Rearrangement of Substrate Radicals

As discussed above, homolysis of AdoCbl and hydrogen abstraction from the substrate to form a substrate radical appear to be closely coupled or concerted reactions. This leaves isomerisation of the substrate radical to 

The amino mutases require pyridoxal phosphate for activity, and although little is known about them mechanistically, Freyis work on lysine 2,3-aminomutase has proved very informative (Lieder et al., 1998 Frey, 1997). Lysine 2,3-aminomutase is not a 1 2 enzyme but it functions very similarly (Section 1). The enzyme uses pyridoxal phosphate to facilitate the 1,2 

FIGURE 19. Mechanism for the rearrangement of propane-1,2-diol radical by diol dehydrase in which the migration of the hydroxyl group if facilitated through electrostatic catalysis by a tightly-bound potassium ion at the active site (Shibata et al., 1999). 

FIGURE 20. Mechanism for the migration of the amino group in Oie reaction catalyzed by lysine-2,3-aniinomutase, for discussion of the mechanism see the text. 

FIGURE 21. Mechanisms for the rearrangement of substrate radicals in the reactions catalyzed by carbon skeleton mutases. For 2-methyleneglutarate mutase and the acyl-CoA mutases both associative (upper pathway) and dissociative (lower pathway) mechanisms have been proposed (Halpem, 1985 Bucket Golding, 1996), whereas for glutamate only a dissociative mechanism appears feasible. 

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An interesting example of the situation in which the preferred addition site does not lead to the preferred ring size has been provided by Miura and coworkers with the extended vinylcyclopropane rearrangement of substrate 2164. Formation of vinylcyclopentyl derivative 22 requires the addition of the triphenylstannyl radical to the unsubstituted terminus... 

The radical rearrangement reaction, serving as a timing device, has been called a free radical clock 2 It provides a means of evaluating the rate constant for reactions of this radical with other substrates. The example shows how the radical-chromium(II) rate constant can be determined. A number of other instances have been summarized.13... 

An experimental probe for the presence of radical intermediates resulting from thermally induced homolytic cleavage of the N-0 bond was derived by incorporating an alkene into a model substrate to act as a potential intramolecular radical trap (Scheme 6.25) [11]. In a control experimental, thermal reaction of 73 gave the desired product 74 in 66% isolated yield. On the other hand, thermal rearrangement of the unsaturated compound 75 under our typical conditions gave the desired hydroxypyrimidinone 76 in only 38% isolated yield. When the vinyl ami-doxime mixture 75Z/E was heated in o-xylene at 125 °C in the presence of a... 

There exist early examples of this transformation [507, 508], but due to the symmetric structure of the alkene part, only isotope labeling, etc., allowed the exclusion of a prototropic rearrangement. Furthermore, due to the high reaction temperatures of 340 °C and above, several different products are formed. A low-temperature version (77 K) of this reaction via the radical cation has been reported [509]. The chirality transfer has been studied and a detailed mechanistic investigation has been conducted [510] typical experiments in that context were the reactions of substrates such as 155 and 157 (Scheme 1.70). 

Electrochemical oxidation of epoxides in absence of nucleophiles, catalyses a rearrangement to the carbonyl compound. The electrolyte for this process is dichlo-romethane with tetrabutylammonium perchlorate. Reaction, illustrated in Scheme 8.7, involves the initial formation of a radical-cation, then rearrangement to the ketone radical-cation, which oxidises a molecule of the substrate epoxide. The process is catalytic and requires only a small charge of electricity [73]. 

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