Showing papers on "Homolysis published in 1974"
TL;DR: In this article, it was shown that strong polar effects were observed not only with strongly polar radicals, but also with moderately polar radicals such as carbon-free radicals, if the reacting substrate has a marked polar nature.
Abstract: Publisher Summary The most interesting development in the field of homolytic aromatic substitution is the realization that polar effects in several reactions play a role important based on the transiton state. The extent of these effects is determined by both the nature of the free radicals and the reacting substrates so that strong polar effects were observed not only with strongly polar radicals, but also with moderately polar radicals, such as carbon-free radicals, if the reacting substrate has a marked polar nature. Homolytic animation is of great interest in the homocyclic aromatic series with a selectivity and versatility comparable to that of electrophilic substitution. Homolytic substitutions with nucleophilic radicals are of great importance for protonated heteroaromatic bases but of lesser interest using nonprotonated heteroaromatic bases and homocyclic aromatic substrates. Among these nucleophilic radicals the use of alkyl, acyl, carbamoyl, α-oxyalkyl, and α- N -alkyl radicals has proved to be very successful. The aryl radicals were used in obtaining a deeper understanding of the homolytic arylation of heteroaromatics, but the sensitivity to polar effects is too low to give significant selectivity from the synthetic point of view.
53 citations
40 citations
TL;DR: In this article, the relationship between nitrogen lone pair interactions and thermodynamic parameters associated with amine basicity is delineated and the use of BH/sup +/ homolytic bond dissociation energies as a measure of nitrogen lone-pair interactions and of resonance stabilization is discussed.
Abstract: Determination of gas phase basicities (defined as the enthalpy change for the reaction BH/sup +/ ..-->.. B + H/sup +/) of diazabicyclooctane and quinuclidine using ion cyclotron resonance spectroscopy is reported. The relationship between nitrogen lone pair interactions and thermodynamic parameters associated with amine basicity are delineated. The use of BH/sup +/ homolytic bond dissociation energies as a measure of nitrogen lone pair interactions and of resonance stabilization is discussed. (DDA)
39 citations
37 citations
TL;DR: In this paper, the C-Mn bond of [RMn(CO)5] or [RCOMn[CO] under u.v. irradiation has been studied using nitrosodurene as a spin-trap for both the radicals formed; irradiation of various metal-metal bonded compounds (LM-ML, M = Mn, Re, Fe, Mo, or Co) gives rise to metal-centred radicals which can similarly be trapped.
Abstract: Homolysis of the C–Mn bond of [RMn(CO)5] or [RCOMn(CO)5] under u.v. irradiation has been studied using nitrosodurene as a spin-trap for both the radicals formed; irradiation of various metal–metal bonded compounds (LM–ML, M = Mn, Re, Fe, Mo, or Co) gives rise to metal-centred radicals which can similarly be trapped.
35 citations
29 citations
TL;DR: In this article, the reaction of cyclopropyl radical to biscyclopropaneformyl peroxide in a series of substituted benzenes was investigated and the reaction was shown to be relatively free from polar effect.
29 citations
TL;DR: In this article, it was shown that the rate of spin trapping and decarboxylation in 2-methyl-2-nitrosopropane (nitrosobutane) is 1·1 × 106 I mol−1s−1 at 40° in di-t-butyl peroxide solvent.
Abstract: From a study of the competition between spin trapping and decarboxylation, the rate constant for addition of the t-butoxycarbonyl radical to 2-methyl-2-nitrosopropane (nitrosobutane) has been determined to be 1·1 × 106 I mol–1s–1 at 40° in di-t-butyl peroxide solvent. The rate constant in benzene must be very similar. Competitive scavenging and decarbonylation of a series of acyl radicals (RĊO) shows that the ease of loss of carbon monoxide increases along the series R = Me < Pri < adamantyl < But < PhCH2. An absolute rate constant (3·9 × 103s–1 at 40°) for the decarbonylation of PriĊO is obtained by assuming that this radical adds to nitrosobutane at the same rate as t-butoxycarbonyl. This rate constant is considerably smaller than that obtained previously in the gas phase and a possible source of error in the gas-phase value is identified. The formation of t-butoxy t-butyl nitroxide by addition of t-butoxyl radicals to nitrosobutane is reversible at ambient temperatures. Fragmentation of this nitroxide to give t-butoxyl radicals occurs with a rate constant of ca. 0·14 s–1 at 40°, whilst fragmentation with loss of t-butyl radicals is shown to occur by N–C, and not O–C, fission with a rate constant of ca. 10–3 s–1 at 40°.
