Showing papers on "Homolysis published in 1986"

Energetics and structure of the 1- and 2-adamantyl radicals and their corresponding carbonium ions by photoelectron spectroscopy.

Abstract: The first photoelectron bands of the 1- and 2-adamantyl radicals, formed by flash vacuum photolysis of 1- and 2-adamantylmethyl nitrite, have been obtained. The adiabatic (IP_a) and vertical (IP_v) ionization potentials of the 1-adamantyl radical are 6.21 ± 0.03 and 6.36 ± 0.05 eV, respectively. IP_a and IP_v for the 2-adamantyl radical are 6.73 ± 0.03 and 6.99 ± 0.05 eV, respectively. The difference in hydride affinities between the 1-adamantyl and tert-butyl cations (Sharma, R. B.; Sen Sharma, D. K.; Hiraoka, K.; Kebarle, P. J. Am. Chem. Soc. 1985, 107, 3747) combined with the difference in IP_a between the tert-butyl and 1-adamantyl radicals (0.49 ± 0.06 eV) yield a value of 99 kcal/mol for the tertiary C-H bond energy in adamantane, 3.7 ± 1.2 kcal/mol greater than the tertiary C-H bond energy in isobutane (assumed to be 95 kcal/mol). The effects of the geometrical constraints imposed by the adamantyl cage on the homolytic and heterolytic C-H bond cleavage energies are discussed for the 1- and 2-adamantyl cases. The width of the Franck-Condon envelope obtained is related to the geometry changes that occur upon ionization. The surprisingly broad envelope observed for the planar 2-adamantyl radical indicates that the Franck-Condon envelope for the 1-admantyl radical should not be interpreted as exclusively due to changes at the bridgehead position. Thermal decomposition products of the 1- and 2-adamantyl radicals are observed, and the pathways for thermal decompositions of the radicals are discussed. To confirm expected trends in ionization potentials and band shapes of tertiary radicals, the first photoelectron band of the 2-methyl-2-butyl radical has been obtained. The IP_a of the 2-metyl-2-butyl radical is 6.65 ± 0.04 eV with IPV = 6.91 ± 0.05 eV.

Studies on the photolytic breakdown of hydroperoxides and peroxidized fatty acids by using electron spin resonance spectroscopy. Spin trapping of alkoxyl and peroxyl radicals in organic solvents.

Abstract: Spin trapping using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) has been used to detect and distinguish between the carbon-centred, alkoxyl, and peroxyl radicals produced during the photolytic decomposition of hydroperoxides. Photolysis of tert-butyl and cumene hydroperoxides, and peroxidized fatty acids, in toluene, with low levels of u.v. light, is shown to lead to the initial production of alkoxyl radicals by homolysis of the oxygen-oxygen bond. Subsequent reaction of these radicals with excess hydroperoxide leads, by hydrogen abstraction, to the production of peroxyl radicals that can be detected as their corresponding adducts with the spin trap. Subsequent breakdown of these adducts produces alkoxyl radicals and a further species that is believed to be the oxidized spin-trap radical 5,5-dimethyl-1-pyrrolidone-2-oxyl. No evidence was obtained at low hydroperoxide concentrations, with either the cumene or lipid alkoxyl radicals, for the occurrence of beta-scission reactions; the production of low levels of carbon-centred radicals is believed to be due to the alternative reactions of hydrogen abstraction, ring closure, and/or 1,2 hydrogen shifts. Analogous experiments with 3,3,5,5-tetramethyl-1-pyrroline N-oxide (TMPO) led only to the trapping of alkoxyl radicals with no evidence for peroxyl radical adducts, this is presumably due to a decreased rate of radical addition because of increased steric hindrance.

