Alkene Metathesis: Videos & Practice Problems

Alkene metathesis, also known as olefin metathesis, is a chemical reaction where two alkenes exchange their carbon atoms at the double bond, resulting in a mixture of E and Z isomers. This process is facilitated by a catalyst known as Grubbs Catalyst, which contains a central ruthenium transition metal bonded to two chlorines and two ligands, with a double bond to a CH group connected to a phenyl group.

In alkene metathesis, the orientation of the alkenes plays a crucial role. When the alkenes are aligned parallel, the double bonds are effectively "cut," allowing for the formation of new double bonds between the carbon atoms of the two alkenes. This results in various possible alkene products. The E and Z isomers arise from the arrangement of the R groups: if the R groups are on the same side, the product is a Z isomer; if they are on opposite sides, it is an E isomer. Conversely, when the alkenes are oriented oppositely, the resulting alkenes will typically have only one R group each.

Alkene metathesis is characterized as an equilibrium process, with optimal yields achieved when terminal alkenes are utilized. The reaction is driven to completion by the release of ethane gas, which is highly volatile and escapes from the solution, effectively pushing the reaction towards the formation of the desired products.

In the context of olefin metathesis reactions, understanding the orientation of alkenes is crucial for predicting the products formed. When two alkenes react, they can align in different configurations, leading to distinct products known as E and Z alkenes. The E configuration occurs when the highest priority substituents on each carbon of the double bond are on opposite sides, while the Z configuration arises when these substituents are on the same side.

To visualize this, consider two identical alkenes. If they align to form an E alkene, the substituents (R groups) will be positioned across from each other. Conversely, if they align to form a Z alkene, the substituents will be on the same side. For example, if one carbon of the double bond is connected to two methyl groups and the other carbon is connected to different substituents, the orientation will determine whether the product is E or Z.

In summary, the two possible products from this olefin metathesis reaction are the E alkene, where substituents are opposite, and the Z alkene, where they are on the same side. Recognizing these configurations is essential for predicting the outcomes of reactions involving alkenes.

Ring closing metathesis (RCM) is a significant synthetic method used to prepare cycloalkenes, particularly large rings that are challenging to synthesize through other means. This process involves intramolecular alkylmetathesis, where two alkenes within the same molecule react to form a cyclic structure.

For instance, consider a reaction setup with a concentration of 0.5 millimolar of methylene chloride, utilizing Grubbs catalyst at a temperature of 40 degrees Celsius over a duration of 20 hours. This extended reaction time allows for effective ring closure. In this scenario, two alkenes are present, and the reaction can be visualized as forming double bonds between specific carbon atoms. The key focus is on the formation of a large ring by ensuring that the appropriate carbon atoms react to create the desired cyclic product.

To achieve a considerable yield of the target product, the reaction is typically conducted in very dilute solutions. This dilution is crucial as it minimizes the likelihood of intermolecular metathesis, which could lead to undesired side products. By carefully controlling the reaction conditions, chemists can optimize the formation of large rings, resulting in an adequate yield of the desired cycloalkene.

In the context of ring closure metathesis reactions, we can explore the transformation of dodeca-1,1-diene using Grubbs catalyst. This reaction involves the formation of cyclic compounds through the manipulation of double bonds. To visualize the process, we start by positioning the two double bonds of the diene close to each other. For instance, we can represent one double bond oriented in one direction and the other in the opposite direction, while counting the remaining carbon chain, which consists of eight additional carbons.

When numbering the carbon atoms, we can label them sequentially from 1 to 8. The goal is to align these double bonds in a manner that facilitates the formation of six-membered rings. By "cutting" through the double bonds, we allow the two carbons adjacent to each double bond to form new double bonds, resulting in the creation of a cyclic structure. This process yields one of the products of the reaction, along with a non-substituted alkene as a side product.

Alternatively, if we aim to produce an E-type isomer, we can adjust the orientation of the double bonds. In this arrangement, one double bond can be positioned in one direction while the other is oriented in the opposite direction, still maintaining the count of eight carbons in the chain. This configuration will also lead to the formation of a six-membered ring, but with the double bonds arranged to reflect the E-isomer characteristics, where the substituents around the double bond are oriented trans to each other.

In summary, the ring closure metathesis of dodeca-1,1-diene can yield two distinct cyclic products based on the orientation of the double bonds, showcasing the versatility of this reaction in organic synthesis.

Alkylation through alkematathesis occurs in two distinct phases, each critical for understanding the overall mechanism. In the first phase, the Grubbs catalyst interacts with an alkene, leading to the formation of two key intermediates. This initial step is essential as it sets the stage for the subsequent reactions. Once the intermediates are established, the process transitions into phase two, where these intermediates react with another alkene to produce the final products.

Understanding this two-phase mechanism is crucial for grasping how alkematathesis operates at a molecular level. The Grubbs catalyst plays a pivotal role in facilitating these reactions, making it a significant component in the study of olefin metathesis. By focusing on the transformation of alkenes through these phases, one can appreciate the intricacies of this chemical process.

