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The thermokinetics for the tautomerization of a series of methylenedihydroacenes to the corresponding methylacenes (toluene to 6-methylpentacene) have been investigated by means of CBS-QB3 calculations. Only for 6-methylpentacene does the methylenedihydro form predominate at room temperature. The obtained equilibrium ratios are consistent with various theoretical methods, but the agreement with the scarce experimental data is only qualitative. The noncatalyzed thermal tautomerization of the methylenedihydroacene in an inert solvent may proceed by means of a reverse radical disproportionation reaction (RRD) as the rate-determining step. The benzylic BDE(C-H)s and the hydrogen atom affinities (HA) of the tautomers have been used to calculate the reaction enthalpy, ΔRRDH. It appears that the Ea,RRD is substantially higher than ΔRRDH. This implies that the opposite reaction (and the tautomer forming step), a radical-radical disproportionation (RD), is an activated process. This is an often ignored or overlooked kinetic feature. The consequence is that although the RRD reaction may be kinetically feasible at elevated temperatures, the products are not the tautomers but rather dimers stemming from radical-radical recombination reactions, with p-isotoluene as a clear exception. It is further shown that the RRD self-reaction of phenalene is too slow at 298 K, despite claims to the contrary.

Photolysis of an aryl sulfide containing 5,6-dihydropyrimidine (1) at 350 nm produces high yields of thymidine and products resulting from trapping of 5,6-dihydrothymidin-5-yl radical by O2 or thiols. Thymidine is believed to result from disproportionation of the radical pair originally generated from C-S bond homolysis in 1 on the microsecond timescale, which is significantly shorter than other photochemical transformations of modified nucleotides into their native forms. Duplex DNA containing 1 is destabilized, presumably due to disruption of π-stacking. Incorporation of 1 within the binding site of the restriction endonuclease EcoRV, provides a photochemical switch for turning on the enzyme's activity. In contrast, 1 is a substrate for endonuclease VIII and serves as a photochemical off switch for this base excision repair enzyme. Modification 1 also modulates the activity of the 10–23 DNAzyme despite its incorporation into a non-duplex region. Overall, dihydropyrimidine 1 shows promise as a tool that provides spatiotemporal control over DNA structure on the miscrosecond tiimescale.

Ascorbyl radical was generated by the pulse radiolysis method and observed with the fast kinetic spectrophotometry within reverse micelles stabilized by AOT in n-heptane or by Igepal CO-520 in cyclohexane at different water to surfactant molar ratio, w0. Rate constants for the disproportionation of the ascorbyl radicals were smaller than those for intermicellar exchange for both type of reverse micelles and slower than those in homogeneous aqueous solutions. However, they increased with increasing w0 for AOT/n-heptane system, while they decreased for Igepal CO-520 system. The absorption spectra of ascorbic acid AOT/n-heptane reverse micellar system showed that the “pH” sensed by this molecule is lower than that in respective homogeneous aqueous solutions. The obtained results were rationalized taking into account three main factors (i) preferential location of ascorbic acid molecules in the interfacial region of the both types of reverse micelles; (ii) postulate that the pH of the interface is lower than that of the water pool of reverse micelles and (iii) different structure of the interface of the reverse micelles made by AOT in n-heptane and those formed by Igepal CO-520 I cyclohexane. Some possible consequences of these findings are discussed.

Observations of chemically induced dynamic nuclear polarization in low magnetic fields are often complicated by a transfer of samples from the laboratory field to the high magnetic field of the spectrometer. Effects of such transfer are analyzed, and theoretical predictions are compared to the results on low-field CIDNP in strongly coupled protons of styrene, a product of geminate disproportionation in the pair of sec-phenethyl and phenacyl radicals. Selective deuteration was used to decouple these protons and analyze redistribution of polarization formed in the course of disproportionation. A violation of magnetic equivalency in the protons constituting the methyl group in the precursor sec-phenethyl radical was observed. Possible origins of this phenomenon are discussed.

Radicals α to the carbonyl of ethyl 3,4,4-trimethylpentanoate couple with little stereoselectivity while analogous tertiary radicals derived from ethyl 2,3,4,4-tetramethylpentanoate disproportionate at 25°C to give the corresponding alkene and alkane. The alkane is formed by disproportionation with a diastereoselectivity of greater than 20:1.

Magnetokinetic effects on the rates of disproportionation of Cl 2 - and Br 2 - and on their reactions with Mn(OH 2 ) 6 2+ and Mn(EDTAH) - have been investigated as a function of the magnetic field intensity, B, between 0 and 6.5 T. The dependence of the rate constants on B has been rationalized in terms of the spin evolution in paired reactants. The isotropic hyperfine coupling-induced multiplet transitions and the Δg mechanism make significant contributions to such effects.

