Diradical Pathway Research Articles

Concerted vs stepwise mechanism in 1,3-dipolar cycloaddition of nitrone to ethene, cyclobutadiene, and benzocyclobutadiene. A computational study

The problem of competition between concerted and stepwise diradical mechanisms in 1,3-dipolar cycloadditions was addressed by studying the reaction between nitrone and ethene with DFT (R(U)B3LYP/6-31G) and post HF methods. According to calculations this reaction should take place via the concerted cycloaddition path. The stepwise process is a viable but not competitive alternative. The R(U)B3LYP/6-31G study was extended to the reaction of the same 1, 3-dipole with cyclobutadiene and benzocyclobutadiene. The very reactive antiaromatic cyclobutadiene has an electronic structure that is particularly disposed to promote stepwise diradical pathways. Calculations suggest that its reaction with nitrone represents a borderline case in which the stepwise process can compete with the concerted one on similar footing. Attenuation of the antiaromatic character of the dipolarophile, i.e., on passing from cyclobutadiene to benzocyclobutadiene, causes the concerted 1,3-dipolar cycloaddition to become once again prevalent over the two-step path. Thus, our results suggest that, in 1,3-dipolar cycloadditions that involve normal dipolarophiles, the concerted path (Huisgen's mechanism) should clearly overwhelm its stepwise diradical (Firestone's mechanism) counterpart.

The Pressure Dependence of the OH Radical Yield from Ozone−Alkene Reactions

The OH radical is the key oxidizing agent in the troposphere, and ozone−alkene reactions appear to be a significant and sometimes dominant source of new HOx radicals in urban and rural air. In this work, we report the first study of the pressure dependence of the OH radical yield for the ozonolysis of ethene, propene, 1-butene, trans-2-butene, and 2,3-dimethyl-2-butene over the range 20−760 Torr and of trans-3-hexene and cyclopentene over the range 200−760 Torr. Low-pressure experiments were performed in a long-path evacuable FTIR cell or a steady-state flow-tube reactor in series with a gas chromatograph/flame ionization detector and FTIR cell. We have also investigated the effect of adding SF6 at atmospheric pressure for ethene, 1-butene, and trans-2-butene, in a collapsible Teflon chamber. OH formation increased almost 3-fold for ethene at low pressures, from 0.22 ± 0.06 at 760 Torr to 0.61 ± 0.18 at 20 Torr, and increased somewhat for propene from 0.33 ± 0.07 at 760 Torr to 0.46 ± 0.11 at 20 Torr. A pressure dependence of the OH formation yield was not observed for 1-butene, trans-2-butene, 2,3-dimethyl-2-butene, trans-3-hexene, or cyclopentene over the ranges studied. Density functional theory calculations at the B3LYP/6-31G(d,p) level are presented to aid in understanding the trends observed. They lead to the proposal that the formation of a hydroperoxide via a diradical pathway can compete with the formation of the carbonyl oxide for the ethene primary ozonide.

Diradical and Peroxirane Pathways in the [π2 + π2] Cycloaddition Reactions of1ΔgDioxygen with Ethene, Methyl Vinyl Ether, and Butadiene: A Density Functional and Multireference Perturbation Theory Study

The main purpose of this study is to assess the relative importance of diradical or peroxirane (perepoxide) intermediates in the singlet oxygen cycloaddition reactions with alkenes that lead to dioxetanes. The relevant nonconcerted pathways are explored for ethene, methyl vinyl ether, and s-trans-butadiene by CAS-MCSCF optimizations followed by multireference perturbative CAS-PT2 energy calculations and by DFT(B3LYP) optimizations. The two different theoretical approaches gave similar results (reported below). These results show that methoxy or vinyl substitution does not affect qualitatively the reaction features evidenced by the unsubstituted system. Peroxirane turns out to be attainable only by passing through the diradical, due to the nature of the critical points involved. The energy barriers for the transformation of the diradical to peroxirane in the case of ethene (ΔE⧧ = 13−15 kcal mol-1) and methyl vinyl ether (ΔE⧧ = 12−13 kcal mol-1) are higher than those for the diradical closure to dioxetane (...