28 citations
TL;DR: In this article, the structure of the dimer is rationalized in terms of the structures of the Cr-Cr bond, and some facile CO substitution reactions, of [(π-C5H5Cr(CO)3]2]2 and (π -C 5H5)Cr( CO)3I with phosphorus(III) ligands are described.
Abstract: [(π-C5H5)Cr(CO)3]2 is extremely reactive. It readily undergoes reactions which involve homolytic or heterolytic scission of the Cr–Cr bond, or the insertion of another atom into it. These observations are rationalized in terms of the structure of the dimer, and this is discussed in detail. Some facile CO substitution reactions, of [(π-C5H5Cr(CO)3]2 and (π-C5H5)Cr(CO)3I with phosphorus(III) ligands are described.
28 citations
TL;DR: In this article, the structures of 4,5-dibenzoyl-pyridazine were proved by 1H-NMR, IR, and mass spectra.
Abstract: 4-Benzyl-pyridazine (2) is prepared by homolytic benzylation of pyridazine (1). Oxidation of2 with KMnO4 or SeO2 yields 4-benzoyl-pyridazine (3). On treatment with NaBH4,3 is reduced quantitatively to the corresponding carbinol4. Reaction of protonated1 with benzoyl radicals affords mainly 4,5-dibenzoyl-pyridazine (5). The structures are proved by1H-NMR-, IR- and mass spectra.
25 citations
TL;DR: In this article, the rate constants for bimolecular homolytic substitution at the boron center have been determined, and interpreted in terms of electronic and steric effects.
Abstract: Organoboranes readily undergo bimolecular homolytic substitution at the boron centre according to the equation: x• + -* X—B( + R (X = R00, RO, RS, R2N, R2COT etc.) Rate constants for a number of these reactions have been determined, and interpreted in terms of electronic and steric effects.
TL;DR: The homolytic chlorination of ethyl 4-oxochromen-2-carboxylate produces three chlorine-containing esters: the known 3-chloro-derivative (1), 2-chloroencoderative (2), and ethyl cis-2,3-dichloro-4-oxide-roman-2 carboxylates (13).
Abstract: The homolytic chlorination of ethyl 4-oxochromen-2-carboxylate produces three chlorine-containing esters: the known 3-chloro-derivative (1), 2-chloroethyl 3-chloro-4-oxochromen-2-carboxylate (2), and ethyl cis-2,3-dichloro-4-oxochroman-2-carboxylate (13). Ammonia converted the ester (1) into 3-amino-4-oxochromen-2-carboxamide, but 3-chloro-4-oxochromen-2-carbonitrile under the same conditions gave 2-amino-3-chlorochromone (15). The preparation and some reactions of ethyl 3-bromomethyl-4-oxochromen-2-carboxylate are described. Conflicting reports of the characteristics of ethyl 4-(2-hydroxyphenyl)-2,4-dioxobutanoate have been resolved and the corresponding methyl ester has been synthesized; aminoethylation of the latter yielded the 3-dimethylaminomethylchromone (11).
TL;DR: In this article, it was shown that the rate of dediazoniation of the benzene diazonium salt increases with increasing amounts of pyridine and the reaction is no longer first-order with respect to the dizonium ion.
Abstract: There are only two dediazoniation products of benzenediazonium tetrafluoroborate in 2,2,2-trifluoroethanol (TFE), namely phenyl 2,2,2-trifluoroethyl ether (1) and fluorobenzene (2). The reaction kinetics are strictly first-order with respect to the diazonium salt. The addition of increasing amounts of pyridine to the system results in a gradual decrease in the yields of 1 and 2 and an increase in the yields of the homolytically formed products, benzene (3), biphenyl (4), isomeric phenylpyridines (5) and diazo tar (6). The reaction kinetics show that the rate of dediazoniation of the benzene diazonium salt increases with increasing amounts of pyridine. The reaction with added pyridine is no longer first-order with respect to the diazonium ion.
The product analyses and the kinetic data are consistent with the view that in pure TFE this diazonium salt decomposes completely by a heterolytic mechanism. The addition of pyridine brings about a competitive homolytic mechanism which becomes increasingly dominant as the concentration of pyridine increases.
