[1,5] Hydrogen Shift

Definition

The [1,5] hydrogen shift is a type of sigmatropic rearrangement, which involves the migration of a hydrogen atom from one position to another within a molecule. This process is characterized by the movement of the hydrogen atom from the 1-position to the 5-position, or vice versa, in the molecular framework.

5 Must Know Facts For Your Next Test

1. The [1,5] hydrogen shift is a thermally allowed, antarafacial sigmatropic rearrangement that occurs through a cyclic, concerted transition state.
2. This rearrangement is often observed in the context of the Claisen rearrangement, where the migration of a hydrogen atom is a key step in the overall transformation.
3. The [1,5] hydrogen shift can be driven by heat or light, and the activation energy required for the rearrangement is typically lower than that of other sigmatropic shifts.
4. The stereochemistry of the starting material is often retained in the product, as the hydrogen migration occurs in a concerted, suprafacial manner.
5. The [1,5] hydrogen shift is a useful synthetic tool, as it allows for the formation of new carbon-carbon bonds and the introduction of functional groups in organic molecules.

Review Questions

• Explain the mechanism of the [1,5] hydrogen shift and how it is classified as a sigmatropic rearrangement.
  • The [1,5] hydrogen shift is a sigmatropic rearrangement that involves the migration of a hydrogen atom from the 1-position to the 5-position (or vice versa) within a molecule. This process occurs through a cyclic, concerted transition state in an antarafacial manner, meaning the hydrogen atom moves from one side of the $\pi$-system to the other. The [1,5] hydrogen shift is considered a thermally allowed sigmatropic rearrangement, as it satisfies the Woodward-Hoffmann rules for pericyclic reactions. This rearrangement is often observed in the context of the Claisen rearrangement, where the migration of the hydrogen atom is a key step in the overall transformation.
• Discuss the factors that influence the feasibility and selectivity of the [1,5] hydrogen shift in organic reactions.
  • The feasibility and selectivity of the [1,5] hydrogen shift are influenced by several factors. The activation energy required for the rearrangement is typically lower than that of other sigmatropic shifts, making the [1,5] hydrogen shift a kinetically favorable process. Additionally, the stereochemistry of the starting material is often retained in the product, as the hydrogen migration occurs in a concerted, suprafacial manner. Other factors that can affect the [1,5] hydrogen shift include the presence of substituents, the ring size, and the electronic properties of the molecule. The ability to control the selectivity of the rearrangement is crucial in organic synthesis, as the [1,5] hydrogen shift can be used to introduce new functional groups and form new carbon-carbon bonds in a predictable manner.
• Evaluate the synthetic utility of the [1,5] hydrogen shift and discuss how it can be applied in the context of complex molecule synthesis.
  • The [1,5] hydrogen shift is a valuable synthetic tool in organic chemistry, as it allows for the formation of new carbon-carbon bonds and the introduction of functional groups in a controlled manner. The concerted, antarafacial nature of the rearrangement, along with the relatively low activation energy required, makes the [1,5] hydrogen shift a reliable and predictable transformation. In the context of complex molecule synthesis, the [1,5] hydrogen shift can be employed as a key step in the construction of larger, more intricate structures. By strategically positioning the hydrogen atom and controlling the reaction conditions, organic chemists can leverage the [1,5] hydrogen shift to access diverse molecular scaffolds and facilitate the synthesis of complex natural products and pharmaceutically relevant compounds. The versatility and predictability of this sigmatropic rearrangement make it a powerful tool in the arsenal of synthetic organic chemistry.

Related terms

Sigmatropic Rearrangement:

A sigmatropic rearrangement is a pericyclic reaction in which a $\sigma$-bond migrates to a new position within the same molecule.

Pericyclic Reaction: A pericyclic reaction is a type of organic reaction that involves the cyclic movement of $\pi$-electrons or $\sigma$-bonds in a concerted manner.

Cope Rearrangement:

The Cope rearrangement is a sigmatropic rearrangement in which a [3,3] shift of a $\sigma$-bond occurs, typically involving the migration of a carbon-based substituent.