carbenes

What is a carbene?

A neutral, highly reactive, electron-deficient intermediate containing a divalent carbon with 6 electrons in its valence shell.

What hybridisation does the carbene carbon have?

sp² hybridised.

What are the two electronic states of a carbene?

Singlet and triplet.

What is the electronic structure of a singlet carbene?

The two non-bonding electrons are paired in one sp² orbital and the p orbital is empty.

What is the electronic structure of a triplet carbene?

The two non-bonding electrons are unpaired in separate orbitals: one in an sp² orbital and one in a p orbital.

Which carbene has an empty p orbital?

Singlet carbene.

Which carbene behaves like a diradical?

Triplet carbene because it has two unpaired electrons.

Why are carbenes highly reactive?

They are electron deficient and have an incomplete octet, so react readily with sources of electrons.

What effects can stabilise carbenes?

+I inductive effects, hyperconjugation, mesomeric stabilisation and metal coordination.

How does hyperconjugation stabilise a carbene?

Donation from a neighbouring filled σ orbital into an electron-deficient carbene orbital helps stabilise the carbene.

How does mesomeric stabilisation stabilise a carbene?

A neighbouring lone pair or π-system can interact with the carbene orbital, allowing electron density to be delocalised.

How can metal coordination stabilise a carbene?

Carbenes can bind to metals such as Rh or Cu to form metal-carbene or carbenoid species.

What are the two main methods of forming carbenes?

From diazocompounds and by α-elimination.

What happens when a diazocompound decomposes?

It loses N₂ to generate a carbene or metal-bound carbene species.

Why is loss of N₂ favourable?

N₂ is very stable, so its formation provides a strong driving force.

How can diazocompounds be prepared from ketones?

Ketone → hydrazone → diazocompound.

What reagent converts a ketone into a hydrazone?

Hydrazine, NH₂NH₂.

How is a hydrazone converted into a diazocompound?

Oxidation of the hydrazone forms the diazocompound.

What is α-elimination?

Two groups are removed from the same carbon to generate a carbene.

Are carbenes formed by α-elimination necessarily triplet?

No; the spin state is not determined solely by the method of formation.

What happens when a carbene reacts with an alkene?

Cyclopropanation occurs: the carbene carbon forms bonds to both alkene carbons to produce a three-membered ring.

Which carbene gives stereospecific cyclopropanation?

Singlet carbene.

Why is singlet-carbene cyclopropanation stereospecific?

It occurs by a concerted mechanism, so the stereochemistry of the alkene is retained.

What does a cis alkene give in stereospecific singlet-carbene cyclopropanation?

A cis-substituted cyclopropane.

What does a trans alkene give in stereospecific singlet-carbene cyclopropanation?

A trans-substituted cyclopropane.

Is triplet-carbene cyclopropanation stereospecific?

No.

Why is triplet-carbene cyclopropanation not stereospecific?

It occurs stepwise through a diradical intermediate that can rotate before spin inversion and radical recombination.

What products can triplet-carbene cyclopropanation give?

A mixture of stereoisomeric cyclopropanes can form.

How can singlet and triplet carbene reactivity be distinguished experimentally?

React the carbene with a stereochemically pure E- or Z-alkene and analyse the stereochemistry of the cyclopropane products.

What result indicates singlet-carbene reactivity?

Stereospecific cyclopropanation with retention of the starting alkene geometry.

What result indicates triplet-carbene reactivity?

Loss of stereospecificity and formation of different stereoisomeric cyclopropanes.

What is the Simmons–Smith reaction?

Cyclopropanation of an alkene through a carbenoid intermediate.

What is a carbenoid?

A species that displays carbene-like reactivity without being a free carbene.

What is the role of Rh₂(OAc)₄ in reactions of diazocompounds?

It promotes loss of N₂ and formation of a Rh-bound carbene species.

Does reaction of a diazocompound with Rh₂(OAc)₄ require formation of a free carbene?

No; the reactive species can be a metal-bound carbene.

Into which bonds can carbenes undergo insertion?

C–H, O–H and N–H bonds.

What happens during carbene C–H insertion?

The carbene inserts into the C–H bond, producing a new C–C bond.

Why are intramolecular carbene insertion reactions synthetically useful?

They provide powerful methods for C–X bond formation and ring formation.

What happens to the C=C bond during carbene cyclopropanation?

The π bond is replaced by two new C–C bonds to the carbene carbon, forming a cyclopropane.

What happens to substituents on the alkene during cyclopropanation?

They remain attached to their original alkene carbons.

Where do the three carbons of a cyclopropane come from during carbene cyclopropanation?

One carbon comes from the carbene and two come from the alkene.

What stereochemical information should be checked in a carbene cyclopropanation precursor?

The geometry of the starting alkene because singlet-carbene cyclopropanation is stereospecific.

What is the key rule for singlet versus triplet carbene reactions with alkenes?

Singlet = concerted and stereospecific; triplet = stepwise via a diradical and potentially non-stereospecific.