Thermolyses of seven dialkyl, two alkyl-aryl and two diaryl O-benzyl ketoxime ethers, R(1)R(2)C[double bond, length as m-dash]NOCH(2)Ph, have been examined in three hydrogen donor solvents: tetralin, 9,10-dihydrophenanthrene, and 9,10-dihydroanthracene. All the oxime ethers gave the products expected from homolytic scission of both the O-C bond (viz., R(1)R(2)C[double bond, length as m-dash]NOH and PhCH(3)) and N-O bond (viz., R(1)R(2)C[double bond, length as m-dash]NH and PhCH(2)OH). The yields of these products depended on which solvent was used and the rates of decomposition of the O-benzyl oxime ethers were greater in 9,10-dihydrophenanthrene and 9,10-dihydroanthracene than in tetralin. These results indicated that a reverse radical disproportionation reaction in which a hydrogen atom was transferred from the solvent to the oxime ether, followed by [small beta]-scission of the resultant aminoalkyl radical, must be important in the latter two solvents. Benzaldehyde was found to be an additional product from thermolyses conducted in tetralin. This, and other evidence, indicated that another induced decomposition mode involving abstraction of a benzylic hydrogen atom, followed by [small beta]-scission of the resulting benzyl radical, became important for some substrates. Participation by minor amounts of enamine tautomers of the oxime ethers was shown to be negligible by comparison of thermolysis data for the O-benzyloxime of bicyclo[3.3.1]nonan-9-one, which cannot give an enamine tautomer, with that of the O-benzyloxime of cyclohexanone.

1. Kinetic and thermodynamic parameters have been obtained for the dimerization and disproportionation of the phenoxyl radicals (I)–(V). 2. Successive replacement of the o-tert-butyl groups of radical (I) by phenyl groups increases the disproportionation rate constant by several orders, largely by reducing steric hindrance effects. 3. Solvation stabilizes phenoxyl radicals with phenyl substituents in benzene, reducing the tendency to dimerization and disproportionation in these compounds.

Photolysis of the title compound afforded silafluorene and dibenzo-1,2-disilacyclohexadiene as primary photoproducts. Two reactive intermediates, tetramethyldisilene and dimethylsilylene, were detected by trapping. The dimethylgermylene, derived from the germanium analog, was first observed by electronic spectra.

The 3-phenyl-3-pentyl radicals from photolysis of azobis-3-phenyl-3-pentane (APP) in solution give 3,4-diethyl-3,4-diphenylhexane and a photolabile product assumed to arise by para coupling. Photolysis of APP as a crystalline solid at −78° gives the disproportionation products 3-phenylpentane (PPH) and 3-phenyl-2-pentene (PP) as a 3:1 mixture of the E and Z isomers. This stereoisomeric ratio is surprising in terms of the solid-state molecular conformations, for which a topochemical scheme predicts predominance of the Z isomer. Isotopic distributions in PPH, Z-PP and E-PP from photolysis of solid APP which was 82% deuterated in its methylene groups shows: (1) that Z-PP is formed under strong lattice control; (2) that E-PP is formed in a process with greater, but not complete, molecular freedom; and (3) that there is often a dynamic competition for hydrogen-atom donation between the two radicals involved in disproportionation.

Reduction de diaryl-1,2 dibromo-1,2 ethanes en derives ethyleniques (le groupe aryl pouvant etre un groupe pyridyl)

The effect of hydrostatic pressure (up to about 6000 atm) upon the probabilities of disproportionation and combination of ethyl radicals in solution was investigated. Disproportionation is favoured relative to combination at higher pressures. This effect may be described by Δv*=v≠comb–v≠disp·. Thus, Δv* is 2.6 ml/mol in iso-octane and 2.8 ml/mol in toluene.

A radical approach to late-stage functionalization using photoredox and Diversinate™ chemistry on the Open Source Malaria (OSM) triazolopyrazine scaffold (Series 4) resulted in the synthesis of 12 new analogues, which were characterized by NMR, UV, and MS data analysis. The structures of four triazolopyrazines were confirmed by X-ray crystal structure analysis. Several minor and unexpected side products were generated during these studies, including two resulting from a possible disproportionation reaction. All compounds were tested for their ability to inhibit the growth of the malaria parasite Plasmodium falciparum (3D7 and Dd2 strains) and for cytotoxicity against a human embryonic kidney (HEK293) cell line. Moderate antimalarial activity was observed for some of the compounds, with IC50 values ranging from 0.3 to &gt, 20 µM, none of the compounds displayed any toxicity against HEK293 at 80 µM.

The BF3·OEt2-mediated disproportionate coupling reaction of sodium sulfinates was found for the first time. In this reaction, various S–S(O)2 bonds can be formed, efficiently giving thiosulfonates in moderate to excellent yields. As a convenient protocol for the synthesis of symmetrical and unsymmetrical thiosulfonates, its reaction mechanism involves the formation of a thiyl radical and sulfonyl radical via a sulfinyl radical disproportionation. What is more, this transformation can also be applied practically as a gram-scale reaction and to the two-step synthesis of sulfone and sulfonamide in one pot in situ using thiosulfonate as an intermediate.