Carbenes and Cyclopropation

Cyclopropanation is the general name for a series of reactions that all produce very similar products by different mechanisms. You may be responsible for one or more of these reactions on your exam, so let’s get into them.

Concept: General properties of cyclopropanation.      

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Video Transcript

In this video, I'm going to introduce a bunch of new addition reactions that all pretty much do the same thing, so I'm going to group them together in a type of reaction called cyclopropanation.
So a cyclopropanation reaction takes place when a double bond encounters either a carbene or a carbenoid. The product from this reaction is really just a cyclopropane. So we literally get the addition of an alkyl group on that double bond and all you're really getting is a methylene group or a CH2.
So you might be wondering, “Johnny, why would a double bond want to react with a methyl group? I mean, I don't remember methyl groups being strong electrophiles.” Well, that's exactly why we need a carbene or a carbenoid because methyls aren't reactive, but carbenes are.
If you recall, carbenes are reactive intermediates not because of their formal charge. If you look, this is an example of a carbene right here. Does it have a formal charge? No, it has a formal charge of zero. So you might think, “This doesn't look reactive to me.” But remember it has a big problem. It violates the octet rule, so even though it doesn't have a formal charge, it wants to have eight octet electrons around it and right now it only has six. It has that lone pair, those two bonds. It's missing two whole electrons, so carbenes are going to be extremely reactive with pretty much everything, including double bonds, which is why they're going to work to make these triangle-shaped products.
What I'm going to do is I'm just going to go one-by-one down the list of all the reagents that can make a cyclopropanation. 

Concept: Reaction with chloroform (CHCl3) and tert-butoxide.

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Concept: Reaction with diazomethane and light or heat.

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You should not be responsible for the full mechanism of Simmons-Smith, but you should know what the reagents are, and be able to predict that it is a form of cyclopropanation. 

Carbenes and Cyclopropation Additional Practice Problems

Provide the reagent or starting material to perform the indicated transformation. More than one reaction may be required. You should number your steps if it is important for the synthetic transformation to take place.

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Supply the missing reagents for the following transformation. 

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Heating diazomethane generates nitrogen gas and a carbene. This question pertains to the structure and reactivity of the carbene species.

a) Draw a picture describing the geometry of the carbene that is generated, showing any orbitals that contain lone pairs or are empty.

 

b) On your picture from part a, indicate the types of orbitals that are present (s, p, sp 3, etc).

 

c) On your picture from part a, indicate in which orbital the lone pair resides.

 

d) On your picture from part a, identify any orbitals having nucleophilic character. Do the same for orbitals with electrophilic character.

 

 

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Propose a mechanism for the following reaction.

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Provide either the reagent(s) or the product(s) for the reaction below. This is not a mechanism question. Simply write the answer in the box.

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Consider the strucutres below and answer the following questions. 

f. Which disubstituted alkene forms enantiomeric pairs on treatment with CHCl / NaOH?

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Give the product, or products, including stereochemistry of the reaction of (Z)-3-methyl-2-pentene with the reagent below. If the products are a pair of enantiomers, you need to draw only one and state that the other enantiomer is formed. 

CHCl3, NaOH

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The creation of a cyclopropane ring from alkenes can be accomplished by two different methods. 

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The creation of a cyclopropane ring from alkenes can be accomplished by two different methods. 

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Synthesis: Provide the reagents for the following conversions.  Note that some transformations may require more than one chemical step. e.g. the acid step of a Grignard  reaction. For full credit, you must include and number these steps when necessary.

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Insert the missing curved-arrows for the following two reaction transformation.
Include all missing curved arrows.

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How many atoms and electrons are directly involved in the bond-making and bond-breaking of the reaction given below?  Choose the right answer. 

Addition of dichlorocarbene (CCl2) to propene

a) four atoms, two electrons

b) four atoms, three electrons

c) four atoms, four electrons

d) three atoms, two electrons

e) three atoms, three electrons

f) three atoms, four electrons

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Predict the product of the following reaction. Show stereochemistry when applicable. 

 

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Predict the products of the following reaction. Show stereochemistry when applicable. 

 

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Predict the principal organic product of each of the following reactions: 

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