6 The cyclopropanation reaction of an alkene with a carbene takes place in a single step. There is NO intermediate. As such, the geometry of the alkene is preserved in the product. Groups that are trans on the alkene will end up trans on the cyclopropane product. Groups that are cis on the alkene will end up cis on the cyclopropane product. H H R R cis-alkene CH 2 I 2, Zn (Cu) ether H H R R cis -cyclopropane H R R H trans-alkene CH 2 I 2, Zn (Cu) ether H R R H trans-cyclopropane Hydrogenation: Addition of H 2 across the ?- …

2 Hydration: addition of water (H-OH) across a double bond to give an alcohol Dehydration: Loss of water (H-OH) from an alcohol to give an alkene + H 2 O OH H H H H H H+ OH H H H H H + H 2 O H+ Addition of Halogens (X 2 ) to Alkenes: 1,2-dihalides CC CC X X 1,2-dihalide alkene X 2 1,2-dibromide has the anti stereochemistry Bromonium ion intermediate controls the stereochemistry + Br 2 Br Br + Br Br not observed
3 Halohydrin Formation CC CC OH X halohydrin alkene “X-OH” X OH anti stereochemistry Br 2 , H 2 O + HBr Organic molecules are sparingly soluble in water as solvent. The reaction is often done in a mix of organic solvent and water using N-bromosuccinimide (NBS) as the electrophilic bromine source. DMSO, H 2 O N O O Br + Br OH N O O H + Note that the aryl ring does not react!!! For unsymmterical alkenes, halohydrin formation is Markovnikov-like in that the orientation of the addition of X-OH can be predicted by considering carbocation stability more ? + charge on the more substituted carbon Br adds to the double bond first (formation of bromonium ion) and is on the least substituted end of the double bond H 2 O adds in the second step and adds to the carbon that has the most ? + charge and ends up on the more substituted end of the double bond CH 3 Br ? + ?+ ?+ CH 3 Br 2 , H 2 O CH 3 HO Br H
4 Hydration of Alkenes: addition of water (H-OH) across the ?-bond of an alkene to give an alcohol. 1. Acid catalyzed hydration- Markovnikov addition of H-OH Not a good method for hydration of an alkene 2. Oxymercuration- Markovnikov addition H-OH 3. Hydroboration- Anti-Markovnikov addition of H-OH, Syn addition of H-OH CH 3 1) Hg (OAc) 2 , H 2 O 2) NaBH 4 CH 3 HO H H CH 3 1) B 2 H 6 , THF 2) H 2 O 2 , NaOH, H 2 O CH 3 H H HO B H B H H H H H B 2 H 6 (diborane) O tetrahydrofuran (THF) 2 O H 3 B + _ borane-THF complex
5 Reaction of Alkenes with Carbenes to give Cyclopropanes Carbene: highly reactive, 6-electron species. ( sp2 -hybridized) Generation and Reaction of Carbenes: CHCl 3 + KOH Cl 2 C: + H 2 O + KCl dichlorocarbene CH 2 I 2 + Zn(Cu) ether I-CH 2 -Zn-I = “H 2 C:” Simmons-Smith Reaction (cyclopropanation) methylene carbene CHCl 3 , KOH Cl Cl H H CH 2 I 2 , Zn (Cu) H H ether

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