Question

Question: Propose a mechanism for the following reaction:

Short answer

The mechanism of the following reaction is shown below:

Step-by-step solution

Co-enzyme B12

Coenzyme B₁₂ or cobalmin coenzymes are coenzymes that refer to complex macrocycles whose reactivity is associated with their unique cobalt-carbon bond. It is required by enzymes that catalyze the exchange of a hydrogen-bonded to one carbon with a group bonded to an adjacent carbon.

For example, methylmalonyl-CoA mutase (A mutase is an enzyme that transfers a group from one position to another), catalyzes a reaction in which the H bonded to one carbon changes places with a C=OSCoA group bonded to an adjacent carbon.

Propose a mechanism of the given reaction

The chemistry of coenzyme B₁₂ takes place at the bond joining the cobalt and the 5’-deoxyadenosyl group, which is an unusually weak bond.

The mechanism involves the following steps:

In the first step, the Co-C bond breaks homolytically, forming a 5’-deoxyadenosyl radical and reducing Co (III) to Co (II).

In the second step of the following mechanism, the 5’-deoxyadenosyl radical removes the hydrogen atom (in this case, from the C-1 carbon) that will change place with another group, thereby becoming
5’-deoxyadenosine.

The radical at C-2 removes a hydrogen atom from 5’-deoxyadenosine, forming the rearranged product and regenerating the 5’-deoxyadenosyl radical.

The 5’-deoxyadenosyl radical recombines with Co (II) to regenerate the coenzyme and we get the complex succinyl-CoA as shown in the figure given below: