10. Addition Reactions

Drawing on what you know about the stereochemistry of alkene addition reactions,

a. write the mechanism for the reaction of 2-butyne with one equivalent of Br2.

Drawing on what you know about the stereochemistry of alkene addition reactions,

b. predict the configuration of the product of the reaction.

Show how hex-1-yne might be converted to

d. 1,1,2,2-tetrabromohexane.

Predict the products you would get when the following alkenes react under the following conditions: (i) H₂SO₄ , H₂O and (ii) 1. Hg(OAc)₂ , H₂O , 2. NaBH₄

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(••) For the alkynes shows here, show the product(s) expected to form when treated under the following conditions: (i) H₂ , Pd/C; (ii) H₂, Pd/C, Pb(OAc)₂ , CaCO₃ (Lindlar's catalyst); (iii) Na⁰ , NH₃ (liquid), ―33°C ; (iv) HBr (1 equiv.); (v) HCl (2 equiv.); (vi) H₂SO₄, HgSO₄, H₂O (vii) 1. BH₃ 2. H₂O₂, NaOH; (viii) Cl₂ (1 equiv.); and (ix) Br₂ (2 equiv.). If you expect two products, show both.

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(••••) In Section 10.8.1, you learned that alkenes react more quickly with electrophiles than do the corresponding alkynes ( ᵏalkene/ᵏalkyne > 1). Explain why there is a greater disparity in the alkene versus alkyne reactivity in the addition of HBr as compared to the addition of Br₂ [The rate data are not real, but are meant to illustrate a real trend.]

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(••••) α-Pinene is a terpene found in pine trees. Starting from geranyl diphosphate (a terpene), draw the arrow-pushing mechanism to produce α-pinene. Assume that an enzyme active site will provide whatever acids and bases you might need. [We did reactions similar to this in Chapter 8.]

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Predict the products you would get when the following alkenes react under the following conditions: (i) H₂SO₄ , H₂O and (ii) 1. Hg(OAc)₂ , H₂O , 2. NaBH₄

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(•) Predict the product(s) that would result when the alkenes shown here are allowed to react under the following conditions: (i) HBr; (ii) HCl; (iii) HBr, H₂O₂ (iv) H₂SO₄ , H₂O (v) 1. Hg(OAc)₂ , H₂O 2. NaBH₄ ; (vi) 1. BH₃ 2. H₂O₂, NaOH

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What stereoisomers are obtained when 2-butyne undergoes each of the following reaction sequences?

a. 1. H2/Lindlar catalyst 2. Br2/CH2Cl2

Show what reagents would be needed to synthesize the pheromone of the omnivorous leafroller (OLR) using olefin metathesis to assemble the molecule at the double bond.

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Propose a mechanism for the triolefin process using a metal alkylidene as the catalyst.

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Show what products you would expect from the following metathesis reactions, using the Schrock or Grubbs catalysts.

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(•••) IN THE CHEMICAL LITERATURE The synthesis of five-membered lactones (cyclic esters) has been accomplished using the electrophilic addition of I―Cl to an alkyne. Suggest a mechanism for this cyclization reaction. (Structure modification of Yao, T.; Larock, R.C. J. Org. Chem. 2005, 70, 1432–1437.)

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Draw the mechanism for the following reaction:

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What stereoisomers are obtained when 2-butyne undergoes each of the following reaction sequences?

c. 1. Cl2/CH2Cl2 2. Br2/CH2Cl2