Diradical Intermediates Research Articles

Novel photo-rearrangements of 3,3,5-triaryl-1,2-dioxolanes: evidence for 1,5-dioxyl diradical intermediates

3-(p-Methoxyphenyl)-substituted 1,2-dioxolanes undergo novel photo-rearrangements to afford 1,3-diaryl-3-(p-methoxyphenoxy)propan-1-ones along with 1,3,3-triaryl-3-hydroxypropan-1-ones via 1,5-dioxyl diradical intermediates.

Oxygen initiated combustion: Thermochemistry and kinetics of unsaturated hydrocarbons

The thermochemistry and kinetics of the initiation reactions involved in the oxidation of unsaturated fuels are explored. The thermochemistry of intermediate radicals, diradicals, and molecular species involved are estimated using group additivity with some assistance from bond additivity. Kinetic parameters are estimated with the techniques of thermochemical kinetics. In the case of acetylene, estimated rate constants are in excellent agreement with experimental results on the induction period and flame speed. It is shown that the route initiated by O2 addition to an unsaturated carbon atom to produce a 1,4 diradical is faster than any other path available to form radicals capable of propagating a chain. The 1,4 diradicals so produced can generally cyclize to form a dioxetane which exothermically opens to a dialdehyde which is the ultimate radical source. Below 1000 K unsaturates will always initiate oxidation faster than saturated fuels. © 1996 John Wiley & Sons, Inc.

Photoinduced skeletal rearrangement of 1,1-diarylspiropentanes

The photochemical reactivities of 1,1-diarylspiropentanes 1a–c have been investigated under various photolysis conditions. Upon direct photolysis or acetone-sensitized photolysis, 1 underwent skeletal rearrangement to afford methylidenecyclobutanes 2, 3 and 4. A mechanism involving diradical intermediates has been proposed. Also studied were the photochemical and thermal reactions of the electron donor–acceptor (EDA) complexes of 1 with tetracyanoethylene (TCNE). Photoirradiation of the charge-transfer (CT) absorption bands of the EDA complexes resulted in the skeletal rearrangement of 1 to 2 and 3, with concomitant formation of TCNE adducts 11b,c and 12a–c. The X-ray structure of 11c is reported. A mechanism involving ion radical pairs [1˙+, TCNE˙–] is proposed for the photoreaction. In contrast, no skeletal rearrangement was observed in the thermal reaction of the EDA complexes although 11b and 11c were produced.

Ring Expansion of 4-Alkynylcyclobutenones. Synthesis of Piperidinoquinones, Highly Substituted Dihydrophenanthridines, Benzophenanthridines, and the Naturally Occurring Pyrrolophenanthridine, Assoanine

New synthetic routes to a variety of N-heterocyclic quinones and hydroquinones are described. These include thermolyses of 4-hydroxy-4-[4-N-(benzenesulfonyl)-4-aza-1,6-dialkynyl]cyclobutenones to piperidinoquinones and 4-hydroxy-4-[3-(N-phenylamino)-1-propynyl]cyclobutenones to dihydrophenanthridinediols. Included in the array of products available by this method are benzophenanthridines, indolophenanthridines, isoindoloindoles, and pyrrolophenanthridines. The methodology was employed in a five-step synthesis of the alkaloid assoanine starting with dimethyl squarate and indoline. The key step in all of these transformations is the ring expansion of appropriately substituted 4-hydroxy-4-alkynylcyclobutenones. These are envisaged to undergo electrocyclic ring opening to the corresponding enynylketenes which ring close to diradical intermediates that then lead to products via either radical additions to proximal alkyne moieties or undergo homolytic aromatic substitution to appropriately placed aryl groups. The synthetic scope and mechanism of the ring expansion reactions are discussed.

Photo-induced electron transfer in small peptides: glycylalanine

Participation of the peptide bond in photoinduced electron transfer accounts for product distributions in the photolysis of XG dipeptides and predicts the possibility that the side-chain will be involved in similar reactions of GX dipeptides. Product analysis following exposure of aqueous glycyl-DL-alanine to UV radiation shows this prediction is fulfilled. Thus, unlike XG peptides, glycylalanine degrades to acetamide substantially without co-production of CO2 and to products where bonding in the side-chain is involved in relaxation of the intermediate diradical. At a lower pH, decarboxylation radicals from glycylalanine have the opportunity to disproportionate as well as dimerize, and this accounts satisfactorily for the difference in product distribution when compared with that from XG photolysis under the same conditions.