Thermal decomposition of energetic materials. Part 9. A relationship of molecular structure and vibrations to decomposition: polynitro-3,3,7,7-tetrakis(trifluoromethyl)-2,4,6,8-tetraazabicyclo[3.3.0]octanes

Abstract: A qualitative correlation exists between the average N-N bond distance and the average frequency of the NO/sub 2/ asymmetric stretch in nitramines containing C/sub 2/NNO/sub 2/ units. Compounds containing long N-N bonds (high nu/sub as/(NO/sub 2/)) tend to liberate considerable NO/sub 2/ when they are rapidly heated. Other decomposition products replace or are in competition with NO/sub 2/ for nitramines having shorter N-N bonds (lower nu/sub as/(NO/sub 2/)). These conclusions are bolstered by rapid-scan infrared spectroscopy studies of the initial gas decomposition products from 2,4,6,8-tetranitro-3,3,7,7-tetrakis(trifluoromethyl)-2,4,6,8-tetraazabicyclo(3.3.0)octane and its 2,6-dinitro analogue. The initial gas products evolved from the tetranitro compound are relatively independent of pressure (1-1000 psi) suggesting that N-N bond homolysis is the initial fast step that dominates thermal decomposition.

Heat of formation and homolytic bond dissociation energies in the keto-enol tautomers of acetaldehyde and acetone

Abstract: On donne les mesures de △H f pour les radicaux ĊH 2 CHO et ĊH 2 COCH 3 et on en deduit les energies de dissociation homolytique de liaisons pour H-CH 2 CHO, H-OCHCH 2 , H-CH 2 COCH 3 et H-OC(CH 3 )=CH 2

Radical reactions in synthesis: carbon–carbon bond formation from 2-substituted allyl trialkyl stannanes

Abstract: A competing 1,3-rearrangement of allyl stannanes has been demonstrated to occur under the normal thermal homolytic allyl transfer reaction conditions which limits the substitution patterns in these processes; two methacrylyl stannanes are described which allow the transfer of the methacrylyl moiety to alkyl bromides and iodides under mild conditions.

Free-radical pathways to alkyl complexes of a nickel tetraaza macrocycle

Abstract: The cationic nickel(I) macrocycle (1R,4S,8R,11S)-(1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane)nickel(I), abbreviated R,S,R,S-(Ni(tmc))/sup +/, reacts in aqueous, alkaline solutions on the stopped-flow time scale with alkyl halides to form a new series of organonickel complexes. Kinetic data were obtained for a large number of alkyl halides. The trends in the rate constants are benzyl > allyl > secondary > primary > methyl > cyclopropyl, and RI > RBr > RCl. These trends suggest that carbon-centered free radicals R are produced by a bimolecular reaction between Ni(tmc)/sup +/ and RX and are then captured by a second Ni(tmc)/sup +/. Further evidence for free-radical involvement comes from cyclization of the radical produced from 6-bromo-1-hexene, from the yields of products in those instances where dimerization of the free radical competes with its capture by Ni(tmc)/sup +/, and from the nonreactivity of alkyl tosylates. The organonickel complexes slowly hydrolyze in unimolecular processes to yield hydrocarbon and the nickel(II) complex R,S,R,S-(Ni(tmc))/sup 2 +/. The organonickel complexes do not undergo unimolecular homolysis but react with Co(II) macrocycles with a 1:2 stoichiometry to form cobalt-carbon bonds. This reaction most likely occurs not by homolytic displacement but by electron transfer followed by radical capture.

Polar effects in free-radical reactions. Selectivity and reversibility in the homolytic benzylation of protonated heteroaromatic bases

Abstract: On etudie la benzylation homolytique des cyano-4pyridine, quinoleine, methyl-2- et methyl-4 quinoleine, isoquinoleine et quinoxaline. On etudie la grande influence de l'effet polaire et de la reversibilite de l'addition du radical benzyle sur la selectivite de la reaction

Stereochemistry of intramolecular homolytic substitution at the sulphur atom of a chiral sulphoxide

Abstract: The formation of the cyclic sulphoxide (R)-(6) by treatment of the bromoarene (R)-(2) with tributylstannane indicates that intramolecular homolytic substitution at the sulphur centre of the sulphoxide group proceeds with strict inversion of configuration.

