8.9 Addition of Carbenes to Alkenes: Cyclopropane Synthesis
2 min read•may 7, 2024
Carbenes are highly reactive molecules with a divalent carbon atom. They react with alkenes to form rings in a single, stereospecific step. This addition reaction is crucial for creating three-membered carbon rings in organic synthesis.
Carbenes can be generated through various methods, including decomposition of and α-elimination of . The offers a milder alternative using a zinc-copper , allowing for more controlled and selective cyclopropane formation in complex molecules.
Carbene Addition to Alkenes
Carbene reactions with alkenes
Top images from around the web for Carbene reactions with alkenes
organic chemistry - Why is the addition of a singlet carbene to an alkene stereospecific ... View original
Is this image relevant?
Organic chemistry 19: Alkenes - epoxidation, dihydroxylation, cyclopropanation View original
Is this image relevant?
organic chemistry - Why is the addition of a singlet carbene to an alkene stereospecific ... View original
Is this image relevant?
Organic chemistry 19: Alkenes - epoxidation, dihydroxylation, cyclopropanation View original
Is this image relevant?
1 of 2
Top images from around the web for Carbene reactions with alkenes
organic chemistry - Why is the addition of a singlet carbene to an alkene stereospecific ... View original
Is this image relevant?
Organic chemistry 19: Alkenes - epoxidation, dihydroxylation, cyclopropanation View original
Is this image relevant?
organic chemistry - Why is the addition of a singlet carbene to an alkene stereospecific ... View original
Is this image relevant?
Organic chemistry 19: Alkenes - epoxidation, dihydroxylation, cyclopropanation View original
Is this image relevant?
1 of 2
- Carbenes are highly reactive, neutral species containing a divalent carbon atom with two nonbonding electrons
- Can exist in either a singlet or triplet state depending on the spin of the nonbonding electrons (singlet or triplet carbenes)
- React with alkenes through a concerted, reaction forming two new bonds simultaneously in a single step without intermediates
- Results in the formation of a which are three-membered rings containing only carbon atoms
- Reaction is stereospecific retaining the of the alkene in the cyclopropane product (cis alkenes yield cis cyclopropanes, trans alkenes yield trans cyclopropanes)
Methods of carbene generation
- Decomposition of diazo compounds containing an group that can lose upon heating or photolysis
- can generate
- -Elimination of haloforms which are or
- Treatment with a strong base or sodium methoxide leads to the formation of a
- Chloroform treated with generates
- Thermal decomposition of causes the loss of and yielding dichlorocarbene
Simmons-Smith reaction vs other syntheses
- Uses a carbenoid which is a species that reacts similarly to a but has a different electronic structure
- Carbenoid is an formed from and a
- Highly stereospecific and preferentially reacting with alkenes over other functional groups
- Allows for selective of alkenes in the presence of other reactive moieties
- Occurs through a concerted, stereospecific addition similar to carbene addition
- Advantages compared to other cyclopropane synthesis methods:
- Does not require the use of explosive or highly reactive reagents (diazo compounds or free carbenes)
- Organozinc carbenoid is less reactive than a free carbene allowing for better control
- Relatively mild reaction conditions tolerant of various functional groups
Carbene Addition Mechanism and Properties
- Carbene addition to alkenes is a type of reaction
- The stereochemistry of the alkene is preserved in the product due to the concerted nature of the reaction
- The of the carbene (singlet or triplet) affects its reactivity and selectivity
- Chemoselective nature allows for targeted cyclopropane formation in complex molecules
Key Terms to Review (30)
Anti stereochemistry: Anti stereochemistry describes the spatial arrangement in a chemical reaction where two substituents are positioned on opposite sides of a double bond or ring structure after the reaction. It is particularly relevant in the halogenation of alkenes, resulting in products where the added atoms are located across from each other.
Carbene: A carbene is a neutral, divalent carbon species with six valence electrons and two covalent bonds. Carbenes are highly reactive intermediates that play a crucial role in the addition of carbenes to alkenes, leading to the synthesis of cyclopropanes.
Carbene, R2C: A carbene is a highly reactive molecule consisting of a carbon atom with two unshared electrons bonded to two groups (R), where R can be hydrogen, alkyl, or other substituents. In organic chemistry, carbenes are known for their role in inserting into C-H bonds and adding across double bonds, such as in the synthesis of cyclopropanes from alkenes.
Carbenoid: A carbenoid is a reactive intermediate in organic chemistry that resembles a carbene, but is typically more stable and can be generated through various methods. Carbenoids are crucial in the context of addition reactions to alkenes, particularly in the synthesis of cyclopropane rings.
Chemoselective: Chemoselective refers to a chemical reaction that selectively occurs at one functional group or reactive site in the presence of other potentially reactive groups. It is a crucial concept in organic chemistry, particularly in the context of addition reactions to alkenes and the synthesis of cyclopropanes.
Concerted Reaction: A concerted reaction is a chemical transformation that occurs in a single, continuous step without the formation of discrete intermediates. In a concerted reaction, the bond-breaking and bond-forming processes happen simultaneously, leading to the direct conversion of reactants into products.
Cycloaddition: Cycloaddition is a fundamental organic chemistry reaction in which two or more unsaturated molecules, or parts of the same molecule, combine to form a cyclic adduct. This process is a powerful tool for the synthesis of a wide range of carbocyclic and heterocyclic compounds, and it is particularly important in the context of alkene oxidation, carbene addition, the Diels-Alder reaction, and various thermal electrocyclic and cycloaddition reactions.
