Chapter+14-2 - Peter Vollhardt University of California at...

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Unformatted text preview: Peter Vollhardt University of California at Berkeley I, too, star TigreDe Bue Aire 1952 lta, nos s, te d small…….. Chapter 14 Delocalized Pi Systems Delocalized Re thedoublebond call Trigonal Theπ bond is e rich: -rich: The E attack, attack add) add Thelobe of thep-orbitals: s Pe ndicular t o thesigm rpe a Pe f ram and paralle t o e e l paralle ach othe r. othe Ethe F ne R∙ 2-Propenyl (Allyl) 2-Propenyl H Que stion: What about adding a third p-orbital t hird -orbital adjace to thedoublebond? nt adjace I s the som thing spe re e cial? Or: Is the any spe re re cial activity at the Or: carbons adjace to a doublebond? nt carbons Re placing oneof thehydroge in e newith anothe sp2ns the r sp Re hybridize carbon give a prope d s nylic or allylic syste . m hybridize allylic Allylic position Obse rvations: a. a. b. H 87 kcal m -1: We ol We ak! H 101 kcal m -1 ol L S1 N S 1 re activity of allylic carbon likethat of RsecX, X, N e n though it is prim ve ary! B H B 50 c. H pKa ~ 40: Acidic! Acidic! + Clearly: Allylic - are stabilized. · Why? Re sonance ! Why? Re CH 2 H 2 C CH 2 H 2 C CH 2 H 2 C S notation: Dotte line hort ds MO Picture of 2-Propenyl (Allyl) MO 3 p Orbitals Orbitals 3 Mole cular Orbitals of E H+ HNode Now le look again at e ne t’s the Re : Bonds m by ove call Bonds ade rlap orbitals. Le look again at H2. orbitals. t’s .+ H. H HH Bonding Antibonding H · H · +H H+ + H+ I n phase H+ + HOut of Out phase phase S of thewavefunction, ign Not charge ! H2 +H σ bond σ* E π* π σ CH2 CH2 π bond What happe to this picturewhe we ns n What inte with anothe p orbital? ract r Interactions of a singly occupied p-orbital -orbital with each of the π molecular orbitals 1) 1) I nte raction of thep atom orbital with the ic Inte π bonding orbital cause thep orbital le l s ve bonding t o m up and theπ bonding le l t o m ove up ve ove down. down I nte raction of thep atom orbital with the ic Inte π* antibonding orbital cause thep orbital s le l t o m down t o where it was ve ove down le originally and that of theπ* originally antibonding orbital to m up. ove up antibonding Thetwo e cts on thep orbital cancel ffe The cancel e othe out; thetwo π orbitals are ach r pushe apart: “e rgy splitting”; d ne im portantly, theπ bonding orbital goes down. down Ethe ne π* π* 0 up Nonbondin g MO 2) p E π unchange d 3) π down Allyl p Orbital Allylic radical is stabilize by 13.5 kcal m -1. d ol Re sulting picture : # of e de nds on pe +,·,- π* 0 ? p E H 2C π H C CH2 Location Location of +,·,iisat s te ini rm te Reactivity of Allylic Position Reactivity A. Radical Haloge nation C CH H HC Me chanism : I nitiation: 1. Br2 hυ or Δ + Br2 Low conc. C C C Br HHH Faste than addition! r + HBr . 