TL;DR: In this paper, the thermal and thermal-oxidative degradaation of polycyanates based on 4,4′-dicyanatodiphenyl (PC-4) and 9,9-bis-(4-cyanatophenyl)fluorene(PC-9) has been investigated over a wide range of temperatures.
Abstract: The thermal and thermal-oxidative degradaation of polycyanates based on 4,4′-dicyanatodiphenyl (PC-4) and 9,9-bis-(4-cyanatophenyl)fluorene (PC-9) have been investigated over a wide range of temperatures. The kinetics of decomposition of the polycyanates have been studied and the apparent energies of activation for the over-all process of breakdown of the polymers and for the formation of carbon dioxide in degradation in vacuo have been determined. It was found that moisture present in the polymers has a substantial effect on the nature and extent of decomposition. It is suggested that degradation of the polycyanates begins with hydrolytic scission of the ester bond and that this is followed by breakdown of the triazine ring by both heterolytic and homolytic mechanisms.
TL;DR: In this paper, a homolytic t-butylation of 3-substituted pyridines was performed with complete selectivity m position 6, and the relative rates correlate with the Hammett σ p constants (ϱ = 5·5).
TL;DR: In this paper, it was shown that the addition of a particular radical to ethylene is ca. 11 kcal mol−1 more exothermic than the corresponding addition to benzene to give a cyclohexadienyl radical, which enables a common pattern of reactivity for the two reactions to be established.
Abstract: Addition of a particular radical to ethylene is ca. 11 kcal mol–1 more exothermic than the corresponding addition to benzene to give a cyclohexadienyl radical; use of this difference enables a common pattern of reactivity for the two reactions to be established.
TL;DR: The thermal decomposition of dibenz[b,d]iodolium halides and 10H-dibenz [b,e]iodininium halide has been examined in this article.
Abstract: The thermal decomposition of dibenz[b,d]iodolium halides and 10H-dibenz[b,e]iodininium halides has been examined. Observation of products of halogen scrambling and hydrogen transfer suggested a radical mechanism involving 2′-iodobiphenyl-2-yl and 2-(2-iodobenzyl)phenyl radicals. Results of trapping experiments with an acetylenic ester suggested the involvment of a biphenyl diradical.
TL;DR: In this article, a new ionic chlorination method of alkyl sulfones with sulfuryl chloride was described, in which the most noteworthy observation was exclusive or highly selective β-chlorination of diethyl sulfone or sulfolane (tetrahydrothiophene-1,1-dioxide) is described.
TL;DR: In this article, a mechanism incorporating these observations is suggested for the defluorination of the radical intermediate in hexafluorobenzene, where the reaction is caried out in the presence of p-fluorobenzoic acid.
TL;DR: The diphenyllead bispolyfluorobenzoates, Ph2Pb(O2CR)2 (R = C6F5, p-MeOC6F4 or p-EtOC 6F4) as discussed by the authors, have associated structures with bridging bidentate carboxylate groups in the solid state.
Abstract: The diphenyllead bispolyfluorobenzoates, Ph2Pb(O2CR)2 (R = C6F5, p-MeOC6F4 or p-EtOC6F4), obtained from diphenyllead diacetate and the appropriate polyfluorobenzoic acids, are considered to have associated structures with bridging bidentate carboxylate groups in the solid state and to be predominantly monomeric with chelating carboxylate groups at low concentrations in chloroform. Thermal decomposition of Ph2Pb(O2CC6F5)2 in boiling pyridine gave Ph2Pb(C6F5)2 and a low yield of Ph3PbC6F5, whereas Ph2Pb(O2C-p-MeOC6F4)2 and Ph2Pb(O2C-p-EtOC6F4)2 gave low yields of Ph3Pb-p-MeOC6F4 and Ph2Pb(p-EtOC6F4)2 respectively. From decomposition of Ph2Pb(O2CC6F5)2 under vacuum, Ph2Pb(C6F5)2, Ph3PbC6F5, Ph4Pb, PhC6F5, (C6F5)2CO and CO2 were obtained, whilst Ph4Pb, Ph2, PhR, RCO2Ph, R2CO and CO2 were detected from pyrolysis of Ph2Pb(O2CR)2 (R= p-MeOC6F4 or p-EtOC6F4). These compounds are formed by competing decarboxylation, rearrangement, and (under vacuum) homolysis reactions of the diorganolead dicarboxylates, and by further reactions of the rearrangement products.
