10. Addition Reactions

(••••) In addition to radicals, anions, and cations, a fourth class of reactive intermediates is carbenes. A neutral species, the simplest carbene has a molecular formula of CH₂ .

(h) Of the reactions studied here in Chapter 8, cyclopropanation is most similar to which?

To determine the stereochemistry of curacin A by synthesis, it would have been necessary to prepare all stereoisomers of the C₁₇―C₂₀ cyclopropane fragment. How would the reaction in Figure 16.30 be modified to produce the other stereoisomers shown here? [Think about the concepts of stereoselectivity (Chapter 6) and stereospecificity (Chapter 8).]

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(•••) Nitrogen-containing heterocycles form particularly stable carbenes and are commonly used as ligands in organometallic chemistry. (a) Why is the carbene shown particularly stable? [It may be helpful to draw the molecular orbital picture.]

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The reaction of an alkene with diazomethane forms a cyclopropane ring. Propose a mechanism for the reaction. (Hint: It is a concerted reaction.)

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Show how you would accomplish each of the following synthetic conversions. 

a. trans-but-2-ene → trans-1,2-dimethylcyclopropane

Show how you would synthesize each compound using methylenecyclohexane as your starting material. <IMAGE>

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Propose mechanisms consistent with the following reactions.

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Limonene is one of the compounds that give lemons their tangy odor. Show the structures of the products expected when limonene reacts with an excess of each of these reagents. <IMAGE>

m. CHBr3 and 50% aq. NaOH

(••••) In addition to radicals, anions, and cations, a fourth class of reactive intermediates is carbenes. A neutral species, the simplest carbene has a molecular formula of CH₂ .

(f) Cyclopropanation of (E)- and (Z)-3-methylhex-3-ene gives two different products. Rationalize this outcome.

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