Alkyl benzenes under radical conditions

As a result, an alkyl benzene undergoes selective bromination at the weak benzylic C—H bond under radical conditions to form a benzylic halide. For example, radical bromination of ethylbenzene using either Br2 (in the presence of light or heat) or A/-bromosuccinimide (NBS, in the presence of light or peroxides) forms a benzylic bromide as the sole product. 

Halogenations. Bromine is a very powerful brominating agent that has found utility in a variety of systems. While the hromina-tion of alkanes is usually not a viable synthetic method, alkyl-benzenes can be brominated at the henzyhc position under radical conditions (eq I). N-bromosuccinimide (NBS) can also be used for this transformation. 

The intramolecular additions of alkoxyl groups to carbon-carbon double bonds to give P-iodo-alkyl ethers from alkyl hypoiodites under photolytic conditions have been reported. Suginome and collaborators found that the formation of a-iodoepoxides arising from an intramolecular a, P-addition of an allylic alkoxyl radical is a major general process in the photolysis of tertiary and some secondary allylic alcohol hypoiodites in the presence of mercury (11) oxide and iodine in benzene. Thus, 5-hydroxy-SP-cholest-3-ene gave epimeric a-iodoepoxides in a ratio of 10 1 in 86% yield as outlined in Scheme 86. Similar reaction of l-alkylcyclohex-2-en-l-ols gave a mixture of cis- and tr s-a-iodoepoxides (Scheme 87). The addition of pyridine dramatically enhanced the yield of the addition product. The yields of the a-iodoepoxides obtained from the photolysis of l-alkylcyclohex-2-en-l-ols and the ratios of cis- to trans-isomers are summarized in Table 109.1. 

Examples of the intermolecular C-P bond formation by means of radical phosphonation and phosphination have been achieved by reaction of aryl halides with trialkyl phosphites and chlorodiphenylphosphine, respectively, in the presence of (TMSlsSiH under standard radical conditions. The phosphonation reaction (Reaction 71) worked well either under UV irradiation at room temperature or in refluxing toluene. The radical phosphina-tion (Reaction 72) required pyridine in boiling benzene for 20 h. Phosphinated products were handled as phosphine sulfides. Scheme 15 shows the reaction mechanism for the phosphination procedure that involves in situ formation of tetraphenylbiphosphine. This approach has also been extended to the phosphination of alkyl halides and sequential radical cyclization/phosphination reaction. ... 

Generally, the reactions are carried out in refluxing benzene solution, since the yield in benzene is better than that in other solvents. Probably, the radicals formed may be somewhat stabilized by the weak orbital-orbital interaction between the radicals and benzene. However, from the environmental point of view, toluene or dioxane is recently used. As substrates, alkyl bromides or alkyl iodides are used, and the reactivity increases in the order prim-alkyl < seoalkyl < te/t-alkyl. Sugar anomeric bromide (3) is generally not so stable, so the reaction is carried out under irradiation conditions with a mercury lamp at room temperature (eqs. 4.2 and 4.3). There are two types of anomeric glycosyl radicals as shown in Figure 4.1. One is the axial radical [I], and the other is the equatorial radical [I ]. The axial radical is more nucleophilic than the equatorial radical due to the stereoelectronic effect, where this effect comes... 

The reactions are commonly initiated with AIBN at ca. 80 °C in a solvent such as benzene or toluene, but alkyl halides have also been reduced at -60 °C with tributyltin hydride under sonication conditions.133-134 Some reductions of halides have been carried out by generating the tin hydride in situ from a molar equivalent of the reducing agent [lithium aluminium hydride, sodium borohydride, or poly(methylhydrosiloxane)] and a catalytic amount of organotin hydride, halide, or oxide. Reduction of halides has also been carried out under aqueous conditions, using 4,4 -azobis(4-cyanovaleric acid) (ACVA) as a water-soluble radical initiator.60... 

The reaction of a nucleophilic alkyl radical R with benzene affords the a-complex 1, a fairly stable cyclohexadienyl radical, which under oxidizing conditions leads to cation 2 (Scheme 1). Depending on the stability of the attacking radical, the formation of 1 is a reversible process. Deprotonation eventually affords the homolytic aromatic substitution product 3. If the reaction is performed under non-oxidizing conditions, cyclohexadienyl radical 1 can dimerize (—> 4), disproportionate to form cyclohexadiene 5 and the arene 3, or further react by other pathways [3]. 

Diacetoxyiodo)benzene in the presence of tert-butyl hydroperoxide readily oxidizes alkenes at the allylic position (Scheme 3.88) [269]. This reaction proceeds via initial formation of the tert-butylperoxy radical and it can be extended to the oxidation of unactivated C—H bonds in alkyl esters and amides to give the corresponding keto compounds under mild conditions [270]. 

---