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Organic Chemistry William H. Brown Christopher S. Foote Brent L. Iverson 6-1 Reactions of Alkenes Chapter 6 6-2 Characteristic Reactions Descriptive Name(s ) React ion C C + HCl ( HX) C C + H2 O C C + Br2 ( X2 ) C C + Br2 ( X2 ) H C C Cl (X) H C C OH (X) Br C C Br (X) H2 O HO C C Br (X) Hydrochlorination (hydrohalogenation) Hydration Bromination (halogenation) Bromo(halo)hydrin formation 6-3 Characteristic Reactions C C + Hg(OAc) 2 C C + BH3 C C + OsO4 C C + H2 H2 O HgOAc Oxymercuration C C HO C C H BH2 Hydroboration C C HO OH Diol formation (oxidation) C C H H Hydrogenation (reduction) 6-4 Reaction Mechanisms A reaction mechanism describes how a reaction occurs • which bonds are broken and which new ones are formed • the order and relative rates of the various bondbreaking and bond-forming steps • if in solution, the role of the solvent • if there is a catalyst, the role of a catalyst • the position of all atoms and energy of the entire system during the reaction 6-5 Gibbs Free Energy free energy change, DG0: a thermodynamic function relating enthalpy, entropy, and temperature DG0 = DH0 –TDS0 Gibbs • exergonic reaction: a reaction in which the Gibbs free energy of the products is lower than that of the reactants; the position of equilibrium for an exergonic reaction favors products • endergonic reaction: a reaction in which the Gibbs free energy of the products is higher than that of the reactants; the position of equilibrium for an endergonic reaction favors starting materials 6-6 Gibbs Free Energy • a change in Gibbs free energy is directly related to chemical equilibrium 0 DG = -RT ln Keq • summary of the relationships between DG0, DH0, DS0, and the position of chemical equilibrium DS0 < 0 DH 0 > 0 DH 0 < 0 DG 0 > 0; the position of equilibrium favors reactants At lower temperatures whenTDS0 < DH0 and DG 0 < 0, the position of equilibrium favors products DS0 > 0 At higher temperatures when TDS0 > DH0 and DG 0 < 0, the position of equilibrium favors products DG 0 < 0; the position of equilibrium favors products 6-7 Energy Diagrams change, DH0: the difference in total bond energy between reactants and products Enthalpy • a measure of bond making (exothermic) and bond breaking (endothermic) of reaction, DH0: the difference in enthalpy between reactants and products Heat • exothermic reaction: a reaction in which the enthalpy of the products is lower than that of the reactants; a reaction in which heat is released • endothermic reaction: a reaction in which the enthalpy of the products is higher than that of the reactants; a reaction in which heat is absorbed 6-8 Energy Diagrams diagram: a graph showing the changes in energy that occur during a chemical reaction Reaction coordinate: a measure in the change in positions of atoms during a reaction Energy Energy Reaction coordinate 6-9 Activation Energy Transition state: • an unstable species of maximum energy formed during the course of a reaction • a maximum on an energy diagram Energy, DG‡: the difference in Gibbs free energy between reactants and a transition state Activation • if DG‡ is large, few collisions occur with sufficient energy to reach the transition state; reaction is slow • if DG‡ is small, many collisions occur with sufficient energy to reach the transition state; reaction is fast 6-10 Energy Diagram • a one-step reaction with no intermediate 6-11 Energy Diagram A two-step reaction with one intermediate 6-12 Developing a Reaction Mechanism How it is done • design experiments to reveal details of a particular chemical reaction • propose a set or sets of steps that might account for the overall transformation • a mechanism becomes established when it is shown to be consistent with every test that can be devised • this does mean that the mechanism is correct, only that it is the best explanation we are able to devise 6-13 Why Mechanisms? • they are the framework within which to organize descriptive chemistry • they provide an intellectual satisfaction derived from