Question

Describe rhodium catalyzed combined C-H activation and cope rearrangement reactions. Predict the major product for the following reactions.

Short answer

In rhodium catalyzed combined C-H activation and Cope rearrangement reactions, a rhodium catalyst interacts with a C-H bond, breaking it and forming a metal-carbon bond, which then undergoes a [3,3]-sigmatropic rearrangement (Cope Rearrangement) leading to the formation of a carbon-carbon double bond in the substrate. To predict the major product, identify the substrate structure and potential sites for C-H activation. Determine the site of C-H activation by locating electron-rich carbon centers linked with the metal catalyst Rh. Then, predict the formation of the Rhodium-substrate complex, followed by the Cope Rearrangement involving the relocation of a double bond and breaking/forming a new C-C bond. Finally, predict the major product after the Rhodium catalyst is released, having a new carbon-carbon double bond resulting from the rearrangement.

Step-by-step solution

Introduction to Rhodium Catalyzed C-H Activation

Rhodium catalyzed C-H activation is a type of organometallic reaction where a transition metal complex, typically rhodium, interacts with a C-H bond and breaks it, leading to the formation of a metal-carbon bond. This reaction can be considered as one of the strategies for molecule functionalization.

Introduction to Cope Rearrangement

The Cope Rearrangement is a pericyclic reaction that involves a [3,3]-sigmatropic rearrangement of a 1,5-diene. This rearrangement allows the formation of a new carbon-carbon double bond, and consequent functionalization of the molecule.

Combined C-H Activation and Cope Rearrangement

In the rhodium catalyzed combined C-H activation and Cope rearrangement reactions, the initial C-H activation dictated by the Rhodium catalyst enables the formation of a metal-alkyl bond where the metal complex and the substrate are linked. Once the complex is formed, it undergoes a Cope rearrangement, effectively leading to the formation of a new carbon-carbon double bond in the substrate. Now let's discuss how to predict the major product for the given reactions.

Identify the Reaction Components

In order to predict the major product, it is essential to identify the reaction components, such as the substrate and the Rh catalyst. Examine the molecules and recognize their structural pattern to determine the possible interaction sites for C-H activation.

Determine the Site of C-H Activation

After identifying the structure of the substrate, locate the potential sites for C-H activation. Look for electron-rich carbon centers or a C-H bond adjacent to a functional group that would likely undergo the reaction under the influence of the metal catalyst Rh.

Predict the Formation of the Rhodium-Substrate Complex

Based on the site identified for C-H activation, predict the formation of the Rhodium-substrate complex that links the Rhodium catalyst to the substrate via the metal-alkyl bond resulting from the C-H activation.

Predict the Cope Rearrangement

With the complex formation ascertained, determine the possible rearrangement due to the pericyclic transformation of the 1,5-diene system. This will involve relocating a double bond and breaking/forming a new C-C bond.

Predict the Major Product

Upon completing the Cope Rearrangement, predict the structure of the resulting product after the Rhodium catalyst is released from the substrate. This product should have a new carbon-carbon double bond resulting from the rearrangement. By following these steps and applying the concepts of rhodium catalyzed combined C-H activation and Cope rearrangement, you will be able to predict the major product for the given reactions.

Key concepts

C-H activation

C-H activation is a fascinating process in organometallic chemistry, providing a pathway for breaking C-H bonds in organic molecules with the help of transition metal complexes like rhodium. The ability of these complexes to selectively target and activate C-H bonds is highly valuable for organic synthesis.
Rhodium serves as an efficient catalyst due to its rich coordination chemistry and ability to stabilize various intermediates. In C-H activation, the rhodium complex forms a bond with the hydrogen atom of the C-H bond, consequently cleaving it.
This process involves the following key steps:

• Coordination: The rhodium metal interacts with the substrate, specifically targeting a suitable C-H bond.
• Activation: Breaking of the C-H bond and formation of a rhodium-hydrogen bond alongside a carbon-metal bond.

C-H activation is significant because it allows for the functionalization of carbon centers that are otherwise inert, opening doors for constructing intricate molecules.

Cope rearrangement

The Cope rearrangement is an intriguing pericyclic reaction used to transform 1,5-dienes. This [3,3]-sigmatropic rearrangement facilitates a reorganization of molecular structure, enabling the formation of new carbon-carbon bonds.
During the Cope rearrangement, all involved atoms move simultaneously in a concerted manner, switching positions without the formation of intermediates. This process relies on the cyclic transition state that the molecule forms, which is highly stable.
Key characteristics of the Cope rearrangement include:

• Rearrangement Mechanism: Involves a six-membered cyclic transition state, ensuring the systematic shift of double bonds and atoms.
• Molecular Impact: Results in the creation of a new double bond along with a significant redistribution of bonding electrons.

This rearrangement is especially useful in synthesizing and modifying complex organic molecules, enhancing structural diversity.

rhodium catalysis

Rhodium catalysis plays a crucial role in advancing C-H activation reactions and beyond. Rhodium, a transition metal, exhibits unique catalytic capabilities, making it invaluable in organometallic chemistry for a variety of transformations.
With its ability to form stable complexes with organic substrates, rhodium assists in facilitating and enhancing many chemical reactions. It offers selectivity, mild reaction conditions, and reduces the need for excessive reagents.
Important points regarding rhodium catalysis include:

• Nature of Catalyst: Rhodium exhibits a versatile oxidation state, enabling various catalytic cycles.
• Reaction Scope: Extends to hydrogenations, carbonylations, and of course, C-H activations.
• Environmental Aspect: Presents an efficient and often greener alternative to traditional methods.

Rhodium's ease of handling and effectiveness in a wide range of organometallic reactions make it a staple in both academia and industry.

pericyclic reactions

Pericyclic reactions offer a broad overview of organic transformations that occur in a concerted manner without intermediates. These reactions are characterized by the reorganization of bonding electrons through cyclic transition states, making them predictable and generally initiated by thermal or photochemical means.
These reactions can be further classified into types such as cycloadditions, sigmatropic rearrangements, and electrocyclic reactions. They significantly impact the synthesis of complex molecular architectures.
Key features of pericyclic reactions include:

• Concerted Mechanism: The entire reaction progresses in a single step with no intermediates.
• Energy Profile: Generally proceed through a transition state where all atoms move concurrently.
• Diverse Applications: Useful in forming rings, new functional groups, and complex organic structures.

By mastering pericyclic reactions, chemists can smartly design pathways for creating diverse chemical entities, aided by theoretical principles outlined in the Woodward-Hoffmann rules.