Diradical Chemistry Research Articles

Synthesis and Reactivity of a Dialane-Bridged Diradical.

Radicals of the lightest group 13 element, boron, are well established and observed in numerous forms. In contrast to boron, radical chemistry involving the heavier group 13 elements (aluminum, gallium, indium, and thallium) remains largely underexplored, primarily attributed to the formidable synthetic challenges associated with these elements. Herein, we report the synthesis and isolation of planar and twisted conformers of a doubly CAAC (cyclic alkyl(amino)carbene)-radical-substituted dialane. Extensive characterization through spectroscopic analyses and X-ray crystallography confirms their identity, while quantum chemical calculations support their open-shell nature and provide further insights into their electronic structures. The dialane-connected diradicals exhibit high susceptibility to oxidation, as evidenced by electrochemical measurements and reactions with o-chloranil and a variety of organic azides. This study opens a previously uncharted class of dialuminum systems to study, broadening the scope of diradical chemistry and its potential applications.

Diradicals Photogeneration from Chloroaryl-Substituted Carboxylic Acids.

With the aim of generating new, thermally inaccessible diradicals, potentially able to induce a double‐strand DNA cleavage, the photochemistry of a set of chloroaryl‐substituted carboxylic acids in polar media was investigated. The photoheterolytic cleavage of the Ar−Cl bond occurred in each case to form the corresponding triplet phenyl cations. Under basic conditions, the photorelease of the chloride anion was accompanied by an intramolecular electron‐transfer from the carboxylate group to the aromatic radical cationic site to give a diradical species. This latter intermediate could then undergo CO2 loss in a structure‐dependent fashion, according to the stability of the resulting diradical, or abstract a hydrogen atom from the medium. In aqueous environment at physiological pH (pH=7.3), both a phenyl cation and a diradical chemistry was observed. The mechanistic scenario and the role of the various intermediates (aryl cations and diradicals) involved in the process was supported by computational analysis.

Thiophene and its sulfur inhibit indenoindenodibenzothiophene diradicals from low-energy lying thermal triplets.

Many qualitative structure-property correlations between diradical character and emerging molecular properties are known. For example, the increase of diradical character further decreases the singlet-triplet energy gap. Here we show that inclusion of thiophenes within a quinoidal polycyclic hydrocarbon imparts appreciable diradical character yet retains the large singlet-triplet energy gap, a phenomenon that has no precedent in the literature. The low aromatic character of thiophene and its electron-rich nature are the key properties leading to these unique findings. A new indenoindenodibenzothiophene scaffold has been prepared and fully characterized by several spectroscopies, magnetic measurements, solid-state X-ray and state-of-the-art quantum chemical calculations, all corroborating this unique dichotomy between the diradical input and the emerging magnetic properties. New structure-property relationships such as these are not only extremely important in the field of diradical chemistry and organic electronics, but also provide new insights into the versatility of π-electron chemical bonding.

Direct Detection of the Open-Shell Singlet Phenyloxenium Ion: An Atom-Centered Diradical Reacts as an Electrophile.