The C7H10 Potential Energy Landscape: Concerted Transition States and Diradical Intermediates for the Retro-Diels−Alder Reaction and [1,3] Sigmatropic Shifts of Norbornene

The potential energy surfaces for the thermal reactions of bicyclo[3.2.0]hept-2-ene and norbornene have been explored with density functional theory at the Becke3LYP/6-31G* level. Both concerted and diradical pathways for the retro-Diels−Alder reaction of norbornene have been examined, and the activation parameters and 13C primary kinetic isotope effects predicted for the concerted pathway are in excellent agreement with experimental data. The concerted mechanism is favored over the lowest energy stepwise diradical route by 12.4 kcal/mol. For the orbital symmetry-allowed suprafacial-inversion (si) pathway of the [1,3] sigmatropic rearrangement of bicyclo[3.2.0]hept-2-ene to form norbornene, a mechanism involving a transition state which leads to a broad diradical plateau on the potential energy surface is predicted. Implications of these surfaces, which differ substantially from those obtained by semiempirical calculations, are also discussed.

Stereoselective Formation of Formal Exo Diels−Alder Adducts of Silyloxydienes and Allenecarboxylates

stereochemical features of the desired natural products and thereby perhaps simplify its total synthesis.3 However, all attempts at carrying out such a cycloaddition with 2 using Fisher carbene complexes4 3 [Z ) C(OMe)dCr(CO)5], which would have given the exo adducts, or the simple acid chloride 3 (Z ) COCl), which would have given more of the endo adducts, unfortunately gave only starting material, presumably due to the steric hindrance of these dienophiles. We proposed using R-methyl allenecarboxylates as the dienophiles in order to decrease this steric hindrance with a subsequent reduction of the exocyclic double bond to give compounds such as 4. We report herein the thermal cycloaddition of silyloxydienes 2 (X ) OSiR3) and allenecarboxylates which ultimately afford the formal exo Diels-Alder adducts 4. Heating a solution of E-4-methyl-2-silyloxybutadiene 55 with ethyl 2-methylbuta-2,3-dienoate 6 in toluene at 120 °C (sealed tube) for 14 h afforded, after chromatographic separation, a mixture of three compounds, the endo Diels-Alder product 7n (18%), the exo Diels-Alder product 7x (28%), and the [2 + 2] cycloadduct 8 (22%) (Scheme 1). The isolation of [2 + 2] cycloadducts from dienes and allenes was precedented in our own work and that of others.6 However, we have now shown that thermolysis of the purified cyclobutane 8 in toluene at 120 °C for 4 d gave only the exo cycloadduct 7x. Thus heating the diene 5 and dienophile 6 in toluene for 4 d gave only the two [4 + 2] cycloadducts 7n and 7x in 14 and 58% yields, respectively. Although we had observed the thermal rearrangement of a [2 + 2] cycloadduct into a [4 + 2] cycloadduct before,6ab this is the first case where the stereochemical course of this process could be determined. The mechanism of the rearrangement of simple 3-ethenylmethylenecyclobutanes I has been proposed to involve an initial bond homolysis to give a bis(allylic) diradical II which then recombines at the termini to give the 4-methylenecyclohexene III.7 Indeed it was shown that the divinyl compound I (R ) CHdCH2) had a lower activation energy for this reaction (Ea ) 27.3 kcal/mol) than did the parent I (R ) H) (Ea ) 35.7 kcal/mol).7a We believe that the substitution in our case may cause the reaction to proceed through a zwitterionic rather than a diradical pathway although the latter can certainly not be ruled out.

Strained Allenes as Dienophiles in the Diels-Alder Reaction: An Experimental and Computational Study.

Strained allenes, such as 1,2-cyclohexadiene, undergo facile Diels-Alder reactions with otherwise unreactive dienes. Substituted cyclohexa-1,2-dienes add to furan via a Diels-Alder reaction, forming only two of four possible regioisomers and stereoisomers. Alkyl cyclohexa-1,2-diene carboxylates yield the nonconjugated endo adduct as the major product. However, chiral cyclohexa-1,2-dienecarboxylates, such as l-menthyl and l-bornyl cyclohexa-1,2-dienecarboxylate, show no diastereoselectivity in [4 + 2] cycloadditions. DFT (B3LYP/6-31G) calculations were performed in order to explain these results. A comparison to the calculated B3LYP/6-31G transition structures and intermediates along the reaction paths of 1,2-cyclohexadiene with 1,3-butadiene and with furan (as well as propadiene with butadiene) show that the diradical stepwise pathways are preferred over the concerted paths. At the same time, the concerted transition structures are extremely asynchronous. A QM/MM study indicates a minimal influence of the chiral auxiliary even in the concerted scenario.

