Radical reactions group migration

In all these reactions the migrating group moves with retention of configuration. It also explains why concerted [1, 2] shifts to electron rich centres will be unfavourable or why 1, 2 shifts of hydrogen or alkyl groups in radicals or anions are rare. 

It is believed that the reaction starts with homolytic cleavage of the cobalt-carbon bond (at a cost of perhaps 100 kJ mol-1)8 to yield a Co(ll) atom and a 5 -decxy-adenosyl radical. This radical then abstracts a hydrogen atom (in Eq. 19.35 from the methyl group). Migration of the —GO)SR group takes place, followed by return of the hydrogen atom from 5 -deoxyadenosine to the substrate. This regenerates the 5 -deoxyadenosyl radical, which can recombine with the Co( I) atom to form the coenzyme. 

Hi) Reduction of Glycosyl Halides with Radical Rearrangement. The reduction of acetylated or benzoylated halides or selenides with a low concentration of tributylstannane leads to 2-deoxy sugar derivatives (Scheme 11). The driving force of this radical reaction is the 1,2-cA-selective migration of an ester group because of the stabilization of the radical at C-2.46... 

In chapter 24 we saw that carbonyl compounds dimerise by a radical reaction when electrons are transferred to them from metals. The typical pinacol 37 formed from acetone 36 is important because it rearranges6 in acid to give a tertiary alkyl ketone 38 known as pinacolone . The key step is a methyl migration as one of the OH groups is lost 39. 

N-Phthalimide-linked peptides that contain C-terminal a-amidotrimethylsilyl groups undergo macrocyclization reactions via sequential single-electron transfer processes [11, 12]. Thus, electron transfer from the neighboring amide to the excited phthalimide chromophore leads to an amide radical cation. Hole migration to the ot-amidotrimethylsilane center is followed by desilylation to form an 1, co-biradical intermediate, which finally cydizes to the macrocydic peptide (Scheme 9.9). 

A particularly nice conversion is the reaction of a cyanohydrin with h and Pb(OAc)4 (the Heusler-Kalvoda reaction Scheme 3). ° After the abstraction of a 8-hydrogen the cyano group migrates to the resulting radical. The final product has a ketone in place of the original cyanohydrin, which was of course form from that ketone, and a cyano group on the carbon 7 to the ketone. 

The migrating group migrates with nearly complete retention of configuration. Thus, the reaction is intramolecular and concerted. However, there is evidence which supports the free radical dissociation-recombination mechanism (Scheme 3.27). 

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