Chapter 14 ALKENES AND ALKYNES II. Oxidation and Reduction Oxymercuration and Hydroboration Simple Ring Formation

Assigned Sections Sections 14.1 through 14.4 (c. 352) Summary (Table 14-1) Skip section 14.3 Sections will be in c. 353

Problem Assignment Chemistry 352 and 353 In-Text Problems: 1 – 34b End-of-Chapter Problems: – – (all but a) , –

What is Oxidation? Gain of oxygen Loss of hydrogen Skim section between p. 1 and 2 Oxidation

epoxidation cleavage glycol formation complete oxidation n + H 2 O OXIDATIONS OF ALKENES

What is Reduction? Gain of hydrogen Loss of oxygen reduced

Sect Sect EPOXIDES EPOXIDES anti-glycol formation

anti glycolepoxide 1) the formation of epoxides, and First we will look at 2) the opening of epoxides to form glycols generally “trans” EPOXIDATION AND ANTI GLYCOL FORMATION

Carboxylic Acid Hydrogen peroxide Organic Peroxide A PERACID

TWO COMMONLY-USED PERACIDS m-chloroperbenzoic acid (MCPBA) peracetic acid

A CONCERTED REACTION epoxide STEREOSPECIFIC EPOXIDATION WITH A PERACID both bonds formed at the same time

anti-Glycol Formation EPOXIDATION RING OPENING anti ring opening

OPENING EPOXIDES IN BASE See Chapter 12, Section 12.8 S N 2 : hydroxyl attacks at least substituted carbon, anti only SN2SN2 :: : :::: : :.. :: : and on the opposite side STEREOSPECIFIC

OPENING EPOXIDES IN ACID See Chapter 12, Section 12.8 for a review. My Chapter 12 webpage give some examples

Sect Sect syn-GLYCOL FORMATION

syn glycols are made with OsO 4 or KMnO 4 OsO 4 KMnO 4 osmium tetroxide potassium permanganate syn -Glycol Formation

H CH 3 H CH 3 OH OH OsO 4 NaHSO 3 2 HO OsO 4 NaHSO 3 H 2 O C C H H CH 3 CH 3 C C OH OH H H CH 3 CH 3 meso SYN ADDITION GIVES CIS GLYCOLS cis remember: addition is syn result is cis cis -2-butene syn conformation

syn-Glycol Formation (I) syn hydroxylation osmium tetroxide

Both of the hydroxyl oxygens in the glycol come from OsO 4 REDUCED OXIDIZED Mechanism Notice the transfer of 2e - onto Os = REDUCTION

syn-Glycol Formation (II) syn hydroxylation potassium permanganate

Making epoxides via bromohydrins Bromohydrin (Chapter 8) ( peracid )

Skip Sect. 14.3

Sect Sect. 14.4OZONOLYSIS

Ozone electric discharge or cosmic rays.. : : : EQUIVALENT RESONANCE STRUCTURES

FORMATION OF AN OZONIDE Ozonolysis HYDROLYSIS OF THE OZONIDE (WORKUP) unstable aldehydes or ketones

WORKUP PROCEDURES FOR OZONOLYSIS Two types of workup (decomposition of the ozonide) are possible : 1. OXIDATIVEHydrogen peroxide is present 2. REDUCTIVE METHOD A Add Zn and H 2 O or H 3 O + Aldehydes are oxidized to carboxylic acids. Formaldehyde is oxidized to carbon dioxide, which is lost as a gas. METHOD B Reduce the ozonide with Pd / H 2, and then add acid ( H 3 O + ). Aldehydes survive intact and are not oxidized with reductive conditions.

EXAMPLES O3O3 Zn / H 2 O 1)O 3 2) H 2 O 2 H 3 O + OR O3O3 1) Pd/H 2 2) H 3 O + REDUCTIVE WORKUP OXIDATIVE WORKUP + + H 2 O

AT ONE TIME OZONOLYSIS WAS WIDELY USED FOR STRUCTURE PROOF BY DEGRADATION FOR STRUCTURE PROOF BY DEGRADATION Unknown compound Broken apart ( or degraded ) to simpler pieces that are easier to identify. The original structure can be deduced by reassembling the pieces. “At one time” = before spectroscopy.

C 7 H 12 C 7 H 14 1) O 3 / CH 2 Cl 2 2) H 3 O + Pd / H 2 PROBLEM TO SOLVE answer

WHAT WAS THE ORIGINAL STRUCTURE ? oxidative workup H2O2H2O2

ACETYLENES KMnO 4 or 1) O 3, CH 2 Cl 2 2) H 3 O + Oxidation of acetylenes, whether by KMnO 4 or ozone, normally yields carboxylic acids.

Spruce, Cedar, Fir or Pine Forest Terpenes O3O3 NATURAL SOURCES OF SMOG reacts with terpenes temperature inversion traps bioemissions

Sect Hydrogenation of Alkenes + CCHH CC HH catalyst The catalyst is Pt, PtO 2, Pd, or Ni (in special cases Ru, Rh, Re) a syn addition reaction fine powder, very porous, suspended in solution

CATALYTIC HYDROGENATION IS COVERED IN SEVERAL PLACES IN THE TEXT, PRINCIPALLY: Ch3 Section 3.18 Hydrogenation of Alkenes Ch4 Section 4.13 Relative Stabilities of Alkenes Ch6 Section 6.12 Heat of Hydrogenation Section 6.13 Resonance Energy of Benzene Ch8 Section 8.11 Hydrogenation of the Double Bond Pages (Addition to Triple Bonds) REVIEW OF HYDROGENATION Ch 14 Section 14.4 Hydrogenation of Alkynes

Hydrogenation of Alkynes

Lindlar is a special catalyst that allows the hydrogenation of an alkyne to stop after one mole of hydrogen is added. quinoline syn addition Most amines, and compounds containing sulfur, reduce the activity of catalysts or “poison” them. Lindlar Catalyst: syn addition

Sect 14.6 Dissolving Metal Reduction anti This reaction proceeds with anti addition (trans compound). Catalytic reduction proceeds with syn addition, hence we have a choice of methods.

