Di-π-methane rearrangement

What is di-π-methane rearrangement?

In 1966, Zimmerman investigated a rearrangement now known as the Zimmerman rearrangement, or di-π-methane rearrangement, di-π-methane photorearrangement, or di-π-methane reaction. This photochemical reaction involves the migration of one π-moiety bonded to a saturated carbon atom (such as a methylene group) to another π-moiety, resulting in the formation of a cyclopropane ring from the remaining π-moiety and methylene group.

There are currently six known variants of this reaction, including the divinylmethane, arylvinylmethane, oxa-di-π-methane, and aza-di-π-methane rearrangements, as well as two less common variants, the barrelene di-π-methane rearrangement and metalla di-π-methane rearrangement.

Di-π-methane rearrangement - general reaction scheme - divinyl type di-π-methane rearrangement - Zimmerman rearrangement - di-π-methane rearrangement - di-π-methane photorearrangement - di-π-methane reaction
Divinyl type di-π-methane rearrangement

The oxa-di-π-methane rearrangement involves the photochemical reaction of a β,γ-unsaturated ketone to an α-cyclopropyl ketone via a 1,2-acyl shift.

Di-π-methane rearrangement - general reaction scheme - oxa-di-π-methane rearrangement - Zimmerman rearrangement  - di-π-methane rearrangement - di-π-methane photorearrangement - di-π-methane reaction
Oxa-di-π-methane rearrangement
Di-π-methane rearrangement - general reaction scheme - Zimmerman rearrangement  - arylvinyl type di-π-methane rearrangement  - di-π-methane rearrangement - di-π-methane photorearrangement - di-π-methane reaction
Arylvinyl type di-π-methane rearrangement
Di-π-methane rearrangement - general reaction scheme - aza-di-π-methane rearrangement - Zimmerman rearrangement   - arylvinyl type di-π-methane rearrangement  - di-π-methane rearrangement - di-π-methane photorearrangement - di-π-methane reaction
Aza-di-π-methane rearrangement

Each variant of the reaction differs in terms of the involvement of singlets and triplets, with the divinylmethane and arylvinylmethane rearrangements favored from the triplet state for bicyclic molecules and the excited singlet for acyclic compounds, while the oxa-di-π-methane rearrangement occurs solely through triplets, except for a few exceptions. The divinylmethane rearrangement readily occurs when the sp3 carbon between the vinyls is substituted, but the arylvinylmethane rearrangement does not have such a requirement.

The di-π-methane rearrangement displays high regiospecificity, with a preference for the migration of the less conjugated π-moiety to the more conjugated one, and it occurs in two important stages involving a “bridge” step and a “break” step.

It is worth noting that the di-π-methane rearrangement can also be induced by plasma, and under certain conditions, it can be reversible.

The application of di-π-methane rearrangement enables the synthesis of substituted cyclopropanes.

References

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