Addition to alkenes accompanied by 1,2-alkyl shift
Description: When secondary (or primary) carbocations are formed adjacent to a quaternary carbon, 1,2-shifts of alkyl group may occur.
Notes: In addition to 1,2-shifts of simple alkyl groups, 1,2-shifts of ring carbons can lead to changes in ring size (e.g. ring expansion reactions)
Notes: The first two examples show migration of a methyl group. The third example shows preferential migration of a ring carbon, which releases ring strain from the cyclobutane. The fourth example shows that hydrogen migration is generally preferred over alkyl migration.
Mechanism: The first step of the reaction is formation of a carbocation. Here, the formation of a carbocation from addition of HBr to an alkene is shown (Step 1, arrows A and B). Since we have a secondary carbocation adjacent to a quaternary carbon, shift of an alkyl group to C2 results in a more stable tertiary carbocation. Breakage of the C3-C4 or C3-C6 bonds is preferred since both of these result in a release of ring strain to give a five-membered ring (Step 2, arrow C). Finally trapping of the carbocation with bromine ion results in the alkyl halide (Step 3, arrow D).
Notes: Note in this example that migration of a ring carbon is preferred over migration of the methyl group because the latter does not allow for a release of ring strain.
Note that breaking C3-C4 is equivalent to breaking C3-C6 (either could be shown here)