Biradical Processes Research Articles

The triplet state of deprotonated mesoionic carbene Breslow intermediates is thermally accessible: Distal difunctionalization of aldehydes

N-Heterocyclic carbene (NHC)-catalyzed transformations through electron-pair-transfer processes have witnessed remarkable developments in the past decades, especially the umpolung of aldehydes via the Breslow intermediates (BIs). The role of BIs in single-electron transfer (SET) pathways has long been established in biosynthesis, and in the last decade, it was extended to NHC organocatalysis. Here, we report that deprotonated mesoionic carbene BIs (BI - s) have an astonishing small singlet/triplet (S/T) gap of only 4.0 kcal/mol, resulting in a thermally accessible triplet state. This low-lying excited state allows for a series of distal difunctionalizations of aryl aldehydes via biradical processes. At the formyl side, an oxidative esterification reaction takes place concomitantly with a reduction of the aryl side. Here, three types of chemical transformation are demonstrated: hydro(deutero)defunctionalization, hydrogenation, and reductive alkylation. • The triplet state of deprotonated MIC-Breslow intermediates is thermally accessible • Catalytic distal difunctionalization of aryl aldehydes via biradical pathways • Redox-neutral reactions with simultaneous oxidative esterification and reduction Over the past decades, N-heterocyclic carbene (NHC)-based organocatalysis, which involves the umpolung of aldehydes via Breslow intermediates (BIs), has been developed as an important approach in organic synthesis. Among them, NHC-catalyzed electron-pair reactions and radical reactions have been established via BIs and BI − s, respectively. In this article, we disclose that the triplet state of deprotonated mesoionic carbene BIs is thermally accessible. Taking advantage of this property, we describe a mesoionic-carbene-catalyzed distal difunctionalization reaction of aryl aldehydes via biradical pathways. These reactions feature redox neutrality with simultaneous oxidative esterification and reduction. Yan and co-workers report that deprotonated mesoionic carbene Breslow intermediates (BIs) have an astonishing small singlet/triplet (S/T) gap of only 4.0 kcal/mol, resulting in a thermally accessible triplet state. This low-lying excited state allows for a series of distal difunctionalizations of aryl aldehydes via biradical processes. At the formyl side, an oxidative esterification reaction takes place concomitantly with a reduction of the aryl side.

QM/MM Investigations on the Bioluminescent Decomposition of Coelenterazine Dioxetanone in Obelin

The bioluminescent mechanism of colenterazine dioxetanone(CZD) in the photoprotein of Obelia(obelin) was investigated by the combined quantum and molecular mechanics(QM/MM) method at TD-DFT level, which involved the real protein environment in decomposition of 1,2-dioxetanones. The anionic decomposition of CZD in (CZD+H2O)– model can go through a charge transfer(CT) catalyzed asynchronous-concerted process, which can be elucidated by the gradual reversible CT initiated luminescence(GRCTIL) mechanism. The neutral CZD in (CZDH+H2O) decomposes through an uncatalyzed non-CT biradical process. The anionic decomposition catalyzed by CT, in which the S0/S1 surface “double crossing” hence has ability to provide high quantum yield of singlet chemiexcitation is thus more possible in bioluminescence of photoprotein.

The trimerization of acetylenes involves a cascade of biradical and pericyclic processes.

Thorough computational studies were performed on mechanisms and energies for the thermal trimerizations of neutral or electron-rich acetylenes used as cross-linkers in organic hard-masks for lithography applications. These studies indicate that the operative mechanism proceeds through initial cyclobutadiene formation via a biradical mechanism. Cyclobutadienes form Dewar benzenes via Diels-Alder cycloadditions, or biradical processes, or both, before producing benzenes by electrocyclic ring-opening reactions. These pathways are preferred to alternatives involving concerted trimerizations or mechanisms involving carbene intermediates.

Biradical processes in reactions between benzyne and tropone

The title reactions were investigated experimentally and theoretically. The benzyne--tropone pair has been known to give a Diels–Alder [4+2] cycloadduct. However, in the present experiment using two sources of benzyne, many unexpected products were obtained. In particular, a zwitterion with a C–Cl bond is strikingly a major product, the structure of which was determined by the X-ray analysis. The bond is formed by quenching the Cl atom from the solvent CH2Cl2, which demonstrates that the reactions proceed via radical processes. DFT calculations revealed the elementary processes of the reactions. The presence of the novel zwitterion in the solid state has been interpreted in terms of the permanent dipole–dipole attractions/interactions/stabilization.

