enantiomers 2. Simmons-Smith Reaction: insertion of zinc into a dihaloalkane "I CH 2 Zn I" C CH 2 I 2 Zn

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1 eactions of Alkenes II eading: Wade chapter 8, sections Study Problems: 8-47, 8-48, 8-55, 8-66, 8-67, 8-70 Key oncepts and Skills: Propose logical mechanisms to explain the observed products of alkene addition reactions, including regiochemistry and stereochemistry Use clues provided by the products of reactions such as ozonolysis to determine the structure of an unknown alkene When more than one method is available for a chemical transformation, choose the better method and explain its advantages Lecture Topics: I. Additions to pi bonds: arbenes arbenes react with pi bonds to form cyclopropanes. There are three methods for generating carbenes: 1. heating of diazo compounds: Diazomethane produces methylene upon heating: N N 2 N N 2 heat N 2 methylene enantiomers 2. Simmons-Smith eaction: insertion of zinc into a dihaloalkane Zn 2 I 2 "I 2 Zn I" methylene 3. α-elimination reaction: 2 I 2 Zn 3-3 dibromomethylene Stereochemistry of cyclopropanation: cis groups in the alkene remain cis in the cyclopropane; trans groups in the alkene remain trans in the cyclopropane 3

2 II. Alkene alogenation Addition of X-X across the pi bond Pi bond acts as a nucleophile; a bromonium ion intermediate gets attacked by nucleophilic bromine; the resulting dibromide has trans stereochemistry: Backside displacement - omonium ion intermediate trans-1,2-dibromo cyclohexane The reaction is stereospecific: different stereoisomers of starting materical produce different steroisomers of products: meso S S alohydrin synthesis: Involves the interception of the bromonium ion by water (or alcohols), which are stronger nucleophiles than -. Again, anti (trans) stereochemistry is observed in the product; unsymmetrical alkenes give Markovnikov orientation in the products: trans bromohydrin enantiomers 3 l δ l δ chloronium ion The nucleophile attacks the more highly substituted (more electrophilic) carbon atom 3 l l 3

3 II. xidation of Alkenes Addition of oxygen Three flavors: 1. epoxidation 2. dihydroxylation 3. xidative cleavage 1. Epoxidation Epoxide (oxirane) formation takes place when an alkene is treated with an organic peracid. In addition to the epoxide product, a carboxylic acid by-product is formed. Epoxidation is a one-step process (concerted bond formation and bond breaking), and the reaction is therefore stereospecific: groups that are cis in the alkene are cis in the epoxide, and groups which are trans in the alkene are trans in the epoxide. meso 3 3 cis-2-butene 3 3 eagent of choice: l 3 MPBA 3 2 l enantiomeric trans-epoxides Acid-catalyzed opening of epoxides: synthesis of trans-1,2-diols Anti-diols are obtained by backside attack of water on activated epoxides formed by protonation of the epoxide oxygen. To favor formation of the trans 1,2-diol, peracetic acid is used in aqueous acid; to favor formation of just the epoxide, MPBA is used in a non-polar solvent.

4 3 3 trans diol available from reaction of alkenes with peracetic acid in water Anti orientation of hydroxyl groups (±)-trans-1,2-cyclopentanediol epoxides available from reaction of alkenes with MPBA in 2 l 2 MPBA l 4 enantiomer 2. Direct Syn ydroxylation of Alkenes Two methods exist for syn hydroxylation: i. smium tetroxide is used in catalytic quantities in the presence of an oxidant to oxidize alkenes. An intermediate osmate ester is formed; 2 2 hydrolyzes the osmate ester and oxidizes osmium back to s 4. Syn 1,2 diol products are obtained. cis-glycol s s 4 s syn diol product 2 2 s smate ester s 4 is expensive and toxic, but it gives excellent yields of the cis diol from alkenes. ii. Permanganate hydroxylation: treatment of alkenes with dilute, cold aqueous KMn 4 gives moderate yields of the cis-1,2-glycol product. KMn 4 is cheaper and less toxic than s 4

5 KMn 4-3 Mn Mn - Manganate ester 3 50% cis-glycol Mn 2 - serves to hydrolyze the intermediate Manganate ester 3. xidative leavage a. Treatment of alkenes with a warm solution of concentrated KMn 4 leads to an oxidative cleavage of alkenes. An alkene carbon with two alkyl groups is oxidized to a ketone ( 2 =) An alkene carbon with one alkyl group is oxidized to a carboxylic acid ((=)) An alkene carbon with no alkyl groups and two hydrogen atoms is oxidized to 2 and 2. warm, conc. KMn 4 - Mn Aldehydes are oxidized to acids in the presence of KMn 4 KMn 4 KMn b. zonolysis Treatment of alkenes with ozone ( 3 ) leads to oxidative cleavage, giving aldehydes and ketones. An intermediate ozonide is produced; dimethyl sulfide (DMS) is added in a second step to reduce the ozonide to aldehydes and ketones. Dimethyl sulfide prevents aldehydes from being further oxidized to carboxylic acids by ozone. To predict the products of an ozone cleavage, simply erase the double bond and put two carbonyls (=) in its place.

6 ' 3 ' ' - molozonide ozonide 3 S 3 Example: ' 3 S DMS 2 = zonolysis provides confirmation of the location of olefins within a molecule DMS DMS Additional Problems for practice: 3 ketone and aldehyde generated dialdehyde generated 1. Show the structures of alkenes that give the following products upon reaction with warm, concentrated KMn 4 (a) (b) ( 3 ) 2 = (c) ( 3 ) 2 =

7 2.) Show how you could prepare the following compounds from an alkene: (a) (b) (c) (d) (e) 3 3.) Draw reasonable mechanisms for the following reactions: 2, (a) enantiomer (b) 1. 2, 2 2. Na, 2 (c) 2, 2

8 a , Pt b. 1. g(ac) NaB 4, - 1. B 3 TF , c. heat e. 1. B 3 TF , - f. 1. B 3 TF , -

Double Bonds. Hydration Rxns. Hydrogenation Rxns. Halogenation. Formation of epoxides. Syn addition of 2 OH. Ozonolysis

Double Bonds. Hydration Rxns. Hydrogenation Rxns. Halogenation. Formation of epoxides. Syn addition of 2 OH. Ozonolysis Double Bonds What do we do with double bonds? We do addition reactions. In an addition reaction, something is added to both carbons involved in a double bond (or not involved in the double bond, in the