Fischer Carbene vs. Schrock Carbene

Attribute	Fischer Carbene	Schrock Carbene
Structure	Octahedral	Tetrahedral
Preparation	From metal carbonyls	From metal alkylidene complexes
Stability	Less stable	More stable
Reactivity	Less reactive	More reactive
Applications	Organic synthesis	Olefin metathesis

Introduction

Carbenes are highly reactive species that contain a divalent carbon atom with two unshared electrons. They play a crucial role in various organic reactions and catalysis. Fischer carbene and Schrock carbene are two important types of carbenes that differ in their synthesis, reactivity, and applications. In this article, we will explore the attributes of Fischer carbene and Schrock carbene, highlighting their similarities and differences.

Synthesis

Fischer carbene, named after Ernst Otto Fischer, is typically synthesized by the reaction of metal carbonyls with diazo compounds. The metal carbonyl acts as a nucleophile, attacking the diazo compound to form a metal carbene intermediate. This intermediate can then undergo various reactions, such as insertion into C-H bonds or cyclopropanation. Fischer carbene complexes are often stabilized by coordination to a metal center, which enhances their stability and reactivity.

On the other hand, Schrock carbene, named after Richard R. Schrock, is usually prepared by the reaction of a metal alkylidene complex with an alkyl halide or alkyl sulfonate. The metal alkylidene complex acts as a nucleophile, attacking the alkyl halide or alkyl sulfonate to form a metal carbene intermediate. Schrock carbenes are typically highly reactive due to the presence of a metal-carbon double bond, which can undergo various transformations, such as olefin metathesis or cyclopropanation.

Reactivity

Fischer carbenes are known for their ability to undergo C-H bond activation, allowing them to insert into C-H bonds of various organic compounds. This reactivity makes Fischer carbenes valuable in synthetic chemistry, as they can be used to functionalize organic molecules selectively. Additionally, Fischer carbenes can participate in cyclopropanation reactions, where the carbene inserts into a carbon-carbon double bond, forming a cyclopropane ring. This reaction is widely used in the synthesis of complex organic molecules.

Similarly, Schrock carbenes exhibit high reactivity due to the presence of a metal-carbon double bond. One of the most notable reactions of Schrock carbenes is olefin metathesis, where the carbene reacts with an alkene to form a new alkene with rearranged substituents. This reaction has revolutionized the synthesis of complex organic molecules and has found applications in various fields, including pharmaceuticals and materials science. Schrock carbenes can also participate in cyclopropanation reactions, similar to Fischer carbenes, allowing for the formation of cyclopropane rings.

Applications

Fischer carbenes find applications in various areas of organic synthesis. Their ability to selectively activate C-H bonds makes them valuable tools for functionalizing complex organic molecules. Fischer carbenes have been used in the synthesis of natural products, pharmaceuticals, and agrochemicals. Additionally, Fischer carbenes have been employed as catalysts in various reactions, such as cyclopropanation and olefin metathesis, further expanding their synthetic utility.

Similarly, Schrock carbenes have found widespread applications in organic synthesis. The ability of Schrock carbenes to perform olefin metathesis reactions has led to the development of efficient and selective methods for the construction of complex organic molecules. This has had a significant impact on the synthesis of pharmaceuticals, polymers, and fine chemicals. Schrock carbenes have also been utilized as catalysts in various other transformations, including cyclopropanation and alkene functionalization.

Conclusion

In conclusion, Fischer carbene and Schrock carbene are two important types of carbenes that differ in their synthesis, reactivity, and applications. Fischer carbene is typically synthesized from metal carbonyls and diazo compounds, while Schrock carbene is prepared from metal alkylidene complexes and alkyl halides or alkyl sulfonates. Both carbenes exhibit high reactivity and can participate in various transformations, such as C-H activation, cyclopropanation, and olefin metathesis. Fischer carbene finds applications in selective functionalization of organic molecules, while Schrock carbene has revolutionized the synthesis of complex organic compounds through olefin metathesis. Understanding the attributes of these carbenes allows chemists to utilize them effectively in organic synthesis and catalysis.