Product studies hydrogen abstraction

Recent studies conducted by the same group revealed that the radical cation of toluene generated by photoinduced electron transfer can be deprotonated in a protic cosolvent and thus efficient trapping by electrophilic alkenes is feasible, yielding benzylation products. Secondary hydrogen abstraction by the benzyl radical from methanol generates hydroxymethyl radicals, which can also be used for preparative hydroxymethylation of alkenes (Scheme 18) [24],... 

In a series of papers in the early 1980s, Sokolov s group reported relative rate studies which were similar in nature to those of the early Szwarc studies. Sokolov generated various perfluoroalkyl radicals via thermal decomposition of the respective perfluoro diacyl peroxides in heptane containing various olefins [89] or arenes [90]. Determination of the ratio of olefin addition products to hydrogen abstraction products provided the relative rate data given in Table 4 [89]. 

If this mechanism is really operative, the "abstraction product" formed in benzene is not necessarily due to a triplet nitrene precursor. Recently a careful study of the thermolysis of methylazide in substituted benzenes demonstrated that the unsubstituted primary sulfonamide is a product of hydrogen abstraction by the nitrene 72). On the other hand there are remarkable differences in isomer ratios (o m p) of the ring-substituted anilides formed depending on the spin state of the reacting nitrene. The triplet was shown to attach the aromatic nucleus mainly in the o-position, as is expected from a highly electrophilic diradical. Dehydrogenations by carbonylnitrenes have been reported by several authors for a variety of systems. In the direct photolysis of ethylazidoformate 29 in cyclohexene, the amide 30 and the bicyclohexenyl 31 were isolated 35b Both products result from an abstraction reaction. 

This presents an interesting analytical quandary. Epoxides are major products of lipid oxidation and derive from LO cyclization as well as LOO additions (see Section 3.2.2). Consequently, it may be difficult to determine the mechanism that is operative in a given reaction system, and indeed, both may contribute. For example, Hendry (283) reacted a series of ROO with their parent compounds at 60°C and found 40% of the products were epoxides. Rate constants of k = 20 to 1130 M sec were calculated assuming the reactions were aU additions, but at the elevated temperature of the study, hydrogen abstraction to form the hydroperoxides, followed by homolytic scission to alkoxyl radicals, could also have contributed to the yields. 

A wide range of monomers and initiators has now been studied and reported. An example of the versatility of tlie nitroxide trapping technique, particularly with oxygen centred radicals, is the reaction with allyl methacrylate. In contrast with the attack on styrene, the complex pattern of initiation gave rise to a variety of products (addition, hydrogen abstraction and p-scission), which were isolated and characterized. 

With aromatic carbonyls, oxetane formation appears to arise from the carbonyl triplet state, as evidenced by quenching studies. For example, benzaldehyde irradiated in the presence of cyclohexene yields products indicative of hydrogen abstraction reactions and an oxetane ... 

It has been generally accepted that the thermal decomposition of paraffinic hydrocarbons proceeds via a free radical chain mechanism [2], In order to explain the different product distributions obtained in terms of experimental conditions (temperature, pressure), two mechanisms were proposed. The first one was by Kossiakoff and Rice [3], This R-K model comes from the studies of low molecular weight alkanes at high temperature (> 600 °C) and atmospheric pressure. In these conditions, the unimolecular reactions are favoured. The alkyl radicals undergo successive decomposition by [3-scission, the main primary products are methane, ethane and 1-alkenes [4], The second one was proposed by Fabuss, Smith and Satterfield [5]. It is adapted to low temperature (< 450 °C) but high pressure (> 100 bar). In this case, the bimolecular reactions are favoured (radical addition, hydrogen abstraction). Thus, an equimolar distribution ofn-alkanes and 1-alkenes is obtained. 

Two types of addition to pyrimidine bases appear to exist. The first, the formation of pyrimidine photohydrates, has been the subject of a detailed review.251 Results suggest that two reactive species may be involved in the photohydration of 1,3-dimethyluracil.252 A recent example of this type of addition is to be found in 6-azacytosine (308) which forms a photohydration product (309) analogous to that found in cytosine.253 The second type of addition proceeds via radical intermediates and is illustrated by the addition of propan-2-ol to the trimethylcytosine 310 to give the alcohol 311 and the dihydro derivative 312.254 The same adduct is formed by a di-tert-butyl peroxide-initiated free radical reaction. Numerous other photoreactions involving the formation by hydrogen abstraction of hydroxyalkyl radicals and their subsequent addition to heterocycles have been reported. Systems studied include 3-aminopyrido[4,3-c]us-triazine,255 02,2 -anhydrouri-dine,256 and sym-triazolo[4,3-fe]pyridazine.257 The photoaddition of alcohols to purines is also a well-documented transformation. The stereospecific addition of methanol to the purine 313, for example, is an important step in the synthesis of coformycin.258 These reactions are frequently more... 

Not all C-H activation chemistry is mediated by transition metal catalysts. Many of the research groups involved in transition metal catalysis for C-H activation have opted for alternative means of catalysis. The activation of methane and ethane in water by the hexaoxo-/i-peroxodisulfate(2—) ion (S2O82) was studied and proceeds by hydrogen abstraction via an oxo radical. Methane gave rise to acetic acid in the absence of external carbon monoxide, suggesting a reaction of a methyl radical with CO formed in situ. Moreover, the addition of (external) CO to the reaction mixture led to an increase in yield of the acid product (Equation (ll)).20... 

Suppression of the Pummerer reaction (Fig. 24) could also be a manifestation of the stabilization of the persulfoxide which prevents its interconversion to the hydro-peroxysulfonium ylide, HPSY (Fig. 25), which is the intermediate that has been suggested to undergo a 1,2-shift of the hydroperoxy group and ultimately produces the SC bond cleavage products.92 However, the situation is probably more complex since the intrazeolite reaction of /1-chlorosulfide, 29 (Fig. 28A), requires 7-hydrogen abstraction. The complexation motif (Fig. 28B) which favors the extended rather than folded M+-PS may also play an important role. A complete understanding of these reactions will require additional studies. 

A number of reports on the thermal decomposition of peroxides have been published. The thermal decompositions of f-butyl peroxyacetate and f-butyl peroxypivalate, of HCOH and a kinetic study of the acid-induced decomposition of di-f-butyl peroxide in n-heptane at high temperatures and pressures have been reported. Thermolysis of substituted f-butyl (2-phenylprop-2-yl) peroxides gave acetophenone as the major product, formed via fragmentation of intermediate alkoxy radicals RCH2C(Ph)(Me)0. A study of the thermolysis mechanism of di-f-butyl and di-f-amyl peroxide by ESR and spin-trapping techniques has been reported. The di-f-amyloxy radical has been trapped for the first time. jS-Scission reaction is much faster in di-f-amyloxyl radicals than in r-butoxyl radicals. The radicals derived from di-f-butyl peroxide are more reactive towards hydrogen abstraction from toluene than those derived from di-f-amyl peroxide. 

The stereospecific labeling of the anti methyl by deuterium in compound 20 to produce substrate 21 (Table 3) was required in order to study the syn/anti regioselectivity of the ene allylic hydroperoxides. The ene products in different solvents showed a preference for hydrogen abstraction from the methyl syn to the phenyl group. The magnitude of this selectivity depends on solvent polarity. On increasing the solvent polarity, a substantial increase in the amount of syn product occurs (Table 3). 

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