A new photoprecursor to the phenyloxenium ion, 4-methoxyphenoxypyridinium tetrafluoroborate, was investigated using trapping studies, product analysis, computational investigations, and laser flash photolysis experiments ranging from the femtosecond to the millisecond time scale. These experiments allowed us to trace the complete arc of the photophysics and photochemistry of this photoprecursor beginning with the initially populated excited states to its sequential formation of transient intermediates and ultimate formation of stable photoproducts. We find that the excited state of the photoprecursor undergoes heterolysis to generate the phenyloxenium ion in ∼2 ps but surprisingly generates the ion in its open-shell singlet diradical configuration (1A2), permitting an unexpected look at the reactivity of an atom-centered open-shell singlet diradical. The open-shell phenyloxenium ion (1A2) has a much shorter lifetime (τ ∼ 0.2 ns) in acetonitrile than the previously observed closed-shell singlet (1A1) phenyloxenium ion (τ ∼ 5 ns). Remarkably, despite possessing no empty valence orbitals, this open-shell singlet oxenium ion behaves as an even more powerful electrophile than the closed-shell singlet oxenium ion, undergoing solvent trapping by weakly nucleophilic solvents such as water and acetonitrile or externally added nucleophiles (e.g., azide) rather than engaging in typical diradical chemistry, such as H atom abstraction, which we have previously observed for a triplet oxenium ion. In acetonitrile, the open-shell singlet oxenium ion is trapped to generate ortho and para Ritter intermediates, one of which (para) is directly observed as a longer-lived species (τ ∼ 0.1 ms) in time-resolved resonance Raman experiments. The Ritter intermediates are ultimately trapped by either the 4-methoxypyridine leaving group (in the case of para addition) or trapped internally via an essentially barrierless rearrangement (in the case of ortho addition) to generate a cyclized product. The expectation that singlet diradicals react similarly to triplet or uncoupled diradicals needs to be reconsidered, as a recent study by Perrin and Reyes-Rodríguez (J. Am. Chem. Soc. 2014, 136, 15263) suggested the unsettling possibility that singlet p-benzyne could suffer nucleophilic attack to generate a naked phenyl anion. Now, this study provides direct spectroscopic observation of this phenomenon, with an atom-centered open-shell singlet diradical reacting as a powerful electrophile. To the question of whether a nucleophile can attack a singly occupied molecular orbital, the answer is apparently yes, at least if another partially occupied orbital is available to avoid violation of the rules of valence.

Exploring diradical chemistry: a carbon-centered radical may act as either an anion or electrophile through an orbital isomer.

Diradical intermediates, formed by thermolysis of alkynylcyclobutenones, can display radical, anion, or electrophilic character because of the existence of an orbital isomer with zwitterionic and cyclohexatrienone character. Our realization that water, alcohols, and certain substituents can induce the switch provides new opportunities in synthesis. For example, it can be used to shut down radical pathways and to give access to aryl carbonates and tetrasubstituted quinones.

Exploring Diradical Chemistry: A Carbon‐Centered Radical May Act as either an Anion or Electrophile through an Orbital Isomer

AbstractDiradical intermediates, formed by thermolysis of alkynylcyclobutenones, can display radical, anion, or electrophilic character because of the existence of an orbital isomer with zwitterionic and cyclohexatrienone character. Our realization that water, alcohols, and certain substituents can induce the switch provides new opportunities in synthesis. For example, it can be used to shut down radical pathways and to give access to aryl carbonates and tetrasubstituted quinones.

Accurate prediction of diradical chemistry from a single-reference density-matrix method: Model application to the bicyclobutane to gauche-1,3-butadiene isomerization.

Multireference correlation in diradical molecules can be captured by a single-reference 2-electron reduced-density-matrix (2-RDM) calculation with only single and double excitations in the 2-RDM parametrization. The 2-RDM parametrization is determined by N-representability conditions that are non-perturbative in their treatment of the electron correlation. Conventional single-reference wave function methods cannot describe the entanglement within diradical molecules without employing triple- and potentially even higher-order excitations of the mean-field determinant. In the isomerization of bicyclobutane to gauche-1,3-butadiene the parametric 2-RDM (p2-RDM) method predicts that the diradical disrotatory transition state is 58.9 kcal/mol above bicyclobutane. This barrier is in agreement with previous multireference calculations as well as recent Monte Carlo and higher-order coupled cluster calculations. The p2-RDM method predicts the Nth natural-orbital occupation number of the transition state to be 0.635, revealing its diradical character. The optimized geometry from the p2-RDM method differs in important details from the complete-active-space self-consistent-field geometry used in many previous studies including the Monte Carlo calculation.