2 Br . + Br . CCC HHH CCC HHH + HBr . Propagation: 2. C C H H HC CCC HHH . Br2 . C C C Br + HHH CCC HHH . Br Te ination: rm Te . + . CH CH Br 3. C H Br2 BrC C C HH H . + Br . Br . CH CH 2 C H C CH H HC CC C HH H Anything that traps radicals, including the“dirt” on thewalls of Anything theflask, contribute to te ination. s rm t he A convenient solid brominating agent: convenient N-Bromosuccinimide, NBS Trace s, Trace always pre nt in NBS se Low Low conc. conc. Me chanism ? :O : + :O H :: :O H : : N Br : : :O + H+ :Br :− : :N O: : Br : + N Br : :: + :B r :− : : : :: O: : :O : :O H : N: : :O :: S toichiom try: Br2 doe not show up, but is theactual brom e s inating spe s! cie doe : + Br Br: P r oton sh if t (ta utom e ri sm ; Se ct ion 1 3-7 ) AllylicF11 AllylicF11 :: :N : :O H Prope ge rate a sym e ne ne s sym trical allylic radical and only one m one product. For unsym e m m ixture . Ratios de nd s Ratios pe unsym trical syste s: m on %radical characte on e carbon and TS le r ach s ading to products. B. S 1: TheAllylic C ation is S tabilize d B. N 2 + CC C C HH H H + CC C C HH H H C C C HC H H HC l -H lO C H+ C C C H OH H H HC CC HC H HC H OH +H Two products C S 2: Fast! Theallylic TSis stabilize and theallylic carbon . N2: d TS is re lative e ctrophilic. ly le re TSde localize d ‡ sp = e -withdrawing 2 C l .. .. . . - δ δ C C C HC H H HC l 100 tim s faste than e r + NaI CC C C HH H I - Cl δ C C C HI H H HC +C l - D. Allylic Organom tallics e D. CH 3 CH2 Li H 2C C CH 3 O RC H H 2C C OH CH 2 C HR CH 3 + Li CH 3 Alte rnativepre paration: allylic Grignard re nts age Br + Mg M gBr Weshall e ncounte ne r utral analogs of allylic anions: CH2 X isoe ctronic to le X : : X = OR, S NR2 R, Conjugated Double Bonds Conjugated What about CCCC CCCC ? Nom nclature C e : is/trans; E/Z. Re w C vie hapte 11 r Nom S tability: He of hydroge ats nation (kcal m -1) ol ΔH˚ C 3(C 2)3C C 2 HHHH + H2 + 2 H2 1,5-He xadie ne -30.3 -60.5 But: 1,3-Butadie ne + 2 H2 -57.1 Re sonancee rgy of butadie ~ 3.5 ne ne Structure Structure Fast S re hort lativeto an alkane C Csinglebond ― (1.54 Å). But is this a good com parison? Orbitals Orbitals Antibonding Bonding C onjugation stabilize the odynam s rm ically, but it also incre s re ase activity, for e plein e ctrophilic additions (re w C xam le vie hapte 12). r (re Markovnikov addition with a twist: Fast CH CH + HC l Re ason: I nte e rm diatecation is also stabilize d Re 1,2-Addition 1,2-Addition (“kine tic”) (“kine C l - - 1,4-Addition C 1,4-Addition l (“the odynam rm ic”) (“the C l Te inal alke le stable rm ne ss Te than inte t han rnal CH + cis C l More stable HC lAddnF11 CH Less stable Kinetic vs Thermodynamic Control Kinetic Extended Conjugation Extended + HBr Quitere active e n , ve Quitere though t hough stabilize by d conjugation conjugation CH 3 CH 3 Br Thre products e CH 3 Cation also stabilize by conjugation d The odynam stability doe not always rm ic s The e qual lack of re activity Cyclohexatriene is Special Benzene Benzene C yclic array of six e ctrons has spe stability, calle cial d six le arom aticity (C hapte 15). r arom Be neis re nze lative ine to H2-cat, e ctrophile oxidants, in ly rt s, -cat, le com parison with he xatrie . ne com Extended Conjugation in Natural and Unnatural Products Unnatural Orangecolor of carrots Biological Biological de gradation de Vision Organic Conductors Organic He ge MacDiarm S e r, id, hirakawa, Nobe Prize2000 l Light e itting diode (LEDs) s Light m Conjugated Systems Undergo Special Transformations: Pericyclic Reactions Pericyclic Theconjugate π syste can re as a unit, involving both d m act The e . For e ple nds xam , 1. C