Reaction of o-Benzyne with Tropothione Involving Biradical Processes

A benzyne-tropothione reaction was studied experimentally and computationally. Three isomeric products were detected by a careful experiment using two benzyne sources. The three equimolar products were identified. The expected symmetry-allowed [4+2] or [8+2] cycloadduct was not detected. In order to explain the unexpected products, density functional calculations and complete active space self-consistent field (CASSCF) calculations were carried out. The benzyne is, first, added to the tropothione via one-center C-S bond formation. Then a singlet biradical intermediate is formed. In the biradical, an alpha hydrogen atom of the tropothione moiety is moved to the benzyne moiety. A closed-shell intermediate is generated. This allene-type intermediate is isomerized to the second intermediate. The intramolecular proton shift in the latter leads to the three products. The biradical character of the benzyne has a key role in the present reaction and was discussed in reference to other benzyne reactions.

Theoretical Prediction of the Phenoxyl Radical Formation Capacity and Cyclooxygenase Inhibition Relationships by Phenolic Compounds

AbstractDue to the importance of the O‐H bond dissociation in the antioxidant mechanism of anti‐inflammatory phenols, we studied the biradical process Ph‐OH→PhO.+H. for 25 phenolic compounds using ab initio calculations. Enthalpies of reaction (ΔHr), changes in the electron density at the O‐H bond critical point (ρOH) and total atomic charges of ortho and para carbon atoms strongly correlate with the in vitro inhibition of cyclooxygenase activity by phenols. The most active compounds have large values of the electron density at the O‐H bond (ρOH), thus favouring the O‐H bond dissociation. In contrast, inactive compounds have small values of the electron density at the O‐H bond (ρOH), thus reducing the hydrogen donation ability. These results are also supported by the representation of the molecular electrostatic potentials maps. The prediction of the cyclooxygenase inhibitory activity of the proposed QSAR equations is analysed using the multilineal (MLR) method. Finally, the differences in biological activity are examined by analysing the binding interactions of active compounds in the pocket site of human COX‐2 enzyme structure derived from crystallographic X ‐ray data.

The half‐projected Hartree‐Fock model for computing thermally “forbidden” pericyclic reactions and biradical processes. I. Formulation and results for some singlet biradical species

AbstractThe possible use of a half‐projected Hartree‐Fock (HPHF) wave function correlating a single pair of electrons, being defined as a linear combination of two unrestricted Slater determinants which are identical except that all the α and β spin functions are interchanged, is investigated in relation to the computation of the potential energy surfaces involving singlet biradical or biradical‐like species. The results of calculations, within the framework of the MINDO/3 semiempirical Hamiltonian, for a number of prototypical biradical molecules indicate that, in general, the HPHF model provides a geometrical and energetical description that is equivalent to that obtained from a conventional two‐configuration self‐consistent‐field wave function.

Mechanism and kinetics of the radical polymerization of acrylamide in inverse micelles

AbstractA kinetic study of the photoinitiated polymerization of acrylamide in oil/AOT/H2O reverse micelles has been investigated. Determinations of the dependence of the rates of polymerization and the molecular weights of the polymers on initiator and monomer concentrations, as well as the relative light intensity and the identity of the oil employed were performed. Rates of polymerization were measured by dilatometry, and polymer molecular weights were determined by viscometry. The polymerization was shown to be first order in monomer concentration, and the order of the termination step of the polymerization depended strongly upon the oil employed. When the oil was benzene, termination occurred by a biradical process. When benzene was replaced by toluene, a monoradical termination was shown to occur, suggesting that a degradative chain transfer to toluene is occurring in this case. The similarities and differences of polymerization in conventional emulsions and in micelles are discussed on the basis of these results.

Reinvestigation of some thermally “forbidden” pericyclic reactions and biradical processes in the semiempirical TCSCF approach: Part 1. Disrotatory opening of cyclobutene, isomerization of bicyclo[2,1,0]pent-2-ene and dimerization of ethylene to cyclobutane

Two-configuration SCF MINDO/3 calculations on the three prototypical thermally “forbidden” pericyclic reactions of disrotatory openings of cyclobutene, isomerization of bicyclo[2,1,0]pent-2-ene and dimerization of ethylene to cyclobutane indicate that such a procedure provides a satisfactory description of both the biradical and biradicallike species involved in the pathway of these reactions and allows an efficient location of the stationary points on the corresponding potential energy surface. The present reinvestigation of the above three pericyclic reactions, shows that the structural and energetic results obtained by the MINDO/3 TCSCF procedure differe substantially from those reported in earlier MINDO/3 studies, based on either the single-configuration SCF or 2 × 2 CI wavefunctions.