Carbon-carbon bond formation in the reaction of aliphatic radicals with alkylcobaloximes.

Abstract: Benzyl, methyl, ethyl, and isopropylaquocobaloximes, R'Co(dmgH)/sub 2/OH/sub 2/, react with aliphatic free radicals, R/sup /, forming RR' and Co/sup II/(dmgH)/sub 2/OH/sub 2/ This reaction is quite general for aliphatic radicals, except for R = benzyl Rate constants with R = /sup /C(CH/sub 3/)/sub 2/OH and /sup /CH(CH/sub 3/)OC/sub 2/H/sub 5/ exhibit little sensitivity toward the steric bulk of substituents on the cap alpha-carbon atom of the organocobaloxime This discounts homolytic displacement by attack at the organic group It is proposed that the radical addition takes place at the nitrogen end of the N=C bond of the macrocycle cis to the Co-C bond, followed by the reductive elimination of RR'

Thermolysis of model compounds for coal. Part 5. Enhancement of free-radical chain rearrangement, cyclization, and hydrogenolysis during thermolysis of surface-immobilized bibenzyl. Implications for coal chemistry

Abstract: Condensation of p-HOC/sub 6/H/sub 4/CH/sub 2/CH/sub 2/C/sub 6/H/sub 5/ (15c) with the surface hydroxyls of fumed silica gave a surface-immobilized form of bibenzyl (16c) whose thermolysis was compared with that studied earlier for liquid and gaseous bibenzyl (1). The objective was to explore the effects of restricted mobility on a multipathway free-radical reaction, with particular emphasis on modeling dimethylene linking units between aromatic clusters in coal. The initial rate of thermolysis of 16c at 350-400/sup 0/C was accelerated some fourfold compared with 1, and the product composition was notably altered. The major initial product classes in order of decreasing amounts, were the following: (1) rearrangement to form surface-attached 1,1-diphenylethane (18); (2) cyclization-dehydrogenation to form surface-attached 9,10-dihydrophenanthrene (20) (and subsequently surface-attached phenanthrene (21)); (3) symmetrical cleavage to form both surface-attached and gas-phase toluene (19 and 3) in similar amounts; (4) dehydrogenation to form surface-attached stilbene (24); and (5) unsymmetrical hydrogenolysis to form surface-attached ethylbenzene (22) plus gas-phase benzene (10) and the analogous pair, 23 plus 11, in similar amounts. For liquid 1, cyclization and unsymmetrical cleavage had been barely detectable and rearrangement was only a minor pathway. The rate constant for unimolecular C-C homolysis in 16c, based on 3 and 19,more » was 10/sup 15.3/ exp(-62,900/RT) s/sup -1/, not significantly different from that of 1 and not dependent on surface coverage. However, the initial extent of rearrangement, expressed as a chain length, was accelerated some 30-fold compared with that of liquid 1; cyclization and unsymmetrical cleavage were similarly enhanced.« less

Homolytic reactions of ligated boranes. Part 3. Electron spin resonance studies of radicals derived from dialkylamine–boranes

Abstract: Liquid-phase e.s.r. studies show that photochemically generated t-butoxyl radicals abstract hydrogen from secondary amine–boranes R2NH→BH3 to give the corresponding amine–boryl radical R2NH→ḂH2 as the kinetically controlled product. Depending on the nature of the N-alkyl groups, the amine–boryl may undergo β-scission or may rapidly abstract hydrogen from the parent amine–borane to give the isomeric aminyl–borane radical R2Ṅ→BH3 which is thermodynamically more stable. Deuterium labelling experiments exclude 1,2-hydrogen atom migration as the mechanism of the isomerisation. The less electrophilic cyclopropyl radical reacts with R2NH→BH3 to yield the aminyl–borane directly by abstraction of hydrogen from nitrogen. In the reactions with t-butoxyl radicals, intermediate amine–boryl radicals may be intercepted by halogen atom transfer from alkyl bromides or by addition to 2-methyl-2-nitrosopropane; with one amine–borane, But(Pri)NH→BH3, the amine–boryl radical was detected directly by e.s.r. spectroscopy. The e.s.r. spectra of the aminyl–borane radicals indicate appreciable hyperconjugative delocalisation of the unpaired electron onto the BH3 group. Ab initio and/or semi-empirical molecular orbital calculations for H3N→ḂH2, H2Ṅ→BH3, and their N-methylated derivatives support the conclusions reached by experiment.

