2.2- Diphenylcyclopropyl radicals

In its relative reactivity toward toluene, ethylbenzene and cumene the more highly substituted 1-methyl-2,2-diphenylcyclopropyl radicaP , derived from the decomposition of the precursor diacyl peroxide, resembles the chlorine radical more than it does the phenyl radical (Table 3). Similarly, comparison of the relative reactivities of primary, secondary and tertiary aliphatic hydrogens toward chlorine atoms (1.0 3.6 4.2) and phenyl radicals (1.0 9.3 44) with the relative reactivities of the C-H bond in the methanol/ethanol/2-propanol series toward the 1-methyl-2,2-diphenylcyclopropyl radical (1.0 2.4 15) further confirms the low selectivity of the cyclopropyl radical. Again, this radical resembles the chlorine atom in its reactivity more than it does the phenyl radical. 

Further attempts to trap the chiral l-methyl-2,2-diphenylcyclopropyl radical, before inversion, by using excellent radical scavengers as solvents were also abortive. Decomposition of the diacyl peroxide (48) in thiophenol and reduction of (-)-(/ )- -bromo-l-methyl-2,2-diphenylcyclopropane (51) with tri-n-butyltin hydride as solvent resulted in essentially racemic hydrocarbon (49) ... 

Similar rearrangements have been observed with 2,2-diphenylcyclopropyl radicals that have a variety of 1-substituents (Scheme 5). 

Cyclopropyl radicals, anion radicals and anions 727 TABLE 14. Relative reactivity of various solvents toward the l-methyl-2,2-diphenylcyclopropyl radical ... 

By itself, the formation of 97 takes on great significance. Its formation has never been observed in solution, only the products resulting from the reactions a, b and i, are observed Also the l-methyl-2,2-diphenylcyclopropyl radical does not dimerize to 97 even in a cage reaction but instead it follows the usual course of a tertiary o radical, it disproportionates. The only other time that dimerization of this radical has been observed was when the radical was formed on a magnesium metal surface . Thus, the reaction of the aggregates [(S)-(96)] resembles a surface reaction (see below). 

As in direct metalation, the reaction occurs at the metal surface. An electron is transferred from the surface to the <7 antibonding orbital of the carbon-bromine bond to produce the anion radical in the rate-determining step " (equation 1). The anion radical can then dissociate at the surface to the 1-methyl-2,2-diphenylcyclopropyl radical (equation 2). At this point some racemization may occur and the radical can undergo a number of indistinguishable reactions. The radical may pick up another electron to yield the anion (equation 3) or since mercury is such an efficient radical trap, the radical may become adsorbed on the mercury surface (equation 4) from which it can either take another electron to yield the anion (equation 5) or combine with another adsorbed radical to produce a dicyclopropylmercury (equation 6). 

The rearrangement of a cyclopropyl radical to an allyl radical in solution was reported for the first time by Walborsky et al. [18] in the thermal decomposition of l-methyl-2,2-diphenylcyclopropanecarbonyl peroxide [I I] (Scheme 6). The l-methyl-2,2-diphenylcyclopropyl radical [16] reacted by both abstracting hydrogen from the solvent... 

The thermal reaction of chiral (S)-l-(methyl-2,2-diphenylcyclopropyl)copper (96) provides an interesting example of the effect of aggregates on the stereochemistry of the cyclopropyl radical. The thermal decomposition of (S)-96 led lo the formation of a variety of products depicted in Scheme 7. Product analysis, including stereochemistry, led to the mechanism shown in Scheme 8. The aggregate mixture [(S)-96] is assumed to exist in THF solution (colloidal ) where n = 2, 4, or 6. 

The stereochemical results of radicals generated in solution and at metal surfaces can vary greatly. For example, genesis of the l-methyl-2,2-diphenylcyclopropyl a radical in... 

There is no doubt that the 1-methyl-2,2-diphenylcyclopropyl a radical is incapable of maintaining its configuration when it is formed in solution How then can one... 

An entirely different result has been reported by Jacobus and Pensak. They found that the reduction of the optically active 1-bromo-l-methyl-2,2-diphenylcyclopropane (51) with sodium naphthalenide (NaN) in DME (0.5 m) yields the corresponding hydrocarbon 49 of 29% optical purity with net retention of configuration. This observation was interpreted to mean that the l-methyl-2,2-diphenylcyclopropyl (t radical was being captured by a second SET from sodium naphthalenide to give the sodium derivative (which transforms in DME to 49) at a rate faster than its inversion frequency (Scheme 14). 

In a search for radical intermediates, IV, the cw-olefmic substrate (Z)-l,2-bis(/ru s-2, trflfws-5-diphenylcyclopropyl)ethene, VI-Z (scheme IX), incorporating the hypersensitive radical trapping group, trans-2, /raAi5-5-diphenylcyclopropyl, was used in the epoxidation... 

Mansuy et al. employed alkyl peroxides and iodosylbenzene as oxygen donors and examined the oxidation products of cyclohexane in the presence of a series of M(TPP) complexes (M=Fe, Mn, Co, Rh, and Cr) [284]. As listed in Table 10, cyclohexanol and cyclohexanone are the major products however, the ratio of the alcohol and ketone was dependent on the oxidant employed. For the oxidation by peroxides, involvement of alkoxy radical (RO ) due to the homolytic 0-0 bond cleavage of R-OO-M was proposed. In order to trap possibly formed radical intermediates. He and Bruice examined the oxidation of cw-stilbene and (Z)-l,2-bis(fran5-2,tran5-3-diphenylcyclopropyl)ethane by iron porphyrin/f-BuOOH system [285]. In separate experiments, AIBN was used as a radical chain initiator for the oxidation of the alkenes by t-BuOOH. According to the product distribution, they concluded the products to be derived from initial combination of t-BuOO, rather than 0=Fe (por)", with olefin. 

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