Hydroxyalkyl radical

2-migration of hydroxy groups in /3-hydroxyalkyl radicals 21 has been studied repeatedly due to the involvement of these species in the enzyme-mediated dehydration reaction of 1,2-diols 40-44 A detailed review of these results has recently been published by Radom et al.45 

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A radical approach to asymmetnc iildol synthesis, which is based on the radical addition of a chiral hydroxyalkyl radical equivalent to a tutroalkene, has been reported, as shown in Eq 4 93 The radical precursor is prepared from the corresponding carboxyhc acid by the Barton reaction, which has been used for synthesis of new fi-lactams ... 

Ceric ions react rapidly with 1,2-diols. There is evidence for chelation of cerium and these complexes are likely intermediates in radical generation10 106 The overall chemistry may be understood in terms of an intermediate alkoxy radical which undergoes p-scission to give a carbonyl compound and a hydroxyalkyl radical (Scheme 3.59). However, it is also possible that there is concerted electron transfer and bond-cleavage. There is little direct data on the chemical nature of the radical in termediates. 

The reaction fails if the decarboxylation produces a radical that is easily oxidized, such as an a-hydroxyalkyl radical.2 In intermediate cases, such as tert-alkyl or a-alkoxyalkyl radicals,2 the yield based on the parent quinono is usually improved by using an excess of persulfate and carboxylic acid to compensate for the loss of radicals due to oxidation (footnote b, Table I). 

A biomimetic oxidation with perfluorinated porphyrin complexes [(F20TPP) FeCl] showed high catalytic activity with secondary alcohols with over 97% yield in all cases [144]. Furthermore, this catalyst is able to oxidize a broad range of alcohols under mild conditions with wCPBA as terminal oxidant. Here, an a-hydroxyalkyl radical species is proposed as central intermediate. 

The hydroxyalkyl radicals CH3CHCH2OH and CH3CH(OH)CH2 formed by reactions of OH with C3H6 can also be adsorbed on the soot. However, the resulting desorption products are not further useful for NO removal, and therefore, the reactivity of these radicals is decreased. 

The model predicts that in the presence of soot, the removal of NO decreases due to both heterogeneous generation of NO from N02 adsorbed on the surface of the soot particles and decrease of reactivity of the hydroxyalkyl radicals. It was found that NO removal, both in the presence and absence of soot, is about the same, however, the proportion of NO and N02 depends on the soot. With lower densities (107 cm-3) of soot particles, the final NO was primarily N02, whereas, with higher densities (109cm-3) of soot particles, the NO was mainly NO. 

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... 

Alcohols, like hydrocarbons, are oxidized by the chain mechanism. The composition of the molecular products of oxidation indicates that oxidation involves first the alcohol group and the neighboring C—H bond. This bond is broken more readily than the C—H bond of the corresponding hydrocarbon, since the unpaired electron of the formed hydroxyalkyl radical interacts with the p electrons of the oxygen atom. 

High values of the inhibition coefficient (/= 12-28) were detected for the first time in the oxidation of cyclohexanol [1] and butanol [2] inhibited by 1-naphthylamine. For the oxidation of decane under the same conditions, /= 2.5. In the case of oxidation of the decane-cyclohexanol mixtures, the coefficient / increases with an increase in the cyclohexanol concentration from 2.5 (in pure decane) to 28 (in pure alcohol). When the oxidation of cyclohexanol was carried out in the presence of tetraphenylhydrazine, the diphenylaminyl radicals produced from tetraphenylhydrazine were found to be reduced to diphenylamine [3]. This conclusion has been confirmed later in another study [4]. Diphenylamine was formed only in the presence of the initiator, regardless of whether the process was conducted under an oxygen atmosphere or under an inert atmosphere. In the former case, the aminyl radical was reduced by the hydroperoxyl radical derived from the alcohol (see Chapter 6), and in the latter case, it was reduced by the hydroxyalkyl radical. 

In the presence of dissolved dioxygen, the hydroxyalkyl radicals are converted into the hydroxyperoxyl radicals very rapidly therefore, only hydroperoxyl and hydroxyalkylperoxyl radicals participate in the reduction of the aminyl radicals. The higher the temperature, the more effective the decomposition of the hydroxyperoxyl radicals and the higher the proportion of the H02 radicals participating in the regeneration of the amine. 

