- N-Hydroxypyridine-2-thione Esters as Radical Precusors in Kinetic Studies. Measurements of Rate Constants for Hydrogen Atom Abstraction Reactions
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N-hydroxypyridine-2-thione esters were employed as radical precursors in kinetic studies.Radical chain reactions of the precursor esters gave 2,2-dimethyl-3-butenyl and 5-hexenyl.These radicals either were trapped by H-atom donors or rearranged, and the rate constants for trapping were determined from the known rate constants for rearrangement and measured product yields.For hydrogen atom donors that reacted too slowly to trap radicals before rearrangement, an estimate of the rate constants for hydrogen atom transfer was made from the yields of rearranged hydrocarbon and alkyl pyridyl sulfide (formed by scavenging of the alkyl radical by the precursor ester).The methods work for a variety of H-atom donors, including thiols, stannanes, phosphines, silanes, and reactive hydrocarbons.The rate constants determined for reduction of alkyl radicals by dicyclohexylphosphine, 1,4-cyclohexadiene, and THF are important for mechanistic studies of potential electron-transfer processes in ractions of nukleophiles with alkyl halides.
- Newcomb, Martin,Un Park, Seung
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p. 4132 - 4134
(2007/10/02)
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- Hydrogenolysis of Alkanes with Quaternary Carbon Atoms over Pt and Ni Black Catalysts
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Hydrogenolysis of hydrocarbons with quaternary C atoms (neopentane, neohexane, 2,2,3-trimethylbutane, 2,2- and 3,3-dimethylpentanes and 2,2,3,3-tetramethylbutane) has been studied over Pt and Ni black catalysts.The reactivities of different types of C-C bond have been determined.The probability of C-C bond rupture where one of the carbon atoms is quaternary is inversely proportional to the bond dissociation energy.On Pt, two essential types of hydrogenolysis can be distinguished.One reaction is responsible for the breaking of internal C-C bonds attached to the quaternary carbon atom and the other for demethylation.With larger molecules, the former reaction is preferred and the surface intermediate should be 1,4-diadsorbed, while that for the latter reaction is 1,3-diadsorbed.Nickel, as previously suggested, causes terminal C-C rupture, although with branched hydrocarbon reactants internal C-C bond rupture is also possible, presumably via 1,4-adsorption.
- Zimmer, Helga,Tetenyi, Pal,Paal, Zoltan
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p. 3573 - 3586
(2007/10/02)
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- Some uses of silicon compounds in organic synthesis
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1.The amount of γ-phenylthioalkylation of silyl dienol ethers is increased when the triphenylsilyl ether is used in place of the trimethylsilyl ether.Phenylthiomethylation is the least γ-selective carbon electrophile of several tried so far. 2.The acid-catalysed reactions of a range of γ-silyl tertiary alcohols cleanly give rearrangement, in which the silyl group controls the outcome.The reactions are similar in several respects to the rearrangements of the corresponding pinacols, except that the silicon controlled reactions are usually cleaner and give higher yields.The reaction is particularly useful for setting up quaternary carbon atoms.
- Fleming, Ian
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- The Kinetics and Mechanism of Ring Opening of Radicals containing the Cyclobutylcarbinyl System
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The kinetic parameters of β-fission of radicals containing the cyclobutylcarbinyl system have been determined by analysis of the mixtures obtained when suitable chloro-compounds are treated with tributylstannane.Under these conditions ring opening is irreversible and in the rigid bicyclic system (4) is under stereoelectronic control.For ring opening of cyclobutylcarbinyl radical (8) kf = 4.3 x 103 s-1 at 60 deg C, and the best values of the activation parameters appear to be ΔH(excit.) = 12.2 kcal mol-1 and ΔS(excit.) = -7.4 cal mol-1 K-1.Monocyclic systems undergo preferential fission of the more substituted βγ-bond.Methyl substituents at the α-, β-, or δ-positions have little effect but γ-substitution strongly enhances the rate of ring opening.The transition state is reactant-like and has a similar disposition of centres to that (1) for homolytic addition.
- Beckwith, Athelstan L. J.,Moad, Graeme
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p. 1083 - 1092
(2007/10/02)
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