Homolytic reactions of ligated boranes. Part 4. Electron spin resonance studies of radicals derived from secondary phosphine–boranes

Abstract: Radicals produced by reaction of photochemically or thermally generated t-butoxyl radicals with dimethyl-, diethyl-, and diphenyl-phosphine–boranes have been studied in solution by e.s.r. spectroscopy. Hydrogen abstraction from R2PH→BH3 appears to yield the phosphinyl–borane radical R2Ṗ→BH3 as the major product, although its e.s.r. spectrum is difficult to detect. Evidence for the competitive formation of an isomeric phosphoranyl radical [R2P(BH2)H]˙ was found for dimethylphosphine–borane. No phosphine–boryl radical R2PH→ḂH2, analogous to R3P→ḂH2 obtained from the reaction of ButO˙ with a trialkylphosphine–borane, was detected. Like the isoelectronic silyl and phosphonyl radicals, R2Ṗ→BH3 abstracts halogen from alkyl bromides and adds readily to alkenes and isocyanides to give alkyl and imidoyl radical adducts, respectively. Spin-trapping of R2Ṗ→BH3 with 2-methyl-2-nitrosopropane or phenyl-N-t-butylnitrone affords the appropriate nitroxides, whilst with 1-nitroso-2,4,6-tri-t-butylbenzene addition takes place exclusively at oxygen to give an oxyaminyl radical.

Stabilities of highly conjugated radicals from bond homolysis rates

Abstract: On mesure les vitesses d'homolyse de la liaison benzyle C-CH 3 des ethyl-4 styrene, phenyl-1butene-1, diphenyl-1,1 ethane, diphenyl-2,2 propane et methyl-1 indene dans un appareil de pyrolyse a tres basse pression

Multiple decomposition pathways for monoalkylpalladium(II) complexes lacking accessible .beta.-hydrogens.

Abstract: The formation and decomposition of metal alkyl species are ubiquitous steps in a multitude of organic reactions that are mediated by the later transition metals, such as those belonging to groups 8-10. While the mechanistic steps involved in the decomposition of the later transition-metal dialkyl compounds are now fairly well-defined, surprisingly little appears to be known about the decomposition pathways for the corresponding monoalkyl complexes, particularly those that cannot undergo a ..beta..-hydrogen abstraction reaction. These latter compounds are generally believed to decompose predominantly through radical pathways following homolysis of the M-C bond. Herein, they report the diverse radical and nonradical pathways that are involved in the decomposition of monoalkylpalladium(II) complexes that lack accessible ..beta..-hydrogens.

Homolytic reactions of ligated boranes. Part 5. Spin-trapping and other addition reactions of ligated boryl radicals

Abstract: The addition reactions of a variety of amine- and phosphine-boryl radicals (L→ḂH2; L = R3N or R3P) with 2-methyl-2-nitrosopropane (MNP), 2,4,6-tri-t-butylnitrosobenzene (TBN), and phenyl-N-t-butylnitrone (PBN) have been studied in benzene solution using e.s.r. spectroscopy. Spin-trapping of the boron-centred radicals by MNP and PBN affords nitroxides, while addition of Me3N→ḂH2 to the ambident TBN yields a mixture of the nitroxide and oxyaminyl adducts, with the latter predominating. Appropriately constituted amine-boryl radicals undergo β-N–C cleavage to give carbon-centred radicals in competition with their spin-trapping by MNP and PBN. In addition to providing mechanistic information, this competition has been used to measure the rate coefficient for spin-trapping of Pri2EtN→ḂH2 by MNP. At 314 K, ktrap is ca. 1.6 × 108 dm3 mol–1 s–1 and thus amine-boryl radicals are trapped more efficiently than alkyl radicals. Attempts to scavenge H3N→ḂH2 were frustrated by the rapid reduction of the spin-traps by ammonia–borane, but this amine-boryl radical did undergo ready addition to alkyl cyanides and isocyanides, to 1,1-di-t-butylethylene, and to 2,6-di-t-butylpyridine as judged by e.s.r. spectroscopy. Addition to cyanides affords transient iminyl radicals H3N→BH2(R)CN˙ which undergo subsequent β-scission at a rate which increases along the series R = Et < Pri < But in parallel with the stabilisation of the departing alkyl radical R˙.

