Other Useful Radical Rearrangements

Useful bicyclic ring systems are obtained by (TMS)3Si radical-mediated fragmentation of strained ketoalkene precursors. For example, the ketoalkene 64 reacted with 1.5equiv of silane to give 95% of hydrindanone 65 (Reaction 7.67) [78]. (TMS)3Si radical adds first to the terminal alkene and the carbon-centred radical can relieve the strain by cleaving the adjacent C—C bond. 

The addition of (TMS)3SiH in the 3-oxo-l,4-diene steroid-type derivative promoted C(9)—C(10) bond cleavage (Reaction 7.68) [79]. The removal of the silyl group was achieved by treating with dilute aqueous hydrochloric acid at room temperature and the final product was obtained in an 85 % yield. 

The radical adduct obtained by the addition of (TMS)3Si to the keto azide 66 underwent an opening of the cyclopropyl ring, with formation of 67 as intermediate, prior to the intramolecular addition to the azido group [60]. After tosylation the final product was obtained in 61 % yield. 

An interesting neophyl-type radical rearrangement process has been established for the synthesis of azabicycles, which are not readily accessible by other means. Barton McCombie deoxygenation of xanthate 70 under slow addition of (TMS)3SiH and AIBN in refluxing toluene furnished the 2-azabenzonorbor-nane derivative in good yield (Reaction 7.72) [82]. 

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Cyclopentadienyl radical can combine with methyl radicals to form fulvene, along with a subsequent loss of H atom. Fulvene may also be formed by reactions of the iso isomer of C4H5 with aeetylene. Once fulvene adds an H atom, subsequent rearrangement to benzene has been shown to be thermodynamically favorable using quantum chemieal methods.Cyelopentadienyl radicals may also combine with eaeh other and then rearrange to form naphthalene. 

Another important rearrangement is that of cyclopropylmethyl radicals to the corresponding homoallyl radicals. This is an exceptionally fast reaction (t1/2 10 8) and has been used as a radical clock to determine the rates of other free-radical reactions.95 Cyclopropylcarbene also undergoes rearrangement, leading to cyclobutene.96... 

Experimental evidence for the presence of radical intermediates is provided by the identification of expected products from radical rearrangements, by the use of appropriate radical probes and by direct detection by electron spin resonance (ESR). Other mechanistic evidence includes inhibition by radical traps, such as di-t-butylnitroxide (DTBN), TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxy), galvinoxyl and oxygen, and by radical anion scavengers such as p-dinitrobenzene (p-DNB). 

The rate of the ring-opening reaction of 5, " and other substrates have been determined using an indirect method for the calibration of fast radical reactions, applicable for radicals with lifetimes as short as 1 ps/ This radical clock method is based on the use of Barton s use of pyridine-2-thione-Al-oxycarbonyl esters as radical precursors and radical trapping by the highly reactive thiophenol and benzeneselenol/ A number of radical clock substrates are known/ Other radical clock processes include racemization of radicals with chiral conformations, one-carbon ring expansion in cyclopentanones, norcarane and sprro[2,5]octane, a-and p-thujone radical rearrangements, and cyclopropylcarbinyl radicals or... 

Further examples of the norbornenyl-nortricyclyl radical rearrangement used as a free radical probe will be discussed in Section Other... 

In any given sigmatropic rearrangement, only one of the two pathways is allowed by the orbital-symmetry rules the other is forbidden. To analyze this situation we first use a modified frontier-orbital approach. We will imagine that in the transition state the migrating H atom breaks away from the rest of the system, which we may treat as if it were a free radical. 

The reductive elimination of a variety of )3-substituted sulfones for the preparation of di-and tri-substituted olefins (e.g. 75 to 76) and the use of allyl sulfones as synthetic equivalents of the allyl dianion CH=CH—CHj , has prompted considerable interest in the [1,3]rearrangements of allylic sulfones ". Kocienski has thus reported that while epoxidation of allylic sulfone 74 with MCPBA in CH2CI2 at room temperature afforded the expected product 75, epoxidation in the presence of two equivalents of NaHCOj afforded the isomeric j ,y-epoxysulfone 77. Similar results were obtained with other a-mono- or di-substituted sulfones. On the other hand, the reaction of y-substituted allylic sulfones results in the isomerization of the double bond, only. The following addition-elimination free radical chain mechanism has been suggested (equations 45, 46). In a closely related and simultaneously published investigation, Whitham and coworkers reported the 1,3-rearrangement of a number of acyclic and cyclic allylic p-tolyl sulfones on treatment with either benzoyl peroxide in CCI4 under reflux or with... 

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