ycloadditions: TheDiels-Alder reaction, a [4+2] The cycloaddition cycloaddition HC HC CH 2 CH 2 CH 2 + CH 2 Δ 20% HC HC H2 C CH 2 CH 2 Otto Die ls 1876-1954 Kurt Alde r 1902-1958 4π-4C -4C Die ne Die 2π-2C -2C Die nophile Die C H2 Nobe Prize1950 l C ycloadduct The Diels-Alder Reaction is The Chemoselective Chemoselective Die ls-Alde re r actions work be whe wepair an st n Die e -rich (push) die with an e -poor (pull) die nophile , (push) ne die or an e -poor die with an e ne -rich die nophile De nds on substitue pe nts: e -Donating: Alkyl, alkoxy, alkylthio Alkyl 3 3 3 2 Hype rconjugation C, H Re sonance COCH 3 H O, COCH 3 S OCH 3 HC H Eve though O is e gative(inductivee ct), re n -ne ffe sonancewins out. O e -Withdrawing: C , C , C N, NO FR NO F I nductive : CF F O CR H2 C C H O CR O CR H2 C C H O CR H2 C C H O CR 90% Re sonance : Exam : ple + Δ Doe not com tewith die s pe nophile re : lative e ly -rich. S eExam s of theTre in Re om ple nd activity of Die nophile and Die s s ne H3 C < < F3 C < Die nophile H3 C H3 C H3 C Diene Increasing reactivity NC < NC NC < < < H 3CO Mechanism: Concerted + Δ Orbital de scription: sp2 sp2 sp3 Die ls-Alde re r action re quire acce s ssing thele stables-cis ss conform ation CH3 CH3 H3C CH3 H s-cis s- tr ans s-cis s-tr ans Whe s-cis formis hinde d or n re im possible there , action slows or doe not s occur. Whe die is constraine s-cis, the n ne d t ransform ation is acce rate . le d Consequences of Concertedness Consequences S re cific: Re ntion of Die te ospe te nophile S re m te oche istry (ne C w —Cbonds gre n) e O CO CH 3 O CC3 OH 80% + CH3 C3 H C is O COCH 3 C is O COCH 3 90% + H 3C CH3 Trans Trans Re ntion of Die S re m te ne te oche istry Re OCH 3 N C CN OCH3 + OCH 3 Trans,trans Trans,trans (sam for cis,cis-die ) e ne (sam N C CN CN CN CN CN OCH3 O CH3 OCH 3 N C CN + OCH 3 C is,trans N C CN CN CN CN CN H 3CO Whe both partne areste oche ically de d: “Endo rule n rs re m fine ” Whe de rm s the approach. te de ine ir Endo/Exo Addition Endo/Exo S ubstitue point away nts f romdie ne S ubstitue point nts t oward die ne Usually faster, even though product less stable: Kinetic control DAF11 Anothe e ple r xam : CH 3 CH 3 H 3C Endo CN NC NC H H o H3C CN http://csi.che ie m .tu-darm /ak/im e m l/ Ge rally: ne o i i A o i A A + A WalbaC tF Dylan o i Re asons for e rulecom x. ndo ple LipshutzC tF S govia e Alkynes as Dienophiles Alkynes Ge rate1,4-cyclohe ne xadie s ne CO2 CH 3 CO2 CH 3 OCH 3 C re an act again again OCH 3 CO2 CH 3 CO2 CH 3 75% + CH2 O2 CH 3 CH2 O2 CH 3 2. Ele ctrocyclic Re actions: I ntramolecular ring closureand ope nings closure TheC ne ne The yclobute 1,3-Butadie Equilibrium Δ, Ea = 32.9 kcal 32.9 m -1 ol Exothe ic (ring rm strain re ase strain le d) hυ Light drive C be the odynam . n: an at rm ics Light Wave ngth de nde (can go e r le pe nt ithe way). way). Im e ml The1,3-C yclohe xadie 1,3,5ne The He xatrie Equilibrium ne He Light drive C be the odynam . n: an at rm ics Wave ngth de nde (can go e r le pe nt ithe way). way). Δ, Ea = 29.9 kcal 29.9 m -1 ol Endothe ic rm Endothe (C Cbe r than C C tte , and no ring strain pre nt) se hυ Im e ml Electrocyclic Reactions are Stereospecific Stereospecific Δ CH 3 Only! cis–3,4-Dim thylcyclobute