Solved Problems on Carbon-Carbon Bond Formation Between Carbon Nucleophiles and Carbon Electrophiles

PROBLEMS

8.1. Give the products of the following reactions. Indicate the new carbon–car-bon bond that has been formed and identify the carbon nu-cleophile and electrophile.

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8.2. Give reactions which would produce the indicated bond in the following compounds. Give the reacting partners and tell which is the electrophile and nucleophile. Also tell how you would generate any reactants which are not stable compounds.

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8.3. Show how you would prepare each of the following molecules from the indicated starting materials. Where more than one step is required, show each step clearly.

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8.4. Give the products of the following reactions. [The reagent Pd(0) is a generic term referring any one of a number of possible zerovalent palla-dium reagents. The reagent referred to a Grubbs catalyst is the ruthenium carbenoid species referred to in the chapter.]

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8.5. Show a method for constructing the following compounds from the indi-cated starting materials.

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8.6. It was observed that when urea 6A was treated with Grubbs catalyst under high dilution, only dimers and oligomers were produced. However, when urea 6B was reacted under the same conditions, cyclic urea 6C was pro-duced in 61% yield. Provide a rationalization for these results.

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The formation of medium rings requires that the two reacting double bonds be in proximity to one another and that dimerization is controlled by high dilution. Since 6A does not cyclize under high dilution, the rate of cyclization is slow com-pared to dimerization. Placement of a much larger cyclohexyl group on nitrogen causes cyclization to proceed much faster, assuming that the dimerization rate is similar for both, which is a fair assumption. The cyclohexyl group must cause a conformational change about the urea linkage that causes the two double bonds to be in close proximity, thus increasing the cyclization rate greatly.