Cycloaromatization of 6,7-benzobicyclo[8.3.0]trideca-6,10-diene-3,8-diyne-1,5-diols via diradical intermediates

Exposure of 1a to MsCl in the presence of Et 3 N, followed by treatment with H 2 O provided anthracene and phenanthrene derivatives 4 and 5 in an (1:1) product ratio via 1,6-didehydro[10]annulene intermediate. Under the same conditions 2a gave naphthalene derivative 6 in 9% yield via ene-yne-allene intermediate. These transformation could be explained by the rapid diradicalforming cycloaromatization. 1a reacted with MsCl in the presence of Et 3 N, followed by treatment with H 2 O to give 4 and 5 in an (1:1) product ratio. Under the same conditions, 2a gave 6 in 9% yield. Both reactions proceed via diradical intermediates.

The Dimerization of (E)‐1,3‐Diphenyl‐1,3‐butadiene Proceeds Suprafacially with Respect to Both Components: Evidence for a Concerted [4 + 2] Cycloaddition

Despite the potential stabilization of diradical intermediates through the phenyl groups, the thermal dimerization of (E)-1,3-diphenyl-1,3-butadiene does not follow a diradical mechanism as previously assumed, but a concerted pathway. The case examined here is the dimerization of the deuteriated diene 1 giving 2, which proceeds suprafacially with regard to both diene and dienophile.

Rearrangement and cycloreversion of Diels-Alder adducts of cyclic 1,3-dienes to norbornadienes. A novelhomo-cope rearrangement. Evidence for competitive pericyclic and diradical processes

On thermolysis of endo,endo-5, endo,endo-17, and endo,exo-17 (the endo,endo and endo,exo Diels-Alder adducts of cyclopentadiene 7 or dimethylfulvene 15 to norbornadiene 8 or 7-isopropylidenenorbornadiene 20), a novel type of homo-Cope rearrangement leading to 6, 21, and 23a, respectively, competes with the retro-Diels-Alder reactions. According to a force-field analysis of the kinetic parameters, the competitive reactions (rearrangement and retro-Diels-Alder reaction) of endo,endo-5 occur in a pericyclic fashion whereas in the isopropylidene-substituted systems stepwise processes compete with the corresponding pericyclic reactions. Indirect experimental evidence for this assumption comes from a stereochemical analysis of the refro-Diels-Alder reaction in the cis-5,6-dideuterionorbornene derivatives exo-36-d2, endo-36-d2 and exo-39-d2, endo-39-d2 occurring stereospecifically in the case of exo-36-d2, endo-36-d2 and non-stereospecifically in the case of exo-39-d2, endo-39-d2. The change in mechanism is rationalized by a different (allyl vs. pentadienyl) stabilization of the potential diradical intermediates in the stepwise reactions.

Excited State Reactivity as a Function of Diradical Structure. Evidence for Two Triplet Cyclopropyldicarbinyl Diradical Intermediates with Differing Reactivity

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTExcited State Reactivity as a Function of Diradical Structure. Evidence for Two Triplet Cyclopropyldicarbinyl Diradical Intermediates with Differing ReactivityHoward E. Zimmerman, Andrei G. Kutateladze, Yasunari Maekawa, and John E. MangetteCite this: J. Am. Chem. Soc. 1994, 116, 21, 9795–9796Publication Date (Print):October 1, 1994Publication History Published online1 May 2002Published inissue 1 October 1994https://doi.org/10.1021/ja00100a073RIGHTS & PERMISSIONSArticle Views104Altmetric-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 PDF (273 KB) Get e-AlertsSupporting Info (2)»Supporting Information Supporting Information Get e-Alerts

1,3,5‐Cyclohexatrien‐1,4‐diyl und 2,4‐Cyclohexadien‐1,4‐diyl

AbstractThe Energy Well of Diradicals, V[1]. — 1,3,5‐Cyclohexatriene‐1,4‐diyl and 2,4‐Cyclohexadiene‐1,4‐diylThe energy profile of the Bergman rearrangement of (Z)‐3‐hexene‐1,5‐diyne (4) has been established from the NO and oxygen dependance of the trapping rate of the intermediate diradical 1 which leads to a heat of formation for 1,4‐didehy‐drobenzene (1) of δHOf= 138.0 ± 1.0 kcal . morl−1. By the same technique the heat of formation of 1,2,4‐cyclohexatriene (2), generated by thermolysis of (Z)‐1,3‐hexadien‐5‐yne (10), gives δHOf = 105.1 ± 1.0 kcal . mol−1 which indicates a high diradical character for 2.

