Rearrangements, radical-based

A radical-based Wagner-Meerwein-type rearrangement has been reported where the radical was generated from the epoxide using CP2T1CI as the radical source.163... 

Alkene formation also took place upon attempted deoxygenation of ITC derivatives of 2,3-epoxy alcohols. This radical-based alternative to the Wharton rearrangement is shown in equation (17), and is analogous to the rearrangement of cyclopropylcarbinyl radicals. 

The Co—carbon bond in AdoCbl is stable in water, but is inherently labile, with a bond dissociation energy of around 30—35 kcal mol This instability is exploited by the AdoCbl-dependent isomerases to effect radical-based rearrangements which, as pointed out above, are initiated by homolytic cleavage of the Co-carbon bond. In the absence of substrate, the homolysis products are not observed, yet in their presence the homolytic cleavage... 

The facts that thiols are good H-atom donors toward alkyl radicals and that silyl radicals are among the most reactive known species for abstraction and addition reactions suggest that any class of compounds which allows the transformation of a thiyl to a silyl radical via a fast intramolecular rearrangement will potentially be a good radical-based reducing agent. The silanethiols 11 and 12 are found to have this property [84, 85]. The reductions of bromides, iodides and isocyanides by thiol 12 are demonstrated to follow the expected mechanism [85]. 

We saw an example of this process as part of larger mechanistic schemes in the radical based oxidative additions of Section 6.3. We also saw typical radical rearrangements used to detect the presence of radical interm iates (e.g., Eq. 6.20). 

In order to study the strucmre-activity relationship of pharmaceutically active derivatives of baccatin HI, radical-based deoxygenations were used to generate a systematic defunctionalization of the taxole core. When tris(trimethylsilyl)silane was used as reducing agent of the xanthate functionality (equation 17 yields based on the consumption of the starting mataial), a cascade of intramolecular rearrangements took place, due to the slower hydrogen donation of silane relative to tin hydrides. ... 

It was found that protonation facilitates the 1,2-shifts in several reactions involving free radicals. Based on molecular-orbital calculations, Golding and Radom ° proposed the following general scheme for the intermolecular rearrangement catalyzed by adenosine cobalamin (eqs. 7.2.5 to 7.2.8) ... 

The pyrazole ring is resistant to oxidation and reduction. Only ozonolysis, electrolytic oxidations, or strong base can cause ring fission. On photolysis, pyrazoles undergo an unusual rearrangement to yield imidazoles via cleavage of the N —N2 bond, followed by cyclization of the radical iatermediate to azirine (27). 

The converse situation in which ring closure is initiated by the attack of a carbon-based radical on the heteroatom has been employed only infrequently (Scheme 18c) (66JA4096). The example in Scheme 18d begins with an intramolecular carbene attack on sulfur followed by rearrangement (75BCJ1490). The formation of pyrrolidines by intramolecular attack of an amino radical on a carbon-carbon double bond is exemplified in Scheme 19. In the third example, where cyclization is catalyzed by a metal ion (Ti, Cu, Fe, Co " ), the stereospecificity of the reaction depends upon the choice of metal ion. 

These are discussed in (B-71MS4). Oxirane itself shows a strong molecular ion peak and a slightly stronger base peak at mje 29 (CHO ) due to isomerization to ethanal and loss of a methyl radical. Substituted oxiranes tend to show only weak molecular ion peaks, because of rearrangement and fragmentation. 

Mechanistically the rearrangement is formulated to proceed via an intermediate radical-pair or ion-pair. In either case the initial step is the formation of a nitrogen-ylide 2 by deprotonation of the ammonium species with a strong base. The abstraction of a proton from the a-carbon is facilitated by an electron-withdrawing group Z—e.g. an ester, keto or phenyl group ... 

---