New generation of organosilyl radicals by photochemically induced homolytic cleavage of silicon-boron bonds

Unlike bis(diethylamino)organosilylboranes, bis(diisopropylamino)organosilylboranes, which have UV absorption at longer wavelength than 300 nm, undergo photolysis to afford pairs of an organosilyl radical and a bis(diisopropylamino)boryl radical by homolytic scission of the silicon-boron bonds. Generation of organosilyl radical and organoboryl radical was confirmed by trapping experiments using TEMPO (2,2,6,6-tetramethylpiperidine N-oxyl). The organosilyl radical thus generated induces not only silylation of mono-olefins and silylative cyclization of dienes but also polymerization to afford polymers bearing organosilyl termini. On the other hand, the bis(diisopropylamino)boryl radical generated is not incorporated into the olefin adducts.     

Activation barriers for addition of methyl radicals to oxygen-stabilized carbocations

Activation barriers (DeltaHMe(double dagger)) for adding methyl radicals to ions of the general formula CH3CR=OCH3+ have been measured by looking at the threshold energies for the reverse reaction, dissociative photoionization of ethers of the general formula RC(CH3)2OCH3. Dissociation by loss of a methyl radical has more favorable thermochemistry than loss of R*, yet the onset of R* loss occurs at lower energies than loss of CH3*. In other words, the more endothermic dissociation exhibits a lower appearance energy. Contrathermodynamic ordering of appearance energies is observed for R = Et, nPr, iPr, tBu, and neopentyl. The sum of the appearance energy difference, DeltaAE, and the thermochemical difference (DeltaDeltaH, calculated using G3 theory) gives a lower bound for the barrier for adding methyl radical to CH3CR=OCH3+. More specifically, the difference between that activation barrier and the one for adding R* to (CH3)2C=OCH3+, DeltaHMe(double dagger)-DeltaHR(double dagger), equals DeltaAE + DeltaDeltaH and has values in the range 20-24 kJ mol(-1) for the homologous series investigated. There is no systematic trend with the steric bulk of R, and available evidence suggests that DeltaHR(double dagger) does not have a value >5 kJ mol(-1). The difference in barrier heights, DeltaHMe(double dagger)-DeltaHiPr(double dagger) for CH3* plus iPrC(CH3)=OX+ vs iPr* + (CH3)2C=OX+, has the same value, regardless of whether X = H or CH3. Mixing of higher energy electronic configurations provides a qualitative theoretical explanation for some (but not all) observed trends in barrier heights.     

An investigation of the homolytic saccharide cleavage of deprotonated flavonol 3-O-glycosides in a quadrupole ion trap mass spectrometer

The trend in the extent of homolytic saccharide cleavage is reported for a series of deprotonated flavonol 3-O-glycosides upon collision-induced dissociation (CID) in a quadrupole ion trap mass spectrometer. The second-generation product ions from the primary [Y(0)](-) and [Y(0)- H](-.) product ions were also identified. It was determined that the structure of both the aglycon and the saccharide portions of the flavonoid glycoside are pivotal in inducing radical cleavage. In contrast to earlier work on this subject reported for a smaller group of flavonols, the correlation between the degree of B-ring hydroxylation and the extent of radical saccharide cleavage showed several notable exceptions in the present work. Homolytic cleavage was also investigated in the context of using tandem mass spectrometry to identify the aglycon portions of flavonoid glycosides.     

Rates of the halide ion cleavage from halo-9,10-diphenylanthracene anion radicals in DMF

The rate constants k for the cleavage of the carbon-halogen bond in anion radicals of 1- and 2-chloro-9,10-diphenylanthracene, 9,10-dichloroanthracene and 2-bromo-9,10-diphenylanthracene have been determined and compared with the literature data for other anthracenyl halides. There is no significant correlation between log k and the formal potentials of the anion radical formation. However, linear relationships of the experimental RT In k vs. the relative thermodynamic contribution to the activation barrier as well as the relative intrinsic activation energy (calculated on the basis of the recent Savéant model and using the ionic component of the bond enthalpy estimated from the absolute electronegativity and hardness) have been found.     

Stepwise and concerted electron-transfer/bond breaking reactions. solvent control of the existence of unstable pi ion radicals and of the activation barriers of their heterolytic cleavage

Available data from various sources seem to indicate an important role of solvation in the cleavage rates of intermediate pi ion radicals, in the passage from concerted to stepwise electron-transfer/bond breaking reaction pathways and even in the very existence of pi ion radicals. After preliminary computations treating the solvent as dielectric continuum, these expectations are examined with the help of a simple model system involving the anion radical of ONCH(2)Cl and two molecules of water, which allows the application of advanced computational techniques and a treatment of these solvent effects that emphasizes the role of solvent molecules that sit close to the charge centers of the molecule. A pi ion radical minimum indeed appears upon introduction of the two water molecules, and cleavage is accompanied by their displacement toward the leaving anion, thus offering a qualitative mimicry of the experimental observations.     

