Allenes radical reactions

Debierne (1914) was the first to suggest a radical reaction theory for water radiolysis (H and OH). In various forms, the idea has been regenerated by Risse (1929), Weiss (1944), Burton (1947, 1950), Allen (1948), and others. Platzman (1953), however, criticized the radical model on theoretical grounds and proposed the formation of the hydrated electron. Stein (1952a, b) meanwhile had suggested that both electrons and H atoms may coexist in radiolyzed water and proposed a model in which the electron digs its own hole. Later, Weiss (1953, 1960) also favored electron hydration with ideas similar to those of Stein and Platzman. In some respects, the theoretical basis of these ideas is attributable to the polaron (Landau, 1933 Platzman and... 

Allene Radical Cations. The bimolecular reactivity of the radical reactions of allene and propyne has been a longstanding matter of interest. Myher and Harrison179 studied the ion/molecule reactions of ionized C3H4 with the respective neutral precursor in a medium-pressure chemical ionization source. CsH7+ ions were found to be amongst... 

Radiation chemistry of aqueous solutions has also been applied to the study of micellar systems. Considerable micellar effects on the yield of radiolytic products and on rates of radical reactions have been observed by several authors (Gebicki and Allen, 1969 Fendler and Patterson, 1970 Bansal et al., 1971 Patterson et al., 1971, 1972 Fendler et al., 1972 Wallace and Thomas, 1974 Gratzel et al., 1974). These observations led to conclusions on the permeability of micelles to various radicals and on the location of substrates in micelles. Recent experiments have also demonstrated a very efficient trapping of e4q by positively charged micelles even when chemical reaction between them did not take place (L. K. Patterson, personal communication). 

In a Mn(OAc)3-mediated oxidative radical reaction of allenes with dimethyl malonate or ethyl cyanoacetate, an efficient synthesis of A -butenolides is realized (Equation 59) <2007S45>. 

The energy of the 1236 A light is greater than the ionization potential of benzene, and hence additional reaction paths are available. All the above products were observed for the photolysis of benzene at 1236 A, with all except hydrogen having somewhat lower quantum yields. No pressure effect was observed in the photolysis at 1236 A. For the photolysis with either lamp, the addition of 3 torr NO to 1 torr of benzene eliminated allene, cyclohexadienes, biphenyl and dihydrobiphenyl as products, while the other products remained unaffected. Hentz and Rzad conclude that the former are formed by free-radical reactions, while the latter are formed by molecular elimination. In view of the curious pressure effects observed by Shindo and Lipsky at 1849 A, conclusions regarding the effect of added gases must be made with caution. 

In this chapter, we have shown how the recent advances in the crossed molecular beam technique allow us to study complex polyatomic reactions of relevance in astrochemistry. The focus was on the CN radical reactions with simple alkynes, but the same approach has been also applied to the study of other CN radical reactions with unsaturated small organic molecules, such as ethylene, benzene, and allene, which are of relevance in astrochemistry as well [77,81,84]. 

Radiation Chemistry of Solvents Water. The successful design of a radiation chemistry experiment depends upon complete knowledge of the radiation chemistry of the solvent. It is the solvent that will determine the radicals initially present in an irradiated sample, and the fate of all these species needs to assessed. Among the first systems whose radiation chemistry was studied was water, both as liquid and vapor phase, as discussed by Gus Allen in The Story of the Radiation Chemistry of Water , contained in Early Developments in Radiation Chemistry (8), Water is the most thoroughly characterized solvent vis-a-vis radiation chemistry. So to illustrate the power of radiation chemical methods in the study of free radical reactions and electron-transfer reactions, I will focus on aqueous systems and hence the radiation chemistry of liquid water. Other solvents can be used when the radiation chemistry of the solvent is carefully considered as noted previously, Miller et al. (I) used pulse radiolysis of solutions in organic solvents for their landmark study showing the Marcus inversion in rate constants. 

Radical reactions ofallenes are largely unknown [57]. Recently, Ma and coworkers [58] developed the first example of a radical addition/cyclization reaction of allene-enes in an alkene-to-allene manner (Scheme 5.32). The reaction of the allene 146 and polyfluoro alkyl iodide 147 with zinc powder as a cheap, readily available, efficient, and mild initiator led to 148 in moderate to good yields (53-86%), which on treatment with TBAF (tetra-n-butylammonium fluoride) gave the allene 149. 

Free radical reactions have been observed to influence molecular and biochemical processes and to directly cause some of the changes observed in cells during differentiation, ageing, and transformation (Abate et al. 1990, Sohal and Allen 1990, Allen and Tresinii 2000). 

In the case of allene + NO reaction in solid Ar during the visible-light photolysis (585 nm), 2-allylnitrite radical was detected by Fourier transform infrared (FT-IR) spectroscopy, whereas excitation of dimethylacetylene - NO systems (610 nm) gives the acetylmethyliminoxy radical and dimethylketene [75]. As a result of oxygen atom transfer from NO to the central carbon atom, 2-allenylidoxy transient biradical is generated in allene ... 

Other Reactions of Allenes.— Radical-chain additions to allenes are presumed to involve radical intermediates of allylic structure by initial attack at the central carbon atom or of vinylic structure by attack at the terminal carbons. The extent to which these structures are preferred depends upon structure of starting allene, nature of attacking radicals, and reaction conditions. Addition of toluene-/ -sulphonyl iodide to chiral allenes (cyclonona-1,2-diene and penta-2,3-diene) results in racemic products (376), and hence attack on... 

The condensation reaction of y-(trimethylsiIyl)allenylboranes 107 with conjugated allenic aldehydes 108 followed by Peterson oleflnation has been carried out by Wang et al The procedure resulted in the formation of enyne-allenes 111/112 which were then used in intramolecular transformations via radical reactions. The reaction proceeded with excellent stereoselectivity depending on whether acidic or basic conditions were used for the elimination of the P-hydroxysilane. Wang then applied this protocol to form (T-isotoluenes and diene-allenes. ... 

Allen, A.D., Fenwick, M.E, Henry-Riyad, H., and TidweU, T.T., Nitroxyl Radical Reactions with 4-Pentenyl- and Cyclopropylketenes New Routes to 5-Hexenyl- and Cyclopropyhnethyl Radicals, /. Org. Chem., 66, 5759, 2001. 

The quantitative aspects of track reactions are involved some details will be presented in Chapter 7. The LET effect is known for H2 and H202 yields in aqueous radiation chemistry. The yields of secondary reactions that depend on either the molecular or the radical yield are affected similarly. Thus, the yield of Fe3+ ion in the Fricke dosimeter system and the initiation yield of radiation-induced polymerization decrease with LET. Numerous examples of LET effects are known in radiation chemistry (Allen, 1961 Falconer and Burton, 1963 Burns and Barker, 1965) and in radiation biology (Lamerton, 1963). 

Discovery of the hydrated electron and pulse-radiolytic measurement of specific rates (giving generally different values for different reactions) necessitated consideration of multiradical diffusion models, for which the pioneering efforts were made by Kuppermann (1967) and by Schwarz (1969). In Kuppermann s model, there are seven reactive species. The four primary radicals are eh, H, H30+, and OH. Two secondary species, OH- and H202, are products of primary reactions while these themselves undergo various secondary reactions. The seventh species, the O atom was included for material balance as suggested by Allen (1964). However, since its initial yield is taken to be only 4% of the ionization yield, its involvement is not evident in the calculation. 

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