constructing models that accurately reflect the behavior of chemical systems • they are tools with which to search for new information and new understanding 6-14 Electrophilic Additions • • • • • hydrohalogenation using HCl, HBr, HI hydration using H2O in the presence of H2SO4 halogenation using Cl2, Br2 halohydrination using HOCl, HOBr oxymercuration using Hg(OAc)2, H2O followed by reduction 6-15 Addition of HX Carried out with pure reagents or in a polar solvent such as acetic acid Br H CH3 CH=CH2 Propene Addition + HBr H Br CH3 CH-CH2 + CH3 CH-CH2 2-Bromopropane 1-Bromopropane (not observed) is regioselective • regioselective reaction: an addition or substitution reaction in which one of two or more possible products is formed in preference to all others that might be formed • Markovnikov’s rule: in the addition of HX, H2O, or ROH to an alkene, H adds to the carbon of the double bond having the greater number of hydrogens 6-16 HBr + 2-Butene A two-step mechanism Step 1: proton transfer from HBr to the alkene gives a carbocation intermediate slow, rate H determining CH3 CH=CHCH3 + H Br CH3 CH-CHCH3 + Br sec-Butyl cation (a 2° carbocation intermediate) Step 2: reaction of the sec-butyl cation (an electrophile) with bromide ion (a nucleophile) completes the reaction + CH3 CHCH2 CH3 Br Bromide ion sec-Butyl cation (a nucleophile) (an electrophile) fast Br CH3 CHCH2 CH3 2-Bromobutane 6-17 HBr + 2-Butene An energy diagram for the two-step addition of HBr to 2-butene • the reaction is exergonic 6-18 Carbocations Carbocation: a species in which a carbon atom has only six electrons in its valence shell and bears positive charge Carbocations are • classified as 1°, 2°, or 3° depending on the number of carbons bonded to the carbon bearing the positive charge • electrophiles; that is, they are electron-loving • Lewis acids 6-19 Carbocations • bond angles about a positively charged carbon are approximately 120° • carbon uses sp2 hybrid orbitals to form sigma bonds to the three attached groups • the unhybridized 2p orbital lies perpendicular to the sigma bond framework and contains no electrons 6-20 Carbocation Stability • a 3° carbocation is more stable than a 2° carbocation, and requires a lower activation energy for its formation • a 2° carbocation is, in turn, more stable than a 1° carbocation, • methyl and 1° carbocations are so unstable that they are never observed in solution 6-21 Carbocation Stability • relative stability H H C+ H Methyl cation (methyl) H CH3 C+ H Ethyl cation (1°) CH3 CH3 CH3 C+ CH3 C+ H Isopropyl cation (2°) CH3 tert-Butyl cation (3°) Increasing carbocation stability • methyl and primary carbocations are so unstable that they are never observed in solution 6-22 Carbocation Stability • we can account for the relative stability of carbocations if we assume that alkyl groups bonded to the positively charged carbon are electron releasing and thereby delocalize the positive charge of the cation • we account for this electron-releasing ability of alkyl groups by (1) the inductive effect, and (2) hyperconjugation 6-23 The Inductive Effect • the positively charged carbon polarizes electrons of adjacent sigma bonds toward it • the positive charge on the cation is thus localized over nearby atoms • the larger the volume over which the positive charge is delocalized, the greater the stability of the cation 6-24 Hyperconjugation • involves partial overlap of the -bonding orbital of an adjacent C-H or C-C bond with the vacant 2p orbital of the cationic carbon • the result is delocalization of the positive charge 6-25 Addition of H2O • addition of water is called hydration • acid-catalyzed hydration of an alkene is regioselective; hydrogen adds preferentially to the less substituted carbon of the double bond • HOH adds in accordance with Markovnikov’s rule CH3 CH=CH2 + H2 O Propene CH3 CH3 C=CH2 + H2 O 2-Methylpropene H2 SO4 OH H CH3 CH-CH2 2-Propanol CH3 CH3 C-CH2 HO H 2-Methyl-2-propanol H2 SO4 6-26 Addition of H2O • Step 1: proton transfer from H3O+ to the alkene + H O H + CH3 CHCH 