Generation of phosphorus-centered radicals via homolytic substitution at sulfur

A novel radical domino process relying on the homolytic cleavage of P-S bonds allows the preparation of phosphorus-containing molecules through addition of P-centered radicals onto olefins. The key step of this reaction is a homolytic substitution on a sulfur atom. The scope of the reaction is broad. Diaminophosphonyl radicals whose reactivity was unknown react smoothly with olefins. Use of tin hydride can be avoided. A radical thiophosphinoylation of triple bonds has been uncovered. [reaction: see text]     

Electrophilic properties of anodically generated polynitroalkyl radicals in homolytic aromatic substitution

Conclusions Arylpolynitromethanes, the products of the electrochemical oxidation of polynitrocarbanions in the presence of benzene and its substitution products, are formed by homolytic aromatic substitution. During this process the intermediate polynitroalkyl radicals show electrophilic properties. The yield of arylpolynitromethanes depends on radical structure and on the electron donor properties of the aromatic substrate.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 368–374, February, 1988.     

Bimolecular homolytic substitution of dialkyl selenides and tellurides with tri-n-butyltin radicals

The rate constants for the S_H2 reactions of tri-n-butyltin radicals with several dialkyl selenides and tellurides have been measured by EPR spectroscopy.     

Reactions of peroxide radicals with the surface of heterogeneous catalysts in homolytic and heterolytic processes

An interrelation between homolytic and heterolytic stages is found for olefin epoxidation by hydroperoxides and cumene hydroperoxide decomposition in the presence of a heterogeneous catalyst (molybdenum selenide). Peroxide radicals that are formed in homolytic decomposition of the hydroperoxide interact with the catalyst surface. As a result, molybdenum atoms are oxidized to the highest valence state, and new reaction sites are created, on which heterolytic reactions occur. It is shown that olefin, as an electron donor, decreases the catalyst activity in the heterolytic reactions. The limited value for the olefin epoxidation rate, which is independent of the amount of the catalyst used, is explained by the disappearance of active sites on the catalyst surface due to their interaction with the neighboring sites containing adsorbed olefin molecules.     

Character of intermediate radicals in homolytic isomerization of cyclic acetals and their heteroanalogs

Conclusions Employing the EPR method and 2-methyl-2-nitrosopropane as the spin trap, we recorded the formation of the radicals (n = 1 and 2; X = O, N, or a chain composed of C and O atoms; R = H or alkyl) during the photolysis of acetals and their heteroanalogs in the presence of di-tertbutyl peroxide.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2097–2099, September, 1976.The authors are indebted to A. Sh. Mukhtarov for assistance in running the experiments.     

Benzyl radical addition reaction through the homolytic cleavage of a benzylic C-H bond

Direct generation of a benzyl radical by C-H bond activation of toluenes and the addition reaction of the resulting radical to an electron deficient olefin were developed. The reaction of dimethyl fumarate with toluene in the presence of Et(3)B as a radical initiator at reflux afforded 2-benzylsuccinic acid dimethyl ester in good yield.     

Isonitriles as source and fate of imidoyl radicals: a novel homolytic α-fragmentation

Imidoyl radicals 4a-c react with phenylacetylene to give annulation products and nitrile 12, arising from β-scission of the intermediate iminyl radical that is involved in the rearrangement of azaspirocyclohexadienyl 8. In contrast, imidoyls 4d and 15 do not react with the alkyne and give good yields of the corresponding isonitriles through a novel example of homolytic α-fragmentation.     

Unprecedented aromatic homolytic substitutions and cyclization of amide-iminyl radicals: experimental and theoretical study

Amide-iminyl radicals are versatile and efficient intermediates in cascade radical cyclizations of N-acylcyanamides. They are easily trapped by alkenes or (hetero-)aromatic rings and cyclize into a series of new heterocyclic compounds which bear a pyrroloquinazoline moiety. As an illustration of the synthetic importance of these compounds, the total synthesis of the natural antitumor compound luotonin A was achieved through a tin-free radical cascade cyclization process. Not only do amide-iminyl radicals lead to new tetracyclic heterocycles but these nitrogen-centered radical species also react in aromatic homolytic substitutions. Indeed, the amide-iminyl radical moiety unprecedentedly displaces methyl, methoxy, and fluorine radicals from an aromatic carbon atom. This seminal reaction in the field of radical chemistry has been developed experimentally and its mechanism has additionally been investigated by a theoretical study.     

Intramolecular homolytic substitution at the sulfur atom: an alternative way to generate phosphorus- and sulfur-centered radicals

Two efficient procedures involving tin hydride or thiophenol-mediated intramolecular homolytic substitution at the sulfur atom are reported. They lead to the generation of varied P(V)-centered radicals from the corresponding aryl or alkyne thiophosphorus substrates. The radical formed can be trapped by an olefin via an intermolecular addition, leading to the construction of C-P bonds. Thiophosphination of triple bonds was also achieved using a radical cycloisomerization process. Extension of the methodology to sulfur-containing species was examined.     

Tin-free generation of alkyl radicals from alkyl 4-pentynyl sulfides via homolytic substitution at the sulfur atom

Homolytic substitution at the sulfur atom of beta-(phenylsulfanyl)vinyl radicals, obtained by radical reaction of benzenethiol with easily accessible alkyl 4-pentynyl sulfides, is a mild, effective, tin-free route for the generation of all types of alkyl radicals. This protocol can be employed in reductive defunctionalizations as well as cyclizations onto both electron-rich and electron-poor C-C double bonds.