to several monomers have been employed. Intermediate radicals were identified by analyzing the recorded TR-EPR spectra, and the reaction rate constants were determined by the electron spin-echo detection method proposed by Weber and Turro [J. Phys. Chem. A, 2003, 107 (18), 3326 - 3334]. The quantum chemical calculation shows that the rate constants for the addition reactions are well-explained by introducing two factors of "enthalpy effect" and "polar effect" to control the activation barrier height. It was observed that the rate constants of the phosphorus-centered radical were larger than those of carbon-centered radicals for some monomers. The difference in the rate constants was argued on the basis of frequency factor and activation barrier both of which are influenced by an atom of radical center.

Addition-elimination in the reaction of alpha-hydroxyalkyl radicals with 3,5-pyridinedicarboxylic acid and nicotinic acid: example of inner sphere organic electron transfer

Reactions of alpha-hydroxyalkyl radicals with 3,5-pyridinedicarboxylic acid (3,5-PDCA) and nicotinic acid (NA) were studied at appropriate pHs in aqueous solutions by pulse radiolysis technique. At pH 1, CH(3)C*HOH and *CH(2)OH radicals were found to react with 3,5-PDCA by rate constants of 2.2 x 10(9) and 5.1 x 10(8) dm(3) mol(-1) s(-1), respectively, giving radical adduct species. The adduct species formed in the reaction of CH(3)C*HOH radicals with 3,5-PDCA underwent unimolecular decay (k = 9.8 x 10(4) s(-1)) giving pyridinyl radicals. Reaction of (CH(3))(2)C*OH, CH(3)C*HOH, and *CH(2)OH radicals with NA at pH 3.3 gave the adduct species which subsequently decayed to the pyridinyl radicals. At pH 1, wherein NA is present in the protonated form, (CH(3))(2)C*OH radicals directly transfer electrons to NA, whereas CH(3)C*HOH and *CH(2)OH radicals react with higher rate constants compared with those at pH 3.3, initially giving the adduct species which subsequently undergo elimination reaction giving pyridinyl radicals. Reactions of alpha-hydroxyalkyl radicals with 3,5-pyridinedicarboxylic acid and nicotinic acid are found to proceed by an addition-elimination pathway that provides one of the few examples of organic inner sphere electron-transfer reactions. Rate constant for the addition reaction as well as rate of elimination varies with the reduction potential of alpha-hydroxyalkyl radicals.

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.

Ortho Effect in the Bergman Cyclization: Electronic and Steric Effects in Hydrogen Abstraction by 1-Substituted Naphthalene 5,8-Diradicals

We present a detailed theoretical study of geometries, electronic structure, and energies of transition states and intermediates completing the full Bergman cycloaromatization pathway of ortho-substituted enediynes with a focus on polar and steric contributions to the kinetics and thermodynamics of hydrogen abstraction. This study provides a rare unambiguous example of remote substitution that affects reactivity of a neutral reactive intermediate through an sigma framework.

Thermodynamic and Strain Effects in the Competition between 5-Exo-dig and 6-Endo-dig Cyclizations of Vinyl and Aryl Radicals

Electronic and structural factors controlling the competition between 5-exo-dig and 6-endo-dig cyclizations of sp2-radicals were analyzed using a combination of available experimental data and computation. Although the stereoelectronically favored 5-exo pathways usually has the lower activation energy, formation of a new aromatic ring not only makes the 6-endo process favorable thermodynamically in conjugated systems but also lowers its activation barrier to the extent where the 5-exo/6-endo selectivity is controlled by subtle factors such as the different sensitivity of the two pathways to strain effects in polycyclic systems. In particular, the stronger sensitivity of the 5-exo pathway to strain leads to a crossover in selectivity. The 6-endo cyclization is kinetically favored in smaller (and strained) cycles, whereas the 5-exo cyclization has lower barriers in the larger rings.

From Alcohols to SugarsHow Does the Reactivity of α-Hydroxyalkyl Radicals Change with Structure? A Quantitative Examination by Pulse Radiolysis

Rate constants are reported for the 1-electron reduction of the azo dye Orange II in water (pH 7.0) by 10 different alpha-hydroxy radicals. The radicals were created by pulse radiolysis of aqueous solutions of the corresponding alcohol/sugar. The rate constants varied from 1 x 10(8) to 2.7 x 10(9) mol(-1) dm(3) s(-1) and radicals with beta-hydroxy groups had the lowest rate constant. The reaction was found to be controlled by the reduction potentials of the radicals, with steric influences having little effects. Good fits of the data were obtained using the Marcus equation with lambda =140 kJ/mol.

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.

Quantitative rate constants for the reaction of dyes and alkenes with alpha-hydroxyalkyl radicals, measured by laser flash photolysis

Rate constants are measured for the addition reactions of 1-hydroxy-1-cyclohexyl (1HC) and 2-hydroxy-2-propyl (2HP) radicals to 7 alkenes and for the 1-electron reduction of 16 organic dyes by 1HC, and a subset of 5 of these dyes by 2HP. This was done to determine to what extent the many reported rate constants for reactions of 2-hydroxy-2-propyl radicals (2HP) may be used to predict the rates of reactions of other tertiary alpha-hydroxy-alkyl radicals, and to give a better understanding of the factors that control dye reduction. The dyes were chosen to represent a wide range of dye types (azo, anthraquinone, phthalocyanine, triaryl-methane, indocyanine and azine dyes). Radicals were produced by laser flash photolysis of the corresponding tertiary alpha-hydroxyketone giving carbonyl and tertiary alpha-hydroxy-alkyl radicals. Control experiments with a bis-acylphosphine oxide were carried out which clearly demonstrated that the carbonyl radicals did not interfere with the kinetics. On average the addition and reduction rate constants for 1HC are only 20% lower than for 2HP. Larger decreases are observed for sterically congested alkenes due to the increased steric bulk of 1HC. The rate constants for 1-electron reduction of the dyes are in the range 4 x 10(7) to 6 x 10(9) mol-1 1 s-1 and may be predicted, reasonably well using the Marcus equation with a reorganisation energy, lambda = 182 kJ mol-1.

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.