The radical rearrangement mechanism

Mechanisms b - d all involve intramolecular migration of the group X without detachment from the two-carbon unit. The first possibility of this type (reaction b) will be referred to as the concerted mechanism and has the symmetrical bridged species as a transition structure. Such rearrangements are usually 

The distinction between the concerted mechanism (b) and the addition-elimination mechanism (c) depends formally on whether the cyclic species is a transition structure or a stable intermediate. This distinction is not always clear-cut, as there are cases in which the depth of the well containing the intermediate becomes very small, making it difficult to classify the rearrangement one way or 

A further (and final in the context of the current discussion) variation to the intramolecular mechanism arises from protonation of the migrating group (mechanism d). Previous studies have shown that protonation is able to facilitate the 1,2-shift in cases where it may otherwise seem unfavorable [23, 51, 52]. This has been explained as being due to an increase in the cationic nature of the shift [23] or, alternatively, by obviating the need for one-electron occupancy of a high energy orbital [52]. 

Implicit in our approach is, of course, acceptance of the bound free-radical hypothesis. As we have seen in the previous sections, this appears to be the most likely mode of action for coenzyme B12. The widespread acceptance [1] of this pathway demonstrates that it has, so far, been able to stand the test of time. Our attention is now turned to theoretical techniques that can provide accurate estimates of thermochemical data. 

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