Even though the radical attacking ethyl alcohol in the above reaction generated a-hydroxyethyl rather then ethoxy free radical, there seems to be little or no tendency for alkoxy free radicals to rearrange to a-hydroxyalkyl radicals. Thus in the reaction... 

From the above it is clear that DMPO can undergo the addition-oxidation mechanism with water as the nucleophile, provided a suitable oxidant is present. With a primary alcohol competing, the O-connected alkoxy spin adduct is formed in addition to HO-DMPO". On the other hand, with a hydroxyl radical source a competing alcohol will undergo hydrogen abstraction by HO" and form an a-hydroxyalkyl radical which forms a C-connected spin adduct. This criterion clearly can distinguish between the two mechanisms at least in model systems (for recent examples, see Reszka and Chignell, 1995 Janzen et al., 1995 Thomas et al., 1996). 

A somewhat more complex mechanism takes place with other H-atom donors, such as primary and secondary alcohols, either added to the liquid ammonia solution or used as the solvent (Andrieux et al., 1987). Instead of being totally reduced, the hydroxyalkyl radical, resulting from the H-atom abstraction from the alcohol, partly deprotonates, generating the anion radical of the parent carbonyl compound. The latter is then generated by... 

On the basis of the very negative activation entropies, the transition states for the addition are highly ionic, i.e. there is a large degree of electron transfer in the transition state as with the hydroxyalkyl radicals (Sect. 2.1.1). In support of this is the fact that the rate constants for addition depend on the reduction potentials of the nitrobenzenes, varied by the substituent R3 in a way describ-able by the Marcus equation for outer-sphere electron transfer [19]. 

The tertiary and primaiy hydroxyalkyl radicals are produced in the ratio 7.2 1, respectively. The former is rapidly oxidized by Fe (supporting the Fenton process) while a big proportion of the latter is accumulated in solution and terminates the redox chain by dimerization, according to the reactions (15) and (16), respectively ... 

Oxidation of alcohols with a variety of oxidizing agents leads to a-hydroxyalkyl radicals. These attack protonated heteroaromatic bases only when obtained from methanol or primary alcohols, with secondary alcohols no attack takes place, probably owing to the ease with which such a-hydroxyalkyl radicals are oxidized. (This limitation does not apply to radiation-induced oxyalkylation, see later.)... 

The best results have been obtained by oxidation with peroxydi-sulfate. The formation of hydroxyalkyl radicals was originally interpreted according to the mechanism of Eqs. (40) and (41). 

Also, the y-ray-induced reactions in alcohols, which sometimes lead to the same product, support the attack of ot-hydroxyalkyl radicals on the unexcited heteroaromatic compound. Yields are particularly interesting in the imidazole series, e.g., with caffeine (16). ... 

The results indicate that C-6 in purine and 2-aminopurine (17) is more reactive than C-8 or C-2, while the primary attack at C-8 in adenine and 6-ethoxypurine (18) indicates that C-8 is more reactive than C-2. The order of reactivity toward a-hydroxyalkyl radicals in the purine system is therefore C-6 > C-8 > C-2. 

The different behavior of the alcohols probably arises from differences in bond dissociation energies. Experiments show that radical attack on methanol (4) and ethanol (27) leads to rupture of the C—H rather than the O—H bond. There appear to be no direct measurements of C—H bond energies in alcohols. However, D(R—OH) has been determined as 102 kcal. and does not appear to vary greatly with changes in R, provided R is a simple alkyl radical (16). Moreover, the heat of rearrangement of alkoxy radicals to hydroxyalkyl radicals has been determined from electron impact data (12). Considering, for example, 2-propanol and the following reactions... 

The reactions of R02 with NO and with R02 generate alkoxy radicals (RO). Alkoxy radicals have several possible atmospheric fates, depending on their particular structure. These include reaction with 02, decomposition, and isomerization as we shall see, reactions with NO and N02 are unlikely to be important under most tropospheric conditions. Atkinson et al. (1995b) and Atkinson (1997b) have reviewed reactions of alkoxy radicals and /3-hydroxyalkyl radicals ... 

Because the /3-hydroxyalkyl radicals formed are substituted alkyl radicals, they react with 02 to form alkylperoxy radicals, e.g.,... 

At high temperatures (>500 K), hydroxyalkyl radicals react with NsO by way of another chain-reaction. An-oxygen transfer (reaction 35) has been proposed as one of the chain-carrying steps.110,111... 

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