Intramolecular H-Transfer

In poly(acrylic acid), two radicals are also formed upon -OH-attack. Again, the secondary radical undergoes intramolecular H-abstraction, leading to the tertiary radical [reaction (4) Ulanski et al. 1996c], 

The rate of this reaction strongly depends on the pH (Fig. 9.3), i.e., the protonation state of the polymer. With increasing pH, the rate of reaction slows down considerably, an indication that the flexibility of the polymer chain is of importance for this reaction to occur efficiently. Whether this implies that H-abstrac-tion mainly occurs from distant sites and not from a neighboring subunit [as shown in reaction (4)], cannot be decided yet on the basis of the existing data. 

A similar H-abstraction takes place in poly(methacrylic) acid, converting the primary radical into a secondary one [reaction (5) k = 350 s1 at pH 7.5 Ulanski et al. 1999b]. 

When in the poly(acrylic acid) system radical the tertiary radical is converted by 02 into the corresponding peroxyl radical, a chain reaction sets in which yields C02 and a acetylacetone-like product [reactions (6)-(9) Ulanski et al. 1996a for the formation of acetylacetone in the model system 2,4-dimethylglutaric acid, see Ulanski et al. 1996b]. 

The C02 yield considerably exceeds that of the acetylacetone-like product, and although C02 is certainly also formed in other reactions (cf. the low molecular weight model Ulanski et al. 1996b), this observation maybe taken as a hint that the H-abstraction reaction does not proceed with neighboring groups only (via a favorable six-membered transition state) but also with more distant sites. A chain-type autoxidation has also been observed with poly(vinyl methyl ether) and a related model compound (Janik et al. 2000a,b). 

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A key question remains how is the olefin formed in the overall process Molecular tantalum complexes are known to undergo facile a- and transfer processes, leading to tantalumalkylidene and tantalum tt-olefin complexes, respectively (mechanism 9, Scheme 29) [98]. Moreover, olefin polymerization with tantalum complexes belongs to the rare case in which the Green-Rooney mechanism seems to operate (Eq. 10, Scheme 29) [102]. Finally, intramolecular H-transfer between perhydrocarbyl ligands has been exemplified (Eq. 11, Scheme 29) [103,104]. 

This chapter compares the reaction of gas-phase methylation of phenol with methanol in basic and in acid catalysis, with the aim of investigating how the transformations occurring on methanol affect the catalytic performance and the reaction mechanism. It is proposed that with the basic catalyst, Mg/Fe/0, the tme alkylating agent is formaldehyde, obtained by dehydrogenation of methanol. Formaldehyde reacts with phenol to yield salicyl alcohol, which rapidly dehydrogenates to salicyladehyde. The latter was isolated in tests made by feeding directly a formalin/phenol aqueous solution. Salicylaldehyde then transforms to o-cresol, the main product of the basic-catalyzed methylation of phenol, likely by means of an intramolecular H-transfer with formaldehyde. With an acid catalyst, H-mordenite, the main products were anisole and cresols moreover, methanol was transformed to alkylaromatics. 

Laser flash photolysis of phenylchlorodiazirine was used to measure the absolute rate constants for intermolecular insertion of phenylchlorocarbene into CH bonds of a variety of co-reactants. Selective stabilization of the carbene ground state by r-complexation to benzene was proposed to explain the slower insertions observed in this solvent in comparison with those in pentane. Insertion into the secondary CH bond of cyclohexane showed a primary kinetic isotope effect k ikY) of 3.8. l-Hydroxymethyl-9-fluorenylidene (79), generated by photolysis of the corresponding diazo compound, gave aldehyde (80) in benzene or acetonitrile via intramolecular H-transfer. In methanol, the major product was the ether, formed by insertion of the carbene into the MeO-H bond, and the aldehyde (80) was formed in minor amounts through H-transfer from the triplet carbene to give a triplet diradical which can relax to the enol. 

Similar isotope effects for human isoenzyme I (157c) on kc t and Km for C02 hydration are 1.7. Silverman and Tu (161) report an isotope effect of 2.5 for H2180 release and suggest that the intrinsic isotope effect of intramolecular H+ transfer might be significantly smaller in isoenzyme I than in isoenzyme II. Hence, the H20-splitting step might also limit the rate of C02 hydration in isoenzyme I. Human isoenzyme I has three titratable active-site histidines with pKa values... 

With GSH carbon-centered besides thiyl radical are formed upon OH attack, notably at pH > 7 (Sjoberg et al. 1982 Eriksen and Fransson 1988). It has been shown subsequently that this is due to an intramolecular H-transfer [reaction (35) Grierson et al. 1992], When the a-NH2 group is no longer protonated as in neutral solution the C-H is only weakly bound and equilibrium (35) is shifted to the side of the C-centered radical. 

In pulse radiolysis studies of Urd and its derivatives (but not with dUrd), spectral changes are observed after the completion of the S04, reaction [k = 3 x 10s s 1 Bothe et al. 1990] that are not typical for S04 reactions with pyrimidines. On the basis of EPR experiments (Hildenbrand 1990 Catterall et al. 1992), these observations can be interpreted by an (overall) intramolecular H-transfer giving rise to a radical at the sugar moiety. This requires that considerable amounts of Ura are released which is indeed observed (Fujita et al. 1988 Aravindakumar et al. 2003 Table 10.4). Chain reactions occur as with the other pyrimidine/peroxodisulfate systems. This increases the Ura yield beyond that expected for a non-chain process, but when corrections are made for this by carrying out experiments at the very high dose rates of electron-beam irradiation, a... 

Pd-catalysed chelate-directed acetoxylation of meta -substituted arenes has been studied.61 Many substituted groups are tolerated by this process and the reaction shows a high degree of regioselectivity for the less sterically hindered ortfto-position. For example, 2-(3-nitrophenyl)pyridine forms 2-(2-acetoxy-3-nitrophenyl)pyridine. Finally, density functional calculations62 on the palladium acetate-promoted cyclomet-allation of dimethylbenzylamine suggest that reaction occurs via an agostic C-H complex rather than a Wheland intermediate. An intramolecular H-transfer to a coordinated acetate via a six-membered transition state follows. 

Experimental data on the rate constants of equilibrium 16 and the intramolecular reaction of CysS radicals with amino acids are rather sparse. An intramolecular H-transfer reaction between CysS radicals and the N-terminal y-glutamic acid was quantified for glutathione (j- Glu-Cys-Gly). ... 

However, the rate constants for intramolecular H-transfer reaction between CysS radicals and amino acids within a peptide chain were unknown. 

C-tracer studies on the oxidation of n-butane at 315 °C showed that reaction (—14o ) did not participate to any measurable extent [56]. Hence, despite careful thermokinetic considerations the estimated rate coefficients appear to be in error. The reason for this might be in estimation of the equilibrium constant K14. It was assumed that the entropy change for intramolecular H-transfer was close to zero [104] and so fei 4 was taken as being equal to exp(—A///RT). If this assumption is not correct, then the values of Xi 4 will be too low and hence 14 will be too high. 

This compound oxidizes in a flow system at temperatures as low as 120 °C and possesses a region of negative temperature coefficient between about 330 and 370 °C. Products include acetaldehyde, formaldehyde, formic acid and peroxides. Below 250 °C, acetaldehyde is the sole non-peroxidic organic product [95]. This is formed by a 1 5 intramolecular H-transfer followed by j3-scission. 

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