Synthese et transposition thermique d'alcools α-ethyleniques β'-alleniques

Thirteen variously substituted α-ethylenic β'-allenic alcohols (1,5,6-triene-3-ols) have been synthesized by three different methods, for two of which the key step was the reaction of an organo copper reagent with a derivative of a β-hydroxy γ-ethylenic alcohol (1-ene-5-yne-3,7-diol). The[3,3]-sigmatropic rearrangement of these alcohols, following the heating of their solution in diglyme, leads to γ-dienic aldehydes and ketones. The yields are best (60–70%) for tertiary alcohols substituted on carbon 5. Comparisons of the reactivities of these 13 alcohols show that the mechanism of this rearrangement can probably vary, following the substitution from a cyclohexadiyl biradical process to a concerted pathway involving a cyclohexane transition state.

Differentiation of excited-state and biradical processes. Photochemistry of phenyl alkyl ketones in the presence of oxygen

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTDifferentiation of excited-state and biradical processes. Photochemistry of phenyl alkyl ketones in the presence of oxygenR. D. Small Jr. and J. C. ScaianoCite this: J. Am. Chem. Soc. 1978, 100, 14, 4512–4519Publication Date (Print):July 1, 1978Publication History Published online1 May 2002Published inissue 1 July 1978https://doi.org/10.1021/ja00482a032RIGHTS & PERMISSIONSArticle Views131Altmetric-Citations38LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (949 KB) Get e-Alerts Get e-Alerts

Shock Tube Studies of the Rate Constant for Chemical Excitation of Hydroxyl Radicals

Abstract The OH(2Σ+2-Π) chemiluminescence in the partial equilibrium region of the H2–O2 reaction in mixtures with H2/O2/Ar=2/2/96, P1=5, 10, 20 Torr was studied in 1250–2000 K incident shock waves. The comparison between the temperature dependence of the OH(2Σ+) emission and that of the computer simulation of the H2–O2 reaction yields the activation energy of the chemical excitation process, assuming the same bi-radical process as former investigators used. The evaluated OH(2Σ+) profile, based on a simulation in which the chemical excitation, radiative, and non-radiative processes are accounted for, depends on the pre-exponential factor of the chemical excitation process. The correlations between the temperature dependence of the evaluated OH(2Σ+) profile and that of the emission profile in the partial equilibrium region gave the pre-exponential factor of the chemical excitation process. Thus, we could estimate the rate constant for O+H+Ar=OH(2Σ+)+Ar to be 1.0×109×exp(−11000 cal⁄RT).

Orbital isomerism in biradical processes

Orbital isomerism in biradical processes

The [1,5] hydrogen transfer and cis–trans-isomerizations of cis-2,3-dimethylpenta-1,3-diene: the cis–trans-isomerization of penta-1,3-diene. Kinetics and equilibrium measurements

The kinetics of the gas-phase thermal unimolecular isomerization of cis-2,3-dimethylpenta-1,3-diene to 3,4-di-methylpenta-1,3-diene by a sigmatropic [1,5] hydrogen shift have been studied and Arrhenius equations obtained for the rate constants in both directions, viz., log k1/s–1= 11·13 ± 0·10 – 141·6 ± 0·9 kJ mol–1/2·303 RT(cis-2,3-DMP to 3,4-DMP), and log k2/s–1= 10·97 ± 0·10 – 139·1 ± 0·9 kJ mol–1/2·303 RT(3,4-DMP to cis-2,3-DMP). Equilibrium constants have also been measured. These results reinforce previous kinetic data on the [1,5] hydrogen shift and indicate a structure for the transition complex not unlike that of cyclopentadiene.The thermal unimolecular cis–trans-isomerization of cis-2,3-dimethylpenta-1,3-diene has also been studied and Arrhenius equations obtained for the rate constants, viz., log k3/s–1= 12·25 ± 0·13 – 183·7 ± 1·6 kJ mol–1/2·303 RT(cis to trans), and log k4/s–1= 12·36 ± 0·13 – 189·3 ± 1·6 kJ mol–1/2·303 RT(trans to cis).The reaction is interpreted in terms of a mechanism involving the intermediate formation of 1,2,3-trimethylcyclobutene. Also reported is the thermal unimolecular cis–trans-isomerization of cis-penta-1,3-diene. This reaction probably also occurs via cyclobutene formation, but a competing biradical process cannot be completely ruled out.