In the study of organic chemistry, particularly in the context of phase 1 mechanisms, understanding the terms 2+2 cycloaddition and cycloreversion is essential. The 2+2 cycloaddition refers to a synthetic method for constructing a cyclobutane ring, while cycloreversion is the process of breaking down that ring back into its constituent parts.

Phase 1 consists of three key steps. In the first step (1a), an alkene interacts with the Grubbs catalyst, which is a type of transition metal catalyst. The alkene, specifically a mono-substituted alkene, replaces one of the ligands (L) in the Grubbs catalyst. This interaction involves the double bond of the alkene connecting with the catalyst, facilitating the formation of a new bond.

Moving to step 1b, the catalyst undergoes a 2+2 cycloaddition, resulting in the formation of a metallocyclobutane. This structure includes the ruthenium transition metal within the cyclobutane ring. For simplification, the ligands and chlorines associated with the catalyst are often omitted in discussions.

In step 1c, the metallocyclobutane undergoes cycloreversion, leading to the generation of two intermediates. During this process, specific bonds within the cyclobutane break, resulting in the formation of alkenes. The focus is primarily on the alkenes that retain the ruthenium, as these will be crucial for the subsequent phase of the reaction.

Overall, the reaction can be summarized as the cleavage of the alkene bond, with each of the resulting double-bonded carbons being coordinated to a ruthenium atom. This mechanistic overview highlights the transformation of the alkene into a structure that is ready for further reactions in phase 2.

In the context of organometallic chemistry, when an alkene reacts with Grubbs catalyst, the primary focus is on the formation of organometallic intermediates rather than the detailed reaction mechanism. In this specific reaction, the alkene undergoes a transformation where the double bond between the two carbon atoms is cleaved. This results in each of the double-bonded carbons forming a bond with the ruthenium (Ru) present in the Grubbs catalyst.

To illustrate the organometallic intermediates, one can represent the two carbon atoms of the alkene as being connected to ruthenium. This can be depicted as follows:

Let’s denote the two carbon atoms as C₁ and C₂. After the reaction, the structure can be represented as:

C₁-Ru and C₂-Ru

Thus, the organometallic intermediates formed are simply the two ruthenium complexes, each bonded to one of the former double-bonded carbon atoms. This representation succinctly captures the essence of the intermediates without delving into the complexities of the reaction mechanism.

In the phase 2 mechanism of organic reactions, intermediates undergo a systematic cycle of addition and revision, specifically through a reaction cycle with alkenes. Each cycle produces one alkene product and another intermediate. The process begins with the reaction of an alkene, which can interact with another alkene to form a cyclobutane product via a [2+2] cycloaddition reaction. This reaction results in the formation of a four-membered ring structure.

During this mechanism, specific bonds within the cyclobutane can break, leading to the generation of alkenes. The resulting alkenes can exist as either cis (Z) or trans (E) isomers, depending on the orientation of substituents around the double bond. For instance, if the substituents (R groups) are on the same side, the product is a Z isomer; if they are on opposite sides, it is an E isomer.

As the reaction progresses, the breaking of bonds in the cyclobutane allows for the formation of various alkene structures. The mechanism illustrates how the arrangement of substituents and the breaking of specific bonds can lead to different products, emphasizing the importance of stereochemistry in organic synthesis. The presence of catalysts, such as ruthenium, can facilitate these transformations, enhancing the efficiency of the reaction.

Overall, understanding the phase 2 mechanism provides insight into the complexities of organic reactions, particularly in the formation of cyclic structures and the generation of stereoisomers.

In the context of organic chemistry, the formation of a metacyclobutane intermediate is a crucial step in the metathesis reaction involving cycloalkenes. To visualize this process, consider two pi bonds from the reactants that participate in a 2+2 cycloaddition reaction. In this scenario, the pi bond from one reactant breaks and forms a new bond with a carbon atom from the other reactant, while simultaneously, the pi bond from the second reactant breaks to form a bond with another carbon atom.

Specifically, imagine a reaction where a ruthenium catalyst is present. The ruthenium facilitates the interaction between the two cycloalkenes. As the reaction progresses, one of the carbons from the ruthenium complex forms a bond with a carbon from the first cycloalkene, while another carbon from the second cycloalkene connects to the ruthenium. This results in the formation of a metacyclobutane intermediate, characterized by a four-membered ring structure.

The structure of the metacyclobutane intermediate can be depicted as follows: one carbon from the cyclohexane ring remains connected to the newly formed bonds, while the other carbons involved in the reaction maintain their connections to the respective cycloalkenes. This intermediate is essential for the subsequent steps in the metathesis reaction, ultimately leading to the formation of the final product.

Understanding this mechanism is vital for grasping the principles of cycloaddition reactions and the role of transition metal catalysts in facilitating these transformations.