TL;DR: In this article, competitive rates of arylation of the title compounds were measured using hexafluorobenzene as reference, and the results suggest that the species responsible for intermolecular discrimination and those responsible for intramolecular selection are not the same.
TL;DR: In this article, three kinds of t-butyl per (trans-2-substituted cyclopropyl) acetates (R�H, CH3, C6H5) were synthesized from the corresponding acyl chlorides and thermally decomposed in cyclohexane.
TL;DR: In this paper, homolytic phenylation takes place preferentially at position 8 of all systems investigated, whereas a lower selectivity was observed for positions 7 and 3.
Abstract: Imidazo[1.2-b]-,s-Triazolo[4.3-b]- ands-Triazolo-[2.3-b]pyridazines and their derivatives were submitted to homolytic phenylation and the reaction mixtures obtained separated by chromatography into individual components which were then identified. Homolytic phenylation takes place preferentially at position 8 of all systems investigated, whereas a lower selectivity was observed for positions 7 and 3.
TL;DR: Partial rate factors for homolytic amination of alkylbenzenes and biphenyl by N-chloroamines have been determined in an acidic medium as mentioned in this paper.
Abstract: Partial rate factors for homolytic amination of alkylbenzenes and biphenyl by N-chloroamines have been determined in an acidic medium. The resulting polar effects are by far the highest ever observed in homolytic substitution and are of the order of magnitude of those in the common electrophilic reactions. The reaction is sensitive to steric and polar effects (conjugative and inductive, but not hyperconjugative), as previously observed for hydrogen absractions. The transition state of the reaction is discussed and a hypothesis is put forward on inductive and Baker–Nathan effects on electrophilic substitution.
TL;DR: In this paper, the α-scission of a series of photolytically produced alkoxyl radicals to alkoxyalkylphosphines has been studied by e.g. spectroscopy in solution.
Abstract: The structures and the kinetics of α-scission of a series of alkoxyalkylphosphoranyl radicals have been studied by e.s.r. spectroscopy in solution. These radicals were generated by the addition of photolytically produced alkoxyl radicals to alkoxyalkylphosphines. The stability towards α-scission of the radicals EtnṖ(OR)4 –n exhibits a maximum for n= 2. Phosphoranyl radicals R2Ṗ(OBut)2 fragment by loss of an alkyl radical, R·, with increasing rate along the series R = Me < But < Et ∼ Prn < Pri∼ Bus < allyl. These results are interpreted in terms of a preference for departure of an alkyl radical from an apical rather than an equatorial site, in α-scission of the trigonal bipyramidal phosphoranyl radical. Factors which influence the relative rates of α- and β-scission of alkoxyalkylphosphoranyl radicals are discussed.
TL;DR: The first step of this photoreaction was a homolytic fission of the C-OC2H5 bond to 2-(2-ethoxy-1,3-dioxo-2,3dihydro)phenalenyl radical (9) and ethoxyl radical as discussed by the authors.
Abstract: Irradiation of 2,2-diethoxy-2,3-dihydrophenalene-1,3-dione (6) with a pyrex-filtered high-pressure Hg arc lamp afforded acetaldehyde, 2-ethoxy-3-hydroxyphenalene-1-one (7) and bis-2-(2-ethoxy-l,3-dioxo-2,3-dihydro) phenalenyl (8). Mechanistic studies revealed that the first step of this photoreaction was a homolytic fission of the C–OC2H5 bond to 2-(2-ethoxy-1,3-dioxo-2,3-dihydro)phenalenyl radical (9) and ethoxyl radical. This homolytic fission occurred mostly from the triplet and in part from the singlet excited state of 6. Dimerization of 9 gave 8 and the disproportionation between 9 and ethoxyl radical gave acetaldehyde and 7. The photochemical behaviors of a variety of triketone derivatives were discussed.
TL;DR: In this article, the reaction of chlorine monoxide with cyclohexene at −20° in the dark was found to proceed by an addition reaction to yield trans-2-chlorocyclohexyl hypochlorite.
Abstract: The reaction of chlorine monoxide with cyclohexene at −20° in the dark was found to proceed by an addition reaction to yield trans-2-chlorocyclohexyl hypochlorite. Concomitant with the addition, a ...