MECHANISM OF Na-LIQUID-NH 3 REDUCTIONS ( from (NH 3 ) n ). - anti addition electron transfer 1 electron transfer 2 +2e +2H + :NH 2 - All intermediates prefer the trans geometry. radical- anion radical anion

SODIUM IN LIQUID AMMONIA SODIUM AMIDE IN LIQUID AMMONIA TWO DIFFERENT REAGENTS ! Reducing Agent Strong Base = = NaNH 2 / NH 3 (l) Na / NH 3 (l)

Sect. 14.7: Oxymercuration of an Alkene

Mechanism of Oxymercuration Formation of Bridged Ion: step 1

Step 2: Attack by water

Step 3: Reduction

Another example Continued next slide

Sect. 14.8: Hydroboration of an Alkene

Herbert Brown Source: Michigan State University, Department of Chemistry Nobel Prize, 1979

Preparation of Diborane Electron deficient

DIBORANE ADDS TO ALKENES ANTI-MARKOVNIKOFF sp 2 empty 2p orbital Since there are 3 B-H bonds, addition can occur three times (3 moles of alkene) Boron adds to the carbon with the most hydrogens (!) which is the reverse of the Markovnikoff Rule. concerted syn addition electrophile RR R substituent stabilizes transient carbocation center hydride transfer anti-Markovnikoff

++  Concerted reaction, no intermediate; syn addition Carbocation intermediate would allow rotation; the reaction would not be stereospecific, giving both syn + anti addition. Concerted versus Carbocation Intermediate RR R

Hydroboration of an Alkene a trialkylboron compound ( or a trialkylborane ) 3 moles note abbreviation above bubble B 2 H 6 gas through the solution ““

Stereochemistry of Hydroboration Syn-additiion

Sect. 14.9: Hydroboration - Protonolysis

Sect : Hydroboration - Oxidation

H2O2H2O2 OH - B(OH) 3 + trialkylborane alcohol ( 3 moles) basic hydrogen peroxide trialkoxyborane acid H3O+H3O+ This work up step hydrolyzes the trialkoxyboron compound.

.. :: H-O-O-H + OHO-O-H + H-O-H ::.. : hydroperoxide ion trialkoxyborane 2 more times internal S N 2 - adds into empty orbital cast-off

Stereochemistry of Hydroboration-Oxidation syn addition retains cis stereochemistry cis syn H3O+H3O+ (work up)

Summary of Hydroboration Reactions

Hydroboration-Protonolysis of Alkynes Only one pi bond reacts A second reaction sequence is required to react the second pi bond.

Hydroboration - Oxidation of Alkynes unfavored tautomer ! Enol Ketone ‘ Only one pi bond reacts immediate tautomerism CCR R' CC R H R' B ‘ CC R H R' OH 3+ BH 3 3 H 2 O 2 / OH _ H3O+H3O+

OXYMERCURATION VS. HYDROBORATION-OXIDATION OXYMERCURATION HYDROBORATION-OXIDATION 1) Hg(OAc) 2 2) NaBH 4 1) B 2 H 6 2) H 2 O 2, NaOH Markovnikoff Additionanti-Markovnikoff Addition antisyn result similar to 3M H 2 SO 4 /H 2 O 3) H 3 O +

SYNTHESIS PROBLEM Show how to synthesize all five compounds from acetylene and methanol. Requires both Na / NH 3 ( liq ) and NaNH 2 / NH 3 ( liq ). and cis isomer

Sect : The Diels-Alder Reaction

Otto Diels and Kurt Alder Source: Michigan State University, Department of Chemistry

HOMO LUMO Diels-Alder Reaction Diels-Alder Reaction The reaction is concerted - all of the orbitals are aligned in a 6-ring. The HOMO of the diene donates elec- trons into the LUMO of the dienophile. Push Pull

      LUMO HOMO BUTADIENEETHYLENE

DieneDienophileA Cyclohexene  Works best if the dienophile has electron-withdrawing groups and the diene has electron-donating groups. The HOMO of the diene donates electrons into the LUMO of the dienophile. pull EXAMPLE - WITH ELECTRONIC FACTORS push

FORMATION OF A BICYCLIC COMPOUND 

 

Sect : Carbene formation and reactions Carbenes are divalent carbon atoms Carbenes are electrophiles and react readily with double bonds Carbenes add syn to alkene Carbene addition is usually stereospecific

Formation of dichlorocarbene

Reactions of dichlorocarbene with alkenes

Diazomethane reactions

Another diazomethane reaction Light

Sect : Carbenoid reactions These reactions are like carbene reactions, but do not give formal carbenes as intermediates. An example is the Simmons-Smith reaction that uses Zn-Cu couple and CH 2 I 2 Products formed from carbenoid reactions are identical to those from pure carbene reactions. syn-addition to alkene and stereospecific.

Simmons-Smith Reaction