Radical anion clocks

If ET intermediates play any role in representative aldehyde or ketone Wittig reactions, they are too short-lived for detection by the fastest available radical or radical anion clocks. This is conceivable if the geometry of the radical ion pair resembles that of an oxaphosphetane with partially developed bonds (223c). Such a scenario fits within the broad definition of a four-center mechanism and allows little (if any) distinction between the geometry of stereochemistry-determining TS that invoke ET versus those that do not. More precise distinctions may have theoretical significance, but they will not influence the stereochemical issues that are of concern in this review. 

The more reactive ylides discriminate less well among the aldehydes in competition experiments (p, KIE), in accordance with the reactivity-selectivity principle (RSP) (15b, 233). Thus, both the KIE and p values could be small. However, Yamataka et al. (223c) question the validity of the RSP in an ET sequence. Their arguments depend on the interpretation of KIE and model reactions for comparison, but it is not clear what experiment would be decisive. Objections to ET mechanisms based on the radical anion clock experiments discussed earlier may also not be decisive because of differences in the counterions. Futhermore, radical clock evidence against ET is available for aliphatic (not aromatic) aldehydes. 

Earlier investigations pointing to ET mechanisms in reactions of nucleophiles should be consulted in this context. The studies of Meyers and coworkers7 9 on radical anion-radical pair (RARP) transitions in halogenations with perhaloalkanes and nucleophilic (ionic) vs electron-transfer substitution reactions of trityl chloride with thiolates and other anions, and the studies of Chanon and coworkers10,99b which utilized radical-clock traps in efforts to monitor the intermediacy of free radicals in the ET pathways suggested by Meyers and colleagues. 

The radical clock experiments as well as the stereochemical outcome of the reaction along with the reactivity profiles observed pointed to an ET process as the operating mechanism. Linear-free energy relationships were also consistent with this mechanistic pathway (see succeeding text). ET may proceed in two ways, usually referred to as inner-sphere and outer-sphere ET, which can be contemplated as the two extremes of a continuous mechanism [204]. Both processes are dissociative in nature for alkyl halides and presumably do not involve a discrete radical anion, RX" [205]. The situation may, however, be different for aryl halides. Radical anions do exist, and aryl halides probably undergo a stepwise reaction with an electron donor to give rise to RX [206]. 

The reaction of bornyl and isobornyl bromides with the nucleophile (Scheme 18) is another case where the amount of inversion is small and the rate constant close to that observed with an aromatic anion radical of the same standard potential (Daasbjerg et al., 1989) it can therefore be rationalized along the same lines. Cyclizable radical-probe experiments carried out with the same nucleophile and 6-bromo-6-methyl-1-heptene, a radical clock presumably slower than the preceding one, showed no cyclized coupling product. It should be noted, on the other hand, that, unlike the case... 

Hexenyl radicals were used as radical clocks for the indirect measurement of the rate of reduction of radicals to anions using SmI2-HMPA. For example, reduction of primary iodide 4 using SmI2-HMPA resulted in the isolation of coupled product 9 in 20% yield and cyclised-coupled product 7 in 80% yield. As the rate of cyclisation of the intermediate primary hexenyl radical 6 was known, a rate constant of k= 106 M 1 s 1 could be estimated for the reduction... 

In contrast to this proposal, A.L.J. Beckwith has recently established the photostimulated SRN1 mechanism using diphenylphosphide or phenylthiolate anions and an organic bromide as an accepting substrate. A radical clock was used whose rearrangement was the demonstration of the radical intermediate [101]. 

The photochemical cleavage of naphthylmethyl alkanoates in methanol is reported to proceed by homolytic cleavage to naphthylmethyl radical and acyloxy radical,the latter decarboxylates in competition with electron transfer to give naphthylmethyl cation and carboxylate anion. Using known rates of electron transfer as a clock the rate constants for decarboxylation of the acyloxy radicals has been estimated.The light induced homolysis of 1-chloromethyl-naphthalene has also been studied using chemically induced dynamic electron polarisation (CIDEP) spectroscopy to detect the naphthyl-... 

The pioneering work on the calibration of intramolecular cy-clization of the 5-hexenyl radical by Ingold and co-workers provided the basis for the development of a large number of radical clocks." These are now used both for the calibration of rate constants for intermolecular radical reactions and as mechanistic probes to test for the intermediacy of radical intermediates in a variety of processes. Furthermore, the ready availability of bimolecular rate constants from competitive product studies using free radical clocks without the use of time-resolved experiments has greatly enhanced the synthetic utility of free radical chemistry. The same concept has recently been extended to radical ion chemistry. For example, rate constants for carbon—carbon bond cleavage reactions of a variety of radical cations and anions derived from substituted diarylethanes have been measured by direct time-resolved techniques. " ... 

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