3 : CH3 CH= CH 2 : + slow, rate determining + :O H A 2o carbocation intermediate H H • Step 2: reaction of the carbocation (an electrophile) with water (a nucleophile) gives an oxonium ion CH3 CHCH 3 : + + : O- H fast CH3 CHCH 3 O+ H : H H An oxonium ion • Step 3: proton transfer to water gives the alcohol H H + CH3 CHCH 3 + H O H : OH H : O: O+ : H : H fast : CH3 CHCH 3 6-27 Carbocation Rearrangements In electrophilic addition to alkenes, there is the possibility for rearrangement Rearrangement: a change in connectivity of the atoms in a product compared with the connectivity of the same atoms in the starting material 6-28 Carbocation Rearrangements • in addition of HCl to an alkene Cl + HCl 3,3-Dimethyl1-butene Cl + 2-Chloro-3,3-dimethylbutane 2-Chloro-2,3-dimethylbutane (the expected product; 17%) (the major product; 83%) • in acid-catalyzed hydration of an alkene + H2 O 3-Methyl-1-butene H2 SO4 OH 2-Methyl-2-butanol 6-29 Carbocation Rearrangements • the driving force is rearrangement of a less stable carbocation to a more stable one CH3 CH3 C- CHCH3 + : Cl : : + : Cl : CH3 : : CH3 CCH= CH2 + H H 3-Methyl-1-butene slow, rate determining H A 2° carbocation intermediate • the less stable 2° carbocation rearranges to a more stable 3° one by 1,2-shift of a hydride ion CH3 CH3 C- CHCH3 + H fast CH3 CH3 C- CHCH3 + H A 3° carbocation 6-30 Carbocation Rearrangements • reaction of the more stable carbocation (an electrophile) with chloride ion (a nucleophile) completes the reaction CH 3 : CH 3 C- CH 2 CH 3 + : Cl : + fast CH 3 : : CH 3 C- CH 2 CH 3 : Cl : 2-Chloro-2-methylbutane 6-31 Addition of Cl2 and Br2 • carried out with either the pure reagents or in an inert solvent such as CH2Cl2 Br Br CH3 CH=CHCH3 2-Butene + Br2 CH2 Cl2 CH3 CH-CHCH3 2,3-Dibromobutane • addition of bromine or chlorine to a cycloalkene gives a trans-dihalocycloalkane Br + Br2 Cyclohexene CH2 Cl2 Br + Br Br trans-1,2-Dibromocyclohexane (a racemic mixture) • addition occurs with anti stereoselectivity; halogen atoms add from the opposite face of the double bond • we will discuss this selectivity in detail in Section 6.7 6-32 Addition of Cl2 and Br2 • Step 1: formation of a bridged bromonium ion intermediate Br Br C C Br Br C C C Br C These carbocations are major contributing structures C C + Br - The bridged bromonium ion retains the geometry 6-33 Addition of Cl2 and Br2 • Step 2: attack of halide ion (a nucleophile) from the opposite side of the bromonium ion (an electrophile) opens the three-membered ring to give the product Br Br C C C C Br Br - Anti (coplanar) orientation of added bromine atoms C C C C Br Br - Br Newman projection of the product Br Br Br Br Anti (coplanar) orientation of added bromine atoms Br Newman projection of the product 6-34 Addition of Cl2 and Br2 • for a cyclohexene, anti coplanar addition corresponds to trans diaxial addition • the initial trans diaxial conformation is in equilibrium with the more stable trans diequatorial conformation • because the bromonium ion can form on either face of the alkene with equal probability, both trans enantiomers are formed as a racemic mixture Br + Br2 Br Br Br (1S,2S)-1,2-Dibromocyclohexane Br Br Br Br (1R,2R)-1,2-Dibromocyclohexane 6-35 Addition of HOCl and HOBr Treatment of an alkene with Br2 or Cl2 in water forms a halohydrin Halohydrin: a compound containing -OH and -X on adjacent carbons CH3 CH=CH2 + Cl2 + H2 O Propene HO Cl CH3 CH-CH2 + HCl 1-Chloro-2-propanol (a chlorohydrin) 6-36 Addition of HOCl and HOBr • reaction is both regiospecific (OH adds to the more substituted carbon) and anti stereoselective • both selectivities are illustrated by the addition of HOBr to 1-methylcyclopentene Br2 / H2 O OH Br 1-Methylcyclopentene + OH + HBr Br H H 2-Bromo-1-methylcyclopentanol ( a racemic mixture ) • to account for the regioselectivity and the anti stereoselectivity, chemists propose the three-step mechanism in the next screen 6-37 Addition of HOCl and HOBr Step 1: formation of a bridged halonium ion intermediate : : Br : : Br : : H R C C H - Br H : : Br : - : Br : C C H H R H bridged bromonium ion C C H H R H minor contributing structure Step 2: attack of H2O on the more substituted carbon opens the three-membered ring : H O: C H R C : Br : C H C + H H O : H H R : : Br : H H H 6-38 Addition of HOCl and HOBr • Step 3: proton transfer to H2O completes the reaction H R C C H H Br C + O • • H R Br H H H O • • + H3 O+ C H H O H As H the elpot map on the next screen shows • the C-X bond to the more substituted carbon is longer than the one to the less substituted carbon • because of this difference in bond lengths, the transition state for ring opening can be reached more easily by attack of the nucleophile at the more substituted carbon 6-39 Addition of HOCl and HOBr • bridged bromonium ion from propene 6-40 Oxymercuration/Reduction Oxymercuration followed by reduction results in hydration of a carbon-carbon double bond • oxymercuration OH + Hg(OAc) 2 + H2 O 1-Pentene O + CH3 COH HgOAc An organomercury compound Mercury(II) acetate Acetic acid • reduction OH OH NaBH4 HgOAc H 2-Pentanol O + CH3 COH + Hg Acetic acid 6-41 Oxymercuration/Reduction • an important feature of oxymercuration/reduction is that it occurs without rearrangement 1 . Hg(OAc) 2 , H2 O 2 . NaBH4 OH 3,3-Dimethyl-2-butanol 3,3-Dimethyl-1-butene • oxymercuration occurs with anti stereoselectivity Hg(OAc) 2 H H Cyclopentene H2 O NaBH4 OH H H HgOAc (Anti addition of OH and HgOAc) OH H H H Cyclopentanol 6-42 Oxymercuration/Reduction • Step 1: dissociation of mercury(II) acetate • Step 2: formation of a bridged mercurinium ion intermediate; a two-atom three-center bond 6-43 Oxymercuration/Reduction • Step 3: regioselective attack of H2O (a nucleophile) on the bridged intermediate opens the three-membered ring • Step 4: reduction of the C-HgOAc bond 6-44 Oxymercuration/Reduction Anti stereoselective • we account for the stereoselectivity by formation of the bridged bromonium ion and anti attack of the nucleophile which opens the three-membered ring Regioselective • of the two carbons of the mercurinium ion intermediate, the more substituted carbon has the greater degree of partial positive character • alternatively, computer modeling indicates that the CHg bond to the more substituted carbon of the bridged intermediate is longer than the one to the less substituted carbon • therefore, the ring-opening transition state is reached more easily by attack at the more substituted carbon 6-45 Hydroboration/Oxidation Hydroboration: the addition of borane, BH3, to an alkene to form a trialkylborane H H B CH2 CH3 + 3 CH2 = CH2 H Borane Borane CH3 CH2 B CH2 CH3 Triethylborane (a trialkylborane) dimerizes to diborane, B2H6 2 BH 3 B2 H6 Borane Diborane 6-46 Hydroboration/Oxidation • borane forms a stable complex with ethers such as THF • the reagent is used most often as a commercially available solution of BH3 in THF 2 : O: + B2 H6 Tetrahydrofuran (THF) 2 + :O BH3 BH3 • THF 6-47 Hydroboration/Oxidation Hydroboration is both • regioselective (boron to the less hindered carbon) • and syn stereoselective + H CH 3 1-Methylcyclopentene BH 3 H BR2 H3 C H (Syn addition of BH3) (R = 2-methylcyclopentyl) 6-48 Hydroboration/Oxidation • concerted regioselective and syn stereoselective addition of B and H to the carbon-carbon double bond H B H CH3 CH2 CH2 CH= CH2 B CH3 CH2 CH2 CH-CH 2 • trialkylboranes are rarely isolated • oxidation with alkaline hydrogen peroxide gives an alcohol and sodium borate R3 B + H2 O2 + NaOH A trialkylborane 3 ROH + Na3 BO3 An alcohol 6-49 Hydroboration/Oxidation Hydrogen peroxide oxidation of a trialkylborane • step 1: hydroperoxide ion (a nucleophile) donates a pair of electrons to boron (an electrophile) R R + R B O-O-H R A trialkylborane Hydroperoxide ion (an electrophile) (a nucleophile) R B O O H R • step 2: rearrangement of an R group with its pair of bonding electrons to an adjacent oxygen atom R R B O O H R R R B O + - O-H R 6-50 Hydroboration/Oxidation • step 3: reaction of the trialkylborane with aqueous NaOH gives the alcohol and sodium borate ( RO) 3 B + 