DNA Cleavage by 4-Alkynyl-3-methoxy-4-hydroxycyclobutenones

4-Alkynyl-3-methoxy-4-hydroxycyclobutenones, a readily available class of compounds, cleave supercoiled DNA by a mechanism that appears to require at least some involvement of diradical intermediates formed in the ring expansion of the cyclobutenones

Effects of .alpha.-tert-butyl group substitution on the reactivity and dimerization products of furan-based o-quinodimethanes

Flash vacuum pyrolysis (FVP) of 2-neopentyl-3-furylmethyl benzoate (8) produces 3-methylene-2-(tert-butylmethylene)-2,3-dihydrofuran (5), the major product as shown by low-temperature 1 H NMR spectroscopy. Upon warming to room temperature, 5 dimerizes giving mostly two stereoisomeric [4+2] dimers 11a and 11b in addition to a small amount of the [4+4] dimer 12. FVP of a mixture of the two [4+2] dimers 11a and 11b gives the thermodynamically more stable [4+4] dimer 12. The rate constants for the dimerization of 5 in solution at temperatures from -29 to +5 o C were determined by 1 H NMR spectroscopy. The rate constants and activation parameters (ΔH * =10.8 kcal/mol, ΔS * = -28.8 eu) are very simlar to those reported for the unsubstituted furan-based o-quinodimethane. FVP of (2-methyl-3-furyl)(tert-butyl)methyl benzoate (13) and (2-neopentyl-3-furyl)(tert-butyl)methyl benzoate (18) give as the major product, 2-methylene-3-(tert-nutylmethylene)-2,3-dihydrofuran (6) and 2,3-bis(tert-butylmethylene)-2,3-dihydrofuran (7), respectively. Compounds 6 and 7, in contrast to 5, are stable at room temperature apparently because for each of these compounds a bulky tert-butyl group is on the more reactive methylene, the 3-methylene. These results offer further support for the mechanism for the dimerization of furan-based o-quinodimethanes which proceeds in two steps via a transient diradical intermediate

Regio and stereochemistry in 2+2 intramolecular photocycloaddition of substituted olefins to cyclopentenones

The influence of substituents on the regio- and stereochemistry of the intramolecular [2+2] photocycloaddition of compound 1–8 has been studied. All photoadducts are formed via parallel approach and syn-addition. The endo/exo ratio was used as indicator for the structure of the diradical intermediates.

Zur Energiedelle des orthogonalen Trimethylenmethans. – 1‐Methylen‐2‐phenylcyclopropan‐Thermolyse

Energy Well of the Orthogonal Trimethylenemethane. – 1-Methylene-2-phenylcyclopropane Thermolysis From the heat of hydrogenation of 5, the activation enthalpy for the racemization of the title compound, and the oxygen dependance of the trapping rate of the intermediate diradical 8 the energy profile for the degenerate methylene-cyclopropane rearrangement can be constructed, which leads to heats of formation for the triplet and singlet state of the diradical 8 of ΔHf0 · 93.9 and 95.6 kcal mol–1, respectively.

ChemInform Abstract: Experimental and Theoretical Exploration of the Detailed Mechanism of the Rearrangement of Barrelenes to Semibullvalenes: Diradical Intermediates and Transition States.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Diastereomerically Pure 3‐(Silyloxy)oxetanes by a Selective Paternò‐Büchi Reaction

AbstractDiastereomerically pure 3‐[(trimethylsilyl)oxy]oxetanes 3 were prepared in moderate to good yields (45–72%) by the Paternò‐Büchi reaction of silyl enol ethers 2 with benzaldehyde. The photocycloaddition exhibits a high degree of regio‐ and diastereoselectivity. The substituents R in the silyl enol ether have been varied [R = Me, Et, iPr, tBu, Ph, CH(OMe)2, CH(OCH2)2, C(OCH2)2Me], and it was found that steric bulk is mainly responsible for enhanced selectivity (diastereoselectivity from 70:30 up to 95:5). The regiochemical control is perfect (regioselectivity >95:5) except for silyl enol ether 2a (R = Me) in the case of which a 90:10 ratio of regioisomers was observed. Irradiation of the reaction mixture at lower temperature (–25°C) led to a further improvement of diastereoselectivity. The relative configuration of the products obtained was elucidated both by 1H‐NMR spectroscopy and by chemical degradation. As a mechanistic hypothesis to explain the high observed diastereoselectivity we propose that the steric environment in the intermediate diradical 11 determines the selectivity according to two possible reaction pathways, i.e. bond formation and retrocleavage.