2+2+2]-Cycloreversion reactions: a theoretical elucidation of thermodynamic and through-bond coupling effects on activation energies †‡

[2+2+2]-Cycloreversion reactions of cyclohexane and ten fused cyclohexanes were studied computationally with B3LYP/6-31G* and CASSCF methods. Reactions involving cleavage of bonds in three- and five-membered rings show distinctly lower barriers to cycloreversion than cleavage of four-membered rings. The lower activation energies for the cleavage of odd-membered rings arise from interactions of the sigma framework of the odd-membered ring with the orbitals of the breaking bond. NICS values were calculated to determine the aromaticity of the different rings involved in bond cleavage. In addition to concerted mechanisms, the stepwise diradical pathways for the [2+2+2]-cycloreversions of cyclohexane and cis-tris-cyclopropacyclohexane were studied.

Substituent Effect on Stereospecificity and Energy of Concert of the Retro-Diels−Alder Reaction of Isopropylidenenorbornene

The synchronous concerted and stepwise diradical pathways of the retro-Diels−Alder reaction of isopropylidenenorbornene have been studied theoretically at the B3LYP/6-31G* level. The effect of the isopropylidene substituent on the competition between the two mechanisms was determined by comparing these results to those obtained earlier with norbornene. The ease of internal rotation of the methylene radical center relative to the C−C bond cleavage in the diradical intermediate is compared with experimental data on stereoselectivity in the retro-Diels−Alder reaction of stereospecifically dideuterated isopropylidenenorbornene. Experimental and theoretical evidence indicates that the lifetime of the diradical is long enough for the stereochemical scrambling in the thermal retro-Diels−Alder reaction.

Structure-activity relationships of cyclic enediynes related to dynemicin A—II. Synthesis and antitumor activity of 9- and 12-substituted enediynes equipped with aryl carbamate moieties

Novel enediyne compounds 4–8, simple analogues of dynemicin A ( 1) equipped with the phenyl or 4-chlorophenyl carbamate moiety, were synthesized and evaluated for DNA-cleaving ability, in vitro cytotoxicity, and in vivo antitumor activity. As a result of the SAR study, it was revealed that the size and character of the substituents (R 1 and R 2) at the C9 position critically influenced both the stability and antitumor activity of the enediyne compounds. We found that the 9-deoxy compound 6a, a stable and less bulky enediyne having a hydrogen as the R 1 and R 2 substituents, showed a significant in vivo activity with a T/C of 215% at a daily dosage of 2.0 mg/kg for 4 days. The incorporation of an oxygen-containing functional group as the R 3 substituent on a benzene ring resulted in considerable abolishing of both the in vitro and in vivo potencies. In a series of 9-acyloxy compounds, incorporation of the basic aromatic moiety such as 8e was effective for the in vitro activity, but it was ineffective for the in vivo activity. Furthermore, for the stereochemistry-activity relationships at the C9 position, the (9R ∗)- isomers of 8c, 8e, and 8f were found to show higher both in vitro and in vivo than the corresponding (9S ∗)- isomers . For the mechanistic studies, compound 6a underwent Bergman cycloaromatization via a diradical pathway under acidic conditions, whereas it scarcely showed DNA-cleaving activity due to the chemical stability of the aryl carbamate moiety under neutral conditions.

Dioxetane decomposition revisited: A semi‐empirical study of the potential energy surface

AbstractThe potential energy surface describing the unimolecular thermolysis of 1,2‐dioxetane into two formaldehydes is explored via semi‐empirical calculations using the PM3 Hamiltonian with multi‐electron configuration interaction. An active space of the four highest occupied orbitals and the two lowest virtual orbitals is used. Several reaction coordinates were examined, and the in‐plane OO distance was used most extensively. The results indicate that the activation barrier to fission is on the ground state surface and is about 18 kcal mol−1, in good agreement with experimental results. Furthermore, calculations of the vertical triplet‐state energies show that the ground and triplet states are nearly degenerate for OO distances beyond that corresponding to the 18 kcal mol−1 barrier, until the molecule abruptly dissociates into two formaldehydes (at ca 2·55 A). This picture is indicative of a diradical pathway, where the activation energy is associated with motion of the system on the ground‐state surface, not the triplet surface, but where generation of a triplet product can be expected. Similar results were obtained for dimethyl‐ and tetramethyldioxetanes, which require higher Ea for dissociation, and for dimethyldioxetanone. Thermochemical calculations pertaining to these reactions and to the formation of triplet products are also presented.

