Metal Carbene complexes Flashcards
What are metal carbenes and carbynes?
Metal carbenes are complexes containing a divalent carbon atom with a lone pair of electrons directly bonded to a metal. Metal carbynes have a trivalent carbon atom bonded to a metal.
Who reported Fischer carbenes and in what year?
E. O. Fischer first reported Fischer carbenes in 1964.
What are the characteristic features of Fischer carbenes?
Fischer carbenes are characterized by a metal in a low formal oxidation state and the M=C bond is stabilized by a lone pair on one of the substituents on carbon, typically a strong electron-donating group like OR or NR2.
Describe the valence isomers of Fischer carbene and their stability factors.
Valence isomers of Fischer carbene are stabilized by a C=X π-bond where X is OR or NR2; this contributes a positive charge on the carbon and susceptibility to nucleophilic attack.
Outline the synthesis method of Fischer carbenes.
Fischer carbenes are typically synthesized through nucleophilic attack at CO, CS, or CNR ligands, like the reaction where an alkoxide attacks a chromium hexacarbonyl precursor to form a Fischer carbene complex.
Explain the bonding in Fischer carbenes, focusing on the energy levels.
In Fischer carbenes, the M(dp) levels are lower in energy and filled, leading to stable complexes.
What is the effect on chemistry due to the characteristics of Fischer carbenes?
The lower energy and filled M(dp) levels make the carbene carbon electrophilic, reacting readily with nucleophiles.
Give an example of a reaction involving Fischer carbene.
A common reaction involving Fischer carbene is the addition of nucleophiles to the carbene carbon, The carbene carbon attacks the electrophilic carbon of the aldehyde, leading to an addition product that can be further manipulated chemically.
When were Schrock carbenes discovered and by whom?
Schrock carbenes were discovered by Richard R. Schrock in 1983.
Describe the characteristic features of Schrock carbenes.
Schrock carbenes are characterized by a metal in a high formal oxidation state and are generally Lewis acidic, with empty M(dp) levels.
Outline the synthesis method for Schrock carbenes.
Schrock carbenes are synthesized through deprotonation strategies, often involving nucleophilic substitution at a metal complex.
Discuss the bonding differences between Schrock and Fischer carbenes.
Schrock carbenes have empty M(dp) levels at higher energy, making them nucleophilic, while Fischer carbenes have lower energy, filled M(dp) levels, making them electrophilic.
Explain the effect on chemistry due to Schrock carbenes’ characteristics.
The high energy and empty M(dp) levels make the carbene carbon of Schrock carbenes nucleophilic, which reacts with electrophiles.
Provide an example of a Schrock carbene reaction
A typical reaction involving Schrock carbenes is their use in alkene metathesis. For example, the reaction between a tungsten-based Schrock carbene and cyclooctene results in ring-opening metathesis, forming polyoctenamer.
Compare the bonding and reactivity between Fischer and Schrock carbenes.
Fischer carbenes exhibit lower energy, filled M(dp) levels making them electrophilic. Schrock carbenes have higher energy, empty M(dp) levels, making them nucleophilic.
What is olefin metathesis?
Olefin metathesis is a chemical reaction in which two alkenes exchange their alkylidene groups, catalysed by metal carbene complexes. This reaction is crucial in the synthesis of various polymers and chemicals.
Describe the role of the Grubbs catalyst in olefin metathesis, including the mechanism.
The Grubbs catalyst, a ruthenium-based metal carbene, facilitates olefin metathesis by forming a metallacyclobutane intermediate that reverts to new alkene products. For instance, the self-metathesis of propene can produce 2-butene and ethylene.
What are metal carbene catalysts and their common uses?
Metal carbene catalysts are used in various organic synthesis processes, including olefin metathesis, where they facilitate the rearrangement of alkenes.
What is the SHOP process and its significance?
The SHOP process involves the oligomerization of ethene to produce 1-alkenes ranging from C10 to C20 using a nickel-alkyl catalyst.
Describe the steps involved in the SHOP process to produce C12 – C16 alkenes.
Initially, ethene is oligomerized using a nickel catalyst to produce a mix of 1-alkenes. These alkenes are then isomerized to internal olefins, which undergo metathesis to selectively produce alkenes in the C12 to C16 range. This process is vital for producing specific alkenes used in detergents and other industrial applications.
Explain the mechanism of a Fischer carbene transformation with an aldehyde.
The carbene carbon of a Fischer carbene reacts with the carbonyl carbon of an aldehyde, forming a new C-C bond and resulting in an alcohol after a subsequent reduction step.
Describe a specific example of olefin metathesis using the Grubbs catalyst.
The reaction between 2-butene and 2-pentene using the Grubbs catalyst results in the formation of 3-hexene and 2-butene, demonstrating the ability of this catalyst to rearrange carbon-carbon double bonds.