ChemInform Abstract: Electronic Structure of Diradical and Dicarbene Intermediates in Short‐ Chain Polydiacetylene Oligomers

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Experimental and theoretical exploration of the detailed mechanism of the rearrangement of barrelenes to semibullvalenes: diradical intermediates and transition states

The equation of which stage of the triplet di-r-methane rearrangement controls the regiochemistry and excited-state rate was resolved in the present investigation. In this study, using m-cyanodibenzobarrelene, the m-cyano substituent was located in a strategic location such that bridging between the vinyl and substituted benzo moiety would lead to stabilized diradical I but nonstabilized diradical II. Conversely, the substitution was such that with bridging between the vinyl and unsubstituted benzo ring, diradical II would be stabilized but diradical I would not. The study of the di-r-methane rearrangement of this system thus weighed the relative importance of stabilization of diradical I versus stabilization of diradical II along the excited triplet hypersurface

Electronic structure of diradical and dicarbene intermediates in short-chain polydiacetylene oligomers

Semiempirical molecular orbital (MO) calculations on short-chain (n=2 to n=8 monomer units) diradical and dicarbene intermediates obtained in the solid state polymerization reaction of diacetylene monomer crystals have been performed. With increasing chain length a transition from the diradicals with two unpaired electrons to the dicarbenes with four unpaired electrons is found in agreement with experimental results. The energetic ordering of the low-lying multiplets (singlet and triplet for the diradicals; singlet, triplet, and quintet for the dicarbenes) is discussed in terms of a model Hamiltonian containing a Hubbard term which takes into account the electron transfer between the radical electrons on different chain ends, and a Heisenberg term describing the exchange interaction of radical electrons on the same chain end in the case of the dicarbenes. All intermediates have a singlet ground state. The small energy gaps to the higher triplet and quintet states decrease rapidly with increasing chain length due to a decrease in the ratio t/U, with t and U being the transfer integral and the on-site Coulomb repulsion of the Hubbard model, respectively. The fine structure arising from the magnetic dipolar interaction of the unpaired electrons is discussed.

Stereochemical features of the (2+2) cycloaddition reactions of chiral allenes. V. Cycloaddition of 1‐tert‐butyl‐3‐methylallene with 1,1‐dichloro‐2,2‐difluoroethene

AbstractThe (2 + 2) cycloaddtion reactions of 1‐tert‐butyl‐3‐methylallene (tBMA) with radicophiles were investigated. The attempted cycloaddition reactions with N‐phenylmaleimide, acrylonitrile and methyl acrylate produce only (4 + 2) cycloadducts of 1‐tert‐butyl‐1,3‐butadiene which is formed by the more rapid [1.3] hydrogen sigmatropic rearrangement of the tBMA. The (2 × 2) cycloaddition of tBMA with 1,1‐dichloro‐2,2‐difluoroethene (1122) occurs more rapidly than does the sigmatropic rearrangement, and produces a mixture of the four cycloadducts 1–4. The cycloaddition of 1122 with enantioenriched tBMA produces one cycloadduct (3) in which ca 91% of the enantiomeric excess (ee) of the tBMA is transferred to the cycloadduct. The other three cycloadducts are formed retaining much less of the ee of the starting tBMA. The results are interpreted on the basis of molecular modeling calculations carried out on the 1122–1,3‐dimethylallene system reported previously. It is suggested that cycloadduct 3 is formed by essentially only one continuous minimum‐energy reaction pathway, while cycloadducts 2 and 4 are formed by two competitive minimum‐energy reaction pathways which result in the formation of cycloadducts possessing opposite absolute configurations. The combined contributions of the two competitive pathways result in much lower overall degrees of transfer of the ee of the tBMA to the diradical intermediates and cycloadducts.