Carbon-centered radicals case studies

Studies with simple radicals show that carbon-centered radicals react with phenols by abstracting a phenolic hydrogen (Scheme 5.14). The phenoxy radicals may then scavenge a further radical by C -C or C-O coupling or (in the case of hydroquinones) by loss of a hydrogen atom to give a quinone. The quinone may then react further (Section 5.4.4). Thus two or more propagating chains may be terminated for every mole of phenol.I9j... 

Rate constants for reactions of Bu3SnH with some a-substituted carbon-centered radicals have been determined. These values were obtained by initially calibrating a substituted radical clock on an absolute kinetic scale and then using the clock in competition kinetic studies with Bu3SnH. Radical clocks 24 and 25 were calibrated by kinetic ESR spectroscopy,88 whereas rate constants for clocks 26-31 were measured directly by LFP.19,89 90 For one case, reaction of Bu3SnH with radical 29, a rate constant was measured directly by LFP using the cyclization of 29 as the probe reaction.19... 

Based on the analysis of the reactions in Scheme 3 and on previous studies (46, 47), a mechanism for the reaction was proposed in which the /x-peroxo complex, 16, may simultaneously abstract two hydrogen atoms from iso-propyl groups on the pyrazolyl ligands. Alternatively, because of the weak 0—0 bond, 16 may homolytically dissociate to form two Tp"Co(0-) oxo-radical moieties, and these species would then abstract hydrogen from the iso-propyl groups. In either case, the resulting carbon-centered radical can either react with solvent, as was observed for the Tp complex (46), or with another carbon-centered radical so as to regenerate the Tp"Co(OH) complex and produce a derivative of the Tp" complex with an iso-propenyl substituent, 18. Ultimately, either route would produce the (/x-OH)2 complex, 17. 

Carbon-centered radicals play a significant role in a number of processes of technological and biological relevance, but as most of the radicals are unstable and very reactive species, difficult to study by experimental techniques. Many available experimental studies focused on the physicochemical properties of radicals come from the spectroscopic techniques, not only electronic spin resonance (ESR) but also IR/Raman and UV-vis, often obtained in matrices at low temperature. Unfortunately, in most cases, experimental results are extremely difficult to interpret, even in terms of appropriate identification of molecular species. For example, the entire UV-vis... 

Indeed, radical probe studies have very decisively excluded the radical cyclization mechanism in at least one typical case. This experiment relies upon the circumstance that the cyclization of a carbon-centered radical to an aldehyde carbon group is known to occur at approximately the same rate as the exo-trig cyclization of such a radical to a carbon-carbon double bond. In a probe molecule designed to provide an opportunity for a hypothetical radical intermediate to add to either or both of these functionalities, no addition to the vinyl double bond was observed (Scheme 75). 

In basic solution, deprotonation at the hydroxyl group (typically) leads to a very rapid elimination of the superoxide radical anion [65, 69]. The deprotonation reaction was found to be the rate-limiting step in all of those cases where the kinetics were studied in sufficient detail. The peroxyl radical derived from a carbon-centered radical such as 53 apparently eliminates superoxide in a similar fashion [18] this probably happens following the pathway (39), (40). 

The behavior of vinyl radicals (5) generated by addition of a variety of thiyl radicals to butynedioic or propynoic acids has been studied by ESR spectroscopy The intermolecu-lar abstraction of a thiol hydrogen is in competition with l, -hydrogen shifts. An unusual 1,4-shift (equation 7) is shown to occur in cases where the resulting carbon-centered radical bears a-sulfur and a-carboxy substituents, whereas in other examples a 1,5-shift predominates (equation 3). 

As discussed in detail elsewhere, fragmentation of a a bond (C-H, C-C, C-Si, C-Sn, etc., sometimes dubbed as mesolytic fragmentation) is a common process among radical cations and is a mild method for the generation of carbon-centered radicals [52-54]. Feasibility has been predicted through thermochemical cycles and the kinetics and mechanism have been studied in detail in a number of significant cases [55]. 

In this case, the information was taken from the study of the n-BusSnH-media-ted cyclization of substituted A -(phenylthio)amides 8, detailed in Scheme 39.6. The reaction is a radical process in which the nitrogen-eentered radical 11 formed in the first instance, cyclizes onto the C=C double bond to give carbon-centered radical species 13 which, after quenching, yields cyclized compound 9. The cyclization step follows the exo-mode in agreement with the Baldwin rules (5-exo-trig). 

With the NDDO methods, tautomeric equilibria,22o especially in heterocycles,216-219,223,224,227,232,233 have been a favorite topic for study using the BKO approach. The tautomeric equilibria of many heterocyclic systems are exquisitely sensitive to solvation,i i3>2i4 making them interesting test cases for the validation of any solvation model. A detailed comparison is presented later in the section on relative free energies in heterocyclic equilibria. A comprehensive study of the stabilization of a wide variety of carbon radical and ionic centers has also been reported.21 ... 

The energy of the SOMO of an ethene derivative which is substituted with conjugating, electron withdrawing substituents is of course much lower than that of ethene itself. An extreme case of such an anion radical is that of tetracyanoethylene (TCNE), which has been isolated as the tetrabutylammonium salt. A reasonably direct, experimental examination of the SOMO distribution of this anion radical was possible through polarized single crystal neutron diffraction studies [108]. Interestingly, the pi SOMO was found not to be centered directly around the two alkene carbon nuclei, but rather to be bent back, away from the alkene C-C bond, as is theoretically expected for an MO which is anti-bonding between these two carbons (Scheme 65). 

Matsuoka and Szwarc (1961) photolyzed diazomethane in the presence of isooctane and styrene in one case, and in the presence of isooctane and styrene-a,j8,j8-d3 in the other case, and thus determined the methyl affinities of styrene and of trideuteriostyrene with respect to isooctane by the method of competing reactions. The isotope effect ( d/ h) was 1-07-1 ll. Matsuoka and Szwarc used these data to justify the conclusion that in radical addition reactions the initial structure of a reactive center is preserved in the transition state. In styrene and in trideuteriostyrene the reactive centers are both terminal carbon atoms doubly bonded to another carhon. In the product the reactive carbon atom becomes tetrahedral, and thus the transition state could conceivably resemble either reactant (trigonal) or product (tetrahedral). It can be calculated, however, that a change of configuration from trigonal to tetrahedral in the transition state should exhibit an isotope effect kj)jk ) of about 1-8 in the reactions studied by Matsuoka and Szwarc the tetrahedral configuration was therefore excluded by these authors. 

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