The high temperature pyrolysis of ethylbenzene

Abstract: The thermal decomposition of ethylbenzene has been investigated behind reflected shock waves over the temperature and pressure ranges of 1350–2080 K and 0.25–0.5 atm using a 1.6% C8H10 Ne mixture. Major products of the pyrolysis are C7H8, C7H7, C6H6, C4H2, C2H4, C2H2, and CH4; C8H8 appears throughout the temperature range as a minor product. Comparison of the product profiles obtained by time-of-flight mass spectrometry and the results of model calculations strongly supports the initiation step of β CC bond homolysis for C8H10 dissociation. A 51 kinetic step reaction mechanism with 24 species was formulated to model the temperature and time dependence of the major products observed in our experiments.

Rhenium carbonyl biradicals. Formation, recombination, and halogen atom transfer

Abstract: The organometallic biradicals (OC)/sub 4/ReLLRe(CO)/sub 4/ (LL = R/sub 2/P(CH/sub 2/)/sub n/PR/sub 2/; R = Me, Ph, Cy; n = 2-6 or cis-Ph/sub 2/PCH=CHPPh/sub 2/) have been observed as transient species following laser flash photolysis of the bridging ligand substituted dirhenium octacarbonyls, in which homolytic metal-metal bond cleavage is a dominant primary photoprocess and CO dissociation is negligible. The intramolecular recombination of these biradicals follows simple first-order kinetics. The rate constants for recombination show a slight solvent dependence probably related to viscosity and the sizes of solvent molecules. The rate constants are affected by steric requirements of the substituents on phosphorus atoms and geometry in the ligand backbone. The rate constants for halogen atom transfer to the biradicals, determined either by laser flash photolysis or by continuous photolysis, are characteristic of mononuclear rhenium carbonyl radicals with similar substituent environments. The metal-centered biradicals thus show typical monoradical behavior in their reactions such as recombination, halogen atom transfer, and electron transfer.

Photoaddition of 4(3H)-Quinazolinone Derivatives to Olefins : Effects of the 2-Substituent

Abstract: The photochemical behavior of 3-(3-butenyl)-4(3H)-quinazolinone (2a) and its 2-chloro(2b) and 2-trifluoromethyl derivatives (2c) was examined in methanol at a variety of wavelengths (254, 300, and 350 nm). The intramolecular 2+2 photoadducts (10 and 14) were obtained only when 2c and its higher methylene homologue (13) were irradiated. Though the 2-unsubstituted quinazolone (2a) was photostable, is 2-chloro derivative (2b) afforded the cyclized product (4) via homolytic fission of the C-Cl bond. An enhancement of the photocycloaddition reactivity of the C=N bond in the quinazolone ring by introduction of a trifluoromethyl group was also demonstrated by the formation of the intermolecular adducts from 2-trifluoromethyl-4(3H)-quinazolinone (1c) by irradiation in the presence of olefins. The reactions due to C-N bond fission of the azetidine ring in these adducts are also described. Finally, by utilizing photo-induced C-Cl bond fission as found in 2b, rutecarpine (26) was synthesized by irradiation of 2-chloro-3-[2-(indol-3-yl)ethyl]-4-(3H)-quinazolinone (25).