Diradical Intermediates Research Articles

Absence of tetramethylene diradical intermediates in Lewis acid catalyzed group transfer polymerization

A diradical tetramethylene intermediate in Lewis acid catalyzed Group Transfer Polymerization of methyl acrylate is unlikely based on the lack of incorporation of styrene during methyl acrylate polymerizations.

Photochemical Reaction of Benzophenones with Allylic Silanes

Abstract Photocycloaddition and hydrogen abstraction reactions between benzophenones and allylic silanes have been investigated. The high regioselective oxetane formations were explained in terms of an additional stability due to the σSi–C–pπ hyperconjugation of β-silyl radicals in diradical intermediates. Allyl radical substituted by a trimethylsilyl group, which was generated from a hydrogen abstraction reaction, recombined with benzophenone ketyl radical at the γ position. No electron transfer products were observed in the reactions.

ChemInform Abstract: Studies in the Cycloproparene Series: Approaches to Cyclopropaheteroarenes.

AbstractPhotolysis of the compounds (I) gives rise to the intermediate diradicals (II), which do not undergo ring‐closure reaction to afford the corresponding cyclopropane derivatives but react to give the unexpected products (III) via 1,4‐hydrogen transfer.

Medium and substituent effects on the photochemistry of phenanthridine N-oxides. Is an intermediate of diradical character involved in the photorearrangement of heterocyclic N-oxides?

The photochemistry of several 6-substituted phenanthridine N-oxides has been investigated, or reinvestigated, in benzene and ethanol. The main processes observed are: (a) 1,2-oxygen and substituent shift to yield N-substituted phenanthridones (2) and (b) ring enlargements to dibenzo[d,f]-1,3-oxazepines (7). With 6-diphenylmethylphenanthridine N-oxide (1b) rearrangement (a) predominates and occurs with 45% substituent loss in benzene (but only 2% in ethanol). With the 6-phenyl derivative (1c) process (a) predominates in ethanol and process (b) in benzene and with the 6-p-nitrophenyl derivative (1d) the latter process predominates in both solvents. With 6-cyanophenanthridine N-oxide (1e) rearrangement (b) predominates in benzene; in the presence of 2,3-dimethylbutene (but not of cyclohexene) addition products are obtained; with dienes deoxygenation is the main process. Medium and substituents may change the nature of the lowest excited singlet state, but more importantly affect the stability of an intermediate of diradical character occurring along the reaction pathway, thus driving it towards rearrangement (a) or (b). Intermediate diradicals are unambiguously indicated only in particular cases [elimination of a diphenylmethyl radical from (1b), alkene addition in the case of (1e)] but their role is probably more general.

Reactive scattering of a supersonic fluorine atom beam: F + OCS

Reactive scattering of F atoms with OCS molecules has been studied at an initial translational energy E≈ 30 kJ mol–1, using a supersonic beam of F atoms seeded in He buffer gas generated by a high-pressure microwave discharge source. The centre-of-mass differential cross-section peaks in the forward direction with a subsidiary peak in the backward direction of relative height ca. 0.75 ± 0.2. The product translational energy for scattering in the forward and backward directions is greater than that for the sideways scattering. A model based on RRKM theory for the angular distribution of reactive scattering arising from short-lived collision complexes formed in large impact parameter collisions is proposed. The fit of this model to the experimental data indicates that the product transition state constitutes a prolate symmetric top in accord with the structure predicted to arise from the addition of an F atom to the CS double bond of OCS. The electronic structure of the short-lived doublet FSCO complex is compared with that of the OSCS triplet diradical intermediate of the O + CS2 reaction, which exhibits stripping dynamics.

Electron-transfer-induced photocyclization reactions of arene-iminium salt systems. Effects of cation-diradical deprotonation and desilylation on the nature and efficiencies of reaction pathways followed

Photocyclization reactions of the N-xylyl-1-pyrrolinium perchlorates 3-6, induced by excited-state electron transfer, have been explored with the intent of uncovering mechanistic details and developing synthetic applications. Both of the silicon-substituted salts 4 and 6 undergo photocyclization to produce the respective benzoindolizidine products, 30 and 13, exclusively via mechanisms involving sequential electron transfer-desilylation. On the other hand, the non-silicon-containing 2-phenyl-1-pyrrolinium perchlorate 5 undergoes conversion to both the indolizidine 13 and 2-phenyl-1-pyrroline (10) upon irradiation. Photofragmentation generating 10 is proposed to arise by cleavage of the intermediate diradical cation 14, which occurs in competition with deprotonation and 1,6-diradical coupling to form 13. Finally, the benzopyrrolizidines 22 and 23 are produced when the 2-methyl-N-xylyl- and 2-methyl-N-benzylpyrrolinium perchlorates 3 and 21 are irradiated. Deuterium labeling studies have shown that these photocyclization reactions proceed through diradical cation coupling processes. The features of these photochemical processes whose chemical outcomes are dependent upon the nature of reaction pathways available to cation diradical intermediates are discussed. Finally, the photoinduced, diradical cyclization methodology for transformation of the silicon-containing N-xylylpyrrolinium perchlorates 4 and 6 has been compared to the alternative dipolar pathway promoted by fluoride-induced desilylation. The indolizidines 30 and 13 are formed when these salts are treated with cesiummore » fluoride at high temperature. However, the yields are much lower than those observed for the photocyclization processes.« less