Cyclopropanation: Cyclopropanation is a chemical reaction in organic chemistry where a cyclic three-membered ring, known as a cyclopropane, is formed by the addition of a carbene species to an alkene. This reaction is a key method for the synthesis of cyclopropane-containing compounds, which have unique structural and reactivity properties.
Cyclopropane: Cyclopropane is a three-membered cyclic alkane with the chemical formula C3H6. It is a highly strained hydrocarbon that exhibits unique chemical and physical properties, which are central to its role in various organic chemistry topics.
Cyclopropane Ring: A cyclopropane ring is a three-membered carbocyclic ring structure consisting of three carbon atoms and six hydrogen atoms. This unique ring system has significant implications in the context of addition reactions involving carbenes and alkenes.
Diazo Compounds: Diazo compounds are a class of organic compounds that contain a diazo group (-N=N-) attached to a carbon atom. They are versatile intermediates in organic synthesis, particularly in the context of addition reactions to alkenes and the formation of cyclopropanes.
Diazomethane: Diazomethane is a highly reactive organic compound with the chemical formula CH2N2. It is a key reagent used in organic synthesis, particularly in the addition of carbenes to alkenes and the preparation of ethers.
Dichlorocarbene: Dichlorocarbene is a highly reactive intermediate species containing a carbon atom with two chlorine substituents and a lone pair of electrons. It is a key reactive species involved in the addition of carbenes to alkenes, leading to the synthesis of cyclopropane rings.
Diels–Alder cycloaddition reaction: The Diels–Alder cycloaddition reaction is a chemical process in organic chemistry where a conjugated diene reacts with a substituted alkene (dienophile) to form a six-membered ring. This reaction occurs through a single, concerted step without the formation of intermediates.
Dihalocarbene: Dihalocarbene is a highly reactive organic compound containing a carbon atom bonded to two halogen atoms, typically chlorine or bromine. It is a key intermediate in various organic reactions, particularly the addition of carbenes to alkenes to form cyclopropanes.
Diiodomethane: Diiodomethane, also known as methylene iodide, is a colorless, dense, and highly reactive organic compound with the chemical formula CH2I2. It is commonly used as a reagent in organic chemistry, particularly in the context of addition reactions involving carbenes.
Electron configuration: Electron configuration describes the distribution of electrons in an atom's atomic orbitals. It follows a set of rules, including the Pauli exclusion principle and Hund's rule, to show how these electrons are arranged around the nucleus.
Electron Configuration: Electron configuration refers to the arrangement of electrons in an atom's orbitals, which determines the atom's chemical properties and behavior. This concept is central to understanding the structure and behavior of atoms, as well as the formation of chemical bonds and the properties of molecules.
Haloforms: Haloforms are a class of organic compounds that contain a single carbon atom bonded to three halogen atoms (usually chlorine or bromine) and one hydrogen atom. These compounds are named for their characteristic structure, which resembles a tetrahedron with the halogen atoms occupying three of the vertices and the hydrogen atom at the fourth vertex.
Methylene Carbene: A methylene carbene is a highly reactive organic compound consisting of a carbon atom with two hydrogen atoms attached, and a lone pair of electrons. This species is an important intermediate in many organic reactions, particularly in the context of addition reactions to alkenes.
Organozinc Compound: Organozinc compounds are a class of organometallic compounds containing a carbon-zinc bond. They are versatile synthetic intermediates used in various organic transformations, including the addition of carbenes to alkenes to form cyclopropanes.
Simmons-Smith Reaction: The Simmons-Smith reaction is a method for the synthesis of cyclopropanes from alkenes using a carbenoid reagent. It involves the addition of a carbene species, generated in situ, to an alkene to form a three-membered ring structure known as a cyclopropane.
Simmons–Smith reaction: The Simmons–Smith reaction is a chemical process where a methylene (CH2) group is introduced into an alkene, converting it to a cyclopropane. This transformation involves the use of a zinc-copper couple and diiodomethane as reactants.
Singlet Carbene: A singlet carbene is a neutral, divalent carbon species with two nonbonding electrons occupying the same orbital, resulting in a closed-shell configuration. This reactive intermediate is a key player in the addition of carbenes to alkenes, leading to the synthesis of cyclopropanes.
Sodium Trichloroacetate: Sodium trichloroacetate is a chemical compound that is commonly used in organic chemistry reactions, particularly in the context of the addition of carbenes to alkenes to synthesize cyclopropanes.
Stereochemistry: Stereochemistry is the study of the three-dimensional arrangement of atoms in molecules and how this arrangement affects the chemical and physical properties of the substance. It examines the spatial orientation of atoms and their relationship to one another, which is crucial in understanding many organic chemistry concepts.
Stereospecific Addition: Stereospecific addition is a type of chemical reaction where the stereochemistry of the reactants is retained in the products. This is an important concept in the context of addition reactions, particularly the addition of carbenes to alkenes to form cyclopropanes.
Trihalomethanes: Trihalomethanes are a group of organic compounds that contain three halogen atoms (typically chlorine or bromine) bonded to a single carbon atom. They are commonly formed as byproducts during the disinfection of water supplies, particularly through the use of chlorine-based disinfectants.
Triplet Carbene: A triplet carbene is a neutral, divalent carbon species with two unpaired electrons, resulting in a triplet spin state. These reactive intermediates are important in organic chemistry, particularly in the context of addition reactions to alkenes to form cyclopropanes.
Zinc-Copper Couple: The zinc-copper couple is a reducing agent used in organic chemistry, particularly in the context of addition reactions involving carbenes and alkenes to synthesize cyclopropanes. It serves as a source of the carbene species, which can then undergo addition to alkenes, leading to the formation of cyclopropane rings.