The photorearrangement of o-vinyl diaryl ethers into o-hydroxy stilbenes evidence against dipolar intermediates

On irradiation of the o-vinyl diaryl ethers 4, 7, and the D-labeled derivative 16, a rearrangement takes place leading to the phenol 6 (from 4) and the corresponding vinyl-bridged hydroxy stilbenes 8/17 (from 7/16) as primary products. The latter compounds are further transformed into the phenanthrenes 9 and 18 after 6π-cyclization and subsequent loss of methanol and water, respectively. A mechanism involving inter- or intramolecular diradical pathways is discussed.

Electron-transfer-induced photochemical reactions of (silylallyl)iminium and benzylpyrrolinium salts by dual diradical and diradical cation cyclization pathways

The electron-transfer-induced photocyclization reactions of a series of N- and C-silylmethallyl-substituted iminium and 1-benzyl-1-pyrrolinium salts have been investigated in order to gain information about their mechanistic course and to probe their synthetic potential. The results obtained from these studies demonstrate that diradical cation intermediates formed in these processes can be transformed to products via two competing mechanistic pathways involving radical coupling or loss of allylic or benzylic electrofugal groups

Thermolysis of the sulfur dioxide adducts of benzobenzvalene. The 1.3-dipolar behaviour of a sulfene

Pyrolysis (fvp) of a strained γ-sultine ( 3) produced 1H-indene-1-carboxaldehyde ( 8) and naphthalene. The key step in the production of aldehyde ( 8) involves a 1,3-dipolar cycloreversion, as evidenced by the trapping of an intermediate sulfene ( 6) with methyl acrylate. A diradical pathway is proposed for the formation of naphthalene.

Photocyclization reactions of 1-benzyl-1-pyrrolinium salts by diradical and diradical cation pathways. Novel photochemical Pictet-Spengler cyclizations

ADVERTISEMENT RETURN TO ISSUEPREVArticlePhotocyclization reactions of 1-benzyl-1-pyrrolinium salts by diradical and diradical cation pathways. Novel photochemical Pictet-Spengler cyclizationsIn Seop Cho and Patrick S. MarianoCite this: J. Org. Chem. 1988, 53, 7, 1590–1592Publication Date (Print):April 1, 1988Publication History Published online1 May 2002Published inissue 1 April 1988https://doi.org/10.1021/jo00242a056RIGHTS & PERMISSIONSArticle Views192Altmetric-Citations10LEARN 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 (365 KB) Get e-Alerts Get e-Alerts

Synthesis and rearrangement of alkylaryl- and aryl-substituted dihydrosemibullvalenes by thermolysis of 7,8-fused cyclo-octa-1,3,5-triene derivatives

The thermal cycloaddition of a cyclobuteno-dienophile (1) with cyclopentadienones has been systematically investigated; the stereoisomeric adducts give convenient access by decarbonylation to a variety of tetra-aryl-, methyltriphenyl-, triphenyl-, tri-t-butyl-, and dimethyldiphenyl-cyclo-octa-1,3,5-triene derivatives as the products of electrocyclic ring-opening of the valence-tautomeric primary products of cheletropic bridge-extrusion, viz. bicyclo[4.2.0]octadienes. These compounds provide useful models for investigation of equilibria between the electrocyclic valence tautomers, the scope and mechanism for thermal cross-cyclisations in, for example, unsymmetrically substituted cyclo-octatrienes, and the thermal vinyl–cyclopropane (1,3-allylic shift) isomerisation and/or H-atom transfer disproportionation of the resulting arylated and alkylaryldihydrosemibullvalenes. The results best accord with diradical pathways for cyclo-octatriene–dihydrosemibullvalene conversions and subsequent rearrangements. Useful 13C and 1H n.m.r. structure correlations and new examples of cyclopentadienones are also reported.