The effect of styrene α-substituents on the regiochemistry of [2+2] cycloadditions with difluoroallene.

Consistent with our hypothesis for a mechanism involving two kinetically-distinct diradical intermediates, the observed effects of styrene α-substituents can be explained as deriving either from steric effects or radical stabilizing effects.

Pressure effect on reactions of C 7H 8 hydrocarbons

The reactions of three C 7H 8 valence isomers (quadricyclane, norbornadiene and cycloheptatriene) with activated carbonyl bonds were considered. Quadricyclane and norbornadiene react with mesoxalates to give the same cycloadducts (oxetanes). While this result is normally expected with quadricyclane, it is surprising for norbornadiene. The determination of the volume profile for the reaction indicates a transition state occurring “earlier” on the reaction coordinate. A mechanism involving a common diradical intermediate is proposed. Cycloheptatriene reacts with mesoxalates only under high pressure giving an ene product. There was no evidence of a norcaradiene structure.

ChemInform Abstract: Photochemical Isomerization of (E,Z)‐1,5‐Bis(9‐anthryl)penta‐1,4‐dien‐3‐one and Its Intramolecular Diels‐Alder Adduct.

Photoexcitation of (E,Z)-1,5-bis(9-anthryl)penta-1,4-dien-3-one (2) leads to 4-(9-anthryl)-4,5-dihydro-5,9b[1′,2′]-benzeno-9bH- benz [e]indene-3(3aH)-one (3) which is formed by an intramolecular Diels -Alder addition. Upon irradiation, (3) undergoes a skeletal rearrangement to give a product (4), rationalized in terms of a stepwise reaction involving diradical intermediates. The molecular structure of (4) has been established by X-ray diffraction, and the stereochemistry of (3) deduced from the results of X-ray diffraction studies of its reduction products (5) and (6).

Cycloadditions of methylenecyclopropanes and strained bicyclo[n.1.0]alkanes to radicophilic olefins

The methylenecyclopropanes 1, 2, 11 upon reaction with α-donorsubstituted acrylonitriles 3a, 3c gave two types of products, regular [2+2] cycloadducts 6, 8, 10, 13, 14 and thiacyclopentane derivatives 7, 9. The capto-dative substituted methylenecyclopropane 12 preferentially homodimerized to 15 even in the presence of 2. The capto-dative olefin 3a also added across the central single bonds of a bicyclo[1.1.0.]butane derivative 4b and bicyclo[2.1.0]pentane 5a to yield 16 and 19 via diradical intermediates.

Reactive scattering of a supersonic oxygen atom beam. O + C2H3Cl, C2H3Br, C2H2F2 and C2HF3

Reactive scattering of O atoms with C2H3Cl, C2H3Br, C2H2F2 and C2HF3 molecules has been measured at an initial translational energy E≈ 35 kJ mol–1 using a supersonic beam of O atoms seeded in He buffer gas. The O + C2H3Cl, C2H3Br reactions both exhibit two reaction pathways, one involving H atom displacement and the other involving halogen atom displacement. The O + CH2CF2, C2HF3 reactions also show two reaction pathways, one involving H atom displacement and the other involving carbon–carbon bond scission. All these reaction pathways are related to specific electronic states of the triplet diradical intermediates.

Photochemical Isomerization of (E,Z)-1,5-Bis(9-anthryl)penta-1,4-dien-3-one and its Intramolecular Diels-Alder Adduct

Photoexcitation of (E,Z)-1,5-bis(9-anthryl)penta-1,4-dien-3-one (2) leads to 4-(9-anthryl)-4,5-dihydro-5,9b[1′,2′]-benzeno-9bH- benz [e]indene-3(3aH)-one (3) which is formed by an intramolecular Diels -Alder addition. Upon irradiation, (3) undergoes a skeletal rearrangement to give a product (4), rationalized in terms of a stepwise reaction involving diradical intermediates. The molecular structure of (4) has been established by X-ray diffraction, and the stereochemistry of (3) deduced from the results of X-ray diffraction studies of its reduction products (5) and (6).