3NaOH A trialkylborate 3ROH + Na3 BO3 Sodium borate 6-51 Oxidation/Reduction Oxidation: the loss of electrons • alternatively, the loss of H, the gain of O, or both Reduction: the gain of electrons • alternatively, the gain of H, the loss of O, or both Recognize using a balanced half-reaction 1. write a half-reaction showing one reactant and its product(s) 2. complete a material balance; use H2O and H+ in acid solution, use H2O and OH- in basic solution 3. complete a charge balance using electrons, e- 6-52 Oxidation/Reduction • three balanced half-reactions OH CH3 CH= CH2 + H2 O Propene CH3 CHCH3 2-Propanol HO OH CH3 CH= CH2 + 2 H2 O Propene CH3 CH= CH2 + 2 H+ + 2 e Propene CH3 CHCH2 + 2 H+ + 2 e 1,2-Propanediol CH3 CH2 CH3 Propane 6-53 Oxidation with OsO4 OsO4 oxidizes an alkene to a glycol, a compound with OH groups on adjacent carbons • oxidation is syn stereoselective OsO4 O O Os O O A cyclic osmate OH NaHSO3 H2 O OH cis-1,2-Cyclopentanediol (a cis glycol) 6-54 Oxidation with OsO4 • OsO4 is both expensive and highly toxic • it is used in catalytic amounts with another oxidizing agent to reoxidize its reduced forms and, thus, recycle OsO4 HOOH Hydrogen peroxide CH3 CH3 COOH CH3 tert-Butyl hydroperoxide (t-BuOOH) 6-55 Oxidation with O3 Treatment of an alkene with ozone followed by a weak reducing agent cleaves the C=C and forms two carbonyl groups in its place CH3 CH3 C= CHCH2 CH 3 2-Methyl-2-pentene O 1 . O3 CH3 CCH3 2 . ( CH3 ) 2 S Propanone (a ketone) O + HCCH 2 CH3 Propanal (an aldehyde) 6-56 Oxidation with O3 • the initial product is a molozonide which rearranges to an isomeric ozonide CH 3 CH= CH CH 3 2-Butene O3 O OO CH 3 CH- CHCH3 A molozonide H O H O CH 3 CH C C ( CH3 ) 2 S CH 3 O O Acetaldehyde An ozonide H3 C 6-57 Reduction of Alkenes Most alkenes react with H2 in the presence of a transition metal catalyst to give alkanes + H2 Cyclohexene Pd 25°C, 3 atm Cyclohexane • commonly used catalysts are Pt, Pd, Ru, and Ni • the process is called catalytic reduction or, alternatively, catalytic hydrogenation • addition occurs with syn stereoselectivity 6-58 Reduction of Alkenes Mechanism of catalytic hydrogenation 6-59 Reduction of Alkenes • even though addition syn stereoselectivity, some product may appear to result from trans addition CH3 CH3 H2 / Pt CH3 1,2-Dimethylcyclohexene CH3 + CH3 70% to 85% cis-1,2-Dimethylcyclohexane CH3 30% to15% trans-1,2-Dimethylcyclohexane (racemic) • reversal of the reaction after the addition of the first hydrogen gives an isomeric alkene, etc. CH3 CH3 1,2-Dimethylcyclohexene H2 / Pt H CH3 H Pt H CH3 CH3 CH3 1,6-Dimethylcyclohexene 6-60 DH0 of Hydrogenation Reduction of an alkene to an alkane is exothermic • there is net conversion of one pi bond to one sigma bond DH0 depends on the degree of substitution • the greater the substitution, the lower the value of DH° DH0 for a trans alkene is lower than that of an isomeric cis alkene • a trans alkene is more stable than a cis alkene 6-61 DH0 of Hydrogenation Ethylene Structural Formula CH 2 =CH2 Propene CH 3 CH=CH2 Name DH° [kJ (kcal)/m ol] -137 (-32.8) -126 (-30.1) 1-Butene -127 (-30.3) cis-2-Butene -120 (-28.6) trans-2-Butene -115 (-27.6) 2-Methyl-2-butene -113 (-26.9) 2,3-Dimethyl-2-butene -111 (-26.6) 6-62 Reaction Stereochemistry In several of the reactions presented in this chapter, chiral centers are created Where one or more chiral centers are created, is the product • • • • one enantiomer and, if so, which one? a pair of enantiomers as a racemic mixture? a meso compound? a mixture of stereoisomers? As we will see, the stereochemistry of the product for some reactions depends on the stereochemistry of the starting material; that is, some reactions are stereospecific 6-63 Reaction Stereochemistry We saw in Section 6.3D that bromine adds to 2butene to give 2,3-dibromobutane CH3 CH=CHCH3 2-Butene + Br2 CH2 Cl2 Br Br CH3 CH-CHCH3 2,3-Dibromobutane • two stereoisomers are possible for 2-butene; a pair of cis,trans isomers • three stereoisomers are possible for the product; a pair of enantiomers and a meso compound • if we start with the cis isomer, what is the stereochemistry of the product? • if we start with the trans isomer, what is the stereochemistry of the product? 