Photochemical studies. 143. Diradical pathways in vinylcyclopropene triplet rearrangements; mechanistic and exploratory organic photochemistry

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPhotochemical studies. 143. Diradical pathways in vinylcyclopropene triplet rearrangements; mechanistic and exploratory organic photochemistryHoward E. Zimmerman and Steven A. FlemingCite this: J. Org. Chem. 1985, 50, 14, 2539–2551Publication Date (Print):July 1, 1985Publication History Published online1 May 2002Published inissue 1 July 1985https://doi.org/10.1021/jo00214a027RIGHTS & PERMISSIONSArticle Views357Altmetric-Citations16LEARN 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 (2 MB) Get e-Alerts Get e-Alerts

Photochemical studies on bicyclic βγ-unsaturated ketones: II1. Okadi-π-methane rearrangements originating from some state other than T1 (π-π*)

the unquenchable ODPM reaction observed on direct irradiation of three βγ-unsaturated ketones has been studied in solution and in the vapour-phase. The results suggest that the higher triplet state might be the origin of this reaction. Thermal generation of this excited state in one case supports the conclusion. The vapour-phase results further suggest the existence of both diradical and concerted pathways for the [1,3]-acyl shift from the excited singlet state, and the possibility of a similar pattern from the higher triplet state.

A thermal [1,3] sigmatropic rearrangement: thermolysis of 2,2-bis(ethylthio)-3,3-dimethyl-4-pentenal

2,2-Bis(ethylthio)-3,3-dimethyl-4-pentenal was found to quantitatively isomerize to 2,2-bis(ethylthio)-5-methyl-4-hexenal over a wide temperature range (130–170 °C). This rearrangement can formally be regarded as a [1,3] sigmatropic shift. The reaction, which could be conveniently monitored by 1Hmr spectroscopy, was found to obey first order kinetics. The substantial positive entropy of activation (+ 61.5 J deg−1 mol−1) for the reaction and the negligible solvent effect (decane vs. DMF) are both consistent with a proposed diradical pathway.

Thermal fragmentation of 3-alkyl-2-phenyloxetanes, 3,3-dimethyl-2-aryloxetanes, and related compounds. A case study of 2-aryl-substituted oxetanes

Thermolyses of epimeric 3-alkyl-2-phenyloxetanes (1c, 1t, 2c, 2t, 3c, and 3t), 3,3-dimethyl-2-aryloxetanes (4, 5, 6, 7, and 8), 3,3,4,4-tetramethyl-2,2-diphenyloxetane (9), and 3,3-dimethyl-2,2-diphenyloxetane (10) were studied in degassed N,N,N′,N′-tetramethylethylenediamine at 270–350 °C. Although the fragmentation of 9 and 10 can be understandable on the basis of a diradical mechanism, there were several observations, in the reaction of certain other oxetanes, which could hardly be explained simply in terms of such a mechanism. Namely, (1) less strained 1t reacted faster than more strained 1c; (2) a major mode of the fragmentation for 1c, 2c, and 3c was "B" (forming an alkene and benzaldehyde), whereas that for 1t, 2t, and 3t was "A" (forming an alkenylbenzene and formaldehyde); (3) the apparent energy of activation for the "B" process seemed to be larger than that for the "A"; (4) a dramatic change of the major fragmentation mode from "B" to "A" was brought about by a substituent on the phenyl group, as was observed in 4–8. These results may be explained reasonably by assuming that the fragmentation proceeds, at least, in dual reaction courses. In competition with an anticipated diradical pathway, there will be another process, which is energetically little more favorable than the diradical fragmentation, rather specific to the "A" mode of fragmentation, and important particularly in the reaction of the trans isomers. Probable candidates for the second process are discussed.

ChemInform Abstract: ORGANIC TRANSITION STATES PART 2, THE METHYLENECYCLOPROPANE REARRANGEMENT, A TWO‐STEP DIRADICAL PATHWAY WITH A SECONDARY MINIMUM

Chemischer InformationsdienstVolume 5, Issue 37 Preparative Organic Chemistry ChemInform Abstract: ORGANIC TRANSITION STATES PART 2, THE METHYLENECYCLOPROPANE REARRANGEMENT, A TWO-STEP DIRADICAL PATHWAY WITH A SECONDARY MINIMUM W. J. HEHRE, W. J. HEHRESearch for more papers by this authorL. SALEM, L. SALEMSearch for more papers by this authorM. R. WILLCOTT, M. R. WILLCOTTSearch for more papers by this author W. J. HEHRE, W. J. HEHRESearch for more papers by this authorL. SALEM, L. SALEMSearch for more papers by this authorM. R. WILLCOTT, M. R. WILLCOTTSearch for more papers by this author First published: September 17, 1974 https://doi.org/10.1002/chin.197437130AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume5, Issue37September 17, 1974 RelatedInformation