Synthesis and Thermal Rearrangement of 4‐Heterohepta‐1,2,5,6‐tetraenes

Abstract4‐Heterohepta‐1,6‐diynes can be rearranged by base catalysis under carefully controlled conditions to the corresponding 4‐heterohepta‐1,2,5,6‐tetraenes. Thermal rearrangment of these tetraenes gives the corresponding dimers by a process in which the rate‐determining step is first order. The rate constants for these reactions have been determined and the relative rates accounted for in terms of an aromatic contribution afforded to the presumed diradical intermediates. In the two cases investigated it was found that when the thermolyses were conducted in the presence of 3O2 then cyclic peroxides were formed.

The polymerization of xylylene bisdialkyl sulfonium salts

AbstractParaxylylene bisdialkyl sulfonium salts can be polymerized in several different ways, yielding addition polymers as well as condensation polymers. The chemistry leads to a variety of polymers including poly(p‐xylylene), poly(p‐xylylidene), poly(xylylene sulfide)s, poly(xylylene oxide)s, and various α‐substituted polyxylylenes such as poly(p‐xylylene‐α‐dialkylsulfonium chloride). Addition polymers form via xylylene diradical intermediates. Condensation polymerization can be effected either by coupling with aldehydes or by nucleophilic displacement reactions.

Flash vacuum thermolysis of spirocyclohexadienones

In an attempt to prepare short-bridged hydroxymetacyclophanes 1b-d , the spirocyclohexadienones 2b-d were pyrolyzed by flash vacuum thermolysis (FVT). Instead of 1b-d , variable amounts of 4-(5-hexenyl)phenol ( 4b ), β-hydroxybenzocycloalkenes ( 5b-d ) and 4-( trans-1-alkenyl) phenols ( 6c-d ) were obtained. The formation of these products is explained by invoking cleavage of a spiro bond in 2 under formation of the intermediate diradical 3 which, depending on the length of the aliphatic chain and on the temperature, has several pathways open for isomerization to spin-paired products.

Addition of dimethylsilylene to substituted 1,3-butadienes; evidence for concerted 1,2-addition followed by nonconcerted rearrangements via diradical intermediates

The mechanism for addition of dimethylsilylene to substituted 1,3-butadienes has been established as consisting of a concerted 1,2-addition followed by ring-opening of vinylsilacyclopropane intermediates to diradicals that can cyclize or disproportionate. The substrates studied were cis- and trans-piperylene, isoprene, 2,3-dimethylbutadiene, cis,trans- and trans,trans-2,4-hexadiene, and trans--1,3-hexadiene.

Analysis of the optical absorption spectra of diradical oligomers in diacetylene crystals

Abstract We report on electronic and vibrational properties of the diradical short-chain reaction intermediates of the low-temperature polymerization reaction in diacetylene single crystals. The positions, shapes and intensities of the zero-phonon lines and of vibronic lines of the polarized optical absorption series are analyzed. Theoretical model calculations concerning the vibrational and excitonic states in the molecular aggregates are performed and applied to the experiments.

The Mechanisms of the Polymerization Reactions in Diacetylene Crystals: Esr-Analysis of the Paramagnetic Reaction Imtermediates

Abstract The mechanisms of the solid state polymerization reactions in diacetylene crystals have been investigated by ESR-spectroscopy. The electronic structure of the diradical, carbene and dicarbene intermediates of the low-temperature photopolymerization reaction of diacetylene crystals have been analyzed. From the chain-lengths dependent ESR fine structure and from the isotropic hyperfine structure detailed information concerning the radical and carbene wavefunctions were deduced. A multiplicity change from S=1 to S=2 occurs at the hexamer intermediates.

Theoretical semi-empirical study of concerted oxidative additions to dinuclear complexes

A theoretical semi-empirical study of synchronous addition of dihydrogen and acetylene to dimetallic complexes, d 8- d 8 (Rh 2, Ir 2) or d 7- d 7 (Fe 2) is reported. For ( d 8) 2 complexes, the concerted addition leads to an intermediate diradical which must rearrange before metal—metal bond formation can occur, ( d 7) 2 complexes react more efficiently; it is shown that cleavage of the initial metal—metal bond may either be concerted with addition in the thermal process or lead to a diradical in the excited state. Thermal and photochemical reactivities are compared in both cases. Competitive addition in the case of (d 8) 2 complexes is discussed.

9-Hydroxy [7]metacyclophane

A new pyrolytic access to a metacyclophane has been found by the flash vacuum thermolysis of spiro[5,7]trideca-l,4-dien-3-one ( 4 ) which gave 9-hydroxy[7]metacyclophane (7) , and 4-(6-heptenyl)phenol (8) . The mechanism of formation of 7 and 8 from the intermediate diradical ( 6 ) is discussed.