6-64 Bromination of cis-2-Butene • reaction of cis-2-butene with bromine forms bridged bromonium ions which are meso and identical 6-65 Bromination of cis-2-Butene • attack of bromide ion at carbons 2 and 3 occurs with equal probability to give enantiomeric products as a racemic mixture 6-66 Bromination of trans-2-Butene • reaction with bromine forms bridged bromonium ion intermediates which are enantiomers 6-67 Bromination of trans-2-Butene • attack of bromide ion in either carbon of either enantiomer gives meso-2,3-dibromobutane 6-68 Bromination of 2-Butene Given these results, we say that addition of Br2 or Cl2 to an alkene is stereospecific • bromination of cis-2-butene gives the enantiomers of 2,3-dibromobutane as a racemic mixture • bromination of trans-2-butene gives meso-2,3dibromobutane Stereospecific reaction: a reaction in which the stereochemistry of the product depends on the stereochemistry of the starting material 6-69 Oxidation of 2-Butene • OsO4 oxidation of cis-2-butene gives meso-2,3butanediol H3 C H H 2 C H H3 C 2 3 C C H CH3 cis-2-Butene (achiral) OsO4 3 C CH3 HO OH (2S,3R)-2,3-Butanediol ROOH identical; a meso compound OH HO 2 C 3 C H H CH3 H3 C (2R,3S)-2,3-Butanediol 6-70 Oxidation of 2-Butene OsO4 oxidation of an alkene is stereospecific • oxidation of trans-2-butene gives the enantiomers of 2,3-butanediol as a racemic mixture (optically inactive) H3 C H 2 3 C C H 2 3 C C CH3 H3 C H trans-2-Butene (achiral) OsO4 CH3 H HO OH (2S,3S)-2,3-Butanediol ROOH OH HO 2 C a pair of enantiomers; a racemic mixture 3 C CH3 H H H3 C (2R,3R)-2,3-Butanediol • and oxidation of cis-2-butene gives meso 2,3butanediol (also optically inactive) 6-71 Reaction Stereochemistry We have seen two examples in which reaction of achiral starting materials gives chiral products • in each case, the product is formed as a racemic mixture (which is optically inactive) or as a meso compound (which is also optically inactive) These examples illustrate a very important point about the creation of chiral molecules • optically active (enantiomerically pure) products can never be produced from achiral starting materials and achiral reagents under achiral conditions • although the molecules of product may be chiral, the product is always optically inactive (either meso or a pair of enantiomers) 6-72 Reaction Stereochemistry Next let us consider the reaction of a chiral starting material in an achiral environment • the bromination of (R)-4-tert-butylcyclohexene • only a single diastereomer is formed Br2 (R)-4-tert-Butylcyclohexene Br Br redraw as a chair conformation Br Br (1S,2S,4R)-1,2-Dibromo-4-tert-butylcyclohexane • the presence of the bulky tert-butyl group controls the orientation of the two bromine atoms added to the ring 6-73 Reaction Stereochemistry Finally, consider the reaction of an achiral starting material in an chiral environment • BINAP can be resolved into its R and S enantiomers PPh2 PPh2 BINAP (S)-(-)-BINAP []D2 5 -223 (R)-(+)-BINAP []D2 5 +223 6-74 Reaction Stereochemistry • treating (R)-BINAP with ruthenium(III) chloride forms a complex in which ruthenium is bound in the chiral environment of the larger BINAP molecule • this complex is soluble in CH2Cl2 and can be used as a homogeneous hydrogenation catalyst (R)-BINAP + RuCl3 (R)-BINAP-Ru • using (R)-BINAP-Ru as a hydrogenation catalyst, (S)naproxen is formed in greater than 98% ee CH2 COOH H3 CO CH3 + H2 (R)-BINAP-Ru pressure COOH H3 CO (S)-Naproxen (ee > 98%) 6-75 Reaction Stereochemistry • BINAP-Ru complexes are somewhat specific for the types of C=C they reduce • to be reduced, the double bond must have some kind of a neighboring group that serves a directing group (S)-BINAP-Ru OH H2 (E)-3,7-Dimethyl-2,6-octadien-1-ol (Geraniol) OH (R)-3,7-Dimethyl-6-octen-1-ol (R)-BINAP-Ru OH (S)-3,7-Dimethyl-6-octen-1-ol 6-76 Reactions of Alkenes End Chapter 6 6-77