e ne CH 3 CH 3 cis,trans–2,4-He xadie ne Δ Only! Trans CH 3 Trans,trans Movement of Substituents Movement C onrotatory: sam dire e ction sam CH 3 CH 3 Both e r Both ithe clockwiseor counte rclockwise S e : am product. product. The rotatein the y H C onrotatory H CH 3 H CH 3 Δ H Im e ml CH 3 H CH 3 H C onrotatory (clockwise ) (clockwise CH3 H H3 C H Δ C ounte rclockwiseconrotation in principlepossiblebut ste rically prohibite : d ste CH3 H CH 3 H Δ H H CH 3 CH 3 Fascinatingly, hυ goe disrotatory (rotation s disrotatory in oppositedire opposite ctions) CH 3 CH 3 hυ dis CH 3 CH 3 CH 3 CH 3 CH 3 hυ dis CH 3 Eve m startling: Thehe n xatrie /cyclohe ne xadie inte ne rconve rsion is Eve ore also ste ospe re cific, but follows theoppositerule of se of rotation, nse ste opposite s com d to thebutadie /cyclobute syste : pare ne ne m com Δ = dis dis Im e ml hυ = con con Robe B. Woodward rt Robe 1917-1979 Roald Hoffm ann b. 1937; NP 1981 Electronic Spectroscopy Electronic (Ultraviolet-Visible or UV) White(sun) light is com d of the pose White visiblespe ctrum Re e be mm r spe ctroscopy (C hapte 10): r (C Excite state d E Ground state ΔE = hυ = hc/ λ UV-Vis spe ctroscopy re quire m highe e rgy than NMR (kcals vs s uch r ne calorie doe not ne d e rnal “condition” (m t). Ele s), s e xte agne ctronic e xcitation f rombonding to antibonding le ls, particularly e for π syste s, be ve asy m cause occupie ounoccupie ΔE re d d lative sm ly all. occupie re For e ple look at a sim π bond, as in e ne ple the : For xam , No π bond le bond ft! I .e Light cause ., s cis-trans isom rization, e radical re radical actions, …. UV Spectrum of Ethene UV Quote as λmax d Quote Broad, be Broad, causeof rotational and vibrational state Ele s. ctronic spe ctroscopy is fast, no “ave raging” is A 171 nm 9 Wave ngth λ (give in nm units of 10--9 m not in fre ncy υ = c/ λ, as we le n nm , ; que Wave did in NMR, whe λ ~ 100 m to 1m re 100 m !) E (kcal m -1) = 28,600/ λ (nm ol ) UV spe ctroscopy be 200 nmre low quire vacuum be s , causeair absorbs. Norm (in atm ally osphe ) onescans 220-400 (UV), 400-800 nm(visible re ). This allows lowe e rgy transitions to bere r ne corde e 1,3-butadie : d, .g. ne This E Re lative ly Re low e rgy low ne λmax Pe he ak ights arere porte as ε : d Pe Extinction coe nt which Extinction fficie , which is absorbancenorm d by conce alize ntration: absorbance conce S houlde sh r, λmax = 222.5 nm (ε = 10,800) ε = A/ c Visible Absorption: Color Visible Ne wton Ne Light e rs theprismfromthetop right, and is nte re fracte by theglass. Theviole is be m than d t nt ore t heye llow and re so thecolors se d, parate . Absorption in Absorption thevisible t he 450 nmorange re 450 orange d 550 nmviole t 550 viole 650 nmblue gre n 650 blue e C of olor substance substance I n e nde π syste s m transitions are possible giving rise xte d m any , t o m com x and not re ore ple adily inte tablespe rpre ctra, but HOMO-LUMO gap ge sm r: Longe wave ngth ts alle st le absorption is indicativeof thee nt of conjugation, e xte .g., Gre r conjugation: ate S alle HOMO/LUMO gap mr CH 3 λ max = 271 nm C onjugate trie d ne λ max = 217 nm Unconjugate trie d ne Azule ne Azule ...
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