New Approaches to Singlet−Triplet Spin−Orbit Coupling in Photochemistry and Diradical Chemistry. Mechanistic and Exploratory Organic Photochemistry1,2

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTNew Approaches to Singlet−Triplet Spin−Orbit Coupling in Photochemistry and Diradical Chemistry. Mechanistic and Exploratory Organic Photochemistry1,2Howard E. Zimmerman and Andrei G. KutateladzeView Author Information Department of Chemistry, University of Wisconsin Madison, Wisconsin 53706Cite this: J. Am. Chem. Soc. 1996, 118, 1, 249–250Publication Date (Web):January 10, 1996Publication History Received5 September 1995Published online10 January 1996Published inissue 1 January 1996https://doi.org/10.1021/ja953052aCopyright © 1996 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views277Altmetric-Citations17LEARN 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 Read OnlinePDF (76 KB) Get e-AlertsSupporting Info (2)»Supporting Information Supporting Information SUBJECTS:Approximation,Excited states,Group theory,Mathematical methods,Molecules Get e-Alerts

On the mechanism of the benzophenone‐sensitized photolysis of 2,3‐diazabicyclo[2.2.1]hept‐2‐ene in the laser jet: Evidence for intermolecular triplet diradical reactions

AbstractThe benzophenone‐sensitized laser jet photolysis of 2,3‐diazabicyclo[2.2.1]hept‐2‐ene (1) affords, besides the previously reported cyclopentene and housane (2), also cyclopentane, cyclopentadiene, and the dimers bicyclopent‐2‐en‐1‐yl (7), 3‐cyclopentylcyclopent‐1‐ene (8), and 1,1'‐bicyclopentyl (9). As a model reaction, the pyrolysis of dimer 8 at 600°C/20 Torr leads to the other dimers 7 and 9 together with cyclopentadiene, cyclopentene, and traces of cyclopentane. Control experiments showed that H abstraction by the cyclopentane‐1,3‐diyldiracidal (3) from cyclohexene (as model substrate for cyclopentene) and addition to housane (2) with formation of diradical 6 are unlikely pathways. Instead, the product data available can be best explained in terms of an intermolecular disproportionation of two diradicals 3 to give the cyclopent‐2‐en‐1‐yl (4) and cyclopentyl (5) radical pair, which is subsequently converted to the observed products by in‐cage and out‐of‐cage coupling and H transfer reactions. Such intermolecular diradical chemistry becomes feasible due to the high steady‐state concentrations (ca, micromolar) generated in the laser jet. Two‐photon processes take place, but are of subordinate importance.

ChemInform Abstract: Reactivity of the Triplet State of the Tetrakis(μ‐pyrophosphito)diplatinate(II) Tetraanion with Alkenes and Alkynes. Comparison with the Energy‐Transfer Photosensitizer and Diradical Chemistry of Ketone Triplets.

ChemInform Abstract: Reactivity of the Triplet State of the Tetrakis(μ‐pyrophosphito)diplatinate(II) Tetraanion with Alkenes and Alkynes. Comparison with the Energy‐Transfer Photosensitizer and Diradical Chemistry of Ketone Triplets.

Reactivity of the triplet state of the tetrakis(.mu.-pyrophosphito)diplatinate(II) tetraanion with alkenes and alkynes. Comparison with the energy-transfer photosensitizer and diradical chemistry of ketone triplets

Reactivity of the triplet state of the tetrakis(.mu.-pyrophosphito)diplatinate(II) tetraanion with alkenes and alkynes. Comparison with the energy-transfer photosensitizer and diradical chemistry of ketone triplets

Chemistry of diradicals. V. Hot molecule effects in 1,4-diradicals. Thermal generation of a Norrish type II intermediate

Chemistry of diradicals. V. Hot molecule effects in 1,4-diradicals. Thermal generation of a Norrish type II intermediate

Chemistry of diradicals. IV. Thermochemistry, calculations, and the barrier to ring closure in short-chain-length diradicals

Chemistry of diradicals. IV. Thermochemistry, calculations, and the barrier to ring closure in short-chain-length diradicals

Chemistry of diradicals. III. Calculation of spin correlation effects on the rotational barriers in 1,4 diradicals

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTChemistry of diradicals. III. Calculation of spin correlation effects on the rotational barriers in 1,4 diradicalsL. M. Stephenson and Thomas A. GibsonCite this: J. Am. Chem. Soc. 1972, 94, 13, 4599–4602Publication Date (Print):June 1, 1972Publication History Published online1 May 2002Published inissue 1 June 1972https://doi.org/10.1021/ja00768a032RIGHTS & PERMISSIONSArticle Views42Altmetric-Citations14LEARN 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 (439 KB) Get e-Alerts Get e-Alerts