Reactivity Barton esters

Motherwell and Potier have been interested in the reactivity of thionitrite esters as a potential surrogate of nitric oxide toward carbon-centered radicals. Tertiary thionitrite esters react with Barton esters to give after decarboxylation the corresponding oximes or the nitroso-dimers in moderate yield [57]. 

Structure and Reactivity PTOC Esters Related Thiohydroxamate Systems Preparation of Barton Esters 67-1... 

The thione function of the Barton ester 1 can be replaced by other elements such as Se (13) without significant alteration of the reactivity (Scheme 6). Although the starting esters are more air sensitive than the analogous N-hydroxypyridine 2-thione derivatives (1), the method has some advantages for example, the adducts of the free-radical fragmentation reactions of N-hydroxypyridine-2-selenone derivatives 13 (i.e., alkylpyridylselenides) can usually be more easily modeled and transformed to other chemical functions than thioethers. 

Radicals formed by fragmentation of xanthate and related thiono esters can also be trapped by reactive alkenes.217 The mechanism of radical generation from thiono esters was discussed in connection with the Barton deoxygenation method in Section 5.4. 

Fluoroxytrifluoromethane, a source of electrophilic fluorine, has recently been employed by Barton et ah for the fluorination of unsaturated steroids.143,144 It is more reactive than perchloryl fluoride and therefore not without danger. It forms, for instance, a highly explosive product with pyridine.143 Like perchloryl fluoride it reacts with enol ethers, esters and enamines, but at lower temperature (— 78°) to yield the fluorinated ketones as well as addition... 

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... 

Alkenyl boronic esters can also be used to trap nucleophilic carbon-centered radicals, which can be generated by the tin hydride method or by decomposition of an organomercurial derivative. The influence of the olefin and boron substituents on the reactivity and regioselectivity has been determined. Vinyl-9-BBN displayed significantly better reactivity than the boronic ester while the directing effect of the boronic ester group is weaker than that of an ester function. The Barton radical chain procedure furnished very stable adducts due to an intramolecular complexation between boron and nitrogen (Scheme 9.12) [29]. Radical additions of various xanthates also occurred smoothly in the presence of lauroyl peroxide [30]. Intermolecular ver-... 

Barton DHR, Lacher B, Zard SZ (1986) The invention of new radical chain reactions part XIII. Generation and reactivity of perfluoroalkyl radicals from thiohydroxeunic esters. Tetrahedron 42 2325-2328. doi 10.1016/80040-4020(01)90613-1... 

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