Phenyl radicals, reactions rates with substituted

The competitive method employed for determining relative rates of substitution in homolytic phenylation cannot be applied for methylation because of the high reactivity of the primary reaction products toward free methyl radicals. Szwarc and his co-workers, however, developed a technique for measuring the relative rates of addition of methyl radicals to aromatic and heteroaromatic systems. - In the decomposition of acetyl peroxide in isooctane the most important reaction is the formation of methane by the abstraction of hydrogen atoms from the solvent by methyl radicals. When an aromatic compound is added to this system it competes with the solvent for methyl radicals, Eqs, (28) and (29). Reaction (28) results in a decrease in the amount... 

In the decay of the radical cation of 4-methyldiphenylamine (44), for which an electronic absorption spectrum with A.max = 689 nm was observed, the main reaction route is the formation of a benzidine-type dimer, similarly to that in the case of 39+. The dimerization rate constant is 2.3 x 104 M 1 s 1. However, in the presence of a large excess of parent molecules acting as a base, the formation of cyclized dimers was also suggested. In contrast, the formation of cyclic structures was found to be characteristic of the radical cations of 3 -substituted (45) and 3,3 -disubstituted (46) derivatives of diphenylamine. On the basis of CV measurements, the formation of dihydrodiphenylphenazine derivatives may be anticipated and, consequently, 47 is assumed to be the product of dimerization of 45. In both cases the rate constants were as fast as ca 1 x 107 M 1 s-1 the 3-methyl substituent promotes visibly the reaction between the 6-position of the phenyl ring of... 

Trradiation of aqueous solutions of aromatic solutes is believed to lead initially to adding H atoms and OH radicals to the ring to form cyclo-hexadienyl radicals, which then react further to form the observed products (17, 21, 34). The hydrated electron can also react to form transient negatively-charged entities (8, 29, 40) which may either be protonated by reaction with the solvent molecules (8, 40, 41) or else dissociate to form a stable anion and a substituted phenyl radical (8). In some systems (12, 41) the lifetime of the electron adduct is sufficient for its detection by pulse radiolysis techniques, but in many cases only the protonated species is seen. The rates of reaction of H atoms, OH radicals and hydrated electrons with aromatic solutes vary widely, but some success... 

The HAS reaction proceeds via a sigma (a) complex (1) with substitution being completed by the loss of the leaving group Y, which is usually hydrogen (Scheme 9.1, Y = H). Examples where the cyclohexadienyl radicals become trapped by fast reductants to form cyclohexadiene [2] and the detection of radical intermediates by ESR or CIDNP provide evidence that the cyclohexadienyl radicals are intermediates in this reaction [3]. In some systems, the addition of a radical onto the arene is the rate-determining step, because of the loss of aromaticity. For example, the rate constant for the addition of the ferf-butyl radical to benzene at 79°C is 3.8 x 10 M s [4], which is clearly at the lower end of a useful radical reaction. The arene needs to be used at high concentration, or as the solvent, in order to compensate for poor rates. On the other hand, as the rate of addition of the phenyl radical to benzene is 4.5 x 10 s" [5], it is more useful in these kind of reactions. 

A distinction between these four possibilities can be made on the basis of the kinetic isotope effect. There is no isotope effect in the arylation of deuterated or tritiated benzenoid compounds with dibenzoyl peroxide, thereby ruling out mechanisms in which a C5— bond is broken in the rate-determining step of the substitution. Paths (ii) and (iii,b) are therefore eliminated. In path (i) the first reaction, Eq. (6), is almost certain to be rate-determining, for the union of tw o radicals, Eq. (7), is a process of very low activation energy, while the abstraction in which a C—H bond is broken would require activation. More significant evidence against this path is that dimers, Arz, should result from it, yet they are never isolated. For instance, no 4,4 -dinitrobiphenyl is formed during the phenylation of... 

Table 4.5 Rate constants (M s ) at 27 °C for the reaction of phenyl substituted silyl radicals with a variety of halides [98,99]...

Sequences in which addition precedes cyclization are not as straightforward to conduct as the reverse however, they are very important because a net annulation results (that is, a new ring is formed by the union of two acyclic precursors in one experimental step). The intermediate radical is differentiated from the other radicals provided that the cyclization reaction is rapid, but it can be difficult to differentiate the initial radical from the final radical. As illustrated in Scheme 57, this is particularly true in the tin hydride method because many different types of radicals react with tin hydride at similar rates. Reaction of (69) under standard radical addition conditions produces (70), which results from a sequence of addition/cy-clization/addition.233 That the last C—C bond is formed actually results from a lack of selectivity the initial and final radicals are not differentiated and they must undergo the same reaction. Of course, this lack of selectivity is of no consequence if the product contains the desired skeleton and the needed functionality for subsequent transformations. Such sequences are very useful for forming three carbon-carbon bonds, and they can also be conducted by Barton s thiohydroxamate method.234 Structural modifications are required to differentiate the initial and final radicals, and, as illustrated by the conversion of (71) to (72), phenyl groups can provide the needed differentiation (probably by retarding the rate of addition more than they retard the rate of hydrogen abstraction). Clive has demonstrated that phenyl-substituted vinyl radicals also provide the needed selectivity, as illustrated by the second example in Scheme 57.233... 

---